In the boreal winter of the Northern Hemisphere, a weakening of the surface Indonesian throughflow (ITF) is commonly observed. The intraseasonal mechanism of the weakening, namely, the impact of the atmospheric Madden–Julian Oscillation (MJO), is well-known and has been extensively studied. However, a significantly low volume transport of ITF (<100 m in depth) was also observed in the Makassar Strait during the traverse of tropical cyclones (TCs). The observed transport decrease is 0.31 Sv (1 Sv = 106 m3/s) on average, which is ~70% of the estimated influence of the MJO. The time scale of the incurred variation is up to 30 days, comparable to the time of 20–90 days caused by the MJO. The winds in the TC circulation have a major impact on the Makassar Strait’s ITF transport reduction. Numerical experiments reveal that the reduction is due to the along-strait sea level anomaly (SLA) variability that is forced by the winds from the upstream region. The mechanism involves the propagation of coastal Kelvin waves along the Sulawesi Sea generated by the TCs and is confirmed by theoretical analysis. Based on the numerical experiments, this mechanism contributes ~40% to the total ITF transport reduction, while the large-scale guiding circulation surrounding the TCs may contribute to the remaining ITF transport reduction. These results support that TCs are also important forcing components in the intraseasonal variation in surface ITF.

Deep-diving surveys enable observations of new features of behavioural and functional dimensions of the fish that are typically missing from specimen-based taxonomy. This study documents three rare armoured searobins (Peristediidae: Scalicus engyceros, Paraheminodus murrayi, Peristedion liorhynchus) in Xianbei Seamount, Zhongsha Islands and Shenhu Canyon in the South China Sea. Not seen in the dead specimen, the fish in life exhibits bizarre shrimp-like (in common sense) morphology: the lip barbels formed a laterally rake-like structure, and the pectoral fins functioned as legs that literally 'walked' on the sediment. The fish S. engyceros showed both sideways and backward walking. The paired pectoral fins extrude outwards laterally like flat plates, supporting body balance during walking and swimming. When disturbed, individuals escaped rapidly using both pectoral appendages and tail fins, combining shrimp-like jerky movements with typical fish swimming. The same species have been reported to occur in both the North and South Pacific oceans, suggesting that their oceanic distribution is by larval dispersal via oceanic currents such as equatorial and Indonesian throughflow currents. The co-occurrence of three congeners in the same sea may hint local geographic speciation at work after settlement in a seamount habitat.

For most wind-driven ocean currents, velocity is typically surface-intensified with a peak at the surface. However, in a notable exception, the Makassar Strait throughflow (MST) and the Mindanao Current (MC) two dynamically linked boundary currents exhibit their maximum velocities in the subsurface (upper thermocline) layer. In this study, the driving mechanisms of subsurface velocity maxima are investigated, using mooring observations, ocean reanalysis products, and regional model simulations, with particular emphasis on the role of wind forcing. Both the in situ measurements and reanalysis data show that the subsurface velocity maxima in the MST and MC in boreal summer are stronger and shallower than in winter. We find that the higher-order baroclinic modes (modes 3 and 4) are crucial in shaping vertical structure of velocity and forming the subsurface velocity maximum. The meridional dynamic height difference along MC is identified as a driver of the subsurface velocity maximum, through balancing with frictional forces. The zonal density gradient, being negative at the surface but positive at depth, is consistent with the subsurface velocity maximum. Numerical experiments with a regional model indicate that the sea surface winds play an important role in driving the seasonality of subsurface velocity maxima of the MC and MST. Wind stress causes downwelling (upwelling) over the MC and deepens (lifts) zero contour of horizontal density gradient and leads to a deeper (shallower) subsurface velocity maximum. Wind forcing can also influence the subsurface velocity maximum of MST by modulating the seasonal freshwater transport from the South China Sea.

The Indo-Pacific Maritime Continent (MC) is the site of intense atmospheric convection, associated with the Walker Circulation, which significantly influences global climate. Observations reveal decreasing sea surface salinity (SSS) in the MC region since 1960, which was a consequence of enhanced freshwater flux. The observed sea surface temperature (SST) increasing trends of the tropical Pacific in the past decades exhibit La Niña-like patterns, with enhanced western tropical Pacific warming and eastern cooling, which diverge from many climate model results. This study explores how MC freshening influences the La Niña-like pattern under global warming. Sensitivity experiments demonstrate that imposed MC freshening enhances near-surface stratification, shoaling the mixed layer and amplifying western Pacific warming. The intensified zonal SST gradient strengthens the Walker circulation, enhancing easterly winds and equatorial upwelling, thereby cooling the eastern Pacific. CMIP6 simulations analysis corroborates that salinity-modulated stratification significantly influences Pacific SST patterns, underscoring the need for improved observations and model performance of the MC region water cycle, as well as associated upper-ocean thermohaline stratification patterns.

The Makassar Strait throughflow (MST), the major component of the Indonesian Throughflow, exhibited an unprecedented strong northward anomaly in the upper 150 m from January to February 2017. The anomalous northward MST was associated with two Madden-Julian oscillation (MJO) events originating in the Indian Ocean. The first MJO successfully traversed the Maritime Continent (MC) to the Pacific Ocean, while the second MJO was impeded. The main driver for the anomalous MST was Kelvin waves propagating from the Indian Ocean, with local wind stress and precipitation associated with the MJO over the MC having minor effects. During the first MJO event, coastal downwelling Kelvin waves affected sea level anomalies in the southern Makassar Strait, resulting in a northward pressure gradient and transport in the MST's upper layer. In contrast, the second blocked MJO event was characterized by the propagation of an upwelling Kelvin wave, which decreased the mixed layer thickness and subsurface layer temperature in the southern Indonesian Seas. The results highlight the importance of MJO remote influences on the Indonesian Throughflow via Kelvin wave propagation. SIGNIFICANCE STATEMENT: The Indo-Pacific Maritime Continent (MC) plays a crucial role in the global climate system. As the primary current system of the MC region, the Indonesian Throughflow (ITF), mainly through the Makassar Strait, transfers mass and heat from the tropical Pacific into the Indian Ocean across a wide range of time scales. At the intraseasonal time scale, the MC interacts with the Madden-Julian oscillation (MJO) propagating from the Indian Ocean into the Pacific Ocean. In this study, we investigate the local and remote impacts of the MJO on the ITF, focusing on the anomalous northward transport event in the Makassar Strait in early 2017. We highlight the remote influences of the MJO on the ITF mediated by Kelvin wave propagation. Our findings contribute to a broader understanding of the ocean circulation response to MJO propagation over the MC, thereby enhancing our knowledge of the intricate interplay between atmospheric and oceanic processes in this critical region.

The tropical Indian Ocean (IO) exhibits persistent biases in sea surface temperature (SST) climatology across Coupled Model Intercomparison Project Phase 6 (CMIP6) models, particularly during boreal winter. These biases manifest as a meridional dipole with warm SSTs in northern tropics and cool SSTs in south. Surface wind induced by interhemispheric SST gradient could intensify southward Ekman transport and reshape the shallow meridional overturning circulation (SMOC). Over the10 degrees S-20 degrees S region, the downward motion of SMOC leads to anomalous subsurface warming, while enhanced evaporation drives surface cooling-weakening vertical thermal structure. Over the equator-10 degrees S, despite favorable upwelling conditions, subsurface remains warm. We attribute this to an overly strong simulation of Indonesian Throughflow, which suppresses the cooling effect of SMOC upwelling. Our results highlight the crucial role of SMOC in connecting the SST bias and subsurface thermal state in climate models and emphasize the need to constrain tropical IO air-sea interactions for improving SST simulations.

The Indonesian Throughflow (ITF), as the only oceanic connection between the tropical Pacific and Indian Oceans, significantly influences the ocean heat distribution and Indo-Pacific climate system. Using the Connectivity Modeling System particle tracking method and GLORYS reanalysis data, we find that the spreading of ITF water after entering the eastern Indian Ocean is strongly altered by extreme Indian Ocean Dipole (IOD) events. The ITF water spreads more southward during extreme negative IOD (nIOD) events and northward with increased westward transport during extreme positive IOD (pIOD) events. The meridional displacement is approximately 0.5 degrees of latitude relative to the climatological mean. These variations are primarily driven by direct wind stress forcing and sea surface height (SSH) gradients induced by the wind stress curl and Ekman pumping. Compared with moderate IOD events, the impact of extreme IOD events on ITF spreading is more pronounced since extreme IOD events are typically characterized by stronger anomalies in the eastern Indian Ocean. The IOD induced ITF shifts significantly alter heat transport in the southeastern tropical Indian, and also modulate the Leeuwin Current (LC), enhancing it during nIOD and weakening it during pIOD events. Furthermore, the changes in the ITF significantly modulate the LC heat transport and play an important role in regulating the occurrence of Ningaloo Ni & ntilde;o events along the western coast of Australia.

The South China Sea throughflow (SCSTF) serves as a heat and freshwater conveyor, playing a pivotal role in regulating physical processes and biogeochemical cycles in the SCS. Because of limited observations, the long-term changes of the SCSTF under a warming climate and underlying dynamics remain unclear. In this study, we reconstructed centennial-scale variability of the SCSTF (1894-2022) using coral oxygen isotope (δ18O) and satellite observations. Besides prominent interannual and decadal variabilities, there exists a marked long-term decreasing trend at -0.14 ± 0.02 Sv (1 Sv = 106 m3 s-1) per decade for the total SCSTF volume transport. Consequently, the SCSTF transport decreased by 35 ± 5% over the past 129 years in comparison with the mean value of 5.15 Sv. Using the Time-dependent Island Rule theory, we further reveal that the intensified trade wind in the tropical western Pacific drives the SCSTF slowdown. The findings substantially contribute to investigating the physical, ecological, and biogeochemical changes in the SCS and the Indo-Pacific Oceans.

The Indonesian Throughflow (ITF), mainly through the Makassar Strait, transports amounts of water and salt from the tropical Pacific Ocean to the Indian Ocean, playing a crucial role in modulating heat and energy budget between two oceans. The South China Sea Throughflow (SCSTF) significantly contributes to the net transport of the ITF via Karimata Strait and Mindoro-Sibutu Passage. However, the specific proportion and variability of South China Sea (SCS) water joining the ITF are still unclear. Based on high-resolution reanalysis data and a Lagrangian particle tracking method-Connectivity Modelling System (CMS), we quantified the proportion and variability of SCS water joining the ITF in the Makassar Strait. The results show that about 16.41

The Pacific shallow meridional overturning circulations, known as Subtropical-tropical Cells (STCs), serve as oceanic channels that link the tropical and subtropical regions. Using Argo observations, here we find distinct seasonal variations in the subsurface branches of the Pacific STCs. The STCs pycnocline transports at each latitude exhibit remarkable seasonal variation, especially the interior transport. The seasonal variation differs between the near-equatorial (5–8°) and far-equatorial regions (9–15°), exhibiting an equatorward enhancement. In the near-equatorial region, the Northern STC pycnocline transports show a strong transport in spring and weak transport in summer, while in the South Pacific, there are two marked peaks in both spring and autumn, with a weakening in summer. In both hemispheres, wind stress curl perturbations in the far-equatorial regions directly drive seasonal subsurface transport perturbations of the STCs. In the Northern Hemisphere, Ekman transport plays a more important role, while equatorial upwelling is the primary mechanism in the Southern STC. The spring peak of the pycnocline transports in the South Pacific is primarily driven by equatorial upwelling, while the autumn peak results from the combined influence of upwelling and Ekman transport. Additionally, the westward propagation of Rossby waves, induced by local wind stress curl, significantly impacts sea surface height, subsequently affecting the subsurface meridional transport of the STCs. The Rossby wave signal is more pronounced in the near-equatorial region of the Northern Hemisphere, having a greater impact on the Northern STC.

The Luzon Strait Transport (LST) plays an important role in the heat and salt budgets, circulation variations, eddy generation, and biogeochemical cycles in the South China Sea (SCS). Due to lack of long-term observations, the LST interannual variations and the underlying dynamics remain ambiguous. Using satellite altimeter data, the longterm upper LST (upper 500 m) over the past 30 years (1993-2022) was first estimated in this study. The mean value of the LST is estimated at 4.18 Sv (1 Sv ; 106 m3 s21) with a standard deviation of 1.03 Sv at the interannual time scale. The LST interannual variations are not correlated with the upstream Kuroshio changes against the well-known "teapot effect" adopted by most previous studies but well correlated with the migration of the North Equatorial Current bifurcation (Yb) and wind stress curl anomaly (WSCa) east of the Philippines. It is found that positive/negative WSCa in the off-equatorial region associated with westerly/easterly wind anomalies in the western tropical Pacific could induce westward upwelling/ downwelling Rossby waves, corresponding to negative/positive sea level anomalies (SLAs). When approaching the eastern coast of the Philippines, negative/positive SLAs were transferred along the Philippines coast as coastal Kelvin waves through the Mindoro-Sibutu pathway. Thus, cyclonic/anticyclonic circulation anomaly formed to the east and around the Philippines, resulting in strong/weak LST. Using the time-dependent island rule theory, the LST interannual variations driven by large-scale and regional wind were quantified. This dynamic framework can interpret approximately two-thirds of the LST interannual variations, providing new insights into the dynamics of low-frequency variations of the Kuroshio intrusion into the SCS. SIGNIFICANCE STATEMENT: The strong Kuroshio intrudes into the South China Sea (SCS) through the Luzon Strait. The LST plays an important role in the physical oceanography and biogeochemical cycles in the SCS. Due to lack of long-term observations, dynamics on the interannual variations of the LST remain ambiguous. In this study, we first calculated long-term LST over the recent three decades. Using the time-dependent island rule theory, interannual variations of the LST were quantified by linking large-scale wind and oceanic waves (Rossby and Kelvin waves). The detailed dynamical processes controlling the LST interannual variations are illustrated. This dynamic framework can interpret most of the LST interannual variations, providing new insights on the dynamics of the western boundary current leaping across a gap.

The Mindanao eddy (ME) and Halmahera eddy (HE) are major mesoscale systems in the Indonesian Throughflow (ITF) source region, yet their interactive influence on ITF transport remains unclear. Using GLORYS12V1 reanalysis data (1993-2020) from the Copernicus Marine Environment Monitoring Service and a Self-Organizing Map (SOM) approach, this study classifies the ME-HE coevolution into four typical flow patterns and explores their seasonal and interannual impacts on ITF transport. Results show that the cooperative pattern of ME's eastward expansion with HE's southeast retreat (Pattern 1) minimizes Makassar Strait transport by weakening the Mindanao Current (MC) intrusion into Sulawesi Sea. Conversely, Patterns 2-4 enhance ITF transport through different mechanisms: Pattern 2 involves a weakened ME and northward-shifted HE; Pattern 3 features a strong northwestward HE that compresses the MC and yields the strongest transport; Pattern 4 represents a transitional configuration with enhanced HE and moderate ME strength. Seasonal patterns shifts are mainly controlled by local wind stress and Ekman pumping. Strong northeast monsoons in boreal winter favor Pattern 1 and weak ITF, while boreal summer southwest winds support Patterns 2-4 and strong ITF. On interannual timescale, ENSO-driven wind anomalies and westward-propagating Rossby waves jointly modulate eddy evolution through sea surface height anomalies. El Ni & ntilde;o event promotes Pattern 1, while La Ni & ntilde;a event favors Pattern 3. Patterns 2 and 4 serve as transitional states due to the asynchronous responses of the ME and HE to ENSO forcing. Additionally, the confluence of the South China Sea Throughflow and MC further modulates ITF transport.

AbstractThe interannual variability of the Indonesian Throughflow (ITF) in the Makassar Strait in 2015–2017 is analyzed using observations and ORAS5 reanalysis data. Strong northward and ensuing strong southward current anomalies in the sub‐thermocline Makassar Strait are identified during and after the 2015/2016 El Niño, respectively. However, the upper layer current anomalies are weakly southward in September–October 2015 and strongly northward in April 2016 through April 2017. These anomalies of the Makassar Strait throughflow in the upper layer are found to be induced mainly by Indian Ocean Kelvin waves forcing sea surface height anomalies off the south Java coasts. The sub‐thermocline current anomalies are found to be generated by the westward and downward propagation of interannual Rossby waves from the central equatorial Pacific during and after the 2015/2016 El Niño. The results underline the importance of the planetary wave propagation into the Indonesian seas in forcing the ITF anomalies.

The Indonesian Throughflow (ITF) serves as the only tropical branch of the Great Ocean Conveyer Belt, and plays an important role in Indo-Pacific Basin interactions. By calculating the heat budget during single and triple La Ni & ntilde;a events (1973-1976, 1998-2000, 2020-2023), we quantified the contribution of enhanced ITF heat transport to prolonging the recharge process during triple La Ni & ntilde;a events. During triple La Ni & ntilde;a events, the accumulated heat transported by the ITF is about -15.01 ZJ (more than twice that for single events), nearly offsetting the positive contribution of net heat flux from the atmosphere over the Pacific Ocean. Single La Ni & ntilde;a events are confined to thermal processes within the Pacific Ocean, while triple events are products of basin interactions, and the ITF serves as a crucial oceanic link between the tropical Indian and Pacific Oceans. The enhanced heat transport of the ITF is concentrated in the subsurface layer (100-200 m), significantly during the second year of a triple La Ni & ntilde;a event. The accumulation of enhanced ITF heat transport in the subsurface layer effectively expands the recharge-discharge area from the western Pacific to the eastern Indian Ocean, with the ensuing discharge process of the tropical Indian Ocean along with sustained La Ni & ntilde;a conditions in the Pacific Ocean.

Mooring observations in the Halmahera Sea during September 2018 through October 2020 show a mean transport of 0.37-0.76 Sv through the Gebe Strait into the Indonesian seas. This estimate, together with mooring measurements in the Makassar Strait, the Maluku Channel, and the Jailolo Strait of the Halmahera Sea, suggests the total mean transport of the Indonesian Throughflow (ITF) to be 16.8 Sv +/- 1.0 Sv (1 Sv = 106 m3 s-1), larger than the 15 Sv mean transport previously measured at the exit straits. Mooring data in the Maluku and Halmahera Seas, combined with Lagrangian tracking of the Makassar Strait throughflow using reanalysis velocity, show that 40% of the total ITF transport comes directly from the South Pacific Ocean. The above findings have revised the traditional concept that the ITF comes mainly from the North Pacific.One Sentence Summary The direct South Pacific to Indian Ocean transfer into the Great Ocean Conveyor is revealed.

In general, tropical cyclones (TCs) will inject energy into oceanic inertial motion‒a prevalent phenomenon in the ocean. Under global warming, the intensity of TCs is on the rise, while their frequency has exhibited a decline since 2000. However, the long-term trend of this energy infusion is an underexplored problem in this context. Using a damped-slab model, we computed the wind work exerted by TCs on the ocean’s mixed-layer inertial motions. Our results show that the global wind work has increased by approximately 50% from 1979 to 2023. The wind work increase of strong TCs (Saffir–Simpson levels 4–5) is the major contributor to the increasing trend of global wind work, primarily due to their increasing frequency and substantial wind stress. At basin scale, the wind work input of the North Atlantic TCs has increased by 2 times, owing to an increase in both their intensity and frequency. Specifically, in the South Indian and the eastern North Pacific basins, the rise in wind work is primarily attributed to the enhanced wind energy of TCs within the inertial bands.

Forecasting the Indian Ocean Dipole (IOD) is crucial because of its significant impact on regional and global climates. While traditional dynamic and empirical models suffer from systematic errors due to nonlinear processes, convolutional neural networks (CNN) are nonlinear in nature and have demonstrated remarkable El Niño Southern Oscillation (ENSO) and IOD forecasting skills based on oceanic predictors, particularly sea surface temperature and heat content. However, it is difficult to measure heat content and easily introduces uncertainties, prompting the need to explore atmospheric predictors for IOD forecasts. Based on sensitivity prediction experiments, we identified the sea level pressure (SLP) signal as a crucial predictor, which forecasts IOD at a 7-month lead. In addition, the CNN model improves monthly forecasting accuracy while reducing errors by 13.43%. Utilizing the heatmap analysis, we elucidated that the multi-seasonal predictability of the IOD primarily originates from mid-latitude climate variability. Besides ENSO signals in the Pacific Ocean, our study highlights the significant impact of remote climate forcing in the South Indian Ocean, tropical North Indian Ocean, and Northwest Pacific Ocean on IOD forecasts. By introducing the SLP precursor and extratropical zones into IOD forecasts, our study offers fresh insights into the underlying dynamics of IOD evolution.

Understanding the impacts of the Indonesian Throughflow (ITF) on the eastward propagation of the Madden-Julian Oscillation (MJO) is crucial for accurately simulating the MJO and achieving high-skill sub-seasonal predictions. Our analyses demonstrate a significant enhancement of MJO eastward propagation due to the strong ITF. Blocking the ITF decreases the eastward sea surface temperature (SST) gradient over the tropical Indian Ocean, hindering MJO propagation across the Maritime Continent (MC). Removing the MJO circulation-induced intraseasonal variability of the ITF transport also weakens the eastward propagation of the MJO, as the MJO easterly winds enhance the ITF transport and warm the eastern tropical Indian Ocean. These experiments reveal that mean and intraseasonal variability of the ITF transport contribute to 73% and 42% of the eastward propagation of the MJO over the MC, respectively. The findings presented in this study highlight the significant role of the ITF in shaping the propagation of the MJO.

The Makassar Strait throughflow (MST) is the major component of the Indonesian Throughflow (ITF), transferring Pacific water into the Indian Ocean. In our previous study, we identified a new zonal pathway, a. k.a. the North Equatorial Subsurface Current (NESC), which carried equatorial water into the MST sub-thermocline (>300 m) in the summer 2016 following the 2015/16 El Nino. We now show continued strong southward MST in the sub-thermocline during the winter of 2016-2017, with salinity higher than that in the summer 2016, due to direct South Pacific water intrusion into the Sulawesi Sea. The origin of the intrusion is identified from the New Guinea Coastal Undercurrent (NGCUC) and from an anomalous westward flow along 3 degrees N in the western equatorial Pacific. The identified interannual variability of the western Pacific Ocean circulation is particularly strong in the winter following super El Nino events.

The rare triple-dip 2020–2023 La Niña event has resulted in a series of extreme climate events across the globe. Here, we reveal the role of tropical Indo-Pacific oceanic interactions in driving the first triple-dip La Niña of the twenty-first century. Specifically, we found that the eastern Indian Ocean subsurface warming anomalies were associated with the re-intensification of the subsequent La Niña event. The subsurface warming anomaly signals were propagated eastward by equatorial and coastal subsurface Kelvin waves from the eastern Indian Ocean to the western Pacific Ocean through the Indo-Pacific oceanic pathway, which contributes to the accumulation of heat content and deepens the thermocline in the western tropical Pacific. The westward Indonesian Throughflow (ITF) transported more heat during multi-year La Niña events from the western Pacific Ocean to the eastern Indian Ocean than during single-year events, resulting in the injection of more warm water into the eastern Indian Ocean. The combination of subsurface Kelvin wave propagation and increased ITF volume transport in the Indo-Pacific region acted to prolong the heat content in the western Pacific during the decay phase of La Niña, ultimately leading to the rare triple-dip 2020–2023 La Niña event.