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Summer 8-2007 A 3D REGIONAL MODEL OF THE INDONESIAN SEAS CIRCULATION Kieran Thomas Anthony O'Driscoll University of Southern Mississippi
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Recommended Citation O'Driscoll, Kieran Thomas Anthony, "A 3D REGIONAL MODEL OF THE INDONESIAN SEAS CIRCULATION" (2007). Dissertations. 1284. https://aquila.usm.edu/dissertations/1284
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A 3D REGIONAL MODEL OF THE INDONESIAN SEAS CIRCULATION
by
Kieran Thomas Anthony O’Driscoll
Abstract of a Dissertation Submitted to the Graduate Studies Office of The University of Southern Mississippi in Partial Fulfillment o f the Requirements for the Degree of Doctor of Philosophy
August 2007
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. COPYRIGHT BY
KIERAN THOMAS ANTHONY O’DRISCOLL
2007
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT
A 3D REGIONAL MODEL OF THE INDONESIAN SEAS CIRCULATION
by
Kieran Thomas Anthony O’Driscoll
August 2007
This study describes the ocean circulation of the Indonesian Seas based on results
using a 3D regional model. The study is divided into 3 parts. In the first part, Chapter
2, the basic properties o f a developed regional model o f the circulation o f the
Indonesian Seas are outlined. It is well known that the complex topography of the
region strongly influences temperature, salinity and current distributions there. One o f
the significant properties of this model is that all basic topographic features are
resolved. The model has four open ports to simulate inflow o f North Pacific Water
from the Mindanao Current, inflow o f South Pacific Water from the New Guinea
Coastal Current, outflow to the Pacific Ocean due to the North Equatorial Counter
Current, and outflow to the Indian Ocean due to the Indonesian Throughflow. Total
transports through the open ports and port normal velocities are specified from
observations. Orlanski's conditions are employed at the open ports with port normal
velocity nudged to observed values and temperature and salinity to climatology. Port
channels are introduced so the effects of open boundary conditions do not impact the
dynamics of the main region. An additional friction was included in the vicinity of
some narrow passages and sills as a proxy for specific processes such as tides and
internal waves that occur within these topographic features. Four experiments are
discussed: seasonally varying and annual mean transports and port normal velocities
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. both with and without local winds. All experiments are totally spun up after 10 years.
This analysis uses data from the post spin up period only. The basic properties of
simulated total transports through the main passages in the region, surface circulation
and sea surface heights are discussed. The portion of North Pacific Water entering the
Indonesian Seas relative to that leaving through the North Equatorial Counter Current
port is fairly constant throughout the year. Most o f this water takes the western route
through the Makassar Strait. The portion o f South Pacific Water entering the
Halmahera Sea compared to that exiting in the North Equatorial Counter Current
varies considerably with the seasons. Turning off the local winds does not
substantially influence the transport through main passages in the model domain.
Surface circulation patterns change substantially with the seasons. The role of
different terms in the heat and salt equations was investigated by dividing the region
into a number of boxes. For any given box, the sum of the horizontal advective fluxes
o f temperature (salinity) through all sides of the box is on the same order as the
vertical heat (salt) flux at the surface, interior nudging term, and the rate o f time
variation of the box integrated temperature (salinity). The comparison o f the basic
structure o f the model surface circulation, sea-surface heights and total transport
values through the main passages with observations appears satisfactory.
The main objective of the second part, Chapter 3, is to analyze the basic features of
potential temperature and salinity distributions in the Indonesian Seas simulated by
the model. The influence o f bottom topography on the formation o f temperature and
salinity distributions is considered by following the three major routes o f flow of
North Pacific and South Pacific Water through the Indonesian Seas. Major elevations
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of bottom relief, such as the Sangihe Ridge; the topographic rise between the
Sulawesi Sea and Makassar Strait; the Dewakang Sill; the ridge between the Flores
and Banda Seas; the topographic rise between the Pacific Ocean and Morotai Basin;
the Lifamatola Sill; and the northern and southern Flalmahera Sills; break the region
down to separate basins having different temperature and salinity stratifications. The
differences in stratification are caused by these topographic features that act to
impede the advection of cold and salty water from a basin (located upstream) to the
neighboring basin (located downstream). Arguments are included to support this
conclusion. In the upper ocean (500m), the Indonesian Throughflow is primarily
shaped by North Pacific Water taking the route through the Makassar Strait. Deep
Banda Sea water is formed by the overflow of North Pacific Water across the
Lifamatola Passage into the Banda Sea. Below 500m South Pacific Water is blocked
by the Halmahera Sills and does not enter the Indonesian Seas. But in the upper ocean
(0-500m) SPW can probably penetrate into the Seram and Maluku Seas to mix with
NPW there.
There are no substantial structural changes o f potential temperature and salinity
distributions between seasons, though values o f some parameters o f temperature and
salinity distributions (e.g., magnitudes o f maxima and minima) can change. It is
shown that the main structure o f the observed distributions o f temperature and salinity
is satisfactorily displayed throughout the entire model domain. The calculated
transports of internal energy (heat) and salt mass through the Lombok and Ombai
Straits, and Timor Passage in August and February are in reasonable agreement with
published observed and simulated data.
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In the last part, Chapter 4, aspects o f turbulence and mixing in the Indonesian Seas
are presented and discussed. The results are based on the Mellor-Yamada 2.5
turbulence parameterization model. Though the importance of mixing in the
Indonesian Seas has been widely acknowledged, very few observations are available
and there have been no model studies of mixing or turbulent diffusion in the region.
The study is focused on turbulent diffusion and turbulent kinetic energy in the upper
mixed layer, the thermocline and in deep water near topographic features. Very large
turbulent kinetic energies and vertical turbulent diffusivities are seen around
topography and are important for the deep overflows found in the region. Large
turbulent energies and diffusivities found in the thermocline are important for the
diffusion o f temperature and salinity signatures found in the Indonesian Seas.
Monsoon winds and local currents lead to large diffusivities in the upper mixed layer.
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. to my mam and late dad
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS
I would like to thank my PhD advisor Professor Vladimir Kamenkovich. Vladimir
has made a huge investment o f his time in me over the last 5 years for which I am
very grateful. I have certainly learned a lot about oceanography, and about life, from
Vladimir during this time and I hope our collaboration and friendship will continue
for many years to come. Thanks also to my committee members: to Professor Ralph
Goodman for his encouragement, enthusiasm for life and his friendship; to Professor
Dmitri Nechaev for his help with the model, the coding and many questions about
oceanography; to Drs. John Kindle and Pat Hogan from the Naval Research
Laboratory for their help and input into the process. I wish to thank the Naval
Oceanographic Office. NAVO have financed my PhD education while giving me
some time to do this work. Thanks also for making resources such as supercomputers
available to make the model runs. I also thank my colleagues at NAVO for their
continued encouragement and friendship. I thank everybody at USM/DMS for all
their help. Finally, thanks to my friends and family for all their love, friendship and
encouragement. I appreciate it.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS
ABSTRACT...... ii
DEDICATION...... vi
ACKNOWLEDGMENTS...... vii
LIST OF TABLES...... x
LIST OF ILLUSTRATIONS...... xi
LIST OF ABBREVIATIONS ...... xviii
CHAPTER
1. INTRODUCTION...... 1
1.1. Background ...... 2
1.2. O bjectives ...... 5 1.3. A Summary o f the Study ...... 5
2. MODEL DESCRIPTION AND PROPERTIES OF THE SIMULATED VELOCITY FIELDS...... 9
2.1. Introduction ...... 9 2.2. Model Description ...... 5 2.3. Results ...... 30 2.4. Estimate o f Terms in Heat and Salt Equations...... 48 2.5. Comparison with Observations ...... 56 2.6. Summary and Conclusions ...... 62
3. THE INFLUENCE OF BOTTOM TOPOGRAPHY ON TEMPERATURE AND SALINITY DISTRIBUTION ...... 69
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.1. Introduction ...... 69 3.2. Transports of Mass, Internal Energy and S alt ...... 71 3.3. The Analysis of Temperature and Salinity Distributions ...... 12 3.4. Seasonal Variation ...... 96 3.5. Comparison with Observations ...... 107 3.6. Summary and Conclusions ...... 114
4. THE ANALYSIS OF DISTRIBUTIONS OF TURBULENCE CHARACTERISTICS BASED ON THE MODEL RESULTS...... 119
4.1. Introduction...... 119 4.2. The Turbulence Parameterization Scheme: The Mellor-Yamada (M-Y) 2.5 Model ...... 120 4.3. Results ...... 123 4.4. Discussion and Conclusions...... 143
5. SUMMARY...... 147
5.1. Future W ork ...... 149
APPENDICES...... 150
REFERENCES...... 158
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES
Table
1. Subregions of the model domain where additional horizontal friction has been applied as follows: -r(i,j)u(i,j,k) and -r(i,j)v(i,j,k). The values of r are maximal at the centers o f the subregions and decay exponentially toward the boundaries ...20
2. List o f experiments. Transports out o f the model domain are positive; transports into the model are negative. In the first column 0 -, S-climatology, and wind stresses are characterized...... 29
3 . Estimates of different terms in the depth integrated heat, 0, and salt diffusion, S, equations for the Seram Sea box. August values are given for the seasonally varying experiment with local wind. Qs are the total transports through the sides o f the box. Negative values o f Q are into the box and positive values are out of the box. The last line gives the estimate of accuracy of calculations. Values for specific heat and reference density are, respectively, cp = 4 x 103 J °C'' kg'1, Po= 1025 kgnf3 ...... 54
4. The simulated transports o f mass, internal energy (heat), and salt through MC, NECC, and NGCC ports, along with Lombok and Ombai Straits, and Timor Passage for August and February. Additionally, the corresponding transports for the upper 500m are given. The specific heat capacity at constant pressure is cp=4x 103J°C'1kg'1. The mean density is given by p(rT025 kgm'3, 1 PW= 1015Js‘...... 72
C. The same as Table 3 but for different boxes in the region: C. l (above) the Banda Sea box, C.2 the Makassar Strait box and C.3 box to the north and east of Lombok Strait ...... 155
x
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF ILLUSTRATIONS
Figure
2.1(a). Model bathymetry and domain (a) (above) and names o f important topographic features, open ports and islands (b). Note that in (b) the model domain is shown in x, y coordinates (i, j are given), and the depth in the grey region is less thanlOOm. Notice also that the island chain extending from Java in the west to Timor Island in the east is the Lesser Sunda Island Chain 15
2.1(b) ...... 16
2.2. Modeled mid-port normal velocity profiles at the 10 port (a), NGCC port (b), NECC port (c), and MC port (d). The profiles at (a), (c) and (d) are assumed time independent. Profiles for August and February are also given for (b). ...22
2.3. The evolution of Ev and Es with time. The characteristics Ev are shown in (a) for the seasonally varying experiment with local wind (exp 7; solid line) and the annual mean experiment with local wind (exp 1; broken line). The characteristics Es are shown in (b) for the same two experiments as (a). Variability of the integrated value of Ev in exp 1 is ~ 0. Ix l0 17kg m2/s2 and for Es is ~ 0 .5 x l0 13 kg m /s2 ...... 30
2.4(a). Transports across several sections of the model domain including the four open ports. Values for experiment 7 (seasonally varying with local wind) for August and February, and experiment 1 (annual mean with local wind) are shown in (a) (above), (b) and (c), respectively. Values for experiment 8 (seasonally varying with no local wind) for August and February, and experiment 2 (annual mean with no local wind) are shown in (d), (e) and (f), respectively. These snapshots were taken after ten years of model integration. Some arrows are slanted for graphic clarity...... 32
2.4(b) ...... 33
2.4(c) ...... 34
2.4(d) ...... 35
2.4(e) ...... 36
2.4(f) ...... 37
2.5. Transport time series through important passages and straits for four different model runs after 10 years o f model run. The seasonally varying experiments
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. with (exp 7) and without local winds (exp 8) are given by the solid and broken lines, respectively. The annual mean experiments with (exp 1) and without (exp 2) local wind are given by the . and x symbols, respectively. The sections shown are: (a) North Makassar Strait, (b) Lombok Strait, (c) Lifamatola Passage, (d) Halmahera Sea, (e) Obi-Seram passage, (f) Mangole-Buru passage, (g) Timor passage, (h) Ombai Strait. Note the different transport scales on the y-axes. For exps 7 and 8 (lines), transports are calculated every 10 days while for exps 1 and 2 transports are calculated only at times given by . and x symbols ...... 38
2.6(a). Surface velocities in m/s. Vectors for experiment 7 for August and February, and experiment 1 are shown in (a) (above), (b) and (c), respectively. Values for experiment 8 for August and February, and experiment 2 are shown in (d), (e) and (f), respectively. These snapshots were taken after ten years of model integration ...... 39
2.6(b) ...... 40
2.6(c) ...... 41
2.6(d )...... 43
2.6(e) ...... 44
2.6(f) ...... 45
2.7(a). Sea surface height. Values for experiment 7 for August and February, and experiment 1 are shown in (a) (above), (b) and (c), respectively. Values for experiment 8 for August and February, and experiment 2 are shown in (d), (e) and (f), respectively. These snapshots were taken after ten years of model integration ...... 47
2.7(b)...... 49
2.7(c)...... 50
2.7(d)...... 51
2.7(e)...... 52
2.7(f)...... 53
2.8. The eight boxes for which estimates o f terms in the heat and salt diffusion equations have been made. The boxes are: 1. Seram Sea, 2. Banda Sea, 3 Makassar Strait, 4. Lombok, 5. East Maluku Sea, 6. Karakelong-Sangihe- Morotai, 7. Halmahera Sea, and 8. West Sulawesi Sea ...... 55
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.1. The locations of vertical sections AGHIJKF, ABCDEF, and LMND are shown by dots. Vertical profiles o f temperature and salinity are provided at points identified by letters A ,..., N. Sections, at which normal velocity and salinity are analyzed, are shown by solid lines. Numbers give August transports through these sections in Sverdrups (106 m3/s) in the direction of arrows. All arrows show transport directed normally to sections; slanting of some arrows is made for convenience o f plotting. Values o f indices i and j are given on the x- and y-axes. The Mindanao Current (MC), North Equatorial Counter Current (NECC), New Guinea Coastal Current (NGCC), and Indian Ocean (10) ports are seen where transports enter and leave the model domain (compare with Figure 2.1(b)) ...... 73
3.2(a). Vertical profiles o f potential temperature along the western route for August at points A, G, H, I, J, K, and F (a) (above) and the distribution o f potential temperature ort vertical section AGHIJKF (b) (shown in Figure 3.1), The bottom topography along this section is shown. Note the break of vertical scale at 300m. Locations o f points A, G, H, I, J, K, and F are indicated along the top o f the figure. Distance from point A is shown on the x-axis. Geographical names and locations are shown (see also Figure 2.1(b))...... 75
3.2(b) ...... 76
3.3(a). Vertical profiles of salinity along the western route for August at points A, G, H, I, J, K, and F (a) (above) and the distribution of salinity on vertical section AGHIJKF (b) shown in Figure3.1. Further explanations are similar to those in the caption to Figure 3.2...... 78
3.3(b) ...... 79
3.4. Isotachs of normal velocity (color) and isohalines (solid lines) at the Sangihe section (i=156; the values of j are indicated on the x-axis). Velocities are in m/s. For reference see Figure 3.1. The bottom topography is shown. The western route section (Figures 3.2(b), 3.3(b)) passes through this section at j=223 ...... 80
3.5. The same as Figure 3.4 but for the Dewakang section (j= l38; the values o f i are indicated on the x-axis). The vertical line shows the location of the section through the Dewakang sill (j=50) presented in Figure 8. The western route section (Figures 3.2(b), 3.3(b)) passes through this section at i=57 ...... 81
3.6. The same as Figure 3.4 but for the Flores-Banda section (i= 78; values of j are indicated on the x-axis). The western route section (Figures 3.2(b), 3.3(b)) passes through this section at j= l 10 ...... 8 2
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.7. v-velocities in m/s at the section through the Dewakang sill (i= 50; values of j are indicated on the x-axis). See Figure 3.5 for the location o f this section. The bottom topography is shown. The numbers on the x-axis (j) should be reversed with the largest on the left and the least on the right ...... 83
3.8. The distribution o f the POM 'vertical' velocity wpom on vertical section AGHIJKF (see Figure 3.1). Velocities are in m/s. The results are noisy but signs of wpom are provided at some points. Bottom topography along this section is shown. Distance from point A is shown on the x-axis. See Figure 3.3(b) for geographical names ...... 84
3.9(a). Vertical profiles o f potential temperature along the northern route for August at points A, B, C, D, E, and F (a) (above) and the distribution o f potential temperature on vertical section ABCDEF (b) (see Figure 3.1). Further explanations are similar to those in the caption to Figure 3.2...... 85
3.9(b)...... 86
3 .10. Vertical profiles o f salinity along the northern route for August at points A, B, C, D, E, and F (a) (above) and the distribution o f salinity on vertical section ABCDEF (b) (see Figure 3.1). Further explanations are similar to those in the caption to Figure 3.2 ...... 87
3.10(b)...... 88
3.11. The same as in Figure 3.8 but for the vertical section $ABCDEF$ (see Figure 3.1 ...... 89
3.12. The same as in Figure 3.4 but for the North Maluku section (j=187; values of i are indicated on the x-axis). The northern route section (Figures 3.9(b), 3.10(b)) passes through this section at i=151 ...... 90
3.13. The same as in Figure 3.4 but for the Lifamatola section ( p i 50; values o f i are indicated on the x-axis). The vertical line gives the location of the section through the Lifamatola passage (i=l 39) in Figure 3.14. The northern route section (Figures 3.9(b), 3.10(b)) passes through this section at i=137 ...... 91
3.14. v-velocities in m/s at the section through the Lifamatola passage (i= 139; values of j are indicated on the x-axis). See Figure 3.13 for section location. The bottom topography is shown. The numbers on the x-axis (j) should be reversed with the largest on the left and the least on the right...... 92
3.15(a). Vertical profiles of potential temperature along the eastern route for August at points L, M, N, and D (a) (above) and distribution o f potential temperature on vertical section LMND (b) (see Figure 3.1). Further explanations are similar to those in the caption to Figure 3.2 ...... 93
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.15(b) 94
3 .16(a). Vertical profiles o f potential temperature along the eastern route for August at points L, M, N, and D (a) (above) and distribution o f potential temperature on vertical section LMND (b) (see Figure 3.1). Further explanations are similar to those in the caption to Figure 3.2 ...... 95
3.16(b) ...... 96
3.17. The same as in Figure 3.4 but for the northern Halmahera sill ( p i 66; values of I are indicated on the x-axis). The eastern route section (Figures 3.15(b), 3.16(b)) passes through this section at i=171 ...... 97
3.18. The same as in Figure 3.4 but for the Halmahera sea section ( p i 57; values of i are indicated on the x-axis). The eastern route section (Figures 3.15(b), 3.16(b)) passes through this section at i=l 6 4 ...... 98
3.19. The same as in Figure 3.4 but for the southern Halmahera sill (p l4 9 ; values of i are indicated on the x-axis). The eastern route section (Figures 3.15(b), 3.16(b)) passes through this section at i=156 ...... 99
3.20. The same as in Figure 3.8 but for the vertical section LMND (see Figure 3.1). Distances from point L are shown on the x-axis ...... 100
3.21. The same as in Figure 3.4 but for the section between Obi and Seram islands (i=148; values of j are indicated on the x-axis). The eastern route section (Figures 3.15(b), 3.16(b)) passes through this section at j=144 ...... 101
3.22. The same as in Figure 3.4 but for the section between Obi and Seram islands (i=148; values of j are indicated on the x-axis). The eastern route section (Figures 3.15(b), 3.16(b)) passes through this section at j= 144 ...... 102
3.23. The same as in Figure 3.4 but for the section across the Lombok Strait (j=l 10; values of i are indicated on the $x$-axis) ...... 103
3.24. The same as in Figure 3.4 but for the section across the Ombai Strait (i= 90; values of j are indicated on the $x$-axis) ...... 104
3.25. The same as in Figure 3.4 but for the section across the Timor Passage (i= 90; values of j are indicated on the $x$-axis)...... 105
4.1. Locations o f sections presented in the results. Section 1 - 2 is taken across the Northern Sangihe Ridge system from the Sulawesi Sea in the west to the Pacific Ocean in the east; section 3 - 4 is taken across the Dewakang Sill from the Southern Makassar Strait in the north to the Flores Sea in the south;
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. section 5 - 6 is taken across the Lifamatola Sill from the South Maluku Sea in the north to the North Banda Sea in the south; and section 7 - 8 is taken across the Halmahera Sea from the Obi-Seram passage in the south to the Pacific ocean in the north. The dots show the locations o f profiles o f 1, Sm and Sh. 124
4.2. Sections of (a) q2(m2/s2) (top) and (b) Kivi(m2/s) (bottom) across the North Sangihe Ridge. Locations of points 1 and 2 at the top of the figures are given in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 300 m ...... 128
4.2. (cont.). Section of (c) Kn(m2/s) (top) and profiles of (d) l(m) (bottom) across the North Sangihe Ridge, Locations of profiles are given by the dots in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 300 m ...... 129
4.2. (cont.). Profiles of (e) Sm (top) and (f) Sh (bottom) across the North Sangihe Ridge. Locations of profiles are given by the dots in Figure 4.1 ...... 130
4.3. Sections of (a) q^(m2/s2) (top) and (b) K.M(m2/s) (bottom) across the Dewakang Sill. Locations of points 3 and 4 at the top of the figures are given in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 100 m...... 131
4.3. (cont.). Section of (c) KH(m2/s) (top) and profiles of (d) l(m) (bottom) across the Dewakang Sill. Locations of profiles are given by the dots in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 100m...... 132
4.3. (cont.). Profiles of (e) Sm (top) and (f) SH (bottom) across the Dewakang Sill. Locations of profiles are given by the dots in Figure 4.1 ...... 133
4.4. Sections of (a) q2(m2/s2) (top) and (b) Km(ih2/ s) (bottom) across the Lifamatola Sill. Locations o f points 5 and 6 at the top of the figures are given in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 100 m...... 134
4.4. (cont.). Section of (c) KH(m2/s) (top) and profiles of (d) l(m) (bottom) across the Lifamatola Sill. Locations of profiles are given by the dots in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 100 m...... 135
4.4. (cont.). Profiles of (e) SM (top) and (f) Sh (bottom) across the Lifamatola Sill. Locations of profiles are given by the dots in Figure 4.1 ...... 136
4.5. Sections of (a) q2(m2/s2) (top) and (b) KM(m2/s) (bottom) across the Halmahera Sea. Locations o f points 5 and 6 at the top o f the figures are given
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 100 m ...... 137
4.5. (cont.). Section of (c) Kn(m2/s) (top) and profiles of (d) l(m) (bottom) across the Halmahera Sea. Locations of profiles are given by the dots in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 100 m...... 138
4.5. (cont.). Profiles of (e) SM (top) and (f) SH (bottom) across the Halmahera Sea. Locations of profiles are given by the dots in Figure 4.1 ...... 139
4.6. Maps of (a) q2(m2/s2) at the surface with climatological wind (top) and (b) no wind...... 140
4.6. (cont.) Maps of (c) q (m./s ) at 20m with climatological wind (top) and (d) no wind...... 141
4.6. (cont.) Maps of (e) q2(m2/s2) at 50m with climatological wind (top) and (f) no wind...... 142
A.l Some important features o f the smoothed model bathymetry: (a) Sangihe Ridge with north and south sills; (b) South Makassar Strait with the Dewakang Sill in the east and western passage near Kalimantan; (c) Lombok Strait;(d) Ombai Strait; (e) Timor Passage; (f) Halmahera Sea with north and south sills; (g) Seram Sea area with Lifamatola Passage to the northwest; Buru-Seram passage to the south; Obi-Halmahera and Obi-Seram passages to the east; and Mangole-Buru passage to the west; (h) Irian Jaya - Seram Passage; and (i) Labani Channel. Isobaths are in 1000's o f meters...... 152
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF ABBREVIATIONS
ACCS Antarctic Circumpolar Current System
ADCP Acoustic Doppler Current Profiler
ARLINDO Arus Lintas Indonen
BBL Bottom Boundary Layer
BL Boundary Layer
COADS Comprehensive Ocean-Atmosphere Data Set
CTD Conductivity Temperature Depth
CTDO2 Conductivity Temperature Depth Oxygen
ECHO European Centre Hamburg Model
ECMWF European Center for Medium-Range Weather Forecasts
ET0P05 Earth Topography 5km
EUC Equatorial Undercurrent
GFDLMOM2 Geophysical Fluid Dynamics Laboratory Modular Ocean Model2
IES Inverted Echo Sounders
10 Indian Ocean
ITF Indonesian Throughflow
JADE Java Australia Dynamics Experiment
LANL Los Alamos National Laboratory
LLWBC Low Latitude Western Boundary Current
MC Mindanao Current
MIT GCM Massachusetts Institute of Technology General Circulation Model
M-Y Mellor-Yamada
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. NADW North Atlantic Deep Water
NEC North Equatorial Current
NECC North Equatorial Counter Current
NGCC New Guinea Coastal Current
NGCUC New Guinea Coastal Undercurrent
NICU New Ireland Coastal Undercurrent
NORDA Naval Ocean Research and Development Activity
NPW North Pacific Water
NWM North West Monsoon
OGCM Ocean Global Circulation Model
OZPOM Australian Princeton Ocean Model
POM Princeton Ocean Model
POM2K Princeton Ocean Model 2000 code
POP Parallel Ocean Program
PIES Pressure Inverted Echo Sounders
SEC South Equatorial Current
SEM South East Monsoon
SLA Sea Level Anomaly
SPGA Shallow Pressure Gauge Array
SPW South Pacific Water
SSH Sea Surface Height
STD Salinity Temperature Depth
TKE Turbulent Kinetic Energy
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TOPEX POSEIDON/ERS Typhoon Operational Experiment
UML Upper Mixed Layer
WEPOCS Western Equatorial Pacific Ocean Circulation Study
WOCE World Ocean Circulation Experiment
xx
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CHAPTER 1
INTRODUCTION
The circulation of the Indonesian Seas is studied with a high resolution regional
model based on the Princeton Ocean Model (POM) (see Mellor 2004). The model
has been developed as a research tool to investigate important physical processes
occurring in the Indonesian Throughflow (ITF) and the different flow paths taken
by the ITF. Though previous modeling studies have made important contributions
to our understanding of the ITF, most have been concerned with transports of mass,
heat and salt through the Indonesian Seas and have not focused in detail on the
processes causing the transformation of water masses or paths taken by the water
masses within the Indonesian Seas region. In addition, the small number of observa
tions in the region also limits our understanding of the ITF.
The Indonesian Seas are situated at low latitudes between the Indian and Pacific
Oceans. They are unique in that they form the only connection between ocean
basins at low latitudes at which interocean exchange can occur. As a result, the
ITF plays a vital role in earth‘s equatorial climate system. For example, the models
of Hirst & Godfrey (1.993), Verschell et al. (1995) and Murtugudde et al. (1998)
show that Sea Surface Temperature (SST) in the Pacific and Indian Oceans and
upper-layer heat storage are dependent on the ITF. The ITF is also thought to
play an important role in the global climate system, see, e.g., Gordon (2001) and
references therein. It is widely accepted that the ITF is strongly connected with
the global ocean thermohaline circulation cell (’’conveyor belt circulation system”)
whereby North Atlantic surface water sinks at high latitudes to form North Atlantic
Deep Water (NADW), spreads to the Indian and Pacific Oceans via the Antarctic
Circumpolar Current System (ACCS) and ultimately returns as surface water with
a major portion passing through the Indonesian Seas (see Gordon (1986), Broecker
(1991), Schmitz (1996 a, b)). Indeed, Shriver & Hurlburt (1997) and Goodman
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(1998) find there are profound effects on the ITF if the generation of NADW is
shutdown.
ITF water, that is water entering the Indian Ocean from the Indonesian Seas,
bears little resemblance to the water masses entering the Indonesian Seas at the Pa
cific. These Pacific Ocean source waters are transformed while crossing the Indone
sian Seas. The processes causing this transformation are not precisely known. The
topography of the Indonesian Seas region is highly irregular with the major basins
separated from each other by topographic features such as sills and narrow pas
sages. A monsoon climate system is active in the region with a southeast monsoon
coming from Australia in the northern summer and a northwest monsoon coming
from Southeast Asia in the southern summer. The monsoons help dictate transport
through the ITF and flow paths taken through the region. Vertical diffusion of tem
perature and salinity signatures is thought to be important in the thermocline and
near topographic features. Seasonal rain and river runoff from the islands add to the
complexity of the region.
1.1. Background
Observational and numerical studies show there is a transport of mass, heat and
salt through the Indonesian Seas in the direction of the Indian Ocean. Stammer et
al.(2Q03), using the MIT GCM that was fully constrained by WOCE data, found a
mean heat transport of about 1.1 PIT from the Pacific into the Indian Ocean. Piola
& Gordon (1984), using simple box models, found that a transport of 13.7 Sv from
the Indonesian Throughflow into the Indian Ocean with an average salinity of 33.55
was required to balance the heat content of the Indian Ocean. Schneider & Bar
nett (1997), using a coupled ocean-atmosphere model (ECHO), calculated a mean
baroclinic component of heat transport from the western Pacific into the Indonesian
seas of 0.3PIT and a barotropic component of 0.4PIT. The seasonal heat transport
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toward the Indian Ocean is maximal at 1.4PIT in July and minimal at 0.1 PW in
February with fluxes referenced to 3A°C. Ganachaud & Wunsch (2000) used hy
drographic sections, current meters with climatological wind fields, biogeochemical
balances and improved a priori error estimates in an inverse model, to improve es
timates of the global circulation and heat fluxes. They found that a heat flux of
1.4PJT enters the Indian Ocean from the ITF. They also note that the hydrographic
data used in the model at this location came from the Java Australia Dynamics Ex
periment (JADE) program in August 1989 only. Schiller et al. (1998), using a global
ocean circulation model (OGCM) with enhanced tropical resolution, found a mean
heat transport across the 1X1 transect (between Java and NW Australia) towards
the Indian ocean of 1.15PIT with a maximum of 1.86 PW in August and a minimum
of 0.58PIT in March. Gordon & McClean (1999) modeled the regional circulation
of the Indonesian Seas using the Los Alamos National Laboratory (LANL) Parallel
Ocean Program (POP) forced by ECMWF wind stresses for the period 1985 through
1995. They found a heat transport toward the Indian Ocean of 0.79PJT in August
1993 and 0.06PJT in February 1994. They found salt flux of 2.51 x 10 skg/s for
August 1993 and 3.0 x 10 7kg/s for February 1994. Hirst & Godfrey (1993), using
the Cox version of the Bryan-Cox OGCM, found a heat transport of 0.62PIT from
the Pacific Ocean toward the Indian Ocean. Lee et al. (2002) compared solutions
of a near-global circulation model with open and closed Indonesian passages from
1981 to 1997. The open Indonesian passage provided a heat flux of approximately
0.6P1T to the Indian Ocean. Godfrey (1996) in his review paper suggested that
a mean throughflow of lOS'u would transport about 0.5PJF of heat into the In
dian Ocean. Ffield et al. (2000), using measurements of temperature and transport
from instruments deployed in the Labani Channel, calculated an observed heat flux
of 0.50P1T through the Makassar Strait for 1997. This included measurements of
0.63P1T during the La Nina months of December 1996 through February 1997 and
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0.39PW during the El Nino months of December 1997 to February 1998. Vranes et
al. (2002) calculated the heat transport through the Makassar Strait for the period
December 1996 - July 1998 using four different observed temperature profiles. Their
mean calculations for the period were 0.72 PW, 0.58PIT, 0.34 PW, and 0.53PIT.
Estimates of ITF transport from observational, numerical and other studies vary
from about 0 — 30ST toward the Indian Ocean, see, e.g., Wyrtki (1961), Godfrey &
Golding (1981), Murray & Arief (1988), Godfrey (1989, 1996), Kindle et al. (1989),
Cresswell et al. (1993), Hirst & Godfrey (1993), Molcard et al. (1994, 1996, 2001),
Miyama et al. (1995), Gordon et al. (1999), Potemra et al. (1997), Shriver &
Hurlburt (1997), Gordon & McClean (1999), and Potemra (1999).
Inverse solutions also show ITF transport toward the Indian Ocean: Piola &
Gordon (1984) 14 Sv; Toole & Warren (1993) 6.7 Sv; MacDonald & Wunsch (1996)
7 Sv; de las Heras & Schlitzer (1999) 13.2Sv; Ganachaud (1999) 15 ± 3Sv.
There is an urgent need for simulations of the Indonesian Throughflow (ITF)
using high resolution models. Though the limitations of regional models are well
known, they have their uses. This high resolution regional model properly resolves
all known important bathymetric features in the model domain such as sills, passages
and narrow channels and straits. As a result, important questions such as partition of
flow between passages, interaction of different water masses, influence of local winds,
turbulent mixing, and overall momentum and energy balances can be addressed.
Previous models have not had the resolution to properly account for the effect
of topography in the Indonesian Seas. Authors are in agreement that models with
higher resolution would improve results by including the effect of bottom topography.
Such models include the studies of Gordon & McClean (1999); Potemra et al. (2003);
Schneider & Barnett (1997); Schneider (1998); Verschell et al. (1995); Murray et al.
(1990); Kindle et al. (1989); Stammer et al. (2003); Schiller et al. (1998); Shriver
& Hurlburt (1997); Hirst fc Godfrey (1993); and Murtugudde et al. (1998) and are
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described in more detail in Chapter 2.
1.2. Objectives
The primary objectives of this research are to study:
• physical mechanisms controlling the splitting of the ITF between passages and the
effect of topography on flow paths taken;
• the effects of seasonal variation of transports through the open ports;
■ the influence of local winds on the regional circulation;
• the role of topography in the formation of distribution of 6 and S in the region with
an emphasis on the interaction of NPW and SPW;
• features of mixing and turbulence having an important effect on 6, S, and velocity
distributions.
1.3. A Summary of the Study
The study is divided into 3 parts. In the first part, Chapter 2, a description of
the model and properties of the simulated velocity fields are presented and discussed.
In the second part, Chapter 3, the influence of bottom topography on temperature
and salinity distributions is presented and discussed. In the third part, Chapter 4,
turbulence characteristics based on model results are presented and discussed.
In Chapter 2 the basic properties of a developed regional model of the circulation
of the Indonesian Seas are outlined. The model, based on the Princeton Ocean
Model, has 250 x 250 grid cells in the horizontal with grid spacing of ~ 10 km and
29 er-levels in the vertical. It is well known that the complex topography of the region strongly influences temperature, salinity and current distributions there. One
of the significant properties of this model is that all basic topographic features are
resolved. The model has four open ports to simulate inflow of North Pacific Water
from the Mindanao Current, inflow of South Pacific Water from the New Guinea
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Coastal Current, outflow to the Pacific Ocean due to the North Equatorial Counter
Current, and outflow to the Indian Ocean due to the Indonesian Throughflow. Total
transports through the open ports and port normal velocities are specified from
observations. Orlanski’s conditions are employed at the open ports with port normal
velocity nudged to observed values and temperature and salinity to climatology. Port
channels are introduced so the effects of open boundary conditions do not impact
the dynamics of the main region. An additional friction was included in the vicinity
of some narrow passages and sills as a proxy for specific processes such as tides and
internal waves that occur within these topographic features.
Four experiments are discussed: seasonally varying and annual mean transports
and port normal velocities both with and without local winds. All experiments are
totally spun up after 10 years. This analysis uses data from the post spin up period
only. The basic properties of simulated total transports through the main passages
in the region, surface circulation and sea-surface heights are discussed. The portion
of North Pacific Water entering the Indonesian Seas relative to that leaving through
the North Equatorial Counter Current port is fairly constant throughout the year.
Most of this water takes the western route through the Makassar Strait. The portion
of South Pacific Water entering the Halmahera Sea compared to that exiting in the
North Equatorial Counter Current varies considerably with the seasons. Turning off
the local winds does not substantially influence the transport through main passages
in the model domain. Surface circulation patterns change substantially with the
seasons. The role of different terms in the heat and salt equations was investigated
by dividing the region into a number of boxes. For any given box, the sum of the
horizontal advective fluxes of temperature (salinity) through all sides of the box
is on the same order as the vertical heat (salt) flux at the surface, interior nudging
term, and the rate of time variation of the box integrated temperature (salinity). The
comparison of the basic structure of the model surface circulation, sea-surface heights
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and total transport values through the main passages with observations appears
satisfactory.
The main objective of the second part, Chapter 3, is to analyze the basic features
of potential temperature and salinity distributions in the Indonesian Seas simulated
by the model. The influence of bottom topography on the formation of temperature
and salinity distributions is considered by following the three major routes of flow
of North Pacific and South Pacific Water through the Indonesian Seas. Major eleva
tions of bottom relief, such as the Sangihe Ridge; the topographic rise between the
Sulawesi Sea and Makassar Strait; the Dewakang Sill; the ridge between the Flores
and Banda Seas; the topographic rise between the Pacific Ocean and Morotai Basin;
the Lifamatola Sill; and the northern and southern Halmahera Sills; break the region
down to separate basins having different temperature and salinity stratifications.
The differences in stratification are caused by these topographic features that act
to impede the advection of cold and salty water from a basin (located upstream)
to the neighboring basin (located downstream). Arguments are included to support
this conclusion. In the upper ocean (500m), the Indonesian Throughflow is primarily
shaped by North Pacific Water taking the route through the Makassar Strait. Deep
Banda Sea water is formed by the overflow of North Pacific Water across the Lifam
atola Passage into the Banda Sea. Below 500m South Pacific Water is blocked by
the Halmahera Sills and does not enter the Indonesian Seas. But in the upper ocean
(0-500m) SPW can probably penetrate into the Seram and Maluku Seas to mix with
NPW there.
There are no substantial structural changes of potential temperature and salinity
distributions between seasons, though values of some parameters of temperature
and salinity distributions (e.g., magnitudes of maxima and minima) can change.
It is shown that the main structure of the observed distributions of temperature
and salinity is satisfactorily displayed throughout the entire model domain. The
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calculated transports of internal energy (heat) and salt mass through the Lombok
and Ombai Straits, and Timor Passage in August and February are in reasonable
agreement with published observed and simulated data.
In the last part, Chapter 4, aspects of turbulence and mixing in the Indonesian
Seas are presented and discussed. The results are based on the Mellor-Yamada 2.5
turbulence parameterization model. Though the importance of mixing in the In
donesian Seas has been widely acknowledged, very few observations are available
and there have been no model studies of mixing or turbulent diffusion in the region.
The study is focused on turbulent diffusion and turbulent kinetic energy in the upper
mixed layer, the thermocline and in deep water near topographic features. Very large
turbulent kinetic energies and vertical turbulent diffusivities are seen around topog
raphy and are important for the deep overflows found in the region. Large turbulent
energies and diffusivities found in the thermocline are important for the diffusion of
temperature and salinity signatures found in the Indonesian Seas. Monsoon winds
and local currents lead to large diffusivities in the upper mixed layer.
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CHAPTER 2
MODEL DESCRIPTION AND PROPERTIES OF THE
SIMULATED VELOCITY FIELDS
2.1. Introduction
This chapter outlines the main properties of a developed high-resolution baro-
clinic regional model of the Indonesian Seas circulation. This study is a continuation
of previous efforts summarized in Burnett et al. (2003) and Kamenkovich et al.
(2003). The model is based on the Princeton Ocean Model (POM). There are 29
cr-levels in the vertical and the horizontal resolution is ~ 10 km. Four open ports
are introduced to simulate the main currents entering and exiting the region of in
terest. The primary motivation for developing such a model is to take into account
all basic features of the bottom topography in the Indonesian Seas region. Many
strong arguments demonstrate the importance of incorporating the relevant details
of the complex bottom topography to satisfactorily model temperature and salinity
distributions and currents in the region (see, e.g., Gordon & McClean 1999; Gordon
et al. 2003 and references given therein). The region is essentially a series of basins
separated by ridges with many sills,, passages and constrictions. To a great extent pe
culiarities such as the sill at the northern end of the Sangihe Ridge, Labani Channel,
Dewakang Sill, Lombok Strait, North and South Halmahera Sills, and Lifamatola Sill
shape the partition of the main flow between different passages and the structure of
temperature and salinity in separate basins. But the width of these features ranges
from 30 to 50km so a rather fine resolution is required. We will show that with a
horizontal resolution of 10km we are able to properly incorporate all the essential
features of the bottom topography into the model.
A review of published modeling results for the Indonesian Seas show that most ef
forts have been directed toward estimating total transport of the Indonesian Through-
flow (ITF) and its influence on the Pacific and Indian Ocean dynamics. Substantially
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less attention has been paid to the analysis of the dynamics of the Indonesian Seas
per se, although the necessity of fine horizontal resolution has been noted in many
articles. We now summarize previous modeling studies of the ITF by briefly dis
cussing baroclinic models with bottom topography and providing a brief summary
of other models. Kindle et al. (1989) investigated the mean, seasonal and interannual
variability of the ITF and forcing mechanisms responsible for this variability with
the Naval Ocean Research and Development Activity (NORDA) global model forced
by Hellerman & Rosenstein (1983) wind stress climatology and European Center for
Medium Weather Forecasting (ECMWF) 1000m6 winds for the period 1980-1987.
The model grid resolution was 0.5° in latitude and 0.7° in longitude. The NORDA
global model has multiple layers with the topography confined to the lowest layer.
The simulations yielded higher than expected transports through Lombok Strait be
cause of coarse model resolution though total transports and current pathways are
very reasonable. The authors acknowledged that models with finer horizontal resolu
tion should be utilized to investigate roles of the New Guinea Coastal Undercurrent
(NGCUC), intermediate and deep ITF and the effects of bottom topography on the
circulation. Hirst & Godfrey (1993) examined the role of the ITF in an Ocean Global
Circulation Model (OGCM), the main aim of which was to understand the role of the
throughflow in the mean mass and heat balances of the Indian and Pacific Oceans.
They used the Cox version of the Bryan-Cox OGCM. The horizontal grid spacing
was 1.5928° in latitude and 2.8125° in longitude. Schneider & Barnett (1997) were
the first to examine the ITF with a global coupled ocean-atmosphere model. The
horizontal resolution of the ocean model was 0.5° within 10° of the equator increasing
to 2.8° poleward of 20°. The model had two entrance paths to the Indonesian Seas,
one of which is the Torres Strait, and two exit passages. Schneider (1998) modeled
the role of the ITF in the global climate system using the same model. The au
thor acknowledges that the coarse resolution of the coupled model does not resolve
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the complicated topography and pathways in the Indonesian Seas and the problem
needs to be addressed by running simulations with higher resolution. Shriver &
Hurlburt (1997) used a set of global ocean simulations to investigate: (i) pathways
feeding the ITF at different depths and (ii) the role of the thermohaline circulation
there. The model had horizontal resolution of 0.5° with vertical resolution rang
ing from 1.5-layer reduced gravity to six layers. It is possible that confining rough
topography to the bottom layer reduces the effect of bottom topography. Gordon &
McClean (1999) used the output of the Los Alamos Parallel Ocean Program (POP)
1/6° global model forced by ECMWF wind stresses for the period 1985 to 1995 to
compare the simulated results for the Indonesian Seas region with observations made
during the southeast monsoon (August 1993) and the northwest monsoon (Febru
ary 1994). They found some major discrepancies between observations and model
results for potential temperature and salinity distributions largely attributable to
model resolution, e.g., the Dewakang Sill (depth ~ 550m) was replaced by a sill of
~ 200m depth and the Torres Strait is too deep and too wide. They suggest that
some model inadequacies will be improved by increasing model resolution. Schiller et
al. (1998) used the Geophysical Fluid Dynamics Laboratory Modular Ocean Model2
OGCM (GFDL MOM2 OGCM) to model near surface dynamics and thermodynam
ics in the Indian Ocean and the ITF. The model resolution is 2° in longitude and
0.5° in latitude within 8° of the equator increasing to 5.85° at the poles. They in
cluded increased viscosity in the region, centered in the Banda Sea and decreasing in
value outward from there, to account for large tidal currents found over the irregular
topography there. Though their estimates of transport are not unreasonable, they
have a reduced number of paths through the Indonesian Seas. Potemra et al. (2003)
examined the exchange of water between the Pacific and Indian Oceans via the ITF
and its temporal variability using output from the Parallel Ocean Climate Model
(POCM). The model is a global-scale ocean model having horizontal resolution of
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~ 0.4° and 20 z-levels. However, it did not resolve flow through the Lombok Strait
and consistently simulates higher salinities, possibly due to too much salt entering
the region through the Torres Strait. Their Figure 1 also shows the consequences of
limited model resolution at the entrance and exit straits to the region. Stammer et
al. (2003) used the results of a fully constrained GCM to estimate ocean transports
of volume, heat and freshwater and their temporal changes including estimates of
the ITF transport. All estimates are based on the Massachusetts Institute of Tech
nology OGCM (MIT OGCM) and its adjoint (see Marshall et al. 1997 and Marotzke
et al. 1999). The model has horizontal resolution of 2° and 22 vertical levels. The
authors acknowledge that flow estimates are limited by the low resolution of the
model. The following authors have successfully applied the reduced gravity model
with no motion in the lower layer to the analysis of the Indonesian Seas circulation.
Kindle et al. (1987); Godfrey (1989; 1996); Murray et al. (1990); Inoue & Welsh
(1993); Verschell et al. (1995); Qiu et al. (1999); and Murtrrgudde et al. (1998).
The bottom topography was essentially ignored in these simulations though Godfrey
(1996) and Godfrey & Masumoto (1999) did try to estimate its effect.
To achieve a more detailed understanding of the Indonesian Seas dynamics we
have attempted to separate the global and regional problems. It is well known that
regional models have limitations but we are confident that questions such as the par
tition of the main flow between different passages, the influence of local winds, the
interaction of North and South Pacific Waters, overall momentum and energy bal
ances, and properties of turbulent mixing in the region can be successfully addressed
with this model.
In Section 2 the model configuration is outlined. In Section 3 some of the model
results (total transports through main passages, surface circulation, sea-surface heights
(SSH)) are discussed. Estimates of terms in the heat and salt diffusion equations
are presented in Section 4. Section 5 focuses on the comparison of simulated and
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observed data. Finally, Section 6 summarizes the basic results of this study.
2.2 Model Description
The horizontal model configuration coincides with that described in Burnett et
al. (2003; Figures 1 and 2). The model domain and bathymetry are shown in Figure
2.1(a) and the names of important topographic features and open ports are given in
Figure 2.1(b). The domain extends throughout the Indonesian Seas region and has
250x 250 grid cells in the horizontal, identified in Figure 2.1(b) by i and j. These grid
cells are ~ 10km long in both horizontal directions and have been generated using the
GRID program provided with the POM (see Mellor 2004). The model contains four
open ports to simulate the impact of major currents entering and exiting the region.
The orthogonal curvilinear coordinate system used in the POM has been rotated
relative to latitude and longitude so that the open port in the Indian Ocean region
(10 port) coincides with a transect line extending from Java to northwest Australia
along which observations were made (see, e.g., Fieux et al. (1994), Sprintall et al.
(2000)). The ITF exits the model domain and enters the Indian Ocean through this
port. There are three open ports in the Pacific region: the Mindanao Current port
in the north (MC port); the New Guinea Coastal Current port (NGCC port) just
to the north of New Guinea/Irian Jaya; and the North Equatorial Counter Current
port (NECC port) in the east. Pacific waters enter the domain through the MC and
NGCC ports and exit through the NECC port. The grey regions in Figure 2.1(a)
and 2.1(b) are shallow (depth is less than 100m) and are considered as land (see
discussion of model topography below). There are 29 a-levels in the vertical so that
important features of the vertical structure are properly resolved over all types of
topography.
2.2.1. Bottom Topography
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Consider two characteristics of the variability of depth H(i,j): slope in the
x-direction, sx(i,j), and in the y-direction, sy(i,j), which are defined as
The standard recommendation by Mellor et al. (1994) is that the maximum
value of sx and sy should be less than 0.4 over the entire model domain. Otherwise,
the model will not work properly because of the pressure gradient problem. We
started with the ETOP05 dataset, which we interpolated onto the model grid and
calculated maxima of sx and sy over all grid points with > 0.1m. The
maximum value of sx was 1.74 (77(144, 225) = 133m; 77( 143,225) = 1923m) while
that of sy was 1.80 (/T(102, 96) = 2606m; 77(102, 95) = 135m) and the maximum
depth was 77( 165,226) = 9302m. So, to follow the recommendation we needed to
modify the bottom topography.
First, the lOOm-isobath was chosen as the coastline. This choice closed the pas
sages to the Sulu Sea, Java Sea, Torres Strait, and passages around north Irian Jaya.
Small passages in the Lesser Sunda Island Chain were closed but the important Lom
bok and Ombai Straits were retained. Then, without changing the structure of the
topography, we introduced some local modifications in 77 to eliminate overly large
local slopes. For example, we set the maximum depth in the model domain to 6000m
(e.g., the Mindanao Trench is deeper than 9000m); set minimum depth of seamounts
in the Pacific basin to 2000m; removed two islands west of the NGCC port, and so
on.
Second, we introduced a rigid boundary around the model domain of 5-cell width
except at the four open port channels (Figure 2.1); eliminated depth variation along
the port channels; and removed all 1-point, islands.
Third, we smoothed this modified bottom topography by applying the following
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.1. Model bathymetry and domain (a) (above) and names of important topographic features, open ports and islands (b). Note that in (b) the model domain is shown in x, y coordinates (i, j are given), and the depth in the grey region is less than 100m. Notice also that the island chain extending from Java in the west to Timor Island in the east is the Lesser Sunda Island Chain.
formula (Il’in et al. 1974),
(2 .2 )
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250
MindanaoCurrentport
Sulawesi Pacific Sea Ocean 200 Morotai North Equatorial BasML* Countercurrent , ^^®Waluku SeoJKt port M il' Lifamatola m ^Passag^ New Guinea Coastal 150 Current port \ Taliabu y/^Seram Sea Mangole Buru s 3
Banda Sea 100 Indian Ocean port Timor Passage
50
Figure 2.1(b).
where H is the smoothed depth, P is the grid point, a and R are some constants,
Pm is the point whose distance rm from P is less than /?. and M is the total number
of such points. The distances R and rm are scaled by the Earth’s radius; the rm are
calculated after mapping the domain on the Mercator plane.
We used a — 2R2. The first smoothing was made with R = 0.02 (a smoothing
radius of ~ 128 km). We then restored the unsmoothed bathymetry in all pas
sages/sills/straits that, had been oversmoothed with R = 0.02. The next smoothing
was done with R = 0.005 (a smoothing radius of ~ 32km). We then again restored
the unsmoothed bathymetry for all passages/sills/straits that had been oversmoothed
with R = 0.005 and made another smoothing with R — 0.0025 (a smoothing radius of
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~ 16km). Finally, we applied the POM GRID subroutine SLPMIN twice to further
reduce large slopes (slmin —0.19).
This smoothed topography was used in all numerical experiments. For this topog
raphy the maximum value of sx was equal to 0.41 (if (141, 207) = 638m; H (140, 270)
= 970m) while the maximum value of sy was equal to 0.40 (if(8 ,116) = 332m;
if (8,115) = 501m). The number of ocean cells is 31105 (approximately 50% of
all grid points). The maximum depth is Hmax(209, 245) = 5999m. Does such a
smoothed bottom topography retain the dynamically important features? From our
knowledge of the general pattern of the circulation in the Indonesian Seas we conclude
that the topography in the areas of the Sangihe Ridge, Labani Channel, Dewakang
Sill, Halmahera Sea, Lifamatola Passage, Lombok Strait, Ombai Strait, Timor Pas
sage, and the passage between Irian Jaya and Seram Island is critically important.
It is shown in Appendix A that the basic features of the bottom topography in these
subregions are retained.
2.2.2. Boundary Conditions
We describe here the open boundary conditions only. The remaining boundary con
ditions are standard for the POM and are given in Appendix B for the sake of
completeness.
2.2.2.1. Boundary conditions for the depth-averaged motion.
1. Specification of the normal velocity based on the prescribed total transport and
assumed simple distributions of this velocity across the port.
2. Tangential velocity is equal to zero.
2.2.2.2. Boundary conditions for the 3D motion.
1. Orlanski’s condition with nudging for the normal velocity. Following the recom
mendations of Marchesiello et al. (2001), we have for the Indian Ocean IO port:
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«(n+1) (2, j, k) — u{n ^ (2, j, k) 2At = ,. m uM(3, j,fc) ~ 0.5[«(n+1)(2,j, fc) + u ^ 1)(2,j,fc)] __
'•fo[“‘”_1)(2, J, *) - *)]. (2-3)
where c(j\ fc) is calculated according to the P0M2K code (see Orlanski 1976); At is
the time step; AXj is the distance between (2 ,j) and (3 ,j) grid points; superscript n
indicates the time nAt; rjQ is the nudging coefhcient for the normal velocity u; ue is
the port normal velocity prescribed from observations. This formula is used for the
calculation of the boundary value rdn+1)(2, j, k). The nudging coefficient differs
for inflow and outflow points, identified by the sign of ffin)(2, j, k)—c(j, k): for outflow
points (2, j, k) — c(j, k) < 0 while for inflow points u ^ (2,j,k) — c(j, k) > 0. Note
that by definition c(j, k) > 0. Usually the nudging coefficient for inflow points is
larger than for outflow points and to provide a smooth transition from inflow to
outflow points we used the following formula
r^o = rout + 0.5rin(l + tanh[a(u(n)(2, j, k) - c(j, k) - ue)]), (2.4)
where rottt = 10~4s_1; rin = 5 • 10-4s -1; a = 60sm_1; ue = 0.05ras_1. Similar
formulas have been used for calculating ffin+1)(im, j, k) for the NECC and NGCC
ports and v^n+1^ (i, jm, k) for the MC port. Note that im = 250, jm = 250 and k
indicates a*, level.
2. Tangential velocity is set to zero.
3. Orlanski’s conditions with nudging for temperature and salinity. Basically these
conditons are similar to (2.3), where, for example, T (l, j, k) and k) are nudged
to the corresponding climatological values, and the nudging coefficient r \SQ is given by
the same formula, (2.4), but with rout = 5- 10_8s_1; rin = 10-5s-1. Similar formulas
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have been used at the NGCC, NECC, and MC ports.
4. The turbulence energy and length scale are set to zero.
2.2.3. Modifications of the POM Basic Equations
2.2.3.1. Within Port Channels. The port channels have been introduced to allow
for tapering off of any possible effects of nudging at the boundary. Therefore the
basic equations have been modified within the port channels in the following way.
1. On the right-hand side of the momentum equations we added frictional, terms
—r\v\u, v) for the outflow port channels (10, i = 3 — 5; and NECC, i — 246 — 249),
nudging and frictional terms —r^(u — ue, v ) for the inflow port channel NGCC (i —
246 — 249) and —rjV\(u, v — ve) for the inflow port channel MC (j = 246 — 249), with
decreasing toward the main region.
2. On the right-hand side of the temperature and salinity equations we added nudging
terms —rls\ T —Tdim); —r\s\S — Sdim) for the IO port channel (i = 2 — 5), for NGCC
port channel (i — 246 — 249), and so on, with decreasing toward the main region.
2.2.3.2. In the Mam Domain.
1. To control the partition of flow in some important regions we incorporated ad
ditional frictional terms of the form —r^(x,y)u,—r^(x,y)v into the momentum
equations, with fiv\x,y) non-zero in the vicinity of some passages and sills. Details
are given in Table 1, This additional friction is a proxy for some specific processes
(tides, internal waves) that occur within the narrow passages and at sills. The
approach is rather crude but these processes somehow need to be taken into ac
count. Otherwise, due to the western intensification, we obtained overly strong flows
through, for example, the Lombok and Ornbai Straits. A similar approach was taken
by Schiller et al. (1998).
2. On the right-hand side of the temperature and salinity equations we introduced
terms - r (0)(T - Tclim); - Sdim), where Tdim and Sdirn are the corresponding
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Location subregion maximum value (s Lombok i=5-22; j=109-125 5 x 10“5 southwest Makassar i=29-43; j=135-149 5 x 1(T5 Ombai i=90-102; j=86-94 2 x i t r 5 Guinea-Seram passage i= 160-167; j=129-140 2 x i t r 4 Taliabu-Sulawesi passage 1=102-118; j=151-167 2 x i t r 4 Obi-Halmahera passage i=139-155; j=146-162 itr5 Lifamatola passage i= 130-144; j=144-162 i t r 4 southwest Flores Sea i=44-50; j=108-125 i t r 4 Buru-Seram passage 1=131-147; j = 128-138 5 x i t r 5 Obi-Seram passage 1=142-152; j=134-151 2 x i t r 5
Table 1. Subregions of the model domain where additional horizontal friction has been applied as follows: —r(i,j)u(i,j,k) and — r(i, j)v(i, j, k). The values of r are maximal at the centers of the subregions and decay exponentially toward the bound aries.
climatological values. The coefficient r ^ is a function of a: it is equal to 10-3s-1 at
the surface and then decays exponentially to constant value of l(T"8s_1 at cr < —0.04
(e.g., it will be 10-8s-1 for depths greater than 40m if the total depth is 1000m).
These terms introduce additional heat and fresh water fluxes basically within the
upper layer, attracting temperature and salinity to seasonally varying climatological
values. They were required for two reasons. First, the surface fluxes of heat, and
especially fresh water, are highly variable and poorly known from observations. Sec
ond, utilization of climatological winds in the model results in underestimation of
vertical mixing and depth of the mixed layer by the Mellor-Yamada parameteriza
tion. Moreover, it is very difficult to model a very complicated structure of salinity
in the upper layer. Nudging of the upper layer temperature and salinity toward
climatological data appeared to be a more reliable way to set up surface boundary
conditions for temperature and salinity. Note that a similar expression was used by
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Haney (1971) in formulating the boundary condition for the heat equation at the
free surface.
3. To approximate temperature and salinity advection we used the Smolarkiewicz
scheme with two iterations and smoothing parameter sw = 0.5. Some modifications
were introduced in the calculation of the buoyancy frequency following basically
Hunter’s suggestion (OZPOM code). All other parameters were chosen in accordance
with the standard version of the POM2K code. Note that the horizontal diffusivity
is less than the horizontal friction coefficient by a factor of 5.
2.2.4■ The Specification of the Port Normal Velocity ue and Total Port Transports
The grid spacing of the Levitus climatology data does not allow for use of
geostrophic relations to specify ue. Moreover, the NGCC port is located too close
to the equator. Therefore, published observations and some modeling results have
been used to determine the port normal velocity ue and total port transports. We
will estimate annual mean and August and February values of port transports and
port normal velocity distributions. Time deviations from the annual mean of port
transports and port normal velocity were approximated by the cosine of the annual
period.
2.2.4-1- 10 port. The open 10 (Indian Ocean) port is basically the region to which
the net westward transport is confined. First, we will model the mid-port normal
velocity profile um(z ) using the geostrophic calculations of Fieux et al. (1994; see
their Figs 12 and 13) based on the JADE (Java Australia Dynamic Experiment)
data. This profile is shown in Figure 2.2(a). The velocity is everywhere directed
toward the Indian Ocean. A maximum magnitude of 10 cm/s is seen at the surface
decreasing monotonically to zero below 450m. The profile um(z) does not change
over the course of a year. Second, to calculate ue(y,zfi) we apply the following
formula
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V 0 500 1000 1000 - 2000 — annual mean 1500 — August S 3000 2000 ■o — - February 4000 2500 3000 5000 (a) 3500 -0.1 -0.05 0
0
1000 1000
§2000 2000 £ 3000 .§ 3000 4000 4000 5000 5000 0.2 ■0.5 -0.4 -0.3 -0.2 -0.1 0 velocity (m/s) velocity (m/s)
Figure 2.2. Modeled mid-port normal velocity profiles at the IO port (a), NGCC port (b), NECC port (c), and MC port (d). The profiles at (a), (c) and (d) are assumed time independent. Profiles for August and February are also given for (b).
Ue(y,z,t) = 0 m " Sx(y,t), (2.5) J~H Um(z)az
where Sx(y,t) is the depth-integrated port normal velocity determined by using the
specified total transport and assumed simple distribution of velocity across the port.
We have chosen an annual mean throughflow transport of 12. 5Sv with a maximum
of 17.05T in August and a minimum of 8 .OSv in February. Observations of the
transport of the Indonesian Throughflow have been made by several investigators.
Fieux et al. (1994) observed a transport 18.6±7.0ST across the Bali-Australia section
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in August 1989. The transport of the ITF consists of the transports through the
Lombok and Ombai Straits and the Timor Passage. Molcard et al. (1996) estimated
a mean transport through the Timor Passage of 4.3 ± LOST in the upper 1250 db
and 2.5 ± 0.8 Sv in the upper 500db with a maximum of 3.0 — 4.05T in August and
a minimum of 1.0 — 2.0ST in February. The measurements were made from current
meter moorings between March 1992 and April 1993. Molcard et al. (2001) estimated
a mean transport through the Ombai Strait of 5.0 ± 1.QST with a maximum of
7.0 —8.0ST in August and a minimum of 4.05T in February. The measurements were
made from a current meter mooring deployed for one year in December 1995. Murray
& Arief (1990) made both observations and model calculations of the transport
through the Lombok Strait. They observed a mean annual transport through the
Lombok Strait of 1.8ST with a maximum of LOST in August and a minimum of
LOSe in February. The observations were made from current meter arrays deployed
in the north Lombok Strait from January 1985 to March 1986. Model results of
Murray & Arief (1990) gave a mean transport of 2.0ST with a maximum of 3.5 Sv
in August and a minimum of LOST in February. Masumoto & Yamagata (1996)
modeled the seasonal variability of the transport of the ITF and found an annual
mean transport of 9.5 Sv with a maximum of 11.6Se in August and a minimum of
6.0Se in February.
2.2.4-2- NGCC port. We used the data shown in Figures 3, 8 and 9 of Ueki et al.
(2003) to model the typical mid-port normal velocity profile um(z,t ) at the entrance
to the NGCC (New Guinea Coastal Current) port. Profiles for August, February
and the annual mean are shown in Figure 2.2(b). The velocity profile changes with
the seasons. The observed near surface velocity of the NGCC is equal to 0.3 rn/s and
directed westward in summer, is equal to 0.1 m/s and directed eastward in winter, and
is equal to 0.1 m/s and directed westward in the annual mean (all estimates here and
in what follows are approximate). The NGCUC (New Guinea Coastal UnderCurrent)
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core (200m) remains at the same position throughout the year with a maximum
velocity of 0.6m/s directed westward in summer, OAm/s directed westward in winter,
and 0.5 m/s in the same direction in the annual mean. Magnitude decreases to zero
below 400m. The current is about 100 km wide throughout the year which agrees with
the approximate width of our port. These values are in good agreement with other
available observations. To determine port normal velocity ue(y,z,t ) a procedure
similar to that described for the 10 port was applied.
Ueki et al. (2003) deployed a series of ADCP moorings off the coast of New Guinea
for 5 years to investigate the seasonal and interannual variations of the NGCC and
the NGCUC. Two of the ADCP moorings were located off the New Guinea coast
at 2°S', 142°E- and 2.5°S 142°JF, 400 km to the east of our open NGCC port. We
assumed that these measurements provide the correct structure and transport of
currents entering our domain through the NGCC port. The system of currents
outside this port is extremely complicated. The South Equatorial Current (SEC)
splits into several branches as it approaches the southeastern edge of New Guinea.
Part of the SEC is returned eastward through the Solomon Strait as a branch of the
South Equatorial Counter Current (SECC) while the remainder continues westward
through either the Vitiaz Strait or St. George’s Channel (see, e.g., Lindstrom et
al. 1990). Further west another portion of the SEC peels off and returns eastward
around Manus Island forming the New Ireland Coastal UnderCurrent (NICU) that
interacts with the Equatorial Undercurrent (EUC) as it approaches the equator (see,
e.g., Butt & Lindstrom 1994). It seems that there is no further peeling off of the
SEC between the NICU and the location of our port (see, e.g., the schematics of
Lukas et al. (1996) and Godfrey (1996) (Figure 1 in both cases), and papers by
Gouriou & Toole (1993), Qiu & Joyce (1992), and Toole et al. (1988) for a further
discussion). Figure 3 of Ueki et al. (2003) shows the structure of the annual mean
NGCUC and NGCC at the two ADCP moorings located at 142° E. Ueki et al. (2003)
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also conducted 11 shipboard ADCP and CTD cruises in summer and winter between
1994 and 2000 to investigate the seasonal variability of the structure of these currents
(see their Figs 8 and 9).
We have chosen an annual mean volume transport through the NGCC port of
22.5Sv, a summer transport of 28.0£u and a winter transport of 17.0ST. We made
seasonal and annual mean estimates of volume transport at the NGCC port, respec
tively, from the Ueki et al. (2003) calculations based on the summer and winter
shipboard cruise data and the ADCP mooring (see their Figs 12 and 17). Several
other investigators have made observations of the NGCC and NGCUC. Lindstrom
et al. (1987) made several transects of the NGCC and NGCUC during the Western
Equatorial Pacific Ocean Circulation Study (WEPOCS), where the seasonal variabil
ity of Low Latitude Western Boundary Currents (LLWBC) was investigated. Two
expeditions were conducted during June-August 1985 (WEPOCS I) and January-
February 1986 (WEPOCS II) where special attention was paid to sampling of the
LLWBC along the coast of Papua New Guinea. In May 1988, Butt & Lindstrom
(1994) made ADCP and CTD measurements off the coast of New Guinea to investi
gate the structure of the SEC and EUC system there. Murray et al. (1995) provided
annual transport measurements of the SEC at the Vitiaz Strait from five current
moorings deployed from February 1992 to April 1993. We have chosen to use the
observations of Ueki et al. (2003) since they represent the most recently updated
dataset in this region.
2.2.4-3. NECC port. We have modeled the mid-port normal velocity um(z) at the
NECC (North Equatorial Counter Current) port using the geostrophic calculations
of Qiu & Joyce (1992). The profile is shown in Figure 2.2(c). It has the same
structure throughout the year. The velocity is toward the west at all depths with
a maximum value of 27 cm/s in the upper 70m in the annual mean. This value
decreases monotonically to 5cm/s at 350m and drops off to zero at 800m. This
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structure and magnitude is in good agreement with other available observations. To
determine port normal velocity ue(y,z,t ) a procedure similar to that described for
the 10 port was applied.
Qiu & Joyce (1992) analyzed 22 years of data from hydrographic cruises col
lected by the Japanese Meteorological Agency along 137°E in winter (1967-1988)
and summer (1972-1988) to study the interannual variability of the western North
Pacific. Note that the 137° E meridian is located adjacent to the NECC port (Qiu
& Joyce 1992; Figure 1). The hydrographic sections were made from 34°A to 1°S
thereby crossing the full extent of the NECC. Hydrographic stations were spaced
approximately 0.5° apart in the region of the NECC. Qiu & Joyce (1992) results of
the geostrophic calculations relative to lOOOdb between 2 °N and 34°^ are shown in
their Figure 4. They found that the mean position of the NECC is between 2° N
and 7 °N. The mean cross-section of the NECC shows a velocity core of more than
0.5m/s centered at 50m and located between 4 °N and 5 °N (see Figure 4 of Qiu
& Joyce (1992)). The current is symmetrical about the core down to 150m, be
low which it is offset to the south. The geostrophic current extends to a depth of
about 750m but is less than 0.2 m/s below 250m and less than 0.1m/s below 500m.
Other geostrophic calculations include those of Gouriou & Toole (1993) who used a
similar data set to that of Qiu & Joyce (1992). Gouriou & Toole (1993) analyzed
hydrographic data collected by the Japanese Meteorological Agency between 1972
and 1985 along 137°E, using reference levels of 600d6 and 1000 db. They found the
mean position and structure of the NECC to be similar to that found by Qiu & Joyce
(1992) (see Figure 4c of Gouriou & Toole 1993).
We have chosen an annual mean transport of JOS'?;, 46ST in summer and 35 Sv
in winter. Qiu & Joyce (1992) calculated a mean eastward geostrophic transport of
42.1ST. Gouriou & Toole (1993) calculated annual mean NECC geostrophic trans
port of 43.1 and 48.2ST, summer transport of 52.5 and 56.3ST, and winter transport
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of 34.9 and 44.8 Sv referenced to 600 and lOOOdb, respectively.
2.2.4-4- MC port. Because of the low annual variability of the Mindanao Cur
rent (MC) we used a combination of the ADCP observations and the geostrophic
calculations of Lukas et al. (1991) at 10° N (see their Figures 6 and 7) to model
the mid-port normal velocity profile um(z). The profile is shown in Figure 2.2(d).
It does not change throughout the year. The velocity is everywhere into the model
domain with magnitudes exceeding 50 cm/s in the upper 250m, decreasing to 20cm/s
at 500m and is zero below 900m. This structure is in good agreement with other
available observations. To determine port normal velocity ue(y,z,t ) a procedure
similar to that described for the IO port was applied.
Lukas et al. (1991) observed the MC during the WEPOCS in June and July 1988.
Data were collected from drifter launches, ADCP observations and CTD02 casts,
including zonal measurements across the MC at 12° N, 10°N, 8°N, 7°N, 6°N (see
Figure 1 of Lukas et al. 1991). The MC flows along the Philippine coast from about
13.5°N to the southern tip of Mindanao Island at 6°Ar, where part of it extends out
into the Sulawesi Sea (see Figure 5 of Lukas et al. 1991). The current is about lOOfcm
wide at 10° iV near the location of our open MC port. The current stays close to the
coast with velocities concentrated in the upper 200m and a maximum core located at
about 100m (see Figure 6 of Lukas et al. 1991). Geostrophic calculations show that
the current extends to a depth of about 700m: (see Figure 7 of Lukas et al. 1991). The
MC is mainly supplied by the North Equatorial Current (NEC) between 12°^ and
10°N (see Figure 4 of Lukas et al. 1991). Observations at different times of the year
show the MC to be very stable. Wijffels et al. (1995) investigated the mean structure
and variability of the MC at 8°N based on eight surveys between September 1987
and June 1990 and found that the MC is remarkably steady in position and structure
over that time period. Kashino et al. (2005) investigated the variability of the MC
from mooring observations between October 1999 and July 2002 and basically came
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to the same conclusion.
We have chosen an annual mean transport of 30 Sv, 35Sv in August and 25 Sv
in February. Wijffels et al. (1995) estimated a mean transport of 25 ± 4 Sv in the
thermocline and above and 39 ± 105T above 2000db with a maximum in summer
and a minimum in winter. Qu et al. (1998) provided mean transport estimates of
27 — 28Sv from CTD measurements made every September-October between 1986
and 1990. Transport estimates from model results include those of Qiu & Lukas
(1996) who estimated a mean transport of 25 — 40ST increasing by 5 Sv in summer
and decreasing by 5Sv in winter with 1557; transported within the Mindanao eddy.
The model of Yaremchuk & Qu (2004) gave a mean transport of 30 Sv with 355c in
summer and 25 Sv in winter.
2.2.5. Period of integration
To monitor the evolution of experiments two characteristics Ey and Es appeared
useful,
Ey = X (2 .6)
where Ey is integrated over the entire volume of the domain and Es over the horizon
tal area of the domain; pm is the mean density determined by averaging the annual
climatological density over the horizontal; N is the buoyancy frequency calculated
from pm; p' — p — pm; wpom is the POM vertical velocity (see, e.g., Appendix D); g
is gravity. Note that these characteristics are not exactly equal to the total energy
(kinetic and potential) and total potential energy created by the SSH variations.
Table 2 summarizes the integration periods, port transports and port normal
velocity profiles for the experiments that were performed. The time deviations from
the annual mean of temperature, salinity, and wind stress components were
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Port transports Velocity Period Type of Sv (x 106m3/s) profiles of inte Ex solution MC NGCC NECC 10 at ports gration annual mean, annual mean 21 1 with local wind -30.0 -22.5 40.0 12.5 (see Figure 2) years annual mean, 2 no local wind the same as in exp 1 August mean, August mean 6 3 with local wind -35.0 -28.0 46.0 17.0 (see Figure 2) years August mean, 4 no local wind the same as in exp 3 February mean, February mean 6 5 with local wind -25.0 -17.0 34.0 8.0 (see Figure 2) years February mean, 6 no local wind the same as in exp 5 seasonal with Annual mean plus cosine Annual mean 12 7 local wind of annual period plus cosine of years based on annual period annual mean (see Figure 2) and 2 Fourier harmonics seasonal with 8 no local wind the same as in exp 7
Table 2. List, of experiments. Transports out of the model domain are positive; transports into the model are negative. In the first column 9- , S'-climatology, and wind stresses are characterized.
approximated by two Fourier harmonics. Here, we analyze experiments 1, 2, 7, and
8 (annual mean with and without wind and seasonally varying with and without
wind). Experiments 3, 4, 5, and 6 were used for technical purposes. Figure 2.3
shows variations of Ey and Eg with time for experiments 1 and 7. Experiments 7
and 8 were run for 12 years and experiments 1 and 2 for 21 years. Figure 2.3 clearly
shows that these experiments have been completely spun up after 10 years.
To estimate a possible effect of the POM approximation of horizontal pressure
gradients we ran, as is usually done, some experiments without any external factors
and with distribution of potential temperature and salinity dependent on z only. The
errors were not substantial.
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x 10
0 2 4 6 8 10 12 14 16 18 20 years
x 10 14
12
>> 0) 8 0 C UJ 6 v ^ •* ,/*v* * rs» * *"V‘sr
4, 0 2 4 6 8 10 12 14 16 18 20 years
Figure 2.3. The evolution of E v and Es with time. The characteristics Ev are shown in (a) for the seasonally varying experiment with local wind (exp 7; solid line) and the annual mean experiment with local wind (exp 1; broken line). The characteristics Es are shown in (b) for the same two experiments as (a). Variability of the integrated value of Ev in exp 1 is ~ 0.1 x 10 17kgrri2/s2 and for Es is ~ 0.5 x 1013%m 2/s 2.
2.3. Results
As seen in Figure 2.4(a), Pacific water enters the Indonesian Seas region through
the MC and NGCC ports. North Pacific Water (NPW) enters the region through
the MC port before splitting into three branches. The first branch of NPW follows a
western path, flowing over the Karakelong and Sangihe Ridges, through the Sulawesi
Sea, and along the Makassar Strait, at the southern part of which it diverges. Part
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of this flow exits the region through the Lombok Strait, but the main portion passes
through the Flores Sea to the Banda Sea. The second branch of NPW takes the north-
south route through the Morotai Basin, Maluku Sea, Lifamatola Passage, Seram Sea
and finally into the Banda Sea, where it meets with water from the western route.
The third branch of NPW leaves the region through the NECC port. South Pacific
Water (SPW) enters the region through the NGCC port before splitting into two
branches. The first branch of SPW flows through the Halmahera Sea to the Seram
Sea where it meets with NPW taking the north-south route. The second branch of
SPW leaves the region through the NECC port. Finally, Banda Sea water exits the
region through the Ombai Strait and Timor Passage.
2.3.1. Transports
We now consider seasonal variations of the simulated transports in experiments
7 and 8 and compare them with those of experiments 1 and 2. First, we examine
transports through the inflow open ports and main passages for experiment 7 (sea
sonally varying circulation with wind) for August and February and for experiment
1 (annual mean circulation with wind), shown in Figures 2.4(a)-(c), respectively.
Transports through the NIC (inflow of NPW) and NGCC (inflow of SPW) ports as
well as through the Makassar Strait, Maluku Sea, Halmahera Sea, Lombok Strait,
Ombai Strait, and Timor Passage are stronger in August and weaker in February,
when compared with the annual mean. Variations relative to annual mean are ±17%
and ±24% for the inflow through MC and NGCC ports, respectively. Correspond
ingly, the increase (decrease) of outflow transports through the NECC and 10 ports
in August (February) is observed (±15% and ±36% of the annual mean values for
NECC and 10 ports, respectively). A portion of the MC transport flows into the
Indonesian Seas region while the remainder outflows through the NECC port. In
August the total transport of NPW entering the Indonesian Seas region through the
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50 100 150 200 250 j
Figure 2.4(a). Transports across several sections of the model domain including the four open ports. Values for experiment 7 (seasonally varying with local wind) for August and February, and experiment 1 (annual mean with local wind) are shown in (a) (above), (b) and (c), respectively. Values for experiment 8 (seasonally varying with no local wind) for August and February, and experiment 2 (annual mean with no local wind) are shown in (d), (e) and (f), respectively. These snapshots were taken after ten years of model integration. Some arrows are slanted for graphic clarity.
Halmahera-Mindanao section (i — 154), is 12.2Sv (35% of total transport of NPW
through the MC port), while in February the corresponding value is 7.6 Sv (30%
of total transport of NPW through the MC port). Of these values, 78% of NPW
is transported through the Makassar Strait in August and 68% in February. For
experiment 1 a transport of 9ASv enters the Indonesian Seas (31% of the total
transport of NPW through the MC port) with 7.0 Sv flowing through the Makassar
Strait (74% of the transport of NPW into the Indonesian Seas). Note that the
percentage of NPW entering the Indonesian Seas region does not change noticeably.
Unlike the transports of NPW, the percentage of SPW entering the Indonesian Seas
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Figure 2.4(b).
varies substantially. In August 4.8ST of SPW enters the Halmahera (17% of total
transport of SPW through the NGCC port), while in February the corresponding
value is 0.45?; (2.4% of total transport of SPW through the NGCC port). The
annual mean value is 3.1 Sv (14% of total transport of SPW through the NGCC
port). Essentially all of the NPW transported through the Maluku Sea enters the
Seram Sea by way of the Lifamatola Passage. This flow combined with that through
the Halmahera Sea accounts for the total transport into the Banda Sea from the
north-south and eastern routes, amounting to QASv in August, 2.7 Sv in February,
and 4.9 Sv annually. Almost all of this water enters the Banda Sea through the
Mangole-Buru section (i = 129).
It is useful to compare transports through the Makassar Strait and the Mangole-
Buru section. The latter is 67% of the former in August, 52% in February, and
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Figure 2.4(c).
70% annually. The comparison of transports through the Lombok Strait and Flores
Sea section (i = 61) provides the following numbers. The transport through the Lom
bok Strait accounts for 22%, 25%, and 23% of the transport through the Makassar
Strait in August, February, and annually, respectively, with the remainder entering
the Banda Sea through the Flores Sea section.
Relative to the annual mean, the increase (decrease) of transport through the
Lombok Strait, Ombai Strait, and Timor Passage are 31% (19%), 8 % (4%), and
62% (69%) in August (February), respectively. Banda Sea water outflows the In
donesian Seas through the Ombai Strait and Timor Passage. This transport totals
14.9ST in August, 6.7 Sv in February, and 10.9ST annually, with the Timor Pas
sage accounting for 63%, 27%, and 53% of this transport in August, February and
annually, respectively.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35
250
200
150
100
50
50 100 150 200 250 i
Figure 2.4(d).
To disclose the influence of local winds consider experiments 8 (seasonally varying
transport without wind) and 2 (annual mean transport without wind). Total trans
ports from experiment 8 for August and February and experiment 2 are shown in
Figures 2.4(d)-(f), respectively. Note that the specified open port transports already
contain the effect of seasonally varying global-scale wind, so turning off the local
winds can influence the distribution of the main flow between interior passages only.
First, we briefly describe properties of winds over the Indonesian Seas.
The southeast monsoon (SEM), winds out of the southeast, acts from July to
September while the northwest monsoon (NWM), winds out of the northwest, acts
from November to March. In August wind stresses on the order of 0.05 - 0.1 Am ” 2
are typical for the Banda, Flores, Seram, Maluku, and Halmahera Seas, and the
southern part of the Makassar Strait. Note that the northward component of wind
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Figure 2.4(e).
stress during the SEM is rather strong in the Seram, Halmahera and Maluku Seas.
Approximately the same magnitudes of wind stress are observed in February when
the southward component is rather pronounced in the Seram, Halmahera and Maluku
Seas. The annual mean wind stresses in the Banda Sea and the southern part of the
Makassar Strait are in the same direction as the SEM though magnitudes are reduced
by a factor of two, while they are very weak in the Flores, Maluku, Halmahera, and
Sulawesi Seas, and the northern part of the Makassar Strait.
We consider the effect of the wind by comparing transports through the main
passages with and without wind in August and February. We note that transports
through the open ports do not depend on whether we have wind or not. Transports
through the exit passages, Lombok Strait, Ombai Strait and Timor Passage, are not
specified so we can expect that they will depend on the wind field. The results
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50 100 150 200 250 j
Figure 2.4(f).
show that dependence on the wind field is weak. The same is true for transports
through other main passages. For example, turning the winds off in August results in
increased transport through the Makassar Strait of < 1ST and a decrease through the
Halmahera Sea of < 1 Sv, while in February transport through Makassar decreases by
~ 2Sv and increases by ~ 1 Sv each through the Maluku and Halmahera Seas. Figure
2.5 clearly presents the seasonal variations of transports through the main eight
passages. The annual harmonic is dominant in all transports except the Lifamatola
Passage and Ombai Strait. Note that amplitudes of transports vary for different
passages.
2.3.2. Surface Circulation
Surface circulation patterns for experiments 7 in August and February, and for
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 6 7 8 9 10 11 12 5 6 7 8 9 10 11 12
> (0 5 4 2 4.5 5 6 7 8 9 10 11 12 5 6 7 8 9 10 11 12 time(years) time(years)
Figure 2.5. Transport time series through important passages and straits for four different model runs after 10 years of model run. The seasonally varying experiments with (exp 7) and without local winds (exp 8 ) are given by the solid and broken lines, respectively. The annual mean experiments with (exp 1) and without (exp 2) lo cal wind are given by the • and x symbols, respectively. The sections shown are: (a) North Makassar Strait, (b) Lombok Strait, (c) Lifamatola Passage, (d) Halma hera Sea, (e) Obi-Seram passage, (f) Mangole-Buru passage, (g) Timor passage, (h) Ombai Strait. Note the different transport scales on the y-axes. For exps 7 and 8 (lines), transports are calculated every 10 days while for exps 1 and 2 transports are calculated only at times given by • and x symbols.
experiment 1 are shown in Figures 2.6(a)-(c), respectively. In August an anticyclonic
gyre is seen in the Sulawesi Sea with the center displaced to the southeast. In the
northern Makassar Strait flow is predominantly southward with the velocity reaching
~ 40cm/s. In the south Makassar Strait most of the flow exits through the Lombok
Strait while the remainder enters the Flores Sea and then continues eastwards along
the Lesser Sunda Island Chain and finally exits the Indonesian Seas through the
Ombai Strait and Timor Passage. A westward current is seen throughout the Banda
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.6. Surface velocities in m/s. Vectors for experiment 7 for August and February, and experiment 1 are shown in (a) (above), (b) and (c), respectively. Values for experiment 8 for August and February, and experiment 2 are shown in (d), (e) and (f), respectively. These snapshots were taken after ten years of model integration.
Sea. A strong surface current from the NGCC port flows southward along the western
side of the Halmahera Sea and then travels directly west across the Seram Sea into
the Banda Sea, turning mostly southward at the southeastern tip of Sulawesi Island.
A branch of this current flows to the north basically through the Lifamatola Passage
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.6(b).
and Maluku Sea. Note also a northward current through the Seram-New Guinea and
Buru-Seram passages.
In February the center of the anticyclonic gyre in the Sulawesi Sea appears to
bedisplaced to the north, supplying a strong current into the northern Makassar
Strait. In the northern Makassar Strait the flow is everywhere to the south with clear
westward intensification; velocities in the east are ~ 50 cm/s and up to 100 cm/s
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.6(c).
in the west. In the south Makassar Strait the flow is everywhere to the east. This
eastward flow extends across the entire Banda Sea, to the Seram and Halmahera Seas
in the north and to the Timor Passage and the Lesser Sunda Island Chain in the
south. A rather narrow westward current is observed to the south of Burn Island.
Currents in the Maluku Sea and Lifamatola Passage as well as in the Seram-New
Guinea, and Buru-Seram passages are to the south.
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In the annual mean circulation (experiment 1) the center of the anticyclonic gyre
in the Sulawesi Sea is displaced similarly to February. In the northern Makassar
Strait flow is northward in the east and southward in the west with velocities of ~ 20
to 30cm/s in both directions. In the south Makassar Strait most of the flow enters
the Flores Sea, where an anticyclonic gyre is formed. Westward currents are seen
throughout the Banda and Seram Seas but a narrow eastward current is observed near
the Lesser Sunda Island Chain that exits the region through the Ombai Strait and
Timor Passage. Northward flow is seen in the Maluku Sea and Lifamatola Passage
as well as in the Seram-New Guinea, and Buru-Seram passages, similar to August
circulation. In the Halmahera Sea flow is southward in the west and northward in
the east.
Surface circulation patterns with no local wind for experiments 8 in August and
February and for experiment 2 are shown in Figures 2.6(d)-(f), respectively. The lo
cal Ekman transport is 3 to 6m2s~l giving a wind-driven velocity of 0.05 to 0.1ms ' 1
in the upper 50m layer. Therefore the difference in surface velocities can be notice
able. First we observe that velocity patterns with the local wind are more organized
compared to the corresponding patterns without the local wind. The influence of
local winds is clearly seen when comparing the directions of some surface currents in
experiments with and without wind.
In August we observe that surface currents are oppositely directed in experiments
7 and 8 (Figures 2.6(a) and (d)) in the southern part of the Sulawesi Sea, Maluku
Sea and Lifamatola Passage, northern part of the Banda Sea (to the south of the
Seram and Buru Islands), and the eastern part of the Makassar Strait (to the north
of the Labani Channel). These differences can be explained by the action of the local
wind stress. Invoking the geostrophic relations, we can determine the direction of
geostrophic surface currents in the above mentioned areas by estimating the direction
of SSH gradients (Figure 2.7(d)). It is seen that the y-gradient of SSH is positive
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.6(d).
in the Sulawesi Sea and to the south of Seram Island while the ./.-gradient of SSH
is positive in the Makassar Strait and Lifamatola Passage. Such gradients generate
exactly the surface currents shown in Figure 2.6(d). Taking into consideration the
direction of wind during the SEM we conclude that it is the SEM that generates the
differences in the direction of currents in Figures 2.6(a) and 2.6(d). Note that the
directions of gradients of SSH are essentially the same in Figures 2.7(a) (with wind)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.6(e).
and 2.7(d) (without wind).
In February surface currents are oppositely directed in experiments 7 and 8 (Fig
ures 2.6(b) and (e)) in the northern part of the Makassar Strait (to the north of
the Labani Channel), in the Lifamatola Passage and Maluku Sea, and in the north
ern part of the Banda Sea. The difference is also noticeable at the section between
Mangole and Buru Islands. The x-gradient of SSH is positive in the Makassar Strait
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0.0 0.2 0.4 0.6 0.8 1.0 1.2
1 0 °N
5 ° N
0 °
5 ° S
1 0 ° S
1 5 ° S
2 0 ° S 115° E 120°E 125°E 130°E 135°E 140°E
Figure 2.6(f).
and negative in the Lifamatola Passage and at the Mangole-Buru section. The y-
gradient of SSH is negative in the northern part of the Banda Sea. It is easy to see
that the differences in the direction of these currents is generated by the NWM.
In the annual mean circulation (Figures 2.6(c) and (f)) the difference of direction
of currents is clearly seen throughout the Banda Sea. The annual mean wind is weak
and the situation resembles August more than February. Note that the velocity is
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reduced by a factor of two relative to August in the northern Makassar Strait while
the southward component is not as broad in the southern Makassar Strait.
2.3.3. Sea Surface Height
SSH for experiment 7 in August and February and experiment 1 are shown in
Figures 2.7(a)-(c), respectively. First, we compare absolute extremes. In August the
maximum SSH is located to the north of the Halmahera Sea (~ 16cm) while the
minimum is located to the south of the Lombok Strait (~ -20cm). In February the
maximum SSH is to the south of Sulawesi Island (~ 12cm) while the minimum is
to the south of the MC port (~ -24cm). The general structure of the annual mean
SSH is similar to the August pattern. Maximum SSH is located to the north of the
Halmahera Sea (~ 8 cm) and minimum SSH is located to the south of the MC port
(~ -17cm).
In August a high of ~ 8 cm is seen in the north of the Sulawesi Sea with a broad
low in the south. In the Makassar Strait a local maximum of ~ 0cm is seen in
the north with a local minimum of ~ —5cm in the south. In the Maluku Sea SSH
decreases from a maximum of ~ 2cm in the north to a minimum of ~ — 2cm at the
Lifamatola Passage. SSH in the Halmahera Sea also slopes downward from north
to south with a maximum of ~ 6cm in the north and a minimum of ~ 3cm in
the south. In the Banda Sea a local minimum of ~ - 8 cm is seen in the east and
the surface slopes upward across the full extent of the sea to the Flores Sea (local
maximum ~ 0cm). A relatively strong local gradient of SSH extends from the Labani
Channel along the southwestern stretch of Sulawesi Island, to the south Flores Sea
and running eastward along the Lesser Sunda Island Chain to the Ombai Strait and
Timor Passage.
In February, the SSH is essentially constant in the Sulawesi Sea to the west of
the Sangihe Ridge. The Makassar Strait slopes from a high of ~ 10cm in the south
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50 1-0.15
50 100 150 200 250
Figure 2.7. Sea surface height. Values for experiment 7 for August and February, and experiment 1 are shown in (a) (above), (b) and (c), respectively. Values for experiment 8 for August and February, and experiment 2 are shown in (d), (e) and (f), respectively. These snapshots were taken after ten years of model integration.
down to a low of ~ 6cm in the north. The SSH both in the Maluku and Halmahera
Seas slopes weakly down from the south to the north from a high of ~ 5cm to a low
of ~ 3cm. A local high of ~ 10cm is seen in the easternmost Banda Sea, decreasing
to ~ 7 cm in the northern Banda and Flores Seas. This value decreases to a SSH of
~ 0cm in the south Flores and south Banda Seas.
The pattern in SSH for the mean annual experiment is similar to that seen in
August though not as pronounced; the low in the eastern Banda Sea is ~ - 2cm; the
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high in the Flores Sea is ~ 4cm; and the low in the south Makassar Strait is ~ Ocm.
The effect of local wind on SSH is generally not substantial (see Figures 2.7(d)-
(f)). In August the differences in SSH are still noticeable when the winds are removed.
SSH patterns in the Banda Sea are slightly changed with the value in the east in
creasing to ~ -5cm. In February the pattern changes very little with a maximum
increase of ~ 2cm in the eastern Banda Sea. The changes in annual SSH are even
less noticeable.
2.4. Estimate of Terms in Heat and Salt Equations
Before estimating the role of the different terms in the heat and salt-diffusion
equations we clarify some important points. The equation for specific internal energy,
em, is formulated as (see, e.g., Kamenkovich 1977; p. 41)
9 / ^ ( 9X 1 3 W7 (pem) + div pemu + q + = X Paf3ea/3 , (2-7) ot 8 S ) rp / T,p a ,0 = 1 where p is the density, u is the velocity of a fluid particle, q is the density-of-heat-flux
vector, I is the density-of-salt-mass-flux vector, Xm is the specific enthalpy, T is the
temperature in Celsius scale, s is the salinity (as a non-dimensional ratio), paj3 is the
stress tensor, and eap is the rate-of-strain tensor.
The last term in the brackets describes the influence of salt diffusion on the
change of the specific internal energy while the term 011 the right-hand side of the
equation describes the mutual conversion of mechanical and internal energy. Both of
these terms are usually neglected. So, the rate of change of internal energy per unit
volume, pem, is controlled by the advection of this property and the divergence of the
density-of-heat-flux vector. The specific internal energy is defined with respect to
an undetermined reference state, thereby influencing the advection of this property
through an open surface.
Following Warren (1999), we will consider the anomaly of the specific internal
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50 100 150 200 250 i
Figure 2.7(b).
energy, e'm(T,s,p) = em(T,s,p) — e(0,0.035, p0), where p is the pressure, and p0
is the mean atmospheric pressure. Due to the mass conservation equation we can
replace em in (2.7) by e'm. Then the advection of pe'm through any surface can be
unambigously determined. It is important that Warren (1999) has shown that with
sufficient accuracy
e'm = Cp 0 , (2.8)
where the specific heat capacity at constant pressure, cp, can be considered as con
stant (cp = 4 x 103Jkg~1), and 0 is the potential temperature in Celsius scale.
Replacing the density, p, by the mean density, p0, we rewite (2.7) as
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Figure 2.7(c).
~ + div (du + = 0. (2.9) dt \ pocpJ
Equation (2.9) is usually called the heat equation. Substituting the turbulent density-
of-heat-flux vector for the molecular density-of-heat-flux vector q and using the POM
parameterization gives the POM heat equation. Integrating this equation, first over
a from -1 to 0, and then over a fixed horizontal area E, and using the boundary
conditions for wpom (at the surface and bottom) and for 9 (at the bottom) gives
^-tjD9dE + fDT(^i)dT - I ( ~ ~ ) dS + | DrW(0-edim)dZ E r E ^ E
— H A h — (6 — 9dim) dT = 0, (2.10) r
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Figure 2.7(d).
where the bar implies the integral over a from -1 to 0; T is the boundary contour
of E; n is the outward normal vector to this contour; subscript 0 indicates that the
expression is calculated at the sea surface (a = 0); is the nudging coefficient
dependent on a; 9ciim is the climatology of 9; AH is the coefficient of horizontal d turbulent diffusion; and 7 — is the derivative in the n direction; D is the total depth, on D = H + rj, where 77 is the sea-surface height; and KH is the coefficient of vertical
turbulent diffusion. The expression for the last term on the left-hand side of (2.10)
follows from the parameterization used in the POM (see Mellor 2004; formulas (11)
and ( 12)).
In equation (2.10) we have a balance between (from left to right) the rate of time
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50 100 150 200 250 i
Figure 2.7(e).
variation of the volume integrated 0 (or S)\ the sum of horizontal advective fluxes
of 6 (or S) through the side boundaries of the volume; the vertical flux of heat (or
salt mass) through the sea surface integrated over the sea-surface side of the volume
(with the sign changed); the nudging term integrated over the volume; and finally
the sum of horizontal diffusive fluxes through the side boundaries of the volume (with
the sign changed).
We now break the area down into boxes and consider equation (2.10) and a similar
equation for salinity S for these boxes. All terms in (2.10) are calculated directly
from the simulated fields of the horizontal components of velocity (ms-1), potential
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.7(f).
temperature (°C) or salinity, and coefficients of vertical and horizontal diffusion (all
data are for August). The results for the most important boxes are presented in
Tables 3 and C.l - C.3 in Appendix C. The boxes considered are shown in Figure
2. 8.
The sum of horizontal advective fluxes through all sides of a given box, vertical
flux at the sea surface, nudging term, and rate of time variation appear to be basically
of the same order. However, the sum of horizontal diffusive fluxes through all sides
of the box is at least two orders of magnitude smaller than all other terms. Note, the
sum of horizontal advective fluxes is several orders of magnitude smaller than the
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calculated values 9 S rate of time variation (xlO8) -8.9xl0 -3 1.3xl0 - 3 horizontal advective fluxes (xlO8) Lifarnatola passage (j=152), Q=-2.7Sv -7.6xl(T 2 -9.3xl0 _1 Between Mangole and Buru (i== 130), Q=6.3Sv 1.2 2.2 Between Buru and Seram (j=134), Q=0.6Sv 1.2 xlO_I 1.9xl0 -1 Between Obi and Seram (i=145), Q=-4.2Sv -1.2 -1.5 Sum of horizontal advective fluxes 6.8 xlO - 3 -5.7xl0 ~4 horizontal diffusive fluxes (xlO8) Lifarnatola straits (j = 152) -1.6 x 10-4 1.4xl0 - 6 Between Mangole and Buru (i=130) 1.9xl0 -4 -1.2 x l 0 -6 Between Buru and Seram (j=134) ~3.3xl0 - 5 2 .8 x 10-7 Between Obi and Seram (i=145) -2.9xl0 “ 5 -5.0xl0 “ 7 Sum of horizontal diffusive fluxes -3.4xl0 - 5 l.lx lO -6 vertical flux at the surface (xlO8) -2.3xl0 - 3 -8.4x 10 -3 nudging (xlO8) -2.9x 10-3 7.7xl0 “ 3 sum of all terms (xlO8) -7.2 xlO -3 -4.8 xlO - 5
Table 3. Estimates of different terms in the depth integrated heat, (9), and salt diffusion, (S'), equations for the Seram Sea box. August values are given for the seasonally varying experiment with local wind. Qs are the total transports through the sides of the box. Negative values of Q are into the box and positive values are out of the box. The last line gives the estimate of accuracy of calculations. Values for specific heat and reference density are, respectively, cp = 4 x 103J o C~1kg~1, Po = 1025kgm~3.
leading advective flux through a side of the box, while the sum of horizontal diffusive
fluxes is always on the order of the diffusive flux through one side of the box. It
is well known that the POM parameterization of horizontal diffusion can generate
some false dynamical effects. These effects, if they are present at all, are very small
in our model.
Tables 3 and C.l - C.3 (Appendix C) show these balances in more detail. For the
Seram box (Table 3) the sum of advective fluxes, vertical diffusion at the sea
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Figure 2.8. The eight boxes for which estimates of terms in the heat and salt diffusion equations have been made. The boxes are: 1. Seram Sea, 2. Banda Sea, 3. Makassar Strait, 4. Lombok, 5. East Maluku Sea, 6 . Karakelong-Sangihe-Morotai, 7. Halmahera Sea, and 8 . West Sulawesi Sea.
surface, nudging, rate of time variation, and the sum of all terms (residual) are on
the same order (for 9); for S, the balance is essentially between vertical diffusion
and nudging while the residual is two orders of magnitude smaller. For the Banda
box (Table C. 1) the nudging and rate of change of time variation balance each
other while the residual is one order of magnitude smaller for 9; for S the balance is
essentially between the sum of advective fluxes, vertical diffusion and nudging while
the residual is two orders of magnitude smaller. For the Makassar box (Table C.
2) the balance is essentially between the sum of advective fluxes, nudging, and the
rate of time variation while the residual is on the order of the nudging term (for
9); while for S, the dominant terms are the vertical diffusion and nudging while the
residual is on the order of the vertical diffusion term. For the Lombok box (Table C.
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3) all four terms - the sum of advective fluxes, vertical diffusion at the sea surface,
nudging, rate of time variation, and the residual are on the same order ( 6); for S,
the vertical diffusion, nudging, and the rate of time variation are on the same order
while the residual is one order of magnitude smaller. Similar relations hold for other
boxes (East Maluku, Karakelong-Sangihe-Morotai, Halmahera, and West Sulawesi).
Note, that the sum of all terms in (2.10) (residual) is sometimes on the order of these
terms rather than at least one order of magnitude smaller. This is seemingly due to
the method of calculation of horizontal advective fluxes in (2.10). Running the POM
we used the Smolarkiewicz iterative scheme for the advection terms in the 6- and
5-equations, while in estimating the horizontal advection in the integrated forms of
these equations we applied the corresponding expressions in ( 2.10) directly.
2.5. Comparison with Observations
The comparison of simulated and observed basic structures of the surface circu
lation and sea-surface height patterns along with magnitudes of transports through
main passages appears to be satisfactory. We now consider in detail the observed
data.
2.5.1. Transports and Surface Circulation
Molcard et al. (1996) estimated the transport through the Timor Passage with
data from two current moorings deployed near the southwesternmost tip of Timor
Island between March 1992 and April 1993. They also made continuous CTD casts
across the passage for 2 days during deployment. Their estimate of the mean trans
port for this period is between 3.4 and 5.3 ± l.bSv. In the upper 500m, the transport
is at a minimum of 1 — 2 Sv during the NWM and at a maximum of 3 — ASv during the
SEM. They found a mean surface velocity of ~ 40cm/.s directed toward the Indian
Ocean along the axis of the passage. Surface velocity is weakest but in the same
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direction during the NWM. Model transport through the Timor Passage is at a min
imum of 2 Sv during the NWM and increases to almost lOSu during the SEM (Figure
2.5(g)). Model surface velocities are maximal during the SEM (Figure 2.6(a)), mini
mal during the NWM (Figure 2.6(b)) and the mean values are in agreement with the
calculation of Molcard et al. (1996) (Figure 2.6(c)). Molcard et al. (1994) also cal
culated the transport through the Timor Passage between August 1989 and August
1990 from current meter moorings and Indonesian coastal tidal gauges. They calcu
lated a mean transport for the period of 7 Sv. In early September 1989 the transport
was as high as IlSu, supplying at least half of the total transport of 22Sv between
Bali and Australia (see Fieux et al. 1994). Molcard et al. (1994) conclude that
the Timor Passage may supply more than 50% of the Indonesian Throughflow (ITF)
during the SEM. The model gives a maximum transport through the Timor Passage
of almost lOSu during the SEM which is more than half of the total transport of the
ITF (17ST) at this time (Figure 2.4(a)).
Cresswell et al. (1993) also estimated a mean transport through the Timor Pas
sage of 7Sv from shipboard ADCP measurements made in October 1987 and March
1988 and from two current moorings deployed between March and December 1988.
An analysis of drifters show that surface velocities exceeded 1 m /s in July 1983.
These observations are in good agreement with model results (see Figure 2.5(g);
Figure 2.6(a)).
Molcard et al. (2001) calculated the mean transport through the Ombai Strait
from a current mooring deployed there for one year in December 1995. They found
a transport of 5 ± 1ST with a maximum of 6Sv in August and a minimum of 4 Sv.
Maximum surface velocities of ~ 60cm /s are seen in August. The model gives a mean
transport of ~ 5.5 Sv (Figure 2.4(c); Figure 2.5(h)), with a maximum in August of
~ 5.1 Sv (Figure 2.4(a); Figure 2.5(h)). Model surface velocities reach a westward
maximum during August as well (Figure 2.6(a)).
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In the Lombok Strait, Murray & Arief (1988) observed a distinct annual cycle in
transport from current meter moorings, deployed between January 1985 and January
1986. They observed a minimum transport of ~ 1 Sv from February to May, a
maximum of ~ 4.05T in the northern summer months and a mean value of 1.7 Sv
for the duration of the deployment. The model gives a similar cycle (Figure 2.5(b))
though maximum transports do not exceed 2.2 Sv. From the same data set, Murray
et al. (1990) observed southward surface velocities throughout the year with brief
periods of reversal towards the north. Model surface currents are always to the south.
Hautala et al. (2001) estimated mean transports through the exit passages of the
ITF from shallow pressure gauge arrays (SPGA) deployed in the passages between
December 1995 and May 1999 and from ADCP surveys during original and subse
quent deployments and recovery. They estimated mean total transports for the two
year period (1996 - 1997) of 2.6 ± O.SSv through the Lombok Strait; 2.6 ± 0.8ST
through the Ombai Strait; and 3.2 ± 1.8ST through the Timor Passage. These esti
mates compare reasonably well with model results (Figure 2.5 (b, h, g)).
Susanto & Gordon (2005) estimated total transport through the Makassar Strait
with data from two current meter moorings including near surface ADCP measure
ments deployed in the Labani Channel between December 1996 and July 1998. They
found a mean transport of 8.1 ± l.5Sv for the entire period and 7.9 ± 1.2Sv for
the 1997 year. The results represent an improvement on the previous estimates
of Gordon & Susanto (1999), Gordon et al. (1999) and Wajsowicz et al. (2003),
since those estimates were hindered by the absence of a surface layer record from
the ADCP. For the same data set, Gordon & Susanto (1999) and Gordon et al.
(1999) estimated transports of 9.2 and 9.3 Sv, respectively, from four simple velocity
profiles (maximum, minimum, average, and seasonal dependence). Wajsowicz et al.
(2003) estimated a mean transport of 6.4 Sv using a normal mode approach based on
monthly data; while Vranes et al. (2002) estimated a mean transport of 8 ,9Sv using
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data only from the upper 700m of the westernmost mooring. Maximum estimates of
transport briefly exceeded 14 Sv in June - July 1997 and August 1998 while minimum
transports dipped below 4Sv in December - January 1997 and during the NWM of
1998. All of these estimates are in good agreement with the model results (see Figure
2.5(a)).
Wajsowicz et al. (2003) also found maximum and minimum transport values of
16.0 and 4.7Sv, respectively. The ADCPs lasted only 3 months on the westernmost
mooring and 8 months on the easternmost. Nevertheless, surface velocity values were
extrapolated from the ADCP data and the shallowest current meter for the entire
deployment period. Comparing ADCP observations between December 1996 and
February 1997 yields mean surface velocities of 35 cm/s southward for the western
most mooring, and of 22 cm/s for the easternmost. This appears to be the result
of the western boundary effect and is also produced b}^ the model (see, e.g., Figure
2.6(b)). Mean surface southward velocities of ~ 25 cm/s at the Labani Channel are
in good agreement with model results (Figure 2.6(c)).
Van Aken et al. (1988) estimated a deep transport of ~ 1.5SV across the Lifam-
atola Passage from a current mooring deployed between January and March 1985,
in good agreement with model results (see, e.g., Fig 2.5(c)).
Wyrtki (1961) was the first to give a comprehensive description of the oceanogra
phy of the Indonesian Seas and of the entire Southeast Asian region. Using both his
and historic observations, including data from merchant ships (see Wyrtki (1960) for
further discussion), he found a high correlation between the monsoon winds and the
ocean circulation. Just as monsoon winds practically reverse direction twice a year,
the ocean currents are also reversed in large areas of the region. W yrtki’s (1961) de
scription of the surface circulation is as follows. In February the MC extends to the
southern tip of Mindanao Island where it splits into two parts. The larger portion
continues southward to the Morotai Basin before turning sharply to the east, form
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ing the cyclonic Mindanao eddy and the core of the NECC. The remainder enters
the Sulawesi Sea, though this is partially returned toward the NECC as a portion of
the broader Mindanao eddy retroflection. This circulation pattern essentially per
sists throughout the year though the extent and location of the Mindanao eddy may
change. The area to the north of New Guinea is marked by the near surface seasonal
variations of the NGCC. This area is influenced by the southeast trade winds from
May until October and by a branch of the northeast trade winds between November
and April. As a result, the surface currents are strongest out of the southeast in
August, when the Halmahera eddy is developed, and strongest toward the southeast
in February, when the Halmahera eddy is not developed.
Wyrtki (1961) showed also that in February part of the MC crosses the central
Sulawesi Sea to enter the Makassar Strait. In addition, there is an anticyclonic cell
to the north, and cyclonic cell to the south. This system of circulation is supported
from February to September. The Mindanao eddy shifts to the east in October and
no surface flow from the Sulawesi Sea enters the Makassar Strait.
In the Makassar Strait the surface flow is directed to the south throughout the
year with small velocities as a rule. Currents are strongest in February and March,
and weakest from July to September. In the south of the Makassar Strait, waters
leaving the strait flow to the west during the SEM and to the east during the NWM.
In the Flores Sea, Wyrtki (1961) described the surface flow as being uniform
and eastward from December to March, when the winds are westerly, and westward
for the remainder of the year, when the winds are southeasterly. He noted that a
weak eastward current just to the north of the Lesser Sunda Island Chain persists
throughout the year.
Mean surface flow across the Lifarnatola Passage is southward at this time. This
current system is reversed during the SEM when surface waters from the Halmahera
Sea enter the Seram Sea and the Banda Sea before reaching the Flores Sea and mean
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surface flow across the Lifarnatola Passage is to the north.
A southwestward current prevails throughout the year in the Timor Passage with
its axis running close to Timor. The current extends to Australia during the SEM
though velocities decrease across the passage. The current retreats toward Timor
during the NWM and a northeastward current forms in the south. During the NWM
the Timor current is supplied by the current flowing to the east along the north coast
of the Lesser Sunda Island Chain. During the SEAI the Timor current is supplied by
upwelling in the eastern Banda Sea.
The model fairly well reproduces the circulation patterns described by Wyrtki
(1961).
2.5.2. Sea Surface Height
Based on sea-level observations, Wyrtki (1961) concluded that the SSH of the
Banda Sea is intimately linked to the southeast Asian monsoon system. During the
NWM, surface water enters the Banda Sea from the west resulting in accumulation
of surface water there and raising of SSH. During the SEAI, this surface water enters
the Indian Ocean as part of the ITF. Wyrtki (1961) estimated the range in SSH in
the Banda Sea at ~ 10 — 20cm. Gordon & Susanto (2001) investigated surface-layer
divergence and sea level anomaly (SLA) in the Banda Sea. They calculated a time
series of SLA from the combined TOPEX POSEIDON/ERS altimeter data for the
period December 1992 to December 1999. An annual range of ~ 20cm was found
with a maximum of ~ 10cm in February and a minimum of ~ — 10cm in August.
The model is in good agreement with both Wyrtki (1961) and Gordon & Susanto
(2001), having a similar range of SSH between these months with a maximum in
February of ~ 10cm and a minimum in August of ~ 8 cm (see Figures 2.7(b) and
2.7(a)).
Bray et al. (1996) investigated large-scale sea level variations in the Indonesian
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Throughflow region. They used data from the Tropical Ocean-Global Atmosphere
(TOGA) sea level archive of stations corrected by barometric pressure data from the
Comprehensive Ocean-Atmosphere Data Set (COADS). Bray et al. (1996) found
that most of the variability in SSH is contained in the annual signal. Annual values
of variability were least in the northern Indonesian Seas with a minimum amplitude
of ~ 2cm in the Sulawesi and Maluku Seas as well as in the Makassar Strait. The
amplitude increases to the south and east to a value of ~ 4cm in the Flores and
northern Banda Seas, to ~ 8 cm in the Timor passage and to a maximum value of
20cm in the southeastern Banda Sea. Maximum SSH in the Banda Sea region occurs
in January-February. SSH maxima occur later in the year in the northern Indonesian
Seas with a maximum occurring in the Sulawesi Sea several months later. Model SSH
is in good agreement with these observations. Model SSH annual amplitude is least
in the north with a minimum value of ~ 2cm in the Sulawesi Sea region, increasing
to a maximum value of ~ 20cm in the southeast Banda Sea.
Waworuntu et al. (2001) investigated the dynamics of the Makassar Strait by
analyzing inverted echo sounders (IES) and bottom pressure data (PIES) for the pe
riod from October 1996 to March 1998 combined with TOPEX/POSEIDON satellite
derived sea height anomaly for the period October 1992 to April 1998. Both the
PIES and TOPEX/POSEIDON data show an annual signal in SSH in the Makassar
Strait with maximum SSH during the NWM and minimum SSH during the SEM.
TOPEX/POSEIDON data show greater annual amplitude of ~ 20cm in the north
ern Makassar Strait, reducing to ~ 10cm in the southern Makassar Strait. Model
results are in good agreement with maximum values of SSH in the Makassar Strait
during the NWM and minimum values of SSH during the SEM. Annual simulated
amplitudes are ~ 10cm.
2.6. Summary and Conclusions.
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The basic features of a developed high-resolution regional model of the Indone
sian Seas circulation have been discussed. The model, based on the Princeton Ocean
Model (POM), has horizontal resolution of ~ lOfcra and 29 a-levels in the vertical. It
is well known that the regional dynamics of the Indonesian Seas is strongly controlled
by peculiarities of the bottom topography. Thus the primary motivation for devel
oping the model is to take into account all basic features of the bottom topography
in the region. It is demonstrated that by using a horizontal resolution of 10km we
have successfully incorporated all important topographic features into this model.
Although the limitations of regional models are known, such models are useful in
gaining insight into the regional dynamics. We feel confident that important ques
tions such as the partition of the main flow between passages and interaction of NPW
and SPW, the influence of local winds, overall momentum and energy balances, and
properties of turbulent mixing in the region can be successfully addressed with the
developed model.
The model has 250 x 250 cells in the horizontal and four open ports to sim
ulate major currents into and out of the domain: the MC (Mindanao Current)
port through which North Pacific Water (NPW) enters the region; the NGCC (New
Guinea Coastal Current) port through which South Pacific Water (SPW) enters the
region; the NECC (North Equatorial Counter Current) port through which part of
both NPW and SPW leave the region; and the 10 (Indian Ocean) port through
which the Indonesian Throughflow (ITF) enters the Indian Ocean. Model topog
raphy, interpolated from the ET0P05 database, has been edited and smoothed to
satisfy the POM recommendation for maximal permissible slopes, thereby decreasing
possible effects of the pressure gradient representation in the POM. The smoothed
topography has been thoroughly checked to ensure that all essential details have been
retained.
At the open ports, the normal velocity for the 2D (depth averaged) motion is
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specified based on the prescribed total transport through the port. The tangential
2D velocity is set to zero. For the 3D motion, Orlanski’s condition for the normal
3D velocity with nudging to the observed normal port velocity has been employed.
Different nudging coefficients for the inflow and outflow ports were used. The tan
gential 3D velocity is set to zero. The boundary conditions for potential temperature
and salinity are similar to the 3D normal velocity condition. The turbulence energy
and length scale are set to zero. The remaining boundary conditions are standard
for the POM.
The POM basic equations have been slightly modified both in the port channels
and in the interior domain. The open port channels (5 cells in length) were intro
duced to allow for tapering off of nudging effects outside the interior of the model
domain. For the momentum equations, friction and nudging terms have been added
in the inflow channels (MC and NGCC) and frictional terms only have been added
in the outflow channels (10 and NECC). For the temperature and salinity equa
tions, nudging terms have been added at all ports. Nudging and friction coefficients
decrease toward the interior domain for all ports. In the main domain, additional
friction has been added to the momentum equations at some sills and passages. This
measure was used to take into account some specific processes, e.g., tidal effects, not
included in the model. Temperature and salinity have been nudged to climatology
near the surface to control the Mellor-Yamada parameterization of vertical turbulent
fluxes in the layer of strong salinity extrema. Weak nudging was included below
the thermocline to reduce the effect of the POM horizontal diffusion. Advection of
temperature and salinity has been modeled with two iterations of the Smolarkiewicz
scheme.
At the open ports normal velocities and total transports are specified from pub
lished observations and some modeling results. At the 10 port we have basically
used the observed data of Fieux et al. (1994) along with other observations at the
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exit passages of the Indonesian Seas region (Lombok Strait; Ombai Strait; Timor
Passage). We have chosen transport values of 17Sv in August, 8 Sv in February and
an annual mean value of 12.5ST. At the NGCC port we have basically used the
observations of Ueki et al. (2003). We have chosen transport values of 28 Sv in Au
gust, 17 Sv in February and 22.5 Sv for the annual mean. At the NECC port we have
basically used the observations of Qiu & Joyce (1992). We have chosen transports
of 46ST in August, 35.ST in February and an annual mean value of 40SV. At the
MC port we have basically used the observations of Lukas et al. (1991). We have
chosen transports of 35 Sv in August, 25 Sv in February and a mean annual transport
of 30Sv. Note that the port normal velocity profiles at the 10, NECC and MC ports
are assumed constant throughout the year, while the port normal velocity profiles
at the NGCC port are supposed different for August and February. All port normal
velocity profiles are scaled by the specified total transports. All seasonally varying
external factors (port transports, port normal velocities, local wind stress, tempera
ture and salinity climatology) were approximated in time by the annual mean value
plus two Fourier harmonics.
The four experiments discussed in this paper - annual mean with and without
wind and seasonally varying with and without wind - were run for at least 12 years.
All of the experiments were fully spun up after 10 years.
The basic properties of simulated total transports through the main passages in
the region, surface circulation and sea-surface heights (SSH) have been discussed.
According to calculated transports, NPW splits into three branches upon entering
the model domain through the MC port. The first branch takes the western route
over the Sangihe Ridge, across the Sulawesi Sea and along the Makassar Strait before
diverging at the southern part of the Makassar Strait. The minor part of the flow
exits through the Lombok Strait while the major part enters the Banda Sea through
the Flores Sea. The second branch of NPW takes the north-south route through
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the Morotai basin to the Maluku Sea, and crosses the Lifarnatola Passage into the
Seram Sea before finally entering the Banda Sea. The third branch of NPW exits
through the NECC port. SPW splits into two branches upon entering the model
domain through the NGCC port. The first branch takes the eastern route through
the Halmahera Sea into the Seram Sea where it encounters NPW on the north-south
route. The second branch exits through the NECC port. Banda Sea water exits the
Indonesian Seas through the Ombai Strait and Timor Passage. Transports through
the Makassar Strait, Maluku Sea, Halmahera Sea, Lombok and Ombai Straits, and
the Timor Passage are stronger in August and weaker in February relative to the
annual mean. The portion of NPW entering the Indonesian Seas relative to that
leaving through the NECC port is fairly constant throughout the year. Most of
this water takes the western route. The portion of SPW entering the Halmahera
Sea compared to that exiting through the NECC port varies considerably with the
seasons. Almost all of the NPW taking the north-south route through the Maluku Sea
and SPW crossing the Halmahera Sea enters the Banda Sea through the Mangole-
Buru Passage. The total transport through Mangole-Buru is about 2/3 of that
through the Makassar Strait in August and the mean annual experiment, and about
1/2 that through Makassar in February. The flow through Lombok is fairly constant
at about 1./4 of the total Makassar transport with the remainder entering the Banda
Sea. At the exit passages, the relative changes in transport with the seasons are
strongest for the Timor Passage, moderate for the Lombok Strait and insignificant
for the Ombai Strait. Turning off the local winds does not substantially influence the
transport through the main passages in the model domain. For almost all passages
the annual harmonic appears dominant with substantially different amplitudes.
Surface circulation patterns change substantially with the seasons. In August the
surface current is everywhere to the west in the interior of the Indonesian Seas in
response to the south-east monsoon (SEM) with the exception of a narrow eastward
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band along the Lesser Sunda Island Chain. In February the flow is everywhere to
the east in the Banda Sea. Flow in the Makassar Strait is to the south with clear
westward intensification seen to the north of Labani Channel. For the mean annual
experiment, flow is to the west in the Banda Sea due to the rather strong mean
local wind there. Turning off the local wind reveals a noticeable change in surface
circulation. First, the surface circulation patterns are not as organized. Surface
circulation patterns are oppositely directed for large areas of the region. Invoking
the geostrophic relations shows that differences in current velocities are generated
by the local wind stress.
SSH values are very reasonable throughout the model domain. Values are affected
by seasonally changing transports and local wind stresses. In the Banda Sea region,
the SSH slopes up from the east to the west during the SEM and down from the east
to the west during the north-west monsoon (NWM). In the annual mean the SSH is
similar to August though amplitudes are reduced due to the decrease in local wind
stress and transports. SSH amplitudes change when the local winds are turned off
but the effect is not as substantial as the seasonal one.
The role of different terms in the heat and salt equations was investigated by
dividing the region into a number of boxes. For any given box, the sum of the
horizontal advective fluxes of temperature (salinity) through all sides of the box is
on the same order as the vertical heat (salt) flux at the surface, nudging term, and
the rate of time variation of the box integrated temperature (salinity). The sum of
horizontal diffusive fluxes of temperature (salinity) through all sides of a given box
are two orders of magnitude smaller than all other terms. The leading advective
flux term for a given box is several orders of magnitude greater than the sum of
the advective fluxes through the box while the sum of the horizontal diffusive fluxes
through all the sides of the box is always on the same order as the flux through one
side. It is well known that the POM parameterization of horizontal diffusion can
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generate false dynamical effects. These effects, if they are present at all, are very
small in our model.
The comparison of the basic structure of the model surface circulation, SSH
and total transport values through the main passages with observations appears
satisfactory. At the exit passages (Timor Passage, Ombai Strait and Lombok Strait),
seasonal variabilities of transport and surface velocities in the model are in good
agreement with available data. Model values of annual mean transport through these
exit passages are also in good agreement with transports estimated from observations.
Model transports in the Makassar Strait are in good agreement with the results of
several investigators. The western boundary effect seen in the model Makassar Strait
was also observed in ADCP data. The model transport at the Lifarnatola Passage
is in reasonable agreement with available observations. Details of the model surface
circulation are in reasonable agreement with Wyrtki’s (1961) description. Model
results in the region show that the SSH is modulated by the monsoons. This result
is verified by observations. Observations in the Banda Sea show SSH values in the
east are least during the SEM and greatest during the NWM. SSH slopes up toward
the west in August and down to the west in February. This pattern is reproduced
by the model and the amplitudes are of the same scale. Similar agreement is found
in the Makassar Strait.
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CHAPTER 3
THE INFLUENCE OF BOTTOM TOPOGRAPHY ON TEMPERATURE
AND SALINITY DISTRIBUTIONS
3.1. Introduction
One of the most important features of the Indonesian Seas circulation is the
transport of mass, internal energy (heat), and salt through the region from the Pacific
to the Indian Ocean. The estimate of these transports depends on the distribution
of potential temperature, salinity and velocity at the entering and exiting passages
of the region. The numerical model and properties of simulated velocity fields have
been discussed in Chapter 2. The main objective of this chapter is to describe the
basic properties of the simulated potential temperature and salinity distributions
and identify physical processes responsible for their formation. The simulated and
observed properties of these distributions will be compared to validate the model.
An outstanding feature of the Indonesian Seas is the complexity of the bottom
topography. This topography has a profound effect on the distribution of tempera
ture and salinity in the region. In setting up the model we have taken into account
all important topographic features by using high horizontal resolution and relevant
smoothing of the bottom relief (see Chapter 2). There are two entering types of
water recognized basically by their vertical salinity structure. North Pacific Water
(NPW) entering the Indonesian Seas region as a branch of the Mindanao Current is
characterized by a surface value of ~ 34.10; a salinity maximum of ~ 34.74 at a depth
of ~ 150m; a deep minimum of ~ 34.46 at a depth of ~ 350m; and a deep value
of ~ 34.66. South Pacific Water (SPW) entering the Indonesian Seas as a branch of the New Guinea Coastal Current is characterized by a surface value of ~ 34.15;
a strong maximum of ~ 35.50 at a depth of ~ 100 — 150m; a weak minimum of
~ 34.61 at a depth of ~ 1500m; and a deep value of ~ 34.65. The temperature
and salinity characteristics of NPW and SPW change as waters flow through the
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Indonesian Seas region. What factors control these changes? In general, the water
properties are advected and diffused. We will show that below 500m the advection is
totally controlled by topographic features in the region. Large elevations of bottom
relief break the region down to separate basins having different temperature and
salinity stratifications. These topographic elevations impede the advection of cold
and salty water from a basin (located upstream) to the neighboring basin (located
downstream). We will consider temperature and salinity sections along the paths of
NPW and SPW. By comparing upstream and downstream temperature and salinity
distributions for each substantial topographic elevation we will identify the impact
of the topographic feature. Finally, we will choose salinity as an important tracer
and estimate the salinity advection through the sections located across the main flow
to see what salinities are really advected into the basin.
There are two paths of flow of NPW and one path of flow of SPW to the Banda
Sea from where waters leave the region, essentially through the Ombai Strait and the
Timor Passage. The first route of NPW (the western route) is through the Sulawesi
Sea. Makassar Strait arid Flores Sea. Along this route, the rather shallow Dewakang
Sill of depth ~ 600m is encountered, blocking the flow of deep saline water towards
the Flores Sea and thereby the Banda Sea. We will show that in the upper ocean
(500m) the Indonesian Throughflow primarily consists of NPW taking the western
route through the Makassar Strait and into the Banda Sea. The second route of
NPW (the northern route) is through the Morotai basin, Lifarnatola Passage and
Seram Sea. We will show that basically the deep salinity of the Banda Sea is formed
through this route. We will also discuss the long standing question regarding the
role of SPW (see, e.g., Aken et al. 1988; Godfrey et al. 1993; Gordon 1995; Godfrey
and Wilkin 1995; Gordon & Fine 1996; Gordon et al. 2003a; and references therein).
Essentially, SPW enters the region through the Halmahera Sea. We will show that
the northern and southern Halmahera Sills completely block the flow of deep SPW
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into the Indonesian Seas. But in the upper layer (0 — 500m) some SPW can probably
penetrate to the Maluku and Seram Seas to mix there with NPW and finally reach
the Banda Sea.
In Section 3.2 we will estimate the simulated transports of mass, internal energy
(heat), and salt through the Indonesian Seas region and contributions of deep parts
of the seas to these transports. In Section 3.3 we will analyze the August simulated
temperature and salinity distributions in the region and processes that form these
distributions. In Section 3.4 we will describe the seasonal variations of the simulated
distributions. In Section 3.5 a detailed comparison of simulated and observed char
acteristics will be provided to validate the model. Finally, in Section 3.6 the basic
results of the study are summarized.
3.2. Transports of Mass, Internal Energy and Salt
We begin with the calculation of transports of mass, internal energy (heat), and
salt through the Ombai Strait, Timor Passage, and Lombok Strait along with trans
ports through the Mindanao Current (MC), New Guinea Coastal Current (NGCC),
and North Equatorial Counter Current (NECC) ports. The results for August and
February are presented in Table 4. In general, the values of total transports are
in agreement with published simulated and observed data. A short review of this
comparison is given in Section 3.5.
Analyzing Table 4, we notice first the substantial seasonal changes of transports
caused by seasonally varying water transports through the MC, NGCC, and NECC
ports and by local winds. Second, transports of internal energy (heat) and salt in the deep parts (below 500m) of the Ombai Strait and Timor Passage appear rather
substantial. Therefore, the analysis of processes responsible for the formation of
distribution of potential temperature and salinity in the deep parts of Indonesian
Seas (especially in the Banda Sea) is very important. Such an analysis will be
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mass transp heat transp salt transp ( x l 09kg/s) (PW) ( x l 08 kg/s) Aug Feb Aug Feb Aug Feb Ombai (i=90) total 5.55 5.06 0.31 0.18 1.91 1.75 0-500m 3.07 1.79 0.26 0.12 1.05 0.62 Timor (i=90) total 9.55 1.94 0.69 0.30 3.28 0.67 0-500m 9.75 4.23 0.69 0.35 3.35 1.46 Lombok (j=110) total 2.18 1.36 0.19 0.12 0.75 0.47 0-500m 3.14 1.77 0.21 0.13 1.08 0.61 MC port (j=246) total -35.70 -25.80 - 2.22 -1.51 -12.32 -8.91. 0-500m -30.90 -21.71 -2.19 -1.40 -10.66 -7.50 NGCC port (i=246) total -28.51 -17.62 -2.02 -0.98 -9.97 -6.17 0-500m -26.93 -16.16 -1.96 -0.93 -9.42 -5.67 NECC port (i=246) total 46.94 35.06 3.85 2.83 16.25 12.19 0-500m 44.97 34.28 3.76 2.79 15.57 11.92 Table 4. The simulated transports of mass, internal energy (heat), and salt through MC, NECC, and NGCC ports, along with Lombok and Ombai Straits, and Timor Passage for August and February. Additionally, the corresponding transports for the upper 500m are given. The specific heat capacity at constant pressure is cp = 4 x 103 J °C~l kg“T The mean density is given by p0 = I025kgm~3, IPW = 1015 Js~l.
presented in the next section. We remind readers that the Banda Sea is the main
source for water flowing through the Ombai Strait and Timor Passage.
3.3. The Analysis of Temperature and Salinity Distributions
Consider the aforementioned three major routes of flow of Pacific water through
the Indonesian Seas, which we will identify as the western, northern, and eastern
routes. The western and northern routes begin at the MC port, through which
NPW enters the region before splitting into three branches. The first branch of NPW
takes the western route flowing over the Karakelong and Sangihe Ridges, through
the Sulawesi Sea, and along the Makassar Strait, at the southern part of which it
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Figure 3.1. The locations of vertical sections AGHIJKF, ABCDEF, and LMND are shown by dots. Vertical profiles of temperature and salinity are provided at points identified by letters A, ... , N. Sections, at which normal velocity and salinity are analyzed, are shown by solid lines. Numbers give August transports through these sections in Sverdrups (10 6m3/s) in the direction of arrows. All arrows show transport directed normally to sections; slanting of some arrows is made for convenience of plotting. Values of indices i and j are given on the x- and y-axes. The Mindanao Current ( MC), North Equatorial Counter Current (NECC), New Guinea Coastal Current (NGCC), and Indian Ocean (10) ports are seen where transports enter and leave the model domain ( compare with Figure 2.1(b)).
diverges. Part of this flow exits the region through the Lombok Strait, but the main
portion passes through the Flores Sea to the Banda Sea (see path AGHIJKF in
Figure 3.1). The second branch of NPW takes the northern route flowing through
the Morotai Basin, Maluku Sea, Lifarnatola Passage, Seram Sea and finally into
the Banda Sea, where waters taking the western and northern routes mix (see path
ABCDEF in Figure 3.1). The third branch of NPW leaves the region through the
NECC port. The eastern route begins at the NGCC port, through which SPW enters
the region. Part of this water leaves the region through the NECC port while the
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remainder flows through the Halmahera Sea to the Seram Sea where it meets the
northern route of NPW (see path LMND in Figure 3.1). Finally, Banda Sea water
exits the region through the Ombai Strait and Timor Passage.
What factors shape the simulated potential temperature, 9, and salinity, S, dis
tributions in the Indonesian Seas region? We will show that 9 and S distributions
are strongly influenced by the bottom topography of the region. The main impact
of bottom topography is manifested in the deep layers (greater than 500m depth),
although some influence of the bottom topography on the salinity distribution can be
also traced in the upper 500m. We will disclose the role of some sills and passages in
the formation of 6 and S distributions. In particular, we will come to the conclusion
that 9 and S distributions in the deep layers of the Banda Sea are essentially shaped
by NPW flowing across the Lifarnatola Passage.
It is important to stress that the main structure of the observed distributions
of 9 and S, namely the monotonic decrease of temperature with depth and the
presence of a maximum and minimum in salinity profiles, is satisfactorily reproduced
by the model throughout the entire region. In the upper layer (0 — 500m) the model
and observed profiles are very similar. In the deeper layers temperature deviations
can reach 1 °C while salinity deviations are essentially on the order of several 0.01,
although in some layers, e.g. the layer of salinity minimum at the NGCC port, they
can reach as much as 0.1. Note that 9 and S distributions at the ports are closely
related but not identical to climatology (Conkright et al. 2002) - a result of the
chosen boundary conditions at the open ports (see Chapter 2). The comparison of
simulated and observed data is discussed in detail in Section 5.
Elevations of bottom relief break the Indonesian Seas region down to separate
basins having different 9 and S stratifications. We will analyze the August simulated
data and begin with the western route.
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500
1000
1500
2000 Mindanao port(A) E. Sulawesi Sea(G) f 2500 x N. Makassar(H) £ * S. Makassar(l) + Flores(J) 3 3000 O W. Banda(K) 0 S. Banda(F) 3500-
4000-
4500
5000
5500. 8
Figure 3.2. Vertical profiles of potential temperature along the western route for August at points A, G, H, /, J, K, and F (a) (above) and the distribution of potential temperature on vertical section AGHIJKF (b) (shown in Figure 3.1). The bottom topography along this section is shown. Note the break of vertical scale at 300m. Locations of points A, G, H 1 /, J, K, and F are indicated along the top of the figure. Distance from point A is shown on the x-axis. Geographical names and locations are shown (see also Figure 2.1(b)).
3.3.1. Western Route
Consider first the transformation of temperature profiles between points A, G, H,
I, J, K, and F (Figure 2a). In the upper 50m layer the profiles differ slightly due to
basically the surface boundary condition. Then in the layer of strong thermocline (50-
300m) and even deeper (up to 500m) all profiles are similar. Noticeable differences
appear at ~ 1000m. For depths greater than 1000m G-profile has substantially higher
temperature than initial /I-profile; G-. H-. and I- profiles practically coincide; and
J-profile (the Flores Sea) slightly diverges from /-profile. Below 2000m K- profile
(west Banda Sea) diverges from J-profile, but K- and F- profiles within the Banda
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 76
AG HI J K F
100
200
300
1000
J 7 T ,4.75
,4 5 4.5
® 2000 Dewakang 4 25
3000 N Makassar Sangihe strait ridge
4000 Flores sea 175/ Karakelong ridge and basin Sulawesi Banda sea 5000 Pacific Ocean o 500 1000 1500 2000 2500 3000 distance(km)
Figure 3.2(b).
Sea coincide. Temperature in the deep Flores Sea is higher than in the Sulawesi Sea
and Makassar Strait. Notice that bottom water in the Banda Sea is substantially
colder than that in the Flores Sea (compare J-profile with K- and F-profiles). The
above mentioned features are clearly seen in the vertical section of temperature taken
along AGHIJKF (Figure 3.2b). In the upper 500m isotherms are not interrupted
by the bottom topography. So the thickness of the thermocline does not change sub
stantially although isotherms appear elevated over the Sangihe Ridge and Dewakang
Sill.
Examine the divergence of A- and G-profiles below 1000m. Note that the section
goes through a passage in the Sangihe Ridge with a depth of ~ 1200m. Water
above the depth of the passage can easily overflow the ridge and enter the adjacent
basin. Below the depth of the passage, it is likely that fluid particles to the east
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 77
of the Sangihe Ridge also overflow the ridge due to the strong upwelling seen there
(see further the discussion of Figure 3.8 ). Due to the overflow all isotherms with
6 < 4°C are blocked by the ridge while isotherms with 6 > 4°C pass over it. It
is convenient to identify the 4°C isotherm as a threshold isotherm. Thus, a quite
different 6 stratification is shaped downstream of the ridge relative to upstream.
What characteristics of the upstream flow and underwater topography determine
such a stratification? There is a vast literature on overflow dynamics (see, e.g., Pratt
& Lundberg 1991; Killworth 1994; Pratt & Chechelnitsky 1997; Nielsen et al. 2004
and references therein), but we will not attempt to answer this question in detail.
We will simply demonstrate different types of overflows in the Indonesian Seas region
and provide some estimate of advection (of, say, salinity) into the basin considered.
Note that; the approach based on revealing blocked and passed isolines is often used
in the interpretation of observed hydrographic data (see, e.g., Gordon et al. 2003(a)
and references therein).
The slight distinction between I- and J-profiles is due to the topographic rise
between the Sulawesi Sea and the Makassar Strait and Flores-Banda Ridge. The
topographic rise blocks isotherms with 6 < 4.5°C' while, as we show later, some cold
water flows around and over the Flores-Banda Ridge into the Flores Sea from the
Banda Sea. Following our approach, we cannot explain the departure of J- and K-
profiles by assuming that the overflow of Flores water across the ridge between the
Flores and Banda Seas shapes the distribution of temperature in the Banda Sea. The
only possible explanation is that colder bottom water in the Banda Sea is formed
along the northern route ABCDEF.
We now turn to the analysis of salinity profiles (Figure 3.3a). Because these
profiles contain more structure than their temperature counterparts, they will provide
us with some additional information about the dynamics. The distinguishing feature
of kl-profile, characteristic of NPW, is the presence of a maximum (34.74) at a
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 78
500
1000
1500
Mindanao port(A) 2000 E. Sulawesi Sea(G) x N. Makassar(H) 1 2500 • S. Makassar(l) £ + Flores(J) -g 3000 O W. Banda(K) 0 S. Banda(F) 3500
4000
4500
5000
34.1 34.2 34.3 34.4 34.5 34.6 34.7 34.833.9 S
Figure 3.3. Vertical profiles of salinity along the western route for August at points A, G, H, I, J, K, and F (a) (above) and the distribution of salinity on vertical section AGHIJKF (b) shown in Figure 3.1. Further explanations are similar to those in the caption to Figure 3.2.
depth of ~ 150m and a minimum (34.46) at a depth of ~ 350m. The structure of
G-, H-, /-, J-, and A'-profiles in the upper ocean is basically the same. They have
maxima and minima at approximately the same depths, however the magnitudes of
these extrema differ substantially. The magnitudes of maxima markedly decrease
(when one moves from A-profile to /7-profilc). while the magnitudes of minima grad
ually increase. It is quite possible that this is due to the effect of vertical diffusion.
F-profile has practically no extrema. A more detailed picture is given by the vertical
section of salinity along AGHIJKF (Figure 3.3b). The high salinity signature of
NPW extends as far as the Dewakang Sill but is interrupted at the Karakelong and
Sangihe Ridges probably due to the upwelling that occurs near the Sangihe Ridge.
The strong lowering of isohaline 34.5 over the Dewakang Sill causes the signature of
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AG H I J K F
34-4
100 165
200 34.55
300
1000
34 58
® 2000 Dewakang
3000 N. Makassar
4000
Flores Karakelong Sulawesi ridge and basin Banda 5000 Pacific Ocean o 500 1000 1500 2000 2500 3000 distance(km)
Figure 3.3(b).
high salinity to end there.
In general, Figures 3.2 and 3.3 demonstrate a gradual evolution of NPW in the
upper 500m along the western route. No intrusion of water with substantially dif
ferent temperature and salinity characteristics can be identified. Because the main
part of the Indonesian Throughflow passes through the Makassar Strait we conclude
that basically it is NPW that shapes the temperature and salinity structure of the
upper part of the Indonesian Throughflow, as was stated by Wyrtki (1961); Gordon
(1986); and Gordon & Fine (1996).
We will analyze now in detail salinity G-, H-, J-, K-, and F- profiles below
500m. First, G-, H-, /-, J-, K-, and F-profiles appear to be saltier than Wprofile
in the depth range 500 — 1100m. Then we observe that G'-profile becomes less salty
than Wprofile below 1500m. This is also clearly seen in Figure 3.3b. There is no
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. j=214-226
Figure 3.4. Isotachs of normal velocity (color) and isohalines (solid lines) at the Sangihe section (*=156; the values of j are indicated on the x-axis). Velocities are in m/s. For reference see Figure 3.1. The bottom topography is shown. The western route section (Figures 3.2(b), 3.3(b)) passes through this section at j=223.
doubt that the deviation is caused by the blocking effect of the Sangihe Ridge,
although such a separation begins at a deeper level than for temperature profiles.
The determination of threshold isohalines and levels, at which the corresponding
salinity profiles diverge, is helpful in identifying the impact of topographic barriers
dividing the region into separate basins. Yet these characteristics do not allow us to
estimate a range of salinities advected into a basin. To explain the role of topographic
dividers in shaping distinct stratifications in different basins it is more informative
to consider the distribution of normal velocity and salinity on sections across the
flow. As seen in Figure 3.4, the deep flow at the passage through the Sangihe Ridge
is directed westward. The highest salinity found near the bottom of this passage is
S ~ 34.59. The next barrier is the Dewakang Sill across which J-profile (Flores Sea)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 81
Figure 3.5. The same as Figure 3.4 but for the Dewakang section (j = 138; the values of i are indicated on the x-axis). The vertical line shows the location of the section through the Dewakang sill (j=50) presented in Figure 8 . The western route section (Figures 3.2(b), 3.3(b)) passes through this section at i=57.
becomes slightly more salty than /-profile below ~ 600m. The salinity section in
Figure 3.3(b) as well as the temperature section (Figure 3.2(b)) do not distinctly
reveal the effect of the Dewakang Sill. The section across the Dewakang Sill (Figure
3.5) appears more informative. It shows that the southward flow carries water of
S < 34.54, so the blocking effect of the sill is quite obvious. Note that salinity in
the deep Flores Sea is higher than 34.58 which is due to the penetration of saltier
water from the Banda Sea, as was indicated by Gordon et al. (1994); Top et al.
(1997); and Gordon et al. (2003(a)). This was also suggested by the consideration
of temperature profiles but not in such a pronounced way. Indeed, the cross-section
through the Flores-Banda ridge (Figure 3.6) shows some deep westward flow in the
south (j=103-105) and north (j=112-135) carrying water of salinity S ~ 34.58 and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 82
i
Figure 3.6. The same as Figure 3.4 but for the Flores-Banda section (/= 78; values of j are indicated on the x-axis). The western route section (Figures 3.2(b), 3.3(b)) passes through this section at j= l 10.
higher. As in the case of #, the distribution of salinity in the deep layers of the Banda
Sea demonstrates that saltier waters, relative to the Flores Sea, can come from the
northern Banda Sea only. Finally, we provide Figure 3.7 to show that the entire
water column moves over the Dewakang Sill from the north to the south without any
return flows.
It is known that the calculated ’vertical’ velocity wpom used in the Princeton
Ocean model (POM) is very noisy. The results of calculation of real vertical velocity
w will probably be even noisier. Nevertheless, we will try to extract some information
from the calculated wpom (Figure 3.8). First, it follows from formula (D.3) in the
Appendix that wpom is the difference of the normal components of velocities of the
fluid particle at a point M of the a-surface and of the point M of that surface itself.
The factor N in (D.3) is very close to unity (see (D.2)). Second, near the bottom
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 83
100
2 0 0 -
300
•£■400
■8 500
600 0.1 m/s
700
800
135 136 137 138 139 140 141 142
Figure 3.7. v-velocities in m/s at the section through the Dewakang sill (*= 50; values of j are indicated on the x-axis). See Figure 3.5 for the location of this section. The bottom topography is shown. The numbers on the x-axis (j) should be reversed with the largest on the left and the least on the right.
cr-surfaces do not practically move. Taking into account that the direction of the
normal vector n a (see (D.2)) is close to the positive vertical direction, we conclude
that both velocities wpom and w almost coincide near the bottom. Now consider
Figure 3.8, restricting ourselves to the analysis of signs of the bottom vertical velocity
only. We see that there is indeed upwelling over the Sangihe Ridge and downwelling
over the southern part of the Dewakang Sill. Downwelling also occurs over the eastern
side of the Flores-Banda ridge. Thus, NPW water first overflows the Sangihe Ridge,
and then the Dewakang Sill (Figure 3.7) and the Flores/Banda ridge. Note that high
values of wpom are caused by very steep bottom topography. As we see from Figure
3.8 the slope of the bottom topography is on the order of 10~2.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 84
distance(km)
Figure 3.8. The distribution of the POM ’vertical’ velocity wpom on vertical section AGHIJKF (see Figure 3.1). Velocities are in m/s. The results are noisy but signs of wpom are provided at some points. Bottom topography along this section is shown. Distance from point A is shown on the x-axis. See Figure 3.3(b) for geographical names.
3.3.2. Northern Route
Consider now the formation of 6 and S in the Seram Sea (point D) beginning
with part ABCD of the northern route, see Figure 3.1. Compare first temperatures
of A- and B-profiles (Figure 3.9a). These profiles basically coincide down to a depth
of 1500m and slightly diverge in deeper layers. Figure 3.9b clearly shows that the
threshold isotherm caused by the steep rise of the bottom topography is 3°C. The
water of C-profile is warmer than water of 5-profile below 2000m. This can be
explained by the blocking effect of the topographic rise between these profiles. C-
and D-profiles are separated by the Lifamatola Sill. We see that C- and D-profiles
diverge below 1300m. Figure 3.9b shows that isotherms between 5 — 4 °C appear to
be inclined over the Lifamatola Sill, leading to the separation of profiles at a depth
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500
1000
1500
2000
£ 2500 Mindanao port (A) N. Morotai basin (B) 3000 • S. Maluku sea (C) + Seram sea (D) 3500 x N. Banda sea (E) O S. Banda sea (F) 4000
4500
5000, e
Figure 3.9. Vertical profiles of potential temperature along the northern route for August at points A, B, C, D, E, and F (a) (above) and the distribution of potential temperature on vertical section ABCDEF (b) (see Figure 3.1). Further explanations are similar to those in the caption to Figure 3.2.
less than that of the sill. Note that we did not see such an effect across the Dewakang
Sill. As seen in Figure 3.9b, the threshold isotherm across the Lifamatola Sill is
3.75°C.
The salinity profiles have a more complicated structure in the upper layer (Figure
3.10a). The same maximum that was discussed in the analysis of the western route
is seen in all profiles at depths between 100 — 200m, and in C- and D-profiles the
magnitude of this maximum is even greater, probably due to the penetration of
saltier water through the Serarn-Obi, Obi-Halmahera, and Lifamatola Passages. But,
as shown below, this effect is not properly described by the model. The minimum
is rapidly destroyed owing to the action of vertical diffusion. In Figure 3.10b, the
elongated patch of high salinity is interrupted over the topographic rise between A-
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A B C D E F
100
200
300
1000
425.
3000
Maluku Morotai basin 4000
Seram Pacific 5000 S. Banda N. Banda
o 200 400 600 800 1000 1200 1400 1800 2000 distance(km)
Figure 3.9(b).
and 5-profiles (due to upwelling, see Figure 3.11). The comparison of A- and B-
profiles, B- and C-profiles and C- and 5-profiles below 500m does not give any
substantial new information about the dynamical role of the topographic rise and
barriers between A- and 5-profiles, when compared with the analysis of temperature
profiles. Note that C- and 5-profiles diverge below 1300m. The effect of the Lifam
atola Sill works below 1300m as in the case of temperature (we observe here some
inclination of isohalines as well). The topographic rise between C- and 5-profiles
(Lifamatola Sill) blocks isohalines with S > 34.62. Figure 3.13 shows very clearly
that the highest salinity advected southward across the sill is 34.61. We will show
that this overflow shapes the salinity in the deep layers of the Seram Sea. The veloc
ity structure of the water overflowing across the Lifamatola Sill is seen in Figure 3.14.
It is interesting to indicate that seemingly not all water can overflow this sill. We
see a near bottom recirculation on the northern side of the Lifamatola Sill implying
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O O fl1' l"1 Xf 500
1000
1500
2000
f 2500
u 3000
3500 Mindanao port (A) N. Morotai Basin (B) * S. Maluku sea (C) 4000 + Seram sea (D) x N. Banda sea (E) 4500 O S. Banda sea (F)
5000
34.234.1 34.3 34.534.4 34.6 34.7 S
Figure 3.10. Vertical profiles of salinity along the northern route for August at points A, B, C, D, E, and F (a) (above) and the distribution of salinity on vertical section ABCDEF (b) (see Figure 3.1). Further explanations are similar to those in the caption to Figure 3.2.
that probably there is not a complete flushing of the basin located to the north of
the sill.
3.3.3. Eastern Route
Consider now the eastern route, LMND , see Figure 3.1. L- and M-profiles of
temperature are very similar down to a depth of 3000m (Figure 3.15a). A-profile
(Halmahera Sea) diverges from M-profile below 500m due to the northern Halmahera
Sill of depth 600m. Below the sill, the temperature of iV-profile is higher than that
of M-profile. D-profile (Seram Sea) diverges from A-profile at the depth of the
southern Halmahera Sill (600m). But the temperature of D-profile is colder than
that of A-profile (Halmahera Sea). It seems that the deep Halmahera basin has its
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A B C D E F
34.4 34.2
100
200
I.55 300
1000
34.6
2000 34.62 J4.51
3000
Maluku Morotai basin 4000 1.66
Seram S. Banda 5000 Pacific
o 200 600400 800 1000 1200 1400 1600 1800 2000 distanee(km)
Figure 3.10(b).
own dynamics, being isolated from the Pacific Ocean to the north and the Seram
Sea to the south by the northern and southern sills (of about the same depth). This
is why both the northern and southern sills impact the flow towards the Seram Sea
jointly as if the Halmahera Sea does not exist. These features are clearly seen in
Figure 3.15b.
The analysis of salinity profiles gives some additional information about dynam
ical processes in this area (Figure 3.16a). First, we see a strong salinity maximum
of 35.50 in L-profile located at ~ 100 — 150m with a weak minimum below 1000m.
This structure is characteristic of SPW. M- and D-profiles also have maxima at the
same depth but with markedly reduced magnitudes. As discussed previously, this is
probably explained by vertical diffusion. It is interesting to note two strong lowerings
of isohalines over the northern and southern Halmahera Sills (Figure 3.16b). We
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 200 400 600 800 1000 1200 1400 1600 1800 2000 distance(km)
Figure 3.11. The same as in Figure 3.8 but for the vertical section ABCDEF (see Figure 3.1).
observed a similar lowering over the Dewakang sill. Note also that we have not seen
anything similar in the temperature section (Figure 3.15b).
A weaker salinity maximum is seen in the Halmahera Sea compared to L- and
M-profiles. It is interesting to note that the magnitude of the salinity maximum in
the Seram Sea is slightly higher than that in the Halmahera Sea, but below 500m
the salinity in the Halmahera Sea becomes higher than the Seram Sea. This again
enforces the idea of an isolated Halmahera basin. Below 1500m, the Seram Sea
water (D-profile) is fresher than water to the north of the northern sill (M-profile).
A similar anomaly was observed in the analysis of temperature profiles.
Next we analyze the structure of currents and salt transports through the Halma-
hera Sea in more detail. Consider Figure 3.17. There are three main branches of the
flow: the near bottom southward current advects salinity 34.66; the near surface
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 90
135 140 145 150 155 160 i
Figure 3.12. The same as in Figure 3.4 but for the North Maluku section (j=187; values of i are indicated on the x-axis). The northern route section (Figures 3.9(b), 3.10(b)) passes through this section at i—151.
southward current (in the western part of the section) advects salinities 34.66 and
less; and the northward current (in the eastern part of the section) advects higher
salinities (up to 34.90). Figure 3.18 shows a section across the Halmahera Sea. To
the west and to the east of the section, the continuation of southward and northward
currents identified in Figure 3.17 can be observed. In the central deep part of the
sea we see anticyclonic circulation that supports the idea of the isolated Halmahera
Sea. Consider now the section across the southern Halmahera Sill (Figure 3.19). We
see the previously identified southward and northward currents in the western and
eastern parts of this section. It appears that the overflow from the Halmahera Sea
has a rather complicated character. According to Figure 3.20, water sinks on the
southern side of both sills but downwelling and upwelling are intermittent on the
northern side of the southern sill. Finally the Obi-Seram section (Figure 3.21)
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distance(km)
Figure 3.13. The same as in Figure 3.4 but for the Lifamatola section (j = 150; values of i are indicated on the x-axis). The vertical line gives the location of the section through the Lifamatola passage (i=139) in Figure 3.14. The northern route section (Figures 3.9(b), 3.10(b)) passes through this section at i=137.
shows a more organized velocity pattern.
In the upper 500m water of rather high salinity (up to 34.72) is advected into
the Seram Sea. A major part of this flow enters the Banda Sea through the Buru-
Mangole passage (see Figure 3.22) with a maximum salinity of 34.75 while most
of the remainder enters the Maluku Sea as a weaker northward current across the
Lifamatola Passage also with salinity maximum 34.75 (see Figure 3.13). The analysis
of isohalines and isotachs of normal velocity at the New Guinea-Seram, Buru-Seram,
and Flalmahera-Obi sections (not shown) also suggests the existence of a secondary
circulation pattern. A slow northward motion through the New Guinea-Seram pas
sage carries water of salinity 34.50 - 34.70. This current continues northward before
making a loop and joining the main flow near the deep Halmahera basin and the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 92
500
1000
£ 1500
2000
0.1 m/s 2500
146 148 150 152 154 156
Figure 3.14. v-velocities in m /s at the section through the Lifamatola passage (i= 139: values of j are indicated on the x-axis). See Figure 3.13 for section location. The bottom topography is shown. The numbers on the ./-axis (j) should be reversed with the largest on the left and the least on the right.
south Halmahera Sill and returning towards the Seram Sea. One branch of this
current leaves the area between Obi and Halmahera islands (carrying salinity 34.60 —
34.65) while the remainder enters the Seram Sea. We mention also a weak southward
current between Buru and Seram Islands having salinity 34.50 — 34.65. We did not
find any noticeable transport of SPW entering the Maluku Sea from the north of
Halmahera Island.
The appearance of high salinity water in the Obi-Seram section, in the Seram Sea
and to the north of the Lifamatola Passage is observed and usually interpreted as a
signature of SPW (see, e.g., Aken et al. 1988; Gordon and Fine 1996). Unfortunately,
the results of our model appear to be inconclusive on this point. This is connected
with some specifics of our model: the nudging of S to the climatology used in our
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 93
------500
1000
2000 NGCC port (L) N.E. of Halmahera Sea (M) • Halmahera Sea (N) 2500 * Seram (D)______
3000
3500, 9
Figure 3.15. Vertical profiles of potential temperature along the eastern route for August at points L, M, N, and D (a) (above) and distribution of potential temperature on vertical section LMND (b) (see Figure 3.1). Further explanations are similar to those in the caption to Figure 3.2.
simulations. This climatology was based on the representation of realistic climatology
by a certain number of Fourier harmonics (see Chapter 2) that resulted in some
increase of climatological values of S in the upper 0 — 500m in this area. This is
the reason why we observed high simulated salinities in the Obi-Seram section, in
the Seram Sea and to the north of the Lifamatola Passage (see, e.g., profile C in the
South Maluku Sea, Fig 3.11a), although high values of salinity are not seen on the
section through the southern Halmahera Sill. Nevertheless, based on the direction of currents in the upper layers of the Obi-Seram section and Lifamatola Passage it
is plausible that some SPW can penetrate to the Seram and Maluku Seas.
But the correspondence between the two climatologies below 500m is satisfactory.
Below 500m water with S = 34.61 enters the Seram Sea largely through the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 94
100
200
300
■o
2000 4-25 —_ ------F-'-y3.25 S. Halmahera
3000 i £ - Halmahera
4000, 200 400 600 800 1000 1200 distance(km)
Figure 3.15(b).
Lifamatola Passage (see Figure 3.13). In the Obi-Seram section (Figure 3.21) a
current, with salinity 34.61, is directed eastward (out of the Seram Sea). Furthermore,
this water flows northward through the south Halmahera Sill and eastward through
the New Guinea-Seram passage. There is a weak northward current between Buru
and Seram Islands with salinity 34.61, which essentially plays no role in the salinity
budget of the Seram Sea. The net flow between Mangole and Buru Islands is out of
the Seram Sea and into the Banda Sea. The flow of bottom water from the Seram
Sea into the Banda Sea is located to the south of the Mangole-Buru section and has
a salinity of 34.61. There is, however, a weak gyre in the northern part of the section
which clearly is of no importance to the salinity budget of the Seram Sea. Thus, we
conclude that the salinity in the deep part of the Seram Sea is formed by the flow of
NPW across the Lifamatola Passage. This conclusion agrees with the presently
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 95
x« 500
1000
NGCC port (L) N.E. of Halmahera Sea (M) 2000 • Halmahera Sea (N) * Seram (D)______
2500-
3000
3500 34.2 34.4 34.6 34.8 35.2 35.4 S
Figure 3.16. Vertical profiles of salinity along the eastern route for August at points L, M, N, and D (a) (above) and the distribution of salinity on vertical section LMND (b) (see Figure 3.1). Further explanations are similar to those in the caption to Figure 3.2.
accepted view (see, Aken et al. 1988, 1991; Gordon et al. 2003(a)). It is the advection
of water of salinity 34.61 through the passage between Mangole and Buru that forms
the salinity of the deep Banda Sea (see also Figure 3.3b).
We now complete the discussion of the northern route. Below 2700m A-profile
(north Banda Sea) is colder than D-profile water (Seram Sea). We cannot explain
this following the approach based on the consideration of overflows. In Figure 3.11
we saw that in the deep north Banda Sea there is upwelling that is probably balanced
by vertical diffusion. Due to this balance, the temperature of this profile decreases to
the bottom. It is possible that bottom temperature in the Banda Sea is established
by this effect. The salinities of E- and D-profiles are very similar.
Finally, we consider velocity and salinity distributions across the Lombok and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 96
L M N D
J4.4 c 100
200 14.66
300
1.64, f 1000
W.63
2000
S. Halmahera
3000
Pacific Seram
200 400 600 800 1000 1200 distance(km)
Figure 3.16(b).
Ombai Straits and Timor Passage (Figures 3.23 - 3.25). Basically, the normal velocity
at all exit passages is unidirectional (outflow) although very weak opposing currents
are seen in the Ombai Strait (between 500 and 1000m) and Timor Passage (near
the bottom, deeper than 400m). In all three cases salinity increases with depth and
no internal maxima or minima are observed. The maximum salinity in the Lombok
Strait is 34.55, in the Ombai Strait 34.59 and in the Timor Passage 34.61.
3.4. Seasonal Variation
There are no substantial structural changes of potential temperature, 0, and
salinity, S, distributions between seasons, though values of some parameters of these
distributions (e.g., magnitudes of maxima and minima) can change. The comparison
of corresponding profiles shows that seasonal variability is greatest near the surface.
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Figure 3.17. The same as in Figure 3.4 but for the northern Halmahera sill {j~ 166; values of i are indicated on the z-axis). The eastern route section (Figures 3.15(b), 3.16(b)) passes through this section at i=171.
Surface temperature changes seasonally by up to ~ 2°C in the northernmost and
southernmost regions but these changes are less pronounced away from these regions.
Seasonal changes in surface salinity are generally greatest near land, particularly in
the Makassar Strait region and around the Halmahera and Seram Seas. In the
thermocline, temperature changes on the order of ~ 0.5°C' occur while isotherms
can be displaced vertically by several tens of meters. Salinity extrema generally
exist throughout the year but their values can change somewhat and they can be
vertically displaced. However, the salinity maximum seen in the Seram and Maluku
Seas in August is much weaker in February and disappears in May and November.
Strong raising and lowering of isohalines, generally associated with topography, is
usually seen throughout the year. Below the thermocline, blocking of some isotherms
and isohalines is observed throughout the year. Though changes of temperature and
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154 156 158 160 162 164 166 168 170 i
Figure 3.18. The same as in Figure 3.4 but for the Halmahera sea section (j=157; values of i are indicated on the x-axis). The eastern route section (Figures 3.15(b), 3.16(b)) passes through this section at i=164.
salinity values are very small, isotherms and isohalines can be displaced by several
tens of meters. Velocity values across the major deep overflows vary little by season
but there is some change in the magnitude and extent of deep downwelling and
upwelling cells that may explain the displacement of deep isotherms and isohalines.
Thus, our conclusions regarding the formation of deep salinities in the Seram and
Banda Seas based on August data remain valid throughout the year. The following
comparisons refer to results in February and all changes are considered relative to
August unless otherwise stated.
3.4-1- Western Route
The structure of 6 profiles along the western route does not change seasonally.
Temperature change is greatest in the upper mixed layer ranging from an increase
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Figure 3.19. The same as in Figure 3.4 but for the southern Halmahera sill (j = 149; values of i are indicated on the x-axis). The eastern route section (Figures 3.15(b), 3.16(b)) passes through this section at i=156.
of ~ 1 °C at the MC port (A-profile) to a decrease of ~ 2°C in the south Banda Sea
(F-profile). In the thermocline, there is very little seasonal variability at the MC
port and in the Banda Sea. Temperatures decrease by ~ 0.1°C in the east Sulawesi
Sea (G-profile) and by ~ 0.5°C in the Flores Sea (J-profile) and increase by ~ 0.1°G
in the west Banda Sea (F-profile). A comparison of temperature sections shows
that isotherms within the thermocline are raised by ~ 10 — 20m from the MC port
(A-profile) to the Sangihe Ridge. Between the Sangihe Ridge and the Dewakang Sill
the isotherms are lowered by ~ 10 — 20m in the upper thermocline while they are
raised by ~ 10 — 20 m in the lower thermocline. In the Flores Sea and west Banda
Sea isotherms within the main thermocline are lowered by ~ 20m. In deep water,
some notable vertical seasonal displacement of isotherms occurs. In the Sulawesi Sea
isotherms are raised by ~ 50m down to a depth of ~ 1750m. Below this depth
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distance(km)
Figure 3.20. The same as in Figure 3.8 but for the vertical section LMND (see Figure 3.1). Distances from point L are shown on the ;/:-axis.
isotherms are displaced upwards by ~ 100m except in the region of the Sangihe
Ridge, and the conjunction of the Sulawesi Sea with the Makassar Strait, where
displacement is markedly reduced.
For salinity profiles, there is some seasonal change in surface and extrema values
but essentially no change in the deeper structure. In the mixed layer, the maximum
seasonal salinity change occurs in the south Makassar Strait where it increases by
~ 1.5 while the Flores Sea freshens by ~ 0.7. Elsewhere, seasonal changes are not
as pronounced. Surface values increase at the MC port (A-profile) and the south
Banda Sea (F-profile), while they decrease in the east Sulawesi Sea (G-profile) and
the north Makassar Strait (//"-profile). Seasonal changes seen near the surface and in
the mixed layer in the south Makassar region are the direct result of monsoon wind
forcing, see, e.g., Gordon et al. (2003(b)). In February, northwest monsoon winds
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i
Figure 3.21. The same as in Figure 3.4 but for the section between Obi and Seram islands (£=148; values of j are indicated on the x-axis). The eastern route section (Figures 3.15(b), 3.16(b)) passes through this section at j=144.
move near surface low salinity water into the south Makassar region. In August,
the southeast monsoon winds force freshwater out of the south Makassar Strait.
The salinity maximum at A-profile is located at 100m (125m in August) while the
minimum is located at 400m (300m in August). The salinity minimum of G'-profile
is also situated at 400m (300m in August). In the south Makassar Strait (I-profile),
a very weak Smax at 200m replaces the strong maximum seen at 100m in August
while a fresher weak minimum at 300m replaces the weak minimum seen at 500m
in August. In the Flores Sea (J-profile), the Smax is weaker and is located at 150m
(100m in August). In the west Banda Sea (.A-profile) the Smax is seen at 150m
(200m in August). Comparing seasonal sections of salinity (figure not shown), we
see a deeper Smax between the Sangihe Ridge and north Makassar. There is no Smax
at the entrance to the Makassar Strait because of the strong upwelling cell there.
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i
Figure 3.22. The same as in Figure 3.4 but for the section between Mangole and Buru (i— 128; values of j are indicated on the x-axis). The eastern route section (Figures 3.15(b), 3.16(b)) passes through this section at j=144.
In August, a slight deepening of the Smax is seen in the Sulawesi Sea, while it is
raised strikingly at the Labani Channel. Analysis of vertical velocity along the
western route reveals large seasonal variability in upwelling and downwelling and
it is hypothesized that this is responsible for salinity changes between the Sangihe
Ridge and the Dewakang Sill. An upwelling cell that extends through the water
column at the conjunction of the south-west Sulawesi Sea and northern Makassar
Strait is much stronger in August; a weak upwelling cell at the location of the Smax
interruption in the north Makassar Strait is not present in August; the upwelling cell
in the Labani Channel is much stronger in August. The patch of low salinity water
seen above the Dewakang Sill in August is more extensive in February due, perhaps,
to freshening of the sea surface but also due to the weakness of the upwelling cell.
In August, the winds are directed towards the north along the Makassar Strait and
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Figure 3.23. The same as in Figure 3.4 but for the section across the Lombok Strait (j = 110; values of i are indicated on the x-axis).
can generate the upwelling cells at the Dewakang Sill and Labani Channel. In deep
water, isohalines are generally lowered in the eastern Sulawesi Sea, raised in the
western Sulawesi Sea and close to the Makassar Strait but there is little or no dis
placement close to the Sangihe Ridge.
3.4-2. Northern Route
We now discuss 6 profiles and sections along the northern route. Seasonal tem
perature change is greatest in the upper mixed layer with Banda Sea water warmer
by 2°C, Seram and Maluku Seas water warmer by ~ 1.0 — 1.5°C, and MC port
and north Morotai water cooler by ~ 1.0° C. In the thermocline, seasonal variability
is greatest in the north Banda Sea where water is warmer by more than 0.6°C and
north Morotai basin where water is cooler by ~ 0.4°C. Changes are smaller in the
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i
Figure 3.24. The same as in Figure 3.4 but for the section across the Ombai Strait (i= 90; values of j are indicated on the x-axis).
Seram and south Maluku Seas. Isotherms are raised in the thermocline from the MC
port to the Morotai basin, while they are lowered in the Maluku and Banda Seas. In
deep water, isotherms are displaced upwards adjacent to the topography separating
the Pacific Ocean from the Morotai basin in response to the strengthening of the
upwelling cell there. In the Seram Sea and northern Banda Sea isotherms between
the depth of the Lifamatola Passage and ~ 3000m are displaced downward. This
result is consistent with a large downwelling cell that is both stronger and more
extensive in February. In the south Banda Sea isotherms are displaced upward consistent with stronger upwelling at that time.
Next, we compare salinity profiles and sections along the northern route. A
notable seasonal change in surface salinity is observed. Surface salinity decreases by
~ 0.3 in the Seram Sea and by ~ 0.25 in the south Maluku Sea and north Banda
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I
Figure 3.25. The same as in Figure 3.4 but for the section across the Timor Passage (i= 90; values of j are indicated on the x-axis).
Sea while it increases by ~ 0.1 in the north Morotai basin. The salinity maximum of
the Seram Sea and the south Maluku Sea is much weaker and there is no maximum in
the north Banda Sea. The salinity section shows that there is no Smax in the Banda
Sea, a very weak deeper Smax in the Seram Sea, and a weaker, broader and deeper
Smax in the Maluku Sea that almost disappears between the north Maluku Sea and
Morotai basin. In the northern Banda and south Maluku Seas isohalines at and
below the depth of the Lifamatola Passage (to a depth of ~ 3000m) are displaced
downward, similar to isotherms. In the deep Banda Sea isohalines are displaced
upward, similar to the upward displacement of isotherms there.
3-4-3. Eastern Route
Comparing temperature profiles and sections along the eastern route shows the
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upper mixed layer is cooler by ~ 0.4°C at the NGCC port (L-profile) and by ~ 0.3°(7
to the northeast of the Halmahera Sea (M-profile) while it is warmer by ~ 0.2°C in
the Halmahera Sea (/V-profile). In the thermocline, seasonal temperature change is
~ 0.5°C; cooler in the Halmahera Sea and warmer to the north of the Halmahera Sea.
The temperature section shows that in the thermocline, isotherms generally slope
downwards from the Halmahera Sea to the Seram Sea, whereas they slope upwards
in August. Therefore, in February, isotherms are shallower in the Halmahera Sea and
are deeper on the Pacific side of the north Halmahera Sill. A notable temperature
change at depth is the cooling of the deep Halmahera basin by ~ 0.2°C.
In the upper mixed layer, the NGCC port (L-profile) is saltier by ~ 0.3, M-profile
(northeast of the Halmahera Sea) is saltier by ~ 0.5 while Ar-profile (Halmahera Sea)
is fresher by ~ 0.1. The salinity maximum in the Halmahera Sea is located at 150m
(100m in August). Between the south Halmahera Sill and the Seram Sea the Smax is
deeper and much weaker. The area of relatively low salinity at the north Halmahera
Sill is much weaker. In the deeper Halmahera Sea salinity is reduced to 34.63 — 34.64
compared with 34.65 in August.
3-4-4■ Exiting Passages
In the Ombai Strait, surface temperature is greater than 29°C in February but
is less than 27 °C in August. Isotherms are displaced upwards by ~ 25 m above
~ 200m and downwards by approximatley the same amount between ~ 200 — 500m.
Below 500m the isotherms have the same location in February and August but are
displaced downwards in May and upwards in November by ~ 50 - 100m. Surface
salinity varies very little with a maximum of S ~ 34.4 in February and a minimum
of S ~ 34.0 in August. No salinity extrema are seen in the Ombai Strait. Below
the thermocline the 34.58 and 34.59 isohalines are displaced downwards in February
relative to all other seasons.
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In the Timor Passage, surface temperature is greater than 28 °C in February but
is less than 27 °C in August. The thermocline is displaced downwards by ~ 25 m. In
the lower thermocline (below ~ 500m) the isotherms have a greater downward slope
toward the south. Surface salinity exceeds 34.8 in February, has a value of ~ 34.0 in
May and ~ 34.5 in August and November. No salinity extrema are seen in August
or in February. However, there is a weak localized Smax ~ 34.64 at ~ 125m in May
and Smax ~ 34.65 at ~ 100m in November. Below ~ 1000m water of salinity 34.61
is observed but a patch of slightly saltier (S ~ 34.62) water is seen in May.
In the Lombok Strait, surface temperature is greater than 28°(7 but is less than
27°C in August. Below the mixed layer, the isotherms are displaced downward by
~ 20m. The surface salinity is S ~ 34.0 in February and August, slightly less in May
and slightly more in November. No salinity extrema are seen for the whole year.
3.5. Comparison with Observations
In general, the developed model correctly reproduces the basic features of ob
served potential temperature and salinity distributions. We consider now in detail
results of the comparison of simulated and observed data.
3.5.1. Heat and Salt Transports
Stammer et al. (2003) using the Massachusets Institute of Technology (MIT)
General Circulation Model (GCM) fully constrained by the World Ocean Circula
tion Experiment (WOCE) data, found mean heat transport of about 1.1 PW from
the Pacific Ocean into the Indian Ocean. Schneider & Barnett (1997) using a cou
pled ocean-atmosphere model found that heat transport towards the Indian ocean is
maximal in July (1APW) and minimal in February (0.1 PW). Ganachaud & Wunsch
(2000) applying an inverse model to hydrographic and current meter data and using
climatological wind fields and some biogeochemical balances calculated a heat flux
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of 1APW entering the Indian Ocean. Schiller et, al. (1998) using an Ocean Global
Circulation Model (OGCM) with enhanced tropical resolution found a mean heat
transport of 1.15PIE across the section between western Australia and Java towards
the Indian Ocean with a maximum of 1.86PIE in August and a minimum of 0.58PIE
in March. Hirst & Godfrey (1993) applying the Bryan-Cox Ocean General Circula
tion Model found a heat transport of 0.62PIE out of the Pacific Ocean toward the
Indian Ocean. Lee et al. (2002) calculated a heat flux of approximately 0,6 PIT to
the Indian Ocean. Godfrey (1996) in his review paper suggested that a Throughflow
of 10ST would transport about 0.5PIE of heat into the Indian Ocean. Ffield et al.
(2000) using ARLINDO (ARus Lintas INDOnen) measurements of temperature and
transport from instruments deployed in the Labani Channel estimated a mean heat
flux of 0.50PIF through the Makassar Strait in 1997. They also provided estimates
of 0.63PIE during the La Nina months of December 1996 through February 1997
and of 0.39PIF during the El Nino months of December 1997 to February 1998.
Vranes et al. (2002) calculated a heat transport through the Makassar Strait for the
period December 1996 - July 1998 using four different observed temperature profiles.
Their mean values are 0.72PIT, 0.58PIT, 0.34PIF, and 0.53PJF respectively. Gordon
& McClean (1999) analyzed the regional circulation of the Indonesian Seas provided
by the Los Alamos National Laboratory (LANL) Parallel Ocean Program (POP)
forced by ECMWF (the European Center for Medium Weather Forecasting) wind
stresses for the period 1985 through 1995. They found a heat transport towards the
Indian Ocean of 0.79PIE in August 1993 and 0.06PILr in February 1994 and a salt
transport of 2.51 x 10 skgs~1 in August 1993 and 3.0 x 10 7kgs~'L in February 1994.
Piola and Gordon (1984) applying simple box models suggested a total salt flux from
the Pacific Ocean to Indian Ocean of 4.71 x lO ^ps"1.
3.5.2. Western Route
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We begin with the western route. Essentially, there is satisfactory agreement
between the simulated and observed (Gordon et al. (2003a)) estimates of the effective
depths of topographic features (the effective depth is the depth of the threshold isoline
upstream of the obstacle). There is a good fit of observed to simulated features such
as the location of the salinity maximum of S ~ 34.75 and minimum of S ~ 34.48
at ~ 125m and ~ 300m respectively and the lower boundary of the thermocline
at ~ 250m in the north Makassar Strait (//-profile); the location of the salinity
maximum of S ~ 34.64 at ~ 100m and minimum of S ~ 34.51 at ~ 300m, and the
lower boundary of the thermocline at ~ 300m in the Flores Sea (see Gordon 2005;
Figure 4). However, simulated deep temperatures of 9 ~ 4.2°C in the East Sulawesi
Sea (G'-profile) and 9 ~ 2.0°C' in the MG port region (/I-pro file) are slightly higher
than observed; in contrast, the corresponding simulated salinities of S ~ 34.58 and
5 ~ 34.65 are slightly lower than observed (see Gordon et al. 2003(a); Figure 2).
In shallower waters in the Makassar Strait and the Flores Sea, the model results
are very similar to the data of Ilahude & Gordon (1996; Figure 3). For salinity, the
simulated results agree with data at depth provided by Gordon et al. (2003a; Figure
5(b)), where the deep Makassar Strait and southwest Sulawesi Sea are fresher than
the Flores Sea which in turn is fresher than the western Banda Sea. In shallower
waters, the position of isohalines characterizing the salinity maximum and minimum
along the Makassar Strait are satisfactorily reproduced by the model (Gordon et
al.2003a; Figure 5b; Ilahude & Gordon 1996; Figure 3). The same is true for the low
salinity cell over the Dewakang Sill seen in Ilahude & Gordon (1996; Figure 3). The
weaker maximum seen in February in Gordon et al. (2003a; Figure 5b) and Ilahude
6 Gordon (1996; Figure 3) is also reproduced by the model. The relatively weak
salinity maximum and minimum in the Flores Sea observed by Gordon et al. (1994;
Figure 4) and the absence of salinity extrema in the western Banda Sea (Gordon et
al. 2003a, Figure 5b; Gordon et al. 1994, Figure 4; Ilahude & Gordon 1996, Figure
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4) are adequately displayed by the model. We did not find appropriate observations
to check the validity of the lack of a salinity maximum near the Karakelong and
Sangihe Ridges.
3.5.3. Northern Route
Along the northern route, the model profile in the south Maluku Sea (G'-profile)
gives salinity maximum of S ~ 34.77 at ~ 150m, in good agreement with that seen
in Gordon (2005; Figure 4). The same is true for the model salinity minimum of
S ~ 34.56 at ~ 800m and model salinity of S ~ 34.62 below ~ 1600m (compare
with Gordon et al. 2003a; Figure 2). In the Seram Sea (D-profile), the model
gives a salinity maximum in August of S ~ 34.76 at ~ 150m, in contradiction to
observations (Gordon 2005; Figure 4). But below this maximum the model salinity
increases monotonically with depth to a deep value of S ~ 34.61, in very good
agreement with Gordon et al. (2003a; Figure 2). The salinity maximum seen at
D-profile is much reduced in February and is not apparent in November and May,
in close agreement with observations. In the south Banda Sea (F-profile), the model
shows no salinity extrema in the upper ocean agreeing with observations of Gordon
(2005; Figure 4). The deeper salinity of S ~ 34.61 also agrees with observations
of Gordon et al. (2003a; Figure 3). Comparing temperatures at depth between
the south Maluku Sea (C-profile) and the Seram Sea (D-profile) gives a simulated
effective Lifamatola Sill depth of ~ 1500m., in good agreement with the effective
depth of ~ 1650m found by observations (Gordon et al. 2003a).
Comparing 9- and S- sections along the northern route, we see that the temper
ature structure in the upper ocean agrees with that of Gordon et al. (2003a; Figure
6a). At depth, the model shows that isotherms slope down over the Lifamatola Pas
sage into the Seram Sea, and even into the Banda Sea, and that the deep waters of
the south Maluku Sea are much cooler than deep waters of the Seram and Banda
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Seas, in accord with Gordon et al. (2003a; Figure 6a) and Aken et al. (1988; Figure
9). For salinity at depth, the model isohalines slope downward across the Lifama
tola Passage and that is why the deep south Maluku Sea appears saltier than the
Seram and Banda Seas, again in agreement with Gordon et al. (2003a; Figure 6b).
The Banda Sea has almost constant salinity at depth also in accord with Gordon et
al. (2003a; Figure 6b) and Aken et al. (1988; Figure 10). In the upper ocean, the
model shows no salinity extrema in the south Banda Sea, corresponding well with
the data of Gordon et al. (2003a; Figure 6b). In the south Maluku Sea, both model
extrema were observed by Gordon et al. (2003a; Figure 6b), although the model
salinity maximum has a higher value especially in August. In the north Maluku
Sea the model salinity extrema have also been observed by Ilahude & Gordon (1996;
Figure 5), although the model salinity maximum is a little higher than the observed
value. Again, we can say nothing certain about the validity of the strong raising
of isohalines between A- B-profiles (Figure 3.10b) because of a lack of appropriate
observations.
3.5.4- Eastern Route
Along the eastern route, we find an effective sill depth across the northern
Halmahera Sill of ~ 500m, somewhat less than ~ 600m observed by Gordon et
al. (2003a; Figure 2). As a result, the model temperature at depth within the
Halmahera Sea is warmer than observed (see also Cresswell & Luick 2001), while the
simulated temperature difference in M- and A-profiles match the observed difference
below the effective depth of the sill. For the deep Halmahera Sea, the model gives
salinity of S ~ 34.65, more than the observations of Gordon et al. (2003a, Figure 2)
who found S ~ 34.60 (see also Riel 1943). The fact that the model effective depth of
the North Halmahera Sill is less than observed appears to be the main reason for the
relatively warmer and saltier deep water seen in the model in the deep Halmahera
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basin. We did not find appropriate observations to check the validity of massive
isohaline lowering over the northern and southern Halmahera Sills (Figure 3.16b).
For completeness sake we mention here the appearance of high salinity water in
the Obi-Seram section, in the Seram Sea and to the north of the Lifamatola Passage
which is an artifact of the model (see discussion in section 3.3).
3.5.5. Exiting Passages
A very limited number of observations of 9 and S have been made at the exit
passages of the Indonesian seas (Timor Passage, Ombai and Lombok Straits). Fieux
et al.(1996) made CTD and CFC measurements in the Ombai Strait and Timor Pas
sage during the north-west monsoon in 1992 as part of the Java Australia Dynamics
Experiment (JADE) program. Molcard et al.(1996) measured the throughflow from
2 current meter moorings placed in the Timor Passage from March 1992 - April 1993.
They also made a series of CTD casts in the Timor passage for two days in March
1992. Cresswell et al. (1993) investigated the circulation of the Timor Sea using ship
board CTD, ADCP and dropsonde measurements from October 1987 to Alarch 1988
with complementary data from satellite tracked drifters, continental shelf moorings
and a Nansen bottle survey during 1976. Molcard et al. (2001) deployed a current
meter mooring in the Ombai Strait in November 1995 for one year. They made
CTD casts at four stations across the Strait in December 1995. Murray et al. (1990)
report on observations made in the Lombok Strait and surrounding regions from
January 1985 until March 1986. They deployed 7 current meter moorings, 5 in the
Lombok Strait and 2 in the west Flores Sea, and made 234 CTD casts in the Lombok
Strait, the adjacent area of the Indian ocean and the west Flores Sea. Sprintall et al.
(2003) studied 5 major exit passages of the Lesser Sunda Islands between December
1995 and May 1999 by using shallow pressure gauge arrays mounted with salinity
and temperature sensors. They also made underway measurements with ADCP and
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CTD packages during deployment of the sensor arrays and three subsequent surveys.
The model results in the Timor Passage agree well with the March 1992 CTD
profiles made by Molcard et al.(1996; Figure 12). They found a surface salinity of
S ~ 34.8 in the upper ~ 50m. As one moves down the water column, the salinity
first decreases (S < 34.5) but then increases monotonically to S ~ 34.61 at ~ 1400m.
The temperature profiles show a surface value of 9 ~ 30oC with the 10°C isotherm
located at a depth of ~ 400m. A closer comparison between CTD profiles of Fieux
et al. (1996; Figure 6) and model results between 1000 — 1500m shows that the
salinities match well but the model potential temperature is warmer than observed
by up to 1 °C. Comparing the model results with Cresswell et al. (1993) we find that
the temperature and salinity sections for the upper 300m in October 1987 (Cresswell
et al. 1993; Figure 6a) practically coincide with the model results. Most notably,
the locations and values of the localized salinity extrema are almost identical. For
March 1988, the observed surface temperature is greater than 29°C (Cresswell et al.
(1993; Figure 6b)) is slightly warmer than the model result (8 > 28 °C) but the basic
features of temperature distribution are similar. They also found a surface salinity
of S > 34.8 and salinity minimum of S < 34.45 at depth ~ 100m to the north of
the passage, a feature clearly seen in the model at this time. Sections of 6 and S
down to 200m for August 1976 (Cresswell et al. (1993 Figure 9)) are also in good
agreement with the model results.
The CTD measurements in December in the Ombai Strait by Molcard et al.
(2001; Sect. 3.1, Figure 4) are in good agreement with the model output. In both
cases, the surface potential temperature is greater than 28°(7, the boundaries of
thermocline layers coincide and the 8 °C isotherm is located at 500m. In deeper waters
(below 1500m) the model is slightly warmer than the observed profiles. Salinities are
also in good agreement; surface salinities coincide, but the observations show very
weak extrema of Smax ~ 34.55 at ~ 200m and 5m,n ~ 34.51 at ~ 300m- whereas the
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model does not contain any extrema. The simulated salinity increases monotonically
from a surface value of S ~ 34.4 to match the observation below a depth of ~ 300m.
Murray et al. (1990; Figs 7 and 6) show that the salinity extrema of Makassar
water are destroyed upon passing through the Lombok Strait both in June and
September 1985. This feature is well reproduced by the model. Sprintall et al.
(2003) found that surface waters are warmer and fresher in the main 5 exit passages
of the Lesser Sunda Islands from March to May compared to other times of the year.
These results are in agreement with the model output.
3.6. Summary and Conclusions
The formation and distribution of temperature and salinity in the Indonesian
Seas region have been studied with a high-resolution model based on the Princeton
Ocean Model. One of the distinctive properties of the model is an adequate repro
duction of all major topographic features of the region by the model bottom relief.
It has been stressed in several publications that the formation of temperature and
salinity distributions in the Indonesian Seas, especially in deep water, is strongly
controlled by the bottom topography (see, e.g., Gordon & McClean 1999; Gordon
et al. 2003a and references therein). The model displays satisfactorily the main fea
tures of potential temperature and salinity distributions in the region. It was shown
that the contribution of the deep parts (below 500m) of the Ombai Strait and Timor
Passage to the total transports of internal energy (heat) and salt mass appears rather
substantial.
To analyze the influence of bottom topography in detail we have considered the
three major routes of flow of Pacific water through the Indonesian Seas, which have
been identified as western, northern, and eastern routes. The western and northern
routes begin at the Mindanao Current (MC) port, through which North Pacific
Water (NPW) enters the region. A branch of NPW, taking the western route, flows
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over the Karakelong and Sangihe Ridges, through the Sulawesi Sea, and along the
Makassar Strait, at the southern part of which it diverges. Part of this flow exits the
region through the Lombok Strait, but the main portion passes through the Flores
Sea to the Banda Sea. Another branch of NPW takes the northern route flowing
through the Morotai Basin, Maluku Sea, Lifamatola Passage, Seram Sea and finally
into the Banda Sea, where waters taking the western and northern routes mix. The
eastern route begins at the New Guinea Coastal Current (NGCC) port, through
which South Pacific Water (SPW) enters the region, flows through the Halmahera
Sea to the Seram Sea, where waters taking the eastern and northern routes mix.
Finally, Banda Sea water exits the Indonesian Seas region through the Ombai Strait
and Timor Passage.
Major topographic features break the region down into separate basins having
different temperature and salinity stratifications. These are the Sangihe Ridge, the
topographic rise between the Sulawesi Sea and Makassar Strait, the Dewakang Sill,
and the ridge between the Flores and Banda Seas along the western route; the topo
graphic rise between the Morotai Basin and Maluku Sea and the Lifamatola Sill
along the northern route; and the northern and southern Halmahera Sills along the
eastern route. To clearly demonstrate the changes in stratifications we analyzed a set
of temperature and salinity profiles and the corresponding sections along each route.
What are the reasons for these changes? The water spills over major topographic el
evations, as exemplified by the calculation of vertical velocity over the slopes of these
elevations. The dynamics of the overflow is such that topographic features impede
the advection of low temperature and high salinity from upstream of the topographic elevation to downstream. It seems likely that there is some relation between blocked
ranges of temperature and salinity and characteristics of upstream flow and bottom
elevations. But we did not attempt to seek such a relation and have restricted our
selves to a description of different types of overflows observed in the Indonesian Seas.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 116
For this purpose we have chosen salinity as a tracer and considered sections above
topographic features or, in other words, at the boundaries between basins. Isohalines
and isotachs of normal velocity at these sections have been presented and discussed.
Our assumption was that the highest salinity in the downstream basin, below the
depth of the topographic feature, should be advected from the neighboring basins.
In this way, we have disclosed, for example, the blocking effect of the Dewakang Sill.
This effect was hardly seen from a traditional consideration of the threshold isohaline
or isotherm. Following such an approach we were able to prove that the difference in
temperature and salinity stratifications is really caused by the impact of the bottom
topography.
The salinity stratification of NPW and SPW entering the Indonesian Seas region
are quite different. NPW is characterized by a salinity maximum of ~ 34.74 at
a depth of ~ 150m, a deep minimum of ~ 34.46 at a depth of ~ 350m, and a
deep value of ~ 34.66. SPW is characterized by a strong maximum of ~ 35.50 at
a depth of ~ 100 — 150m, a weak minimum of ~ 34.61 at a depth of ~ 1500m,
and a deep value of ~ 34.65. Thus, it was shown that the deep temperature and
salinity distributions in the basin between the Sangihe Ridge and the topographic
rise at Dewakang Sill are shaped by the spilling of NPW, taking the western route,
over the Sangihe Ridge. The distributions in the basin between the Dewakang Sill
and the Flores-Banda ridge are formed by spilling of western route NPW over the
sill and flow of northern route NPW around the ridge. The deep temperature and
salinity distributions in the Morotai Basin and Maluku Sea are controlled by the
topographic rise just to the Pacific Ocean side of the Morotai Basin. The Banda
Sea deep temperature and salinity distributions are basically shaped by NPW taking
the northern route through the Lifamatola Passage and Seram Sea. It appears that
deep SPW is blocked by the northern and southern Halmahera Sills, so the deep
temperature and salinity distributions in the Seram Sea is basically formed by NPW
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 117
passing over the Lifamatola Sill. The relatively shallow Dewakang and Halmahera
Sills (~ 600m) essentially block the advection of temperature and salinity of those
layers in upstream regions of these sills that are below ~ 600m.
The analysis of observations presents some evidence of SPW in the upper 500m
in the Seram Sea, Banda Sea and to the north of the Lifamatola Passage (Aken et al.
1988; Gordon & Fine 1996). This seems to be the only contribution of SPW to the
formation of stratification of upper Banda Sea water. Unfortunately, due to some
reasons, discussed in Section 3, the model results of salinity distribution appeared
inconclusive on this point. Nevertheless, based on the direction of currents in the
upper layers of the Obi-Seram and Mangole-Buru sections and Lifamatola Passage
we can conclude that the model results do not contradict observations.
Note that we demonstrated a gradual evolution of NPW in the upper 500m along
the western route. No intrusion of water having substantially different temperature
and salinity characteristics could be found. Because the main part of the Indonesian
Throughflow passes through the Makassar Strait, we arrive at the conclusion that
basically it is NPW that shapes the temperature and salinity structure of the upper
part of the Indonesian Throughflow, as was stated by Wyrtki (1961); Gordon (1986);
and Gordon & Fine (1996).
There are no substantial structural changes of potential temperature and salinity
distributions between seasons, though values of some parameters of temperature and
salinity distributions (e.g., magnitudes of maxima and minima) can change. Thus,
our conclusions regarding the formation of deep temperatures and salinities in the
Seram and Banda Seas based on August data remain valid throughout the year.
It is important to stress that the main structure of the observed distributions of
temperature and salinity, namely the monotonic decrease of temperature with depth
and the presence of a maximum and minimum in salinity profiles, is reproduced very
well throughout the entire model domain. In the upper ocean (0-500m) the model
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 118
and observed profiles are very similar. In the deeper ocean temperature deviations
can reach 1 °C while salinity deviations are essentially on the order of several 0.01,
although in some layers, e.g., the layer of salinity minimum at the NGCC port, they
can reach as much as 0.1. The total transports of internal energy (heat) and salt
mass through the Lombok and Ombai Straits, and Timor Passage in August and
February are in reasonable agreement with published observed and simulated data.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 119
CHAPTER 4
THE ANALYSIS OF DISTRIBUTIONS OF TURBULENCE
CHARACTERISTICS BASED ON THE MODEL RESULTS
4.1. Introduction
Features of mixing are analyzed based on results of the integration of a regional
model of the Indonesian Seas using the Mellor-Yamada 2.5 turbulence model. Tur
bulent mixing in the ITF system is well known to be important, see, for example,
the discussion of Gordon (2005). Two outstanding questions concerning the ITF and
the Indonesian Seas region are: i) how are T, S characteristics of thermocline NPW
and SPW transformed across the Indonesian seas and ii) how are deep overflows in
the Indonesian Seas sustained? The trivial answer to these questions is, in a word,
mixing. The objective of this chapter is to discuss systematically turbulence charac
teristics within the ITF system. To the best of our knowledge, such a discussion for
the ITF region has not previously been given in the literature.
Turbulence measurements in the Indonesian Seas are a very scarce commodity.
The first discussion of turbulence in the Indonesian Seas we are aware of was by
Stommel & Fedorov (1967) who discovered microstructure in temperature and salin
ity profiles from STD recorders near Timor and Mindanao. Alford et al. (1999)
measured microstructure in the Banda Sea near 100 m depth over a 2 week period.
Some investigators, such as Gordon et al. (2003), Van Aken et al. (1988), Berger
et al. (1988) and Bennekom (1988) have used simple models to estimate turbu
lent diffusivities in the Indonesian Seas. Thus, the need to model turbulence in the
Indonesian Seas is urgent.
The results show that turbulent kinetic energy (TKE) is large at topographic
features such as sills. TKE is small in the interior ocean except in areas of strong
currents in the upper thermocline. Values are large near the surface dire mostly to
the monsoon winds but this effect is seen only to a depth of ~ 50m. Surface currents
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 120
are also quite energetic. Diffusivities are large in regions of high turbulence kinetic
energy. Model results seem realistic when compared with available observations and
other models. In the thermocline, large diffusivities may be sufficient to alter the
signature of NPW and SPW in the Indonesian Seas, while, at deep sills, they may be
sufficient to maintain the temperature gradient across sills thereby sustaining deep
overflows.
The material presented is as follows. The Mellor-Yamada 2.5 model is briefly
described in section 2, turbulence model results are presented in section 3 and a
discussion and conclusions are presented in section 4.
4.2. The Turbulence Parameterization Scheme:
The Mellor-Yamada (M-Y) 2.5 Model
The M-Y turbulence model has demonstrated skill in solving a wide range of
ocean and atmosphere turbulence problems. The full model consists of i) the energy
redistribution hypothesis of Rotta (1951), ii) the Kolmogorov (1941) hypothesis of
local, small scale isotropy and iii) adding closure expressions for third order moment
turbulent velocity diffusion and turbulent pressure diffusion terms. The Rotta and
Kolmogorov hypotheses lead to the definition of four length scales assumed to be
proportional to a master length scale. The resultant five constants are thus unam
biguously related to measured laboratory turbulence for neutral flows. The M-Y 2.5
version is derived from a hierarchy of model versions by a process of formula sim
plification based on assumed small departures from isotropy and application of the
boundary layer (BL) approximation. The M-Y 2.5 is a very popular second-moment
turbulence closure scheme widely used in research and operational models (see, e.g.,
Mellor & Yamada (1974, 1982), Mellor (2001), Galperin et al. (1988), Mellor (2004)).
The model solves for twice the turbulence kinetic energy (TKE), q2, and the master
turbulent length scale, /, as follows:
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. dqHD 8Uq2lD dVq2lD duq2l d \Kq dq2l dt ^ dx dy ^ da da D da
2 Do3 ~ - —£-W$t(GH) + F,. (4.2) t> 11
The terms on the left side of (4.1) are the tendency and advective terms, and the
terms on the right hand side are the vertical turbulence diffusion, shear production,
buoyancy production, dissipation, and horizontal diffusion terms, respectively. D
is total depth of the water column; U and V are horizontal velocities; oj is the
POM vertical velocity; a is the vertical coordinate; Km is the calculated vertical
diffusivity for momentum (eddy viscosity); KH is the calculated vertical diffusivity
for temperature and salinity; Kq is the calculated vertical turbulence diffusivity and
is proportional to KH] g is gravity; p0 is the reference density; is the vertical
density gradient minus the adiabatic lapse rate; B\ is an empirical constant; and
I is the turbulence master length scale. In (4.2), W = 1 + F 2( ^ ) 2 where L^1 =
(■rj — z)~l + (H — z)~x is a wall proximity function and Ei,E2 and E?t are non-
dimensional constants.
1.0 if Gh > 0
% G h ) = 1.0 - 0.9M if G hc < G h < 0
0.1 if Gh < Ghc
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 122
where Gh = is a Richardson number Ghc — -6.0 is a constant and M —
(GH/Gh c Y ^2■ For weak turbulence, Gh < G hc , the dissipation is small. Note that
although equation (4.2) is the most empirical part of the model, it is used because it
has given reasonable results for multiple turbulent regions of the ocean. The solutions
to (4.1) and (4.2) above, lead to expressions of the turbulent diffusivities, KM and
I(a. where
Km — qISm, Kh = qlSn- (4.3)
The coefficients Sm and Sh are stability functions of Gh and are given by
6^! S h [1 — (3A2R2 + 18^4i^42) G h ] — -42 1 - 9(67 H, (4.4) b T
Sm [1 — §A\A2Gh\ — Sh [(1842 + G/r]
= Al [l-3Cl -6A lB1]^(GH). (4.5)
The five constants in (4.4) and (4.5) were evaluated from near-surface turbulence data
and decaying homogeneous turbulence data. They are (Mellor & Yamada (1982)):
(A\, Bi, A2, B2,C\) = (0.92,16.6,0.74,10.1,0.08). The stability functions Sm and
Sh limit to infinity as Gn approaches the value 0.0288, but this is not physically
possible since stratification would have been destroyed by the very large diffusivity
values associated with very large values of Sm and Sh - We note that a background
diffusivity of 2 .10- 5m2/,s is added to KM and Kh to represent internal waves and other mixing processes not modeled by the M-Y scheme. Solutions to (4.3), (4.4) and
(4.5) result in values of KM and KH that are used to solve the momentum and heat
and salinity equations.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 123
4.3 Results The results are taken from the 1st of August during the South Eastern Monsoon
(SEM) after 10 years of model integration.
In deep water, large values of q2, KM and K h are seen near topographic features
such as sills. Values are greatest adjacent to the sills but patterns are complicated
and perhaps a little noisy. Maximum values of q2 are ~ 10-1 — lm 2/ s 2. The values
are greatest at the sills and along sloping topography but significant values can
extend in both upstream and downstrean directions. At deep sills large values of q2
extend above sill depth on the downstream side. In the interior ocean, away from
and above sill depth, q2 is reduced to less than 10_ 8 m2/s2. Patterns of KM and
Kh are very similar to q2. Maximum values of K m and KH are ~ 10-2 — lm 2/s
at the sills and along topography, and can be as large as 10_3m2/s for significant
distances downstream and upstream of the sills. In the thermocline q2 and the
diffusivities are generally small but increase noticeably where currents are known
to be strong with maximum values of q2 ~ 5.10~3m 2/s 2, K m ~ 5.10~2m2/s and
Kh ~ 5.10“3m 2/s. In the upper mixed layer (UML) large q2 and diffusivities are
generally confined to the upper 50m with maximum values of q2 ~ 10~3m 2/,s2 and
Km and K h ~ 5.10~2m 2/s. Turbulence master length scale, I, is large and generally
constant in the interior (~ 100m) and is small in the boundary layers. Stability
parameters, Sm and Sh , are greatest in the boundary layers where they approach 1.
They are generally small in the interior.
The results are given in Figures 4.1 to 4.6. Figure 4.1 shows the locations of
sections and profiles presented and discussed. The sections focus on 4 areas over
complex topography. Results across the North Sangihe Ridge area are presented in
Figure 4.2 and include (a) a section ( 1-2 in Figure 4.1) of q2] (b) KM', (c) KH; (d)
profiles of I at points across the section; and (e) and (f) profiles of stability factors
Sm and Sh , respectively. Note that the scale on the colorbar in the sections is
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.1. Locations of sections presented in the results. Section 1 - 2 is taken across the Northern Sangihe Ridge system from the Sulawesi Sea in the west to the Pacific Ocean in the east; section 3 - 4 is taken across the Dewakang Sill from the Southern Makassar Strait in the north to the Flores Sea in the south; section 5 -6 is taken across the Lifamatola Sill from the South Maluku Sea in the north to the North Banda Sea in the south; and section 7 - 8 is taken across the Halmahera Sea from the Obi-Seram passage in the south to the Pacific ocean in the north. The dots show the locations of profiles of 1 ,S m and SH.
logarithmic and profiles are shown on er-surfaces. All results are from August of
the seasonally varying experiment after 10 years of the model run. Figures 4.3 -
Figures 4.5 give the results across the Dewakang Sill, Limatola Sill and Halmahera
Sea, respectively. Figure 4.6 shows the effect of the wind.
4-3.1. North Sangihe Ridge Section
Across the North Sangihe Ridge (Figure 4.2) values of q2 in the vicinity of bottom
topography are ~ 10~3m 2/s2, or greater. The entire Karakelong Basin below the
depth of the Sangihe ridge, to the west, and the Karakelong Ridge, to the east, shows
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 125
q2 greater than 2.10“4m 2/ s 2. Below 1000m values generally greater than 2.10™4m 2/ s 2
extend 150 to 200 km downstream of the North Sangihe Ridge and into the Sulawesi
Sea. Upstream of the Karakelong Ridge, in the Pacific Ocean, q2 generally becomes
small within 100/cm of the sloping topography. In the thermocline, above ~ 1000m,
values of 10“ 8 m2/s 2 or less are seen except around the Karakelong Basin and ridge
where values are greater and maximum values of ~ 2.10_3m 2/ s 2 are found in the
upper 300m. In the UML values of ~ 2.10“ 4m 2/,s2 are confined to the upper 50m.
The spatial pattern in K m and Kh is very similar to q2. Near topography values
of Km are ~ 10- 2m2/s and often increase to 10” 1 to 1 rri2/s at the slope. Km values
are ~'5.10_3m2/s in the Karakelong Basin. Below 1000m, values of KM ~ 5.10“ 4
are seen downstream of the North Sangihe Sill. Km decreases to the background
value of 2.10“5m2/s away from topography. K m is small in the thermocline except
near the Karakelong Basin and Ridge where values are greater and reach 10- 2m2/s
between 200 and 300m. Values of KH are generally an order of magnitude less than
Km except in the UML (upper 50m) where both are ~ 5.10_ 3ra2/s.
The master length scale, I, is generally large in the interior ~ 1.00m and small
in the BLs, though the interior value is reduced at Karakelong Ridge (black profile).
Profiles of Sm and Sh have similar characteristics with maximum values approaching
1 in the UML and bottom boundary layer (BBL) and negligible values in the interior.
4-3.2. Dewakang Sill Section
Across the Dewakang Sill (Figure 4.3) q2 values of 2.10~ 4m 2/s 2 or greater are
seen. These values extend to 200m above the sill and increase to at least 10~ 3m 2/ s 2
along the passage between the sill and the Flores Sea. Along sloping topography,
values are generally reduced to background diffusivities on the downstream side to
wards the Flores Sea, while patches of extremely high q2 ~ 10_2m 2/s 2 are seen on
the upstream side, South Makassar Strait. Large values are seen near the bottom
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 126
in the Flores Sea. Patches of high q2 extend through the water column at the De
wakang Sill. Values in the UML are ~ 2.10_ 4m 2/s2. Away from topography values
are reduced to ~ 10“8 m2/s 2.
The pattern in KM and KH is very similar to q2. Maximum values of KM are
~ 5.10~3 to 10“2m2/s downstream of the sill to the Flores Sea. Values are generally
small along the upstream and downstream slopes and rapidly decrease to background
diffusivities. Values of Kh are generally an order of magnitude less than K m but are
similar in the UML where typical values are 10 ~3 or greater.
Profiles of Sm and Sh show large values in the boundary layers only, while I is
everywhere large outside the boundary layers.
4-3.3. Lifamatola Passage Section
The section across the Lifamatola Passage (Figure 4.4) extends from the South
Maluku Sea (Batjan Basin) across the Seram Sea into the Northern Banda Sea. In
deep water, downslope and downstream of the Lifamatola sill q2 values ~ 2.10~ 4m2/ s2
are seen. These values extend across the Seram Sea and into the North Banda Sea
where values can reach 10_ 2m 2/ s 2 in patches. Values of ~ 2.10_ 4m 2/ s 2 mostly ex
tend throughout the lower 1000m of the water column between the sill and the North
Banda Sea. Upstream, in the Maluku Sea, very large values are seen in patches with
substantial areas having q2 ~ 2.10_4m 2/ s 2 or greater. Above ~ 1600m values are
small. In the UML q2 ~ 2.10_4m2/s 2.
The pattern in both K m and K h is similar to q2. Km values of 5.10~ 2m2/s
extend immediately downstream of the Lifamatola sill to the North Banda Sea.
Values greater than ~ 10“ 3m2/s are seen in the lower 1000m of the water column
in the Seram Sea, while values of ~ 5.10~ 4m2/s are seen to sill depth extending into
the North Banda Sea. Large values are also seen in the Maluku Sea, upstream of
the sill. Above ~ 1600m values reduce to the background value. Maximum Kh
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 127
values of 10 ~lm2/s or greater are seen downstream of the sill in the lower 500m of
the Seram Sea. In the UML values are as high as 5.10~ 2m 2/s. In the UML values
reach 5.10_ 2m2/s for both KH and KM.
Profiles of Sm and Sh show large values at the boundaries and small values in
the interior, while I is small in the boundaries and large outside.
4.3.4. Halmahera Sea Section
The Halmahera Sea section (Figure 4.5) extends from the Pacific Ocean to the
passage between Obi and Seram islands. Highest values of q2 ~ 5.10~3m 2/s 2 are seen
at the North Halmahera Sill in the upper 200?n and extend across the Halmahera
Sea to the southern sill where q2 is slightly less. Below sill depth, q2 ~ 2.10_4m 2/ s 2
throughout the Halmahera Basin. Downstream of the south sill along the slope in
the Seram Sea values are ~ 2.10“ 4m 2/s 2 and reach 5.10_ 3m 2/ s 2 in patches. Similar
patches are seen on the upstream slope in the Pacific Ocean. Away from topography
q2 is generally small. In the UML q2 ~ 2.10 4m2/s2.
Patterns in KM and KH are very similar to q2. KM is largest in the Halmahera
Basin where values ~ 5.10 ~~2 to 10- 1m2/s. In the upper 200m KM ~ 5.10- 2m2/s at
the Northern Halmahera Sill and ~ 10_ 3m2/s at the Southern Halmahera Sill. Km
is greater than 10~3m2/s in patches along the downstream slope. Kh is ~ 10-1 in
the Halmahera Basin, ~ 10 -3 in the upper 200m at the Northern Halmahera Sill and
~ 5.10-4 at the Southern Halmahera Sill. K m and Kh are ~ 5.10-2 in the UML.
Profiles of Sm and Sh show relatively large values outside the boundary layers,
particularly at the north sill (black profile) and I is also reduced in the interior,
particularly at the sills (black and blue).
4-3.5. Effect of the Wind on Turbulence Kinetic Energy (TKE)
Contour maps of q2 at different depths are given in Figure 4.6. The results show
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. distance(km)
5.10*2
10*2
1000 5.10*3
-S. 2000
3000 5.10*4 Karakelong Ridge 4000 North Sangihe Ridge Karakelong\ Basin 5000 5 .0 * 5 Sulawesi Pacific Ocean S ea 2 .0 * 5 200 250 300 350 400 distance(km)
Figure 4.2 Sections of (a) q2{m?/s2) (top) and (b) KM(m2/s) (bottom) across the North Sangihe Ridge. Locations of points 1 and 2 at the top of the figures are given in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 300m.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 50 100 150 200 250 300 350 400 distance(km)
— i=138 j=221 i=146 j=221 10 1=154 j=221
$ 15
20
25
30. 100 200 300 400 500 600 700 800 900 1000 I ( m )
Figure 4.2 (cont.) Section of (c) KH(m2/ s) (top) and profiles of (d) l(m) (bottom) across the North Sangihe Ridge. Locations of profiles are given by the dots in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 300m.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 130
10
1=138 j=221 [=146 j=221 j=15 4 j=221
20
25
30, 0.05 0.15 0.25 0.3 0.35 0.4
t i i ..... \ i ...... -r------r------1------1------
/ ...... _ ------..
-
“ ------i=138j=221 ~ i=146j=221 ------[= 154j=221
-
N ) I------1------_i—1------..... i 1______i i______i i______i i______j i ______i i______ti______0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Figure 4.2 (cont.) Profiles of (e) Sm (top) and (f) Sh (bottom) across the North Sangihe Ridge. Locations of profiles are given by the dots in Figure 4.1.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 100 200 300 distance(km)
IDe-2
S.IOe-3
IOe-3
£ 750 5.IOe-4
1000 Dewakang Sill 10e-4 1250 Flores S ea
1500 South M akassar Strait 2.108-5
150 200 250 300 350 distance(km)
Figure 4.3 Sections of (a) q2(m2/s 2) (top) and (b) KM{ni2/s) (bottom) across the Dewakang Sill. Locations of points 3 and 4 at the top of the figures are given in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 100m.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 50 100 150 200 250 300 350 distance(km)
— — i=50 j=149 )=50 )=139 i=52 j=134
10
« 15
20
25
30. 10 20 3040 50 60 70 80 90 100 I (m )
Figure 4.3 (cont.) Section of (c) KH(m2/s) (top) and profiles of (d) l(m) (bottom) across the Dewakang Sill. Locations of profiles are given by the dots in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 100m.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 133
10
= 5 0 j=149 S 15 =50 j=139 = 5 2 j=134
20
25
30, 0,05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 SM
1 1 1 1 ...... 1 1 1 1 1
— ------—
-
I ...... ------j=50 j=149 i=5Q j=139 ------i=52 j=134
-
v - ~ ------^
\ I______JI______...... - __ L1 ______...... 1.I______1I ______1I ______- /I______I t______It------...... i1 ------...... 1 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Figure 4.3 (cont.) Profiles of (e) Sm (top) and (f) Sh (bottom) across the De wakang Sill. Locations of profiles are given by the dots in Figure 4.1.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 50 100 150 200 250 300 350 distance(km)
5.t0e*2
2000
2500
3000 Lifamatola Strait 5.i0e*4
3500 Seram S e a 4000 Maluku S e a North Banda S ea 100 150 200 250 300 350 distance(km)
Figure 4.4 Sections of (a) q2(m2/s 2) (top) and (b) KM(,m2/s) (bottom) across the Lifamatola Sill. Locations of points 5 and 6 at the top of the figures are given in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 100m.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 50 100 150 200 250 300 350 distance(km)
=138 j=156 10 =138 j=150 =131 j~144
20
25
30, 50 100 150 2 00 250 300 I (m)
Figure 4.4 (cont.) Section of (c) KH(m2/s) (top) and profiles of (d) l(m) (bottom) across the Lifamatola Sill. Locations of profiles are given by the dots in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the change in scale at 100m.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 136
20
25
30, 0.05 0.15 0.25 0.3 0.35 0.4
10 =138 j=221 = 1 4 6 j=221 = 1 5 4 j=221 S> 15
20
25
30, 0.05 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
SH
Figure 4.4 (cont.) Profiles of (e) Sm (top) and (f) Sh (bottom) across the Lifam atola Sill. Locations of profiles are given by the dots in Figure 4.1.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 137
|5.10e-3
2 .100-3
108-3 1000 5.108-4
1500 South Haimahera Sill North Haimahera Sill 2.108-4
2000 IOa-4
Haimahera Basin 2500 ,108-5
Seram S ea Pacific/ Ocean 2 .108-9 3000*- 100 150 200 250 350 distance(km)
IHf
_ 1000 5.108-3
South Haimahera Sill1500
North Haimahera Sill 2000 5.108-4
Haimahera Basin 2500
Seram S e a Pacific Ocean 2.10e-5 200 250 300 350 400 450 distance(km)
Figure 4.5 Sections of (a) q2(m2/s 2) (top) and (b) KM(m2/s) (bottom) across the Haimahera Sea. Locations of points 7 and 8 at the top of the figures are given in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the the change in scale at 150m.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 50 100 150 200 250 300 350 400 450 distance(km)
10
$ 15
20
i=155 j=149 i= 162j= 156 25 —i=170j=164
30, 20 40 60 80 100 120 140 160 180 200 I (m )
Figure 4.5 (cont.) Section of (c) KH(m2/s) (top) and profiles of (d) l(m) (bottom) across the Haimahera Sea. Locations of profiles are given by the dots in Figure 4.1. Note the logarithmic scale on the colorbar. Note also the the change in scale at 150m.
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i= 155j= 149 i= 162j= 156 £ 15 •j=170 j=164
20
30, 0.05 0.15 0.25 0.3 0.35 0.4
I 10 f =155 j=149 =162 j=156 £ 15 =170 j=164
20
25
30J- 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 S u
Figure 4.5 (cont.) Profiles of (e) SM (top) and (f) Sh (bottom) across the Haima hera Sea. Locations of profiles are given by the dots in Figure 4.1.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 140
i
Figure 4.6 Maps of (a) q2(m2/s 2) at the surface with climatological wind (top) and (b) no wind.
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Figure 4.6 (cont.) Maps of (c) q2(m2/s2) at 20m with climatological wind (top) and (d) no wind.
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Figure 4.6 (cont.) Maps of (e) q2(m2/s 2) at 50m with clirnatological wind (top) and (f) no wind.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 143
that the effect of the winds is felt to a depth of ~ 50m. Values of g 2 at the surface with
and without winds are given in Figures 4.6(a) and (b), respectively. These results
are given by the surfce boundary condition q2 = B \^ u 2 for experiment 7 (with wind)
where u2 is the friction velocity and q2 = 0 for experiment 8 (no wind). At 20m, (c)
and (d), values of ~ 10_3m 2/s 2 are seen around Haimahera, North Sangihe Ridge,
and Labani Channel for both experiments. Elsewhere, values of ~ 5.10~ 4m 2/s 2 are
generally seen for experiment 7 while values are very small for experiment 8 . At
50m, (e) and (f), the results are very similar. Values greater than 10~ 2ra2/s 2 are
seen near Haimahera, Sangihe and Makassar Strait. Values are small elsewhere. A
similar pattern is seen at 75 m and 100m (figures not shown).
4.4. Discussion and Conclusions
Turbulence and mixing are important in the Indonesian Seas. Of particular
interest are processes causing the transformation of thermocline water masses, NPW
and SPW, and processes maintaining deep overflows that are not well understood.
In addition, processes occurring at deep topographic sills in this region may help in
our understanding of the dynamics of deep overflows in general, which is important
for climate and ocean circulation studies.
In deep water in the Indonesian Seas, large values of q2 are found near sills and
around topography. Values are greatest along sloping topography but can be large for
substantial distances both downstream and upstream of the sill, though large values
generally extend for greater distances downstream. Values are generally small in the
thermocline except in areas of known currents. Patterns in K m and l\H are similar
to q2 and it appears that the turbulence mean velocity, q, is the major contributor
to large diffusivities. Large diffusivities in the upper thermocline in areas of known
currents are important for the diffusion of NPW and SPW, and to the maintenance
of overflows at sills and around topogaphy in the Indonesian Seas. The master length
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scale, I, is small in the boundary layers and generally large in the interior, while the
stability parameters Sm and Sh are large in the boundary layers and generally small
in the interior. Sm and Sh can contribute to large diffusivities in the interior where
currents are strong, e.g., at the Haimahera Sills and Karakelong. However, I does
not contribute to large diffusivities since its value decreases where diffusivities are
large.
Distributions of KM and KH are very different at the four sills discussed, and
give an indication of the complex flow dynamics at the sills. The Sangihe Ridge
and the Lifamatola Sill are alike in that strong bottom overflows carry water into
the downstream basin, the Sulawesi and Seram Sea, respectively. Large values of
q2 associated with the overflows result in high diffusivities downstream of the sill.
These large diffusivities can mix warm water down to sill level thus ensuring the
temperature (pressure) gradient across the sill is maintained. At Dewakang, a process
similar to both Sangihe and Lifamatola occurs at the sill, ensuring the maintenance
of the overflow. However, large diffusivities are not seen along the sloping topography
of the Flores Sea and it is not clear how deep the overflow can penetrate or mix.
The Haimahera system is also very complex. Large values of q2, K M and KH are
seen at the Southern Haimahera Sill and along the bottom topography downstream.
A bottom current is also directed downstream across the southern Haimahera Sill
(Chapter 3).
Recall, from Chapter 3, that 0 and S distributions vary across the sill, i.e., 6 and
S are different on the upstream sloping topography, at the sill and on the downstream
sloping topography, respectively. Then fluid particles will change their 9 and S char
acteristics as they upwell and downwell across the sill. The only possible mechanism
for this is diffusion. This seems quite plausible given the large diffusivities at the sill.
So, turbulent flows over steep topography are highly complex and variable at
and around the sills. Using available observations both Gordon et al. (2003) and
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Van Aken et al. (1988) calculated a mean vertical diffusivity for the Banda Sea
region below 300m of ~ 10“3m 2/s from Munk's (1966) abyssal recipe. Based on
radon profiles and deep silicate distribution, Berger et al. (1988) and Van Bennekom
(1988), respectively, found diffusivities as high as 2.10“~ 1m2/s in the deep Banda
Sea. Studies of the Faeroe Bank Channel Overflow show turbulent diffusivities with
values of 10~ 3m2/s to 10"2m2/s see, e.g., Saunders (1.990), Duncan et al. (2003),
Mauritzen et al. (2005). Mauritzen et al. (2005) also found that strong mixing
extends downstream at the Faeroe Bank Channel overflow while mixing is more
abrupt over the Denmark Strait overflow. Using the M-Y 2.5 turbulence model in an
idealized sloping basin, Mellor & Wang (1996) found diffusivities between 5.10“ 2m2/s
and 10_ 1m2/s in a layer extending 500m off the bottom along sloping topography.
These studies support our results of large q2 and diffusivities around topography.
The model diffusivities reduce to the background values of 2.10“ 5m2/s in the
interior ocean away from topography. Toole et al. (1994) using microstructure pro
files found diffusivities of 5.10- 4m2/s near topography, decreasing to 10- 5m2/s away
from topography. Polzin et al. (1997) found values of l(V 5m2/s in the abyssal ocean
away from topography while Ledwell et al. (1993) also found values of 10~ 5m2/s in
the thermocline in the open ocean.
Large diffusivities are often seen in the upper thermocline in regions of strong
currents. In the Sangihe Ridge system, particularly large values are seen in the
eastern portion of the Karakelong basin and over the Karakelong Ridge to a depth
of 500m. These diffusivities are a result of the large q2 generated by the Mindanao
Current. In the Haimahera Sea system large diffusivities in the thermocline are due to the q2 generated by the New Guinea Coastal Current. Large diffusivities in the
thermocline are also seen at the Dewakang Sill and result from large q2 generated by
the strong southward current along the Makassar Strait.
Large diffusivities and q2 in the UML generally result from the SEM winds.
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However, energetic near surface currents entering the model domain through the
open ports, such as the Mindanao Current and the NGCC, and interior currents
such as in the Makassar Strait also lead to large values of q2.
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CHAPTER 5
SUMMARY
The circulation of the Indonesian Seas has been studied with a high resolution
regional model based on the POM. The model was developed as a research tool
to examine important physical processes occurring in the Indonesian Seas and the
different flow paths taken by the ITF. The primary objectives of this research were
to study: physical mechanisms controlling the splitting of the ITF between passages
and the effect of topography on flow paths taken; the effects of seasonal variation
of transports through the open ports; the influence of local winds on the regional
circulation; the role of topography in the formation of distributions of 0 and S in
the region with an emphasis on the interaction of NPW and SPW; and features of
mixing and turbulence having an important effect on 9, S and velocity distributions.
Physical mechanisms controlling the splitting of the ITF between passages and
the effect of topography on flow paths taken, the effects of seasonal variation of
transports through the open ports, and the influence of local winds on the regional
circulation were studied in Chapter 2 (objectives 1,2,3). All important topographic
features within the Indonesian Seas have been included in the developed regional
model which has horizontal resolution of ~ 10 km within a 250 x 250 grid and 29
cr-levels in the vertical. The model has four open ports to simulate inflow of North
Pacific Water from the Mindanao Current, inflow of South Pacific Water from the
New Guinea Coastal Current, outflow to the Pacific Ocean in the North Equator
ial Counter Current, and outflow to the Indian Ocean in the ITF. Total transports
through the open ports and typical normal velocities at the ports are specified from
observations. Four experiments were discussed: seasonally varying and annual mean
port transports and normal velocities both with and without local winds. It was
shown that the portion of North Pacific Water entering the Indonesian Seas relative
to that leaving through the North Equatorial Counter Current port is fairly constant
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throughout the year. Most of this water takes the western route through the Makas
sar Strait. We have shown that the portion of South Pacific Water entering the
Haimahera Sea compared to that exiting in the North Equatorial Counter Current
varies considerably with the seasons.
It was shown that turning off the local winds does not substantially influence
transport through the main passages in the model domain. Surface circulation pat
terns change substantially with the seasons. The comparison of the basic structure of
the model surface circulation and total transport values through the main passages
with observations appears satisfactory.
The role of topography in the formation of 6 and S distributions in the region
with an emphasis on the interaction of NPW and SPW, was examined in Chapter 3
(objective 4). This was considered by following the three major flow routes through
the Indonesian Seas: the western route from the Mindanao Port through the Sulawesi
Sea, along the Makassar Strait, across the Dewakang Sill, into the Flores Sea and
finally to the South Banda Sea; the northern route also from the Mindanao Port
across the Morotai Basin, the Maluku Sea, across Lifamatola into the Seram Sea,
the north Banda Sea and finally the South Banda Sea; and the eastern route from
the NGCC Port to the Northern Haimahera Sill, across the Haimahera Basin to the
Southern Haimahera Sill, and across the Obi-Seram passage to the Seram Sea. We
have shown that differences in stratification are caused by topographic features that
impede the advection of cold and salty water from an upstream basin to the basin
downstream. It was shown that in the upper 500m the ITF is primarily shaped by
NPW taking the western route through the Makassar Strait. Deep Banda Sea water
is formed by the overflow of NPW at the Lifamatola Sill, the deepest connection
between the Pacific and the Banda Sea. Finally, it was shown that the Haimahera
Sills block SPW from entering the Indonesian Seas below 500m. In the upper 500m,
SPW enters the Banda Sea through the Haimahera Sea and mixes with NPW in the
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Banda Sea region.
Features of mixing and turbulence having an important effect on 9, S and velocity
distributions, were outlined and analysed in Chapter 4 (objective 5). The turbulence
kinetic energy and the master turbulence length scale are calculated with the M-Y
2.5 turbulence parameterization model. It was shown that large turbulence kinetic
energies are found near topographic features such as sills. This energy results in
large diffusivities leading to mixing that can sustain overflows at sills. The results
show that the dynamics and processes are complicated and vary from sill to sill.
Known currents in the upper thermocline lead to strong diffusion of NPW and SPW
temperature and salinity signatures. The effect of the SEM winds on mixing is
limited to the upper 50m of the water column.
5.1. Future Work
One of the most interesting features of the Indonesian Seas circulation is the
deep overflow of water from a basin upstream to the neighboring basin downstream.
This is made possible by the complex bottom topography of the Indonesian Seas
region. The processes and flow dynamics of the overflow are very complicated and not
completely understood. We would like to investigate these overflows in more detail.
This could be done, perhaps, using very high resolution models around the area
of a known overflow and also with simple conceptual overflow models. This would
contribute to our knowledge of the overflow in general and also to our understanding
of the formation of deep water masses such as North Atlantic Deep Water, thereby
improving our understanding of the ocean circulation.
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Appendix A
The smoothed bottom topography in the areas of the Sangihe Ridge, Labani
Channel, Dewakang Sill, Haimahera Sea, Lifamatola Passage, Lombok Strait, Ombai
Strait, Timor Passage, and the passage between the Irian Jaya and Seram Island is
shown in Figure Al. We now compare it with the unsmoothed detailed topography
given by Smith & Sandwell (1997), hereafter S & S:
a) The outstanding topographic features at the north and south of the Sangihe Ridge
are sills. At the northern sill, the location of the 1250m and 1500m isobaths agree
with S &S and the sill depth is 1300 — 1400m in both cases. At the southern sill, the
location of the 1000m and 1250m isobaths agree also with S &S and the sill depth
is between 1100 — 1200m in both cases.
b) The outstanding topographic feature at the Labani Channel is a sill. Locations
of the 1500m and 1750m isobaths agree with S&S. The sill depth is 1800 — 1900m
deep in our smoothed topography and 2000 — 2100m in S &S.
c) The outstanding topographic feature at Dewakang in the south Makassar Strait is
the Dewakang Sill. The locations of the 600m and 800m isobaths agree with S &S.
Our sill depth is 500 — 600m and S &S sill depth is ~ 600m. The passage to the west
of the Dewakang Sill and to the east of Kalimantan Island has depth 400 — 600m
and the location of the passage and depth range agree with S&S.
d) The outstanding topographic features in the Haimahera Sea are sills at the north
ern and southern entrances to the sea itself. The 500m and 1000m isobaths agree
with S&S at both sills and the sill depths of 500 — 600m agree with S&S.
e) The outstanding topographic feature at the Lifamatola Passage is a sill. The
location of the smoothed 2000m and 1500m isobaths agree with S&S. Our smoothed
bathymetry gives a sill depth of ~ 1750 m whereas S&S give a sill depth of 1800 —
1900m.
f) The outstanding topographic feature between Burn and Seram Islands is a sill. The
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location of 2000m and 1500m, isobaths agree with S&S. A sill depth of 1100 — 1200m
agrees with S&S. Isobaths between islands in the other 3 passages in this region agree
well with S&S.
g) The outstanding topographic feature between Irian Jaya and Seram Island is a
passage. The location of 1000m, 1500m and 2000m isobaths agree with S&S.
h) The outstanding topographic feature at the Lombok Strait is a sill. The location
of the 400m and 600m isobaths agree with S&S. The sill depth is ~ 400m in both
cases.
i) The outstanding topographic feature at Ombai Strait is a deep sill. The location of
1500m and 2000m isobaths in our smoothed topography agree with S&S. The depth
of our sill is 1600 — 1700m compared with 1400 — 1500m, in S&S.
j) The outstanding topographic feature between Timor Island and Australia is a deep
passage. The 1000m, and 1500m isobaths generally agree with S&S. The depth of
the passage is ~ 1500m in our smoothed topography whereas it is 1600 — 1800m in
S&S.
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155160165170175 (i)
130 140 150 160 180 110 115 i
Figure A.I. Some important features of the smoothed model bathymetry: (a) Sangihe Ridge with north and south sills; (b) South Makassar Strait with the De wakang Sill in the east and western passage near Kalimantan; (c) Lombok Strait;(d) Ombai Strait; (e) Timor Passage; (f) Haimahera Sea with north and south sills; (g) Seram Sea area with Lifamatola Passage to the northwest; Buru-Seram passage to the south; Obi-Halmahera and Obi-Seram passages to the east; and Mangole- Buru passage to the west; (h) Irian Jaya - Seram Passage; and (i) Labani Channel. Isobaths are in 1000‘s of meters.
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Appendix B
Horizontal boundary conditions at rigid walls for the depth-averaged motion :
1. Impermeability condition (normal component of velocity is equal to zero).
2. A special expression for the viscous flux of momentum in the direction normal to
the wall. This is sometimes called the ’half-slip’ condition. This condition has been
chosen because of the simplicity of its formulation in the C-grid.
Horizontal boundary conditions at rigid walls for the 3D motion
1. Impermeability condition (normal component of velocity is equal to zero).
2. A special expression for the viscous flux of momentum in the direction normal to
the wall (’half-slip’ condition).
3. Diffusive fluxes of temperature and salinity in the direction normal to the wall
are set to zero.
4. Diffusive fluxes of turbulence energy and length scale in the direction normal to
the wall are set to zero.
Vertical boundary conditions for the 3D motion
Surface:
1. The vertical turbulent flux of horizontal momentum is set equal to the prescribed
wind stress.
2. The kinematic boundary condition for the vertical velocity.
3. The prescribed temperature and salinity are set equal to the climatological value
at the surface
4. The turbulence energy is set proportional to the wind stress.
5. A special relation is assumed between the turbulence length scale and energy
at the second a-level. The turbulence length scale is set equal to zero at the first
cr-level.
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Bottom:
1. The bottom stress is set proportional to the square of the bottom velocity.
2. The kinematic boundary condition for the vertical velocity.
3. The vertical fluxes of heat and salt are set equal to zero.
4. The turbulence energy is set proportional to the bottom stress.
5. The turbulence length scale is set equal to zero.
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Appendix C
Table C.l calculated values e 5 rate of time variation (xlO8) -1.3x 10” 1 2.7x 10 ” 2 horizontal advective fluxes (xlO8) west Banda sea (i=90) including Timor passage, Ombai strait 9.7xl0 -1 2.5 and Flores sea, Q=7.3Sv north Banda sea (j=134), Q=-7.lSv -9.8X1CT 1 -2.4 east Banda section (i=145), Q=-0.2Sv 3.8x 10 ” 2 -7.4x10“ 2 Sum of horizontal advective fluxes 2.3xl(T 2 -9.4x 10” 3 horizontal diffusive fluxes (xlO8) west Banda sea (i=90) including Timor passage, Ombai strait -6.5xl0 ” 5 3.6x 10” 6 and Flores sea north Banda sea (j=134) -9.2xl0 ” 5 -3.4 xlO ” 6 east Banda section (i=145) 7.4xl(T 5 -5.4xl0 ” 6 Sum of horizontal diffusive fluxes -8 .2 x 10” 5 -5.2 xlO ” 6 vertical flux at the surface (xlO8) -6.3xl0 ” 2 -2.8X10 ” 1 nudging (xlO8) 1.8X10 ' 1 2.6 xlO ” 1 sum of all terms (xlO8) 2.6 x l 0” 3 7.8xl0 ” 3
Table C. The same as Table 3 but for different boxes in the region: C.l (above) the Banda Sea box, C.2 the Makassar Strait box and C.3 box to the north and east of Lombok Strait.
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Table C.2 calculated values e S rate of time variation (xlO8) -9.5xl0 “ 3 5.5xl0 -3 horizontal advective fluxes (xlO8) south of Makassar box (j=142), Q=9.5Sv 1.7 3.3 north of Makassar box (j=191), Q=-9.5Sv -1.7 -3.3 sum of horizontal advective fluxes 2.4x 10-2 4.8xlCT 3 horizontal diffusive fluxes (xlO8) south of Makassar box (j=142) 2.7xl0 ‘ 4 1.7xl0 “ 5 north of Makassar box (j=191) 2.6 x l 0“ 5 3 .8xl0 “ 6 sum of horizontal diffusive fluxes 2.9xl0 “ 4 2 .1 x l 0“ 5 vertical flux at the surface (xlO8) -4.6 xlCT 3 1.3xl0 “ 2 nudging (xlO8) 3.0xl0 -2 -2.4xl0 - 2 sum of all terms (xlO8) 4.0xl0 “ 2 -l.lx lO - 2
Table C.3 calculated values e S rate of time variation (xlO8) -3.6x10-2 l.lx lO ” 2 horizontal advective fluxes (xlO8) north of Lombok box (j=142), Q=-0.4Sv -3.5xl0 - 2 -1.2 x 10- ' east of Lombok box (i=41), Q=-1.7Sv -3.4 xlO -1 -6 .1 x 10- ' south of Lombok box (j=114), Q= 2.lSv 4.5x10-' 7.3x10“ ' sum of horizontal advective fluxes 7.7xl0 - 2 -2.5 x H r 3 horizontal diffusive fluxes (xlO8) north of Lombok box (j=142) -3.2x 10“ 5 -8.1 x 10"6 east of Lombok box (i=41) -3.1 xlO " 4 -5.9xl0 “ 5 south of Lombok box (j=114) -3.8 xlO " 4 1.2 x l 0“ 5 sum of horizontal of diffusive fluxes -7.1xl0 -4 -5.5x 10“ 5 vertical flux at the surface (xlO8) -2.5x10-2 -3.4xl0 - 2 nudging (xlO8) -3.2x10-2 2 .6 x 10-2
sum of all terms (xlO8) -1.7x10-2 l.OxlO -3
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Appendix D
It is useful to write down the ’vertical’ velocity wpom used in the POM in the following form. First, consider cr-surfaces (surfaces at which a is constant). Their equation is
z = crD + r], D = H + r), (D.l) where H is the depth of the ocean and r) is the sea surface height. Introduce the unit normal vector to the o- surface,
1/2 dz dz ' dz ' ' dz ' , 1 , N + + 1 (D.2) N mxdx rriydy m Tdx mTdx
where mx and my are scale factors for the curvilinear orthogonal coordinate system x,y. Consider the velocity of an arbitrary point of the cr-surface and denote the normal component of this velocity by Va. Then it is easy to show that
wpom N (u • n„ - K,), (D.3) where u is the velocity of a fluid particle.
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