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The south Indian Ocean (SIO) is a region of strong air‐sea heat loss due to the unique ocean circulation pattern influenced by the Indonesian Throughflow. In this study, the seasonal variation of the surface layer heat budget in the eastern SIO is investigated using 2 years of measurements from a mooring at 25°S, 100°E, the only colocated upper ocean and surface meteorology time series in the subtropical Indian Ocean. The mooring data are combined with other in situ and satellite data to examine the role of air‐sea fluxes and ocean heat transport on the evolution of mixed layer temperature using heat budget diagnostic models. Results show that on seasonal timescales, mixed layer heat storage in the eastern SIO is mostly balanced by a combination of surface fluxes and turbulent entrainment with a contribution from horizontal advection at times. Solar radiation dominates the seasonal cycle of net surface heat flux, which warms the mixed layer during austral summer (67 Wm‐2) and cools it during austral winter (‐44 Wm‐2). Entrainment is in good agreement with the heat budget residual for most of the year. Horizontal advection is spatially variable and appears to be dominated by the presence of mesoscale eddies and possibly annual and semi‐annual Rossby waves propagating from the eastern boundary. Results from the 2‐year mooring‐based data analysis are in reasonably good agreement with a 12‐year regional heat budget analysis around the mooring location using ocean reanalysis products.

Results from an eddy-resolving ocean model simulation, an ocean reanalysis data, and reduced-gravity ocean (RGO) model simulations reach consensus in suggesting prominent decadal fluctuations of the Mindanao Current (MC) during 1960-2010. The MC was evidently enhanced during 1974-1982 and 2002-2008 and weakened during 1983-1990 and 1994-2001, with typical amplitude of similar to 2.0Sv (1Sv10(6)m(3)/s) for decadal volume transport anomaly. These variations are closely associated with the strength of the westward flowing North Equatorial Current at 130 degrees E and not evidently related with its bifurcation latitude. Experiments using a 1.5-layer RGO model are performed to understand the mechanisms. Decadal variations of the MC are primarily caused by off-equatorial wind changes in the tropical North Pacific, and winds in the latitude range of the MC (3-13 degrees N) play the dominant role. Equatorial winds between 3 degrees S and 3 degrees N generally act to attenuate the MC variability, and subtropical winds have little contribution. For zonal distribution, local wind forcing between 120 and 160 degrees E explains 67.8% of the MC decadal variability, while those in the central and eastern Pacific have weaker effects (25.0% and 7.2%, respectively). RGO model runs forced by nine wind data sets confirm the tight relationship between the MC and the western Pacific winds since the 1980s. The anomalous westerly winds under El Nino-like condition drive upwelling in the Philippine Sea through off-equatorial Ekman pumping that strengthens the MC. The relationship prior to 1980 is sensitive to the choice of wind data set, with four data sets yielding low or negative correlations. Plain Language Summary The Mindanao Current (MC) is important in supplying water to the western Pacific warm pool and the Indonesian Throughflow. The MC variabilities on seasonal and interannual time scales have been intensively investigated, whereas decadal variability of the MC was rarely explored. In this study, we used ocean model simulations and ocean reanalysis data to explore the decadal variability of the MC during 1960-2010 and the underlying causes. It is found that the MC was stronger during 1974-1982 and 2002-2008 and weaker during 1983-1990 and 1994-2001. The typical amplitude is similar to 2.0Sv (1Sv10(6)m(3)/s). We also found that wind forcing in the tropical northwest Pacific region is most important in controlling the MC transport. These variations are largely determined by the strength of the North Equatorial Current at 130 degrees E rather than the North Equatorial Current's bifurcation latitude off the Philippine coast. Subtropical wind forcing north of 23 degrees N has little contribution, whereas equatorial winds between 3 degrees S and 3 degrees N generally act to attenuate the MC variability. A tight relationship is found between the MC and the western Pacific zonal winds in decadal variability, particularly after similar to 1980. By contrast, the correlation between the MC and El Nino-Southern Oscillation is low on decadal time scale.

International audience; Drastic paleogeography changes in the Indonesian archipelago over the Plio-Pleistocene, either in response to sea level oscillations or vertical land motion, enabled the periodic emergence of the Sunda shelf. When emerged, this wide continental platform in the heart of the Maritime Continent may have modified regional and global climate systems. We investigate the effect of the exposure of the Sunda shelf on climate dynamics using a set of numerical simulations with (i) atmosphere-land surface and (ii) fully coupled general circulation models. We first explore the impact of convection schemes on the rainfall regime simulated over the Maritime Continent and show how they could explain the discrepancies among previous studies. We further depict a robust and common mechanism that prevails. We show that diurnal heating of the surface of the continental platform enhances low-level convergence and local convection, and fosters local precipitations. This effect, to a second order, is modulated by the radiative effect and increased turbulent heat flux driven by vegetated surface properties such as albedo or roughness. Increasing precipitations over the exposed platform also impacts freshwater export into seawater, making salinity of the Indian Ocean and Indonesian Throughflow highly dependent on the routing scheme over the exposed shelf.

The Indo-Australian Basin (IAB) constitutes a crucial water route for the global ocean's circulation between the Pacific and Indian Oceans. Locally generated eddies contribute to the continued westward transport of water away from the IAB. In this paper, results from the first comprehensive regional ocean eddy census are discussed. A hybrid eddy census and tracking algorithm is used to analyze satellite altimetry data from 1993 to 2018. A total of 2,792 eddies were identified and tracked. Cyclonic eddies and anticyclonic eddies exhibit geographical segregation in terms of their dynamic characteristics. Larger amplitudes, radii, and absolute vorticities are associated with high eddy kinetic energy regions located in the vicinity of the South Java Current, South Equatorial Current, and Indonesian Throughflow. The eddies are grouped into three duration-based classifications providing the first systematic insight into understanding their physical characteristics, formation, and distribution in the IAB.

Climate models consistently project a robust weakening of the Indonesian Throughflow (ITF) and the Atlantic meridional overturning circulation (AMOC) in response to greenhouse gas forcing. Previous studies of ITF variability have largely focused on local processes in the Indo‐Pacific Basin. Here, we propose that much of the centennial‐scale ITF weakening is dynamically linked to changes in the Atlantic Basin and communicated between basins via wave processes. In response to an AMOC slowdown, the Indian Ocean develops a northward surface transport anomaly that converges mass and modifies sea surface height in the Indian Ocean, which weakens the ITF. We illustrate these dynamic interbasin connections using a 1.5‐layer reduced gravity model and then validate the responses in a comprehensive general circulation model. Our results highlight the importance of transient volume exchanges between the Atlantic and Indo‐Pacific basins in regulating the global ocean circulation in a changing climate.

Ocean gateways facilitate circulation between ocean basins, thereby impacting global climate. The Indonesian Gateway transports water from the Pacific to the Indian Ocean via the Indonesian Throughflow (ITF) and drives the strength and intensity of the modern Leeuwin Current, which carries warm equatorial waters along the western coast of Australia to higher latitudes. Therefore, ITF dynamics are a vital component of global thermohaline circulation. Plio‐Pleistocene changes in ITF behavior and Leeuwin Current intensity remain poorly constrained due to a lack of sedimentary records from regions under its influence. Here, organic geochemical proxies are used to reconstruct sea surface temperatures on the northwest Australian shelf at IODP Site U1463, downstream of the ITF outlet and under the influence of the Leeuwin Current. Our records, based on TEX86 and the long‐chain diol index, provide insight into past ITF variability (3.5–1.5 Ma) and confirm that sea surface temperature exerted a control on Australian continental hydroclimate. A significant TEX86 cooling of ~5°C occurs within the mid‐Pliocene Warm Period (3.3–3.1 Ma) suggesting that this interval was characterized by SST fluctuations at Site U1463. A major feature of both the TEX86 and long‐chain diol index records is a strong cooling from ~1.7 to 1.5 Ma. We suggest that this event reflects a reduction in Leeuwin Current intensity due to a major step in ongoing ITF constriction, accompanied by a switch from South to North Pacific source waters entering the ITF inlet. Our new data suggest that an additional ITF constriction event may have occurred in the Pleistocene.

The origins of the upper branch of the Atlantic meridional overturning circulation (AMOC) are traced with backward‐in‐time Lagrangian trajectories, quantifying the partition of volume transport between different routes of entry from the Indo‐Pacific into the Atlantic. Particles are advected by the velocity field from a recent release of “Estimating the Circulation and Climate of the Ocean” (ECCOv4). This global time‐variable velocity field is a dynamically consistent interpolation of over 1 billion oceanographic observations collected between 1992 and 2015. Of the 13.6 Sverdrups (1 Sv = 106 m3/s) flowing northward across 6°S, 15% enters the Atlantic from Drake Passage, 35% enters from the straits between Asia and Australia (Indonesian Throughflow), and 49% comes from the region south of Australia (Tasman Leakage). Because of blending in the Agulhas region, water mass properties in the South Atlantic are not a good indicator of origin., Key Points The upper limb of the meridional overturning circulation originates primarily from the Tasman Leakage and the Indonesian ThroughflowNinety‐seven percent of the upper limb of the meridional overturning circulation enters the Atlantic from the Agulhas regionBecause of blending in the Agulhas region, water mass properties in the South Atlantic are not a good indicator of origin

A regional data-constrained coupled ocean-sea ice general circulation model and its adjoint are used to investigate mechanisms controlling the volume transport variability through Bering Strait dur...

The Indonesian Throughflow (ITF) inflow through the Sulawesi, Maluku, and Halmahera Seas and the ITF outflow into the eastern tropical Indian Ocean, based on reanalysis and model data, are used to investigate the interannual to decadal response of the ITF vertical profile to Indo‐Pacific forcing. The thermocline (upper 300 m) inflow, driven by the North Pacific Ocean, negatively responds to El Niño‐Southern Oscillation (ENSO) with almost no lag; while the thermocline outflow lags by 5–7 months. The sub‐thermocline (300–760 m) inflow, which is influenced by both the North and South Pacific, positively responds to ENSO and Interdecadal Pacific Oscillation (IPO) indices with 13 months lag and displays a long‐term trend tracking the IPO index. Influenced by eastern Indian Ocean variations, the sub‐thermocline outflow positively correlates with the ENSO and IPO indices with about 7–9 months lag, a shorter lag time than the inflow.

The Bering Strait throughflow has important implications for the Arctic freshwater, heat, and nutrients. By keeping the interannual variabilities of the atmospheric forcing only inside or outside the Arctic Ocean in numerical simulations, we can quantify their relative contributions to the interannual variability of the throughflow. We found that winds play a much more important role for the throughflow interannual variability than buoyancy forcing. Winds over the western Arctic Ocean and North Pacific determine the direction of Ekman transport, thus changing the sea surface height gradient between the two basins, and consequently influencing the volume transport strength. Although winds over the two basins are similarly important for the variance of ocean volume transport, the North Pacific winds cause stronger variability in freshwater and heat transports through modifying the inflow temperature and salinity. After 1994, winds over the western Arctic Ocean explain a larger part of the variability of Bering Strait volume transport than the winds outside the Arctic Ocean.

El Nino events are characterized by anomalously warm tropical Pacific surface waters and concurrent ocean heat discharge, a precursor of subsequent cold La Nina conditions. Here we show that El Nino 2015/2016 departed from this norm: despite extreme peak surface temperatures, tropical Pacific (30°N-30°S) upper ocean heat content increased by 9.6 ± 1.7 ZJ (1 ZJ = 1021 J), in stark contrast to the previous strong El Nino in 1997/1998 (-11.5 ± 2.9 ZJ). Unprecedented reduction of Indonesian Throughflow volume and heat transport played a key role in the anomalous 2015/2016 event. We argue that this anomaly is linked with the previously documented intensified warming and associated rising sea levels in the Indian Ocean during the last decade. Additionally, increased absorption of solar radiation acted to dampen Pacific ocean heat content discharge. These results explain the weak and short-lived La Nina conditions in 2016/2017 and indicate the need for realistic representation of Indo-Pacific energy transfers for skillful seasonal-to-decadal predictions.

Deciphering the evolution of Southern Ocean circulation during the Eocene and Oligocene has important implications for understanding the development of the Antarctic Circumpolar Current and transition to Earth's "icehouse" climate. To better understand ocean circulation patterns in the Indian Ocean sector of the Southern Ocean, we generated a new fossil fish tooth neodymium isotope record (εNd) from the upper Eocene to upper Oligocene sections (36-23 Ma) of Ocean Drilling Program Sites 744 and 748 (Kerguelen Plateau, Indian Ocean). Reconstructed seawater εNd values from fossil fish teeth are used to trace changes in water masses across ocean basins. The records from Site 748 and Site 744 reveal a gradual shift from εNd values around -6.5 to -7.5 in the late Eocene to εNd values between -7.5 and -8.3 by the late Oligocene, consistent with a Circumpolar Deep Water (CDW) influence at the Kerguelen Plateau throughout the Oligocene. We interpret the shift to less radiogenic values to reflect the increased export of Northern Component Water to the Southern Ocean, likely into the proto-CDW. However, the records show no major change in water mass composition around the Kerguelen Plateau that would accompany an increase in Pacific throughflow related to the opening of Drake Passage and imply that Pacific throughflow via the Drake Passage occurred by the late Eocene. High-frequency variability in eNd values at Site 744 is interpreted as an imprint of Oligocene glacial activity, with a particularly pronounced excursion at 32.6 Ma roughly coinciding with other glacial weathering indicators around Antarctica.

The Indonesian Throughflow (ITF) waters are significantly transformed within the Indonesian Archipelago and consequently influence the large-scale ocean circulation such as Agulhas and Leeuwin Currents. Existing ocean general circulation models (OGCMs) are, however, incapable of reproducing the transformation of the ITF waters, since tidal forcing is neglected in such models. In the present study, we first conduct high-resolution non-hydrostatic three-dimensional numerical experiments focusing on the transformation of the ITF waters in the Halmahera Sea which is thought to be the most important bottleneck in simulating the ITF water-mass properties. It is shown that intensive vertical mixing induced by breaking of internal tides in the shallow regions in the Halmahera Sea dilutes the ITF waters, significantly reducing model biases found in the existing OGCMs. We next evaluate quantitatively the effect of tide-induced vertical mixing on the transformation of the ITF waters. It is shown that tide-induced vertical mixing dominates the transformation of the ITF waters, although some supplementary processes such as horizontal mixing associated with the sub-mesoscale eddies resulting from tidal interaction with land configurations cannot be ignored.

The rapid Indian Ocean warming during the early-21th century was a major heat sink for the recent global surface warming slowdown. Analysis of observational data and ocean model experiments reveals that during 2003–2012 more than half of the increased upper Indian Ocean heat content was concentrated in the southeast Indian Ocean (SEIO), causing a warming “hot spot” of 0.8–1.2 K decade−1 near the west coast of Australia. This SEIO warming was primarily induced by the enhancements of the Pacific trade winds and Indonesian throughflow associated with the Interdecadal Pacific Oscillation's (IPO) transition to its negative phase, and to a lesser degree by local atmospheric forcing within the Indian Ocean. Large-ensemble climate model simulations suggest that this warming event was likely also exacerbated by anthropogenic forcing and thus unprecedentedly strong as compared to previous IPO transition periods. Climate model projections suggest an increasing possibility of such strong decadal warming in future.

We present a high-resolution (~20 to 100 years temporal resolution) reconstruction of hydrological changes in the Makassar Strait over the last 14 kyr from Core SO217-18517 retrieved off the Mahakam Delta (1°32.198′S, 117°33.756′E; 698 m water depth) during the SO217 Makassar-Java Cruise. Sea surface temperatures, based on Mg/Ca of Globigerinoides ruber and alkenone UK′37, and seawater δ18O reconstructions, based on G. ruber δ18O and Mg/Ca, in combination with sortable silt grain size measurements and X-ray fluorescence (XRF) core scanner derived elemental data provide evidence for increased precipitation during the Bolling-Allerod (BA) and early Holocene and for warmer and more saline surface waters and a decrease in the intensity of the Indonesian Throughflow (ITF) during the Younger Dryas (YD). XRF derived Log (Zr/Rb) records, sortable silt data and increased sedimentation rates indicate decreased winnowing, interpreted as a slowdown of the ITF thermocline flow during the YD. We attribute this decline in ITF intensity to slowdown of the Atlantic meridional overturning circulation during the YD. We suggest that changes in Makassar Strait surface hydrology during this interval of Northern Hemisphere cooling and Southern Hemisphere warming were related to a southward displacement of the Intertropical Convergence Zone.

We examine the consistency of Indo-Pacific decadal sea level variability in 10 gridded, observation-based sea level products for the 1960–2010 period. Decadal sea level variations are robust in the Pacific, with more than 50% of variance explained by decadal modulation of two flavors of El Nino–Southern Oscillation (classical ENSO and Modoki). Amplitude of decadal sea level variability is weaker in the Indian Ocean than in the Pacific. All data sets indicate a transmission of decadal sea level signals from the western Pacific to the northwest Australian coast through the Indonesian throughflow. The southern tropical Indian Ocean sea level variability is associated with decadal modulations of ENSO in reconstructions but not in reanalyses or in situ data set. The Pacific-independent Indian Ocean decadal sea level variability is not robust but tends to be maximum in the southwestern tropical Indian Ocean. The inconsistency of Indian Ocean decadal variability across the sea level products calls for caution in making definitive conclusions on decadal sea level variability in this basin.

The Indonesian Throughflow (ITF) is an important component of the global overturning circulation. In this study, we amend Godfrey's Island Rule to estimate the ITF transport by including contributions from deep ocean vertical transport. Simulations using a near-global 1/10° ocean general circulation model are used to verify the amended Island Rule. We show that deep ocean circulation is as important as wind-driven processes to the ITF transport and variability. The centennial weakening of the ITF by 32% during the 21st century, under the high greenhouse gas emission scenario, is primarily associated with reductions in net deep ocean upwelling in the tropical and South Pacific. Deep ocean circulation of the Pacific may become less connected with the ITF transport in a warm climate.

This paper reports upper ocean thermohaline structure and variability observed during the life cycle of an intense Madden Julian Oscillation (MJO) event occurred in the southern tropical Indian Ocean (14°S–Eq, 70°E–81°E). Water column measurements for this study were collected using airborne expendable probes deployed from NOAA's WP-3D Orion aircraft operated as a part of Dynamics of MJO field experiment conducted during November–December 2011. Purpose of the study is twofold; (1) to provide a statistical analysis of the upper ocean properties observed during different phases of MJO and, (2) to investigate how the upper ocean thermohaline structure evolved in the study region in response to the MJO induced perturbation. During the active phase of MJO, mixed layer depth (MLD) had a characteristic bimodal distribution. Primary and secondary modes were at ∼34 m and ∼65 m, respectively. Spatial heterogeneity of the upper ocean response to the MJO forcing was the plausible reason for bimodal distribution. Thermocline and isothermal layer depth deepened, respectively, by 13 and 19 m from the suppressed through the restoring phase of MJO. Thicker (>30 m) barrier layers were found to occur more frequently in the active phase of MJO, associated with convective rainfalls. Additionally, the water mass analysis indicated that, in the active phase of this MJO event the subsurface was dominated by Indonesian throughflow, nonetheless intrusion of Arabian Sea high saline water was also noted near the equator.

Seasonal surface freshening of the Makassar Strait, the main conduit of the Indonesian Throughflow (ITF), is a key factor controlling the ITF. Here we present a 262 year reconstruction of seasonal sea-surface-salinity variability from 1742 to 2004 Common Era by using coral δ18O records from the central Makassar Strait. Our record reveals persistent seasonal freshening and also years with significant truncations of seasonal freshening that correlate exactly with South Pacific Convergence Zone (SPCZ) zonal events >4000 km to the east. During these events, the SPCZ dramatically rotates ~15° north to near the equator and stronger westward flowing South Pacific boundary currents force higher-salinity water through the Makassar Strait in February–May halting the normal seasonal freshening in the strait. By these teleconnections, our Makassar coral δ18O series provides the first record of the recurrence interval of these zonal SPCZ events and demonstrates that they have occurred on a semiregular basis since the mid-1700s.

In this study, seasonal and interannual variations of the mixed layer salinity (MLS) in the southeast tropical Indian Ocean (SETIO) are analyzed using satellite observations, historical data sets, and data-assimilating ocean model outputs. On the seasonal cycle, the MLS in the SETIO becomes fresher in austral winter and saltier in austral summer: between the Java-Lesser Sunda coast and the South Equatorial Current (SEC, 12 degrees S), where positive entrainment and fresh advections counterbalance each other, the annual cycle of the MLS closely follows the variation of the air-sea freshwater forcing; off the northwest and west Australian coasts, the MLS variations are influenced by the annual cycles of the Indonesian Throughflow (ITF) and Leeuwin Current (LC) transports as well as the air-sea freshwater forcing, with eddy fluxes acting to freshen the MLS along the SEC, the Eastern Gyral Current, and the LC. On the interannual-scale, El Nino (La Nina) events are typically associated with saltier (fresher) MLS in the SETIO. Composite and budget analyses reveal that interannual variations in precipitations drive the MLS anomalies off the Java-Lesser Sunda coast; between 12 degrees S and the northwest Australian coast, the MLS variations are influenced by both advection anomalies and local precipitation anomalies; whereas anomalous meridional currents contribute to the MLS variations off the west Australian coast. Both enhanced local precipitations and the ITF transport anomalies have substantial contributions to the drastic freshening of the Indonesian-Australian Basin between the Java-Lesser Sunda coast and the northwest Australian coast during the extended La Nina events in 1999-2001 and 2010-2012.

By combining volume transport and salinity analysis from 1958 to 2014, this paper investigates how the transient Equatorial Undercurrent (EUC) sustains the summer-fall equatorial eastern Indian Ocean (EIO) upwelling. On seasonal time scales, the EIO upwelling is mainly supplied by the salty water from the western basin through a buffering process: The winter-spring EUC carries the salty water from the western basin eastward, induces downwelling in the EIO, and pushes portion of the salty water below the central thermocline, which subsequently upwells to the central thermocline during summer-fall and sustains the EIO upwelling. On interannual time scales, enhanced upwelling occurs during positive Indian Ocean Dipole (+IOD) years. The strong summer-fall EUC associated with the +IOD supplies water for the intensified upwelling. This research provides new knowledge for basin-scale mass and property exchanges associated with the EIO upwelling, contributing to our understanding of three-dimensional ocean circulation and climate variability.

Sea Surface Height (SSH) data from the European Centre for Medium-Range Weather Forecasts-Ocean Reanalysis System 4 (ECMWF-ORAS4) are used to determine the pressure difference in connection with variability of the South China Sea ThroughFlow (SCSTF) from 1958 to 2007. Two branches of SCSTF, the Karimata-Sunda Strait ThroughFlow (KSSTF) and the Mindoro Strait ThroughFlow (MSTF), are examined. Using the ensemble empirical mode decomposition method (EEMD), time series of pressure difference and volume transport are decomposed into intrinsic mode functions and trend functions, with the corresponding variability on different time scales. Pressure difference agrees with the KSSTF volume transport on decadal time scale; while for the MSTF, pressure difference varies similarly with volume transport on interannual time scale. Separating the dynamic height difference into the thermal and haline terms, for the KSSTF more than half of the dynamic height difference (32 cm) is due to the thermal contributions; while the remaining dynamic height difference (23 cm) is due to the haline contributions. For the MSTF, the dynamic height difference (29 cm) is primarily due to the thermal contribution (26 cm).

The Indonesian Throughflow (ITF) region possesses strong mixing and experiences significant freshwater input, but the role of salinity variability in the Indonesian Seas remains unclear. The goal of this study is to understand how salinity variability influences the ITF transport on interannual time scales. The ITF transport is calculated using observations and assimilation data sets and verified using direct ITF transport estimates. We find that the halosteric component of the ITF transport contributes (36 +/- 7)% of the total ITF variability, in contrast to (63 +/- 6)% by the thermosteric component. Thus, while not dominant, this result nonetheless implies that the salinity variability in the Indonesian Seas is of remarkable importance in determining the interannual variability of ITF transport. Correlation analysis indicates that the interannual variability of the total ITF transport is mainly influenced by the El Nino-Southern Oscillation (ENSO) rather than the Indian Ocean Dipole. Under the ENSO cycle, the Walker Circulation shifts longitudinally resulting in fluctuations in precipitation over the Indonesian Seas that modulates salinity and subsequently influences the interannual variability of ITF transport. This result signals the importance of precipitation and the subsequent salinity effect in determining the interannual variability of the ITF transport. The role of wind forcing and oceanic planetary waves is also revisited using this newly calculated ITF transport series. ENSO-related wind forcing is found to modulate the ITF transport via Rossby waves through the wave guide in the Indonesian Seas, which is in agreement with previous studies.

Remarkable warming of the Southern Indian Ocean during the recent two decades is assessed using a heat budget analysis based on the Estimating the Circulation and Climate of the Ocean version 4 release 3 model results. The annual mean temperature averaged in the upper-700m Southern Indian Ocean during 1998-2015 has experienced significant warming at a rate of 1.03×10-2℃/yr. A heat budget analysis indicates that the increase is mostly driven by decreased cooling from net air-sea heat flux and increased warming from heat advection. Increased ITF advection is the largest contributor to warming the upper 700m of the Southern Indian Ocean, while the reduction of surface turbulent heat flux is of secondary importance. These results expand our understanding of the decadal heat balance in the Indian Ocean and of Indo-Pacific decadal climate variability.

Based on 30-year repeated expendable bathythermograph (XBT) deployments between Fremantle, Western Australia and the Sunda Strait, Indonesia from 1984 to 2013, interannual variability of geostrophic transport of the Indonesian Throughflow (ITF) and its relationships with El Nino Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) are investigated. The IOD induced coastal Kelvin waves propagate along the Sumatra-Java coast of Indonesia, and ENSO induced coastal Kelvin waves propagate along the northwest coast of Australia, both influencing interannual variations of the ITF transport. The ITF geostrophic transport is stronger during La Nina phase and weaker during El Nino phase, with the Nino3.4 index leading the ITF variability by 7 months. The Indian Ocean wind variability associated with the IOD to a certain extent offset the Pacific ENSO influences on the ITF geostrophic transport during the developing and mature phases of El Nino and La Nina, due to the co-varying IOD variability with ENSO. The ITF geostrophic transport experiences a strengthening trend of about 1 Sv every 10 years over the study period, which is mostly due to a response to the strengthening of the trade winds in the Pacific during the climate change hiatus period. Decadal variations of the temperature-salinity relationships need to be considered when estimating the geostrophic transport of the ITF using XBT data. This article is protected by copyright. All rights reserved.

During the 2010–2011 La Nina and Ningaloo Nino, excessive precipitations in the Maritime Continent and Indonesian-Australian Basin caused surface waters to freshen by 0.3 practical salinity unit in the southeast Indian Ocean. The low-salinity anomalies are observed to be carried westward by the Indonesian throughflow and the South Equatorial Current and transmitted into the poleward flowing eastern boundary current, the Leeuwin Current, along the Western Australian coast. Low-salinity anomalies contribute to about 30% of the anomalous increase of the southward Leeuwin Current transport during the evolution of the 2010–2011 Ningaloo Nino, resulting in unprecedented warming off the coast of Western Australia. Episodical freshening of the Leeuwin Current has been observed at the Rottnest coastal reference station of Western Australia during extended La Nina conditions over the past several decades; low-salinity anomalies at the station during the 2010–2011 Ningaloo Nino are comparable with strong historical events.

Tidal mixing in the Indonesian Archipelago contributes to regulation of the tropical atmospheric circulation and water-mass transformation in the Indonesian Throughflow. The present study quantifies the vertical diffusivity in the Indonesian Archipelago by driving a high-resolution three-dimensional numerical model and investigates the processes of internal tide generation, propagation, and dissipation. The numerical experiment shows that M2 internal tides are effectively generated over prominent subsurface ridges. The conversion rate from M2 barotropic to baroclinic energy over the whole analyzed model domain is estimated to be 85.5 GW. The generated internal tides dissipate 50–100% of their energy in close proximity to the generation sites (“near-field”), and the remaining baroclinic energy propagates away causing relatively large energy dissipation far from the generation sites (“far-field”). The local dissipation efficiency q, therefore, has an extremely nonuniform spatial distribution, although it has been assumed to be constant in the existing tidal mixing parameterization for the Indonesian Archipelago. Compared with the model-predicted values, the existing parameterization yields the same order of vertical diffusivity averaged within the Indonesian Archipelago, but significantly overestimated (or underestimated) vertical diffusivity in the near-field (or the far-field). This discrepancy is attributable to the fact that the effects of internal wave propagation are completely omitted in the existing parameterization, suggesting the potential danger of using such parameterized vertical mixing in predicting the distribution of SST as well as water-mass transformation in the Indonesian Seas.

A growing body of research on stream thermal regimes has highlighted the importance of heat advection associated with surface water and groundwater interactions, such as hyporheic exchange, groundwater discharge, and hillslope throughflow inputs. Existing catchment models that predict stream temperature use a variety of approaches to estimate throughflow temperatures, but none has been evaluated against field measurements of throughflow temperature. In this study, throughflow temperatures were monitored over two winters at 50 locations adjacent to a headwater stream (11 ha catchment area) located in the rain-on-snow zone of the Pacific Northwest. Existing approaches to estimate throughflow temperature under or overpredicted throughflow temperatures by up to 5°C, or were unable to represent the influence of transient snow cover. Therefore, a conceptual-parametric model that is computationally efficient was developed that simulates hillslope hydrology and throughflow temperatures. The model structure includes an upslope reservoir that drains into a downslope reservoir that, in turn, drains into the stream. Vertical and lateral energy and water fluxes are simulated using simplified process representations. The model successfully predicts throughflow temperatures and highlights the dominant role of throughflow advection and the influence of snow cover on stream thermal regimes during high flow periods and rain-on-snow events.

The Indonesian throughflow (ITF) from the Pacific to the Indian Ocean plays an important role in global ocean circulation and climate. Yet, continuous ITF measurement is difficult and expensive. The longest time series of in situ measurements of the ITF to date were taken in the Makassar Strait, the main pathway of the ITF. Here we have demonstrated a plausible approach to derive the ITF transport proxy using satellite altimetry sea surface height (SSH), gravimetry ocean bottom pressure (OBP) data, in situ measurements from the Makassar Strait from 1996 to 1998 and 2004 to 2011, and a theoretical formulation. We first identified the optimal locations of the correlation between the observed ITF transport through the Makassar Strait and the pressure gradients, represented by the SSH and OBP differences between the Pacific and Indian Oceans at a 1° × 1° horizontal resolution. The optimal locations were found centered at 162°E and 11°N in the Pacific Ocean and 80°E and 0° in the Indian Ocean, then were used in the theoretical formulation to estimate the throughflow. The proxy time series follow the observation time series quite well, with the 1993–2011 mean proxy transport of 11.6 ± 3.2 Sv southward, varying from 5.6 Sv during the strong 1997 El Nino to 16.9 Sv during the 2007 La Nina period, which are consistent with previous estimates. The observed Makassar mean transport is 13.0 ± 3.8 Sv southward over 2004–2011, while the SSH proxy (for the same period) gives an ITF mean transport of 13.9 ± 4.1 Sv and the SSH + OBP proxy (for 2004–2010) is 15.8 ± 3.2 Sv.

Steric sea level change has been identified as one of the major contributors to the regional variability of sea level trends observed by satellite altimetry for the past two decades. This contribution varies in space and time. The temperature (thermosteric) contribution to sea level has generally been found to be more important than the salinity (halosteric) effect. Based on sea level measurements from satellite altimetry and temperature and salinity data from Argo floats during 2005–2013, we found that the southeast Indian Ocean experiences a large halosteric contribution to sea level change. The conspicuously large halosteric contribution is associated with a freshening in the upper 300 m. Neither local atmospheric forcing such as Ekman pumping and E − P nor halosteric signal transmitted from the western tropical Pacific can explain this freshening. An enhanced precipitation in the Maritime Continent region and the observed strengthening of the Indonesian throughflow are the likely causes.

It is typically stated that the Pacific-to-Arctic oceanic flow through the Bering Strait (important for Arctic heat, freshwater, and nutrient budgets) is driven by local wind and a (poorly defined) far-field “pressure head” forcing, related to sea surface height differences between the Pacific and the Arctic. Using monthly, Arctic-wide, ocean bottom pressure satellite data and in situ mooring data from the Bering Strait from 2002 to 2016, we discover the spatial structure of this pressure head forcing, finding that the Bering Strait throughflow variability is dominantly driven from the Arctic, specifically by sea level change in the East Siberian Sea (ESS), in turn related to westward winds along the Arctic coasts. In the (comparatively calm) summer, this explains approximately two thirds of the Bering Strait variability. In winter, local wind variability dominates the total flow, but the pressure head term, while still correlated with the ESS-dominated sea level pattern, is now more strongly related to Bering Sea Shelf sea level variability.

Previous studies have shown that ENSO's anomalous equatorial winds, including the observed southward shift of zonal winds that occurs around the event peak, can be reconstructed with the first two Empirical Orthogonal Functions (EOFs) of equatorial region wind stresses. Using a high-resolution ocean general circulation model, we investigate the effect of these two EOFs on changes in warm water volume (WWV), interhemispheric mass transports, and Indonesian Throughflow (ITF). Wind stress anomalies associated with the first EOF produce changes in WWV that are dynamically consistent with the conceptual recharge oscillator paradigm. The ITF is found to heavily damp these WWV changes, reducing their variance by half. Wind stress anomalies associated with the second EOF, which depicts the southward wind shift, are responsible for WWV changes that are of comparable magnitude to those driven by the first mode. The southward wind shift is also responsible for the majority of the observed interhemispheric upper ocean mass exchanges. These winds transfer mass between the Northern and the Southern Hemisphere during El Niño events. Whilst water is transferred in the opposite direction during La Niña events, the magnitude of this exchange is roughly half of that seen during El Niño events. Thus, the discharging of WWV during El Niño events is meridionally asymmetric, while the WWV recharging during a La Niña event is largely symmetric. The inclusion of the southward wind shift is also shown to allow ENSO to exchange mass with much higher latitudes than that allowed by the first EOF alone.

The Indonesian throughflow (ITF) is a chokepoint in the upper ocean thermohaline circulation, carrying Pacific waters through the strongly mixed Indonesian Seas and into the Indian Ocean. Yet the influence of the ITF on biogeochemical fluxes into the Indian Ocean is largely unknown. This study determines the first depth- and time-resolved nitrate, phosphate, and silicate fluxes at the three main exit passages of the ITF: Lombok Strait, Ombai Strait, and Timor Passage. Nutrient flux as well as its variability with depth and time differs greatly between the passages. We estimate the effective flux of nutrients into the Indian Ocean by accounting for existing nutrients in the basin and find it largest in the upper 300–400 m. This suggests that the majority of ITF nutrient supply to the Indian Ocean is to thermocline waters, where it is likely to support new production and significantly impact Indian Ocean biogeochemical cycling.

A new milestone has been reached with the launch of two dedicated satellite missions to routinely measure the sea surface salinity (SSS) fields from space at global and regional scales. In the present work, a thorough analysis of the first two years of Aquarius SSS data in the South Indian Ocean is performed. This analysis is focused on three questions: How accurate is Aquarius SSS related to in situ data from the fresh Indonesian Throughflow and salty subtropical waters? Can Aquarius give a spatial context for the data measured by the RAMA mooring system? Are westward propagating annual-period signals described in recent model simulations reproduced by Aquarius-derived SSS? We find Aquarius observations to be highly correlated with those of Argo floats, with small disagreements occurring near oceanic fronts. Aquarius gives fresher SSS than in-situ data in the tropical region due to rainfall effects, except in the eastern basin where the freshening seems to be related to sharp localized leakages of very fresh waters from the Indonesian seas that the Aquarius product is not able to properly resolve. Aquarius data is shown to reproduce quite well the annual cycle obtained from RAMA and Argo gridded datasets. The annual cycle in Aquarius is characterized by SSS propagating features with different characteristics west and east of the Ninety East Ridge. These features are strikingly different from sea surface height waves. Our results suggest that SSS annual propagation might be reflecting coupled ocean-atmosphere dynamics and surface-subsurface processes operating over the entire South Indian Ocean.

Ongoing acoustic Doppler current profilers (ADCP) observation in the eastern equatorial Indian Ocean and a recent International Nusantara Stratification and Transport (INSTANT) experiment in the Indonesian Throughflow (ITF) straits have shown coherent intraseasonal oceanic variations in this region. The intraseasonal variations are dominated by 30–70 day variations, with a tendency for the observed currents in the eastern equatorial Indian Ocean to lead those at the Lombok and Ombai Straits. Phase speed of these eastward propagating signals estimated using lag correlation analysis does not correspond to one particular baroclinic mode, though it is in the range expected for the first two baroclinic modes. In this study, the dynamics underlying this intraseasonal coherency is evaluated using output from a high-resolution ocean general circulation model developed for the Earth Simulator (OFES). The results from model simulation of January 2001 through December 2007 show that the first two baroclinic modes dominate the intraseasonal variations in this region. While the first and second baroclinic modes have comparable contribution to the intraseasonal oceanic variations in the eastern equatorial Indian Ocean and in the Ombai Strait, the intraseasonal oceanic variations in the Lombok Strait are dominated by the first baroclinic mode. Moreover, the analysis reveals that the intraseasonal variability at all mooring sites is mostly confined in the upper layer above ∼100 m depth. Both equatorial wind from the Indian Ocean and alongshore winds off Sumatra and Java play important roles in generating intraseasonal variations in the Lombok and Ombai Straits.

The upper ocean circulation of the Pacific and Indian Oceans is connected through both the Indonesian Throughflow north of Australia and the Tasman leakage around its south. The relative importance of these two pathways is examined using virtual Lagrangian particles in a high-resolution nested ocean model. The unprecedented combination of a long integration time within an eddy-permitting ocean model simulation allows the first assessment of the interannual variability of these pathways in a realistic setting. The mean Indonesian Throughflow, as diagnosed by the particles, is 14.3 Sv, considerably higher than the diagnosed average Tasman leakage of 4.2 Sv. The time series of Indonesian Throughflow agrees well with the Eulerian transport through the major Indonesian Passages, validating the Lagrangian approach using transport-tagged particles. While the Indonesian Throughflow is mainly associated with upper ocean pathways, the Tasman leakage is concentrated in the 400–900 m depth range at subtropical latitudes. Over the effective period considered (1968–1994), no apparent relationship is found between the Tasman leakage and Indonesian Throughflow. However, the Indonesian Throughflow transport correlates with ENSO. During strong La Niñas, more water of Southern Hemisphere origin flows through Makassar, Moluccas, Ombai, and Timor Straits, but less through Moluccas Strait. In general, each strait responds differently to ENSO, highlighting the complex nature of the ENSO-ITF interaction.

The Indonesian Throughflow (ITF) is the only open pathway for interocean exchange between the Pacific and Indian Ocean basins at tropical latitudes. A proxy time series of ITF transport variability is developed using remotely sensed altimeter data. The focus is on the three outflow passages of Lombok, Ombai, and Timor that collectively transport the entire ITF into the Indian Ocean, and where direct velocity measurements are available to help ground-truth the transport algorithm. The resulting 18 year proxy time series shows strong interannual ITF variability. Significant trends of increased transport are found in the upper layer of Lombok Strait, and over the full depth in Timor Passage that are likely related to enhanced Pacific trade winds since the early 1990s. The partitioning of the total ITF transport through each of the major outflow passage varies according to the phase of the Indian Ocean Dipole (IOD) or El Nino-Southern Oscillation (ENSO). In general, Pacific ENSO variability is strongest in Timor Passage, most likely through the influence of planetary waves transmitted from the Pacific along the Northwest Australian shelf pathway. Somewhat surprisingly, concurrent El Nino and positive IOD episodes consistently show contradictory results from those composites constructed for purely El Nino episodes. This is particularly evident in Lombok and Ombai Straits, but also at depth in Timor Passage. This suggests that Indian Ocean dynamics likely win out over Pacific Ocean dynamics in gating the transport through the outflow passages during concurrent ENSO and IOD events.

Deep water circulation in the Luzon Strait, which connects the Pacific deep circulation with the South China Sea throughflow, is investigated using a set of oceanographic observations combined with results from three numerical experiments. Both the in situ observations and the model show a deep water overflow in the Luzon Strait. Their results suggest that the deep Pacific water first flows into the Luzon Strait through the Bashi Channel (1.2 Sv, 1 Sv = 1 × 106 m3 s−1) and the Taltung Canyon (0.4 Sv), then turns southward along the Luzon Trough, and finally enters the South China Sea primarily through two gaps in the Heng-Chun Ridge. Overall, the mean transport of the Luzon Strait overflow is about 1.5 Sv. Results from numerical experiments suggest that strong diapycnal mixing in the South China Sea and Luzon Strait, which sustains the baroclinic pressure gradient across the Luzon Strait, is the primary driving mechanism of the deep circulation in the Luzon Strait.

[1] The structure and variability of the Halmahera and Mindanao Eddies (HE and ME), which are thought to be quasi-stationary eddies, are described based on onboard and time-series observations in the gateway region of the Indonesian Throughflow (ITF). The HE was found to have a clear anticyclonic eddy structure during all cruises. It tilted northward and/or westward with increasing depth. The HE mixes waters from the northern and southern hemispheres and transfers this mixed water to the eastern route of the ITF. The HE shifted northwestward in the boreal summer and the cold phase of ENSO (El Nino and Southern Oscillation) and southeastward in the boreal winter and the warm phase of ENSO. In contrast, the ME, with its cyclonic eddy structure, was observed only once and seems not to be a quasi-stationary eddy. It appears in the mean field and is confined above the thermocline. The stationary northward undercurrent, the Mindanao Undercurrent, was also not found at 7°N east of Mindanao, but the northern tip of the HE was found at this latitude.