The Interannual Variability of The Leeuwin Current Ryan Warrington
i - The Interannual Variability of The Leeuwin Current Ryan Warrington
Abstract
Previous numerical modeling studies have replicated the mean flow characteristics of the warm poleward current off the coast of Western Australia, known as the Leeuwin Current and subsequently raised questions about the current’s interannual variability. This thesis used the Regional Ocean Modeling System (ROMS) to investigate the effect that pronounced Southern Oscillation events have on the mean flow of the Leeuwin Current.
The modeling consisted of three ROMS simulations running from July-December to compare current behavior during the strong 1997 El Niño, the 1998 La Niña response and 1996, a neutral year. ROMS was forced using temperature, salinity and baroclinic velocity data from the OCCAM global ocean model. The critical decision of deciding when to perform the run was based on OCCAM plots, satellite altimetry and southern oscillation composite images. The timing of the run (July - December) was intended to incorporate both the latter phase of the current’s peak and the peak phase of Southern Oscillation events.
A comprehensive analysis of OCCAM’s reproduction of both Southern Oscillation events and mean flow characteristics of the Leeuwin Current revealed that both interannual and seasonal forcing of the Leeuwin Current were adequate. However it was revealed that 1996 showed weak to mild La Niña characteristics in the forcing data, which reduced the effectiveness of the comparison between 1996 and 1998.
After a stable ROMS solution was generated the reproduction of the Leeuwin Current’s mean flow characteristics was verified using satellite altimetry. An analysis of model results revealed that Southern Oscillation events had a significant impact on both the North West Shelf source waters and the Leeuwin Current itself. It was also found it took approximately one month before the effect of warm, low salinity inflow onto the North West Shelf was felt by the Leeuwin Current.
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The overall climate of the domain was significantly affected by El Niño as reflected by cooler temperatures and higher salinity. The El Niño Leeuwin Current was observed to be distinctly cooler and more saline then the other two years by approximately 1-1.5 ° C and 0.1 - 0.2 PSU on average, which was attributed not only to a reduced geopotential gradient along the Western Australia coast, but a lack of pooling in the North West Shelf.
The La Niña Leeuwin Current was distinctly warmer then the El Niño year but was very similar to the ‘neutral’ year. This was put down to the likeness of the neutral year to a La Niña event. The increase in temperature in the latter part both 1996 and 1998 reflected the behaviour of warm Indonesian Throughflow discharge onto the North West Shelf. Whilst the difference between a La Niña and neutral year could not be fully quantified, the dynamic behaviour of the Indonesian Throughflow-Leeuwin Current interaction was sufficient to suggest that La Niña year produces a warmer Leeuwin Current then normal.
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Acknowledgements
There any many people to thank generously providing their time, resources and support all throughout the year and without who this project would not have gone ahead.
A big thankyou to Greg Ivey for overseeing the project and for all the guidance and support he provided.
I am indebted to Michael Meuleners, who generously gave his time all throughout the year and provided answers to my endless questions about modelling. Without his help this project would have taken substantially longer, possibly in the order of years.
I would also like to thank iVEC for their generous use of their facilities, without which the project would have been unable to go ahead. Also thanks to Darren Carey the systems administrator for helping me to gain access to the system.
I would also like to thank to Geosciences Australia for providing the bathymetry data.
Also thankyou to David Griffin from CSRIO for providing satellite imagery and suggesting relevant work to look at.
Finally I would like to thank my family and extend a special thankyou to my girlfriend Jessica for all of her support and for understanding.
iv - The Interannual Variability of The Leeuwin Current Ryan Warrington
Table of Contents
1 INTRODUCTION...... 1 2 GLOSSARY AND ABBREVIATIONS...... 1 3 LITERATURE REVIEW...... 3 3.1 INTRODUCTION...... 3 3.2 DRIVING FORCE ...... 5 3.3 THE SOUTHERN OSCILLATION...... 6 3.4 PREVIOUS STUDIES...... 9 3.4.1 Observational Studies...... 9 3.4.2 Modelling Studies ...... 13 3.5 EDDY GENERATION...... 15 3.6 ROMS STUDIES ...... 15 4 METHODS...... 18 4.1 ROMS MODEL DESCRIPTION...... 18 4.1.1 Primitive Equations ...... 18 4.1.2 Transformed Co-ordinates...... 20 4.1.3 Method of Solutions and Forcing ...... 21 4.2 OCCAM MODEL DESCRIPTION ...... 22 4.3 VISUALISING DATA...... 23 4.4 MODEL DOMAIN ...... 24 4.5 EVALUATING RUN TIMEFRAME...... 25 4.6 SATELLITE IMAGERY...... 26 4.7 OCCAM VALIDATION: SOUTHERN OSCILLATION ...... 26 4.8 OCCAM’S REPRODUCTION OF THE LEEUWIN CURRENT SIGNATURE ...... 27 4.9 MODEL SETUP...... 28 4.10 MODEL RUN...... 30 4.11 MODEL ANALYSIS...... 32 4.12 VERIFICATION ...... 34 5 RESULTS ...... 34 5.1 SATELLITE IMAGERY...... 34 5.1.1 AVHRR PATHFINDER ...... 34 5.1.2 TOPEX/POSIDEN ERS ½ ...... 36 5.1.3 Summary of Observations...... 37 5.2 OCCAM VALIDATION: OUTHERN OSCILLATION...... 38 5.2.1 Pacific Signature ...... 39 5.2.2 Indonesian Signature...... 40 5.3 OCCAM’S REPRODUCTION OF THE LEEUWIN CURRENT SIGNATURE ...... 40 5.4 ROMS MODEL RESULTS...... 44 5.4.1 Run 1: Tides...... 44 5.4.2 Run 2: No Tides ...... 45 5.4.3 Run 3: No Western Boundary or Tides...... 46 6 DISCUSSION...... 59 6.1 SATELLITE ALTIMETRY...... 59 6.2 OCCAM ...... 60 6.3 ROMS...... 62 6.3.1 Tides ...... 62 6.3.2 No Tides...... 62 6.3.3 No Western Boundary or Tides...... 63
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6.3.4 Comparison of El Niño Peak (August/September/October)...... 65 6.3.5 Comparison of the La Niña peak (November/December)...... 66 6.4 PROBLEMS AND LIMITATIONS ...... 67 7 CONCLUSIONS ...... 69 8 REFERENCES...... 72 9 FIGURES...... 77 10 APPENDIX 1: ENSO COMPOSITE IMAGES ...... 87 11 APPENDIX 2: OCCAM SOUTHERN OSCILLATION PLOTS...... 89 11.1 SEPTEMBER ...... 89 11.2 OCTOBER ...... 90 11.3 NOVEMBER ...... 92 11.4 DECEMBER ...... 93 12 APPENDIX 3: COMPARATIVE PLOTS OF MODEL RESULTS ...... 94 12.1 SATELLITE IMAGERY COMPARISON...... 94 12.2 JULY...... 95 12.3 AUGUST/SEPTEMBER/OCTOBER...... 96 12.4 OCTOBER/NOVEMBER...... 99 12.5 LEEUWIN CURRENT/LEEUWIN UNDERCURRENT COUPLING...... 101
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Figures Figure 9-1 A schematic showing the ocean circulation that drives the LC (Nof et al. 2002) ...... 77 Figure 9-2 The ROMS model domain shown with WA coastline and 100m bathymetry contours...... 77 Figure 9-3 The distribution of the sigma layers in the vertical direction of the ROM model grid ...... 78 Figure 9-4 The example of the global satellite altimetry with the two regions of interest circled (California Institute of Technology: Jet Propulsion Laboratory 2006) ...... 78 Figure 9-5 The partitioning of the ROMS grid chosen for analysis of results ...... 79 Figure 9-6 The 1997 El Niño peak in September as shown by satellite altimetry (California Institute of Technology: Jet Propulsion Laboratory 2006) ...... 79 Figure 9-7 The La Niña peak in November 1998 and corresponding plot of 1996, showing a weak La Niña pattern as shown by global satellite altimetry (California Institute of Technology: Jet Propulsion Laboratory 2006)...... 80 Figure 9-8 OCCAM plot of potential temperature & baroclinic velocity of May 1996 showing peak LC signature and highlighting the meandering/eddying behaviour of the current ...... 80 Figure 9-9 OCCAM plot of potential temperature & baroclinic velocity of May 1997 showing peak LC signature...... 81 Figure 9-10 OCCAM plot of potential temperature & baroclinic velocity of May 1998 showing peak LC signature...... 81 Figure 9-11 OCCAM sea surface height average of May 1996 showing a dipole eddy pair ...... 82 Figure 9-12 OCCAM v velocity plot of the surface layer showing the LC signature in July 1996, 1997 and 1998 ...... 82 Figure 9-13 OCCAM v velocity plot at 27 ° S in July 1996 showing the LC-LUC coupling...... 83 Figure 9-14 ROMS Sea surface height of initial simulation day 2 with first observable oscillations ...... 83 Figure 9-15 ROMS Sea surface height of initial simulation day 40 with the oscillations greatly amplified...... 84 Figure 9-16 V velocity in the surface layer of the second simulation without tides, showing the propagation of baroclinic velocity across the western boundary and reflection at the southern end of the domain. The LC signature is still observable.....84 Figure 9-17 The contrast in visibility of the LC signature seen from a 3 ° C range reduction of the MATLAB colour bar...... 85 Figure 9-18 Satellite SST image showing distinct western inflow into the shelf break region around Shark Bay (Griffin 2001)...... 86 Figure 10-1 Southern oscillation composite image showing May-October (year -1) (Earth System Research Laboratory: Physical Sciences Division 2006)...... 87 Figure 10-2 Southern oscillation composite image showing November-April (year 0) (Earth System Research Laboratory: Physical Sciences Division 2006)...... 87 Figure 10-3 Southern oscillation composite image showing May-October (year 0) (Earth System Research Laboratory: Physical Sciences Division 2006)...... 88 Figure 10-4 Southern oscillation composite image showing November-April (year +1) (Earth System Research Laboratory: Physical Sciences Division 2006)...... 88 Figure 11-1 OCCAM plot of equatorial Pacific in September 1996, 1997 and 1998 .89
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Figure 11-2 OCCAM plot of ITF region in September 1996, 1997 and 1998 ...... 90 Figure 11-3 OCCAM plot of equatorial Pacific in October 1996, 1997 and 1998...... 90 Figure 11-4 OCCAM plot of ITF region in October 1996, 1997 and 1998...... 91 Figure 11-5 OCCAM plot of equatorial Pacific in November 1996, 1997 and 1998..92 Figure 11-6 OCCAM plot of ITF region in November 1996, 1997 and 1998 ...... 92 Figure 11-7 OCCAM plot of equatorial Pacific in December 1996, 1997 and 1998 ..93 Figure 11-8 OCCAM plot of ITF region in December 1996, 1997 and 1998...... 93 Figure 12-1 Comparison of satellite imagery (Griffin 2001) and model results on the 17th September 1996 showing similar temperature patterns...... 94 Figure 12-2 ROMS potential temperature at 5 m depth on the 21st July 1996, 1997 and 1998 (° C)...... 95 Figure 12-3 ROMS v velocity at 5 depth on the 21st July 1996, 1997 and 1998 (m/s) ...... 95 Figure 12-4 ROMS salinity at 5 m depth on 21st July, 1996, 1997 and 1998 respectively (PSU) ...... 96 Figure 12-5 ROMS potential temperature at 5 m depth on the 20th August 1996, 1997 and 1998 (° C)...... 96 Figure 12-6 ROMS potential temperature at 5 m depth on the 19th October 1996, 1997 and 1998 (° C)...... 97 Figure 12-7 ROMS salinity at 5 depth on the 20th August 1996, 1997 and 1998 (PSU) ...... 97 Figure 12-8 ROMS salinity at 5 depth on the 19th October 1996, 1997 and 1998 (PSU)...... 98 Figure 12-9ROMS v velocity at 5 depth on the 29th September 1996, 1997 and 1998 (m/s) ...... 98 Figure 12-10 ROMS potential temperature at 5 depth on the 18th November 1996, 1997 and 1998 (° C)...... 99 Figure 12-11 ROMS potential temperature at 5 depth on the 18th December 1996, 1997 and 1998 (° C)...... 99 Figure 12-12 ROMS salinity at 5 depth on the 18th November 1996, 1997 and 1998 (PSU)...... 100 Figure 12-13 ROMS salinity at 5 depth on the 18th December 1996, 1997 and 1998 (PSU)...... 100 Figure 12-14 ROMS v velocity at 5m depth on the 18th December 1997. Highlights the development of two large cyclonic features on the NWS...... 101 Figure 12-15 V velocity vertical cross section across 29 ° S transect on the 29th September 1997, showing the LC-LUC coupling...... 101 Figure 12-16 V velocity vertical cross section across 24 ° S transect on the 29th September 1996, showing the lack of LC-LUC coupling...... 102
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Tables
Table 3-1 The monthly Southern Oscillation Index from 1996 to 1999 (Bureau of Meteorology 2006)...... 7 Table 4-1 Definitions of variables used in the primitive equations (Rutgers IMCS Ocean Modelling Group 2006) ...... 19 Table 4-2 ROMS model parameters ...... 29 Table 4-3 The format of the table used for comparison of temperature and salinity results across the domain ...... 33 Table 5-1 July average LC temperatures for all regions...... 48 Table 5-2 August/September/October average LC temperatures for all regions...... 50 Table 5-3 November/December average LC temperatures for all regions...... 53 Table 5-4 July average LC salinity for all regions...... 54 Table 5-5 August/September/October average LC salinity for all regions...... 56 Table 5-6 November/December average LC salinity for all regions...... 57
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Equations
Equation 4-1 Momentum equation for the x direction...... 19 Equation 4-2 Momentum equation for the y direction...... 19 Equation 4-3 Momentum equation for the z direction...... 19 Equation 4-4 The advective-diffusive equation for temperature ...... 19 Equation 4-5 The advective-diffusive equations for salinity...... 19 Equation 4-6 The equation of state for temperature, salinity and pressure ...... 19 Equation 4-7 The continuity equation for an incompressible fluid...... 19 Equation 4-8 The ROMS sigma coordinate transformation ...... 21
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1 Introduction This honours dissertation is prepared in partial fulfilment of the criteria for the degree of Bachelor of Engineering (Environmental) at the School of Environmental Systems Engineering at The University of Western Australia. This project was jointly supervised by Professor Greg Ivey and Michael Meuleners.
2 Glossary and Abbreviations
Sverdrup units (Sv) – A unit of measuring ocean transport equivalent to 0.001 km3/s
Indonesian Throughflow – An ocean current that transports the upper ocean water between the Pacific and Indian Oceans through the Indonesian Archipelago driven by sea level differences
Geostrophic flow – The balance between Coriolis and horizontal pressure forces resulting in flow along surfaces of each pressure
Southern Oscillation – A complex global interaction between the ocean and atmosphere, with a warm phase known as El Niño and a cold phase La Niña.
Mesoscale – The scale of meteorological phenomena (approximately 2 – 200 km)
Baroclinic – Pertaining to stratified fluids or fluid layers
Barotropic – Pertaining to fluids with uniform depth profiles or depth averaged fluids
Ekman Transport – Transport resulting from a balance between Coriolis force and frictional stress
WA – Western Australia NWS – North West Shelf NW Cape – North West Cape
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ITF – Indonesian Throughflow LC – Leeuwin Current LUC – Leeuwin Undercurrent AVHRR – Advanced Very High Resolution Radiometer. T/P - Topex Poseidon SST – Sea surface temperature SSH – Sea surface height SSTA – Sea surface temperature anomaly SSHA – Sea surface height anomaly PSU – Practical Salinity Units ROMS – Regional Ocean Modelling System OCCAM – Ocean Circulation and Climate Advanced Modelling ENSO – El Niño Southern Oscillation SOI – Southern Oscillation Index NOAA - National Oceanic and Atmospheric Administration
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3 Literature Review
3.1 Introduction
The major eastern boundary currents of the world namely: Peru, California, Benguela and Canary are all characterised by a wide (~1000km) slow moving (< 10 cm s-1) equatorward surface flow. In addition these currents are associated with relatively shallow thermoclines, cold upwelling at the surface and high biological production (Batteen et al. 1992). However there is an eastern boundary current that exists off the west coast of Australia atypical of these regular eastern boundary current properties. The Leeuwin Current (LC) is a unique poleward eastern boundary current which transports warm low salinity water south along the coast of Western Australia (WA) (Fieux et al. 2005). In contrast to other eastern ocean boundary currents (Batteen et al. 1992) the LC is a strong (periodically exceeding 150 cm s-1), narrow (<100km wide) current with abnormally warm surface water and a deep thermocline.
The current itself was named after the Dutch vessel the Leeuwin by Cresswell and Golding (1980) and is characterised by downwelling, meanders, jet like streams and eddies and exhibits high seasonal, interannual and mesoscale variability (Meuleners et al. 2005a; Griffiths & Pearce 1985a; Batteen et al. 1992; Fang & Morrow 2003). Observations (Smith et al. 1991) have shown LC water to be low in dissolved oxygen and high in nutrients. The current is strongest along the continental shelf edge and generally runs parallel to bottom topography (Fieux et al. 2005) although as will be discussed in this thesis, eddy fields tend to distort the local mean flow dynamics and often redirect LC water offshore (Meuleners et al. 2005a). The current is also thought to be responsible for the numerous tropical marine species being observed off the WA coast in areas (Batteen & Butler 1998).
Early research by (Cresswell & Golding 1980) concluded that the LC was a fast flowing current which appeared to induce cyclonic mesoscale features on the seaward side. They observed that the flow continued eastwards around Cape Leeuwin and flows as far south as the Great Australian Bight. Other observations of the current include it’s intensification as it flows southwards and it’s strong seasonal signature
3 - The Interannual Variability of The Leeuwin Current Ryan Warrington with a maximum flow that occurs from May to June, although the signature is still strong through most of autumn and winter (Fang & Morrow 2003; Batteen et al. 1992) The major reason for the timing of the LC peak is the low pressure system over Australia occurring around December through to March, which drives prevailing northerly winds that suppress the LC. There is also a secondary maximum LC flow which occurs in November although it is not as pronounced as the main peak in May (Fieux et al. 2005).
Beneath the LC, seawards of the shelf break there is an equatorward undercurrent known as the Leeuwin Undercurrent (LUC) (Weaver & Middleton 1990). The LUC has been found to be strongest during summer which coincides with the weaker signature of the LC. The LUC has also been described as a slow equatorward flow feeding an offshore geostrophic flow (Godfrey & Weaver 1991). Specifics about the undercurrent tend to differ, Smith et al. (1991) proposes that the LUC exists at depths greater then 400m, while Godfrey & Weaver (1991) points to observations showing the current exists below a depth of 250m, while Thompson (1984) puts the LUC at depths below 150 m. However these discrepancies can be put down to different sampling locations and transects, due to the high spatial variability of the LC system. The characteristics of the LUC tend to be the reverse to the surface current, with observations by (Thompson 1984) showing LUC water consists of high salinity water, high in dissolved oxygen and low in nutrients. (Cresswell 1991) also observed that the LUC consisted of high salinity waters and South Indian Central waters, which was confirmed by (Smith et al. 1991). The LUC has been described as slow (Godfrey & Weaver 1991) and Batteen & Butler (1998) confirmed the undercurrent was significantly slower then the surface current, although the velocity was largely dependant on location with the maximum velocities being observed near Cape Leeuwin. Other then these few observations there is little reliable information on the undercurrents velocity structure and dynamics (Meuleners et al. 2005a).
Measurements by (Smith et al. 1991) indicate the LC transports an estimated 5 Sv from The ITF source waters annually. Estimates of that the total outflow from Indonesian seas into the Southern Indian Ocean amount to approximately 5-7 Sv
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according to (Nof et al. 2002), who also argues this outflow separates into two branches, with the branch driving the LC accounting for 87% of this source water.
3.2 Driving Force
The first thing to note about the LC is that prevailing winds in the region counter the mean current flow direction. The predominant wind direction off the WA coast is equatorward (northerly) which would normally suggest a equatorward current and coastal upwelling (Smith et al. 1991), however the current flows poleward and there is no observable evidence of coastal upwelling (Meuleners et al. 2005a; Morrow et al. 2003) (Smith et al. 1991; Fang & Morrow 2003). Furthermore these prevailing winds should drive offshore surface Ekman transport however geostrophic transport is observed instead (Fieux et al. 2005).
The flow of the LC is driven by a large alongshore geopotential gradient which results from the discharge of warm, low salinity water from the ITF (Feng et al. 2005). Smith et al. (1991) found the magnitude of this gradient to be much larger then for other eastern boundary regions, with value exceeding 2 x 10-6 m s-2. Onshore geostrophic transport results from this geopotential gradient which then overwhelms the offshore Ekman transport induced by the prevailing winds and, in turn, drives the LC (Godfrey & Weaver 1991; Feng et al. 2005).Figure 9-1 A schematic showing the ocean circulation that drives the LC shows the simplified passage of water feeding the LC (Figure 9-1). It has been suggested that the momentum balance of the LC is dominated by the alongshore pressure gradient, the prevailing winds and bottom stress (Thompson 1984; Smith et al. 1991). Observations by (Feng et al. 2005) indicate that the alongshore pressure gradient reverses its sign at a depth of 200m and that this deeper pressure gradient may be responsible for the existence of the LUC.
Seasonal changes in the strength of the LC are a result of variable wind stress and has little to do with alongshore the geopotential gradient which has minimal seasonal dependence (Smith et al. 1991). However, the ITF itself is also responsible for remotely forcing the many seasonal and interannual variations experienced not just by
5 - The Interannual Variability of The Leeuwin Current Ryan Warrington the LC itself but also by the larger eastern boundary region of the Indian Ocean (Fang & Morrow 2003)
3.3 The Southern Oscillation
Commonly referred to as ENSO (El Niño Southern Oscillation), the Southern Oscillation is a major interannual climatic phenomenon with the largest amplitude over the Indian and Pacific Oceans, the effects of which are felt in many parts of the world. The Southern Oscillation, a complex interaction between the ocean and atmosphere which has a warm phase known as El Niño and a cool phase known as La Niña. Strict definitions of these phases and the oscillation itself cannot be formed as the conditions under which phase occurs vary quite significantly and the oscillation itself can only be described as an imperfect correlation of conditions between many points around the globe (Philander 1990). The El Niño phase is usually associated with high surface pressure over the western and low surface pressure over the southern Pacific, coinciding with heavy rainfall, unusually warm surface waters and relaxed trade winds in the central and eastern tropical pacific and cooler surface waters over the Pacific/Indian ocean interface around Indonesia. La Niña on the other hand usually displays the opposite conditions and is commonly associated with unusually cool surface waters over the Pacific and warm ocean waters around Indonesia, as well as many other trends. Sea surface temperature anomaly from global composite images, show the development of both phases of the Southern Oscillation (see Appendix 1 (Earth System Research Laboratory: Physical Sciences Division 2006)).
The Southern Oscillation Index (SOI), calculated from the monthly and seasonal fluctuations between Tahiti and Darwin, has been developed to determine when El Niño and La Niña events occur. Several other indices have been derived, although they are not as widely known as the SOI (Pattiaratchi et al. 2006). Using this index, 1997 has been identified as a particularly strong El Niño year and 1998 found to be a La Niña year((Pattiaratchi et al. 2006) (Vranes et al. 2002),(Sprintall et al. 2003)). The monthly SOI values for the years 1996 to 1999 are shown below in Table 3-1. An El
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Niño event is determined by a sustained negative index, whereas a La Niña event is shown by a sustained positive index (Bureau of Meteorology 2006). Therefore from Table 3-1, 1997 is a strong El Niño and 1998 is a strong La Niña and 1996 is neutral to a weak La Niña event.
Table 3-1 The monthly Southern Oscillation Index from 1996 to 1999 (Bureau of Meteorology 2006). Au Year Jan Feb Mar Apr May Jun Jul Sep Oct Nov Dec g 13. 1996 8.4 1.1 6.2 7.8 1.3 9 6.8 4.6 6.9 4.2 -0.1 7.2 13. 1997 4.1 3 -8.5 -16 -22 -24 -10 -20 -15 -18 -15 -9.1 1998 -24 -19 -29 -24 0.5 9.9 15 9.8 11 10.9 12.5 13.3 18. - 1999 15.6 8.6 8.9 5 1.3 1 4.8 2.1 0.4 9.1 13.1 12.8
A study of the ITF used temperature and ocean current time series from December 1996 to early July 1998 to calculate heat transport and assess the influence on the Indian Ocean (Vranes et al. 2002). This data coincides with strong ENSO events over 1997/1998 and it was found that heat transport varies with ENSO phase and as expected is lower during El Niño and higher during La Niña.
Another observational study running from 1995-1999 also noted a strong response of the ITF to ENSO events in both temperature and salinity data. It was observed that from mid 1997 to early 1998 that the region was cooler and saltier then normal which related to the 1997 El Niño event. The region then observed warmer conditions from mid-1998 onwards which was attributed to the La Niña response (Sprintall et al. 2003). An investigation into the vertical structure of the water column at the entry and exit passages of the throughflow by (Potemra et al. 2003), using a numerical model show that differences of inflow/outflow on interannual timescales is correlated to ENSO. It was also suggested that generally local winds affect the layer of water above
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the thermocline but remote forcing (e.g. ENSO) affects the flow in the subsurface layer.
The observed response of the ITF to ENSO events, poses the question of ENSO’s effect of the LC, due to the importance of the throughflow to the geopotential gradient driving the LC. There have been several studies to determine the effect of ENSO on the LC. An investigation into the effect of climate change on the LC by (Pattiaratchi & Buchan 1991) confirmed that the LC is weaker during El Niño years. (Feng et al. 2003) investigated annual and interannual variations of the LC at 32 ° S (the latitude of Perth) and found that the LC was distinctly weaker during and El Niño year and stronger during a La Niña year. Geostrophic transport was also found to be significantly affected by the Southern Oscillation with a mean transport of 3.4 Sv and El Niño and La Niña transports of 3.0 and 4.2 Sv respectively. The use of the Fremantle sea level as a strength index for the LC is also justified and calibrated.
More recently (Pattiaratchi et al. 2006) investigated the latitudinal response of the current to interannual forcing and found that along the maximum transport location, the surface height slope provides a suitable quantification of the currents remote forcing. This forcing appeared to be correlated to the Southern Oscillation and was stronger during La Niña and weaker during El Niño. Net transport along the coast was also found to respond to ENSO forcing north of the eddy kinetic energy (EKE) maximum, namely stronger during La Niña and weaker during El Niño, however, there was no response south of the EKE maximum. In general, more eddies with more energy were found to be generated during La Niña years. Overall the LC was found to be stronger at lower latitudes (north of 29-30 ° S) during La Niña years and weaker during El Niño years, while at higher latitudes the influence of ENSO was found to be insignificant. Temporally the LC starts responding to ENSO as early as May, with the greatest difference in transport between El Niño and La Niña years occurring from the end of winter to early summer, in which case La Niña observed stronger transport for the most part. There were no significant differences in summer however during autumn and winter the net southward transport was found to be similar or even weaker in La Niña years.
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3.4 Previous Studies
3.4.1 Observational Studies
In the period from 1980-1991 there were several observational studies of the LC, starting with (Cresswell & Golding 1980)who investigated the current suspected of existing off the continental shelf and provided a basic description of the currents flow patterns and dynamics. They achieved this using tracking buoys equipped to measure flow velocities and sea surface temperature (SST). The study area went from 29-32 ° S which only extends slightly further north then the Abrolhos islands. This study also observed the generation of cyclonic eddies on the seaward side of the current and also named the LC.
The investigation of the LC by (Legeckis & Cresswell 1981)involved the application of infrared satellite imagery to the LC. This was done by using National Oceanic and Atmospheric Administration (NOAA) satellite TIROS-N. The computer enhanced data taken came from the satellites Advanced Very High Resolution Radiometer (AVHRR), although this would be considered primitive technology to what is available today, the results were still useful, as they confirmed known characteristics’ of the LC and also yielded new ones. This study showed clearly that the source region was off the NWS as it was one of the first studies to extend this far north and also noted the existence of seaward offshoots of the current. This study was hindered somewhat by cloud cover and satellite orbit delays. This study also noted the eastward diversion of the current at Cape Leeuwin and the continuing flow into the Bight.
The next major observational study was (Thompson 1984) which ranged from 22-28 ° S with data being collected along 5, 300km transects. This study measured temperature, salinity, dissolved oxygen and nutrient levels as well as water velocity profiles and surface velocity. The mixed layer depth was also calculated, which was on average 25m deep in summer and 50m deep in winter, providing a possible explanation for the seasonality of the current. Where the mixed layer depth was greater then 30m, the geopotential gradient was sufficiently strong enough to overcome wind stress and drive the LC. It was also found that a warm poleward
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current would generate a deeper mixed layer in winter, so it is unclear on whether the mixed layer depth was increased by the presence of the LC, or if it was responsible for driving it. The study also confirmed some of the basic current properties mentioned in section 3.1 and was the first study to calculated transport rate for the LC, giving values of 4 Sv for the surface current and 5 Sv for the undercurrent.
(Godfrey & Ridgway 1985) collected Hydrology data from CSRIO and the Royal Australian Navy between 10-40 ° S and 105-130 ° E. The authors divided the area up into a series of “bins” from which temperature-salinity relationships were quantified and steric height data was calculated. This managed to confirm many of the aspects of the larger scale picture of the LC including the geopotential driving force. Transport was also estimated at 3-5 Sv for the surface current and 1-2 Sv for the undercurrent.
In 1985 there were two studies conducted using AVHRR satellite imagery, with one focusing on instability and eddy pairs (Griffiths & Pearce 1985a) and the other tracking a single warm core eddy (Griffiths & Pearce 1985b). The study region for both papers lies along the south coast of WA and cloud cover limited temporal observations to several months. An important observation of the studies was the formation process of an eddy dipole pair. It was found that large eruptions from the outer edge of the current temporarily blocked current flow were associated with the formation of cyclonic and anticyclonic eddy pairs. Analysis of eddy characteristics in both studies confirmed observed eddies and instabilities to be baroclinic in origin.
As part of the Leeuwin Current Interdisciplinary Experiment (LUCIE) in 1986/87 a large scale observational study was conducted using current meters and CTD surveys (Smith et al. 1991). The study area was between 22 – 25 ° S and made detailed observations about the LC. The LC was found to flow strongly within 100km of the shelf edge year round with January as the only exception. As described in section 3.1, the surface current was found to transport 4 Sv poleward annually. It was also found that the current showed strong seasonal variation, with the current signature being strongest near the shelf edge through March -May and seaward of the shelf edge through June-August. It was found to be weaker and closer to the shelf edge through September-January and almost nonexistent in January. The seasonal variation was
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attributed to wind stress which had a strong northerly component in spring and summer. Also observed was a year round local temperature maximum at the current core, along the entire WA coast and a local salinity minimum again at the core at north of 32 ° S. A local salinity maximum was observed south of 34 ° it was found the between the minimum and maximum that the LC entrained high salinity water from the seaward side of the current. It was mentioned that the strength of the current may have some dependence on the Southern Oscillation phenomenon, although due to lack of long term data a relationship could not be established. Another finding was that onshore geostrophic transport in the upper 300m driven by the pressure gradient was balanced by offshore Ekman transport in a deeper boundary layer close to the shelf break.
A study using a combination of acoustic Doppler current profiler (ADCP), temperature-salinity (CTD) data, moored current meters and AVHRR satellite data was used to establish current behaviour on the NWS in May-June of 1993 (Holloway 1995). This data (taken at latitudes of 17 and 19 ° S) found the current signature to be wider then 250km, deeper then 440m and flowing with an average velocity of 0.2 m s- 1. The study also observed a typical transport of 4 Sv with a maximum of 7 Sv. The study also observed a poleward current at 12 ° S for most of the year (with a maximum from January through April) which turned and flowed northward in May- June, from current meter data from 1983-85. This flow correlated to NWS current meters at 20 ° S from December – March. The overall conclusion of the study was warm, low salinity water wells onto the NWS and a significant portion of the current water originates from this shelf source during the current peak of May-June. It was also found that the current is a wide meandering/eddying flow at latitudes of around 17 ° S but is a single core south of 19 ° S. A comparison of the data in this study with observations from (Smith et al. 1991) south of the NWS show observations and transport estimates to correlate quite well.
Since these earlier observational studies have quantified basic flow characteristics quite well, more recent observational studies have focused on eddy behaviour and dynamics and their effect on the mean flow. Because of the large spatial scale of the
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LC and the rapid advancement of modelling tools, the majority of larger scale studies of late have used a modelling approach.
Two comprehensive studies resulting from the Transport Indo-Pacific cruise in 2000(Fang & Morrow 2003; Morrow et al. 2003) have provided a much needed insight into the detailed eddy dynamics of the LC. These studies used a combination of altimeter data for 1995-2000 from TOPEX/POSIDEN and ERS satellites as well as hydrology data from four CTD transects. Anticyclonic (warm core) eddies were the focus of the research and it was noted that their paths were strongly influenced by bathymetry. Eddies were found to either slow down or be deflected as they travel over topography and the mixing induced by this topography can result in structural deformation leading to the dissipation of the eddy. It was also found that most long lived eddies are generated when the LC peaks (May-June) and that they are generated in three preferential locations, namely near (1) The NW Cape (2) Shark Bay and (3) The Abrolhos Islands, with the largest amplitude around 32 ° S, near the Abrolhos islands. The production rate of these warm core eddies was estimated for each year of data and is as follows: 1995 (6), 1996 (6), 1997 (3), 1998 (6), 1999 (9) and 2000 (7). This result is of particular interest for this study because of deviation from the average in 1997 and 1999. Which coincide with pronounced ENSO events, which will be further discussed in section 3.5. Most of the warm core eddies spawned by the current did not drift far offshore and were usually entrained back into the mean flow or dissipated in the Perth basin (Fang & Morrow 2003).
The second study (Morrow et al. 2003) is more detailed then the first and focuses mainly on the internal structure of several warm core eddies. The most important finding was that once eddies disconnect from the current they begin to increase in size in the vertical direction with an average depth of 1500m and influence being observed up to 2500m in depth. It was also observed that when these eddies become trapped inshore and they can change the local flow dynamics and divert the warm LC water offshore for several months or even longer.
Some of this data was used in the follow-on study (Feng et al. 2005) in which the momentum balance of the LC was analysed. It was found that the Abrolhos islands
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marked the point where the balance changed dramatically. North of the Abrolhos the current was balanced by pressure gradient and wind stress but south of the islands the current was found to be more complex as offshore of the current the momentum balance was composed of an increased pressure gradient, weakened wind stress and Reynolds stress exerted by eddies. The eddies grew through a mixture of baroclinic and barotropic instability with the energy being drawn from the pressure gradient.
3.4.2 Modelling Studies
Numerical modelling studies of the LC have progressed drastically in the last two decades. Early models such as the Bryan-Cox Ocean General Circulation Model (GCM) were first used to describe mechanisms for the generation and flow of the LC using both an analytical and a numerical model (Weaver & Middleton 1989). }. The numerical model imposed a geopotential gradient which produced an eastward flowing geostrophic flow which upon reaching the continental shelf turns poleward and intensifies as it moves southwards. The model also showed the development of cyclonic circulations off the NW Cape as well as the equatorward undercurrent off the shelf break. These features all agree with previous observations of the LC behaviour and, upon comparison with observation data it was found that amplitudes of currents, advection of the temperature and salinity contours and the width and structure of the current were reproduced “adequately”. The analytical model was more limited then a numerical model and unable to cope with certain aspects of the natural behaviour of the current such as the advection of temperature and salinity. Another similar study using an analytic model (Weaver & Middleton 1990) produced unrealistic current velocities and hence even at this early stage numerical models proved to be superior over analytic models, however as analytical models provide insight into the behaviour of numerical model they are still useful for validation of flow characteristics. These studies and several of those that followed used a fairly coarse model which neglected features such as detailed bathymetry and a realistic coastline (Meuleners et al. 2005a).
A high resolution model, primitive equation model was applied to investigate the role of thermal forcing for the current focusing on both the NWS waters and the Indian
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Ocean temperature field by Batteen & Rutherford (1990). She found that the Indian Ocean temperature gradient alone is suffice to drive the current-undercurrent system, but when coupled with the NWS waters it was found the shelf waters dominated close to the equator source region. This dominance weakened as the current moved further south but the flow was still significantly stronger as a result of the inclusion of the NWS waters. This study also reproduced some of the mesoscale eddies of scales comparable to observations and found the generation to be a result of mixed baroclinic and barotropic instability. A very similar study using the same model with both wind and thermal forcing was applied during the currents peak in the austral autumn and winter (Batteen et al. 1992). It was found that the Indian Ocean thermal forcing dominates wind at the poleward end of the chosen model domain (near Cape Leeuwin) and the current accelerates against the prevailing wind. At the equatorial end of the domain (near the NW cape) the NWS waters completely dominate the wind forcing.
In an extension of the above two studies a (Batteen et al. 1992) investigated the development of the currents characteristics and eddying behaviour with a similar model with a higher resolution. Four runs were performed with NWS waters turned on and off in combination with a 1) an idealised coastline and 2) a irregular coastline. The most realistic behaviour was observed with NWS waters on with irregular coastline which agreed well with observational data showing that the currents characteristics and mesoscale features can be qualitatively modelled accurately.
A numerical model of the ITF driven by a forcing term representing the difference in sea level between the Pacific and Indian Oceans, which includes the effects of islands, sills and continental shelves gave a mean throughflow transport estimate of 7.5 Sv (Dearnaley 1990). Another important outcome of this study was that seasonal forcing of the model produced a pulse of outflow during March-July through the Timor Sea, which was confined to the NWS. This finding was important as the timing of this pulse coincides with the intensification of the LC. The importance of the throughflow to the Indian Ocean climate is confirmed by (Hughes 1991) with the use of a idealised box model. Another modelling study with Sverdrup circulation deliberately omitted in order to observe the buoyancy driven circulation also confirms the connection of the
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LC with the ITF (Godfrey & Weaver 1991). An interesting result of running the model with much lower Pacific temperatures results in the LC disappearing, and a current system very similar to Peru being observed. However with realistic temperature values this model produces the LC and LUC system with deep mixed layers near Cape Leeuwin. This study also flags the importance of climate change to the LC.
3.5 Eddy Generation
A major interest in the LC is the large, energetic eddies that it generates and their interaction of these eddies with other features such as local flow dynamics. This is of potential significance for both biological and other scientific purposes (Meuleners et al. 2005a). Eddy generation from the LC occurs because of instability of the mean flow although the importance of an irregular coastline has also been suggested for generation to occur (Fang & Morrow 2003). As a result of numerous modelling studies the specific processes responsible for eddy shedding were found to be a mixture of barotropic and baroclinic instability (Feng et al. 2005). The relationship between the irregularity of the coastline and eddy generation is highlighted by the fact that regions of the most eddy generation coincide with some of the major coastal features of WA (Fang & Morrow 2003). There are three preferential latitudinal corridors for long lived eddies to drift offshore namely: 20-22 ° S, 24-25 ° S and 28- 32 ° S, which in geographical terms is the NW Cape, Shark Bay and the Abrolhos Islands. There is some uncertainty in whether the topography contributes to cause or effect of this westward eddy propagation, as these topographic features block the passage of these eddies in a southerly direction and this may be the reason for their western movement.
3.6 ROMS Studies
As previously stated all of the above studies use primitive equation models lacking detailed realistic bathymetry and coastline features. The motivating studies for this
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paper (Meuleners et al. 2005a) applied the Regional Ocean Modelling System (ROMS) to the LC. ROMS is a complex free surface, terrain following (sigma) coordinate primitive equation model which can incorporate detailed bathymetry, coastal features and uses high order closure schemes to resolve grid and subgrid scale processes (Rutgers IMCS Ocean Modelling Group 2006; Meuleners et al. 2005a). A more detailed model description will be given in the section 4.
Prior to (Meuleners et al. 2005a), ROMS was applied to the LC by (Griffin et al. 2001) using idealised bathymetry and a data assimilation technique to model forcing. The LC’s general features were reproduced but there were some discrepancies between in situ observations and model results, which occurred as a result of idealised bathymetry.
Meuleners et al. (2005a) firstly modelled the mean flow characteristics of the LC system. Meuleners et al. (2005b) secondly studied the eddying characteristics of the system and a third investigation was conducted to determine the affect of bathymetry and the shape of the coastlines on LC. The study domain ran from just above Perth (32° S) to just past Shark Bay (24 ° S) and between 108-116 ° east, with the eastern boundary within the domain being considered closed and the other boundaries open. The study consisted of two 500-day model runs, with the first omitting wind stress forcing and heat exchange across the air-water interface. The second simulation included wind stress to investigate seasonality, but neither model run did included tides, due to the fact that tidal excursion in the study area was small. There model depth was set between 15-1600m depth over 25 sigma layers, with increased resolution in the upper 800m to give a more detailed reproduction of the surface current. To get the solution running an initial simulation was internally forced using temperature and salinity data from (Levitus & Boyer 1994; Levitus et al. 1994). It was found from the first study that ROMS was able to reproduce the LC-LUC current systems mean flow and eddying characteristics at the correct spatial, temporal and migratory scales.
The main sites of eddy generation were near Shark Bay and the Abrolhos Islands,
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which agrees well with observations with (Fang & Morrow 2003). The other main result of this first simulation was the observation of a strong coupling between the LC and LUC, especially observable from the existence of the persistent anticlockwise eddy at depth, being maintained by the surface flow’s meandering nature. There was also a suspected link between the redirection of the Flinders Current which was maintained by easterly wind stress and the origin of the LUC (Meuleners et al. 2005a).
Meuleners et al. (2005b) was conduced primarily to observe eddying characteristics involved an initial 1000 day simulation over a slightly modified domain, with shorter, focused runs being performed at a finer temporal resolution where eddy generation was observed in the initial run. Strong mesoscale variability was observed in the currents annual signature which was predominantly generated by baroclinic instability of the frontal region along the current western edge, although near the shelf break topographic features shear instability was also significant. Topographical interaction with coastline features was unimportant due to the LC migrating west of the shelf break. It was subsequently concluded that there was no evidence of standing or transient eddies forming downstream of coastal features such as capes or ridges (Meuleners et al. 2005b).
The study by Meuleners was unable to address questions about the interannual variability of the current, in particular the influence of 1997 El Niño and 1998 La Niña on the current and the ability for ROMS to highlight this influence. This study expands upon the above work by running the simulation over a neutral year, a strong El Niño year, and strong La Niña year. In addition to this the forcing data from the OCCAM model, which will be described in section 4.
These findings about the relationship between ENSO, the ITF and LC current system will be a useful comparison for model results to try and quantify the link between ENSO and the LC.
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4 Methods
4.1 ROMS Model Description
As mentioned in section 3.6, ROMS is a fully 3D, hydrostatic, terrain following primitive equation model, solving equations for momentum, temperature, salinity and the equation of state. ROMS is the most recent development of the S-Coordinate Rutgers University Model (SCRUM) which was formulated from the original terrain following model SPEM (semi-spectral primitive equation model) (Haidvogel et al. 2000). An important feature of the model is the ability for the user control the distribution of the vertical layers to provide increased resolution where it is required, known as nonlinear stretching. Some of ROMS features include the masking of land and promontories and optional inclusion on multiple thermodynamic and passive tracers.
The ROMS code itself is written in Fortran 90/95 code and uses C-pre-processing to active user specified physical and numerical options. These options allow the user to choose between various advection schemes, pressure-gradient algorithms, turbulence closures and types of boundary conditions (Warner et al. 2005). There are also logical switches in the input file which allow the user to specify which variables to output into the solution file, such as 2D and 3D velocity, surface/bottom wind and wave stress in both the U and V directions and various heat fluxes. ROMS also incorporates alternatives for high order upstream-biased advection and subgridscale parameterisation (Haidvogel et al. 2000). The input and output data from ROMS is via the NetCDF format which allows for cross-platform compatibility, enabling the user to easily analyse and visualise output using packages such as MATLAB.
4.1.1 Primitive Equations
The basic hydrostatic primitive equations themselves used in the model are those governing the momentum balance in the x, y and z directions, the advective-diffusive equations, equations of state and the continuity equation for an incompressible fluid.
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These equations are shown below in Cartesian coordinates in Equation 4-1 through to Equation 4-8. The definitions of variables used in these equations are given in Table 4-1. Although within the model these equations are transformed from cartesian coordinates to the s (stretched) coordinate system in the z (vertical) direction and to curvilinear coordinates in the x and y (horizontal) directions (Rutgers IMCS Ocean Modelling Group 2006).
δu δφ ⎯+ ⎯→ fvu ++−=−⋅∇ DF δt v δx uu Equation 4-1 Momentum equation for the x direction δv δφ ⎯+ ⎯→ fuv ++−=−⋅∇ DF δt v δy vv
Equation 4-2 Momentum equation for the y direction δφ − ρg = δz ρ0 Equation 4-3 Momentum equation for the z direction δT ⎯+ ⎯→ +=⋅∇ DFT δt v TT Equation 4-4 The advective-diffusive equation for temperature δS ⎯+ ⎯→ +=⋅∇ DFS δt v SS Equation 4-5 The advective-diffusive equations for salinity ρ = ρ PST ),,(
Equation 4-6 The equation of state for temperature, salinity and pressure δu δv δw =++ 0 δx δy δz
Equation 4-7 The continuity equation for an incompressible fluid.
Table 4-1 Definitions of variables used in the primitive equations (Rutgers IMCS Ocean Modelling Group 2006)
, ,, FFFF STvu - Forcing terms g - Acceleration of gravity yxf ),( - Coriolis parameter
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, ,, DDDD STvu - Diffusive terms yxh ),( - Bottom depth ν ,κ - Horizontal viscosity and diffusivity
, , KKK STm - Vertical viscosity and diffusivity
P - Total Pressure ≈ −ρ0 gzP
φ tzyx ),,,( - Dynamic pressure φ = P /( ρ0 ) ρ + ρ tzyx ),,,( - Total in situ density tzyxS ),,,( - Salinity t - Time tzyxT ),,,( - Temperature
,, wvu - The zyx ),,( components of velocity vector ⎯⎯v→ x, y - Horizontal coordinates z - Vertical coordinate ζ tyx ),,( - The surface elevation
4.1.2 Transformed Co-ordinates
Horizontal Curvilinear Coordinates
In the horizontal direction ROMS uses orthogonal curvilinear coordinates on an “Arakawa C grid”. The reason for using these coordinates is to allow boundaries to follow irregular boundaries (such as coastlines) and to allow for variation in the grid resolution for small sections of the physical domain where there are processes of interest, such as boundary currents (Rutgers IMCS Ocean Modelling Group 2006). This type of coordinate system allows the model domain to represent the detail of the WA coastline at the grid scale.
The Cartesian Naiver Stokes equations of motion shown in above are transformed according to the relationship below. The coordinates then become ξ yx ),( and η yx ),( and the relationship between horizontal arc length differential distance is given by
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ξ = )/1()( dmds ξ and η = )/1()( dnds η (subject to scale factors m ξ η),( and n ξ η),( which relate differential distance to the actual arc length) (Haidvogel et al. 2000).
Stretched Vertical Coordinates
The general transformation applied in the vertical direction to is according to Equation 4-8 (below), where H(x,y) is defined as the resting thickness of the water column and ζ(x.y.z) is the instantaneous sea surface height. In addition to this the transformation applied to modify the distribution of the vertical layers to give increased resolution close to the surface is = s + − s ()( sChhhz ) where h(s) is a constant defined by the user representing the typical surface mixed layer depth. C(s) a
function of θ,s and θb, where the values of θb and θ determine the distribution of the vertical layers, larger values of θ result in increased resolution at the surface and as the value of θb approaches 1 the resolution close to the ocean floor is increased (Haidvogel et al. 2000). Vertical transformations of this kind allows for better representation of bottom topography and topographically driven ocean dynamics.
⎛ − ζ tyxz ),,( ⎞ = ss ⎜ ⎟ − ≤ s ≤ 01 ⎝ + ζ tyxyxH ),,(),( ⎠ Equation 4-8 The ROMS sigma coordinate transformation
4.1.3 Method of Solutions and Forcing
In the horizontal direction ROMS resolves solutions using a centred second-order finite difference approximation (an Arkawa “C” grid) (Meuleners et al. 2005a). In the vertical direction a centred second-order finite difference approximation over the staggered vertical grid (Rutgers IMCS Ocean Modelling Group 2006).
A source of error with a stretched terrain following vertical grid is the truncation error associated with the pressure gradient term, which increase with the steepness of topography. When the vertical grid surface is long aligned with isopryncals (constant density) or geopotential surfaces, the horizontal pressure gradient is split into along-
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coordinate term and a hydrostatic correction term, which tend to cancel (Warner et al. 2005). The cancellation of these terms is not exact due non-cancelling approximation errors in the two terms and this yields an associated pressure gradient error. However algorithms within ROMS are designed to minimise pressure gradient errors (Haidvogel et al. 2000; Rutgers IMCS Ocean Modelling Group 2006). As the slope of the continental shelf off WA is relatively gentle, these errors are not expected to significant in this thesis.
ROMS can be configured for various combinations of open or closed boundaries and if there are open boundaries, ROMS must be constantly provided with values of u and v velocity, temperature, salinity and surface elevation. These values are stored in a boundary file, in NetCDF format with each boundary being classified under a separate variable. Values of these same variables must also be supplied for ROMS to give the initial condition at time zero. This comes in the form of an initial condition file which provides values of the aforementioned variables for all points of the grid at time zero. Having the boundary and initial condition files in the NetCDF format allows for the use of a variety of forcing models, such as OCCAM, which is used in this study to provide the initial condition data and constant boundary input, derived from 5-day averages. ROMS must be supplied with a climatology data input file, which is used to generate the boundary file and initial condition file. In this case the climatology file is compiled from OCCAM data, but this may vary in other applications depending on what is being used for force ROMS. The other files needed by ROMS are a grid file, which contains the bathymetry and a 3D grid of the model domain and a forcing file, which contains the wind stress and tidal forcing data.
The information above provides a brief description the processes behind the ROMS model and how the solutions are determined, a full description of the model can be found in (Haidvogel et al. 2000) and (Rutgers IMCS Ocean Modelling Group 2006).
4.2 OCCAM Model Description
Forcing data for ROMS was obtained from the Ocean Circulation and Climate Advanced Modelling Project (OCCAM). OCCAM is a 66 layer model with
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resolutions of ¼°, 1/8° and 1/12 °. OCCAM is based on the Bryan-Cox-Semtner numerical model with the temperature and salinity fields originally being initialised using data from (Levitus et al. 1994; Levitus & Boyer 1994) datasets. This initialisation data was also used to force ROMS in (Meuleners et al. 2005a) because OCCAM was not available at the time. Hence by using OCCAM to force ROMS in this thesis, results will be comparable with (Meuleners et al. 2005a) because both OCCAM and Meuleners used the Levitus dataset.
OCCAM has been refined to include finer resolutions and changes were made to the online datasets available during the course of this thesis, however as the data used was only the ¼ ° resolution, the physical data did not change. A description of the current model run can be found in (Coward & de Cuevas 2005) and a description of the original model run in (Webb et al. 1998). The original model consisted of 36 vertical layers, with a varying thickness of 20m at the surface to 250 m near the ocean floor, a 1/4 ° resolution and 15 minute baroclinic timestep. The model also had realistic topography consistent with the ¼ ° resolution and interpolated wind forcing stress vectors (Saunders et al. 1999).
Experiments to verify the OCCAM model were conducted by (Saunders et al. 1999) and it was found that the model reproduced the large scale distribution of sea surface height and near-surface currents well, although their variance was underestimated. As OCCAM is the best available global ocean model and datasets were freely available for use it was a suitable choice for forcing data.
4.3 Visualising Data
Input/Output data were visualised using a MATLAB toolbox known as ODVT (oceanographic data visualisation tool). Using this toolbox both horizontal and vertical sections of the water column can be plotted. Within the horizontal sections, there is an option to plot either at a constant depth or along a sigma layer. The toolbox uses the NetCDF libraries to process the model output from ROMS and OCCAM solutions. This package can also be used to create movies of ROMS output which
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help to provide an overview of the behaviour of the model over the entire simulation. Movies of both sea surface height and absolute horizontal velocity were used to assess the behaviour of the system as a whole. This was done in addition to plotting relative variables, such as potential temperature and salinity at 10 day intervals throughout the simulations in order to analyse the behaviour of the system in detail.
4.4 Model Domain
In this study the model domain consisted of three open boundaries, on the study areas northern, southern and western extents. The eastern boundary adjacent to the coast was considered a closed boundary. The study area can be seen in Figure 9-2.
The chosen model domain has its western boundary focused along the coast of WA from just north of Barrow Island to just north of Perth. The grid itself follows the shape of the coastline and hence is not directly aligned in a north-south/east-west direction. The gird runs between 16.2 ° S and 31.2 ° S and between 108.2 ° W and 116.5 ° W although these are not the co-ordinates of the corners of the grid. Figure 9-2 shows land masking at the NW Cape, Barrow Island, Shark Bay and the Abrolhos Islands.
The reason for choosing this domain lies in the fact that this study is an extension of the work done in (Meuleners et al. 2005a). The domain chosen for this study not only encapsulates the area modelled in (Meuleners et al. 2005a) but also extends past the NW cape as far as Barrow Island, to include part of the NWS. The reason for extending the study area was to attempt to fully capture the influence ENSO on the LC, as ENSO has a strong signature observable in ITF waters, which feed the LC’s source water onto the NWS.
This domain also contains the three main locations where eddy shedding occurs (discussed in section 3.5). Although the primary aim of this study is to quantify the impacts of ENSO on the LC’s mean flow characteristics, including the preferential
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eddy shedding locations within the domain allows eddy generation dynamics to used as an additional comparison between Southern Oscillation events.
The grid has a variable resolution which is finest closest to the coastline, with a resolution of approximately 1.8 km near Shark Bay and 4km offshore. Bathymetry contours are shown in black and are underlain in all plots of ROMS output data at are at 100m increments. The bathymetry was supplied by Geosciences Australia.
4.5 Evaluating Run Timeframe
Three years were selected to perform model runs, namely; 1996, 1997 and 1998. The significance of these three years is that 1997 was a strong El Niño year, 1998 was a La Niña year and 1996 was a ‘neutral’ year, meaning that no significant Southern Oscillation events occurred as determined by the SOI (see section 3.3). By comparing ROMS output from these three years a clear picture of the behaviour in each of the three possible interannual scenarios can be formulated. Although the behaviour of the current in these three years will no mimic all similar events, i.e. the current behaviour seen in the 1997 El Niño will not be the same as all other El Niño’s, it was possible to compare the forcing characteristics of the Southern Oscillation events on the flow dynamics along the W.A coastline. A reason for the choice of 1996 as the neutral year was the availability of satellite data from CSIRO which can be used to verify ROMS output.
For each of the years the modelled solution was run for 166 days between the 11th July and 23rd December. Because ROMS is such a computationally intensive model, running the model for an entire year was not practical, so a smaller period had to be selected. This was done using a combination of Satellite Altimetry, OCCAM data and information from section 3.1, including the Southern Oscillation composite images that are included in Appendix 1 (section 10). The observations that led to this period being selected are covered below in section 5, however the main reasons were because of the timing of the temperature anomalies in 1997 and 1998 are expected to cause a deviation in the strength of the geopotential gradient along the WA coast. The
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El Niño ITF set-down occurred in August/September/October 1997 and the La Niña ITF set-up occurred in November/December/January 1998/1999. Hence to see the effect of both phases of the ENSO on the LC, the simulation needed to encompass these months. The timing of these different phases meant that the effect on the LC could not be compared directly between El Niño and La Niña years. Hence a neutral year was used to compare to the El Niño peak in August/September/October and also to the La Niña peak in November/December/January.
4.6 Satellite Imagery
Global satellite altimetry running from January 1996- October 1999 was used to discern the peak period of the 1997 El Niño and 1998 La Niña events. The imagery studied consisted of sea level anomaly data from TOPEX/POSEIDON (T/P) and ERS 1/2 and sea surface temperature anomaly data from AVHRR PATHFINDER (California Institute of Technology: Jet Propulsion Laboratory 2006). This altimetry was used to determine the magnitude and timing of Pacific and ITF temperature and sea surface height anomalies associated with the Southern Oscillation events. The two regions that were of interest are circled in Figure 9-4.
The sea surface temperature/height anomalies are generated using the following formula: SSH/SST anomaly = SSH/SST – long term SSH/SST average. It is expected that a positive temperature anomaly over the ITF would increase the pooling of warm water on the NWS and subsequently increasing the geopotential gradient along the coast of WA and vice versa with a negative anomaly.
4.7 OCCAM Validation: Southern Oscillation
A key element of the OCCAM data that needed to be verified was the replication of the El Niño and La Niña sea surface temperature signatures, in particular over the eastern Pacific Ocean and over the ITF. In order to do these OCCAM monthly averages of potential temperature were collected for the years 1996, 1997 and 1998.
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Because of data sizes, there were two separate sets of data collected for each year, with one focused on the eastern Pacific Ocean (between 30 ° N and 30 ° S latitude and 180 ° E and 90 ° W longitude) and one focused on the ITF(between 30 ° N and 30 ° S latitude and 90 ° E and 180 ° E longitude).
The potential temperature images were plotted using ODVT and comparisons were made between the three years to ascertain when the 1997-1998 Southern Oscillation peaks occurred and to determine when to best perform the ROMS simulations. The results of these comparisons are described in section 5.2.
4.8 OCCAM’s Reproduction of the Leeuwin Current Signature
In addition to verifying that the El Niño/La Niña signatures are reproduced by OCCAM, it had to be verified that OCCAM was reproducing the LC signature in order to validate OCCAM as a suitable input for ROMS. This involved collecting both the 5 day average and monthly average OCCAM datasets (between 14 ° S and 32 ° S latitude and between 107 ° W and 118 ° E longitude) for the entire years 1996, 1997 and 1998. This region encompasses the entire study area as well as the area around the boundaries, which is important as OCCAM provides constant input data at the boundaries through the entire model run. Consequently it is important that the dynamics of OCCAM provide a reasonable representation of the LC at a coarse scale and that the main features of the LC are observable in the input data for ROMS.
In both the 5 day average and monthly average datasets there were a number of variables that were plotted, these were: potential temperature, salinity, baroclinic v velocity (N/S velocity), baroclinic u velocity (E/W velocity), sea surface height and absolute baroclinic velocity in the surface layer. Emphasis was on the comparison of potential temperature and salinity, chiefly because these was being used to force ROMS and secondly as they were the most easily comparable. The other variables such as sea surface height and absolute velocity were used to verify that OCCAM was reproducing LC characteristics.
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All months of the year were analysed to determine if the model reproduced key current properties such as a southward intensification and its peaks in May/June and November. It was felt that it was more important to verify that OCCAM could reproduce LC current behaviour over the entire year rather than just focusing on the model timeframe. The main variables that were analysed were the temperature and salinity, as these can best highlight the LC signature, due to its meandering nature which is not always obvious in u (E/W) and v (N/S) velocity plots. Analysis of the velocity data was predominantly focused on N/S velocity, as this can best highlight the southern propagation of the LC. A very effective way of tracking the actual path of the LC was to plot the baroclinic vectors velocity in the surface layer. This is derived from the magnitude and direction of both the baroclinic u and v velocity and displays velocity vectors superimposed over potential temperature. Baroclinic velocity vectors whilst clearly showing the path of the LC, did not provide a good basis for comparison due to the fact that spatial variability of the LC velocity is very high and the current meanders and eddies are random and also highly variable. Finally sea surface height data was analysed briefly to determine the magnitude of the steric height gradient along the WA coast produced by OCCAM.
Although eddy generation and propagation is not of paramount importance to this thesis, it was important that it be verified OCCAM was displaying the meandering and eddy shedding behaviour associated with the LC signature. Consequently sea surface height and baroclinic velocity/potential temperature data were compared where eddies were observed.
4.9 Model Setup
ROMS was run on COGNAC, a 168 processor SGI Altix 2700 Bx2 high performance computer, located at the iVEC’s facility at the Australian Resources Research Centre in Technology Park, WA. The ROMS source code was compiled using a FORTRAN compiler on COGNAC. Depending on the system usage the simulation was performed using either 4 or 16 processors, with 8 and 32 gigabytes of RAM respectively.
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Three 166 day simulations were performed on COGNAC, with the model was set to write output data for each day of the simulation into the output file. The physical setup of the model itself involved compiling a climatology file, which was used to generate a boundary file and initial condition file. This climatology file was generated for each of the years from a set of 5 day snapshots of the OCCAM solution. The forcing file contained wind forcing data that was obtained from the OCCAM solution and interpolated onto the model grid.
Internally the ROMS was setup the same as in (Meuleners et al. 2005a) use a third- order upstream bias, G-Scheme for advection of momentum and tracers (Kantha & Clayson 2000). (Song & Wright 1998) weighted Jacobian pressure gradient scheme was used to evaluate the baroclinic hydrostatic pressure gradient term. The horizontal diffusion for active tracers utilised a Laplacian scheme for the mixing coefficient, which is for the largest cell in the domain. The mixing coefficients for momentum and tracers along geopotential surfaces were then scaled by the grid size. The parameterization of vertical turbulent mixing for momentum and tracers is based on the first-order vertical closure turbulent model (K profile parameterisation – KPP) (Large et al. 1994) modified to represent the surface and boundary layer. The horizontal viscosity mixing scheme used a Laplacian form suggested in (Haidvogel et al. 2000). An active comparison of this Laplacian form and an alternative Biharmonic scheme is detailed in (Meuleners et al. 2005a), which suggests the Laplacian form is best suited to this particular application.
The following fields were written into the output files: potential temperature, sea surface height, salinity, turbulent kinetic energy and u and v velocity. The number of sigma layers in the vertical direction was set to 30 and the stretching parameters were set so there was increased vertical resolution in the upper and lower layers (Figure 9-3). A summary of model parameters can be seen below in Table 4-2.
Table 4-2 ROMS model parameters ∆x, M Nominal 4km, 120 Resolution, number of grid points (zonal direction) ∆y, L Nominal 4km, 300 Resolution, number of grid points (meridional direction) N 30 Number of sigma coordinate levels
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Θs 3 Sigma coordinate stretching factor
Θb 0.1 Sigma coordinate bottom stretching factor
∆t(baroclinic) 200 s Baroclinic time step
∆t(barotropic) 40s Barotropic time step vv 5.5 m2s-1 Horizontal viscosity coefficient vd 5.5 m2s-1 Horizontal diffusivity coefficient r 2.5 x 10-3 Quadratic bottom drag coefficient
4.10 Model Run
The run time of each simulation was approximately 26 hours using 16 processors and 44 hours using 4 processors. An initial run was performed using a baroclinic timestep of 200 seconds with a barotropic timestep of 40 seconds. The selection of this timestep needed to be small enough to accurately resolve dynamic processes such as the propagation of tidal fronts. If the time step is too large then processes such as waves cannot be resolved, for example if wave is moving with a speed of 5m/s it will take 360 seconds to travel across a cell of length 1.8 km, which means if the time step is greater then 360 seconds then this wave will not be resolved by the model. Hence with a minimum grid scale of 1.8 km 200 seconds is adequate to resolve all necessary features. The Courant number is a good indication the effect of the chosen timestep on the stability of the solution. The Courant number is a ratio of time step to cell residence time, and is a function of grid size and resolution, time step and fluid velocity. In this application the Courant number is a function of the barotropic timestep and the maximum and minimum values of this are automatically calculated by ROMS. In general the solution will be stable with a courant number less then 1 for an implicit solution. The maximum Courant number was found to be 0.42, hence the model results are likely to be stable.
There were initial run was performed only for the neutral and El Niño years, the aim being to validate the model output first and then to complete the La Niña run. However there were problems with this initial simulation, due to large surface oscillations which are described in detail in section 5.4.1, and consequently the La Niña run was not completed. This problem was initially suspected of being a time step issue, caused by tidal fronts skipping grid cells because the barotropic time step was
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too large, despite the courant number being within acceptable range. Subsequent partial simulations were run with the following timesteps: - 130 s (baroclinic) 26 s (barotropic) - 100 s (baroclinic) 20 s (barotropic) - 50 s (baroclinic) 10 s (barotropic) However this appeared to have little effect on the oscillations and it was concluded that they were a numerical issue.
After it was determined that the oscillations in the run with tides could not be eliminated within the project timeframe, the ROMS source code was recompiled to run exactly the same except without tides. It was expected that without the large variation in tidal amplitude across the model domain that the simulation would be more stable. The tidal aspect of the model was included for completeness only and removing tides should have no net effect on the aim of the study.
The second simulation without tides was run to completion and initial analysis of the results showed large velocity fields that were propagating through the western boundary, being reflected off the southern boundary and suppressing the LC, hence dominating the dynamics of the system. Therefore in an attempt to correct this problem the baroclinic velocity along the western boundary was turned off, as it was deemed that this should not affect the reproduction of the LC or the aims of the study. This was thought to be a better option then turning off the baroclinic velocity on the southern boundary as this would have removed the forcing of the LUC and not addressed in the large velocity flux entering the system from the west. Temperature and salinity forcing were left unaltered.
The third and final simulation was run with no tides and no baroclinic velocity on the western boundary. Instead an internal boundary condition was applied with linear nudging to define the baroclinic velocity at the western boundary. This simulation ran to completion with no major areas of instability and was validated to produce the mean flow characteristics of the LC. Hence this simulation was run for the three years and subsequently analysed in depth.
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4.11 Model Analysis
As there was a large amount of data to analyse, the temperature and salinity data were the main variables considered as they are the best indicators of presence of the LC.
The v velocity and sea surface height were used mainly to verify and investigate observations made from the temperature and salinity data. Whilst ODVT has the facility to plot both the barotropic and baroclinic velocity, these were very memory intensive for the size of the domain and only represented by vectors and not quantities, like the v velocity plots, therefore these were not used in the analysis.
After initially viewing the ROMS model output, it was decided to divide the area up into four sections of latitude (Figure 9-5). These zones were chosen as areas that had similar dynamics and temperature/salinity throughout the duration of the simulation. The 1st region in between 16-22 ° S and encompasses the NWS, the 2nd region is between 22-24 ° S and includes the NW Cape Ningaloo Reef. The 3rd region runs from 24-28 ° S and is centred at Shark Bay and the 4th region runs from 28-32 ° S and contains the Abrolhos Islands, one of the preferential eddy shedding locations.
For each significant monthly period including June (initial condition), August/September/October (El Niño peak) and November/December (La Niña peak) a table of averaged temperature and salinity values for each of the regions is provided.
The values in this table reflect the average temperature or salinity of the LC signature only within these regions, not the average temperature of the entire regions themselves. As there is no distinct LC signature on the NWS, values for region 1 are the average temperature/salinity of the water on the NWS itself, as this is these are the source waters for the LC.
This table contains the average values at both the start and end of the particular period (with the exception of July) so that it is clear how much each years temperature and salinity values have changed over each period. The start and end values are estimated using the first two and last two plots of each period respectively to try and eliminate
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any short lived temperature and salinity changes from the results. Also included in the table is the temperature difference of each region and the total temperature difference across the domain, as the sum of these differences to gain an overall picture of how the LC temperature/salinity changes with each time period. An example of the table format can be seen below (Table 4-3).
Table 4-3 The format of the table used for comparison of temperature and salinity results across the domain Monthly period: August/September/October Average temperature Average temperature at start of period ( ° C) at end of period ( ° C) Difference Year 1996 1996 1996 Region 1:NWS 25.5 25 -0.5 Region 2: Ningaloo 24.5 23.5 -1 Region 3: Shark Bay 24 23 -1 Region 4:Abrolhos Islands 23 22 -1 Total Change -3.5
In addition to the tables, the behaviour of the system as a whole will be described in each of the three years and then contrasted. This will include the main features and LC flow characteristics, however will be more focused on the bigger picture, rather specifics details of mesoscale features. This description will be supported with plots of temperature, salinity and in some cases v velocity.
In order to briefly establish whether the LC-LUC relationship was being reproduced in the ROMS solution, there were several vertical sections of the v velocity plotted along transects at 24, 27 and 30 ° S, which was in the region where the LC-LUC relationship was observed in OCCAM data.
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4.12 Verification
The model results from 1996 were compared with SST and SSTA satellite imagery, with superimposed geostrophic velocity fields, as this was an easily accessible source with detailed images of the time period provided by David Griffin at CSRIO Marine and Atmospheric Research (Griffin 2001). However this imagery only extended as far north as 22 ° S (just past the NW Cape). This imagery was used to identify the LC System’s key features and mean temperature fields rather then an absolute comparison.
5 Results
5.1 Satellite Imagery
5.1.1 AVHRR PATHFINDER
To determine the difference in temporal variability in the source water of the LC, satellite altimetry of SSH/SST anomalies occurring near the ITF and Western Australia will be described. In addition those related to the El Niño/La Niña Pacific signature will also be included. The reason for this is that the altimetry covers the entire globe and there are countless anomalies that may occur throughout a year but are mostly insignificant. Therefore only those anomalies that are near the region of interest will be considered as others are irrelevant with respect to the LC. SSH/SST anomalies in equatorial Pacific region are significant as they help to identify the ENSO state, and relate it back to the patterns shown in Appendix 1 (section 10).
The El Niño/La Niña signatures are less pronounced in AVHRR PATHFINDER sea surface temperature anomaly imaging, however observations from this data are more important then T/P & ERS ½ observations because ROMS is driven by temperature data and not sea surface height.
1996
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There is medium amplitude ( < -2 ° C ) cold anomaly in March over the ITF and NWS appearing in January, peaking in February and dissipating in early March. There is also a small amplitude cold ( < -1.25 ° C ) anomaly encapsulating the entire coast of WA along with the ITF exit passages beginning in May which recedes down the WA coast past Cape Leeuwin in August/September. By December the anomaly has moved into the Great Australian Bight. The southward propagation of this anomaly is most likely a result of this colder water becoming entrained in the LC.
There is also a small warm anomaly (< 1 ° C ) around the ITF exit passages and along the coast of WA down to Cape Leeuwin through October/November/December, which is similar to the typical ITF La Niña signature, as seen in Figure 9-7. It is suspected that this anomaly may have a positive effect on the strength of the LC, resulting from the higher temperature water being discharged onto the NWS.
A large cold anomaly with a central magnitude of -2.5 ° C is observable off the upper west coast of South America between July and February, which propagates west slightly before disappearing. This is of a similar pattern to the La Niña signature however it is much smaller then the observed La Niña in 1998 and more localised..
1997
There is a significant (~ -1.5 - -2.25 ° C) cold anomaly over the entire ITF from January through to March, which extends its influence along the WA coast through May-June. The anomaly then expands again to encompass the entire ITF and WA coast and intensifies, peaking in August and dissipating by the end of October (Figure 9-6). This is identifiable as the Indonesian El Niño signature. The pattern first appears in March over the central Pacific, intensifies and expands eastwards through June/July, reaching a sustained peak of magnitude 2.5 ° C through August-December.
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1998
A significant cold anomaly ( < -2 °C ) appears over upper half of WA and most of the ITF through January and February, which is then replaced by a significant warm anomaly ( ~ 0.5 – 2 ° C) propagating from west of the throughflow exit passages, which lasts from March until then end of May. The area remains relatively neutral until October when another significant amplitude (< 2 ° C) warm anomaly appears over the ITF and NWS, with its focus over the exit passages of the ITF. This anomaly disappears by the start of December and can be identified as the Indonesian La Niña signature. It also coincides with the La Niña signature over the Pacific which appears in July and peaks over November/December/January having a maximum amplitude of -2.5 ° C (Figure 9-7).
5.1.2 TOPEX/POSIDEN ERS ½
1996
There is a medium amplitude (~ 6 cm) positive anomaly over the Pacific end of the ITF which is first observable in September increasing noticeably in size and slightly in intensity through October-November (Figure 9-7). However the anomaly does not expand south of Timor and hence maintains a significant distance from the source region of the LC. This anomaly recedes through November and December, travelling east as it decreases in size to form the start of the 1997 El Niño pacific signature.
There is also a small amplitude (< 4 cm), positive anomaly running along the coast of WA from just above the NW Cape to down to Cape Leeuwin which appears in November and disappears in January, however it does not propagate far from the coast and stays separate from the lager anomaly to the north.
1997
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A significant (magnitude > 6cm), positive sea level anomaly focused over the central Pacific ocean is observable in January. This is the start of the El Niño signature, which proceeds to increase in size and intensity through February and March. This anomaly beings to propagate east through April and May and expands to the coast of Central and South America. This continues to intensify and grow in size, reaching a sustained peak of magnitude > 14 cm in September/October until it begins to recede in late November (Figure 9-6). It retains its peak intensity in some areas until April 1998 but eventually recedes completely.
There is also a negative anomaly along the coast of WA of magnitude ~ -10cm between March and May, which coincides with the development of the El Niño’s Indonesian signature, which is observable over the entry passages to the ITF with a central magnitude of ~ - 18cm. This signature develops in size through June and July to encapsulate the entire ITF region, NWS and coast of WA. It peaks in September/October and beings to recede in November (Figure 9-6).
1998
This negative anomaly in the ITF region continues to recede through to May and beings to propagate east in June and July to form the Pacific La Niña signature. This Pacific signature has a sustained peak in September/October/November and recedes in December, having maximum amplitude of -18 cm (Figure 9-7).
A positive anomaly propagating SW reaches the ITF in September and intensifies through to its peak in November of magnitude ~ 14cm, by then having expanded to cover the entire ITF region and its surrounds, the NWS and WA coast. It recedes significantly by late December.
5.1.3 Summary of Observations
The satellite imagery shows that there are several anomalies in 1996, however both are of comparatively small amplitude to the El Niño/La Niña years and but disqualify 1996 slightly, as a neutral year, instead suggesting a weak La Niña pattern. However
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the anomalies in 1996 sea surface temperature data are of a similar pattern to a La Niña event, but the sea surface height data does not always correlated to temperature during these anomalies. In the El Niño and La Niña events the sea surface height and sea surface temperature data correlate well, hence these temperature anomalies in 1996 should not be classified as a true Southern Oscillation event on this basis. However as 1996 has a neutral to weak La Niña pattern it is expected that this may be associated with a slightly larger geopotential gradient then normal, although not as large as the expected gradient in a La Niña year.
The 1997 El Niño is pronounced in both sea surface temperature and height and has a sustained peak through August, September and October. The lower sea surface temperatures over then ITF through this period are expected to have a substantial effect of the strength of the LC (Figure 9-6).
The 1998 La Niña is not as large in magnitude as the 1997 El Niño and has a strong sea surface height signature and a less pronounced sea surface temperature signature (Figure 9-7). The peak of this La Niña can be seen to occur in November, December and January.
5.2 OCCAM Validation: outhern Oscillation
OCCAM plots of the Pacific Ocean clearly show a El Niño signature in 1997 and a La Niña signature in 1998. Temperatures in the 1996 dataset are in between the 1997 and 1998 values for most of the year, although they are closer to 1998 values, sometimes exceeding them. This suggests that 1996 will display weak to significant La Nina characteristics.
In line with the observations from the Satellite Altimetry described in section 5.1, monthly average plots of the four months in which the ENSO events are strongest are shown, to highlight the reproduction of the El Niño/La Niña signatures in OCCAM solution. The months shown are; September and October, in which the strongest phase
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of the El Niño event of 1997 occurs and November and December, during which the 1998 La Niña peaks.
Whilst all months of three years were compared, the greatest reproduction of the Southern Oscillation signatures were observed in these four months and hence only these four months will be described in detailed. Images supporting the observations are provided in Appendix 2 (section 11).
5.2.1 Pacific Signature
The El Niño signature is observable in all four months and is distinctly warmer then both 1996 and 1998 in Figure 11-1,Figure 11-3,Figure 11-5 and Figure 11-7. The neural and La Niña year are characterised by a similar temperature pattern in the Pacific, with two wide bands extending to approximately 20 ° N and S respectively with temperature greater then ~25 ° C and a narrow band of cooler water (~ 22 ° C ) along the equator. However the El Niño year is characterised by a wide area with a temperature greaten then 25 ° C between 20 ° N and 20 ° S latitude, with its peak along the equator.
These model responses relate well to the Southern Oscillation composite images (Figure 10-3, Figure 10-4). These images show the El Niño to be strongest along the equator and to extend approximately 15-20 ° on either side of the equator. The magnitude of the El Niño signature does not change markedly over the four month period, which is consistent with observations made in section 5.1 as the El Niño does not start to recede until the end of December. Hence OCCAM is reproducing the El Niño signature as observed by satellite altimetry. This is also consistent with the images in Appendix 1, showing that El Niño typically has a sustained peak of uniform temperature from August through to January.
The reproduction of the La Niña is not as easy compare, as the OCCAM images are potential temperature and the Southern Oscillation images are temperature anomaly. However the La Niña peak signature shown in Appendix 1 is centred along the
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equator, extending again approximately 15-20 ° on either side. The La Niña signature shown by OCCAM data is also centred along the equator but is warmer then the neutral year, but a possible explanation for this is given below.
The difference in magnitude between the Neutral and La Niña years is only small and analogous to expectations, as 1998 appears to be slightly warmer then 1996 in all four months. However this could be explained by the cold Pacific anomaly described in section 5.1.1. Hence whilst observations were not as expected, they are still consistent with observations from satellite data.
5.2.2 Indonesian Signature
In all four months, the El Niño year is significantly colder then the other two years, which is consistent with expectations. The La Niña signature is also more evident then in the Pacific data, as 1998 is noticeably warmer then 1996 in all months except October. 1996 shows slightly warmer temperatures in October then 1998, although this may be the result of the small warm anomaly observed in satellite imagery covered in section 5.1.1. The ENSO composite images (Figure 10-3, Figure 10-4) show the El Niño and La Niña Indonesian signatures to be strongest in August/September/October/November, which is consistent with this OCCAM result.
Over the four months the warmest mass of water was observed to propagate south across the ITF and began to pool out of the exit passages of the ITF and onto the NWS, this was observable in all years although most prevalent in 1998. In all cases by December the temperature at the NW Cape is at least 5 ° C warmer then in September. This may be a result of warming average temperatures due to the transition into summer and more of a seasonal pattern then an ENSO pattern.
5.3 OCCAM’S Reproduction of the Leeuwin Current Signature
Plots of potential temperature in the surface layer clearly showed a LC signature in all three years and plots of absolute baroclinic velocity in the surface layer clearly show the path of LC with distinct meanders and eddies. Baroclinic velocity vectors were
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automatically calculated from the u and v velocity using the ODVT toolbox and superimposed over potential temperature, although these velocity vectors were not used as a comparison as they are not as quantitative as temperature. These plots also showed significant variability between the years as expected. It was found that the model output agreed well with LC properties described in section 3.1, such as its peak in May/June and its intensification as it flows south as well as its general width (< 100km).
The following summarises the observable LC current behaviour in OCCAM data, which is best illustrated through the use of the neutral year. Potential temperature images of 1996 show that in January there is a warm body of water over the NWS ( ~ 27 ° C) which is beginning to extend around the NW Cape, however the is no significant LC signature observable. By February the warm body of water on the shelf has contributed to the generation of the LC signature which has propagated as far as 29 ° S. Also observable in February an inflow of even warmer water (~ 29 ° C) onto the NWS from the northeast. This LC signature intensifies in March and April in combination with the southern movement of the warmer NWS water although the signature does not propagate any further south. May sees the a cooling of the NWS waters but a continued intensification of the current and propagation to past 30 ° S. The temperature of the current begins to reduce in June and July along the continued cooling of the NWS source waters. After this the temperature of the current remains fairly consistent until November when the current signature begins to dissipate and is no longer distinct in December. Also occurring in November is a flux of warmer water onto the NWS from outside of the domain to the northeast, which continues during December.
The baroclinic velocity shows a meandering LC signature throughout all months of the year, although it is least prevalent in January as expected, but the intensity of the LC in other months is difficult to compare due to the varying current paths. The meandering/eddying nature of the LC is also highlighted very well by the baroclinic velocity, an example of which can be seen in Figure 9-8. There was a large number of observable eddies in the OCCAM data and the common warm and cold core dipole relationship was often observable and can be seen in the potential
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temperature/baroclinic velocity plot of May 1996 in Figure 9-8 and is further highlighted the corresponding plot of sea surface height in Figure 9-11. There were too many other eddies observed within the data to analyse in detail however they were observed to originate predominantly in the three locations detailed in section 3.5, namely off the NW Cape, Shark Bay and the Abrolhos Islands.
Now that the behaviour of the current in a neutral year has been elaborated, a comparison with 1997 and 1998 can be formed. Monthly average plots of May for each of the three years show clearly the LC temperature signature at its peak and also highlight the currents meandering nature (Figure 9-8, Figure 9-9, Figure 9-10). These three plots highlight the nature of the variability between the three years, whilst 1996 and 1997 are similar, the neutral year exhibits a wider, slightly warmer signature. The La Niña year is distinctly stronger then the other two years, with the most noticeable difference being the increased temperature in the NWS source waters, as well as a stronger general temperature signature.
The general trend in LC strength between the three years of OCCAM data is that 1998 is stronger then normal and 1997 is weaker then normal, assuming that 1996 is considered normal. This was observed in April, May, June, July, September, October and December, although in August 1996 and 1998 were both of similar strength.
In both January and February the signature was strongest in 1997 and weakest in 1998. However it is expected that the current will be weak for the first few months of 1998 due to the persistence of the 1997 El Nine, but the fact that 1997 is warmer then 1996 is analogous to expectations. March shows a similar strength LC signature in the neutral and El Niño year with a slightly stronger La Niña year. In November the signature extends furthest south in 1996, although 1998 is slightly warmer and 1997 is still weakest.
The salinity data from OCCAM was also plotted and analysed and it was found that it showed the same trends as potential temperature data, although due to the low variability of salinity (< 3 PSU across the entire domain) it was harder to compare. The salinity on the NWS was distinctly lower then any other region within the domain
42 - The Interannual Variability of The Leeuwin Current Ryan Warrington and this low salinity water was observed to propagate down the coast as a typical LC signature. The timing and magnitude the LC salinity signature was the same as the LC temperature signature. As with the temperature the general trend was that 1998 displayed the strongest signature, although there was not a great deal of difference between 1996 and 1998. 1997 was observed to exhibit the weakest signature. This was again subject to inconsistency in January and February and March, probably due to the El Niño persistence in early 1998. This was one of the reasons for electing to run ROMS from July-November, with the major reason being related to observations from satellite data.
The u(E/W) and v (N/S) velocity components showed that the representation of the LC was evident in the horizontal profile (Figure 9-12) and in the vertical profile of the water column, south of 22 ° S extending to a depth of 200m . When the LC was observable, the signature was evident south of 24 ° S which is consistent with the southward intensification of the LC described in section 3.1. The magnitude of the LC current core was found to vary between the three years, and was generally strongest in 1996 and weakest in 1997, having a maximum velocity in localised areas of approximately 1 m/s in 1996, 0.7 m/s in 1997 and 0.9 m/s in 1998, in monthly average plots with the maximum velocity occurring in July in each of the three years (Figure 9-12).
The LU-LUC relationship was usually observable in the vertical velocity profile, as can be seen from Figure 9-13. Plots of vertical sections of different latitudes showed that the LUC was consistently observable at 24-32 ° S, with its core located at approximately 400m depth and a maximum velocity of 0.2 m/s. The LUC was observable throughout the entire year although did not have a discernable seasonal peak, although did become noticeably weaker in August in each of the three years. As with the surface current there were large areas with velocities often greater the LUC that dominated the dynamics of the system, observable in many of the plots. Sea surface height data shows both the LC signature and the steric height gradient along the WA coast driving the flow of the LC (although this is not the complete region over which the gradient exists.) The water on the NWS is constantly the highest water level across the entire domain for the entire year, although the
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magnitude does vary in conjunction with the variation of the temperature profile as described above.
It is clear that from the above data that OCCAM is reproducing the key characteristics of the LC required to drive ROMS. In addition observations made on the interannual variability suggest that the best period for comparison would be later in the year to overcome the persistence of the 1997 El Niño in early 1998.
5.4 ROMS Model Results
5.4.1 Run 1: Tides
The initial run of using a timestep of 200 seconds and a baroclinic timestep of 30 seconds ran to completion with no major stability issues. However there were issues with the results in the form numerical oscillation of the free surface, which amplified in magnitude and spread across the domain quickly. This oscillation was initially suspected of being a time-stepping issue caused by tidal fronts skipping grid cells in between time steps. However experimenting with different time steps had no bearing on the oscillations and hence this theory was discredited. These oscillations did not display wave like behaviour so were most likely a numerical issue.
The problem observable after simulation day 1 was an oscillation in sea surface heights which had an initial amplitude of 0.25m and quickly amplified as time went on. These oscillations had an immediate influence on the u and v velocity in the upper sigma layers. There oscillations on simulation day 2 can be seen in Figure 9-14 and the amplification of these oscillations by simulation day 40 are shown in Figure 9-15. It is clear from these two plots that these oscillations are quickly amplified and values become unrealistic, as the range of sea surface height by day 40 is between -8 and 2 m (Figure 9-15), with the oscillations themselves having a maximum amplitude of ~ 3.5 m. Plots of the associated u (E/W) and v (N/S) velocities in the surface layer show clearly that these oscillations in sea surface height are driving the momentum of the nearby system, which is also amplified as the oscillations get larger. Overall this was generating both sea surface height and velocity values that were not realistic. Plots of
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the potential temperature and salinity of the system showed a more reasonable output, but the only way in which they could be compared was through plotting of layers rather then at specific horizontal depths, due to the fact that the sea surface was at -8 m in some areas and this produced areas on horizontal plots where no values exited.
5.4.2 Run 2: No Tides
It was found that overall as a result of switching off the tides that the oscillations in previous results were completely eliminated and the model had become much more stable. Also more realistic values of sea surface height and baroclinic velocity had been generated. Sea surface height was found to not exceed +- 2 m and velocity found to stay under 2 m/s for the majority of the simulation, which are within the realms of believability.
Having rid of the oscillation problem a new challenge presented itself, which was the manifested from the OCCAM generated boundary conditions. Throughout the simulations there was a large momentum flux into the system in two locations along the western boundary and travelling initially in a south-westerly direction. This momentum was unable to propagate out through the southern boundary, due to the predefined boundary conditions. Hence as this system could not remove this momentum through the boundaries it was contained within the system and being redirected northwards, closer to the continental shelf. This was creating a net northward moment of the water column over the continental shelf that was dominating the regional dynamics and overpowering the LC, although a slight LC signature was still observable (Figure 9-16). A key reason for questioning the validity of the momentum output of this simulation was a continual increase of the volume average kinetic energy of the system.
The origin of this western inflow was unknown as an analysis of OCCAM wind fields showed there was not a prevailing westerly component and that the magnitude and direction had high temporal variability. Reviewing the baroclinic velocity in OCCAM results showed a jet like stream forming offshore of the continentally shelf and
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propagating eastwards which may be responsible for this western inflow into the ROMS domain. An analysis of the ROMS boundary file also indicated the overall momentum into the system on the western boundary exceeded the momentum out of the system, which was likely why the kinetic energy of the system was increasing over time. Satellite imagery shows that there is a western inflow that propagates onto the continental shelf around Shark Bay, but is entrained into the LC and not redirected north (Figure 9-18).
5.4.3 Run 3: No Western Boundary or Tides
In order to counteract the large velocity flux into the system from the western boundary, baroclinic velocity forcing on this boundary was turned off. The baroclinic velocity was instead defined by an internal boundary condition, meaning that the momentum at the western boundary was defined by the cells adjacent to the boundary inside the domain, rather then being externally forced. All of the plots of this simulation that have been included in this paper can be found in Appendix 3 (section 12).
5.4.3.1 Validation
Firstly it had to be verified that the simulation was displaying LC current characteristics correctly before the three years could be compared. This was done through the use of a neutral year, as in the previous cases.
It was found that there was generally a general southern flow of warm, lower salinity water along the continental shelf, generally following the 2 and 3 km bathymetry contours. This flow was found to intensify with distance south, especially south of the Abrolhos Islands. This flow was identified as the LC signature and was observed to form meanders and induce eddies on the seaward side. The LC had a width of between 25 and 200km in regions 3 and 4 where it was most distinct, but in region 2 was often observed as a wide band of water rather then a single current core. As these
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features produced by the simulation coincide with observed LC features the solutions from this simulation were analysed in more detail.
A comparison with satellite imagery showed that the solution displayed similar temperature patterns and mesoscale features as observed in interpolated remotely sensed SST data. An important observation from the satellite imagery which supported model results was the flow of warm water onto the continental shelf around Shark Bay, which then was entrained in the LC (Figure 9-18). The similarity between the model results and satellite imagery is highlighted by temperature distribution in Figure 12-1.
An unexpected phenomenon which was frequently observable through the three simulations was a strong (~ 1.5 m/s on average) southerly velocity field propagating from the western boundary at the top of region 1. This velocity field did not appear to have a widespread influence on the domain as it was confined to within 100km of the boundary in all except one location, where it encroached further the domain and travelled parallel to the bathymetry (Figure 12-9). This velocity field was found to extend approximately 500 m into the water column.
5.4.3.2 Temperature
Potential temperature plots were plotted every 10 days, starting on the 11th of July. The LC signature was observable along the WA coast and was predominantly confined to the continental shelf along the 2km bathymetry contour, although it did meander several hundred km offshore in some locations. The strength of the current weakened as the year progressed which is consistent LC properties. Comparing the three years based on 10 day increments between plots was considered a better option then monthly averages as this can be used to identify specific features such as meanders and eddies developing over time which would not be produced as well by monthly averages. Supporting plots for the second and second last snapshot of each period are provided in Appendix 3 (section 12).
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July
Both 1996 and 1997 show a wide meandering LC which generally follows the 2 and 3 km bathymetry contours, with an average width of around 200km (Figure 12-2). The LC begins to narrow to 100km south of the Abrolhos Islands in region 4, which is highlighted by the v velocity profiles (Figure 12-3). The temperatures of the LC were fairly similar in each of the four regions and can be seen in Table 5-1. The biggest difference between these two regions was on the NWS.
The 1998 LC showed a very strong signature between 50-100 km width, following the 2km bathymetry contour down from the NW cape in regions 2, 3 and 4 (Figure 12-2), which is also evident in the v velocity profile (Figure 12-3). July also shows a large (diameter ~200km) warm core eddy developing off the NW Cape region 2. The temperatures are also shown in Table 5-1.
In general 1998 was warmest and 1997 coolest across the entire domain throughout July, this trend was especially obvious in region 1 on the NWS itself. The La Niña LC signature was warmer then 1996/1997 in region 4, where the El Niño and neutral year were of similar temperature. The 1996 LC was only mildly warmer then 1997, with a 0.5 ° difference in regions 1, 2 and 3 being the most significant.
Table 5-1 July average LC temperatures for all regions Monthly period: July Average temperature at start of period ( ° C) Year 1996 Region 1:NWS 25.5 Region 2: Ningaloo 25 Region 3: Shark Bay 24.5 Region 4:Abrolhos Islands 23
Year 1997 Region 1:NWS 25 Region 2: Ningaloo 24.5
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Region 3: Shark Bay 24 Region 4:Abrolhos Islands 23
Year 1998 Region 1:NWS 26.5 Region 2: Ningaloo 25.5 Region 3: Shark Bay 24.5 Region 4:Abrolhos Islands 24.5 August/September/October
This period sees the 1996 LC narrow significantly in region 3, although the current is still a highly meandering flow of width 100-200km in regions 3 and 4 and a large mass of southerly propagating warmer water in region 2 (Figure 12-5, Figure 12-6). There are large meanders with develop into eddies off both Shark Bay and the Abrolhos Islands in late September/October. This period also sees a significant cooling of the domain by an average of 1 ° C (Table 5-2). There is also a warm mass of water (25 ° C) which moves off the NWS into the open ocean to the south in early August. Some of this water moves back across the continental shelf in September to feed the LC in region 2.
In 1997 there is a very significant cooling in all regions in this period and the LC signature becomes generally weaker, with the average temperature drop in ASO is 2 ° C (Table 5-2, Figure 12-5, and Figure 12-6). The LC beings to move offshore in region 3 and becomes very hard to distinguish from the open ocean water, although it does still maintain a faint signature. The signature is still clear in region 4 south of the Abrolhos Islands and a short lived warm core eddy is shed off the island chain in October.
The La Niña LC remains confined on the continental shelf edge through this period but begins to meander and increases in width to in excess of 100km in late August/early September (Figure 12-5, Figure 12-6). The 1998 LC also drops noticeably in temperature over this period, by an average of 1.5 ° C (Table 5-2). By
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the end of October the LC begins to narrow again, mainly in region 4, just south of the Abrolhos Islands. There is also stream of water at 25 ° C that stems from the NWS edge in July, has travelled adjacent to the western boundary and is driven over back over the continental shelf, forming eddies, which induce mixing with LC water and entrain this higher temperature water and strengthen the current in region 4 from August through to the end of September.
August/September/October highlights the development of the ROMS solution, moving away from its initial OCCAM characteristics as the difference between the three years becomes much more apparent. The temperature of the LC signature drops noticeably in all three years, especially 1997. 1998 remains as the strongest year and 1997 the weakest, and the contrast between 1996 and 1997 has increased distinctly since July, especially on the NWS, which is most likely a result of the El Niño peak in the ITF. 1996 has continued to increase in comparative strength and is as warm as 1998 in some parts, particularly on the NWS, however 1998 still generally displays a warmer LC signature. The 1997 LC has become distinctly weaker then the other two years, which may the result of the lack of warm inflow onto the NWS from the northern boundary which the other two years are still displaying. This is a likely result of the ITF El Niño peak reducing the geopotential gradient. V velocity profiles highlight the diminished LC signature in region 3 in 1997 and the generally stronger 1998 LC, as well as showing the similarities between 1996 and 1998 (Figure 12-9).
Table 5-2 August/September/October average LC temperatures for all regions Monthly period: August/September/October Average temperature Average temperature at start of period ( ° C) at end of period ( ° C) Difference Year 1996 1996 1996 Region 1:NWS 25.5 25 -0.5 Region 2: Ningaloo 24.5 23.5 -1 Region 3: Shark Bay 24 23 -1 Region 4:Abrolhos Islands 23 22 -1 Total Change -3.5 Year 1997 1997 1997 Region 1:NWS 25 23 -2
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Region 2: Ningaloo 24.5 23 -1.5 Region 3: Shark Bay 24 22 -2 Region 4:Abrolhos Islands 23 21 -2 Total Change -7.5 Year 1998 1998 1998 Region 1:NWS 26 24.5 -1.5 Region 2: Ningaloo 25 23.5 -1.5 Region 3: Shark Bay 24.5 23 -1.5 Region 4:Abrolhos Islands 23.5 22.5 -1 Total Change -5.5
November/December
There is a slight increase in the temperature of the 1996 LC to the south throughout November/December due to a warm temperature front crossing the continental shelf around 24 ° S, which subsequently feeds the LC in regions 3 and 4. This front originates from a water mass that moved off the NWS in August (Figure 12-10, Figure 12-11). This results in a temperature increase in region 4, but the temperatures in the other regions remain fairly constant over this period. The width of the LC does not change significantly over this period.
In 1997 there is an inflow of cold water (20 ° C) onto the NWS from the northwest as well as an inflow of warmer water onto the NWS form the northeast (24.5 ° C) and the two temperature fronts interact to form two large scale cyclonic features with a length scale in the order or 300km that dominates the NWS dynamics (Figure 12-14). There is no observable effect of these features on the LC although the simulation ends before the full effect of these features can be seen. The LC widens slightly in regions 3 and 4 and maintains a constant temperature except for region 4 where there is a mild reduction (Table 5-3, Figure 12-10 and Figure 12-11). The signature does narrow from October, although the ambient temperature does not change but the convergence of cooler water from the open ocean onto the shelf confines the LC and makes it easier to distinguish.
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During 1998 there is a region of warm water (25.5 ° C) near the NWS edge at the top of region 1 that is transported across the shelf through November to feed the LC in December. This raises the temperature of region 2 and region 3 significantly (Table 5-3) and starts to meander off the shelf just south of Shark Bay in late December (Figure 12-11). However it does not reach region 4 before the end of the simulation and this region remains at a fairly constant 22.5 ° C. The warm water from the NWS combines with another flow that crosses the continental shelf between 24 and 26 ° S and strengthens the LC in region 3. This second warm flow originated from the NWS in late October and has travelled south along the western boundary before being transported back onto the shelf, a flow pattern which was becoming frequently observable in the results.
November/December shows the El Niño being characteristically weaker then the other two years, which are found to be of very similar temperatures. Both 1996 and 1998 have a relatively similar size warm body of water to the north of the domain, but it is distributed differently, with 1996 having the warmest pool of water on the NWS itself which beings to recede towards the northern boundary in December. Conversely 1998 shows a warm mass closer to the edge of the NWS, which then moves south east and feeds the LC. This transportation of warm water across the NWS that feeds into the LC in 1998 is a likely result of the set-up created by warm ITF temperatures from the November/December La Niña peak, described in section 5.1. Also occurring in 1996 is the warmer water that moved off the NWS in August and has crossed the continental shelf again near 24 ° S, although this also occurs in 1998.
The El Niño year displays a significantly cooler body of water on the NWS then the other two years. December 1997 also displayed significant intrusion of cooler water (20 ° C) from the top of the western boundary, which generates two large cyclonic features. This was a particular area of questionable dynamics due to the internal boundary condition on the western extent of the domain. However there was nothing observed in temperature data that was considered overly unrealistic hence there is no reason to suggest this intrusion of cooler water could not actually occur.
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Table 5-3 November/December average LC temperatures for all regions Monthly period: November/December Average temperature Average temperature at at start of period ( ° C) end of period ( ° C) Difference Year 1996 1996 1996 Region 1:NWS 25 25 0 Region 2: Ningaloo 24 24 0 Region 3: Shark Bay 23 23 0 Region 4:Abrolhos Islands 21.5 22 0.5 Total Change 0.5 Year 1997 1997 1997 Region 1:NWS 23.5 22.5 -1 Region 2: Ningaloo 23 23 0 Region 3: Shark Bay 22 22 0 Region 4:Abrolhos Islands 22 21 -1 Total Change -2 Year 1998 1998 1998 Region 1:NWS 25 25 0 Region 2: Ningaloo 23.5 24.5 1 Region 3: Shark Bay 23.5 24 0.5 Region 4:Abrolhos Islands 22.5 24 1.5 Total Change 3
5.4.3.3 Salinity
July
The salinity shows a distinct LC signature in all three years south of 22 ° S, with 1998 displaying the strongest signature which follows the 2km bathymetry contour (Figure 12-4). There was also several jet like offshoots observable in 1998 above region 4 where the LC intensified significantly.
In region 3 the LC is more of a direct southward flow in 1998 and 1996 whereas 1997 is meandering current. On the NWS there is an inflow of low salinity water from the
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north west corner of the study domain, which is right on the NWS edge. This inflow follows the shelf edge just past 22 ° S. This inflow was seen in all three years but was strongest in 1998 with a value of 34.4 PSU, but was closer to 34.6 PSU in 1996 and 1997.
Table 5-4 July average LC salinity for all regions Monthly period: July Average salinity at start of period (PSU) Year 1996 Region 1:NWS 34.9 Region 2: Ningaloo 35 Region 3: Shark Bay 35.1 Region 4:Abrolhos Islands 35.2
Year 1997 Region 1:NWS 34.7 Region 2: Ningaloo 34.9 Region 3: Shark Bay 35 Region 4:Abrolhos Islands 35.1
Year 1998 Region 1:NWS 34.6 Region 2: Ningaloo 35 Region 3: Shark Bay 35.1 Region 4:Abrolhos Islands 35.2
August/September/October
In August 1996 there is a very large pool of low salinity water (34.8 PSU) covering almost all of region 1. This was observed to feed LC water that meanders along the continental shelf break, mainly following the 3km bathymetry contour and gradually increases in salinity both with time and distance south (Figure 12-7, Figure 12-8). There were several warm core eddies generated off the Abrolhos Islands in region 4 which are more apparent the in temperature data. Another major feature of the system which is more apparent in salinity data is a stream of water from the low salinity pool
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in region 1 that has followed the western boundary and been redirected west across the continental shelf during August in region 3. This was observed to strengthen the LC and induce mixing to reduce the density front created by this low salinity inflow. By October the salinity in all regions and had increased significantly (Table 5-5). The LC also maintained a fairly wide flow in region 3 of approximately 200km.
The 1997 LC also increased in salinity steadily from August to the end of October, with the greatest changes in regions 1-3 (Table 5-5, Figure 12-7 and Figure 12-8). The LC narrowed significantly in region 4, reducing its width to 75km. There is also a large warm core eddy off shed the Abrolhos Islands in October, with a length scale of around 200km. The LC also becomes harder to distinguish in region 3 during this period. There is also some onshore flow feeding into the current around 28 °, similar to that observed in 1996 although it has a less pronounced effect due to higher salinity. The LC in region 3 narrowed as time progressed having a width of between 100-150km by the end of October.
1998 showed the same western inflow into the current around 28 ° S as the other two years, but was ~0.4 PSU less saline then 1996 and consequently had a greater effect on the intensification of the LC which persisted until mid-September (Figure 12-7). The 1998 LC maintained a fairly direct flow along the 2km bathymetry contour and maintained a constant width in regions 2 and 3 but narrowed significantly in region 4 to 25-50km. As with the other two years, the salinity slowly increased over the three months in all regions, with the greatest change on the NWS (Table 5-5, Figure 12-7 and Figure 12-8).
As seen in temperature data, there is a general increase in the salinity of the system in all years from August to October (Table 5-5). 1997 has evolved to become distinctly more saline in general then the other two years, especially in region 1. Overall the salinity data showed the same trends as those shown by temperature data during this time period, although generation of mesoscale features and mixing were more obvious.
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Table 5-5 August/September/October average LC salinity for all regions Monthly period: August/September/October Average salinity at start of Average salinity at end of period (PSU) period (PSU) Difference Year 1996 1996 1996 Region 1:NWS 34.7 34.8 0.1 Region 2: Ningaloo 34.8 35 0.2 Region 3: Shark Bay 35.05 35.15 0.1 Region 4:Abrolhos Islands 35.15 35.25 0.1 Total Change 0.5 Year 1997 1997 1997 Region 1:NWS 34.7 35 0.3 Region 2: Ningaloo 34.9 35.1 0.2 Region 3: Shark Bay 35 35.25 0.25 Region 4:Abrolhos Islands 35.2 35.35 0.15 Total Change 0.9 Year 1998 1998 1998 Region 1:NWS 34.7 34.9 0.2 Region 2: Ningaloo 34.9 35 0.1 Region 3: Shark Bay 35 35.1 0.1 Region 4:Abrolhos Islands 35.15 35.2 0.05 Total Change 0.45
November/December
At the start of November 1996 there is a pool of 34.8 PSU water covering the NWS in region 1, which flows into two existing streams of water, one which feeds the LC directly in region 2 and the other which travel off the shelf and along the western boundary. The southern movement of this water mass causes the density in regions 1 and 2 to increase. Also at the start of November, a density front originating from the western boundary stream of NWS waters in October is transported onshore across the continental shelf, causing mixing and a short lived intensification of the LC in region 3. Although as observed in temperature data, this does not affect region 4 before the
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end of the simulation and does not change the density of region 3 as it is too short lived. Overall the density of the LC increases slightly over November/December (Table 5-6, Figure 4-12 and Figure 4-13).
The transport of a 35 PSU density front across the shelf at the top of region 3 causes the density of regions 3 and 4 to decrease slightly over November/December as lower salinity water is entrained in the LC. The southward propagation of this lower density water also causes the density in regions 1 and 2 to rise slightly (Table 5-6). The width of the LC remains uniform over this period (Figure 4-12, Figure 4-13).
There is an overall density reduction in 1998 as a result of the transport of significant low density (34.6 PSU) inflow in the north west corner of the domain fist appearing in October. This is transported in both the western boundary and continental shelf southern flow paths and intensifies the LC regions 2 and 3 in what is becoming a commonly observable pattern of the salinity front crossing the shelf and being entrained in the mean current flow, reducing its salinity (Table 5-6, Figure 4-12 and Figure 4-13).
Overall in November/December there was only a small ° of change in the salinity of each of the three years. There was however significant inflow onto the NWS in 1998 which is likely result of the 1998 La Niña. This meant that 1998 was the only year with a decrease in salinity over November/December, which coincides with the La Niña peak set-up of ITF waters.
Table 5-6 November/December average LC salinity for all regions Monthly period: November/December Average salinity at start of Average salinity at end of period (PSU) period (PSU) Difference Year 1996 1996 1996 Region 1:NWS 34.8 34.9 0.1 Region 2: Ningaloo 35 35.1 0.1 Region 3: Shark Bay 35.15 35.15 0
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Region 4:Abrolhos Islands 35.25 35.25 0 Total Change 0.2 Year 1997 1997 1997 Region 1:NWS 35 35.1 0.1 Region 2: Ningaloo 35.1 35.2 0.1 Region 3: Shark Bay 35.25 35.2 -0.05 Region 4:Abrolhos Islands 35.35 35.25 -0.1 Total Change 0.05 Year 1998 1998 1998 Region 1:NWS 34.9 34.8 -0.1 Region 2: Ningaloo 35 34.9 -0.1 Region 3: Shark Bay 35.1 34.9 -0.2 Region 4:Abrolhos Islands 35.2 35.3 -0.05 Total Change -0.30
5.4.3.4 Velocity
The velocity analysis revealed that the LC-LUC was most clearly observable around 27-29 ° S in the ROMS solution (Figure 12-15), however it was not visible some other areas of the domain, where it was dominated by other velocity fields that had propagated down through the water column (Figure 12-16). Where visible the LUC had a velocity of about 0.15 m/s and was centred at a depth of 600m. The LC velocity core extended through the water column to as deep as 400m in some cases and had a velocity ranging form 0.1 m/s to 1 m/s.
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6 Discussion
The fist major point to note about comparing the LC in different years is that the description of the current itself is based on warm, low salinity water meandering in a general southerly direction along the coast of WA. There is no specific characteristic that can be used to identify the current as it doesn’t have a uniform flow path, width, velocity or temperature/salinity values. Hence the only way to actually identify the LC signature is by the satisfaction of the mean flow characteristics that define the current. Therefore identifying a specific body of water as the LC is subjective and any attempt to form a comparison between different years must take this into account.
After reviewing OCCAM initially it was determined that the best way to identify the LC was to look for a south moving band of water in close proximity to the continental shelf that had a higher temperature and lower salinity then ambient water. So long as this method of identification was consistent between the three different years then the comparison overcomes the subjectiveness of defining the LC.
6.1 Satellite Altimetry
A key observation from the global satellite altimetry was that 1996 resembled a La Niña year more then initially thought. As shown by the altimetry there are certain anomalies occurring in 1996 that are reminiscent of a La Niña event, although they are neither as long lived nor as pronounced, but will be seen as a weak La Niña by ROMS.
There is a warm temperature anomaly over the ITF in October/November/December 1996 which coincides with a similar warm anomaly in 1998 which was classified as the La Niña peak in November/December/January. This particular anomaly was expected to significantly reduce any variation seen as a result of the 1998 La Niña, although the La Niña anomaly is slightly warmer and was expected to show a slightly stronger signature, which agreed with the model results.
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1997 displayed typical El Niño characteristics in both SSTA and SSHA data. The temperature was drastically lower then the other two years in the ITF region and this was expected to significantly weaken the geopotential gradient along the WA coast and show large interannual variations from both 1996 and 1998, this also agreed with model results.
6.2 OCCAM
The ROMS solution depended highly on the quality of input data, so verifying that OCCAM was reproducing the mean flow characteristics was very important for validation of results. It was also very important to verify that OCCAM was reproducing ENSO dynamics observed in satellite data to provide the required driving force for interannual variations in the LC.
The reproduction of the ENSO patterns by OCCAM in both the Pacific and ITF was consistent with historical patterns. In general the equatorial Pacific region showed that the correct phase relationship of ENSO signatures in the two regions was being reproduced by OCCAM, which was the main purpose of including this region in the analysis. The El Niño signature was prevalent in 1997 in the ITF, but the variation between 1996 and 1998 in the ITF was only minor. This was most likely a result of the La Niña characteristics of 1996. 1998 was the warmest year in every month except October, which is when the 1996 ITF anomaly started to develop in satellite imagery, a month ahead of the warm 1998 ITF anomaly. It was also noted that 1996 and 1998 were very similar in the equatorial Pacific region, with 1996 being slightly cooler, but this response was consistent with the observations from satellite imagery. The similarities between these two years is recognised as being the main reason the 1996 and 1998 ROMS solutions didn’t show a large magnitude of difference.
Another observation in the ITF region was the southerly propagation of warm water across ITF from September to December, results in pooling of this water onto the NWS, which was seen in the ROMS solution as inflow from northern boundary of the model domain in October/November/December in all three years. Although in 1997
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this was mixed with a cool inflow from the western boundary. This movement of warm water across the ITF and onto the NWS is most likely a seasonal phenomenon as it coincides with the change from spring to summer, but this is an example of the remote forcing of seasonal variations in the current (Fang & Morrow 2003). This also provides a reason for the secondary peak of the LC occurring in November, as this warmer water on the NWS should increase the WA alongshore gradient enough to induce increased LC flow as a response (Fieux et al. 2005).
The OCCAM solution was found to both reproduce the mean flow characteristics of the LC at the correct temporal and spatial scales and show significant interannual variability between the three years. OCCAM also showed current meanders and eddy shedding, especially within the three preferential corridors described in section 3.5. Because the OCCAM models results were freely available, a whole yearly comparison was possible.
An interannual comparison of OCCAM data for the three years showed that 1997 was generally a much weaker LC then both 1996 and 1998. 1998 did exhibit a slightly stronger signature then 1996 for most of the year, although the 1996 LC was stronger in November and August showed very similar values. This comparison was consistent with expectations based on satellite altimetry. This comparison posed the question whether the 1996 anomalies were increasing the strength of the LC more then expected, or whether there was less difference between a La Niña and neutral LC then expected. From on the small magnitude of difference between 1996 and 1998 ITF temperatures, and the differences between the 1996 and 1997 LC based on the relative strength of the El Niño in the ITF, it was concluded that the 1996 LC strength had been increased significantly by the La Niña like anomalies in the ITF. Therefore the 1996 LC and was more like a weak La Niña LC then a completely neutral LC. This was kept in mind when analysing the ROMS solution.
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6.3 ROMS
6.3.1 Tides
The first simulation incorporating tides was found to be very unstable and whilst the model did not crash due to the surface oscillations, these were amplified quickly and dominated the dynamics of the system. Hence analysis of LC characteristics within these results was kept to a minimum. The generation of the oscillations within these results was interesting in itself as it raises questions about ROMS ability to handle changes in tidal amplitude across the domain. This problem is perhaps more apparent within this domain as opposed to other ROMS applications because of the large scale of the domain and the magnitude of difference in tidal amplitude along the WA coast. This problem also highlights the how isolated areas of instability are quickly magnified, as the values in these problem cells are used in the calculation of adjacent cells for the subsequent timestep in the finite difference approximation approach used by ROMS.
6.3.2 No Tides
Whilst being a stable simulation, there were problems manifested in this simulation caused by the fixed internal boundary conditions. The western boundary contributed to a new inflow of momentum into the system which was observable in the volume average kinetic energy of the domain, which gradually increased over time. The large velocity fields propagating across the western boundary moved in a general southerly direction, are reflected on the southern boundary and were redirected northwards. These velocity fields were dominating the dynamics of the system and suppressing the LC on the continental shelf edge, as the redirected northward flow was on the seaward side of the shelf break.
In a quasi steady state system this volume average KE would normally peak at some stage of the simulation, unless momentum was unable to escape the system, which is unlikely in a natural system as there is always some form of energy dissipation
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mechanism. We know that with the LC-LUC system that there should not be a constant increase in the kinetic energy of the system as there should be substantial transport through the southern boundary.
This problem is caused by the fact predefined boundary conditions allow only for the input/output of temperature/salinity and baroclinic velocity as provided by the OCCAM solution. Because the ROMS solution that evolves within the domain is different to the OCCAM solution, flows that ROMS has resolved that are destined to propagate out through the boundaries may be confined within the system if they do not match the OCCAM boundary conditions. It is difficult to overcome this problem if the external boundary forcing is to be kept constant at its original values. The ROMS code has been written to process temperature, salinity and baroclinic velocity separately along the entire length of each boundary. Hence one or more of these variables can be switched on or off along one or more of the boundaries, but this affects the entire length of boundary and may not be appropriate if there is only a small problem area.
In this case because the baroclinic velocity on the western boundary was not central to the reproduction of the LC itself, although it does contribute through a large scale geostrophic inflow that is located more to the north of the domain. However as the interannual variation caused by ENSO is caused by the ITF pooling water on the NWS, removing the geostrophic inflow into the system should only have pronounced any variations between the three years. Turning off the baroclinic velocity on the western boundary was also considered a better option then turning it off on the southern boundary, which was the second option. However this other option did not adequately address the problem of large velocity fields propagating through the western boundary, which would have still dominated the system. It would have also removed the LUC momentum input which feeds into the system from the south, possibly changing the overall dynamics of the surface current.
6.3.3 No Western Boundary or Tides
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The final simulation showed a better representation of the mean flow characteristics of the LC and did not produce the large velocity flux across the western boundary which surpassed the current in the previous simulation. The volume average kinetic energy of the system also appeared to reach a quasi-steady state by simulation day 24.
There were some large velocity fields directly adjacent to this western boundary, however the remained confined within a small area and did not have a large impact on the overall dynamics of the LC. These velocity fields could have been caused by the system trying to equilibrate the difference between the boundary where the baroclinic velocity was zero and the internal cells which had a baroclinic velocity component, despite the fact that nudging was applied. A possible explanation with this is that there was not significant nudging applied at the boundaries, however there was not sufficient time to investigate this further due to the large runtime of the simulations. Another avenue of thought it that these velocity fields could have been caused by the temperature/salinity values on the boundary, which were inducing velocity fields due to large density differences, however this was unable to be verified.
A verification the LC mean flow characteristics showed that the LC generally was composed of warm, low salinity water and followed the 2 and 3 km bathymetry contours. The LC also showed significant meandering that lead to eddy shedding in most cases.
A feature that was observed in each of the three years that may have been a result of the fixed western boundary temperature and salinity was the propagation of warm low salinity water off the NWS in region 1 and its passage south along this western boundary. This water was then redirected southwards in region 3 between 24 and 28 ° and crossed the continental shelf, creating a sharp density front which induced mixing as the water was entrained into the LC. This behaviour could be a result of the fixed boundary conditions, having seen the potential results of this in the second ROMS simulation. However this behaviour was contrasted against satellite data and it was found that it wasn’t uncommon for warm, low salinity water to be redirected over the confidential shelf around Shark Bay.
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6.3.4 Comparison of El Niño Peak (August/September/October)
The temperature data in Table 5-2 shows that whilst there is a general cooling of the LC from August to October, consistent with the seaonsonality of the LC. 1997 experiences more a greater amount cooling then the other years, especially the neutral year (1996 cools 3.5 ° and 1997 cools 7.5 °). As well as increased cooling the 1997 LC has an average temperature 1-1.5 ° C lower then the other regions both at the start and end of this period. It is also easy to see from Figure 12-5 and Figure 12-6 that the general temperature of the entire domain is significantly cooler then the other two regions.
The same trend is observable in the salinity data, with the salinity the entire study domain, including the LC increasing in all years over this period. The greatest increase is in the El Niño year (0.9 PSU) compared to 0.5 PSU in the neutral year and 0.45 PSU in the La Niña year. The salinity values are very similar in each of the three yeas at the start of August but by the end of October the ROMS solution developed and 1997 has slightly higher values then the other years.
All of this suggests that El Niño is having the expected cooling effect on the current, especially with notable decrease in temperature and increase of salinity of the NWS source waters. These sources waters essentially determine the maximum temperature/minimum salinity of the LC across the domain, assuming there is no western inflow not of NWS origin. It is obvious that there is not the same magnitude of warm inflow into the system onto the NWS as 1996 and 1998 during this period. This is explained well by the ITF OCCAM plots, which show 1997 as a distinctly cooler year, with less pooling of warm water on the NWS. Therefore it can be concluded that the comparative reduction in the strength of the LC seen between August and October is a result of the ITF El Niño peak.
The differences between 1996 and 1998 in salinity are very small in this period and whilst region 1 is less saline in October 1996, regions 3 and 4 are less saline in 1998.
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The difference in region 1 may be affected by the fact that the ITF temperatures in October 1996 were warmer then in 1998. This low salinity pooling onto the NWS would not have time to reach regions 3 and 4 in October, with average velocities being less then 1 m/s, explaining why 1998 is less saline in these regions, as September ITF temperatures were greater in 1998, hence there was slightly warmer water on the NWS in September 1998, which has travelled south through the domain to reach regions 3 and 4 by the end of October.
A temperature comparison of the neutral and La Niña years between August and October similarly shows that the difference between these two years is not great, although 1998 does experience a greater temperature drop then 1996 by a total of 2 ° across the domain. The greatest decrease is at the bottom end of the domain in 1996 and conversely at the top of the domain on the NWS in 1998. This supports the suggestion that the ITF 1996 ITF warm anomaly described in section 5.1.1 is having some effect on the LC in October 1996, although it only appears to be mild. In general the temperatures of these two years are very similar and the main difference between the two years being the width of the flow, with 1996 having a wider more meandering flow then the more direct 1998 LC which is mainly confined to the 2km bathymetry contour.
6.3.5 Comparison of the La Niña peak (November/December)
The effect of the inflow of warm, low salinity water onto the NWS in October 1996 on the LC in the lower part of the domain can been seen in November, with a short lived increase of the LC signature. This clearly shows the mechanism by which warm water transported onto the NWS from the ITF feeds the LC flow. The same mechanism, with a more pronounced effect was also seen in November/December 1998. This increase in temperature and decrease of salinity correlates well with La Niña peak in the ITF, as shown by satellite altimetry and OCCAM. This is highlighted by the overall increase in temperature of 1998 in Table 5-3, which is 3 ° in 1998 and only 0.5 ° in 1996. The salinity values in Table 5-6 also confirm the increase in the difference between 1996 and 1998, with a salinity increase of 0.2 PSU in 1996 a net
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decrease of 0.3 PSU in 1998. Overall it was concluded from the results that the warm ITF anomaly in October/November/December 1996 had the effect of mildly increased pooling on the NWS in October which strengthened the LC in November. However this strengthening was not as significant as that caused by the 1998 La Niña peak in November/December which showed a greater increase of the LC signature consistent with the greater magnitude of the warm ITF temperature anomaly in 1998 (compared to 1996) generated by La Niña.
The persistence of the 1997 El Niño can also be seen in November and December. Both 1996 and 1998 show a net increase in temperature over this period but the 1997 LC continues to decrease in temperature. By the end of the simulation in December average LC temperatures are much cooler in 1997 then the other two years. However this is not confirmed by salinity data, with regions 1 and 2 increasing in salinity but regions 3 and 4 showing a decrease. This means there is minimal net salinity change in 1997 (0.05 PSU increase), whereas there was a 0.2 PSU increase in 1996 and 0.3 PSU decrease in 1998. The salinity increase in 1996 occurs in regions 1 and 2, which is a likely result of the transport south of the warm northern inflow onto the NWS that occurred in October. However as there was not a net decrease in 1997 then it can be concluded that El Niño is still having some form of effect on the LC in November/December 1997, especially in the temperature signature.
6.4 Problems and Limitations
A major problem which hindered the comparison of the three years was that first a simulation with acceptable dynamics had to be generated, which took longer then initially thought. Due to the available timeframe for the thesis, this had a some bearing on the overall outcome of results. However a substantial comparison between the three years was able to be formed.
A difficulty with analysing the ROMS model output was the range of the colour bar. This is normally determined by the respective minimum and maximum temperatures within the domain for each plot, but since a comparison between different years and time periods was required this range had to be locked. The range of this colour bar
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had to incorporate the overall maximum and minimum temperature from the combined three runs and hence had a significant range. This meant that the LC was hard to discern from the open ocean in some plots and this is highlighted in Figure 10- 17. This was easy to overcome by changing the range in plots where the LC was hard to distinguish. However these additional plots have not been included in this paper and so the LC is not as clear as it could be in some of the images in Appendix 3 (section 12), however this scale was changed during the analysis to clarify the exact location of the LC where required.
A major issue with this thesis was found to be the large volumes of data generated by the models. With three lots of 166 days of output from the ROMS simulations, there was far too much data to consider everything. The main focus was on the temperature and salinity of the system as these are the key indicators of the presence of the LC. The velocity profile of the system was also of interest although it was found that ROMS could not produce a realistic velocity profile across the entire domain with tides or western boundary baroclinic velocity input. The same problem of too much data was experienced with the OCCAM data; hence analysis of output had to be conducted at a broader scale because of this. Therefore the detailed dynamics of meanders and eddy formation were not covered in detail in this thesis as it was more focused on looking at the bigger picture of the effect of El Niño and La Niña of the NWS source waters and LC over the entire domain.
There was also some instability generated in the system because of the difference between the ROMS and OCCAM coastline, however this did not have a major impact on the overall performance of the model. There was some instability generated in two areas, one near the Abrolhos Islands and the other landwards of Barrow Island, but this was very localised and hence was not included in results as it didn’t have an effect of the LC dynamics within the results.
Differences in the LC were very hard to actually quantify, due to the spatial variability of the LC over the domain. The ROMS domain was divided up into four regions of similar dynamics and temperature and salinity values in an attempt to make quantitative analysis easier.
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A major problem in comparing differences between the three years was the meandering nature of the current, which meant there wasn’t often a single core current to compare, instead a wider band of water. This was especially prevalent in regions 2 and 3, as this is the area where a wide band of high temperature, salinity water slowly narrows to form the LC. There was no attempt to identify a specific LC signature on the NWS in region 1, as this region acts as a source for the LC and exhibits a general southerly propagation of water to ‘feed’ the LC rather then actually containing a LC core. The dynamics in this NWS region did show a general southern propagation of surface water but as expected there was no sign of a specific LC core. It was found that the best location to compare the LC was in regions 3 and 4, due to the southern intensification of the LC. However the greatest influence on the system as a result ENSO was found to occur on the NWS in region 1, which then in turn affected the rest of the system within the next few months.
Despite having an internal boundary condition for baroclinic velocity the western boundary still produced some unexpected dynamics, although these were localised. The important point to note about any unrealistic dynamics generated within the vicinity of this boundary is that they were consistent across each of the three years so they did not have an effect of the comparison of the three years, which was the focal point of this study. Although there were not values of temperature, salinity, velocity of otherwise within the results to suggest this boundary was exhibiting behaviour which may not actually occur in the real life system, it was simply noted that the dynamics observed in this region had not been observed in satellite imagery or any other sources before.
7 Conclusions
As the LC had been successfully modelling using ROMS previously, there was not expected to be much trouble getting the simulations themselves going. However there was significant difficultly getting ROMS to produce a stable set of results, deemed theoretically realistic enough to form a comparison. Hence a much larger amount of time was spent on getting a valid ROMS solution from OCCAM forcing.
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The OCCAM reproduction of the ENSO events in the ITF region correlated well with satellite altimetry and hence it was found that OCCAM could adequately reproduce the characteristics associated with El Niño and La Niña. An important finding from this work was the identification of a weak La Niña temperature pattern in 1996, which reduced the effectiveness of comparison between 1996 and 1998. This comparison also identified the peak of the 1997 El Niño in August through October and the peak of the 1998 La Niña between November and January. Also the peak of the weak La Niña like signature in 1996 occurred through October to December.
The first ROMS solution with tides highlighted the sensitivity of stability of ROMS solutions and provides an indication of the possible inability of ROMS to handle large changes in tidal amplitude across a domain. The second solution highlighted the problems with reflection of flow on boundaries, due to the difference between the p5redefined boundary conditions and the evolving ROMS solution.
Once a stable solution was generated and the mean flow characteristics verified an analysis revealed that ENSO events had a significant impact of the NWS source waters and the LC itself. It was also found it took approximately one month before the effect of warm, low salinity inflow onto the NWS was felt by the LC.
The El Niño LC was observed to be distinctly cooler and more saline then the other two years and the magnitude of this difference was 1-1.5 ° C and 01.-0.2 PSU on average. Overall it was not just the LC but the overall climate of the domain that was affected by El Niño, which was attributed not only to a reduced geopotential gradient along the WA coast, but a lack of inflow into the NWS source waters.
The La Niña LC was distinctly warmer then the El Niño year but was very similar to the ‘neutral’ year. This was put down to the likeness of the neutral year to a La Niña event. The increase in temperature in the latter part both 1996 and 1998 reflected the behaviour of warm ITF discharge onto the NWS. The timing of this was consistent with the peak of warm anomalies in the ITF region which was suspected on increasing the geopotential gradient to increase flow of the LC. Whilst the difference between a
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La Niña and neutral year could not be fully quantified because of the likeness of 1996 to a La Niña year, the dynamic behaviour of the ITF-LC interaction was sufficient to suggest that La Niña year produces a warmer LC then normal.
It is a recommendation of this study that modelling of the LC using ROMS continues due to ROMS ability to adequately reproduce the LC’s mean flow characteristics and its generation of mesoscale features at the correct spatial and temporal scales. This study has raised questions about ROMS ability to handle large changes in tidal amplitude across a domain and it is a recommendation of this study that this be investigated in further detail, not just in WA but in other regions where this occurs. Furthermore it is suggested that future ROMS studies on the LC allow a significant amount of time for model development, as it was a major finding of this study that obtaining a valid ROMS solution required several runs to refine the input and model parameters.
It is also recommended that interannual dynamics of the LC be studied on a smaller scale, to investigate the detailed dynamical differences between El Niño and La Niña LC patterns, rather then broad scale differences. Finally a investigation into the effect of ENSO events along WA’s South Coast, especially around Cape Leeuwin, would be interesting to clarify how far south the influence of ENSO on the LC is significant.
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8 References
Batteen, M. L. & Butler, C. L. 1998, 'Modelling Studies of the Leeuwin Current off Western and Southern Australia', Journal of Physical Oceanography, vol. 28, no. 11, pp. 2199-2221. Batteen, M. L. & Rutherford, M. J. 1990, 'Modelling Studies of Eddies in the Leeuwin Current - the Role of Thermal Forcing', Journal of Physical Oceanography, vol. 20, no. 9, pp. 1484-1520. Batteen, M. L., Rutherford, M. J. & Bayler, E. J. 1992, 'A Numerical Study of Wind- and Thermal-Forcing Effects on the Ocean Circulation off Western Australia', Journal of Physical Oceanography, vol. 22, no. 12, pp. 1406-1433. Bureau of Meteorology, Southern Oscillation Index Archives: 1876 to present [Online], Government of Australia, Available: http://www.bom.gov.au/climate/current/soihtm1.shtml [28th July 2006]. California Institute of Technology: Jet Propulsion Laboratory, Ocean Surface Topography from Space - Gallery [Online], NASA, Available: http://sealevel.jpl.nasa.gov/gallery/videos.html [15th April 2006]. Coward, A. C. & de Cuevas, B. A., The OCCAM 66 Layer Model: physics, initial conditions and external forcing [Online], University of Southampton, Available: http://www.noc.soton.ac.uk/JRD/OCCAM/ [2nd October 2006]. Cresswell, G. R. 1991, 'The Leeuwin Current - observations and recent models', Journal of the Royal Society of Western Australia, vol. 74, pp. 1-14. Cresswell, G. R. & Golding, T. J. 1980, 'Observations of a south-flowing current in the southeastern Indian Ocean', Deep Sea Research Part A. Oceanographic Research Papers, vol. 27, no. 6, pp. 449-466. Dearnaley, M. P. 1990, Throughflow from the Pacific to the Indian Ocean through South East Asian Waters, DX95458, University of Southampton (United Kingdom). Earth System Research Laboratory: Physical Sciences Division, Plots of the SST anomaly and wind fields of the development patterns for El Nino and La Nina events [Online], National Oceanic and Atmospheric Administration,
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Available: http://www.cdc.noaa.gov/cdc/reanalysis/reanalysis.shtml [16th April 2006]. Fang, F. & Morrow, R. 2003, 'Evolution, movement and decay of warm-core Leeuwin Current eddies', Deep Sea Research Part II: Topical Studies in Oceanography Physical Oceanography of the Indian Ocean: from WOCE to CLIVAR, vol. 50, no. 12-13, pp. 2245-2261. Feng, M., Meyers, G., Pearce, A. & Wijffels, S. 2003, 'Annual and interannual variations of the Leeuwin Current at 32 degrees S', Journal of Geophysical Research, vol. 108, no. C11, p. 3355. Feng, M., Wijffels, S., Godfrey, S. & Meyers, G. 2005, 'Do Eddies Play a Role in the Momentum Balance of the Leeuwin Current?' Journal of Physical Oceanography, vol. 35, no. 6, p. 964. Fieux, M., Molcard, R. & Morrow, R. 2005, 'Water properties and transport of the Leeuwin Current and Eddies off Western Australia', Deep Sea Research Part I: Oceanographic Research Papers, vol. 52, no. 9, pp. 1617-1635. Godfrey, J. S. & Ridgway, K. R. 1985, 'The large-scale environment of the poleward- flowing Leeuwin Current, Western Australia: longshore steric height gradients, wind stresses and geostrophic flow', Journal of Physical Oceanography, vol. 15, no. 5, pp. 481-495. Godfrey, J. S. & Weaver, A. J. 1991, 'Is the Leeuwin Current driven by Pacific heating and winds?' Progress In Oceanography, vol. 27, no. 3-4, pp. 225-272. Griffin, D. A., WA [Western Australia] ocean movies 1993-2000 [Online], CSIRO Marine and Atmospheric Research, Available: http://www.marin.csiro.au/~griffin/WACD/CIP.htm. Griffin, D. A., Wilkin, J. L., Chubb, C. F., Pearce, A. F. & Caputi, N. 2001, Mesoscale oceanographic data assimilative modelling with application to Western Australian fisheries. Fisheries Research and Development Corporation final report 97/139, CSIRO Marine and Atmospheric Research. Griffiths, R. W. & Pearce, A. F. 1985a, 'Instability and eddy pairs on the Leeuwin current south of Australia', Deep Sea Research Part A. Oceanographic Research Papers, vol. 32, no. 12, pp. 1511-1534.
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Griffiths, R. W. & Pearce, A. F. 1985b, 'Satellite images of an unstable warm eddy derived from the Leeuwin Current', Deep Sea Research Part A. Oceanographic Research Papers, vol. 32, no. 11, pp. 1371-1380. Haidvogel, D. B., Arango, H. G., Hedstrom, K., Beckmann, A., Malanotte-Rizzoli, P. & Shchepetkin, A. F. 2000, 'Model evaluation experiments in the North Atlantic Basin: Simulations in nonlinear terrain-following coordinates', Dynamics of Atmospheres and Oceans, vol. 32, pp. 239-281. Holloway, P. E. 1995, 'Leeuwin current observations on the Australian North West Shelf, May-June 1993', Deep Sea Research Part I: Oceanographic Research Papers, vol. 42, no. 3, pp. 285-305. Hughes, T. 1991, Indonesian Throughflow and its effect on the climate of the Indian Ocean, MM67804, McGill University (Canada). Kantha, L. H. & Clayson, C. A. 2000, Numerical Models of Oceans and Oceanic Processes, Academic Press, San Diego. Large, W. G., McWilliams, J. C. & Doney, S. C. 1994, 'Oceanic vertical mixing: a review and a model with a nonlocal boundary layer parameterization', Reviews of Geophysics, vol. 32, pp. 363-404. Legeckis, R. & Cresswell, G. 1981, 'Satellite observations of sea-surface temperature fronts off the coast of western and southern Australia', Deep Sea Research Part A. Oceanographic Research Papers, vol. 28, no. 3, pp. 297-306. Levitus, S. & Boyer, T. P. 1994, World Ocean Atlas 1994, vol 4: Temperature, NOAA Atlas NESDIS 4. US Government Printing Office, Washington. Levitus, S., Burgett, R. & Boyer, T. P. 1994, World Ocean Atlas 1994, vol 3: Salinity, NOAA Atlas NESDIS 3. US Government Printing Office, Washington. Meuleners, M. J., Pattiaratchi, C. B. & Ivey, G. N. 2005a, 'Numerical Modelling of the Mean Flow Characteristics of the Leeuwin Current System'. Meuleners, M. J., Pattiaratchi, C. B. & Ivey, G. N. 2005b, 'A Numerical Study of the Eddying Characteristics of the Leeuwin Current System'. Morrow, R., Fang, F., Fieux, M. & Molcard, R. 2003, 'Anatomy of three warm-core Leeuwin Current eddies', Deep Sea Research Part II: Topical Studies in Oceanography Physical Oceanography of the Indian Ocean: from WOCE to CLIVAR, vol. 50, no. 12-13, pp. 2229-2243.
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Nof, D., Pichevin, T. & Sprintall, J. 2002, '"Teddies" and the origin of the Leeuwin current', Journal of Physical Oceanography, vol. 32, no. 9, p. 2571. Pattiaratchi, C. B. & Buchan, S. J. 1991, 'Implications of long-term climate change for the Leeuwin Current', Journal of Royal Society of Western Australia, vol. 74, pp. 133-140. Pattiaratchi, C. B., Feng, M., Berthot, A., Li, Y. & Meyers, G. 2006, Latitudinal response of the Leeuwin Current to interannual forcings using SODA reanalysis data, School of Environmental Systems Engineering, The University of Western Australia, Perth. Philander, S. G. 1990, El Nino, La Nina, and the Southern Oscillation, Academic Press, Inc., New Jersey. Potemra, J. T., Hautala, S. L. & Sprintall, J. 2003, 'Vertical structure of Indonesian Throughflow in a large-scale model', Deep Sea Research Part II: Topical Studies in Oceanography Physical Oceanography of the Indian Ocean: from WOCE to CLIVAR, vol. 50, no. 12-13, pp. 2143-2161. Rutgers IMCS Ocean Modelling Group, Regional Ocean Modelling System [Online], Rutgers University, Available: http://marine.rutgers.edu/po/index.php?model=roms [15th April 2006]. Saunders, P. M., Coward, A. C. & de Cuevas, B. A. 1999, 'Circulation of the Pacific Ocean seen in a global ocean model: Ocean Circulation and Climate Advanced Modelling project (OCCAM)', Journal of Geophysical Research- Oceans, vol. 104, no. C8, pp. 18281-18299. Smith, R. L., Huyer, A., Godfrey, J. S. & Church, J. A. 1991, 'The Leeuwin Current off Western Australia, 1986-1987', Journal of Physical Oceanography, vol. 21, no. 2, pp. 323-345. Song, Y. T. & Wright, D. G. 1998, 'A general pressure gradient formulation fro ocean models. Part II: Energy, momentum and bottom torque consistency.' Monthly Weather Review, vol. 126, pp. 3231-3247. Sprintall, J., Potemra, J. T., Hautala, S. L., Bray, N. A. & Pandoe, W. W. 2003, 'Temperature and salinity variability in the exit passages of the Indonesian Throughflow', Deep Sea Research Part II: Topical Studies in Oceanography
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Physical Oceanography of the Indian Ocean: from WOCE to CLIVAR, vol. 50, no. 12-13, pp. 2183-2204. Thompson, R. 1984, 'Observations of the Leeuwin Current off Western Australia', Journal of physical oceanography, vol. 14, no. 3, p. 623. Vranes, K., Gordon, A. L. & Ffield, A. 2002, 'The heat transport of the Indonesian Throughflow and implications for the Indian Ocean heat budget', Deep Sea Research Part II: Topical Studies in Oceanography World Ocean Circulation Experiment, vol. 49, no. 7-8, pp. 1391-1410. Warner, J. C., Geyer, W. R. & Lerczak, J. A. 2005, 'Numerical modelling of an estuary: A comprehensive skill assessment', Journal of Geophysical Research, vol. 110, p. C05001. Weaver, A. J. & Middleton, J. H. 1989, 'On the Dynamics of the Leeuwin Current', Journal of Physical Oceanography, vol. 19, no. 5, pp. 626-648. Weaver, A. J. & Middleton, J. H. 1990, 'An analytic model for the Leeuwin Current off western Australia', Continental Shelf Research, vol. 10, no. 2, pp. 105-122. Webb, D. J., Coward, A. C. & de Cuevas, B. A., The first main run of the OCCAM Global Ocean Model [Online], University of Southampton, Available: http://www.noc.soton.ac.uk/JRD/OCCAM/occam_papers.html [2nd October 2006].
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9 Figures
Figure 9-1 A schematic showing the ocean circulation that drives the LC (Nof et al. 2002)
Figure 9-2 The ROMS model domain shown with WA coastline and 100m bathymetry contours
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Figure 9-3 The distribution of the sigma layers in the vertical direction of the ROM model grid
Figure 9-4 The example of the global satellite altimetry with the two regions of interest circled (California Institute of Technology: Jet Propulsion Laboratory 2006)
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Region 1: North West Shelf
Region 2: Ningaloo
Region 3: Shark Bay
Region 4: Abrolhos Islands
Figure 9-5 The partitioning of the ROMS grid chosen for analysis of results
Figure 9-6 The 1997 El Niño peak in September as shown by satellite altimetry (California Institute of Technology: Jet Propulsion Laboratory 2006)
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Figure 9-7 The La Niña peak in November 1998 and corresponding plot of 1996, showing a weak La Niña pattern as shown by global satellite altimetry (California Institute of Technology: Jet Propulsion Laboratory 2006)
Figure 9-8 OCCAM plot of potential temperature & baroclinic velocity of May 1996 showing peak LC signature and highlighting the meandering/eddying behaviour of the current
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Figure 9-9 OCCAM plot of potential temperature & baroclinic velocity of May 1997 showing peak LC signature
Figure 9-10 OCCAM plot of potential temperature & baroclinic velocity of May 1998 showing peak LC signature
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Figure 9-11 OCCAM sea surface height average of May 1996 showing a dipole eddy pair
Figure 9-12 OCCAM v velocity plot of the surface layer showing the LC signature in July 1996, 1997 and 1998
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Figure 9-13 OCCAM v velocity plot at 27 ° S in July 1996 showing the LC-LUC coupling
Figure 9-14 ROMS Sea surface height of initial simulation day 2 with first observable oscillations
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Figure 9-15 ROMS Sea surface height of initial simulation day 40 with the oscillations greatly amplified
Figure 9-16 V velocity in the surface layer of the second simulation without tides, showing the propagation of baroclinic velocity across the western boundary and reflection at the southern end of the domain. The LC signature is still observable.
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Figure 9-17 The contrast in visibility of the LC signature seen from a 3 ° C range reduction of the MATLAB colour bar
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Figure 9-18 Satellite SST image showing distinct western inflow into the shelf break region around Shark Bay (Griffin 2001)
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10 Appendix 1: ENSO Composite Images
The following composite images use sea surface temperature anomaly, superimposed with wind vectors to show the typical El Niño and La Niña signatures. The date index at the top of each image is composed of the initials of the three monthly period and the Southern Oscillation year. This Southern Oscillation year runs from November through to October. Year zero is the year the event itself occurred and -1 and +1 are the years before and after the event respectively (Earth System Research Laboratory: Physical Sciences Division 2006).
E L
N I N O
L A
N I N A
Figure 10-1 Southern oscillation composite image showing May-October (year -1) (Earth System Research Laboratory: Physical Sciences Division 2006)
E L
N I N O
L A
N I N A
Figure 10-2 Southern oscillation composite image showing November-April (year 0) (Earth System Research Laboratory: Physical Sciences Division 2006)
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E L
N I N O
L A
N I N A
Figure 10-3 Southern oscillation composite image showing May-October (year 0) (Earth System Research Laboratory: Physical Sciences Division 2006)
E L
N I N O
L A
N I N A
Figure 10-4 Southern oscillation composite image showing November-April (year +1) (Earth System Research Laboratory: Physical Sciences Division 2006)
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11 Appendix 2: OCCAM Southern Oscillation Plots
11.1 September
1996 1998
1997
Figure 11-1 OCCAM plot of equatorial Pacific in September 1996, 1997 and 1998
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1996 1998
1997
Figure 11-2 OCCAM plot of ITF region in September 1996, 1997 and 1998
11.2 October
1996 1998
1997
Figure 11-3 OCCAM plot of equatorial Pacific in October 1996, 1997 and 1998
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1996 1998
1997
Figure 11-4 OCCAM plot of ITF region in October 1996, 1997 and 1998
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11.3 November
1996 1998
1997
Figure 11-5 OCCAM plot of equatorial Pacific in November 1996, 1997 and 1998
1996 1998
1997
Figure 11-6 OCCAM plot of ITF region in November 1996, 1997 and 1998
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11.4 December
1996 1998
1997
Figure 11-7 OCCAM plot of equatorial Pacific in December 1996, 1997 and 1998
1996 1998
1997
Figure 11-8 OCCAM plot of ITF region in December 1996, 1997 and 1998
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12 Appendix 3: Comparative plots of model results
12.1 Satellite Imagery Comparison
Figure 12-1 Comparison of satellite imagery (Griffin 2001) and model results on the 17th September 1996 showing similar temperature patterns
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12.2 July
Temperature
Figure 12-2 ROMS potential temperature at 5 m depth on the 21st July 1996, 1997 and 1998 (° C)
V Velocity
Figure 12-3 ROMS v velocity at 5 depth on the 21st July 1996, 1997 and 1998 (m/s) Salinity
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Figure 12-4 ROMS salinity at 5 m depth on 21st July, 1996, 1997 and 1998 respectively (PSU)
12.3 August/September/October
Temperature
Figure 12-5 ROMS potential temperature at 5 m depth on the 20th August 1996, 1997 and 1998 (° C)
96 - The Interannual Variability of The Leeuwin Current Ryan Warrington
Figure 12-6 ROMS potential temperature at 5 m depth on the 19th October 1996, 1997 and 1998 (° C)
Salinity
Figure 12-7 ROMS salinity at 5 depth on the 20th August 1996, 1997 and 1998 (PSU)
97 - The Interannual Variability of The Leeuwin Current Ryan Warrington
Figure 12-8 ROMS salinity at 5 depth on the 19th October 1996, 1997 and 1998 (PSU)
V Velocity
Figure 12-9ROMS v velocity at 5 depth on the 29th September 1996, 1997 and 1998 (m/s)
98 - The Interannual Variability of The Leeuwin Current Ryan Warrington
12.4 October/November
Temperature
Figure 12-10 ROMS potential temperature at 5 depth on the 18th November 1996, 1997 and 1998 (° C)
Figure 12-11 ROMS potential temperature at 5 depth on the 18th December 1996, 1997 and 1998 (° C)
99 - The Interannual Variability of The Leeuwin Current Ryan Warrington
Salinity
Figure 12-12 ROMS salinity at 5 depth on the 18th November 1996, 1997 and 1998 (PSU)
Figure 12-13 ROMS salinity at 5 depth on the 18th December 1996, 1997 and 1998 (PSU)
100 - The Interannual Variability of The Leeuwin Current Ryan Warrington
Velocity
Figure 12-14 ROMS v velocity at 5m depth on the 18th December 1997. Highlights the development of two large cyclonic features on the NWS
12.5 Leeuwin Current/Leeuwin Undercurrent coupling
Figure 12-15 V velocity vertical cross section across 29 ° S transect on the 29th September 1997, showing the LC-LUC coupling
101 - The Interannual Variability of The Leeuwin Current Ryan Warrington
Figure 12-16 V velocity vertical cross section across 24 ° S transect on the 29th September 1996, showing the lack of LC-LUC coupling
102 - The Interannual Variability of The Leeuwin Current Ryan Warrington
103 -