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Instability/stability in the North Equatorial Current (NEC) basin is studied based on data obtained from nine moorings deployed at 8.5°N, 10.5°N, 11.0°N, 12.5°N, 13.0°N, 15.0°N, 15.5°N, 17.5°N, and 18.0°N along 130.0°E during cruises in 2015–2017. In low latitudes, the Coriolis parameter and stratification ratio play important roles in NEC stability, whereas velocity shear and the layer depth ratio are important for NEC stability in high latitudes. Beneath the westward NEC, eastward zonal jets occur intermittently centered around 8.5°N, 12.5°N, and 17.5°N along 130.0°E. Similar to the NEC, the main body of these zonal jets also deepens with latitude. In the boundary layer comprising the bottom NEC and upper zonal jets, the growth rate of the NEC is attributed not only to velocity shear but also to zonal jet velocity based on the longwave assumption. Based on the shortwave assumption, the growth rate is proportional to zonal jet velocity but has no relationship with velocity shear. Climatologically, the growth rate in the boundary layer is not zero at 8.5°N, 12.5°N, and 13.0°N, where the velocity shear and zonal jets are larger than at other stations. The instability also occurs at the time node when the zonal jets are strong enough, although the mean zonal jets may disappear at this station.

Herein, the spatial characteristics of the Equatorial Undercurrent (EUC) in the region between Mindanao and New Guinea and the origins of the EUC and the North Equatorial Countercurrent (NECC) were investigated using a combination of high-resolution Argo hydrographic data and numerical modeling. The Mindanao Current (MC) results in the formation of a strong EUC-depth eastward current that splits into two branches near the Halmahera Eddy west of 132 degrees E, with one branch directly flowing eastward without experiencing the Halmahera Eddy rotation effect, and the other branch flowing southwestward and then turning northeastward. The EUC and NECC were difficult to distinguish between 132 degrees E and 136 degrees E, since the former combined with the latter to form a continuous equator-tilted current. At similar to 137 degrees E, the EUC axis started turning toward the equator, and an isolated EUC core was observed at 143 degrees E and similar to 200-m depth. The Pacific EUC was shown to originate at similar to 130 degrees E, where it featured an isolated current core with a maximum velocity of 5-10cm/s at similar to 1 degrees N and 200- to 400-m depth. The EUC and NECC in the western tropical Pacific were found to have the same water sources, namely, the MC and the New Guinea Coastal Undercurrent, with the shallow parts (centered at similar to 100-m depth) of MC and New Guinea Coastal Undercurrent finally reaching the NECC and their deeper parts (centered at similar to 200-m depth) finally reaching the EUC.

Seasonal and interannual variability of the New Guinea Coastal Current (NGCC) and New Guinea Coastal Undercurrent (NGCUC) were investigated with 3 years of mooring measurements off the northern coast of New Guinea and outputs from the Ocean General Circulation Model for the Earth Simulator during 1980-2018. Acoustic Doppler Current Profilers mounted on the two moorings captured variations of the currents in the upper 800 m off the New Guinea coast during 2015-2018. NGCC is a seasonally reversing current in the upper 100 m, which flows southeastward in boreal winter with maximum velocity of 63 cm/s near the surface, and flows northwestward in boreal summer with maximum velocity of -55 cm/s at 80 m. NGCUC flows northwestward all year round between 100 and 400 m, and its temporal mean velocity reaches -40 cm/s at 200 m. A seasonally reversing current named New Guinea Coastal Intermediate Current with speed of 10 cm/s is detected below the NGCUC, which is in phase with the NGCC on seasonal time scale. Seasonal variation of the NGCUC is also in phase with that of the NGCC, and it is strong in boreal summer and weak in winter. Such seasonal signal reaches down to the depth of 800 m. Both mooring measurements and model outputs indicate that NGCUC demonstrates significant interannual variations associated with El Nino-Southern Oscillation (ENSO), with its velocity core shoaling during El Nino and deepening during La Nina, but the net transport of NGCUC exhibits no significant relationship with ENSO. Plain Language Summary The currents north of the New Guinea coast transport water masses crossing the equator from the Southern Hemisphere to the Northern Hemisphere, and modulate the thermohaline structure in the equatorial Pacific, playing an important role in the El Nino-Southern Oscillation cycle and the interhemisphere water exchange. To better understand the structure and variability of these currents, we deployed two subsurface moorings and obtained the velocity time series measurements down to 800 m. A seasonally reversing current is detected below the traditionally known upper-ocean currents, and the monsoon wind forcing is noticed to reach the depth of 800 m, much deeper than the previously acknowledged 150 m. The velocity core of the New Guinea Coastal Undercurrent is revealed shoaling and deepening during El Nino and La Nina, respectively, but the transport of this current exhibits no close relationship with ENSO, which is probably due to the transport restriction through Vitiaz Strait. This research improved our understanding of the vertical structure and variability of the currents off the New Guinea coast.

An ocean downscaling model product, forced under the RCP8.5 future climate change scenario, has been used to understand the ocean heat balance of the Indian Ocean in a warming climate. Towards the end of the 21th century, the model simulates a significant reduction of Indonesian Throughflow (ITF) transport, which reduces the Pacific to Indian Ocean heat transport by 0.20 PW; whereas across S in the southern Indian Ocean (SIO), the southward heat transport is reduced by 0.28 PW, mainly contributed from the weakening western boundary current, the Agulhas Current (0.21 PW). The projected weakening of the Agulhas Current is to compensate for the reduction of the ITF transport, with additional contribution from the spin-down of the SIO subtropical gyre. Thus, being amplified by the ocean circulation changes in the SIO, the projected Indian Ocean warming trend is much faster than the direct air-sea heat flux input.

The Western Pacific Warm Pool (WPWP) plays an important role in the global climate through modulating deep convections, ENSO, monsoon onsets, etc. Due to the vast spatial range and huge heat storage of the WPWP, near-real-time monitoring of its three-dimensional variations remains challenging. Based on Argo observations and three reanalysis data sets, we find that the Nino4 sea surface temperature (SST) index captures the interannual variability of the WPWP well. The Nino4 SST can explain approximately half of the variance of the WPWP heat content and almost all the variance of the east-west migration of the WPWP. An assessment of 31 CMIP5 models also reveals that models with larger interannual spectral powers and amplitudes of the Nino4 SST tend to simulate larger variations in the heat content and east-west migration of the WPWP. A surface heat budget analysis further shows that the Nino4 SST and WPWP are physically connected through basin-scale horizontal advections of mean temperatures by anomalous horizontal currents, which dominate the interannual variations of both the Nino4 SST and WPWP. Our results indicate that the Nino4 SST can efficiently estimate the interannual WPWP changes and a reliable predictor of the onset time of the South China Sea summer monsoon and Bay of Bengal summer monsoon, without the need to calculate the eastern boundary location and heat content of the WPWP. Moreover, a better simulation of the SST and horizontal currents in the Nino4 region can help to reduce model bias when reproducing the WPWP's interannual variabilities.

Intermediate water exchange in the northwest tropical Pacific is explored with the temperature, salinity, and current measurements of a mooring system deployed at 8 degrees N, 127.05 degrees E during 2010-2014. For the first time, prominent semiannual variability (SAV; with the maximum power at similar to 187days) of subthermocline meridional flow along the Mindanao coast is revealed. A significant correlation between meridional flow and salinity is found at intermediate depths. This provides direct evidence for the alternating transports of South Pacific and North Pacific Intermediate Waters by northward and southward undercurrents, respectively. Further analysis with an eddy-resolving ocean general circulation model demonstrates that the SAV is generated locally near the western boundary, manifesting as large-scale subthermocline recirculation and leading to alternating northward and southward flows near the Mindanao coast, which plays an efficient role in the intermediate water exchange of the northwest tropical Pacific. Mechanisms underlying the observed SAV are discussed.

Direct measurements of Kuroshio at its origin (18 degrees N, east of the Luzon Island) are conducted from November 2010 to October 2012. It is found that the depth-averaged Kuroshio between 200 and 700 m has increased over 15 cm s(-1) during the 2 year observational period and it is accompanied by the pronounced southward shift of the North Equatorial Current (NEC) bifurcation. Further analysis indicates that the Kuroshio's strengthening is confined to the upstream segment east of the Luzon Island while the Kuroshio decreased as it passed the Luzon Strait due to a dipole-like sea surface height (SSH) trend between 15 degrees N and 23 degrees N. It is demonstrated that the 2 year strengthening of the Kuroshio, as well as the dipole-like SSH trend can be adequately reproduced by a 1.5 layer nonlinear reduced gravity model, suggesting an important role of upper ocean response to low-frequency wind forcing in the western Pacific. Salinity at 500 m depth is also found to increase during the concurrent 2 years. This subthermocline salinity increase is a combined outcome of vertical (basin-scale isopycnal surface movement) and horizontal advections (i.e., strengthened Kuroshio) due to changes in the large-scale wind-driven ocean circulation.

The subsurface countercurrents in the Philippine Sea and their roles in water mass transportation have been reported in previous studies. Their existence is still controversial, and the underlying dynamics remains unclear. This study investigates the climatological structures and relationships of three subsurface countercurrents, namely the Mindanao Undercurrent (MUC), the Luzon Undercurrent (LUC), and the North Equatorial Undercurrent (NEUC), using recently available hydrographic and satellite altimeter data. The three subsurface currents below and opposite to the surface currents are confirmed by multisections analysis. The MUC, as traced at zonal sections between 6.5 degrees N and 10.5 degrees N, shows two northward velocity cores, both with maximum speed larger than 10 cm s(-1). The LUC exhibits an obscure core with southward velocity larger than 2 cm s(-1) under the Kuroshio at 18 degrees N and 16.25 degrees N sections. The eastward flowing NEUC also has two separated cores at 128.2 degrees E and 130 degrees E sections with velocity larger than 1 cm s(-1). Analyses of -S relationship suggest that the southern part of NEUC is fed by the MUC with the South Pacific water and South/North Pacific water mixture, while the northern NEUC is likely a destiny of the North Pacific water carried by the LUC. Tightly associated with the opposite horizontal gradients between sea surface height (SSH) and the depth of thermocline (DTC), the subsurface countercurrents exist in connected zones where the baroclinic adjustment below the thermocline overcomes the barotropic forcing at the sea surface, which indicates the dynamical linkages among the three subsurface countercurrents.

The intraseasonal variability (ISV) of the subthermocline current east of Mindanao was characterized and shown to be caused by the activity of subthermocline eddies using mooring observations at 8 degrees N, 127.03 degrees E and a high-resolution numerical model. The ISV of the observed current east of Mindanao is vertically coherent in the upper 940 m but is significantly intensified below the thermocline. The ISV amplitude (8 cm s(-1)) of zonal subthermocline current is comparable with that (11 cm s(-1)) of the meridional current, revealing the nature of active eddies. The ISV of the subthermocline current was caused by the subthermocline eddies from three different pathways. The subthermocline eddies propagating along approximately 10 degrees N-11 degrees N contributed more to the ISV of the subthermocline current east of Mindanao than did those eddies propagating westward along 8 degrees N or northwestward from the New Guinea coast. Subthermocline eddies mainly exist south of the bifurcation latitude of the North Equatorial Current in the western tropical Pacific, and their generation and propagation mechanisms are briefly discussed.

The role of the western tropical Pacific Ocean heat content in the South China Sea summer monsoon (SCSSM) onset is investigated in the present paper, by using atmospheric data from NCEP and ocean subsurface temperature data from Japan Meteorology Agency. It is showed from the result that the heat content (HC) of the upper 400 m layer in the western tropical Pacific (WTP), especially in the region of (130 degrees E-150 degrees E, 0 degrees N-14 degrees N) in the last four decades, is a good predictive indicator for the SCSSM onset. Positive (negative) HC anomalies can induce a strong (weak) convection over the WTP, leading to stronger (weaker) Walker circulation and weaker (stronger) western North Pacific subtropical high (WNPSH) in the boreal spring. Consequently, the anomalous westerly (easterly) in the tropical Indian Ocean is favorable (unfavorable) for the airflow into the SCS and for an early (late) WNPSH retreat from the SCS and hence for an early (late) SCSSM onset. It is elucidated that the long-term trend of SCSSM onset changes its sign around 1993/94 from decline to rise, which is responding and attributed to the WTP HC trend. During the period of 1971-1993, the WTP HC shows a significant decrease trend. In particular, a significant decline trend is observed in the HC difference between the WTP and western tropical Indian Ocean, which causes an easterly trend in the SCS and strengthened WNPSH trend, leading to a late onset trend of SCSSM. The situation is reverse after 1993/94.

Significant intraseasonal variability (ISV) of about 45-80 days and seasonal variation of the Luzon Undercurrent (LUC) at 18 degrees N are studied using direct current measurements and a high-resolution global Hybrid Coordinate Ocean Model. The variations of the LUC are vertically coherent with those of Kuroshio Current both on intraseasonal and seasonal time scales. The ISV of the LUC is dominated by eddies with diameters of about 200-300 km and extending from sea surface to intermediate layer east of Luzon Island. The LUC becomes strong (weak) when cyclonic (anticyclonic) eddies occur. The eddies east of Luzon Island mainly originate from the bifurcation point (similar to 13 degrees N) of the North Equatorial Current. These eddies propagate northwestward at a typical propagation speed of about 0.16 m s(-1) along the east coast of Philippines, gradually strengthen and pass the Luzon coast, and continue northward to Luzon strait. On seasonal time scale, the LUC is strong (weak) in boreal winter (summer), and this variation is related to the seasonal evolution of large-scale ocean circulation east of Philippines mainly controlled by local wind forcing.

The mean structure and variability of the currents east of Mindanao are investigated through 2 year mooring observations at about 8 degrees N, 127 degrees 3' E from December 2010 to December 2012. The strong southward Mindanao Current (MC) exists in the upper 600 m with a maximum mean velocity of 73 cm/s and a standard deviation of 17 cm/s at 100 m. A northward mean flow is observed below 600 m to the depth deeper than 1000 m, which has been called the Mindanao Undercurrent (MUC) with a maximum mean velocity of about 10 cm/s at 950 m and a standard deviation of 19 cm/s. Further analysis with hydrographic data and an eddy-resolving model outputs also suggests this northward mean current to be the MUC. Intraseasonal variability with a period of 60-80 days is revealed through the whole water column from 200 m down to about 900 m. This intraseasonal variability appears to be closely related to subthermocline eddies, which translate westward and intensify near the Mindanao coast.

This study investigates the long-term change of the Pacific South Equatorial Current (SEC) bifurcation latitude (SBL) over 1950-2010 with Simple Ocean Data Assimilation version 2.2.4. Results indicate that the SBL averaged within upper 200 m has migrated southward at 0.020 degrees S yr(-1), comparable in magnitude with -0.024 degrees N yr(-1) for the North Equatorial Current bifurcation latitude (NBL). The SEC transport into the Coral Sea has increased. Due to the southward SBL migration, most of the increased SEC water was transported equatorward, contributing to the Equatorial Undercurrent intensification. Experiments with a nonlinear 1.5 layer reduced gravity model indicate that the southward migration of SBL is mainly caused by positive Ekman flux divergence trend in the eastern tropical South Pacific, while that of NBL is caused by negative Ekman flux divergence trend in the western tropical North Pacific.

In this study, we examine the decadal variations of the Pacific North Equatorial Current (NEC) bifurcation latitude (NBL) averaged over upper 100 m and underlying dynamics over the past six decades using 11 ocean products, including seven kinds of ocean reanalyzes based on ocean data assimilation systems, two kinds of numerical simulations without assimilating observations and two kinds of objective analyzes based on in situ observations only. During the period of 1954-2007, the multiproduct mean of decadal NBL anomalies shows maxima around 1965/1966, 1980/1981, 1995/1996, and 2003/2004, and minima around 1958, 1971/1972, 1986/1987, and 2000/2001, respectively. The NBL decadal variations are related to the first Empirical Orthogonal Function mode of decadal anomalies of sea surface height (SSH) in the northwestern tropical Pacific Ocean, which shows spatially coherent variation over the whole region and explains most of the total variance. Further regression and composite analyzes indicate that northerly/southerly NBL corresponds to negative/positive SSH anomalies and cyclonic/anticyclonic gyre anomalies in the northwestern tropical Pacific Ocean. These decadal circulation variations and thus the decadal NBL variations are governed mostly by the first two vertical modes and attribute the most to the first baroclinic mode. The NBL decadal variation is highly positively correlated with the tropical Pacific decadal variability (TPDV) around the zero time lag. With a lead of about half the decadal cycle the NBL displays closer but negative relationship to TPDV in four ocean products, possibly manifesting the dynamical role of the circulation in the northwestern tropical Pacific in the phase-shifting of TPDV.

Hydrographic observations, ocean state estimates, and ocean objective analyses are combined to investigate the decadal variations of the North Equatorial Current (NEC) in the Pacific at 137 degrees E during the last three decades (1975-2005). Observations show that the decadal NEC transport has three maxima around 1980/1981, 1994/1995, and 2004/2005, and two minima around 1989/1990 and 1999/2000, respectively. Associated with these maxima/minima, the sea surface height (SSH) falls/rises and the subsurface isopycnals shoal/deepen in the southern part of NEC, resulting in westward/eastward zonal velocity anomalies. Results from the ocean state estimates and ocean objective analyses show good agreement with observations. Further analysis indicates that the observed zonal velocity anomalies at 137 degrees E are part of the cyclonic/anticyclonic gyre anomalies formed in the tropical northwestern Pacific east of the Philippines, coinciding with the tropical gyre. Results from a 11/2 layer reduced gravity model suggest that these oceanic variations are mainly controlled by the decadal wind forcing in the tropical western Pacific and can be attributed to both local Ekman dynamics and baroclinic Rossby wave propagation.

The interannual variability of the North Equatorial Current (NEC) transport in the tropical northwestern Pacific Ocean is investigated with the output from ECMWF Ocean Analysis/Reanalysis System 3 (ORA-S3). The results show that the amplitude and root mean square (RMS) of interannual NEC transport anomalies increase from about 3.0-4.0 Sv and 2.0 Sv at 170 degrees E to above 5.0 and 3.4 Sv at 135 degrees E, respectively. The NEC transport variation agrees well with the variation of the sea surface height (SSH) anomaly difference between the southern and northern boundaries of the NEC region. Further analysis near the Philippine coast suggests that their good agreement mainly comes from the agreement of the NEC transport and SSH variations south of the gyre boundary. Around the bifurcation point off the Philippine coast, the southern branch of the NEC transport is highly related to El Nino-Southern Oscillation (ENSO) events. During El Nino/La Nina years, westerly/easterly wind anomalies and positive/negative wind stress curl anomalies develop in the tropical northwestern Pacific Ocean south of 20 degrees N before the mature phase. The wind forcing center moves eastward with time and reaches the easternmost position around 170 degrees E several months before the mature phase. This wind forcing generates upwelling/downwelling Rossby waves, which propagate westward to result in negative/positive SSH anomalies, hence inducing a cyclonic/anticyclonic gyre anomaly, which is responsible for the increase/decrease of the NEC transport. The northern branch of the NEC transport near the Philippine coast has no significant simultaneous relation with ENSO events.

Analysis of results from an eddy-resolving general circulation model showed two subsurface velocity cores in the mean within the depth range between 400 and 1000 m below the Mindanao Current (MC). One is confined to the inshore edge at about 126.8 degrees E and connected with the Sulawesi Sea. The other takes place somewhat offshore around 127.7 degrees E, being closely related to the intrusion of South Pacific water. Both cores are referred to as the Mindanao Undercurrent (MUC). The MC/MUC is approximately a geostrophic flow, except on the inshore edge of the MUC where up to 50% of the mean flow can be explained by ageostrophic dynamics. In contrast with the well-defined southward flowing MC, the MUC is of high velocity variance relative to the mean. Empirical orthogonal function (EOF) analysis shows that approximately 60% of the total velocity variance is associated with two meandering modes, with their major signatures in the subthermocline. The dominant time scale of variability is 50-100 days. An ensemble of these meso-scale fluctuations provides a northward freshwater flux on the offshore edge of the Philippine coast, which to a certain extent explains why water of South Pacific origin appears to extend farther northward than the mean MUC. In the offshore velocity core of the MUC, for example, eddy induced freshwater flux is equivalent to a mean flow of about 0.3 m s(-1) in the density range between 26.9 and 27.3 kg m(-3), which is greater than the mean current by a factor of 6.

The origin and pathway of Luzon Undercurrent (LUC) water are investigated using a simulated adjoint tracer, based on circulation estimates of a global general circulation model. The results demonstrate that most of the LUC water comes from the subtropical North Pacific, confirming the earlier hypothesis on its relation to the northward shift of the North Equatorial Current (NEC) bifurcation at subsurface. Of the total volume of initially tracer-tagged LUC water, approximately 41% is traced back to the winter mixed layer in the Kuroshio extension after 50 years of integration, coinciding with the formation of subtropical mode waters, while the rest is trapped in the northern subtropical gyre with its density >26.8 kg m(-3), indicative of a significant South Pacific origin. As these waters move toward the western boundary, they get mixed and finally reach the density range of the LUC water. This result provides quantitative evidence for the dramatic impact of mixing on the route of subtropical water to becoming the LUC water.

Both spatial and seasonal variation of turbulent diapycnal mixing in the subtropical northwestern Pacific are evaluated by employing a fine-scale parameterization method based on profiles of potential density, which are obtained from CTD measurements during our recent hydrographic surveys implemented by the China National Key Basic Research Project from 2008 to 2010 and the World Ocean Circulation Experiment. Over smooth seafloor, the value of diffusivity away from the boundary is comparable with the values observed in the stratified midlatitude ocean interior, i.e., O (10(-5) m(2) s(-1)). On the other hand, enhanced diapycnal mixing, i.e., O (10(-4) m(2) s(-1)) or larger has been found over rough topography such as the Central Basin Trough, Okidaito Ridge, the origin of the Kuroshio Current, and especially Luzon Strait, which might result from dissipation of baroclinic energy generated when barotropic tides rub over rough topography. Over flat bathymetry, mixing is probably stirred by the wind work on near inertial motions in the upper 600 m and enhanced downward propagating energy has been found in the presence of anticyclonic eddies, which points to the important role of anticyclonic eddies in enhancing the diapycnal mixing at greater depth. The diffusivity also displays a distinct seasonal variation with strong (weak) mixing corresponding to strong (weak) wind-input energy in winter (summer), which, however, is only confined to upper 600 m. This is different from the midlatitude northwestern Pacific, where seasonality of diffusivity can be found at 1500-m depth.

Anti-cyclonic eddies northwest of Luzon of the Philippines in summer-fall are identified in the merged data products of satellite altimeters of Topex/Poseidon, Jason-1 and European Research Satellites. The generation and propagation of the anti-cyclonic eddies, which are confirmed by satellite ocean color data, are found to be a seasonal phenomenon that is phase-locked to the onset of the southwesterly monsoon and the relaxation of the cyclonic wind curl in the northeastern South China Sea. The eddies originate from northwest of Luzon in summer, move across the northeastern South China Sea to reach the China continental slope in fall, and propagate southwestward along the continental slope in fall-winter, inducing shelfbreak current variations in the western South China Sea in fall-winter. The anti-cyclonic eddy discovered by Li et al. (1998) in the northern South China Sea is found to originate from northwest of Luzon and carry primarily the South China Sea waters. It does not appear to be an eddy shed from the Kuroshio in the Luzon Strait area as alluded by Li et al. (1998) and others.

[ 1] Satellite ocean color, sea surface temperature, and altimeter data are used to study the surface Kuroshio path in the Luzon Strait area. The results suggest that the dominant path of surface Kuroshio intrusion in winter is a direct route from northeast of Luzon to southwest of Taiwan and then westward along the continental slope of northern South China Sea. Anticyclonic intrusions of the Kuroshio in the Luzon Strait area are observed during less than 30% of the time on average and in all four seasons of the year. Winter is the most favorable season for the formation of the anticyclonic intrusions. However, the Kuroshio is observed to deviate from the dominant path during only a little over one third of the wintertime on average. The loop currents of the Kuroshio, which feature prominent inflow-outflow currents in the Luzon Strait during the anticyclonic intrusions, are observed only occasionally, with more episodes in summer than in winter. The observation of more frequent loop currents of the Kuroshio in summer than in winter is a revision to the existing conclusion. These results demonstrate that the anticyclonic intrusion of the Kuroshio is a transient phenomenon rather than a persistent circulation pattern in the Luzon Strait area as suggested by some of the existing numerical model simulations. The growth and decay of the anticyclonic intrusions of the Kuroshio are closely related to the passages and evolution of mesoscale eddies in the Luzon Strait area. Each anticyclonic intrusion event lasts for a few weeks. Its termination sometimes results in a detached anticyclonic eddy propagating to the western basin along the continental slope of the northern South China Sea.

Using recently collected current and hydrographic data, we provide a high resolution picture of the subinertial flow and estimate the volume transport through the Luzon Strait. The distribution of the subinertial flow shows a strong westward flow around 100 m in the northern part of the Luzon Strait, while the eastward flow is confined to the deeper layers, mostly at depths around 1000 m. The total volume transport is estimated to be 6 +/- 3 Sv during the period of observations from October 4 to 16, 2005. The observations also confirm that the Luzon Strait transport has a sandwiched vertical structure. The net westward volume transport in the deep (> 1500 m) layer of the Luzon Strait reaches 2 Sv.

Variation of sea surface temperature (SST) in the region between Australia and Indonesia is of crucial importance to Australian winter rainfall. To gain insight into how the SST is maintained and changed, the present study provides a comprehensive description of ocean dynamics in the region, using results from a seasonal ocean general circulation model run by Semtner and Chervin [1988,1992]. We also investigate the heat budget of the surface layer (0–50 m). Indonesian topography is heavily simplified in this model, but some simple results which probably do not depend strongly on topographic detail are obtained. The currents in the model are verified by comparison to observed annual mean and seasonal thermal structures and circulations determined from the expendable bathythermograph data collected in the region since 1983. We found a distinctive difference in the model circulations and heat budgets in two regions: one between the coast of Indonesia and 13°S (called the northern region) and the other between the northwest coast of Australia and 13°S (called the southern region). Net surface heat fluxes in the model's northern region are low compared to those in most climatologies, possibly because the model does not allow for strong tidal mixing within the Indonesian archipelago. The annual mean vertical velocity in the upper 500 m shows strong upwelling in the northern region and weak downwelling in the southern region. The annual variation in the vertical motion near the surface (