Indonesian Throughflow Transport Research Articles

Variability of Indonesian Throughflow Transport in the Indonesian Seas during Triple-Dip La Niña

Abstract The Indonesian seas serve as an important conduit for the Indonesian Throughflow (ITF), a critical system of warm water and freshwater transport connecting the Pacific and Indian Oceans. El Niño-Southern Oscillation (ENSO) significantly impacts the regional variability in the Pacific and exerts a profound influence on water mass transport from the Pacific to the Indian Oceans. This study focuses on the influence of triple-dip La Niña events during two distinct periods (1998 – 2001 and 2020 – 2023) on the variability of ITF transport in key areas of the Indonesian seas, including the Makassar, Lombok, and Ombai Straits, Flores and Banda Seas, and Timor Passage. Using forecast and reanalysis data from the Copernicus Marine Environment Monitoring Service (CMEMS), transport calculations are conducted across cross-sectional profiles representing the study areas. The results reveal that the volume transport is significantly higher at depths ranging from ∼50 – 300 m. It also shows seasonal fluctuations in volume transport across all regions, with lower transport during winter (DJF) and higher transport during summer (JJA) and transition period (SON) in the Makassar and Lombok Straits. In the Flores and Banda Seas, Ombai Strait, and Timor Passage, the surface volume transport follows seasonal patterns, while at deeper depths (∼50 – 300 m and 300 – 800 m) exhibits distinct fluctuations compared to the surface. Triple-dip La Niña events lead to an overall increase in volume transport during both periods, with the 2019 – 2023 period showing larger values (∼19 Sv) compared to the 1997 – 2001 period (∼14 Sv). The strengthened ocean current during triple-dip La Niña events results in more significant variations in total transport, leading to larger differences between minimum and maximum transport during these periods. The volume transport in Makassar Strait is further analyzed using the Empirical Orthogonal Function (EOF) that identifies three dominant modes (86.4%), revealing the influence of semi-annual, interannual, and decadal factors on the observed variability.

The Role of Precipitation and Salinity Effect in Multidecadal Changes and Long-Term Trend of the Indonesian Throughflow

Abstract Multidecadal variability of the Indonesian Throughflow (ITF) is crucial for the Indo-Pacific and global climate due to significant interbasin exchanges of heat and freshwater. Previous studies suggest that both wind and buoyancy forcing may drive ITF variability, but the role of precipitation and salinity effect in the variability of ITF on multidecadal time scales remains largely unexplored. Here, we investigate the multidecadal changes and long-term trend of the ITF transport during the past six decades, with a focus on the role of precipitation and salinity effect. The diverse datasets consistently indicate a substantial upward trend in the halosteric component of geostrophic transport of ITF in the outflow region at 114°E during the six decades. We find that the meridional differences of the salinity trend in the outflow region explain the increasing trend of the halosteric component of ITF transport. On a larger scale, the tropical western Pacific Ocean and Indonesian seas have experienced significant freshening, which has strengthened the Indo-Pacific pressure gradient and thus enhanced the ITF. In contrast, the equatorial trade wind in the western Pacific Ocean has weakened over recent decades, implying that changes in wind forcing have contributed to weakening the ITF. The combined effect of strengthened halosteric and weakened thermosteric components has resulted in a weak strengthening for the total ITF with large uncertainties. Although both the thermosteric and halosteric components are associated with natural climate modes, our results suggest that the importance of salinity effect is likely increasing given the enhanced water cycle under global warming.

Contributions of Indo‐Pacific Forcings to Interannual Variability of the Indonesian Throughflow in the Upper and Lower Layers

AbstractThe paper uses reanalysis data and model outputs to quantify relative contributions of the El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) to the vertical structure variability of the Indonesian Throughflow (ITF) using multiple regression analysis. In comparing ENSO to IOD events, it becomes evident that ENSO plays a more significant role in ITF upper layer (0–300 m) transport, while the IOD has a greater impact on ITF lower layer (300–760 m) transport. Given the larger anomaly in ITF upper layer transport, ENSO dominates the interannual variability of ITF transport. Moreover, ITF transport is influenced by different physical processes associated with varying phases of ENSO and the IOD. The month‐by‐month contributions reveal that the IOD primarily controls ITF transport during El Niño development years, while ENSO has more influence during El Niño decaying years.

Wind and Heat Forcings of the Seasonal and Interannual Sea Level Variabilities in the Southwest Pacific

Abstract Sea level variabilities in the southwest Pacific contribute to the variations of equatorial current bifurcation and the Indonesian Throughflow transport. These processes are closely related to the recharge/discharge of equatorial heat content and dynamic distribution of anthropogenic ocean heating over the Indo-Pacific basin, thus being of profound significance for climate variability and change. Here we identify the major features of seasonal and interannual sea level variabilities in this region, confirming the dominance of the first baroclinic mode in the tropics (contributing 60%–80% of the variances) and higher baroclinic modes in the extratropics (40%–60% of the seasonal variance). Seasonally, except in the western Coral Sea where the Ekman pumping is significant, the wind-driven first-mode baroclinic Rossby waves originating to the east of the date line control the sea level variations over most tropical Pacific regions. In the domain where the 1.5-layer reduced gravity model becomes deficient, the surface heat fluxes dominate, explaining ∼40%–80% of sea level variance. For interannual variability, ∼40%–60% of the variance are El Niño–Southern Oscillation (ENSO) related. The wind-driven Rossby and Kelvin waves east of the date line explain ∼40%–78% of the interannual variance in the tropical Pacific. Outside the tropics, small-scale diffusive processes are presumed critical for interannual variability according to a thermodynamic analysis using an eddy-permitting ocean model simulation. Further process and predictive understandings can be achieved with the coupled climate models properly parameterizing the subgrid-scale processes.

Freshwater Contributions to Decadal Variability of the Indonesian Throughflow

AbstractThe Makassar Strait, the main passageway of the Indonesian Throughflow (ITF), is an important component of Indo‐Pacific climate through its inter‐basin redistribution of heat and freshwater. Observational studies suggest that wind‐driven freshwater advection from the marginal seas into the Makassar Strait modulates the strait's surface transport. However, direct observations are too short (<15 years) to resolve variability on decadal timescales. Here we use a series of global ocean simulations to assess the advected freshwater contributions to ITF transport across a range of timescales. The simulated seasonal and interannual freshwater dynamics are consistent with previous studies. On decadal timescales, we find that wind‐driven advection of South China Sea (SCS) waters into the Makassar Strait modulates upper‐ocean ITF transport. Atmospheric circulation changes associated with Pacific decadal variability appear to drive this mechanism via Pacific lower‐latitude western boundary current interactions that affect the SCS circulation.

The intrinsic variability of the Indonesian Throughflow

The Indonesian Throughflow (ITF) carries an annual average of about 15 Sv of water from the Pacific through the Indonesian Seas Into the Indian Ocean, and its year-to-year variation ranges from 1 to 4 Sv. A 10-member ensemble of 41-year integrations of a semi-global eddy-resolving oceanic general circulation model is examined to explore the intrinsic (chaotic) variability of the ITF transport and associated flow. It is found that the annual-mean ITF transport is different by about 1 Sv between the ensemble members at several years. The characteristic vertical and horizontal structures of the ensemble anomaly (deviation from the ensemble average) are described. These structures and the basin-scale spread of the anomaly suggest that the intrinsic variability of the ITF is a genuine increase or decrease of the classical ITF rather than variability due to local eddies or nonlinear currents within the Indonesian Seas. The lagged correlation of the intrinsic component of the ITF transport with sea-surface height and barotropic streamfunction suggests that the intrinsic variability may come from zonal jets in the western subtropical North Pacific.

Relative importance of ENSO and IOD on interannual variability of Indonesian Throughflow transport

IntroductionThe Indonesian Throughflow (ITF) connects the Pacific Ocean and the Indian Ocean. It plays an important role in the global ocean circulation system. The interannual variability of ITF transport is largely modulated by climate modes, such as Central-Pacific (CP) and Eastern-Pacific (EP) El Niño and Indian Ocean Dipole (IOD). However, the relative importance of these climate modes importing on the ITF is not well clarified.MethodsDominant roles of the climate modes on ITF in specific periods are quantified by combining a machine learning algorithm of the random forest (RF) model with a variety of reanalysis datasets.ResultsThe results reveal that during the period from 1993 to 2019, the average ITF transport derived from high-resolution reanalysis datasets is -14.97 Sv with an intensification trend of -0.06 Sv year-1, which mainly occurred in the upper layer. Four periods, which are 1993–2000, 2002–2008, 2009–2012 and 2013–2019, are identified as Niño 3.4, Dipole Mode Index (DMI), no significant dominant index, and DMI dominated, respectively.DiscussionThe corresponding sea surface height differences between the Northwest Tropical Pacific Ocean (NWP) and Southeast Indian Ocean (SEI) in these three periods when exist dominant index are -0.50 cm, 0.99 cm and -3.22 cm, respectively, which are responsible for the dominance of the climate modes. The study provides a new insight to quantify the response of ITF transport to climate drivers.

The island rule with multiple islands and its application to the Indonesian Throughflow

The island rule theory in the case of complex geometry with multiple islands referring to the Indo-Pacific Maritime Continent is investigated on the basis of Godfrey’s island rule theory. The bottom friction and lateral friction of multiple channels are considered by employing the Munk and Stommel boundary layer models. Five idealized cases with various spatial distributions of islands are designed to examine the influence of shape and size of the islands. The analytical solutions of the streamfunctions of the through-flows among the islands are obtained and the volume transport through each channel is estimated. The Indonesian Throughflow (ITF) transport is then calculated using the analytical solutions with wind stress and compared with observations and previous theoretical results. The ITF transport from the multiple island rule is about 14.5 Sv during 2004–2006, which is close to the observed ITF transport (about 15.0 Sv) from the International Nusantara Stratification and Transport. We find that the multiple islands rule reproduces well the mean value and interannual variability of the observed ITF transport, and inclusion of wind stress in the North Pacific Ocean may improve the estimate of ITF transport. Sensitivity experiments indicate that frictional boundary layer thickness and channel size influence the estimated ITF transport under the multiple island framework. These results imply that the multiple island rule shows improvements in estimating the ITF transport relative to previous studies, and the multiple island rule can be used to produce long time series of ITF transport and might have implications for paleo-ITF study.

Indonesian Throughflow Slowdown under Global Warming: Remote AMOC Effect versus Regional Surface Forcing

Abstract The Indonesian Throughflow (ITF) is projected to slow down under anthropogenic warming. Several mechanisms—some mutually conflicting—have been proposed but the detailed processes causing this slowdown remain unclear. By turning on/off buoyancy and wind forcings globally and in key regions, this study investigates the dynamical adjustments underlying the centennial ITF slowdown in the global oceans and climate models. Our results show that the projected weakened ITF transport in the top 1500 m is dominated by remote anomalous buoyancy forcing in the North Atlantic Ocean. Specifically, surface freshening and warming over the North Atlantic Ocean slow the Atlantic meridional overturning circulation (AMOC), and the resultant dynamic signals propagate through the coastal-equatorial waveguide into the southeastern Indian Ocean and western Pacific Ocean, causing the reduction of ITF transport over a deep layer. In contrast, the anomalous surface buoyancy flux in the Indo-Pacific affects the ocean temperature and salinity in a shallow upper layer, resulting in ITF changes in forms of high baroclinic mode structure with negligible impacts on the net ITF transport. A vertical partitioning index is proposed to distinguish the remote forcing via the AMOC and regional forcing in the Indo-Pacific Ocean, which could be useful for monitoring, attributing, and predicting the changing ITF transport under global warming.

Rapid 21st Century Weakening of the Agulhas Current in a Warming Climate

AbstractThe Agulhas Current (AC) is a critical component of the global ocean circulation. Climate models consistently project the AC to decline in the 21st century. Previous studies typically attributed the weakening of AC to changes in Indian Ocean wind and/or a decline of the Indonesian Throughflow (ITF) transport. However, our analysis suggests that changes in local surface wind and ITF can only explain a portion of the model‐simulated AC changes. Using a hierarchy of models, we show that the decline of the Atlantic Meridional Overturning Circulation (AMOC) strength in a warming climate could also contribute substantially to the AC weakening. Following a weakening of the AMOC, the baroclinic structure of the AC is also modified such that the AC is shoaled to a shallower depth, communicated between basins via Kelvin waves. Our results highlight the importance of remote processes in future western boundary current changes.

Using a deep-learning approach to infer and forecast the Indonesian Throughflow transport from sea surface height

The Indonesian Throughflow (ITF) connects the tropical Pacific and Indian Oceans and is critical to the regional and global climate systems. Previous research indicates that the Indo-Pacific pressure gradient is a major driver of the ITF, implying the possibility of forecasting ITF transport by the sea surface height (SSH) of the Indo-Pacific Ocean. Here we used a deep-learning approach with the convolutional neural network (CNN) model to reproduce ITF transport. The CNN model was trained with a random selection of the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations and verified with residual components of the CMIP6 simulations. A test of the training results showed that the CNN model with SSH is able to reproduce approximately 90% of the total variance of ITF transport. The CNN model with CMIP6 was then transformed to the Simple Ocean Data Assimilation (SODA) dataset and this transformed model reproduced approximately 80% of the total variance of ITF transport in the SODA. A time series of ITF transport, verified by Monitoring the ITF (MITF) and International Nusantara Stratification and Transport (INSTANT) measurements of ITF, was then produced by the model using satellite observations from 1993 to 2021. We discovered that the CNN model can make a valid prediction with a lead time of 7 months, implying that the ITF transport can be predicted using the deep-learning approach with SSH data.

The ocean response to El Nino Southern Oscillation (ENSO) in the Bali Sea: a model study

The Bali Sea (BS) is connected to the Java Sea and Makassar Strait in the north and is one of the main outflow straits of the Indonesian Throughflow (ITF) in Lombok Strait (LS). Local ocean dynamics and their variability may be controlled significantly by the ITF. This study aims to investigate the interannual variation of circulation, mixed layer depth, and isotherm of 20°C, by performing a simulation of fine- a resolution 1/84° ocean circulation model of CROCO between 2010-2020 with a focus on El Nino 2015 (EN2015) and La Nina 2020 (LN2020) events. The model validation reproduces well the satellite-derived SST and SSH dataset with a correlation coefficient of >0.9. The model demonstrated an anti-clockwise circulation of the ITF in the BS, entering via the northern and exiting into the LS via the southern area. This circulation changes drastically during the ENSO event, where the ITF transport inflow is weaker and shallower during EN2015, compared to a much stronger and deeper ITF inflow during LN2020. Accordingly, shallower mixed layer depth and depth of isotherm of 20°C are found during EN2015, and vice versa. Coherence of these parameters with NINO3.4 is very significant (>-0.89) with a phase lag of 1.7 months.

Decadal variability of the interannual climate predictability associated with the Indo-Pacific oceanic channel dynamics in CCSM4

The lag correlations between the observed sea surface temperature anomalies (SSTA) in the southeastern tropical Indian Ocean in fall and those in the Pacific cold tongue at the one-year time lag are calculated in running windows of 7–11 years and are shown to have decadal variability. Similar decadal variability has also been identified in the historical simulations of the Community Climate System Model version 4 (CCSM4) that participates in the Coupled Model Intercomparison Project phase-5 (CMIP5). During the positive phases of the decadal variability, the significant lag correlations are diagnosed to be dominated by the oceanic channel dynamics, i.e., upwelling Kelvin waves associated with the Indian Ocean Dipole (IOD) force enhanced Indonesian Throughflow transport anomalies and western Pacific thermocline anomalies to propagate eastward, resulting in SSTA in the central-eastern equatorial Pacific Ocean. During the negative phases of the decadal variability, the subsurface lag correlations suggest that the western Pacific Ocean are still dominated by the oceanic channel dynamics, but do not correlate well with the SSTA in the cold tongue due to a deeper thermocline in the eastern equatorial Pacific. The IOD-ENSO teleconnection is not affected significantly by the anthropogenic forcing during either the positive or the negative decadal phases. The CCSM4 model is found to underestimate Indonesian Throughflow transport variability but overestimate the westerly wind anomalies in the western-central equatorial Pacific, which forces unrealistic anomalies in the equatorial Pacific Ocean associated with the IOD.

Projected future changes in the contribution of Indo-Pacific sea surface height variability to the Indonesian throughflow

The Indonesian throughflow (ITF) transports a significant amount of warm freshwater from the Pacific to the Indian Ocean, making it critical to the global climate system. This study examines decadal ITF variations using ocean reanalysis data as well as climate model simulations from the Coupled Model Inter-comparison Project Phase 5 (CMIP5). While the observed annual cycle of ITF transport is known to be correlated with the annual cycle of sea surface height (SSH) difference between the Pacific and Indian Oceans, ocean reanalysis data (1959–2015) show that the Pacific Ocean SSH variability controls more than 85% of ITF variation on decadal timescales. In contrast, the Indian Ocean SSH variability contributes less than 15%. While those observed contributions are mostly reproduced in the CMIP5 historical simulations, an analysis of future climate projections shows a 25–30% increase in the Indian Ocean SSH variability to decadal ITF variations and a corresponding decrease in the Pacific contribution. These projected changes in the Indian Ocean SSH variability are associated with a 23% increase in the amplitudes of negative zonal wind stress anomalies over the equatorial Indian Ocean, along with a 12º eastward shift in the center of action in these anomalies. This combined effect of the increased amplitude and eastward shift in the zonal wind stress increases the SSHA variance over the Indian Ocean, increasing its contribution to the ITF variation. The decadal ITF changes discussed in this study will be crucial in understanding the future global climate variability, strongly coupled to Indo-Pacific interactions.

VARIATIONS OF INDONESIAN TRHOUGHFLOW TRANSPORT IN MALUKU AND HALMAHERA SEA RELATED TO THE OCCURRENCE OF EL NINO SOUTHERN OSCILLATION

ABSTRACTMaluku and Halmahera Sea are the entry paths for the Indonesian Throughflow (ITF). This study examined the variation of their current speed, current direction, and transport during the ENSO phases on La Nina in 2010-2011 and El Nino in 2015-2016. The data used were the Marine Copernicus reanalysis data of u and v components of current velocity at 155 m and 266 m of depth. The data were processed by using GrADS to see the direction and speed of the current as well as by calculating the ITF transport. During La Nina, both at 155 m and 266 m of depths were found the eddy patterns. When El Nino occurred, the direction of currents in Maluku Sea was different from La Nina conditions, while the direction of currents in Halmahera Sea had no differences. Current velocity in both oceans at each depth was lower during El Nino than La Nina. The condition of ITF transport in the Maluku Sea during La Nina was dominated by southward movement with a smaller value than during El Nino which generally moved northward meanwhile in the Halmahera Sea had the opposite condition.Keywords: current speed, current direction, ENSO, ITF, transport

Intraseasonal variability of the Indonesian throughflow associated with the Madden-Julian Oscillation

Intraseasonal variability (20–90 days) of the Indonesian Throughflow (ITF), which is primarily forced by the Madden-Julian Oscillation (MJO), is investigated using a high-resolution global ocean reanalysis and satellite altimeter data. Previous studies show that during the MJO active phase, downwelling Kelvin waves generated in the central equatorial Indian Ocean propagate along the coast of Sumatra and Java islands, affecting the ITF transport at exit passages in the Indonesian Seas and Makassar Strait. However, the intraseasonal variation of the ITF transport through these straits over the MJO life cycle, especially during the suppressed phase, has not been quantified. To quantify the ITF transport associated with the MJO, composites of ITF transport through major straits in the Indonesian Seas are constructed using a 0.08° global HYbrid Coordinate Ocean Model (HYCOM) reanalysis. A prominent reduction of ITF transport through major straits is found during the MJO active phase, and a transport enhancement comparable to the reduction is evident during the suppressed phase. As a result, the net effect of the MJO on the mean ITF transport is very small due to the cancellation of the enhancement and reduction. The magnitude of the MJO-associated ITF transport variation through all ITF exit passages is about 6 Sv, which is about 50% of the total transport. While the propagation of coastal Kelvin waves during the MJO active phase and their impact on ITF transport is clearly evident in the composite, such Kelvin wave influence on ITF transport is not clearly detected during the suppressed phase. This suggests that anomalous winds over the Maritime Continent (MC) area are mostly responsible for the ITF variation during the suppressed phase in many MJO events. Yet, during some MJO events, remotely-forced upwelling Kelvin waves and their significant impact on the ITF transport are evident during the MJO suppressed phase.

Observed Water Exchange Between the South China Sea and Java Sea Through Karimata Strait

AbstractVolume, heat and freshwater transports from the South China Sea (SCS) to the Java Sea through the Karimata Strait are estimated based on direct measurements of current, temperature, salinity, and satellite observations. Subject to strong seasonal variability, the volume, heat, freshwater transports are −1.98 ± 0.23 Sv (1 Sv = 106 m3/s), −209.68 ± 15.19 TW (1 TW = 1012 W), −99.87 ± 15.11 mSv (1 mSv = 10‐3 Sv) in boreal winter, and 0.47 ± 0.20 Sv, 61.06 ± 15.46 TW, 31.67 ± 8.76 mSv in boreal summer, respectively (negative transport defined from SCS to Java Sea). The annual mean volume, heat and freshwater transports are −0.78 ± 0.12 Sv, −77.31 ± 4.99 TW, and −30.87 ± 6.15 mSv, respectively. The freshwater transport through Karimata Strait is as large as 42% of rainfall input to the entire Indonesian seas in boreal winter, thereby modulating the Indonesian Throughflow (ITF) through the “freshwater plug” effect in the Makassar Strait. The interannual variability of the transports is not significantly correlated to the Indian Ocean Dipole and El Niño‐Southern Oscillation. There is a significant decreasing trend of volume and heat transports from the SCS to Java Sea over the period of 1997–2015, with a reduction of 0.26 Sv and 15 TW, respectively. The freshwater transport decreases 12.29 mSv by 2016 compared with that in 2009, equivalent to 33.2%–49.72% of the annual mean freshwater transport during 2009–2016. This decreasing trend would play contradictory roles in the decadal trend of ITF transport by building a “freshwater plug” in the southern Makassar Strait and a ‘buoyant plume’ in the Indo‐Australian Basin, respectively.

Multi-year marine cold-spells off the west coast of Australia and effects on fisheries

Ocean temperature variability off the west coast of Australia is affected by the El Niño Southern Oscillation from the Pacific, the Indian Ocean Dipole, the Southern Annular Mode, and the regional air-sea coupling mode – the Ningaloo Niño. The modes of climate variability affect the strength of the Leeuwin Current, a warm, poleward-flowing eastern boundary current, and air-sea heat exchanges off the coast, impacting on the upper ocean heat balance and sea surface temperature (SST) variability. After the unprecedented 2010–2011 Ningaloo Niño (extreme marine heatwave) event, the region experienced more marine heatwaves in the following two austral summers, before SST off the coast eventually switched to a multi-year cold phase. This marine cold-spell (MCS) commenced with the 2015–16 El Niño and continued to 2019, when the Niño-4 W region SST was warmed up and Leeuwin Current was weakened. The upper ocean heat content in the southeast Indian Ocean was lowered (thermocline being elevated), due to the reduction of the Indonesian Throughflow transport, and alongshore northward winds strengthened during this period, which allowed thermocline waters to be more efficiently entrained into the surface layer to cool the SST. Thus, several factors might have contributed in the multi-year MCSs. The MCSs was associated with increased primary production in the Leeuwin Current and helped the recovery of benthic species of local fisheries such as Roe's abalone, saucer scallops and Shark Bay's blue swimmer crabs that were significantly affected by the damaging heat stress during the 2011–2013 marine heatwave period. • Marine cold-spells (MCSs) occurred during 2015–2019 off the west coast of Australia • MCSs commenced with 2015–16 El Niño and were tightly related to Niño-4 W SST • MCSs were associated with weaker Leeuwin Current, shallow thermocline depth • MCSs were associated with stronger northward winds and increased primary production • MCCs helped recovery of invertebrate fisheries affected by the 2011 marine heatwave

Contributions of Indonesian Throughflow to eastern Indian Ocean surface variability during ENSO events

AbstractThe contribution of ocean transport to the transition between the Indian Ocean Dipole mode (IOD) and the Indian Ocean Basin mode (IOB) during El Niño–Southern Oscillation (ENSO) years is investigated using reanalysis products. Composite analysis suggests that the IOD–IOB transition is robust among ENSO years, with variations in the eastern Indian Ocean mixed layer playing a key role. Although this transition has typically been attributed to changes in surface heat flux in the eastern Indian Ocean, our results suggest that approximately one third of the mixed layer temperature tendency during the transition results from anomalous ocean heat transport. Enhanced surface heat flux anomalies and oceanic advection starting from October during El Niño developing years combine to warm the eastern Indian Ocean, promoting the decay of the positive IOD cold tongue and the emergence of a positive IOB pattern. The contribution of ocean advection to the IOD—IOB transition is dominated by the Indonesian Throughflow (ITF), which accounts for around 70–80% of total ocean advection contribution in the southeastern Indian Ocean (20–25% of the total warming tendency). These variations in ITF heat transport arise in large part from local wind anomalies. Southeasterly wind anomalies along the northeastern edge of an anomalous Indian Ocean anticyclone associated with El Niño intensify surface‐layer heat transport into the Indian Ocean in the ITF outflow region. This change in surface transport contrasts with total ITF transport anomalies during El Niño years, as total transport is dominated by negative subsurface transport anomalies.

Projected Future Changes of Meridional Heat Transport and Heat Balance of the Indian Ocean

AbstractAn ocean downscaling model product, forced under the Representative Concentration Pathway 8.5 future climate change scenario, has been used to understand the ocean heat balance of the Indian Ocean in a warming climate. Toward 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 32°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 will be faster than the direct air‐sea heat flux input.