986 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29

Models of the Southeast Asian Seas

ROXANA C. WAJSOWICZ Department of Meteorology/JCESS, University of Maryland at College Park, College Park, Maryland

(Manuscript received 23 October 1997, in ®nal form 17 June 1998)

ABSTRACT The mean and seasonal variations in transport through and within the Southeast Asian seas are investigated using a series of simple models. The results are compared with results from a ®ne-resolution, 3D, numerical simulation of the global circulation from 1987 to 1995 [Parallel Ocean Climate Model (POCM)]. For the mean circulation, the models are based on Sverdrup dynamics with the circulation around each island calculated according to the island rule with overlapping islands taken into account. Assuming all of the passages are wide and deep yields an archipelago circulation vastly at odds with observations. A large westward transport through Torres Strait provides the through¯ow between the Paci®c and Indian Oceans. A large westward transport through Luzon Strait passes southward through the South China Sea into the Sulu Sea and exits into the Paci®c Ocean through the Celebes Sea. There is a northwestward transport through the remainder of the archipelago. By successively blocking straits under the assumption that frictional effects are suf®cient to arrest ¯ow in the strait, an understanding is built up of why the mean circulation in the archipelago is as observed and as simulated in POCM. For example, blocking Torres Strait yields a more realistic circulation with southward ¯ow in the archipelago. Greater realism is achieved by blocking off the South China Sea, so making the dominant pathway for the through¯ow from the Paci®c westward through the Celebes Sea and southward through Makassar Strait. The weak through¯ow in POCM (7.5 ϫ 106 m3 sϪ1) is found due to the wind stresses derived from the European Centre for Medium-Range Weather Forecasts 10-m twice-daily winds, which are much weaker than the Hellerman and Rosenstein climatology (HR) used in previous studies. Also, POCM's through¯ow is wholly fed by the South Equatorial Current rather than predominantly by the Mindanao Current, as found in models forced by HR climatology. Analysis of the wind stress datasets and that of the Florida State University from 1961 to 1995 shows that the latitude of the zero-Sverdrup-transport streamline near the Paci®c entrance to the Celebes Sea has shifted poleward over the decades, so decreasing the absolute amount originating from the Mindanao Current. Regarding the seasonal cycle, there is negligible transport below 500 m at annual period within the archipelago in POCM, which suggests that the numerous islands and sills within the archipelago enhance the adjustment to the applied wind stress locally. Assuming a local Sverdrup balance, island-rule-based models of the archipelago show that forcing by wind stresses over the archipelago and Australia give reasonable agreement with POCM for the amplitude of the annual harmonic in depth-integrated transport. Better agreement in phase within the straits and seas is obtained by recognizing that frictional effects within certain straits enables the in¯uence of wind stress variations to be felt in directions other than just to the west, as in the original island rule. It is further noted that the adjustment to semiannual period wind-stress forcing is incomplete within the seas; there is no local quasi-equilibrium response. In POCM, the archipelago ®lls above 500 m in February±June and September±November, and drains in the remaining months. There is compensating ¯ow below. Also, seasonal variability of the currents in the west Paci®c is not suf®cient to alter signi®cantly the gyre closure in the west Paci®c, and the depth-integrated through¯ow is fed by the South Equatorial Current throughout the year, either via a western boundary current or a broad zonal jet.

1. Introduction terannual variability in the transport of mass and heat Several observational programs are underway and nu- through the archipelago from the tropical Paci®c to In- merous groups are setting up models to investigate the dian Ocean, and its consequences, or air±sea interaction circulation within and through the Indonesian archipel- over the archipelago. Therefore, it is timely to review ago. The region is recognized as potentially important results from a state-of-the-art numerical general circu- in the El NinÄo±Southern Oscillation due to either in- lation model (GCM) in the context of theoretical studies and observations. A comparison between observation and GCM is reported by Gordon and McClean (1999). Here, the focus is on a comparison between GCM and Corresponding author address: Dr. Roxana C. Wajsowicz, Dept. theoretical studies, speci®cally Wajsowicz (1993a,b; of Meteorology/JCESS, University of Maryland, 3433 Computer and Space Science Bldg., College Park, MD 20742. 1996). The state-of-the-art numerical GCM considered E-mail: [email protected] here is the global Parallel Ocean Climate Model

᭧ 1999 American Meteorological Society

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(POCM) developed by Semtner and Chervin (1992), Makassar Strait when the prevailing winds are a min- which has its origins in the 3D GCMs developed by K. imum. The importance of friction in spreading the in- Bryan and colleagues at the Geophysical Fluid Dynam- ¯uence of wind stress variations over a range of latitudes ics Laboratory, Princeton [see, e.g., Bryan (1969) and and eastward as well as westward is investigated spe- Cox (1984)]. POCM has a global domain with grid res- ci®cally in connection with the Philippines and Kali- olution 0.4Њ in longitude and 0.4Њcos␪ in latitude (␪). It mantan and the effect on the circulation around Sula- has 20 levels in the vertical with 9 in the upper 500 m. wesi. POCM was run for 1987 to 1995 forced by 3-day av- The semiannual harmonic in transports within the erage wind stresses derived from twice-daily European Southeast Asian seas shows considerable variation in Centre for Medium-Range Weather Forecasts 10-m phase with depth indicating that the adjustment is in- winds (ECMWF), and heat ¯uxes derived from Barnier complete. The second baroclinic mode is responsible, et al. (1995); see Stammer et al. (1996) and Tokmakian and draining and ®lling of the seas, chie¯y the Flores (1996) for further details. and Banda Seas, above 500 m are described in section In section 2, POCM's through¯ow transport is de- 4. Detailed modeling of the ``capacitance effect'' is be- scribed, and the properties of the mean through¯ow, yond the scope of the present study, and only the depth- namely magnitude, path, and origin, are compared with integrated transport is considered in section 5. Results previous estimates from climatology. The magnitude is are summarized and discussed in section 6. much smaller than the island rule estimate of Godfrey (1989) based on Hellerman and Rosenstein (1983) wind 2. Mean through¯ow transport and archipelago stress climatology. Also, the closure of the northern and circulation southern tropical gyres within the western boundary lay- er of the equatorial Paci®c, and so the water mass source In discussing the through¯ow most modelers refer to of the through¯ow, is different from that deduced from the total depth-integrated transport, whereas observa- observations, for example, Western Equatorial Paci®c tionalists refer to the geostrophic transport relative to a Ocean Climate Studies buoy tracks in Lukas et al. depth of no motion, say 500 db. The location of the line (1991) and salt balance analysis in Gordon (1986). The between the Asian and Australian continents, across pathway for the transport within the archipelago is also which the through¯ow is measured, is also differently different from observations reported in Wyrtki (1961) chosen. From POCM, for the 1987±95 average, these and F®eld and Gordon (1992). These results could in- distinctions are minor, as the ¯ow is con®ned to above dicate that POCM does not simulate well the Indonesian 500 m and there is no net stage of mass within the Through¯ow and archipelago circulation, and could lead archipelago on these timescales. As described later in to the recommendation of the catch-all cure of needing section 4, these distinctions must be reexamined for better grid resolution to represent all of the islands and shorter timescales. narrow straits within the archipelago, as well as better The net depth-integrated through¯ow transport is in- physics. In section 3, possible causes are examined with dependent of location of measurement on all timescales the help of Sverdrup models and simpli®ed GCMs, resolved by POCM (seasonal and greater). The ampli- which show that the basic geometric con®guration of tude of the 9-yr mean is only 7.5 Sv (Sv ϵ 106 m3 sϪ1) POCM is adequate, and the properties described in sec- compared with Godfrey's (1989) Sverdrup model esti- tion 2 result from the ECMWF wind stresses used to mate of 16 Ϯ 4 Sv; see Fig. 1a for the spectrum. The force POCM. next largest component is the annual harmonic with am- The seasonal cycle, speci®cally the response at annual plitude 3.4 Sv and maximum southward transport to- and semiannual periods, is examined in detail in section ward the end of July. The semiannual component is not 4. The horizontal distribution of the signal within the a distinct peak and has an amplitude of only 0.85 Sv Southeast Asian seas, and the coherency over depth, and maximum southward transport toward the end of which is a measure of the Rossby adjustment within the February and August. The structure of the seasonal cy- archipelago in response to the forcing, are described. cle is shown in Fig. 1b. It has a maximum southward Although there is considerable seasonal variability in transport of 11.7 Sv in August. The modeled double the structure of the currents in the west Paci®c, the peak reported by Masumoto and Yamagata (1993) is not through¯ow is fed by the South Equatorial Current obvious; the peak is only slightly humpbacked with a (SEC) throughout the year. The variability occurs in secondary maximum in April. The minimum southward whether feeding is by a western boundary current or by transport of 3.2 Sv occurs in February; there is a min- a broad zonal jet. imum of almost similar magnitude in December. These The completeness of the adjustment at annual period compare with maximum (minimum) southward trans- suggests that the multiple island rule could be used to port of 11.6 Sv (6.0 Sv) in August (January) reported explain aspects of the seasonal cycle in transports. In in the revised modeling study of Masumoto and Ya- section 5, simple models forced by global and regional magata (1996) and maximum, net transport relative to seasonal wind-stress variations are used to help explain 400 m toward the west between Australia and Indonesia such aspects as the boreal maximum in transport through of 12 Sv, from XBT observations by Meyers et al.

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FIG. 1. Time series analysis of POCM's net depth-integrated Indonesian Through¯ow transport, ␺Au, for 1987±95. The amplitude ␺n and 53 phase ␾ n of each Fourier component n is shown in (a), where ␺Au(t) ϭ ⌺nϭ0 ␺n cos(2␲nt/108 Ϫ ␾ n), t ϭ 0,1,2,´´´,107. Deviations from the 1987±1995 mean of ␺Au ϭϪ7.46 Sv are shown in (b) for monthly averages (seasonal cycle), and in (c) for each month, with the 12- month running mean overplotted using a dot-dashed line (interannual variability).

(1995). Properties of the seasonal cycle are discussed ways of the ¯ow through and within the archipelago further in sections 4 and 5. over various depths are shown in Fig. 2a. There is almost For completeness, the interannual variability in net no difference in the ¯ow integrated over the total depth depth-integrated through¯ow transport has a broad spec- and that integrated over the upper 510 m. Figure 2a trum with amplitude of 1.0±1.5 Sv; see Fig. 1a. The shows that the main path is southward through the Hal- year-by-year structure is shown in Fig. 1c. As expected mahera Sea (3.2 Sv), and on through the Maluku pas- from earlier theories [e.g., Wajsowicz (1993a)] the sages. A lesser amount (1.4 Sv) enters the Celebes Sea southward through¯ow is about 3 Sv weaker during the from the Paci®c, combines with transport from the Sulu 1991±93 ENSO and stronger during the ensuing La NinÄa basin (0.7 Sv), and ¯ows southward through the Ma- in 1995. Surprisingly, the variabilities expected for the kassar Strait (2.1 Sv) and eastward to the Savu Strait. 1986±87 and 1988±89 events are not distinguishable. Savu Strait is the westernmost, deep exit strait in POCM, This likely re¯ects the time for POCM to adjust to the and its 7.0-Sv southward transport is made up of that switch from monthly climatological to 3-day forcing at from the Maluku passages and Makassar Strait, plus a the beginning of 1987. 1.4-Sv ¯ow through the Torres Strait, which turns north- Returning to properties of the mean ¯ow, the path- ward into the Banda Sea east of Timor. The westernmost

→

FIG. 2. The net transports through major straits and passages calculated from the average of POCM's 1987±95 integration are shown in (a). The transport integrated over the upper 510 m is indistinguishable from that over the total depth, and is denoted by a solid black arrow. A solid gray arrow denotes the transport integrated over the upper 50 m. The net transport into the Southeast Asian seas from the Paci®c Ocean and from the seas into the Indian Ocean are also plotted. A detailed map of the mean transport integrated over the upper 510 m for each grid box of POCM is shown in (b). Addition of these values across a strait yields the value plotted in (a). Results for the depth- integrated transport are summarized in Table 1. Calibration vectors are drawn beneath each plot. The darker gray coastlines drawn denote the grid points where u, ␷ ϭ 0. The lighter gray coastlines are of the neighboring T, S grid points. Note, the strait between Sumatra and Java is represented, but that between Java and Flores is not, for example.

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TABLE 1. Summary of POCM transports integrated over total depth for 1987±95 mean.

Major straits Transport (Sv) Major islands Inferred ␺ (Sv)

Luzon ␺Phϩ Ϫ␺Ta Ϫ2.85

N. Palawan ␺Ph Ϫ␺Pw Ϫ0.39

S. Palawan ␺Ka Ϫ␺Pw 0.28 Irian Jaya (␺Ir) Ϫ6.07

Sulu ␺Ph Ϫ␺Ka Ϫ0.67 N. Philippines (␺Phϩ) Ϫ1.20

Celebes ␺Su Ϫ␺PhϪ Ϫ1.45 S. Philippines (␺PhϪ) Ϫ1.40

N. Maluku ␺Ha Ϫ␺Su Ϫ0.04 Taiwan (␺Ta) 1.65

Halmahera Sea ␺Ir Ϫ␺Ha Ϫ3.18 Australia (␺Au) Ϫ7.46

Maluku ␺Ir Ϫ␺Su Ϫ3.22 Halmahera (␺Ha) Ϫ2.89

Makassar ␺Su Ϫ␺Ka Ϫ2.12 Timor (␺Ti) Ϫ7.33

Arafura ␺Ir Ϫ␺Ti 1.26 Palawan (␺Pw) Ϫ1.01

Torres ␺Au Ϫ␺Ir Ϫ1.39 Sulawesi (␺Su) Ϫ2.85

Timor ␺Au Ϫ␺Ti Ϫ0.13 Kalimantan (␺Ka) Ϫ0.73

Taiwan ␺Ta 1.65 Java±Flores (␺Ja) Ϫ0.33

Sunda Shelf ␺Ka Ϫ0.73

Savu ␺Ti Ϫ␺Ja Ϫ7.00

Through¯ow ␺Au Ϫ7.46 exit strait in POCM is between Sumatra and Java, which wide strait, or the last strait is reached, and that carries is barely a grid point wide (40 km) and only 50 m deep, the remainder of the transport. This explains why the but it carries a southward transport of 0.3 Sv, compared majority of the through¯ow passes through Savu Strait with that through the very wide, deep Timor Strait of instead of Timor Strait, and why any passes through the only 0.1 Sv. The only other signi®cant mean net ¯ow very narrow, shallow strait between Sumatra and Java within the archipelago is a 2.8-Sv in¯ow through Luzon (similarly for Lombok vs. Timor Strait in the Los Al- Strait, between Taiwan and the Philippines, of which amos POP GCM: Gordon and McClean 1999). How- 1.6 Sv returns in a clockwise circulation around Taiwan ever, Makassar Strait is typically described as the major and the remainder ¯ows southward into the South China pathway for the through¯ow; therefore, it is important Sea. The major strait transports are summarized in Table to examine why POCM favors a divided path, with the 1. POCM has a free surface, and so a streamfunction more eastern path having the larger transport. for the depth-integrated ¯ow ␺ is not de®ned. However, Finally, the detailed circulation pattern, Fig. 2b, an approximate value can be inferred for each island shows that the closure of the gyres is not that expected from the neighboring strait transports, according to ␺x from previous climatological studies discussed by God- ϭ ␷ , ␺y ϭϪu, where u, ␷ are the depth-integrated frey et al. (1993), where the South Equatorial Current strait transports at sections shown in Fig. 2a, and it is retro¯ects eastward at Irian Jaya to contribute to the assumed ␺ ϭ 0 on the Asian continent. The values are North Equatorial Countercurrent (NECC), and the Min- given in Table 1. danao Current feeds the through¯ow and the NECC. In Also shown in Fig. 2a is the transport integrated over POCM, the through¯ow is fed by the SEC, and the the upper 50 m, which is a measure of the transport Mindanao Current almost wholly feeds the NECC. The within the Ekman or mixed layer. The transport into the reasons for these discrepancies are explored in section 3. Indian Ocean over this depth is 2.8 Sv compared with an in¯ow from the Paci®c of only 0.7 Sv. Net upwelling of 2.1 Sv into the mixed layer over the archipelago is 3. Dependency of mean through¯ow properties on chie¯y due to topographic effects at the northern edge geometry and wind stress of the Sunda Shelf and the shelf between Taiwan and China, accounting for 1.1 Sv. An additional 0.5 Sv up- There are two likely sources for the discrepancies in wells into the layer over the Sunda Shelf, 0.3 Sv over through¯ow magnitude and composition between the southern ridge blocking the Savu Basin, and 0.2 Sv POCM and previous modeling and observational studies over the entrance to the Maluku passages. Upwelling of identi®ed in section 2. The ®rst is speci®cation of 0.4 Sv over each of the Celebes and Banda Seas is POCM's geometry/topography, (e.g., an open Torres chie¯y due to wind stress curl. Major downwelling, 0.9 Strait). The second is the prescribed surface forcing, Sv, occurs over the Arafura Shelf, as the 1.4 Sv, which wind stress being more likely than heat ¯ux. In this enters the archipelago through the 50 m deep Torres section, a dynamical model, based on the multiple island Strait, descends. rule (Wajsowicz 1993a), forced by the Hellerman and The pathway of POCM's net through¯ow is not in- Rosenstein (1983) wind stress climatology (HR hence- consistent with theory. Wajsowicz's (1996) electrical forth) or POCM's ECMWF 1987±95 climatology circuit theory gives that for a group of parallel straits, (ECMWF henceforth), and with different archipelago the western most strait is selected up to a frictionally con®gurations, is used to illustrate the variety of cir- determined limit, and so on eastward until a dynamically culations readily achievable.

Unauthenticated | Downloaded 09/23/21 06:37 PM UTC MAY 1999 WAJSOWICZ 991 a. Simple models using multiple island rule Sverdrup circulation of Fig. 3, namely, New Zealand, Irian Jaya±PNG, the Philippines, and Taiwan. Second, The ®rst step is to construct the depth-integrated the multiple island rule is used to calculate ␺ on islands transport streamfunction ␺ by integrating the Sverdrup overlapped by these islands, namely Australia (by New relationship (Gill 1982, p 465): Zealand), Halmahera (by Irian Jaya±PNG), Timor (by 1 ␭ Irian Jaya±PNG) and Palawan (by the Philippines). Fi- ␺(␭) ϭ (curl␶)´kR cos␪ d␭ nally, ␺ on Sulawesi (overlapped by Halmahera and ␳␤o ͵ 0 Irian Jaya±PNG) and Kalimantan (overlapped by the R2 ␭ Philippines, Halmahera and Sulawesi) are calculated. ϭ (curl␶)´k d␭, (3.1) Equation (3.2) was derived under the assumption that 2⍀␳o ͵ 0 friction was only important in western boundary layers. where ␭ is longitude, ␪ is latitude, R is the earth's radius, In the following subsections, cases where different ⍀ is the earth's rotation rate, ␳o is a representative den- straits are described as blocked are considered. This is sity, and (curl␶)´k is the vertical component of the equivalent to assuming that friction is suf®cient to arrest wind-stress curl. The boundary condition on the west ¯ow in the particular strait, and is negligible outside coast of the American continent (␭ ϭ 0) is ␺ ϭ 0. The any western boundary layer or the strait where ␺ has results are shown in Figs. 3a,b for HR and ECMWF the value of the adjacent land masses. Suppose that fric- data, respectively. There are considerable differences in tion in the strait between overlapping islands i and i ϩ the magnitudes of the subtropical gyres and meridional 1 is suf®cient to arrest the ¯ow, then (3.2) applied to gradient in ␺ over the tropical latitudes; the HR-forced each island would have an additional frictional term, model is typically stronger. (cf. Wajsowicz 1993a). Equation (3.2) with no addi- The next step is to specify the western boundary layer tional friction term applies to the cojoined island, as ␺ dynamics and archipelago con®guration to close the ϭ ␺i ϭ ␺iϩ1 in the strait. This latter value can be sub- gyres shown in Figs. 3a,b and so determine the mag- stituted in the previous equations to calculate the nec- nitude and composition of the through¯ow. For the mod- essary value of the friction integral for each island. els described herein, the archipelago consists of POCM's Irian Jaya±Papua New Guinea (PNG), Austra- lia, Halmahera, the Philippines (joined to form a single 1) ALL STRAITS OPEN island), Taiwan, Sulawesi, Timor, Palawan, and Kali- In the ®rst model, it is assumed that all of the passages mantan. Java is assumed connected to Sumatra. The within the archipelago are wide and deep. The circu- names of the islands and straits used are given in Figs. lation resulting from forcing with ECMWF wind stress- 3c,d. The value of ␺ on each island is calculated from es is shown in Fig. 4a and summarized for both HR and the multiple island rule (Wajsowicz 1993a), which can ECMWF forcing in Table 2, columns 1 and 4, respec- be expressed in the more numerically accessible form of tively. Although the through¯ow magnitude is not un- Ϫ1 reasonable, 18 Sv into the Indian Ocean, its origin and ␺j ϭ curl␶ ´ k dA the archipelago circulation are completely at odds with ( fNЈ Ϫ f SЈ) ͵ DЈ observation. It is fed by a 23-Sv westward transport 1 through Torres Strait! There is a 6-Sv northward trans- ϩ YOn␺ n, (3.2) port through Savu Strait, which splits into northward 2 Y ͸ Ј nϭ1,jϪ1 Sv and 4 Sv transports across the Sunda Shelf and where ␺j is the value of the streamfunction on island j through Makassar Strait, respectively. There is an enor- sheltered from the Paci®c's interior Sverdrup ¯ow by mous 42-Sv westward transport through Luzon Strait islands n, n ϭ 0,´´´, j Ϫ 1 upon which the stream- between the Philippines and Taiwan, 29 Sv of which function is ␺n; YOn is the meridional distance of overlap turns northward and exits through Taiwan Strait, and between the islands j, n; and YЈ is the meridional extent the remainder of which drives a southward circulation of island j. The area DЈ is that bounded to the north and through the strait directly south of Palawan and eastward south by latitude lines extending eastward from the tips into the Paci®c through the Celebes Sea. Although Fig. of island j, to the west by the west coast of island j, 4a is obviously incorrrect, it serves a useful purpose in and to the east by the west coasts of the islands n, n ϭ showing the pressure gradients that the wind stress 0,´´´,j Ϫ 1, possibly lines of latitude extending east- wants to set up within the archipelago, which friction, wards from their northern and southern tips, and pos- invoked by geometric±topographic features, needs to sibly part of the eastern boundary of the adjacent ocean suppress. if the overlapping is not complete. The Coriolis param- The results for a model forced by HR climatology are eter has the value of f NЈ, f SЈ on the northern and southern similar. The North Equatorial Current (NEC) feeds an latitudes, respectively, of the tips of island j. anticlockwise gyre around the Philippines and Palawan, The order of calculation is important to preserve its through the South China Sea exiting into the North elegance and simplicity. First, the island rule is used to Equatorial Countercurrent. The South Equatorial Cur- calculate ␺ on the islands wholly exposed to the Paci®c rent feeds a clockwise gyre around Irian Jaya, Timor,

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FIG. 3. The transport streamfunction calculated from the Sverdrup balance (3.1) is contoured as a function of latitude and longitude for (a) the Hellerman and Rosenstein's (1983) wind stress climatology and (b) the ECMWF 1987±95 mean used to force POCM. The contour interval is 5 Sv with a positive contour denoted by solid line, negative by a dashed, and the zero contour by a dotted line. The westward integration of (3.1) is stopped at the Australian±Asian boundary and the gyres are left open. The respective close-up plots of the Southeast Asian region are shown in (c) and (d). In (d) for ECMWF forcing, the streamfunction is positive over the latitudes of the Paci®c entrance

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FIG.3.(Continued) to the Indonesian seas, approximately from the equator to 4ЊN, indicating a bias toward an SEC-fed through¯ow in contrast with the negative distribution in (c) for Hellerman and Rosenstein forcing. The gyres are closed in both plots assuming relative vorticity is destroyed at the latitude of creation in western boundary layers and that friction is suf®cient to arrest ¯ows in Torres Strait and straits entering the South China Sea except for Luzon Strait, corresponding to Fig. 4f.

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TABLE 2. Summary of basic multiple-island-rule calculations for annual mean. Hellerman and Rosenstein ECMWF Torres Strait and Torres Strait and All straits Torres Strait Halmahera Sea All straits Torres Strait Halmahera Sea open closed closed open closed closed ␺ on Islands (Sv) New Zealand Ϫ30.10 Ϫ30.10 Ϫ30.10 Ϫ22.85 Ϫ22.85 Ϫ22.85

Irian Jaya (␺Ir) 11.65 Ϫ23.13 Ϫ21.99 5.15 Ϫ15.66 Ϫ14.77

Philippines (␺Ph) Ϫ15.95 Ϫ15.95 Ϫ15.95 Ϫ13.39 Ϫ13.39 Ϫ13.39

Taiwan (␺Ta) 38.16 38.16 38.16 29.07 29.07 29.07

Australia (␺Au) Ϫ28.19 Ϫ23.13 Ϫ21.99 Ϫ17.98 Ϫ15.66 Ϫ14.77

Halmahera (␺Ha) 2.92 Ϫ2.87 Ϫ21.99 3.51 0.04 Ϫ14.77

Timor (␺Ti) 12.08 Ϫ22.70 Ϫ21.55 5.88 Ϫ14.93 Ϫ14.04

Palawan (␺Pw) Ϫ16.26 Ϫ16.26 Ϫ16.26 Ϫ13.48 Ϫ13.48 Ϫ13.48

Sulawesi (␺Su) 10.12 Ϫ16.60 Ϫ21.09 5.59 Ϫ10.40 Ϫ13.88

Kalimantan (␺Ka) Ϫ0.30 Ϫ14.38 Ϫ17.42 1.94 Ϫ6.49 Ϫ8.84 Strait transports (Sv)

Luzon ␺Ph Ϫ␺Ta Ϫ54.11 Ϫ54.11 Ϫ54.11 Ϫ42.47 Ϫ42.47 Ϫ42.47

N. Palawan ␺Ph Ϫ␺Pw 0.32 0.32 0.32 0.08 0.08 0.08

S. Palawan ␺Ka Ϫ␺Pw 15.96 1.88 Ϫ1.16 15.41 6.99 4.63

Sulu ␺Ph Ϫ␺Ka Ϫ15.65 Ϫ1.56 1.48 Ϫ15.33 Ϫ6.90 Ϫ4.55

Celebes ␺Su Ϫ␺Ph 26.07 Ϫ0.65 Ϫ5.14 18.98 3.00 Ϫ0.48

N. Maluku ␺Ha Ϫ␺Su Ϫ7.20 13.72 Ϫ0.90 Ϫ2.08 10.44 Ϫ0.90

Halmahera Sea ␺Ir Ϫ␺Ha 8.72 Ϫ20.26 0.00 1.64 Ϫ15.70 0.00

Maluku ␺Ir Ϫ␺Su 1.52 Ϫ6.54 Ϫ0.90 Ϫ0.44 Ϫ5.27 Ϫ0.90

Makassar ␺Su Ϫ␺Ka 10.42 Ϫ2.21 Ϫ3.66 3.65 Ϫ3.91 Ϫ5.03

Arafura ␺Ir Ϫ␺Ti Ϫ0.43 Ϫ0.43 Ϫ0.43 Ϫ0.73 Ϫ0.73 Ϫ0.73

Torres ␺Au Ϫ␺Ir Ϫ39.83 0.00 0.00 Ϫ23.13 0.00 0.00

Timor ␺Au Ϫ␺Ti Ϫ40.27 Ϫ0.43 Ϫ0.43 Ϫ23.86 Ϫ0.73 Ϫ0.73

Through¯ow ϭ␺Au, Taiwan Strait ϭ␺Ta, Sunda Shelf ϭ␺Ka, Savu Strait ϭ␺Ti.

and Sulawesi, which exits into the South Equatorial 2) TORRES STRAIT CLOSED Countercurrent (SECC) or New Guinea Coastal Current. However, the magnitudes are much larger; the through- Figure 4b shows the effect of blocking off Torres ¯ow is 28 Sv. Strait, which is summarized in Table 2, columns 2 and It is worth noting that Fig. 4 shows the net ¯ow across 5. The pressure gradients across the Sunda Shelf and each section, and not details of the current system within through Makassar Strait are reversed to give 6.5 and 4 each sea. Details of the current systems, and so origin Sv southward transports, respectively. The net through- of different water masses, are determined by assuming, ¯ow is reduced by 1.5 Sv and no longer exits through say, relative vorticity is destroyed at the latitude of cre- Timor Strait, but instead through Savu Strait. The trans- ation, so that streamlines entering the western boundary port through the Celebes Sea is considerably weakened, layer turn either north or south, do not overshoot the but it is still 3 Sv eastward into the Paci®c. Also, as the meridional extent of the island in the western boundary pressure head driving the South Equatorial Current has layer, and new streamlines are not generated in recir- no longer been able to leak through Torres Strait, it is culation gyres. An example of an archipelago streamline now available to drive a considerable clockwise cir- pattern under this assumption, corresponding to case f culation around Halmahera, and southward transport described shortly, is shown in Figs. 3c,d. through the Makulu passages. All three pathways, name-

←

FIG. 4. The multiple-island rule with wind stress given by ECMWF-derived values is used to determine transport within the Indonesian seas for several geometric con®gurations. In (a) all of the passages are assumed wide and deep, that is, frictional effects are negligible outside western boundary layers. In (b) frictional effects are assumed suf®cient to arrest the ¯ow in Torres Strait. In (c) frictional effects are assumed suf®cient to arrest additionally the ¯ow in the strait between Irian Jaya and Halmahera. Results are summarized in Table 2. Additional scenarios are modeled by setting the streamfunction on an island to that on the Asian continent. The basic con®guration is (b). In (d) the Sunda Shelf is blocked, in (e) the Taiwan Strait and the strait between Palawan and Kalimantan are blocked as well, and in (f) the strait between Palawan and the Philippines is blocked also, so giving that ¯ow only enters from the Paci®c through the passages between Irian Jaya and the Philippines. The results are summarized in Table 3. Calibration vectors are drawn beneath the plots.

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TABLE 3. Summary of second-order multiple-island-rule calculations for annual mean strait transport(sv). Hellerman and Rosenstein ECMWF Sunda Shelf, Sunda Shelf, S. Palawan, S. Palawan, and Taiwan and Taiwan Straits S. China Straits S. China closed Sea closed closed Sea closed

Other Sunda Shelf ␺Ka ϭ 0, ␺Ka ϭ 0, Other Sunda Shelf ␺Ka ϭ 0, ␺Ka ϭ 0,

straits closed ␺Pw ϭ 0, ␺Pw ϭ 0 straits closed ␺Pw ϭ 0, ␺Pw ϭ 0

open ␺Ka ϭ 0 ␺Ta ϭ 0 ␺Ta, ␺Ph ϭ 0 open ␺Ka ϭ 0 ␺Ta ϭ 0 ␺Ta, ␺Ph ϭ 0 Torres Strait closed

Luzon ␺Ph Ϫ␺Ta Ϫ54.11 Ϫ54.11 Ϫ15.95 0.00 Ϫ42.47 Ϫ42.47 Ϫ13.39 0.00

N. Palawan ␺Ph Ϫ␺Pw 0.32 Ϫ0.32 Ϫ15.95 0.00 0.08 0.08 Ϫ13.39 0.00

S. Palawan ␺Ka Ϫ␺Pw 1.88 Ϫ16.26 0.00 0.00 6.99 13.48 0.00 0.00

Sulu ␺Ph Ϫ␺Ka Ϫ1.56 Ϫ15.95 Ϫ15.95 0.00 Ϫ6.90 Ϫ13.39 Ϫ13.39 0.00

Celebes ␺Su Ϫ␺Ph Ϫ0.65 Ϫ0.65 Ϫ0.65 Ϫ16.60 3.00 3.00 3.00 Ϫ10.40

Makassar ␺Su Ϫ␺Ka Ϫ2.21 Ϫ16.60 Ϫ16.60 Ϫ16.60 Ϫ3.91 Ϫ10.40 Ϫ10.40 Ϫ10.40 Torres Strait and Halmahera Sea closed

Luzon ␺Ph Ϫ␺Ta Ϫ54.11 Ϫ54.11 Ϫ15.95 0.00 Ϫ42.47 Ϫ42.47 Ϫ13.39 0.00

N. Palawan ␺Ph Ϫ␺Pw 0.32 0.32 Ϫ15.95 0.00 0.08 0.08 Ϫ13.39 0.00

S. Palawan ␺Ka Ϫ␺Pw Ϫ1.16 16.26 0.00 0.00 4.63 13.48 0.00 0.00

Sulu ␺Ph Ϫ␺Ka 1.48 Ϫ15.95 Ϫ15.95 0.00 Ϫ4.55 Ϫ13.39 Ϫ13.39 0.00

Celebes ␺Su Ϫ␺Ph Ϫ5.14 Ϫ5.14 Ϫ5.14 Ϫ21.09 Ϫ0.48 Ϫ0.48 Ϫ0.48 Ϫ13.88

Makassar ␺Su Ϫ␺Ka Ϫ3.66 Ϫ21.09 Ϫ21.09 Ϫ21.09 Ϫ5.03 Ϫ13.88 Ϫ13.88 Ϫ13.88

ly Sunda Shelf, Makassar Strait, and the Maluku pas- the gap between Halmahera and Irian Jaya increases the sages, contribute about 5 Sv each to the through¯ow. westward transport into the Celebes Sea from the Paci®c Expressed in terms of the northern anticlockwise gyre from 0.6 to 5.1 Sv, but this only yields an increase in around the Philippines, and southern clockwise gyre the Makassar Strait transport of 1.4 Sv, whereas the around Irian Jaya±PNG, Timor, Sulawesi, and a transport across the Sunda Shelf increases by 3.1 Sv. through¯ow fed by the SEC through Torres Strait, the To obtain an archipelago circulation and through¯ow southern clockwise gyre has been pushed into a clock- path closer to observations requires ¯ow through pas- wise circulation around Halmahera and the Celebes Sea, sages connecting with the South China Sea to be sub- and the two gyres are now linked with 6 Sv from the stantially blocked. The form of the multiple island rule North Paci®c via the South China Sea feeding the permits this to be done without recomputing ␺ on all through¯ow. The remaining 9 Sv comes from the South of the islands, as demonstrated by the three models be- Paci®c and SEC. low. The results are summarized in Table 3. The results for a model forced by HR climatology are similar. The through¯ow is reduced by 5 to 23 Sv. The 4) SUNDA SHELF CLOSED distribution between the various parallel passages is markedly different. The Sunda Shelf has a southward The effect of closing the Sunda Shelf is found by transport of 14 Sv, Makassar Strait only 2 Sv, and the setting ␺ on Kalimantan to zero (the value on the Asian Maluku passages 7 Sv. Also, there is a small westward continent). This result is shown in Fig. 4d and sum- transport from the Paci®c into the archipelago via the marized in Table 3, columns 2 and 6. The main effect Celebes Sea. is to make Makassar Strait the dominant pathway for the through¯ow, as expected from Wajsowicz (1996); it is now the westernmost available passage. 3) TORRES STRAIT AND HALMAHERA SEA CLOSED Figure 4c shows the effect of additionally blocking 5) SUNDA SHELF,SOUTH PALAWAN, AND TAIWAN off the Halmahera Sea between Irian Jaya and Halma- STRAITS CLOSED hera, which is summarized in Table 2, columns 3 and 6. It is suf®cient to reduce the net southward transports Another easy computation is to close the straits be- through the Maluku and Banda Seas to less than 1 Sv, tween Taiwan and the mainland and between Palawan and there is now a very weak transport from the Paci®c and Kalimantan. The result is shown in Fig. 4e and Ocean into the Celebes Sea. However, the dominant path summarized in Table 3, columns 3 and 7. The effect is is still through Luzon Strait and southward through the to eliminate the unrealistic circulation around Taiwan, South China Sea. but there is still a 16-Sv westward transport through The results for HR climatology are similar. Closing Luzon Strait, which feeds the through¯ow. As expected

Unauthenticated | Downloaded 09/23/21 06:37 PM UTC MAY 1999 WAJSOWICZ 997 from Wajsowicz (1996), this transport passes through iii) The need to block off the straits connecting the the passage to the north of Palawan, which had previ- Philippines to the Asian continent to obtain signi®cant ously carried little transport, as this is now the west- southward transport through Makassar Strait does not ernmost strait. imply that the through¯ow is driven by the pressure head associated with the North Paci®c wind stress curl, which drives the Kuroshio and Mindanao Currents. It 6) SOUTH CHINA SEA CLOSED is driven by the South Paci®c wind stress curl and is Figure 4f shows that a westward transport driving the affected by the assumed latitudinal blocking extent of through¯ow from the Paci®c into the Celebes Sea is Australia. The magnitude of the through¯ow, given by achieved only if the South China Sea is blocked off by ␺ on Australia, differs for each of the experminents in assuming the Philippines is connected to the Asian con- Table 2, but is the same for each of the experiments in tinent. The results are summarized in Table 3, columns Table 3. 4 and 8. Although the path is more realistic, whether iv) The transport through Torres Strait must be al- the streamlines of the through¯ow originate in the North most zero, which con®rms the observations of Wolanski or South Paci®c depends on details of the selected west- et al. (1988), who estimated a net mean transport of less ern boundary layer dynamics at the Paci®c entrance to than 0.01 Sv westward during the boreal winter. As- the Celebes Sea. If relative vorticity is destroyed at the suming frictional effects are suf®cient to block the ¯ow latitude of creation in the western boundary layer, the through Torres Strait represents a minimum for the resulting streamfunction pattern is shown in Figs. 3c,d through¯ow magnitude. Opening Torres Strait, even for HR and ECMWF forcing respectively. partially, acts to increase the through¯ow for the Pa- Several important conclusions can be drawn from ci®c's basic climatological wind stress pattern. these experiments (models): v) No signi®cant depth-integrated transport occurs i) Geometric factors are not responsible for the weak- through Timor Strait unless the straits to the west are ness of POCM's through¯ow. There is a signi®cant dif- substantially blocked. The southward transport between ference between HR's climatology and that derived from Papua New Guinea and Timor, across the Arafura Shelf ECMWF's 10-m winds for 1987±95. In all three basic and through the Timor Trough, is only 0.4 Sv. It is the cases, the ECMWF through¯ow magnitude is only two- same in all experiments, as it is only a function of the thirds that of the HR-forced model. wind stress between PNG and Timor. A simpli®ed island rule, consisting of just the line vi) These experiments (models) demonstrate that for integrals of the wind stress along the equator and 44ЊS, the basic Sverdrup circulation shown in Fig. 3, openings gives a through¯ow of 9.6 Sv with 3.3 Sv from the from the Paci®c into the archipelago poleward of the equatorial wind stresses and 6.3 Sv from the midlatitude Celebes Sea will tend to syphon off the through¯ow. South Paci®c wind stresses for the ECMWF data used Therefore, in modeling the through¯ow, just as Waj- to force POCM. These compare with 13.8 Sv, 5.4 Sv, sowicz (1996) shows that careful consideration needs and 8.4 Sv for the total through¯ow and the equatorial to be given to groups of parallel meridional passages and midlatitude contributions, respectively, for HR cli- and their relative frictional blocking effect, so consid- matology. Therefore, the ECMWF wind stresses are sig- eration must be given to zonal, off-equatorial passages ni®cantly weaker than HR climatology in both the equa- and the blocking effect of friction. torial and southern midlatitudes of the Paci®c. Wajsow- The diagnostic value of the multiple-island-rule- icz (1994) noted that the island rule provided weak val- based models is further emphasized by considering ues for the through¯ow in the second half of the 1980s Metzger and Hurlburt's (1996) nonlinear, primitive- versus the ®rst half, but assumed it was due to a switch equation, shallow-water model results. They found that from using wind stresses derived from ECMWF 1000- the effect of opening the Shibata passage between the mb winds in the ®rst half of the 1980s to ones derived Philippines and Palawan led to a cyclonic circulation from Japan Meterological Agency (JMA) 1000-mb around the Philippines, and so an increase in the Ku- winds for the second half. As de®ned by compilation roshio and decrease in the Mindanao Current transports. dates, Hellerman and Rosenstein's climatology is based The island rule gives a negative streamfunction value on wind stress measurements pre-1982; therefore, the on the Philippines, hence Metzger and Hurlburt's result. ECMWF and JMA results may be a function of recent They also found that opening/closing the Sunda Shelf/ global changes and not necessarily a re¯ection on the Java Sea or the Sulu archipelago had no effect on the through¯ow transport. Once again, this result is easily quality of these products. explained by the island rule, as these islands and straits ii) The Celebes Sea Makassar Strait is not the dom- lie to the north and west and are not close neighbors of inant pathway for the through¯ow unless straits con- Australia. necting the Philippines to the Asian continent are sub- stantially blocked. Models forced by ECMWF and HR climatologies differ in their Celebes Sea responsesÐa b. Local versus remote forcing point which will be addressed later when through¯ow The powerfulness of the simple multiple-island-rule composition is investigated further. model is demonstrated further in tackling the issue of

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TABLE 4. Summary of geometry and contributions from annual mean local forcing in multiple-island-rule calculations. Contribution from forcing over island and adjacent sea/ocean Latitude of Latitude of Contribution from Hellerman and northern tip southern tip overlapping islands Rosenstein (Sv) ECMWF (Sv)

New Zealand (␺Nz) 34.53ЊS 47.20ЊS Ð Ϫ30.10 Ϫ22.85

Irian Jaya (␺Ir) Eq. 10.74ЊS Ð 11.65 5.15 Irian Jaya (ϩ) Љ 43.54ЊS Ð Ϫ16.91 Ϫ10.93 Irian Jaya (*) 2.00ЊN 43.54ЊS Ð Ϫ16.03 Ϫ10.25

Philippines (␺Ph) 18.48ЊN 5.59ЊN Ð Ϫ15.95 Ϫ13.39

Taiwan (␺Ta) 25.17ЊN 22.24ЊN Ð 38.16 29.07

Australia (␺Au) 10.74ЊS 43.54ЊS 0.275␺Nz Ϫ19.92 Ϫ11.70

Australia (ϩ) Eq. 43.54ЊS 0.207␺Nz Ϫ16.91 Ϫ10.93

Australia (*) 2.00ЊN 43.54ЊS 0.198␺Nz Ϫ16.03 Ϫ10.25

Halmahera (␺Ha) 2.00ЊN 0.40ЊS 0.167␺Ir 0.98 2.65 Halmahera (*) 2.00ЊN 43.54ЊS Ð Ϫ16.03 Ϫ10.25

Timor (␺Ti) 7.58ЊS 10.35ЊS ␺Ir 0.43 0.73

Palawan (␺Pw) 11.13ЊN 8.37ЊN ␺Ph Ϫ0.32 Ϫ0.08

Sulawesi (␺Su) 1.60ЊN 5.59ЊS 0.722␺Ir ϩ 0.278␺Ha 0.90 0.90

Kalimantan (␺Ka) 6.79ЊN 4.00ЊS 0.111␺Ph ϩ 0.037␺Ha ϩ 0.519␺Su Ϫ3.89 0.40

ϩ Torres Strait closed. * Torres Strait and Halmahera Sea closed. remote versus local forcing of a current system within Also, Wajsowicz (1993b) noted that time dependency the Southeast Asian seas. The separate terms in (3.2) could be important as the gyres may close differently were calculated for each of the islands using the HR at different times of the year, or in different years. and ECMWF datasets, and the results are displayed in Maps of the transport streamfunction for a Sverdrup Table 4. The total circulation around an island is given model forced by HR climatology and ECMWF wind by adding the contribution from overlapping islands [the stresses averaged for 1987±95 are shown in Fig. 3. For second term in Eq. (3.2)] and the contribution from Godfrey's (1989) Sverdrup model, in which the Indo- direct exposure to the Paci®c gyres, and/or adjacent sea, nesian seas are modeled as a simple open channel and and/or over the island itself [the ®rst term in Eq. (3.2)]. relative vorticity is assumed destroyed at the latitude of From Table 4, for islands within the seas, forcing from creation in the western boundary layer, the through¯ow wind stress over the island itself or adjacent sea is typ- is wholly fed by the SEC if the ␺ ϭ 0 contour at the ically weak. The circulation around an interior island is southern edge of the anticlockwise gyre next to the Phil- driven by overlap with an island exposed directly or ippines approaches the western boundary layer at a lat- indirectly to the Paci®c Sverdrup ¯ow. A notable ex- itude north of the northern tip of Irian Jaya (in the no- ception is Kalimantan subject to HR forcing. tation of Wajsowicz 1993b, Regime III occurs if ␺N Ն 0). From Fig. 3, this is the case for both HR and c. Through¯ow composition from a Sverdrup model ECMWF 1987±95 climatologies. However, from Waj- According to the Sverdrup model of Godfrey (1989), sowicz (1993b), if Halmahera is taken into account, the the composition of the through¯ow is determined by the through¯ow is wholly fed by the SEC only if the ␺ ϭ latitude at which the zero contour of net mass transport 0 contour cuts the western boundary layer to the north intersects the western boundary layer (cf. the zero wind of the northern tip of Halmahera (Regime III occurs if stress curl line for zonally uniform wind stress). From ␺HT Ն 0). This is the case for the ECMWF-forced model Godfrey et al. (1993), this latitude may differ by a cou- (see Fig. 3d) but not that forced by HR climatology (see ple of hundred kilometers between wind stress datasets Fig. 3c). The value of the Sverdrup streamfunction at but not suf®ciently for the Sverdrup model to give a the edge of the western boundary layer in the two mod- through¯ow fed by the Mindanao Current as deduced els and the relevant latitudes are plotted in the upper from observations. However, they noted GCMs pro- panel of Fig. 5a. It is noteworthy that for the ECMWF- duced a through¯ow fed by the Mindanao Current due forced model, the meridional gradient for the stream- to higher order dynamics; the SEC retro¯ected into the function is weak over equatorial latitudes, so even if NECC due to lateral viscous effects or barotropic in- different western boundary layer dynamics are chosen, stability. Further to this discussion, Wajsowicz (1993b) the gyre closure/through¯ow feeding would remain the noted that, if Halmahera were taken into account, then same. In contrast, the HR-forced model has a quite steep a Sverdrup model would give a 30:70 split between the gradient, making its composition much less robust. For Mindanao Current and SEC for feeding the through¯ow. example, suppose the western boundary layer dynamics

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FIG. 5. The Sverdrup transport streamfunction on the gray coastline bounding the west side of the Paci®c in each of Fig. 3 is plotted against latitude in the upper panel of (a). A dotted line is used for Hellerman and Rosenstein climatology (Figs. 3a,c) and a dot-dashed line for ECMWF 1987±95 climatology (Figs. 3b,d). The temporal variability of these values for the ECMWF data is shown in the lower panel of (a) and for the FSU wind stress data in (b). The latitudes of the northern tips of Irian Jaya and Halmahera are marked. The zero contour, whose position determines whether the through¯ow is wholly SEC-fed or not, is denoted by a bold solid line.

Unauthenticated | Downloaded 09/23/21 06:37 PM UTC 1000 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29 were nonlinear allowing for the existence of the Min- icz's (1994) ECMWF/JMA analysis for the 1980s, de- danao eddy, which effectively shifted the streamlines 2Њ scribed in section 3a, supports a sizeable reduction in of latitude equatorward. The composition is then de- total through¯ow transport. Observations are barely suf- termined by ␺ plotted at 4ЊN in Fig. 5a. For the ®cient to make a de®nitive statement about the clima- ECMWF-forced model, ␺ is still positive at 4ЊN, so the tological mean, and certainly not suf®cient to make de- through¯ow is wholly fed by the SEC; but for the HR- ductions about any long term variability. Fine's (1985) forced model, ␺ ഠ 12 Sv at 4ЊN, so the approximate tritium analysis is probably the most conclusive, indi- ratio of North to South Paci®c water mass making up cating that about 5 Sv of the through¯ow is made up the through¯ow increases from 10:90 to 55:45 for the of water ventilated in the North Paci®c for the 1960s, con®guration shown in Fig. 3c (from Wajsowicz 1993b, the remainder being fed directly, or indirectly via the Regime IIb occurs where the North Paci®c component tropical North Paci®c, from the South Paci®c. is Ϫ␺HT). Figure 3c for HR data shows the dif®culty in inter- Both Godfrey (1989) and Wajsowicz (1993b) as- preting observations. In the absence of mixing, less than sumed that ␺ ϭ 0 on the Philippines and Kalimantan. 3 Sv of the 23 Sv through¯ow comes directly from the If there is no frictional obstruction to circulation de- Mindanao Current. However, almost 20 Sv from the veloping around the Philippines, then the island rule Mindanao Current ¯ushes through the Celebes Sea be- gives that ␺Ph Ͻ 0 for the climatological mean wind fore exiting into the North Equatorial Countercurrent. stress pattern, which shifts the bifurcation latitude equa- The water mass analyses and modeling of Gordon torward for the Kuroshio and Mindanao Current. The (1986), F®eld and Gordon (1992), and Fine et al. (1994) relative fraction of North to South Paci®c water mass could/did not differentiate between a direct source and feeding the through¯ow is unchanged. However, the a source resulting from ¯ushing and mixing. Also, if North Paci®c component of the through¯ow comes from buoys were seeded in the Mindanao Current, a pattern the fraction of the Kuroshio that penetrates the South such as Fig. 3c would yield a large fraction entering the China Sea rather than the Mindanao Current. Celebes Sea and heading toward the Makassar Strait, as The variation in latitude at which the ␺ ϭ 0 contour in Lukas et al.'s (1991) observations, in which 8 of 11 approaches the western boundary layer for each of the entered the Celebes Sea. However, in the ¯ow of Fig. years 1987±95 for the ECMWF wind stresses is shown 3c, after traveling southward in the western boundary in the lower panel of Fig. 5a. Although the Sverdrup current next to Kalimantan, the buoys would exit into model is not valid on an annual timescale, as the ad- the NECC. Lukas et al. observed only two of the eight justment timescale over latitudes of the archipelago is entering Makassar Strait. Of course, it is dif®cult to 2±3 yr, it is interesting to note from Fig. 5a that the believe that if the ¯ow pattern were as in Fig. 3c, then wind stress is only favorable for producing a nonwholly none of the seven buoys seeded by Lukas et al. in the SEC-fed through¯ow in only one, possibly two, of the SEC entered either the Celebes or Banda Seas. Simi- years from 1987 to 1995, namely 1987 and possibly larly, it is dif®cult to reconcile the senario depicted in 1992. Hence, it is expected, and found, that the through- Fig. 3d for ECMWF data, in which no North Paci®c ¯ow is chie¯y fed by the SEC throughout the nine years water directly feeds the through¯ow and less than 5 Sv of the POCM integration. from the Mindanao Current ¯ushes through Celebes Sea, The Sverdrup relationship was explored further using with the limited observations. the Florida State University (FSU) wind stresses. The variation in latitude at which the ␺ ϭ 0 contour ap- d. Con®rmation using a barotropic GCM proaches the western boundary layer is shown in Fig. 5b for 1961±95. For the period 1961±82 (cf. HR cli- The above results from the island rule and Sverdrup matology) the linear Sverdrup model gives that Ϫ␺HT model are quite conclusive. The climatologies for the ഠ 3.5 Sv of the through¯ow is from the North Paci®c ECMWF-derived wind stress averaged over 1987±95 and for 1987±95 (cf. ECMWF climatology) none (␺HT and HR are fundamentally different both in magnitude Ն 0) comes from the North Paci®c. (The FSU data is and spatial pattern. The question is whether nonlinearity only to 30ЊS, so the net through¯ow value cannot be could invalidate the island rule or cause suf®cient ex- calculated from the island rule). If the streamlines were cursions of the streamlines in the western boundary lay- shifted 2Њ of latitude equatorward due to the Mindanao er to modify the closure regimes given by the simple eddy, then the North Paci®c component would increase Sverdrup model. Barotropic versions of POCM with the to 12.5 Sv for the 1961±82 mean and to 5.4 Sv for the reduced domain of Antarctica to 40ЊN, 120ЊEto70ЊW 1987±95 mean; the values are given by the average of were spun up with HR climatology and ECMWF 1987± ␺ at 4ЊN in Fig. 5b. 95 mean. The resulting streamfunctions, assuming the Whether there has been a signi®cant shift in the frac- Indonesian passages are closed, are shown in the upper tion of each current feeding the through¯ow is open to panels of Figs. 6a,b. The location and magnitudes of speculation. A decrease in through¯ow magnitude be- the gyres are very similar to their Sverdrup model coun- tween the two time periods could imply that the frac- terparts shown in Fig. 3 despite the obvious deviations tional composition remained unchanged, and Wajsow- due to nonlinearity over the equatorial latitudes. Ex-

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FIG. 6. The 300-day instantaneous transport streamfunction from a barotropic GCM spun up from rest, using (a) Hellerman and Rosenstein climatology and (b) ECMWF 1987±95 climatology. The GCM's horizontal grid resolution is the same as POCM, and the domain spans the Paci®c and Indian Oceans. The different geometries in the upper and lower panels show the robustness of the streamfunction pattern as the Paci®c's western boundary is approached. Nonlinearity produces suf®cient latitudinal excursions of the contours that the through¯ow is fed by the Mindanao Current in (a) and the SEC in (b). A Sverdrup model in which relative vorticity is destroyed at the latitude of creation would give an SEC-fed through¯ow in both cases in the absence of Halmahera. The contour interval is 5 Sv, and regions where the streamfunction is negative are shaded. cursions of up to 3Њ of latitude of the streamlines within modify the source. However, for the HR-forced model, the western boundary layer are found in both models. the more southly extent of the NECC axis and ␺ ϭ 0 For the ECMWF-forced model, the streamfunction is contour enables nonlinearity to modify the source. The positive between the equator and 4ЊN next to the western negative streamlines next to the coast in the closed ar- boundary. However, for the HR-forced model, nonlin- chipelago model connect with those of the SEC, yielding earity and the more southern latitude of the relevant ␺ a through¯ow fed mainly by the Mindanao Current in ϭ 0 contour is suf®cient to make the streamfunction the HR-forced model. Including Halmahera between 1ЊS negative north of the equator next to the western bound- and 2ЊN reaf®rms the gyre closure found above. The ary. ECMWF-forced model's ␺ ϭ 0 contour does not pen- Opening the archipelago, shown in the lower panels etrate far enough southward to signi®cantly affect the of Fig. 6, does not signi®cantly modify the stream- closure. function in the ocean interior; it is still essentially gov- In the lower panels of Fig. 6, Australia±PNG is rep- erned by Sverdrup dynamics. Halmahera is absent in resented by a thin island with a realistic eastern coast- the open archipelago models of the lower panels, and line. Further experiments with thin rectangular islands so according to Wajsowicz (1993b), as the interior spanning the latitudes of the major islands (cf. Wajsow- streamfunction is positive at the tip of Irian Jaya, both icz 1993b) produced very similar patterns (not shown). HR and ECMWF latitude of the northern data give an Retaining the zonal staggering of Irian Jaya and the SEC-fed through¯ow in a simple Sverdup model. The Philippines is important, as the streamlines of the SEC axis of the NECC and latitude of the relevant ␺ ϭ 0 have an opportunity to join up with those of the NECC contour is suf®ciently far northward in the ECMWF- before either reaches the longitude of the Mindanao Cur- forced model that nonlinearity does not signi®cantly rent. The westward order of determining gyre closure

Unauthenticated | Downloaded 09/23/21 06:37 PM UTC 1002 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29 is due to long Rossby wave dynamics, which establish Philippines form a block around which an approxi- the interior ¯ow. mately 3 Sv seasonal circulation ¯ows, with a maximum Finally, higher order dynamics do not modify sig- clockwise circulation in June. The path of the signal ni®cantly the difference in the magnitudes of the cir- integrated over the upper 510 m is the same, except at culation around Australia±PNG, which are 13.64 and the South China Sea and Celebes Sea entrances. In Lu- 7.98 Sv, respectively, for the HR- and ECMWF-forced zon Strait and the north Maluku passage, the depth- models. integrated transport slightly leads that integrated over 510 m, whereas at the Celebes Sea entrance between the Philippines and Sulawesi, vice versa holds. 4. Seasonal through¯ow transport and archipelago The transport integrated over the upper 50 m shows circulation an archipelago-wide seasonal upwelling anomaly giving In contrast to the spectral pro®le for the wind stress, an increase of transport from 1 to almost 2 Sv over that which has distinct peaks at annual and semiannual pe- depth between PT and IT in July. The upper 50-m signal riods, a spectral analysis of the through¯ow transport typically lags that of the total depth, except in the Hal- shows a dominant annual component, but the semian- mahera and Timor Strait, where it leads. nual signal is much weaker, as the energy is divided over a range of frequencies (see Fig. 1a). This suggests b. Semiannual harmonic that the seasonal cycle in through¯ow transport shown in Fig. 1c cannot be explained simply in terms of a The pathways of the semiannual transport signal are response to local forcing. The complexity of the sea- shown in Fig. 7b. The amplitude for the depth-integrated sonal signal within the archipelago is shown in Fig. 7. transport is less than 1 Sv, and the maximum north/east transport occurs in mid-May and mid-November. The path is a combination of ¯ow through the Celebes Sea± a. Annual harmonic Makassar Strait with a north/east maximum in mid-June As noted earlier for the mean through¯ow integrated and ¯ow through the Maluku passages±North Banda Sea over the total depth, the amount that enters the archi- with a maximum in mid-April. The signals combine and pelago through the various gaps from the Paci®c is the ¯ow through Savu Strait. Added to this is a contribution same as the amount that left through the various gaps from Torres Strait±Timor Strait with a north/east max- into the Indian Ocean. However, for the ¯ow integrated imum at the beginning of February. over a fraction of the total depth, the amounts are not There is a considerable discrepancy between PT and necessarily equal and in the following discussion are IT integrated over the upper 510 m for the semiannual referred to as PT and IT, respectively. signal (see Fig. 7b). The amplitude for PT is 3 Sv, and The annual period component in transport in POCM the maximum north/east transport is in mid-June, where- within and through the archipelago is plotted in Fig. 7a. as the amplitude for IT is only 1 Sv and the maximum For the depth-integrated ¯ow, both the amplitude and north/east transport is in mid-May. The path is similar phase of PT equal those of IT. The amplitude is 3.4 Sv, to that of the net depth-integrated transport, but with a and the maximum north/east transport occurs at the end considerable difference in amplitude and phase for the of January. For the ¯ow integrated over the upper 510 signal through the north and south Maluku passages. It m, the amplitudes of PT and IT are approximately equal is smaller and the phase matches that in the Makassar and equal that of the depth-integrated transport, showing Strait. The signals weaken in the Flores and Banda Seas, that the annual cycle may be considered con®ned to reemerging in Savu Strait with a lead in phase. above 500 m. However, PT's maximum north/east trans- The semiannual signal in transport over the upper 50 port occurs at the beginning of January, whereas that m is also shown in Fig. 7b. The amplitude and phase of IT is the end of January, as for the depth-integrated of PT and IT differ, but the difference is much less than transport. for that integrated over the upper 510 m. The path of the depth-integrated signal is through the How much of the above is signi®cant and realistic is Halmahera Sea, into the Maluku passages, and on dif®cult to ascertain, and it is not the purpose of this through Savu Strait. An additional contribution comes manuscript to validate POCM against observations. from Torres Strait into Timor Strait. Kalimantan and the Rather, POCM is a proxy for observations enabling im-

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FIG. 7. The amplitude and phase for (a) the annual component, and (b) the semiannual component, of the net transport through various straits calculated from POCM's 1987±95 integration. A solid black arrow denotes transport integrated over the upper 50 m, a light gray arrow over 510 m, and a mid-gray arrow over the total depth. Arrows denoting the net ¯ow between the Paci®c Ocean and the Southeast Asian seas, and the seas and Indian Ocean, are also plotted. In many instances the arrows overlap: a single black arrow indicates that all of the transport lies above 50 m; a light gray but no mid-gray arrow indicates that all of the transport lies above 510 m. The length of the arrow denotes the amplitude of the harmonic, and its direction the phase with 1 January pointing northward. Calibration vectors are drawn below the plots.

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Unauthenticated | Downloaded 09/23/21 06:37 PM UTC 1004 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29 portant features of the circulation to be identi®ed for the purpose of constructing simple process models. To this end, an important point in understanding the ar- chipelago and its circulation, which can be deduced from comparing Figs. 7a and 7b, and is expected to be model independent, is that at the annual period, there is reasonable correlation in phase of transport at all depths, so the archipelago has adjusted, whereas at the semiannual period there is a signi®cant discrepancy in the amplitude and phase of the signal integrated over the different depths. Therefore, the adjustment is in- complete. This implies that details of the archipelago geometry and topography may be important, as they act to enhance the coherency of the signal with depth. c. Capacitance of the Indonesian Seas There has been speculation that the Indonesian seas may act as a capacitor, that is store mass to be released at a later time, (e.g., Masumoto and Yamagata 1993; Meyers et al. 1995). Analysis of POCM shows that this is not the case for the net depth-integrated ¯ow on any of the timescales resolved by the model. However, from the above analysis for the transport integrated over the upper 510 m, the Indonesian seas ``drain'' from the end of June to mid-September, and from mid-November un- til the end of January. Otherwise, the seas ``®ll'' over this depth (see Fig. 8). The amplitude of this semian- nual-dominant signal is about 3 Sv. A simple explanation of this phenomenon is provided by considering the transport as a function of depth at the entrance and exit to the seas, as shown in Fig. 9. At the Indian exits, Fig. 9b, the transport is con®ned to above 500 m for each month even though the maximum FIG. 8. The transport integrated over the upper 510 m, summed sill depth is 2750 m. The transport signal is composed over all of the Paci®c entrances and over all of the Indian exits in of barotropic and predominantly ®rst baroclinic modes POCM, is plotted as a function of time in (a). The results represent from June to November. The second baroclinic mode is averages for each month for 1987±95. The remaining transport over of equal signi®cance during the remainder of the year. the total depth is plotted in (b). The signal at the Paci®c entrances is much more com- plex (see Fig. 9a). The ¯ow is con®ned to above 1750 this depth. During apparent ®lling, ¯ow enters the ar- m even though the maximum sill depth is 3300 m. Al- chipelago over the upper 500 m, some continues through though the ¯ow is chie¯y southward above 500 m the archipelago and exits into the Indian Ocean, the throughout the year, below 500 m the sign reverses in remainder downwells and returns to the Paci®c at depth. phase with the noted semiannual cycle. Applying con- Analysis of the individual basins within the archi- tinuity to the signals in Figs. 9a,b implies the up/down- pelago shows that the Flores and Banda Seas are re- welling shown in Fig. 9c. During months when the seas sponsible for the semiannual capacitor signal. In the are apparently draining above 500 m, there is net up- South China Sea, the capacitor signal has an annual welling over all depths. During periods of apparent ®ll- period. In the Celebes Sea, the capacitor signal is not ing, there is upwelling over the upper couple hundred dominated by a single frequency and has several ®lling meters with downwelling beneath. and draining periods over the year. Schematics of the archipelago circulation, and raising and lowering of isopycnals, during the ®lling and drain- d. Seasonal variations in through¯ow composition ing phases, as typi®ed by April and July, are shown in Figs. 10a and 10b, respectively. During apparent drain- The annual mean circulation, shown in Fig. 2b, gives ing, ¯ow enters the archipelago from the Paci®c over a a through¯ow path from the New Guinea Coastal Cur- layer much deeper than 500 m. Bottom topography or rent fed by the SEC, through the strait between Irian local wind-stress curl forces this transport to upwell into Jaya and Halmahera, south through the Maluku passages the upper 500 m, and it exits into the Indian Ocean over and Banda Sea, exiting into the Indian Ocean through

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FIG. 9. The variation in transport with depth in POCM is plotted for each month. The values summed over all of the Paci®c entrances are plotted in (a) and over all of the Indian exits in (b). The upwelling/downwelling between levels, implied by continuity, is plotted in (c). The vertical axis is model level in POCM. The actual depths are noted on the rhs axis. The maximum sill depth is 3300 m at the Paci®c entrances and 2750 m at the Indian exits. The scale in Sverdrups is labelled on the top axis. the Savu Sea. A secondary path is through Makassar Wajsowicz (1993b) noted that composition was a non- Strait. Although ¯ow through Makassar Strait appears linear function of time if the gyres closed differently on to be fed by the SEC, mixing between waters of the seasonal or interannual timescales. Inspection of the re- Mindanao Current and SEC at the entrance of the Cel- sults from POCM shows a wide range of behavior on ebes Seas via the Halmahera and Mindanao eddies could seasonal timescales. Over the upper 50 m, the through- considerably freshen this ¯ow. The path is similar ¯ow is fed by the NEC, which enters through Luzon whether discussing the ¯ow integrated over the upper Strait, passes through the South China Sea and Flores 50 m, 510 m, or total depth. Sea, and exits through the Savu Sea from December to

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land within the archipelago may be described by the island rule on seasonal timescales and longer. This class of solution is explored in section 5a. The near coherency of the annual signal with depth described in section 4 suggests that the smallness of the basins and numerous sills within the archipelago have led to a completed local adjustment. In a time-dependent application of (3.2), it is assumed that the barotropic and baroclinic waves trav- el very much faster than the rate of change of the wind stress. Therefore, for (3.2) to be strictly valid for de- scribing the baroclinic response at annual period, the basin within which the relevant island is located and all of the waves are propagating and causing an adjustment to a quasi-steady state, needs to be very small, namely, about 4Њ of latitude square for a 2-month adjustment timescale for the ®rst baroclinic mode. In this spirit, multiple-island-rule models with the forcing region lim- ited to over archipelago and Australia are described in section 5b. In (3.2), wind stress variations to the east of an island bounded by lines of latitude emanating from its northern and southern tips determine transport variations around the island. Effects from other latitudes are only felt if the island is overlapped to the east by another island. Therefore wind stress variations over Kalimantan cannot affect the circulation on the Philippines or Sulawesi, for example. However, if frictional effects outside western boundary layers are taken into account, then in¯uence can extend in directions other than westwards. These effects are explored in section 5c. FIG. 10. Schematics of the circulation during the semiannual (a) ®lling and (b) draining. a. Simple models using multiple island rule February. From March to May, the through¯ow over Figure 12 shows the result of forcing the multiple- the upper 50 m is fed by upwelling in the Flores Sea. island-rule system, described in section 3a, with the For the remainder of the year, it is fed by the SEC ECMWF 1987±95 monthly wind stress climatology. As through the Halmahera Sea, as for the annual mean. in section 3a, cases with Torres Strait and the Halmahera Additional ¯ow comes from Torres Strait via a current, Sea closed are also considered. The circulation around which passes to the north of Timor into the Savu Sea. New Zealand, the Philippines, Palawan, and Taiwan, are Also, from September to November, there is a contri- independent of the closure of the Torres Strait and Hal- bution from upwelling in the Flores Sea. mahera Sea and are shown in Fig. 12a. To help interpret Integration over the upper 510 m yields pathways results from the multiple-island-rule models, the annual each month that are similar to those for the annual mean. and semiannual components of the ECMWF wind stress There is considerable excursion in the equatorward pe- and its curl are shown in Fig. 13. The seasonal cycle neration of the New Guinea Coastal Current, but the in ␺ on New Zealand is due to that in the westerlies locations of the Mindanao and Halmahera eddies are across the Paci®c at the island's northern latitude, which ®xed, so there is little change in the closure of the gyres are a maximum in July. The seasonal cycle in ␺ on at the entrance to the Celebes Sea. The most different Taiwan is remarkably similar to that found for Lombok scenario from the annual mean is typi®ed by April, Strait transport by Murray and Arief's (1988) current shown in Fig. 11. The Celebes Sea with ¯ow branching meter observations and in Masumoto and Yamagata's between Makassar Strait and the Maluku passages is the (1993) GCM. The magnitude is much larger and the main pathway. The ¯ow is not fed by the western bound- phase is lagged by a month, but the humpback boreal ary current, but rather a broad band of westward ¯ow spring/summer extremum is notable. For an island of from the SEC. small meridional extent centered on y ϭ yi, the island rule reduces to 5. Mechanisms of seasonal variability x Ϫ1 E Wajsowicz (1993a) showed that in the absence of ␺ ϭ curl␶ (y )´k dx, (5.1) i ␤ ͵ i topography, the depth-integrated transport around an is- xi

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FIG. 11. As in Fig. 2b but summed for all Aprils from 1987 to 1995.

where xi is the longitude of the island, xE is the longitude the amplitudes of both the annual and semiannual com- of the eastern boundary of the ocean at yi, and ␤ is the ponents to less than 1 Sv and yields a seasonal cycle planetary vorticity gradient. From Figs. 13c,d, the signal in through¯ow transport, which bears little resemblence on Taiwan is due to the quite strong annual and semi- to that in POCM; compare Fig. 12b and Fig. 1c. The annual cycle in the curl in the easterly trades across the effect of closing the Halmahera Sea is small. The Paci®c as much as the local monsoon. The Philippines ``local'' contributions to ␺ on each of the islands, that and Palawan have an almost identical circulation, in- is, the ®rst term in (3.2), are plotted in Fig. 12d. Com- dicating that local forcing over Palawan is negligible. paring the cycle for Australia with that shown in Fig. The annual and semiannual signals are similar in mag- 12b, shows the effect of including the seasonal cycle in nitude, with the former being due to the local monsoons the westerlies to the east of New Zealand. and annual cycle of the trades across the Paci®c and the The seasonal cycles in ␺ on Timor, Sulawesi, and latter due to the trades. Kalimantan are shown in Fig. 12c, with the local con- The seasonal cycles in ␺ on Australia, Irian Jaya± tribution shown in Fig. 12e. The signal on Timor is PNG, and Halmahera are shown in Fig. 12b. When Tor- almost the same as that on Irian Jaya±PNG, as the local res Strait is wide and deep, the annual cycle dominates, contribution is negligible if Torres Strait is open. When and the circulation around Irian Jaya±PNG and Hal- Torres Strait is closed, the local contribution has in- mahera is out-of-phase with that around Australia. The creased signi®cance, but the signal is still dominated by trades, which contribute to the maximum positive anom- Irian Jaya±PNG. Local effects are also negligible for aly in ␺ on Irian Jaya±PNG in July, reinforce the sea- Sulawesi, whose signal is a combination of that on Hal- sonal signal in the southern midlatitude westerlies gen- mahera and Irian Jaya±PNG. However, for Kalimantan, erating a negative anomaly on Australia. The signal on which has a small direct exposure to the Paci®c trades, Irian Jaya±PNG is also due to the strong monsoonal effects in addition to that of overlapping islands, that signal on its west coast. Closing Torres Strait reduces is, local effects, are signi®cant.

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FIG. 12. The seasonal anomalies in depth-integrated transport as described by the multiple-island rule forced by ECMWF's monthly climatology for 1987±95. The values of the transport streamfunction on each of the islands are shown in (a), (b), and (c). Symbols for each island are as indicted in Table 5. New Zealand (Nz), Taiwan (Ta), the Philippines (Ph), Palawan (Pw), Australia (Au), Irian Jaya-PNG (Ir), Halmahera (Ha), Timor (Ti), Sulawesi (Su) and Kalimantan (Ka). The nonoverlapping contributions, or local contributions, for relevant islands are plotted in (d) and (e). The transport through straits, labeled in Fig. 3 and de®ned in Table 5, are plotted in (f), (g), and (h). In each of the plots, the very thick lines are for a model in which all of the passages are assumed wide and deep; the results are summarized in the ®rst model column of Table 5. The medium thick lines are for a model in which frictional effects are assumed suf®cient to arrest the ¯ow in Torres Strait. The thin lines are for a model in which the ¯ows in the Torres Strait and the straits between Irian Jaya and Halmahera and either end of Palawan are assumed arrested by friction; the results are summarized in the ®rst model column of Table 6.

The transport through Taiwan Strait, across the Sunda around the island block of the Philippines and Kali- Shelf, through the Flores Sea, and through Savu Strait mantan (Luzon Strait, Sunda Shelf, Makassar Strait, and are given by ␺ on Taiwan, Kalimantan, Sulawesi, and Celebes Sea entrance) in June (see Fig. 7a). The two Timor, respectively. The transports through other straits pathways are linked by the Flores Sea. are given by differences, and are plotted in Figs. 12f± In the multiple-island-rule model with all of the straits h. The annual and semiannual components of these sig- open (see upper panel of Fig. 14a) the annual component nals are displayed for the southeast Asian seas in Fig. in transport through the seas is characterized by max- 14a and tabulated in Table 5 for the basic case of all imum westward transport through Torres and Timor straits being wide and deep. Straits in July, a maximum anticlockwise circulation around Taiwan in July, a maximum clockwise gyre around Kalimantan, which exits into the Paci®c through 1) ANNUAL COMPONENT the south Palawan Strait and Celebes Sea in August. Its In POCM, the annual component of transport through originating forks are through the Savu Strait, and from the seas was characterized by maximum south/west the equatorial Paci®c through the strait between Hal- transports through the eastern straits (Torres, Timor, mahera and Irian±Jaya, then through the Flores Sea. Savu Straits, Maluku passages, and straits either side of There is a small recirculation around Kalimantan giving Halmahera) in July, and a maximum clockwise gyre a maximum southward transport in Makassar Strait in

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September. Although there is a phase lag, a maximum March. It also contributes to a signal, which has max- southward transport in Makassar Strait in boreal summer imum drainage from the Paci®c Ocean into the Celebes against the maximum northward ¯ow of the prevailing Sea and through the south Palawan strait at the begin- winds agrees with POCM and observations (Wyrtki ning of February. There is a weak signal through the 1961). Flores Sea and Savu Strait, which has a maximum east- Unlike the annual-mean ¯ow simulation, closing Tor- ward and southward transport in mid-January. res Strait does not radically alter the phases and am- The net through¯ow is similar in magnitude but al- plitudes of the annual harmonic in transport in the cen- most out of phase with that of the transport integrated tral and eastern seas and straits. The signals in Savu and over the total depth in POCM (maximum southward at Timor Straits (Figs. 12c and g, respectively) are reduced the beginning of May versus mid-February in POCM). to insigni®cance, and the signal in the Halmahera Sea Blocking off Torres Strait and/or the Halmahera Sea (Fig. 12g) is enhanced. Additionally closing the Hal- does not affect the magnitude of the semiannual com- mahera Sea reduces all of the transports in the eastern ponent of the net through¯ow, but shifts the phase seas and straits (Flores Sea, Maluku passages, straits slightly forward (not shown). either side of Halmahera) to insigni®cance (Figs. 12g Calculations using Hellerman and Rosenstein's and 12h). monthly climatological wind stresses yielded similar re- Blocking off the Sunda Shelf, which is equivalent to sults for the multiple-island-rule model when forced setting ␺Ka ϭ 0, reverses the phase of the Makassar Strait with ECMWF 1987±95 monthly climatology. The phas- signal, contrary to POCM and observations. Addition- es for the annual and semiannual harmonics are similar, ally blocking off the strait south of Palawan and Taiwan but the amplitudes were proportionately larger, as ex-

Strait, that is, setting ␺Pw ϭ 0 and ␺Ta ϭ 0, yields a pected from the annual mean calculations in section 3. seasonal signal characterized by maximum draining of the Celebes Sea through the Sulu Sea and Luzon Strait b. Reduced domain of dependence for wind stress in April and a maximum clockwise gyre around Sula- wesi, entering via the Halmahera Sea and exiting via A model is considered in which the curl of the wind the Celebes Sea. stress east of 130ЊE in the Northern Hemisphere, and If Torres Strait is open, then the amplitude and phase east of 160ЊE in the Southern Hemisphere, is assumed of the net through¯ow in this multiple-island-rule model is zero. Dynamically, this approximates to the assump- is similar to that for the transport integrated over the tion that the archipelago is in equilibrium with local total depth in POCM, namely O(3 Sv) with a maximum forcing on seasonal timescales, and that the circulation southward transport in late July. However, unlike the around Australia, which gives the depth-integrated annual mean model, closing Torres Strait and Halmahera through¯ow, is in equilibrium with the overlying wind

Sea does not improve the simulation of ␺Au; indeed, it stress curl. The results, assuming all of the passages makes it worse; see Fig. 12b. Comparing the results within the archipelago are wide and deep, are displayed summarized in Table 5 for this simple model and in Fig. 14b (note the different scale from Fig. 14a) and POCM, the amplitudes are typically several orders of tabulated in Table 5. For both the annual and semiannual magnitude too large. Limiting the domain of depen- harmonics, the order of magnitude of the transport am- dence of the wind stress and including frictional effects plitudes is in much better agreement with POCM shown outside western boundary layer are considered in sec- in Fig. 7. Phases for the South China Sea and neigh- tions 5b and 5c, respectively. boring passages are in better agreement. However, Savu Strait is still almost 180Њ out of phase at annual period, and ¯ow into the Celebes Sea between the Philippines 2) SEMIANNUAL COMPONENT and Sulawesi is still almost 90Њ out of phase. In the In POCM (see Fig. 7b), the semiannual component cases of Makassar Strait and the Halmahera Sea, the in transport is not in phase with depth. The transport situation is worse; as before the phase of the transports integrated over the total depth is characterized by a roughly agreed with POCM and observations, but now strong signal through the Maluku passages, and through the transports are almost 180Њ out of phase. A possible Savu Strait with a maximum northward transport in mid cause for the discrepancies in phase is examined below. April to May. Additional signals of signi®cance are through Torres and Timor Straits with a maximum west- c. Interisland dependency and frictional effects ward transport in late April, and through the Celebes Sea and Makassar Strait with a maximum westward and The seasonal anomalies in inferred ␺ on POCM's southward transport in mid-March. major islands are shown in Fig. 15, which should be In the multiple-island-rule model, the Torres and Ti- compared with Fig. 12 for the multiple-island-rule mod- mor Strait signal has the same phase, but is much stron- els. According to (3.2), forced by either the total wind ger than in POCM, see lower panel of Fig. 14a. The stress ®eld or the limited version described in section seasonal variability is dominated by the circulation 5b, the seasonal anomalies in ␺ on Australia and Irian around Taiwan, which is maximum clockwise in mid- Jaya±PNG are of almost opposite phase, and closing

Unauthenticated | Downloaded 09/23/21 06:37 PM UTC 1010 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29 hese plots, the amplitude r panel) wind stresses from alibration vectors are drawn beneath . 13. The amplitude and phase for (a: left) the annual component and (b: right) the semiannual component of the zonal (upper panel) and meridional (lowe IG F the ECMWF 1987±95 climatology.the The length plots. of a The vector denotes annual the and amplitude, and semiannual its direction components denotes the of phase, the with 1 vertical January pointing component northward. C of the curl of the wind stress are displayed in (c) and (d), respectively. In t

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FIG. 13. (Continued) is contoured with interval 0.1 ϫ 10Ϫ7 dyn cmϪ3, and the phase is denoted by a uniform length arrow with 1 January pointing northward. Arrows are not plotted if the amplitude is less than 0.2 ϫ 10Ϫ8 dyn cmϪ3.

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Torres Strait yields a joint ␺, which resembles neither It is noteworthy that the expression (5.3) can be gen- of the open strait values. From Fig. 15b, in POCM, eralized to describe the situation of islands overlapping which has a partially open Torres Strait, ␺Au and ␺Ir are with a meridional frictional strait by rede®ning f I ap- similar (cf. Fig. 12b for the multiple-island-rule model, propriately, namely f I ϭ ( f NЈ Ϫ f SЈ)( f N Ϫ f S)/[max( f NЈ, where they are 180Њ out of phase). This is due to fric- f N) Ϫ min( f SЈ, f S)]. tional effects in Torres Strait, as is simply demonstrated. In cases of overlapping islands, distinguishing be- Using the notation of Wajsowicz (1993a), the depth- tween frictional effects and in¯uence due to overlap is integrated circulation around Irian Jaya±PNG and Aus- somewhat arbitrary. The values of ␺ on the Philippines tralia are, respectively, and Palawan in POCM (see Fig. 15a) are almost iden- tical, as expected from (3.2), since the local wind stress Ϫ1 ␶ contribution is negligible. Frictional effects need not be ␺ ϭϩF ´ dᐉ, Ir invoked to explain the similarity, though as will be seen ( fNЈ Ϫ f SЈ) ͵ ΂΃␳o I shortly in the dicussion of Kalimantan and Sulawesi, Ϫ1 ␶ they are important. The value of ␺ on Halmahera in ␺Au ϭϩF ´ dᐉ, (5.2) POCM (see Fig. 15b) is similar to ␺Ir. According to ( fNSϪ f ) ͵ ␳ o A ΂΃ (3.2), ␺Ha ϭ 0.167␺Ir plus a local contribution, which is shown in Fig. 12d. However, from (5.3), where F is the depth-integrated friction term; f N, f NЈ are the values of the Coriolis parameter at the latitudes 1 ␺ ϭ ␺ ϩ (␺ Ϫ ␺ ). (5.4) of the northern tips of Australia and Irian Jaya, respec- Ha IrϪ1 Ha ideal Ir ideal (1 ϩ rfI ) tively; f S, f SЈ at the southern tips; and I, A are the closed circuits across the Paci®c spanning Irian Jaya±PNG and The differences ␺Ha Ϫ 0.167␺Ir and ␺Ha Ϫ ␺Ir are shown Australia, respectively. Assuming that Torres Strait is in Fig. 15d. The former does not compare particularly suf®ciently narrow so that ∫ F ´ dl Ϫ∫ F ´ dl where AITTഡ well with the local contribution plotted in Fig. 12d, but AT, IT are the section of the paths A, I within Torres the latter (with the sign reversed) does compare well Strait, and f N ഠ f SЈ ϭ f T, then with the transport through the Irian Jaya±Halmahera Ϫ1 11 strait, plotted in Fig. 12g, with rf I ഠ 2. ␺Irϭ ␺ Ir idealϩ ␺ closed, Turning to Timor, from (3.2), ␺ on Timor should equal (1 ϩ rfϪ1)(1ϩ frϪ1) II that on Irian Jaya±PNG plus a negligible local contri- 11 bution. From Fig. 15c, in POCM, ␺Ti and ␺Ir are very ␺Auϭ ␺ Au idealϩ ␺ closed, (5.3) similar, and from Fig. 15e, the local contribution is sim- (1 ϩ rfϪ1)(1ϩ frϪ1) II ilar to that plotted in Fig. 12e. Therefore, the opposite where a Rayleigh friction formulation has been assumed phase in the annual harmonic of transport through Savu for F, which yields a constant parameter r, which is the Strait between POCM and the earlier multiple-island- Rayleigh friction coef®cient multiplied by the ratio of rule models (see Table 5) could be explained by dif- the length to the width of the strait. The parameter f I ferences in ␺ on Irian Jaya, rather than an Indian Ocean Ϫ5 Ϫ1 ϭ [( f N Ϫ f T)( f T Ϫ f S)]/( f N Ϫ f S) ഡ 1.98 ϫ 10 s , in¯uence. ␺Ir ideal and ␺Au ideal are the values given by (5.2) with F Finally, from (3.2), ␺ on Sulawesi is 0.722␺Ir ϩ ∫ ϭ 0, and ␺closed ϭ [Ϫ1/( f N Ϫ f S)] AϩI (␶/␳o)´dl, that 0.278␺Ha plus a local contribution. POCM does not is, the ideal value if the Torres Strait is closed. The agree with this dependency, as both Halmahera and Irian magnitude of r could be estimated by substituting the- Jaya±PNG have a negative transport anomaly from July oretical values for ␺ on the rhs of (5.3) and values from through September. A candidate for the discrepancy is POCM on the lhs, and performing a least squares ®t. the frictional in¯uence of Kalimantan, which is exerted

From Fig. 15b, ␺Ir in POCM lacks dominance in the along the length of Sulawesi's western boundary. Sim- annual harmonic, so yielding a temporal variability sim- ilarly, the value of ␺ on Kalimantan is much better ilar to the theoretical prediction when the Torres Strait described by recognizing the frictional in¯uence of the Ϫ1 is closed. Therefore rf I k 1, which assuming the Tor- Philippines via Palawan on Kalimantan. The differences res Strait 10 times longer than its effective width, gives between ␺Su, ␺Ka and the overlapping contributions giv- a Rayleigh friction timescale much smaller than 6 days. en by (3.2), and ␺Su Ϫ ␺Ka, ␺Ka Ϫ ␺Ph as suggested by

←

FIG. 14. The annual (upper panel) and semiannual (lower panel) components of transports in multiple-island rule models are shown in (a) for a model in which all of the passages are assumed wide and deep and forcing is by the total, wind stress ®eld (®rst model column of Table 5), (b) as in (a), but the wind stress curl is assumed zero west of 130ЊN in the Northern Hemisphere and 160ЊS in the Southern Hemisphere (second model column of Table 5), and (c) as in (b), but friction is assumed suf®cient to arrest the ¯ow in the Torres and Taiwan

Straits, the strait between Irian Jaya and Halmahera, and the straits either end of Palawan (second model column of Table 6 with ␺Ta ϭ 0). The length of the vector denotes the amplitude with calibration vectors at the bottom of each panel, and the orientation denotes the phase with 1 January pointing north.

Unauthenticated | Downloaded 09/23/21 06:37 PM UTC 1014 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29 ϭ n/a 1.75 2.44 0.11 1.48 2.34 1.89 2.31 0.62 2.99 0.12 2.40 0.44 0.82 0.83 0.84 2.64 2.42 1.69 1.78 0.59 2.71 Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Semiannual OCM. Amp n/a 1.19 0.29 0.14 0.85 1.56 0.31 0.75 0.24 1.05 1.09 0.94 0.43 0.43 0.85 0.42 0.09 0.26 0.20 1.08 2.04 Amp Ph 1.679 POCM n/a 0.62 2.50 2.75 0.27 1.52 0.34 2.68 2.98 2.96 1.25 0.48 0.28 3.10 0.12 0.19 2.43 0.90 1.59 1.30 0.82 1.11 Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Annual n/a 1.04 3.12 0.69 3.44 1.61 1.77 3.25 0.68 2.98 2.37 1.66 2.35 0.79 2.49 1.68 2.45 0.56 0.90 1.39 2.71 1.67 Amp Ph n/a 2.04 1.28 0.98 1.53 2.02 2.27 1.16 2.09 2.37 2.07 0.93 1.70 0.58 1.18 0.95 1.99 2.55 2.25 0.91 2.05 1.01 Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Semiannual n/a 1.05 0.36 1.38 0.25 0.50 1.78 0.42 1.50 0.92 0.56 0.44 0.66 0.79 1.29 1.99 1.63 0.08 1.32 1.25 1.85 1.00 Amp Ph 0 on 1 January. limited ϭ ␶ on islands MIR- n/a 2.87 1.13 0.07 0.02 2.75 3.06 2.07 2.91 2.93 2.99 0.17 2.86 1.77 0.11 0.04 2.58 0.20 2.68 0.41 2.76 0.16 ␺ Strait transports Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Annual n/a 2.57 0.89 1.87 4.94 1.32 2.38 0.75 4.17 3.00 1.27 1.61 1.17 0.51 7.45 7.31 1.75 0.75 2.89 3.62 4.78 2.87 Amp Ph phase in radians where Ph ϭ . 0.29 2.45 1.22 0.31 1.18 2.61 2.49 1.21 2.42 2.98 0.09 0.79 1.98 0.58 0.92 0.84 2.10 2.55 2.10 1.05 2.07 1.79 Ti Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ ϭ␺ Semiannual 5.12 1.62 5.17 5.91 1.38 2.15 2.40 5.23 2.33 1.18 0.61 0.72 1.48 0.79 2.91 3.66 7.69 0.08 5.90 5.83 7.30 0.46 Amp Ph , Savu Strait Ka amplitude in Sverdrups, Ph MIR±ECMWF ϭ␺ 3.03 3.03 1.70 0.31 0.15 2.56 3.09 1.82 2.87 2.60 1.10 0.68 1.25 1.77 0.13 0.10 3.11 0.20 2.27 0.85 2.42 1.73 Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Ϫ Annual 6.91 9.16 6.54 8.50 3.86 5.18 0.51 9.31 6.54 0.75 5.22 Amp Ph 15.65 16.21 16.06 14.96 19.22 19.17 19.15 19.74 15.10 12.72 16.20 , Sunda Shelf Ta ϭ␺ Ha Pw Pw Su , Taiwan Strait Ϫ␺ Ka Au Ir Ph ) Ϫ␺ Ti Su ␺ Ϫ␺ ) Ϫ␺ ) Ir Ti Ta Ka Ph Ϫ␺ ) ) Ha Ph ϭ␺ ) Ka Ka ) Ha ␺ Ir ␺ Ϫ␺ ␺ ␺ Au Su ␺ Su Ϫ␺ ) Ϫ␺ ␺ ␺ Pw Ϫ␺ Ϫ␺ Ϫ␺ ␺ ␺ ) Ta 5. Summary of amplitude and phase of the annual and semiannual harmonics and multiple-island-rule (MIR) models forced by ECMWF wind stresses and for P ␺ Su Ir ␺ Ir Ti Ϫ␺ ␺ ␺ Ph Au ␺ ␺ Au ␺ ␺ ␺ ␺ Ph ␺ ABLE T Australia ( Timor ( New Zealand Irian Jaya ( Sulawesi ( Luzon Halmahera ( N. Palawan S. Palawan Sulu Celebes N. Maluku Maluku Makassar Arafura Torres Timor Through¯ow Philippines ( Palawan ( Kalimantan ( Halmahera Sea Taiwan (

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FIG. 15. The monthly deviation from the annual mean of the depth-integrated transport through each strait in POCM is calculated and then summed to give the value of the seasonal anomaly in ␺ on each island. The results are plotted in (a), (b), and (c) corresponding to those for the multiple-island-rule models in Fig. 12. The nonoverlapping contributions, or local contributions, de®ned as the value minus the fraction from over-lapping islands, with the fraction as given by the multiple-island rule, are plotted in (d) and (e). These should also be compared with corresponding plots in Fig. 12. Also plotted in (d) is ␺Ha Ϫ ␺Ir, and (e) ␺Ka Ϫ ␺Ir and ␺Su Ϫ ␺Ka. The results are summarized in the third model column of Table 5.

(5.3), are plotted in Fig. 15e. These should be compared the Philippines and Kalimantan need to be calculated with Fig. 12e, and the transport in Makassar Strait plot- before those between Sulawesi and Kalimantan are con- ted in Fig. 12h and the straits either side of Palawan in sidered. Ϫ1 Fig. 12f. For Kalimantan and the Philippines, rf I ഠ 9, Important conclusions drawn from these simple mod- Ϫ1 and for Sulawesi and Kalimantan, rf I ഠ 2. The annual els are: and semiannual harmonics in transports within the In- i) At annual period, wind forcing over the Southeast donesian seas for a multiple-island-rule model forced Asian seas and Australia could account for a major frac- by the limited wind stress ®eld and in which Halmahera, tion of the variation in transport within and through the Irian Jaya±PNG, and Australia form an island block and seas. Wind forcing over the Indian Ocean does not con- Kalimantan, Palawan, and the Philippines form a block tribute to transport variations in the multiple-island-rule- are displayed in Fig. 14c. The results are tabulated in based models. Table 6 along with those for the same geometric con- ii) Frictional effects within neighboring straits enable ®guration forced by the full domain wind stress ®eld. the circulation around an island to be in¯uenced by wind The latter values are included for comparison, as the stress variations in directions other than just to the east. choice of 130Њ and 160ЊE for the boundaries of in¯uence iii) Frictional effects cannot be suf®cient to totally of the wind stress was somewhat arbitrary. For example, arrest seasonal variations in the transport through Torres comparing with POCM values, the amplitude around Strait. Otherwise, the amplitude in the annual harmonic the Kalimantan±Palawan±Philippines block lies some- in transport through Timor Strait is too small. Further, where between the limited-area and full-domain values. they cannot be negligible, otherwise the transport From Fig. 13c, this suggests that the strong curl just to through the Savu Strait is out of phase with POCM and the east of 130ЊE near the Philippines needs to be in- observations. cluded. iv) Frictional effects cannot be suf®cient to totally It is noteworthy that friction in itself only reduces the arrest seasonal variations in transport across the Sunda amplitude of the transport signal within a strait; it does Shelf. Otherwise, the transport through Makassar Strait not modify the phase. However, (5.4) can be repeatedly is out of phase with POCM and observations. applied to an island that has several neighboring straits. v) Frictional effects cannot be suf®cient to totally In which case, the values of ␺ on the rhs are from the arrest seasonal variations in transport through the straits previous iteration. For example, in calculating the trans- at either end of Palawan. Otherwise, the amplitude of port through Makassar Strait, frictional effects between the annual harmonic in transport through the South Chi-

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TABLE 6. Summary of properties of seasonal cycle assuming Australia±Irian Jaya±Halmahera, and Philippines±Palawan±Kalimantan are island blocks. Amp ϭ amplitude in Sverdrups, Ph ϭ phase in radians where Ph ϭ 0 on 1 January. MIR±ECMWF MIR-␶ limited Annual Semiannual Annual Semiannual Amp Ph Amp Ph Amp Ph Amp Ph ␺ on islands (Sv), (rad)

Irian Jaya (␺Ir)

Australia (␺Au) 1.14 Ϫ2.83 1.28 0.95 2.75 Ϫ0.05 0.15 Ϫ2.55

Halmahera (␺Ha)

Philippines (␺Ph)

Palawan (␺Pw) 4.81 3.12 3.11 1.27 0.23 2.03 0.21 2.87

Kalimantan (␺Ka)

Timor (␺Ti) 1.00 2.99 0.62 1.43 2.87 0.13 0.94 Ϫ2.56

Sulawesi (␺Su) 3.10 Ϫ2.92 0.68 0.92 0.93 Ϫ0.51 0.74 Ϫ2.25 Strait transports (Sv), (rad)

Luzon ␺Ph Ϫ␺Ta 23.81 Ϫ2.89 6.70 2.35 2.00 2.97 1.54 2.25 Celebes ␺ Ϫ␺ Su Ph 1.94 Ϫ0.40 2.48 Ϫ1.78 1.13 Ϫ0.63 0.69 Ϫ1.96 Makassar ␺Su Ϫ␺Ka N. Maluku ␺ Ϫ␺ Ha Su 1.96 0.17 0.60 0.97 1.96 0.17 0.60 0.97 Maluku ␺Ir Ϫ␺Ti Arafura ␺ Ϫ␺ Ir Ti 0.51 Ϫ1.77 0.79 0.58 0.51 Ϫ1.77 0.79 0.58 Timor ␺Au Ϫ␺Ti

Through¯ow ϭ␺Au, Taiwan Strait ϭ␺Ta, Sunda Shelf ϭ␺Ka, Savu Strait ϭ␺Ti; ␺Nz, ␺Ta as in Table 5. na Sea is too small. Further, they cannot be negligible, Yamagata et al. (1996) and Masumoto and Yamagata otherwise the transport through Makassar Strait is out (1996) in their GCM studies found that annual-period of phase with POCM and observations (in limited-area- transport variations in their archipelago straits exiting forcing model), and similarly for transport at the Celebes into the Indian Ocean were driven by equatorial Paci®c Sea entrance. wind stress variations and those over Australia in agree- There have been several process-oriented numerical ment with the ®ndings herein. This contrasts with sug- modeling studies of the seasonal cycle in the Southeast gestions that coastal upwelling/downwelling along the Asian region, and it is useful to consider their results south Javan coast is responsible (e.g., see Inoue and in the context of these multiple-island-rule calculations. Welsh 1993). Metzger and Hurlburt (1996) and Inoue and Welsh It is noteworthy that Masumoto and Yamagata's (1993) used reduced-gravity layer models, which need (1996) conclusions do not necessarily agree in detail careful interpretation when discussing time-dependent with the multiple-island-rule model. In the latter model, wind-driven transports, as illustrated by the contrasting the Lombok±Savu Strait transport is given by ␺Ti, which amplitudes and phases for the ¯ow integrated over the equals ␺Au plus a local contribution, whereas the Timor total depth and upper 510 m in POCM shown in Fig. Strait transport is given by ␺Au Ϫ ␺Ti, that is minus the 7. Both studies found that wind stresses over a strait previous local contribution. If ␺Au is divided into com- were not responsible for driving transport variations in ponents driven by equatorial and nonequatorial wind the strait, which is consistent with the ®ndings herein. stresses, then according to Masumoto and Yamagata's Metzger and Hurlburt (1996) found that removing the (1996) ®ndings, the Lombok Strait transport is driven seasonal cycle in wind stress curl over the South China by Paci®c equatorial wind stresses, so the nonequatorial

Sea and western side of the Philippines halved the sea- component of ␺Au is zero or cancels the local contri- sonal cycle in the Luzon Strait transport. This is con- bution. Cancellation seems fortuitous, but would be sistent with the multiple island rule model, as from Fig. needed in the multiple-island-rule model to explain their 13c, the wind stress curl is very large over the north- ®nding that the Timor Strait transport variations are western boundary of the Philippines. They also noted driven by wind stress variations over southwest Aus- that seasonal variations in the Luzon Strait transport tralia. were halved if wind stress variations to northeast of the Philippines were removed, so concluding that wind 6. Summary and discussion stress variations directly to the east of the Philippines were unimportant. This result cannot be explained by In determining the mean ocean state, the Indonesian the multiple-island-rule-based models herein, as coastal Through¯ow is an important source of heat and fresh- Kelvin waves are assumed to propagate in®nitely fast water for the Indian Ocean. Therefore knowledge of the around the domain. magnitude of its transport and mean temperature and

Unauthenticated | Downloaded 09/23/21 06:37 PM UTC MAY 1999 WAJSOWICZ 1017 salinity are of interest. Details of the currents and path- POCM that at annual period the ¯ow within the archi- ways within the Indonesian seas are not crucial unless pelago is con®ned to above 500 m, although the max- they in some way determine the above. The importance imum sill depths are much larger. This suggests that of the seasonal variability of the through¯ow in its im- Sverdrup dynamics could be used to explain the annual- pact on the Paci®c or Indian Oceans is as yet unclear. period transport variations. A multiple-island-rule-based There is a sense that it must be important as the through- model forced by the global wind stress ®eld gave a ¯ow transport varies from almost zero in the boreal circulation that was at least an order of magnitude too winter to the order of 20 Sv in the summer. Also, the large. When reduced to just that over the archipelago pool of high sea surface temperatures (Ͼ28ЊC) shifts and Australia, reasonable agreement was found with the from the southeast archipelago in the boreal winter to amplitude of the annual harmonic in POCM. However, the northern archipelago in summer. the phase of the transport in straits such as Makassar An investigation of the mean ¯ow within and through were made to differ completely. the archipelago was prompted by the ®nding that, in the Recognition that frictional effects, which typically re- 1987±95 ®ne-resolution, global ocean simulation duce the amplitude, but do not affect the phase of trans- (POCM) conducted by Semtner and colleagues, the port through a strait, will modify the phase in the ar- magnitude of the through¯ow was much smaller than chipelago due to so many islands spanning different Godfrey's (1989) ``Island Rule'' estimate and that it was latitudes, and their circulation being driven by different fed by the warm, salty South Equatorial Current in con- wind systems, enabled much better agreement to be trast with feeding by the cool, fresh Mindanao Current made with POCM. Signi®cant conclusions about the reported for an earlier model in Godfrey et al. (1993). grouping and in¯uence of various islands were sum- The pathway through the archipelago was also more multitudinous than Makassar Strait noted by F®eld and marized at the end of section 5c. Gordon (1992), for example. The semiannual signal in POCM showed considerable A series of models based on the multiple-island rule variation in phase with depth, so although there was (Wajsowicz 1993a) showed that the above properties some agreement between the multiple-island-rule mod- were the result of the climatology of the wind stress els and POCM for this harmonic, there is not an un- dataset used to force POCM, which is based on ECMWF derlying dynamical justi®cation for the agreement. Fur- 10-m twice-daily winds. The wind stress is weaker than ther investigation of the signal showed that over the that of Hellerman and Rosenstein (1983) in both the boreal summer and winter the archipelago apparently equatorial and midlatitudes of the Paci®c, and the line drained over the upper 500 m, as water from below of zero Sverdrup transport, which is within the latitudes upwelled giving a greater transport exiting than entering spanned by the archipelago, is shifted suf®ciently pole- the archipelago over the upper 500 m. In the transition ward that not even nonlinear effects can modify the months, there was apparent ®lling of the archipelago source. An investigation of The Florida State University above 500 m, as a fraction of the transport entering from wind stresses from 1961 to 1995 con®rmed that there the Paci®c downwelled and returned at depth. Yamagata had been a reduction in the absolute amount of North et al. (1996), following Clarke and Liu (1993), have Paci®c water mass feeding the through¯ow, but lack of noted that unlike annual variations in the Lombok data poleward of 30ЊS prevented any conclusions being through¯ow, which are monsoonal driven, semiannual made about a shift in fractional composition. The result variations may be remotely forced by zonal winds in is noteworthy, however, because of observed decadal- the central equatorial Indian Ocean. In all of the simple scale North Paci®c climate changes during this period models described herein, Indian Ocean wind stresses are (see e.g., Graham 1994; Latif and Barnett 1994). passive. An informative experiment was provided by a model Finally, it is noted that seasonal variations in the cur- in which all of the straits were assumed wide and deep. rents of the west Paci®c were not suf®cient to alter the The circulation within the archipelago was completely origin of the through¯ow. It continued to be fed by the at odds with observation, but demonstrated the pressure SEC throughout the year, whether indirectly by the New gradients that the wind stress wanted to set up that fric- Guinea Coastal Current or directly by a broad zonal jet tion had to suppress. A series of experiments illustrated entering the archipelago. Wajsowicz's (1996) theory that the through¯ow selects a path through the westernmost straits up to a frictionally determined limit and then progressively ®lls up more Acknowledgments. The author would like to thank eastern paths. The signi®cance of off-equatorial, zonal Professor A. J. Semtner and Dr. Tokmakian for making entrances, notably Taiwan, Luzon, and Torres Strait, was available results from their Parallel Ocean Climate Mod- noted. Important conclusions from these experiments el. Funding for this research was provided by the Of®ce were summarized at the end of section 3a. of Naval Research Grant N00014-96-1-0611. Super- The focus of the investigation on seasonal variability computers at the National Center for Atmospheric Re- was to construct a simple model of transport variations search and DOD High Performance Computing Center, within and through the archipelago. It was noted from Mississippi, were used.

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REFERENCES Current during the Western Equatorial Paci®c Ocean Circulation Study. J. Geophys. Res., 96, 7089±7104. Barnier, B., L. Siefried, and P. Marchesiello, 1995: Thermal forcing Masumoto, Y., and T. Yamagata, 1993: Simulated seasonal circulation for a global ocean circulation model using a 3-year climatology in the Indonesian Seas. J. Geophys. Res., 98, 12 501±12 509. of ECMWF analyses. J. Mar. Syst., 6, 363±380. , and , 1996: Seasonal variations of the Indonesian through- Bryan, K., 1969: A numerical method for the study of the circulation ¯ow in a general ocean circulation model. J. Geophys. Res., 101, of the world ocean. J. Comput. Phys., 4, 347±376. 12 287±12 294. Clarke, A. J., and X. Liu, 1993: Observations and dynamics of semi- Metzger, E. J., and H. E. Hurlburt, 1996: Coupled dynamics of the annual and annual sea levels near the eastern equatorial Indian South China Sea, the Sulu Sea, and the Paci®c Ocean. J. Geo- Ocean boundary. J. Phys. Oceanogr., 23, 386±399. phys. Res., 101, 12 331±12 352. Cox, M., 1984: A primitive equation, 3-dimensional model of the Meyers, G., R. J. Bailey, and A. P.Worby, 1995: Geostrophic transport ocean. GFDL Ocean Group Tech. Rep. 1, Geophysical Fluid of the Indonesian through¯ow. Deep-Sea Res., Part I, 42, 1163± Dynamics Laboratory/NOAA, Princeton University, Princeton, 1174. NJ 08542. F®eld, A., and A. L. Gordon, 1992: Vertical mixing in the Indonesian Murray, S. P., and D. Arief, 1998: Through¯ow into the Indian Ocean thermocline. J. Phys. Oceanogr., 22, 184±195. through Lombok Strait, January 1985±January 1986. Nature, Fine, R. A., 1985: Direct evidence using tritium data for through¯ow 333, 444±447. from the Paci®c into the Indian Ocean. Nature, 315, 478±480. Semtner, A. J., and R. M. Chervin, 1992: Ocean general circulation , R. Lukas, F. M. Bingham, M. J. Warner, and R. H. Gammon, from a global eddy-resolving model. J. Geophys. Res., 97, 5493± 1994: The western equatorial Paci®c: A water mass crossroads. 5550. J. Geophys. Res., 99, 25 063±25 080. Stammer, D., R. Tokmakian, A. J. Semtner, and C. Wunsch, 1996: Gill, A. E., 1982: Atmosphere±Ocean Dynamics. Academic Press, How well does a 1/4Њ circulation model simulate large-scale 662 pp. oceanic observations? J. Geophys. Res., 101, 25 779±25 811. Godfrey, J. S., 1989: A Sverdrup model of the depth-integrated ¯ow Tokmakian, R., 1996: Comparison of time series from two global for the World Ocean allowing for island circulations. Geophys. models with tide gauge data. Geophys. Res. Lett., 23, 3759± Astrophys. Fluid Dyn., 45, 89±112. 3762. , A. Hirst, and J. Wilkin, 1993: Why does the Indonesian through- Wajsowicz, R. C., 1993a: The circulation of the depth-integrated ¯ow ¯ow appear to originate from the Northern Hemisphere? J. Phys. around an island with application to the Indonesian Through¯ow. Oceanogr., 23, 1087±1098. J. Phys. Oceanogr., 23, 1470±1484. Gordon, A. L., 1986: Interocean exchange of thermocline water. J. , 1993b: A simple model of the Indonesian Through¯ow and its Geophys. Res., 91, 5037±5046. composition. J. Phys. Oceanogr., 23, 2683±2703. , and J. L. McClean, 1999: Thermohaline strati®cation of the , 1994: A relationship between interannual variations in the Indonesian Seas: Model and observations. J. Phys. Oceanogr., South Paci®c wind stress curl, the Indonesian Through¯ow, and 29, 198±216. the West Paci®c warm water pool. J. Phys. Oceanogr., 24, 2180± Graham, N. E., 1994: Decadal-scale climate variability in the tropical 2187. and North Paci®c during the 1970s and 1980s: Observations and , 1996: Flow of a western boundary current through multiple model results. Climate Dyn., 10, 135±162. straits: An electrical circuit anaology for the Indonesian through- Hellerman, S., and M. J. Rosenstein, 1983: Normal monthly wind stress over the world ocean with error estimates. J. Phys. Ocean- ¯ow and archipelago. J. Geophys. Res., 101 (C5), 12 295± ogr., 13, 1093±1104. 12 300. Inoue, M., and S. E. Welsh, 1993: Modeling seasonal variability in Wolanski, E., P. Rido, and M. Inoue, 1988: Currents through Torres the wind-driven upper-layer circulation in the Indo±Paci®c re- Strait. J. Phys. Oceaongr., 18, 1535±1545. gion. J. Phys. Oceanogr., 23, 1411±1436. Wyrtki, K., 1961: Physical oceanography of the Southeast Asian wa- Latif, M., and T. P. Barnett, 1994: Causes of decadal climate vari- ters. NAGA Rep. 2, Scripps Inst. of Oceanogr., University of ability over the North Paci®c/North American sector. Science, California, La Jolla, CA, 195 pp. 266, 634±637. Yamagata, T., K. Mizuno, and Y. Masumoto, 1996: Seasonal varia- Lukas, R., E. Firing, P. Hacker, P. L. Richardson, C. A. Collins, R. tions in the Indian Ocean and their impact on the Lombok Fine, and R. Gammon, 1991: Observations of the Mindanao through¯ow. J. Geophys. Res., 101, 12 465±12 475.

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