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198 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29

Thermohaline Strati®cation of the Indonesian : Model and Observations*

ARNOLD L. GORDON Lamont-Doherty Observatory of Columbia University, Palisades, New York

JULIE L. MCCLEAN Department of Oceanography, Naval Postgraduate School, Monterey, California

(Manuscript received 15 October 1996, in ®nal form 5 March 1998)

ABSTRACT The Indonesian Through¯ow, weaving through complex topography, drawing water from near the division of the North Paci®c and South Paci®c water mass ®elds, represents a severe challenge to modeling efforts. Ther- mohaline observations within the Indonesian seas in August 1993 (southeast monsoon) and February 1994 (northwest monsoon) offer an opportunity to compare observations to model output for these periods. The simulation used in these comparisons is the Los Alamos Parallel Program (POP) 1/6 deg (on average) global model, forced by ECMWF wind stresses for the period 1985 through 1995. The model temperature structure shows discrepancies from the observed pro®les, such as between 200 and 1200 dbar where the model temperature is as much as 3ЊC warmer than the observed temperature. Within the 5Њ±28ЊC temperature interval, the model salinity is excessive, often by more than 0.2. The model density, dominated by the temperature pro®le, is lower than the observed density between 200 and 1200 dbar, and is denser at other depths. In the model Strait, North Paci®c waters are found down to about 250 dbar, in agreement with observations. The model sill depth in the of 200 m, rather than the observed 550-m sill depth, shields the model from Makassar Strait lower water, causing the Flores lower thermocline to be dominated by salty water from the . In the , Seram, and Banda Seas the model thermocline is far too salty, due to excessive amounts of South Paci®c water. Observations show that the bulk of the Makassar through¯ow turns eastward into the Flores and Banda Seas, before exiting the Indonesian seas near . In the model, South Paci®c thermocline water spreads uninhibited into the Banda, Flores, and Timor Seas and ultimately into the . The model through¯ow transport is about 7.0 Sv (Sv ϵ 106 m3 sϪ1) in August 1993 and 0.6 Sv in February 1994, which is low compared to observationally based estimates. However, during the prolonged El NinÄo of the early 1990s the through¯ow is suspected to be lower than average and, indeed, the model transports for the non±El NinÄo months of August 1988 and February 1989 are larger. It is likely that aspects of the model bathymetry, particularly that of the , which is too open to the South Paci®c, and the Makassar Strait, which is too restrictive, may be the cause of the discrepancies between observations and model.

1. Introduction 1989; Tourre and White 1995). Furthermore, advective The magnitude and variations of the Paci®c to Indian and tidal-induced mixing may in¯uence the SST and sea±air coupling, with feedback on ENSO (F®eld and Ocean Through¯ow within the Indonesian Seas are con- Gordon 1996; Godfrey 1996). sidered to be key elements in the thermohaline balance The interocean pressure gradient driving the Indo- of the Indian and Paci®c (MacDonald 1993) and nesian Through¯ow is primarily con®ned to the upper perhaps to the global climate system (Gordon 1986). 300 m of the water column (Wyrtki 1987). Observations Local variability of the Indonesian Seas may in¯uence indicate that within this layer the through¯ow is com- ENSO by governing the transfer of the warm tropical posed mostly of North Paci®c thermocline and inter- water between the Paci®c and Indian Oceans (Wyrtki mediate water ¯owing through Makassar Strait (Fine 1985; F®eld and Gordon 1992; Bingham and Lukas 1994; Fieux et al. 1994; Gordon et al. 1994; Ilahude * Lamont-Doherty Earth Observatory Contribution Number 5774. and Gordon 1996; Gordon and Fine 1996). In the deep channels east of , South Paci®c water in®ltrates the lower thermocline and dominates the deeper layers, Corresponding author address: Prof. Arnold Gordon, Lamont-Do- herty Earth Observatory, Columbia University, Palisades, NY 10964- including the over¯ow into the deep Banda Sea (van 8000. Aken et al. 1988; Gordon 1995; Ilahude and Gordon E-mail: [email protected] 1996; Gordon and Fine 1996; Hautala et al. 1996).

᭧ 1999 American Meteorological Society FEBRUARY 1999 GORDON AND MCCLEAN 199

FIG. 1. (a) Bathymetry of the deep Indonesian Seas. The symbols indicate the positions of the Arlindo CTD stations. The 100-, 200-, 500-, 1000-, 2000-, 3000-, and 4000-m isobaths are shown, as are the seas and other features referred to in the text. The stations composing the three sections presented in Figs. 2, 3, and 4 are also displayed. (b) Model bathymetry of the Indonesian showing the 500-, 1000-, 2000-, 3000-, and 4000-m isobaths. 200 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29

FIG. 2. Makassar section (see Fig. 1a) of potential temperature and salinity for the upper 1000 dbar. SEM represents the southeast monsoon of August 1993; NWM represents the northwest monsoon of February 1994. (a) Observational data, the bottom feature shown at 5Њ to 5.5ЊS is the observed shallowest sill encountered along the Makassar section; (b) model output, the same bottom depths as used in (a); these are based solely on the depths at the Arlindo station sites.

While some Makassar transport ¯ows through the Lom- 106 m3 sϪ1: Wyrtki 1961; Godfrey and Golding 1981; bok Strait, most passes through the into the Gordon 1986, Kindle et al. 1989; Murray and Arief Banda Sea. Here it is joined by additional transport 1990; MacDonald 1993; Cresswell et al. 1993; Hirst derived from the deep eastern through¯ow channels and and Godfrey 1993; Miyama et al. 1995; Molcard et al. then enters the Indian Ocean by way of the passages on 1994; Molcard et al. 1996). Measurements in the Lom- either side of Timor (regional geometry is shown in Fig. bok Strait from January 1985 to January 1986 (Murray 1a). and Arief 1988; Murray et al. 1989) show an average Transport estimates based on observations, models, transport of 1.7 Sv with a maximum of 3.8±4.0 Sv to- and conjecture range from near zero to 30 Sv (Sv ϵ ward the Indian Ocean during July and August. A min- FEBRUARY 1999 GORDON AND MCCLEAN 201

FIG.2.(Continued) imum between zero and 1 Sv, also toward the Indian from XBT data for the period 1983±89, with a 12 Sv Ocean, is observed from December 1985 to January August±September maximum, and essentially zero 1986. The most recent observational-based estimates of transport in May±June and again in October±November. transports through the are 18.6 Sv for August Meyers (1996), Fieux et al. (1996), and Gordon and 1989 (Fieux et al. 1994) and Ϫ2 Sv (¯ow toward the Fine (1996) all suggest that the through¯ow is reduced Paci®c Ocean) during February±March 1992 (Fieux et during El NinÄo episodes when sea level stands lower at al. 1996), denoting the seasonal cycle. However, as the tropical Paci®c entrance to the Indonesian seas. Mol- Fieux et al. (1996) stress, interannual variability cannot card et al. (1996), using two moorings between Roti be ruled out, noting that the February±March 1992 pe- Island and Ashmore Reef (March 1992±April 1993), riod falls within an El NinÄo episode. Meyers et al. (1995) estimate a mean transport of 4.3 Sv between the sea estimated the annual mean ¯ow to be 5 Sv through the surface and 1250 m in the Timor Passage, the deepest upper 400 m across a section between and of the exit passages to the Indian Ocean. Of this trans- 202 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29

FIG. 3. Maluku±Seram±Banda section (see Fig. 1a) of potential temperature and salinity for the upper 1000 dbar. SEM represents the southeast monsoon of August 1993; NWM represents the northwest monsoon of February 1994. (a) Observational data; (b) model output. port 1.8 Sv is con®ned to depths below the thermocline, gust). Interannual modulation by ENSO may account between 400 and 1250 m, and may be representative of for as much as a 5 Sv reduction in transport (Meyers the South Paci®c component of the through¯ow. Den- 1996). sity-driven over¯ow of South Paci®c water into the deep For global models to faithfully reproduce the earth's Banda Sea across the Lifamatola Passage (sill depth climate system, interocean ¯uxes of mass, heat, and 1940-m) contributes approximately 1.5 Sv to the total freshwater must be properly represented. The Indone- through¯ow (Broecker et al. 1986; van Aken et al. 1988; sian Seas with their complex topography, located near van Bennekom 1988). the division of the North Paci®c and South Paci®c water Thus, ``prevailing wisdom'' seems to point to 5±10 mass ®elds, represent a severe challenge to model va- Sv annual average through¯ow, with the maximum lidity. The objective of this paper is to compare ther- transport occurring during the southeast monsoon (Au- mohaline strati®cation and water spreading patterns as FEBRUARY 1999 GORDON AND MCCLEAN 203

FIG.3.(Continued) seen in observations made in 1993 and 1994 with the (NWM) of 1994 (25 January±3 March). During this results of the Los Alamos Parallel Ocean Program 1/6 period the Southern Oscillation index remained nega- deg global model. tive, representing a rather prolonged, though mild El NinÄo episode. 2. Arlindo observations 3. Model description The ®rst phase of the Arlindo Project (a joint ocean- ographic research endeavor of and the United The numerical simulation used in this paper is the States) consists of an extensive suite (Fig. 1a) of CTD Los Alamos National Laboratory (LANL) Parallel measurements extending to the sea ¯oor or to 3000 de- Ocean Program (POP) global ocean model. It is an eddy- cibars obtained from the Indonesian Research Vessel resolving, 20-level, primitive equation model con®gured Baruna Jaya I during the southeast monsoon (SEM) of for a one-sixth degree (on average) Mercator grid. It 1993 (6 August±12 September) and northwest monsoon was initialized from a net, 35-yr spinup from Levitus 204 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29

FIG. 4. Timor section (see Fig. 1a) of potential temperature and salinity for the upper 1000 dbar. SEM represents the southeast monsoon of August 1993; NWM represents the northwest monsoon of February 1994. (a) Observational data; (b) model output.

(1982) at lower resolution, followed by 25 years at the 4. Comparison of observational and model data same resolution. Monthly climatological surface heat An array of monthly averaged model temperature and ¯uxes obtained from European Center for Medium- salinity values for August 1993 and February 1994, in- Range Forecasts (ECMWF) analyses (Barnier et al. terpolated to coincide with the positions of the Arlindo 1995) and daily ECMWF wind stresses averaged to CTD stations, is compared to the Arlindo observational three days were used to force the model. The surface results along three sections during each monsoon sea- salinity was restored to Levitus (1982) monthly cli- son: the western through¯ow channel along the Ma- matology with a restoring timescale of 30 days and the kassar Strait (Fig. 2) and along the eastern through¯ow topography is ETOP05. Further details of the model channel from the Maluku to the Banda Seas (Fig. 3). formulation can be found in Fu and Smith (1996), The third section crosses the Timor Sea (Fig. 4), the McClean et al. (1997), and Maltrud et al. (1998). major out¯ow of Indonesian water into the Indian FEBRUARY 1999 GORDON AND MCCLEAN 205

FIG.4.(Continued)

Ocean. The three sections correspond in each case to a the 10ЊC isotherm is near 320 dbar, and the 5ЊC isotherm full line joining stations in Fig. 1a; each observed and falls between 800 and 900 dbar. The gradient between corresponding model section have been projected onto the 20Њ and 25ЊC isotherms is extremely sharp, as these a meridian for comparison purposes. isotherms are separated by less than 50 dbar. The salinity strati®cation displays seasonal changes. The surface sa- a. Potential and salinity sections linity is much lower in the NWM, a response to greater precipitation and river runoff from . The 1) OBSERVATIONAL DATA FOR MAKASSAR SECTION subsurface salinity maximum (Smax), which denotes the The western through¯ow channel along the axis of presence of North Paci®c subtropical water, is more pro- the Makassar Strait (Fig. 2a) marks the primary through- nounced in the SEM, suggesting greater through¯ow of ¯ow route for North Paci®c thermocline water. The tem- North Paci®c thermocline water (to counter the effects perature distribution reveals little variation with season. of vigorous vertical mixing characteristic of that region)

Water in excess of 25ЊC occurs within the upper 75 dbar; of the Smax core in that season. The salinity minimum 206 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29

(Smin) near 300 dbar marks intrusion of North Paci®c enters the from the across a Intermediate Water. 500-m sill (Fig. 1a). In the southern Banda Sea (3.5Њ± 7.5ЊS) the similarity of the thermocline characteristics with those of the Flores Sea re¯ect the circulation link 2) MODEL DATA FOR MAKASSAR SECTION with Makassar Strait. The model salinity strati®cation along the Makassar The observations reveal that during the NWM the Strait (Fig. 2b) differs from the observations, particu- thermocline is deeper than during the SEM in the region larly below 150 dbar. The salinity in the northern Ma- extending from the Seram Sea to the central Banda Sea kassar Strait goes awry below the Smin of the North (1Њ±4ЊS); the average seasonal change in depth is ap- Paci®c Intermediate Water, attaining values above 34.6. proximately 40 dbar. In the Banda Sea, observed sea

The Smin does not spread south of the midpoint of the surface temperature during the SEM is about 4ЊC colder strait near 3ЊS. This depth interval is replaced to the than during the NWM. Seasonal heaving of the ther- south by very saline water, reaching over 34.8 psu, well mocline and seasonality of the SST in the Banda and above observed values. Part of this difference is attri- Seram Seas are associated with strong seasonal wind buted to the model bottom topography. Near 3ЊS the forcing, inducing SEM surface water divergence and model has a very narrow passage (one longitudinal grid NWM convergence (Wyrtki 1961). box wide in velocityÐ0.28Њ) with a 200-m sill (Fig. 1b), whereas the observations show a wider, much deep- 4) MODEL DATA FOR THE MALUKU TO er (Fig. 1a) channel at that site. The observed controlling BANDA SEA SECTION sill is 550 m deep at 5.5ЊS (Gordon et al. 1994). The model's narrow, shallow sill blocks Makassar water The model and observed salinity ®elds differ signif- from passing into the Flores Sea, which is then domi- icantly; the sharp model halocline between 25 and 50 nated by in¯ow from the Banda Sea. dbar and the high salinity (greater than 35) of the 20ЊC The model water column below 200 dbar is warmer water near 100 dbar (Fig. 3b) are not observed in the than the observed water column, indicating more stored Arlindo data (Fig. 3a). The Smax layer around 100 dbar heat in the model ®eld. The model density (not shown) is far more saline than the Smax subtropical underwater is dominated by the temperature and so is less dense of North Paci®c water and is closer to the South Paci®c than the observations from 200 to 1200 dbar. In general, Smax subtropical underwater. It is likely that in the model the model thermocline is thicker (more diffuse) than the the South Paci®c Smax, drawn from the New Guinea observed thermocline, most likely a consequence of the Coastal Undercurrent, is introduced into the Seram and model's vertical mixing scheme. This behavior has been adjacent seas from the Halmahera Sea. This does not observed in models in which constant vertical mixing seem to be a sill depth problem, as 100 dbar is well coef®cients are used (Bryan 1987; Schott and BoÈning above the observational and model sill depth of the 1991). While the Richardson-number-dependent param- Halmahera Sea. eterization used in the POP model tends to alleviate this The Smin of the North Paci®c Intermediate Water problem (Pacanowski and Philander 1981), an isopycnal (NPIW) in the model is more pronounced and penetrates mixing formulation may be more appropriate (R. Bleck farther into the Banda Sea than revealed in the obser- 1996, personal communication). vations. The North NPIW signal is still apparent in the Another cause for the model/data discrepancies may model Timor Sea and hence represents a contrast to the be due to the lack of a tidal mixing parameterization in generally weaker North Paci®c thermocline presence the POP model. F®eld and Gordon (1992) attribute the within the model. At depths below the Smin the model attenuation of the North Paci®c subsurface Smax (near salinity is again well above that of the observations. The 100 dbar) to tidal mixing. Since the salinities in the saline layer greater than 34.8 psu between 500 and 700 southern Makassar channel of the model are all too high, m is too salty to be derived from the North Paci®c the inclusion of tidal mixing is unlikely to produce more although too deep to be derived solely from the Hal- than a minor improvement to the salinity strati®cation. mahera Sea (model sill is slightly less than 500 m, Fig. A major response would result from incorporating more 1b). It may result from mixing in roughly equal amounts realistic topography in the Makassar Strait. of the downwelled South Paci®c Smax within the Banda Sea and upwelled deeper Banda Sea water, perhaps with some input from the Indian Ocean. 3) OBSERVATIONAL DATA FOR THE MALUKU TO As found in the Makassar Strait the model water col- BANDA SEA SECTION umn temperature (Fig. 3b) below the depth of the 20ЊC The salinity ®eld along the eastern channel reveals isotherm is warmer than the observational data, pre-

(Fig. 3a) that the North Paci®c Smax and Smin occurs only sumably a result of the vertical mixing scheme. The in the Maluku Sea (1.5ЊN±0.5ЊS). In the Seram and model thermal strati®cation along the eastern channel northern Banda Seas (2Њ±3.5ЊS), the Smax near 20ЊCis has a 20-m seasonal change in thermocline depth and absent. Instead, a deeper colder Smax derived from the a2ЊC seasonal change in SST, both about half of the South Paci®c lower thermocline is observed. This water seasonal variability seen in the observations. FEBRUARY 1999 GORDON AND MCCLEAN 207

5) OBSERVATIONAL DATA FOR THE to the southeast wind. Southward ¯ow across 2Њ±47ЊS TIMOR SEA SECTION is shown in the Seram Sea and in Makassar Strait. The northward ¯ow along the western edge of the Makassar In the Timor Sea, the observations (Fig. 4a) display Strait lying over shallower water (100±500 m: Fig. 1) a thermocline strati®cation characteristic of the southern is not corroborated by the Wyrtki current map. Strong Banda Sea, which in turn is similar to that found in the westward ¯ow crosses the after ¯owing Flores Sea and Makassar Strait, a natural consequence through the Torres Strait, a shallow passage between of the ¯ow pattern. The warmer, salty surface water Australia and New Guinea along 142ЊE (not shown in observed during the NWM and the cooler, fresher sur- ®gure). Surface water passes southward into the Indian face water of the SEM are a response to local sea±air Ocean through the major channels ( and Alor) ¯uxes. of the Sunda Island chain. Eastward ¯ow marking the South Java Current curls to the south to join the through- 6) MODEL DATA FOR THE TIMOR SEA SECTION ¯ow south of Java. With the exception noted, these ve- locity patterns are all seen in Wyrtki (1961). As expected, the model data (Fig. 4b) mirrors the In February 1994 (Fig. 5b) the surface ¯ow is nearly model Banda Sea, which is far more saline than the opposite to that of August 1993. Eastward ¯ow from observed Banda Sea. The model Smin near 300 dbar in- the advects into the Flores Sea, is augmented dicates a surviving trace of North Paci®c Intermediate by northward ¯ow from , and then passes Water, a trait not as clearly marked within the obser- into the Banda Sea. The South Java Current is present vations. The NWM surface water in the model is very in both seasons, but in February 1994 it extends farther warm, in excess of 30ЊC approximating the observed to the east feeding the northward ¯ow through Lombok SST, but the corresponding high surface salinity is not Strait rather than turning southward to join the through- modeled. A major model±data difference is found where ¯ow. Surface water ¯ows northward in the Makassar water is colder than 4ЊC. The observed data reveal an Strait, in opposition to the Makassar surface currents intrusion of a deep saline layer from the Indian Ocean shown by Wyrtki (1961). Another discrepancy with the below the observed 1200-m sill depth within the Banda Wyrtki map is in the Halmahera Sea, where Wyrtki Sea (Fieux et al. 1994); this feature is not seen in the shows northward ¯ow, and the model shows southward model ®elds. ¯ow. Northward and eastward ¯ow of surface water, derived from the southern boundary (south Indian b. Circulation and transport Ocean), is modeled east of Timor, extending into the Torres Strait. The Arlindo observations were used to determine the The ¯ow at 117.5 m (Figs. 5c,d) re¯ects the ther- water-mass spreading pattern of the thermocline (Gor- mocline circulation, near the Smax core layer. There are don and Fine 1996). This pattern does not portray an a number of differences with the deduced ¯ow pattern instantaneous or monthly circulation but rather is a re- based on the observed water-mass distribution discussed sponse to the advective and diffusive processes averaged by Gordon and Fine (1996). The POP velocities at 117.5 over a period of the residence time, estimated as six m for August 1993 (Fig. 5c) are directed toward the months to 1 yr (F®eld and Gordon 1992). Deeper water south in the Makassar Strait, drawn from the Mindanao mass spreading patterns, discussed by van Aken et al. Current, consistent with the presence of North Paci®c

(1988) and van Bennekom (1988) based on the Snellius Smax subtropical water. The New Guinea Coastal Current II data, re¯ect a longer residence timescale, spanning a (2ЊS±2ЊN, 115Њ±140ЊE) advects South Paci®c thermo- decade or more. The model velocity and transport ®elds cline water toward the Halmahera Sea with a weak are monthly averages, so total similarity is not expected branch continuing into the Seram Sea, which joins a with the water-mass-derived circulation in a time-vary- stronger southward ¯ow from the Maluku Sea extending ing regime. With this limitation in mind, we next relate along the western Banda Sea. The western Banda ¯ow the through¯ow patterns of the upper 1000 m deduced converges with northward ¯ow to feed an eastward cur- from the Arlindo data for August 1993 and February rent between 4Њ and 5ЊS. 1994 with POP transport values for these months. Flow is directed into the Banda Sea from the Timor Passage, part of which is redirected to the Indian Ocean within the Alor Passage and Lombok Strait. This ¯ow 1) VELOCITY pattern is not in agreement with the water mass obser- The reversal of the model surface ¯ow (12.5 m: Figs. vations, which suggest eastward ¯ow of thermocline 5a,b) in response to the monsoon wind forcing is ap- water in the Flores sea with export to the Indian Ocean parent and, for most areas, is in very good agreement both north and south of Timor. However, the water- with the climatic mean surface current ®elds presented mass-based ¯ow patterns may not fully re¯ect subbasin- by Wyrtki (1961). In August 1993 (Fig. 5a) the POP scale recirculation cells. surface water ¯ows westward across the Banda Sea, The February 1994 ¯ow at 117.5 m (Fig. 5d) displays through the Flores Sea, and into the Java Sea in response some differences from the August 1993 model ®eld. 208 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29

FIG. 5. Horizontal velocity from the model at the 12.5 m, (a) and (b), and at 117.5 m, (c) and (d), for the periods of the two Arlindo CTD cruises: August 1993 representing the southeast monsoon and February 1994 representing the northwest monsoon. FEBRUARY 1999 GORDON AND MCCLEAN 209

FIG.5.(Continued) 210 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29

FIG. 6. Transport in Sverdrups across various passages within the Indonesian seas. The solid arrows denote August 1993; the gray arrows denote February 1994. The values are provided in Table 1.

Stronger ¯ux from the Halmahera to the Seram Sea is the 1994 NWM, yielding a 3.9 Sv average. Seasonal seen. The Halmahera in¯ow diverges, feeding north- variability is found by observations (Molcard et al. ward ¯ow along the western margin of the Maluku Sea, 1994; Molcard et al. 1996; Meyers et al. 1995), although with a weaker southward branch along the western mar- observationally based through¯ow annual average es- gin of the Banda Sea. Within the western Banda Sea timates are slightly larger. For example, the August 1989 the Seram Sea in¯ux combines with a westward zonal and February 1992 Timor Sea average is 8.3 Sv (Fieux ¯ow along 5ЊS, which is derived from the Indian Ocean et al. 1994; Fieux et al. 1996), the XBT-based estimate in¯ow south of Timor, to contribute to the export in the by Meyers et al. (1995) is 5 Sv, and a 6.0 Sv total is Alor and Lombok Passages. The zonal ¯ow in the model obtained by summing the current meter values is fed by strong vertical advection in the eastern Banda in the Timor Sea (Molcard et al. 1996) and Lombok Sea (®gure not shown). Strait (Murray and Arief 1989). The Alor Passage trans- port is not included in these estimates. The model Lom- bok Strait transport values of 3.2 Sv toward the Indian 2) TRANSPORTS Ocean in August 1993 and 0.4 Sv toward the north in The model transports across select passages (Fig. 6; February 1994 can be considered to be in excellent Table 1a,b) show year round ¯ow toward the Indian agreement with the 1985/86 measurements (Murray and Ocean: 7.0 Sv during the 1993 SEM and 0.7 Sv during Arief 1989). Flow toward the Indian Ocean through shallow passage between Java and and between Sumatra and may provide an addition 1 Sv TABLE 1a. Model total transports (in Sverdrups) during the Arlindo (Piola and Gordon 1984). ®eld observations, Aug 1993 and Feb 1994. Negative values are westward or southward and positive values are eastward or northward. In August 1993 the primary model paths introducing Paci®c water into the Indonesian seas are across the Passage (Fig. 6) Aug 93 Feb 94 Average TABLE 1b. Total through¯ow transports (Sverdrups). Lombok Ϫ3.2 ϩ0.4 Ϫ1.4 Makassar Ϫ1.1 Ϫ1.0 Ϫ1.1 Aug 93 Feb 94 Average Sulawesi±Irian Ϫ4.1 Ϫ0.3 Ϫ2.2 Out¯ow to Indian* 7.0 0.7 3.0 Flores Ϫ4.4 ϩ2.9 Ϫ0.8 Ϫ Ϫ Ϫ In¯ow from Paci®c** 7.0 0.7 3.9 Timor Ϫ1.1 ϩ2.8 ϩ0.9 Ϫ Ϫ Ϫ Alor Ϫ2.7 Ϫ3.9 Ϫ3.3 * Out¯ow includes Lombok, Timor, and Alor Passages. Java Ϫ2.3 ϩ1.5 Ϫ0.4 ** Paci®c in¯ow includes Makassar, Sulawesi±Irian, Arafura, mi- Arafura Ϫ4.0 ϩ2.2 Ϫ0.9 nus Java. FEBRUARY 1999 GORDON AND MCCLEAN 211

TABLE 2a. Model total transports (Sverdrups) for Aug 1988 and TABLE 2b. Total through¯ow transports (Sverdrups). Feb 1989. Negative values are westward or southward and positive values are eastward or northward. Aug 1988 Feb 1989 Average Passage Aug 88 Feb 89 Average Out¯ow to Indian* Ϫ18.2 Ϫ5.8 Ϫ12.0 In¯ow from Paci®c** Ϫ18.1 Ϫ5.8 Ϫ12.0 Lombok Ϫ5.7 Ϫ2.1 Ϫ3.9 Makassar Ϫ6.0 Ϫ0.6 Ϫ3.3 * Out¯ow includes Lombok, Timor, and Alor Passages. Sulawesi±Irian Ϫ5.0 Ϫ2.2 Ϫ3.6 ** Paci®c in¯ow includes Makassar, Sulawesi±Irian, Arafura, mi- Flores ϩ1.2 ϩ3.7 ϩ2.5 nus Java. Timor Ϫ5.3 ϩ0.6 Ϫ2.3 Alor Ϫ7.2 Ϫ4.4 Ϫ5.8 Java ϩ0.9 ϩ5.2 ϩ3.0 In February 1994 the model through¯ow into the In- Arafura Ϫ6.3 ϩ2.2 Ϫ2.0 dian Ocean weakens to essentially zero (0.6 Sv). The Makassar Strait transport remains about the same as August 1993, but the Sulawesi±Irian Jaya transport Sulawesi±Irian Jaya (4.1 Sv) section and the Arafura drops to near zero. The (dubious) transport over the shelf (4.0 Sv). The latter is derived from surface water Arafura shelf reverses and is now directed toward the transported across the shallow Torres Strait between of the South Paci®c Ocean. The Lombok Australia and New Guinea. The Makassar runs a poor Strait and Timor Sea transports are toward the north, third (1.13 Sv). into the Indonesian Seas. The main pathway of ¯ow into The model Torres Strait (the shallowest connection the Indian Ocean is via the Alor Passage, which is slight- of the South Paci®c to the Indonesian Seas between ly larger in February than in August. The transports Australia and New Guinea, along 142ЊE) is 50 m deep below 360 m across the Sulawesi±Irian Jaya passage and about 120 km wide. This is much deeper than the have reversed sign and are small: 0.3 Sv to the south observed Torres Strait, which is less than 10 m deep in the depth range 360±985 m and 1.9 Sv to the north with the presence of numerous islands and reefs. Based in water deeper than 985 m. on 5 months of current observations, Wolanski et al. The model August 1993 transports show that the (1988) ®nd strong tidal ¯ow, but no evidence for mean channels east of Sulawesi are the dominant through¯ow ¯ow through Torres Strait. Clearly the model allows too pathways. The Torres Strait pathway is unlikely and much communication across the Torres Strait. Simply probably an artifact of the excessively deep bathymetry removing the Arafura transport when considering the in the model. In February 1994 the total interocean total through¯ow may not be reasonable, as closing the through¯ow is essentially zero, with the Makassar Strait model Torres Strait would alter the surface wind driven southward transport combined with the eastward ¯ow transport between the Arafura and Coral Seas as well in the Java Sea balancing the Torres Strait export to the as inducing unrealistic circulation patterns on either side South Paci®c Ocean. The Alor Passage ¯ow to the In- of the Strait (A. Semtner 1996, personal communica- dian Ocean is mostly compensated by northward ¯ow tion). In addition, it may alter the transmission of the within the Lombok Strait and Timor Sea. Coral Sea high pressure head into the southern Indo- nesian Seas, changing the access of the tropical Paci®c water to the northern Indonesian seas via passages closer 3) INTERANNUAL TRANSPORT VARIATIONS to the equator. During 1993 and 1994 the Indonesian region was in The depth distribution of the in¯ow is totally within the midst of a prolonged El NinÄo episode. The Southern the upper 360 m for the Makassar Strait and Arafura Oscillation index (SOI) was negative from December shelf passages. Across the Sulawesi±Irian Jaya passage, 1989 to the middle of 1995. It is possible that the in- 4.6 Sv enters in the upper 360 m; between 300 and 985 terocean through¯ow during such periods may be lower m there is a northward ¯ow of 3.0 Sv, and below 985 than during a positive SOI (Meyers 1996), owing to the m, 2.6 Sv ¯ows into the Indonesian seas. This latter lower sea level at the Paci®c entrance to the Indonesian ¯ow probably includes the over¯ow of the Lifamatola seas. In order to test this possibility we compare model Passage into the deep Banda Basin. The 3.0 Sv north- ®elds from August 1993 with those from August 1988 ward ¯ow between 360 and 985 m consists of saline and February 1994 with those from February 1989 since water apparent in the Maluku to Banda Sea model sec- the SOI was positive in 1998 and 1989 (Table 2) tion (Fig. 3b). This transport may be derived from the The August 1988 through¯ow is 18.2 Sv [agreeing return of upwelled deep Banda Sea water, mixing with nicely with the 1989 value of Fieux et al. (1994)], the warmer salty South Paci®c subtropical underwater sink- February 1989 through¯ow is 5.8 Sv, yielding an av- ing from near 100 m (Fig. 3b). Some contribution of erage of 12.0 Sv. These values are 2.6 and 8.8 times Indian Ocean water may be drawn from the model Alor larger, respectively, than the corresponding months from and Timor Passages, which together show about a 0.9 1993 and 1994. Additionally, the ratio of the transport Sv northward transport in the 360-m to 985-m layer through the various passages has changed: notably the (Alor is 1.3 Sv to the north, Timor is 0.4 Sv to the Java and Flores transports are eastward for both months, south). the Lombok does not reverse sign, and the Makassar 212 JOURNALOFPHYSICALOCEANOGRAPHY VOLUME29 displaysstrongseasonalvariability.Also,inAugust 1988theMakassartransportissomewhatlargerthan thesouthwardtransportintheSulawesi±IrianJayapas- sage.Clearly,interannualvariabilityoftheinterocean transportassociatedwiththeENSOphasemaybeas largeasseasonalvariability.

4)HEATANDSALTFLUXES Toobtainameasureoftheheatandsalttransported betweenthePaci®candtheIndianOceans,¯uxesof thesepropertieswerecalculatedfrommonthlyaveraged POP®eldsthroughazonalsectionalong115ЊEbetween AustraliaandJava.SincethissectionlieswestofLom- bokStraitandtheIndianOceanbasinfarthernorthis closed,allthrough¯owwaterwillcrossthissection. Thesecalculationsweremadeforthemonthscoinciding withtheArlindoexperiment(August1993andFebruary 1994)andfromAugust1988andFebruary1989during thecoldeventinthePaci®cbasin. ThedirectmethodofHallandBryden(1982)was usedtocalculateheattransports;howeversincethenet volumetransportacrossthissectionisnotzero,these transportsdependonthescaleoftemperature.Follow- ingTalley(1984)andSchneiderandBarnett(1997)the transportisbrokendownintobarotropicandbaroclinic contributionswherethebarotropictermisreferenced bythespatiallyaveragedtemperatureofthereturn¯ow intothePaci®cbetweenAustraliaandAntarctica (2.8ЊC).Itiswritten

L ␳Cu␪ dy dx ϭ ␳CH( y)u(␪ Ϫ ␪ ) dy P͵͵ P͵ ref 0

LH(y) ϩ ␳CuЈ␪Ј dz dy P͵͵ 00 with FIG.7.(a)Baroclinic(solidline)andbarotropic(dashedline)ad- vectiveheattransports(PW),(b)totaladvectiveheattransport uϭuϩuЈ and ␪ ϭ ␪ ϩ ␪Ј, (PW),and(c)masstransport(Sv)across115ЊEbetweenAustralia whereu and ␪ aredepth-weightedaveragesofpotential andJavafor1993. temperatureandzonalvelocity, ␪ref isthespatially weightedaverageofthereturn¯ow,andH(y)isthe through¯owtransportwasstrongerduringthe1988/89 oceandepthalongthesection. periodduringaPaci®ccoldevent.Thetotalheattrans- ThetotalheattransportsinAugust1993andFebruary portintotheIndianOceanwas1.77and0.64PWin 1994areϪ0.79andϪ0.06PW(negativevaluesare August1988andFebruary1989;bothhigherthanthe intotheIndianOcean),respectively.Thebarotropiccon- 1993/94values. tributionsduringthesemonthsareϪ0.09and0.12PW, Asmentioned,thequantitiesusedintheabovecal- respectively,whilethebarocliniccomponentsaccount culationare͗u͘and͗␪͘, where angle brackets denote a forϪ0.70andϪ0.18PW,respectively. monthlyaverage.Thetotaladvective¯ux Thebarotropicandbarocliniccontributionswerecal- culatedforeachmonthin1993andareshowninFig. ␳CP ͗u␪͘ dz dy 7aandtheirtotalsareseeninFig.7b.Astrongseasonal ͵͵ cycleisseeninthebaroclinic¯ow(solidline),which isrelatedtotheseasonalcycleofthethrough¯owmass isalsoarchived,andthedifferencebetweenitandthe transport(Fig.7c).Thebarotropiccomponent(dashed ¯uxduetothemeancurrents, line)ismuchsmallerthanthebarocliniccomponentand ␳C ͗u͗͘␪͘ dz dy, lacksaseasoncycle.Aspreviouslynoted,themodel P͵͵ FEBRUARY 1999 GORDON AND MCCLEAN 213

and 8.5ЊS, the latter latitude being the northern end of the through¯ow section, and between these latitudes along 115ЊE. Land forms the western boundary of the box, as well as south of 22ЊS along 115ЊE. Here the 10- yr mean ®elds are used. Temperature ¯uxes are obtained by directly integrating the total advective ¯uxes along each ocean boundary. Temperature ¯uxes can be used here as we are interested in the relative contribution of the through¯ow waters to the southern Indian Ocean heat budget. The in¯ow from the Paci®c is 0.72 PW, while 0.38 PW enters the region from the northern In- dian Ocean across 8.5ЊS. This net input of 1.1 PW is balanced by 0.71 PW exiting the box at 30ЊS and 0.32 PW being lost to the atmosphere over the area of the box. The residual is balanced by the change in heat storage over the time of the run (0.08 PW), which rep- resents a loss to the box. Of the heat input to the region, the through¯ow represents about 65% of the total. The remaining 35% is from the input of solar radiation to the north of the through¯ow. This result is consistent with the recent ®ndings of Garternicht and Schott (1997) who examined the effect of the through¯ow on the In- dian Ocean heat budget in the Semtner and Chervin ¼Њ model. The main differences between the models are resolution and the form of the surface heat forcing; in POP, monthly climatological heat ¯uxes together with a surface temperature restoring term were used, while this version of the Semtner and Chervin model used surface restoring only. In this case, these factors have not signi®cantly changed the effect of the through¯ow on the heat budget of the Indian Ocean. Salt ¯uxes were obtained directly by integrating ␳͗us͘ FIG. 8. (a) Temperature ¯uxes (PW), (b) salt ¯uxes (ϫ106 kg sϪ1) across the Australia to Java section: for August 1993 in the southern Indian Ocean for 1986±95. and February 1994 they are Ϫ251 and Ϫ30 ϫ 106 kg sϪ1, respectively, into the Indian Ocean. The contribu- tion from the mean currents is Ϫ246 and Ϫ22 ϫ 106 represents the eddy ¯ux. The relative contribution of kg sϪ1, respectively. The eddy contribution is again the eddies to the total ¯ux is about two orders of mag- small. nitude smaller than that of the mean term across the It should be noted that there is no loss or gain of 115ЊE section between Australia and Java for the above freshwater in the model; rather salt is added or removed cases. Consequently, the transports derived from the through the effects of the restoring to Levitus (1982) mean ®elds almost entirely represent the total transports salinities. To understand the impact of the through¯ow through the 115ЊE section. salt ¯ux on the southern Indian Ocean, salt ¯uxes are Annual net heat transports into the Indian Ocean have calculated on the boundaries of the box described earlier, been calculated by Hirst and Godfrey (1993) and again using the 10-yr mean ®elds (Fig. 8b). Fluxes input Schneider and Barnett (1997) using global general cir- to the box are 207 ϫ 106 kg sϪ1 from the through¯ow culation models (the latter was coupled to an atmo- and 16 ϫ 106 kg sϪ1 over the surface of the box. Outputs spheric model). They obtained 0.63 and 0.9 PW, re- are 8 and 212 ϫ 106 kg sϪ1 along 8.5ЊS and 30ЊS, spectively, into the Indian Ocean. The net input transport respectively. The residual is due to subgrid-scale trans- from the POP 10-yr mean ®eld is 0.66 PW, in general ports since the salt storage term is only Ϫ0.1 ϫ 106 kg agreement with the other models. Differences may be sϪ1. Clearly, the through¯ow waters dominate the south- attributed to the model formulations (resolution and fric- ern Indian Ocean salt budget. tion) and forcing. As the model is found to inject too much saline South To determine the in¯uence of the Indonesian Paci®c thermocline water into the Indian Ocean via the Through¯ow waters on the Indian Ocean heat budget, Indonesian seas, it is expected that the model requires a region encompassing the pathway of the through¯ow less net Indian Ocean evaporation than in reality. The is examined (Fig. 8a). A closed region is constructed net salt input of 16 ϫ 106 kg sϪ1 requires an equivalent by extracting the total advective heat ¯uxes along 30Њ net evaporation of 0.455 Sv consistent with the region 214 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29 being one of excessive evaporation. Baumgartner and Paci®c thermocline water. The heat budget is also likely Reichel (1975; Table XXXV, 178) indicates net evap- to be somewhat higher than is realistic as a result of the oration of 0.585 Sv for the Indian Ocean between 10Њ vertical mixing scheme, which produces too diffuse a and 30ЊS. Thus, the equivalent model net evaporation thermocline and thus producing warmer temperatures is 78% of the estimated net evaporation. lower in the water column than are observed. Some of these inadequacies may be remedied by in- creased vertical and horizontal resolution, and the use 5. Discussion of a new global topography dataset obtained from sat- Within the Indonesian Seas the Los Alamos Parallel ellite altimetry (Smith and Sandwell 1997). Increasing Ocean Program 1/6 deg global model temperature and the number of vertical levels in the upper water column salinity strati®cation is dominated by relatively salty and limiting the depth of the Torres Strait to the top South Paci®c thermocline water rather than fresher level would reduce the unrealistically high transports North Paci®c thermocline water as found in the Arlindo through this region. In addition, locally increased fric- data. Comparisons of the observations and model ®elds tion to mimic the effects of the strong, observed tidal show that the thermohaline strati®cation within the Min- currents would also retard this ¯ow. At deeper locations, danao Current and the New Guinea Coastal Current, the an increased number of levels would result in more potential sources for the interocean through¯ow, match ``active'' velocity grid points as there is a loss of ve- quite closely. This suggests that the thermohaline dis- locity points adjacent to land. Increased horizontal res- crepancies between the model and observed ®elds with- olution would better resolve the ¯ow in narrow channels in the Indonesian seas stem from regional effects that such as Lombok Strait, which currently is only one ve- allow excessive amounts of South Paci®c water into the locity grid box wide. The Smith and Sandwell (1997) Indonesian seas from north of Irian±Jaya and through topography dataset, produced by combining shipboard Torres Strait, while limiting access of the fresher North depth soundings with gravity data derived from satellite Paci®c thermocline water through Makassar Strait. It is altimetry, may provide a more realistic bathymetry than suspected that the details of the Indonesian seas bottom that currently used. bathymetry may be the cause for the differences. Ex- Another model inadequacy is the lack of a mixed amples of inaccurate model bathymetry in key access layer. Presently, the model is unable to distribute mo- passages are a 200-m sill in the central Makassar Strait mentum to the lower layers, resulting in unrealistically that arti®cially blocks North Paci®c water from freely high Ekman ¯ows in the upper level. Inclusion of a advecting into the Flores Sea; the arti®cially deep Torres mixed layer would remedy this situation and produce a Strait (50-m model depth vs the less than 10-m observed more realistic thermohaline structure in the upper model depth) that allows South Paci®c surface water and, per- water column. In addition, a tidal mixing parameteri- haps what is more important, the South Paci®c sub- zation may produce a more realistic attenuation of water tropical gyre pressure head to pass into the southern mass properties in the through¯ow pathways. Hence, Indonesian Seas; and the relatively wide Alor Passage improved bathymetry, increased vertical and horizontal that can accommodate the interocean through¯ow with- resolution, and the inclusion of a mixed layer and a tidal out requiring the ``use'' of the deep passage south of mixing parameterization will be necessary to more ac- Timor, as observed. curately model the Indonesian region. A consequence of through¯ow transport being dom- inated by the South Paci®c thermocline is that the model North Paci®c residence time would be much longer than 6. Conclusions the real residence time. The O(10 Sv) Indonesian a. Temperature and salinity Through¯ow is the primary exit for the low salinity North Paci®c thermocline and intermediate water, as Relative to the Arlindo observations, the model ther- exports only one-tenth of the suspected mocline for August 1993 and February 1994 reveals Indonesian Through¯ow magnitude. Reduced export of excessive amounts of saline South Paci®c water within North Paci®c thermocline water implies reduced import the Indonesian seas. The model Makassar Strait has a via cross equatorial transport of South Paci®c water. The North Paci®c±dominated thermocline agreeing with the model South Paci®c water streams directly into the In- observational stations to around 250 dbar. Through¯ow dian Ocean, without ®rst folding into the North Paci®c below that depth is inhibited by a 200-m-deep restriction within the series of zonal equatorial currents. The model in the model central Makassar Strait. The model ther- North Paci®c thermocline is expected therefore to have mohaline strati®cation within the Maluku, Seram, and an unrealistically high degree of isolation from the rest Banda Seas is far too salty at all levels except in the of the World Ocean. surface and deep water, indicating an unrealistically South Paci®c±dominated through¯ow waters will large infusion of South Paci®c water. One source of the also impact the salt budget of the southern Indian Ocean, South Paci®c water appears to be through the Halmahera resulting in larger ¯uxes into the region than are likely Sea, though the model sill for this region of 500 m is to result if the water column was dominated by North approximately correct. Apparently the model allows ex- FEBRUARY 1999 GORDON AND MCCLEAN 215 cessive downward mixing of the saline South Paci®c solar radiation north of the through¯ow. In addition, the water to depths of about 700 dbar (near the 5ЊC iso- through¯ow salt ¯ux dominates the input to this region. therm). The model thermocline appears to be too dif- The model through¯ow of excessive amounts of South fusive, a result of the vertical mixing scheme. The model Paci®c thermocline water requires smaller model net exports to the Indian Ocean a water column laced with evaporation in the subtropical Indian Ocean than ex- too much South Paci®c salt. In the Timor Sea below pected from observational-based estimates. the 1200-m sill depth that separates the Indonesian seas from the Timor Sea, the model lacks the observed in- Acknowledgments. A. Gordon's grant support in the trusion of saline south Indian Ocean deep water. preparation of this research paper is derived from the following grants to Lamont-Doherty Earth Observatory of Columbia University: ONR Grant N00014-90-J- b. Mass, heat and salt ¯uxes 1233; NSF OCE-95-29648; and IPA Agreement, Mon- The pathways of the interocean through¯ow within terey Grant NPS CU01548101 while he occupied the the model do not agree with the observationally derived Marine Chair at the Naval Postgraduate School inference, though the observational base is weak. Within in Monterey, California, in 1995. The model analysis the model, North Paci®c thermocline water ¯owing was supported by the Department of Energy's Of®ce of through the upper 200 m of the Makassar Strait exits Health and Environmental Research under the the Indonesian seas in the Lombok Strait (the same is CHAMMP (Computer Hardware, Advanced Mathe- found in the Miyama et al. 1995 model results), whereas matics, and Model Physics) program and NSF Grant the observations show that the bulk turns eastward into OCE-9633049. The simulation was run on the Connec- the Flores and Banda Seas before exiting the Indonesian tion Machine 5 at the Advanced Computing Laboratory seas near Timor. A major discrepancy in the surface of Los Alamos National Laboratory, Los Alamos, New layer velocity ®eld between the model and observations, Mexico. Bert Semtner and Mathew Maltrud are thanked with possible overall transport rami®cations, arises from for helpful discussions. 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