Indonesian Throughflow Variability Research Articles

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.

Indonesian Throughflow Variability and Linkage to ENSO and IOD in an Ensemble of CMIP5 Models

Abstract Understanding variability of the Indonesian Throughflow (ITF) and its links to El Niño–Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD), and how they are represented across climate models constitutes an important step toward improved future climate projections. These issues are examined using 20 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and the SODA-2.2.4 ocean reanalysis. It is found that the CMIP5 models overall simulate aspects of ITF variability, such as spectral and vertical structure, that are consistent with the reanalysis, although intermodel differences are substantial. The ITF variability is shown to exhibit two dominant principal vertical structures: a surface-intensified transport anomaly (ITFM1) and an anomalous transport characterized by opposing flows in the surface and subsurface (ITFM2). In the CMIP5 models and reanalysis, ITFM2 is linked to both ENSO and the IOD via anomalous Indo-Pacific Walker circulation. The driver of ITFM1 however differs between the reanalysis and the CMIP5 models. In the reanalysis ITFM1 is a delayed response to ENSO, whereas in the CMIP5 models it is linked to the IOD associated with the overly strong IOD amplitude bias. Further, the CMIP5 ITF variability tends to be weaker than in the reanalysis, due to a tendency for the CMIP5 models to simulate a delayed IOD in response to ENSO. The importance in considering the vertical structure of ITF variability in understanding ENSO and IOD impact is further underscored by the close link between greenhouse-forced changes in ENSO variability and projected changes in subsurface ITF variability.

Variability in Indonesian Throughflow Upper Hydrology in Response to Precession‐Induced Tropical Climate Processes Over the Past 120 kyr

AbstractThe Indonesian Throughflow (ITF), as the sole low‐latitude conduit connecting the Pacific and Indian Oceans, regulates the thermohaline balance between these oceans. Thus, investigating the variability in the ITF and its relationship with the precessional forcing is crucial for understanding the drivers of tropical climate change. Here, we reconstruct the history of the ITF over the past ∼120 kyr based on high‐resolution (∼400 years) δ18O and Mg/Ca records of Globigerinoides ruber and Pulleniatina obliquiloculata from core SO217‐18540 retrieved from the Flores Sea upwelling region within the main pathway of the ITF. A comparison of these new records with published paleo‐oceanographic and climatological data from the western tropical Pacific Ocean suggests that annual mean conditions in the Flores Sea were controlled by ITF variability rather than by monsoonal upwelling. Our results further indicate that precessional insolation was a major forcing for the hydrological evolution of the ITF during the past 120 kyr. We suggest that the precessional insolation forcing paced ITF variability by modulating the mean state of El Niño‐Southern Oscillation‐like conditions and latitudinal shifts and/or expansion/contraction of the Intertropical Convergence Zone.

Indonesian Throughflow variability over the last two glacial-interglacial cycles: Evidence from the eastern Indian Ocean

We present a Pulleniatina obliquiloculata Mg/Ca-derived thermocline water temperature record (TWT) covering the last 270,000 years at the south of the Lombok Strait, one of the main exits of the Indonesian Throughflow (ITF) into the Indian Ocean. The comparison with TWT records from the Java-Sumatra upwelling system and the Timor Sea suggests that changes in Lombok TWT reflect the balance between the wind-driven Java upwelling and the ITF thermocline transport. We show that the evolution of the TWT gradient (ΔTWT) between the upwelling site and the Lombok site can be used to decipher the relative strength of ITF through time. The 270 ka-long, ΔTWT record shows (i) that the ITF was weaker during MIS 6 and MIS 2–4 compared to the late Holocene, and was enhanced during MIS8, MIS 7, MIS 5 and the early Holocene, and that (ii) it varied with a strong precession-related component. Glacial-interglacial changes may reflect the modulation of ITF by sea-level changes through the modification of ITF pathways in the Indonesian Archipelago, and/or result from changes in the intensity of the Global Conveyor Belt. The strong precession contribution in the relative TWT strength at our Lombok site is interpreted as revealing the importance of southeast monsoon winds on ITF intensity.

Centennial Changes in the Indonesian Throughflow Connected to the Atlantic Meridional Overturning Circulation: The Ocean's Transient Conveyor Belt

AbstractClimate 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.

Retracted: Plio‐Pleistocene Indonesian Throughflow Variability Drove Eastern Indian Ocean Sea Surface Temperatures

AbstractOcean 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.

Structure and Variability of Indonesian Throughflow in Labani Canal

Libani Canal is one of the areas which the flows water masses of the Pacific Ocean toward Indonesian seas. The existence of Mindanao current that affects an input of Indonesian Through-flow should be predicted. The influence of climate change on large scale circulation will affect the variations of physical condition. This study focused to investigate the variation of meridional Libani Current at 30 levels deep based on INDESO models data from January 2007 to January 2014. An area of interested was located at 4.16 °S and 117.92 - 119.42 °E. The results showed a strong current average reaches 0.5 m/s with a north-south orientation as the impact of bottom topography. Variations of current through Fourier analysis showed the annual and inter-annual fluctuations in the 365 days and 120-200 days related impacts strong El-Nino in 2009-2010. Spectrum energy density peaks in the 3 days and 53 days period that indicated as the impact of intra-seasonal variations.

Stepwise Weakening of the Pliocene Leeuwin Current

AbstractThe Indonesian Throughflow (ITF) operates as an important link in global thermohaline circulation, and ITF variability probably modulated Pliocene climate change. Yet, whether ITF variability accounted for oceanographic change south of Northwest Cape remains controversial. Here, we present a multiproxy oceanographic reconstruction from the Perth Basin and reconstruct the Pliocene history of the Leeuwin Current (LC). We show that the LC was active throughout the Pliocene, albeit with fluctuations in intensity and scope. Three main factors controlled LC strength. First, a tectonic ITF reorganization caused an abrupt and permanent LC reduction at 3.7 Ma. On shorter timescales, eustatic sea level and direct orbital forcing of wind patterns hampered or promoted the LC. At 3.3 Ma, for instance, LC intensity plunged in response to a eustatic ITF restriction. Site U1459 then fell outside the extent of a weakened LC, and the latitudinal sea surface temperature gradient along West Australia doubled its steepness.

The Variability of Indonesian Throughflow in Sumba Strait and Its Linkage to the Climate Events

As one of the Indonesian Throughflow (ITF) outflow passages, the Sumba Strait is a meeting point for the Pacific and the Indian Ocean water mass. In order to study the long-term variability of ITF flowing via Sumba Strait, this research uses observational data from the Ekspedisi WIdya Nusantara (EWIN) research cruise conducted in August 2016 to validate the altimetric geostrophic surface current by referencing the shear velocity. Stating the referenced level to 700 m, geostrophic transport is calculated using the Monthly Isopycnal/Mixed-Layer Ocean Climatology (MIMOC) data. Over the period of 1993-2016, the results demonstrate a dominant seasonal pattern for the geostrophic variability. While the total geostrophic transport shows a main westward direction towards the Indian Ocean, the Sumba Strait provides only a small portion (less than 0.1 Sv) for westward ITF geostrophic current. Intraseasonally, the maximum transport occurs during the southeast monsoon. The reversal of South Java Current (SJC), which flows with the eastward direction heading to the Savu Sea, is observed as the intrusion for westward ITF in almost every monsoon season. Despite having an unclear year to year cycle, climate mode of the Indian Ocean may have more influence on the surface geostrophic variability at the Sumba Strait. On the other hand, ocean-atmosphere coupling in the Pacific Ocean has a role in regulating geostrophic transport variation within the Sumba Strait. Using a statistical approach, the findings emphasize that the throughflow may well be impacted as well as feedback on both ENSO and IOD since there is robustness in those variables.

The amplifying effect of Indonesian Throughflow heat transport on Late Pliocene Southern Hemisphere climate cooling

An unusually short glaciation interrupted the warm Pliocene around 3.3 Ma (Marine Isotope Stage (MIS) M2). Different hypotheses exist to explain why this glaciation event was so pronounced, and why the global climate system returned to warm Pliocene conditions relatively quickly afterwards. One of these proposed mechanisms is a reduced equator-to-pole heat transfer, in response to a tectonically reduced Indonesian Throughflow (ITF). The ITF is a critical part of the global thermohaline ocean circulation, transporting heat from the Indo-Pacific Warm Pool to the Indian Ocean. When ITF connectivity is reduced, the water and heat supply for the Leeuwin Current, flowing poleward along Australia's west coast, is also diminished. To assess the possible relationship between mid-Pliocene glaciations and latitudinal heat transport through the Indonesian Throughflow, we constructed a multi-proxy orbital-scale record for the 3.7–2.8 Ma interval from International Ocean Discovery Program (IODP) Site U1463, off northwest Australia. The comparison of the Site U1463 record with paleoclimate records from nearby Site 763 and West Pacific Warm Pool Site 806 allows for a detailed regional reconstruction of Pliocene paleoceanography and thus for testing the proposed hypothesis.An astronomically-paced decrease in potassium content characterizes the late Pliocene interval of U1463. This record documents the increasing aridity of northwest Australia, periodically alleviated by reinforced summer monsoon precipitation under summer insolation maxima. The δ18O record of the planktonic foraminifer Globigerinoides sacculifer correlates exceptionally well with the sea surface temperature (SST) record from Site 806 in the West Pacific Warm Pool, even during MIS M2. Hence, Site U1463 preserves an uninterrupted ITF signal even during Pliocene glaciations. However, the U1463 δ18OG.sacculifer record exhibits a 0.5‰ offset with the nearby Site 763A record around MIS M2. This implies that Site 763A, about 500 km west of U1463, more closely tracks Indian Ocean SST records across MIS M2. The U1463 data reveal that heat-transport through the Indonesian Throughflow did not shut down completely during MIS M2, but rather its intensity decreased prior to and during MIS M2, causing Site 763A to temporarily reflect an Indian Ocean, rather than an ITF signal. We conclude that ITF variability significantly influenced latitudinal heat transport by means of the Leeuwin Current and hence contributed to the relative intensity of MIS M2. We propose the ITF valve between the Pacific and Indian Ocean as a positive feedback mechanism, in which an initial sea level lowering reduces ITF heat transport, in turn amplifying global cooling by advancing the thermal isolation of Antarctica.

Cocos (Keeling) Corals Reveal 200 Years of Multidecadal Modulation of Southeast Indian Ocean Hydrology by Indonesian Throughflow

AbstractThe only low latitude pathway of heat and salt from the Pacific Ocean to the Indian Ocean, known as Indonesian Throughflow (ITF), has been suggested to modulate Global Mean Surface Temperature (GMST) warming through redistribution of surface Pacific Ocean heat. ITF observations are only available since ~1990s, and thus, its multidecadal variability on longer time scales has remained elusive. Here we present a 200 year bimonthly record of geochemical parameters (δ18O‐Sr/Ca) measured on Cocos (Keeling) corals tracking sea surface temperature (SST; Sr/Ca) and sea surface salinity (SSS; seawater‐δ18O═δ18Osw) in the southeastern tropical Indian Ocean (SETIO). Our results show that SETIO SSS and δ18Osw were impacted by ITF transport over the past 60 years, and therefore, reconstructions of Cocos δ18Osw hold information on past ITF variability on longer time spans. Over the past 200 years ITF leakage into SETIO is dominated by the interannual climate modes of the Pacific Ocean (El Niño—Southern Oscillation) and Indian Ocean (Indian Ocean Dipole). Pacific decadal climate variability (represented by the Pacific Decadal Oscillation) significantly impacted ITF strength over the past 200 years determining the spatiotemporal SST and SSS advection into the Indian Ocean on multidecadal time scales. A comparison of our SETIO δ18Osw record to GMST shows that ITF transport varied in synchrony with global warming rate, being predominantly high/low during GMST warming slowdown/acceleration, respectively. This hints toward an important role for the ITF in global warming rate modulation.

Variability of Indonesian Throughflow and Borneo Runoff During the Last 14 kyr

AbstractWe 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 Bølling‐Allerød (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.

Applicability and feasibility studies of coastal acoustic tomography for long-term monitoring of the Indonesian throughflow transport variability

We have been trying to monitor the Indonesian Throughflow (ITF), especially to understand how prominent tides and tidal mixing along the ITF main pathways in the Makassar and Lombok straits could transform the Pacific waters into Indian Oceans in the shorter scale and how the ITF variability in longer time scales affect the climate regimes over the region, using a modern and innovative technology of Coastal Acoustic Tomography (CAT). The first trial will be done in the Lombok sill where two 5-kHz CATs and three tide gauge sensors are put in the sill of depth range 200-300 m, lying between Lombok and Nusa Penida islands in the southern Lombok strait by the end of July 2016. The experimental design is a real-time land-cable CAT system, equipped with a 3G mobile antenna for telemetering CAT data to our Maritime Continent Center of Excellence (MCCOE) monitoring center in Jakarta every 10 minutes interval. Results are expected to verify CAT aplicability and feasibility as a cost effective oceanographic instrument to monitor the ITF transport variability along major straits in the Indonesian seas. This talk will also present the possibility of CAT to study mixing processes as important factors to improve model parametrizations.

Impact of the Madden–Julian Oscillation on the Indonesian Throughflow in the Makassar Strait during the CINDY/DYNAMO Field Campaign

Abstract Previous studies indicate that equatorial zonal winds in the Indian Ocean can significantly influence the Indonesian Throughflow (ITF). During the Cooperative Indian Ocean Experiment on Intraseasonal Variability (CINDY)/Dynamics of the Madden–Julian Oscillation (DYNAMO) field campaign, two strong MJO events were observed within a month without a clear suppressed phase between them, and these events generated exceptionally strong ocean responses. Strong eastward currents along the equator in the Indian Ocean lasted more than one month from late November 2011 to early January 2012. The influence of these unique MJO events during the field campaign on ITF variability is investigated using a high-resolution (1/25°) global ocean general circulation model, the Hybrid Coordinate Ocean Model (HYCOM). The strong westerlies associated with these MJO events, which exceed 10 m s−1, generate strong equatorial eastward jets and downwelling near the eastern boundary. The equatorial jets are realistically simulated by the global HYCOM based on the comparison with the data collected during the field campaign. The analysis demonstrates that sea surface height (SSH) and alongshore velocity anomalies at the eastern boundary propagate along the coast of Sumatra and Java as coastal Kelvin waves, significantly reducing the ITF transport at the Makassar Strait during January–early February. The alongshore velocity anomalies associated with the Kelvin wave significantly leads SSH anomalies. The magnitude of the anomalous currents at the Makassar Strait is exceptionally large because of the unique feature of the MJO events, and thus the typical seasonal cycle of ITF could be significantly altered by strong MJO events such as those observed during the CINDY/DYNAMO field campaign.

Interannual variability of the Indonesian Throughflow: The salinity effect

AbstractThe 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 Niño‐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.

Interannual variability of the Indonesian Throughflow transport: A revisit based on 30 year expendable bathythermograph data

AbstractBased 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 Niño 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 Niña phase and weaker during El Niño phase, with the Niño3.4 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 Niño and La Niña, due to the covarying 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.

On the Energy Exchange between Tropical Ocean Basins Related to ENSO*

Abstract Vast amounts of energy are exchanged between the ocean, atmosphere, and space in association with El Niño–Southern Oscillation (ENSO). This study examines energy budgets of all tropical (30°S–30°N) ocean basins and the atmosphere separately using different, largely independent oceanic and atmospheric reanalyses to depict anomalous energy flows associated with ENSO in a consistent framework. It is found that variability of area-averaged ocean heat content (OHC) in the tropical Pacific to a large extent is modulated by energy flow through the ocean surface. While redistribution of OHC within the tropical Pacific is an integral part of ENSO dynamics, variability of ocean heat transport out of the tropical Pacific region is found to be mostly small. Noteworthy contributions arise from the Indonesian Throughflow (ITF), which is anticorrelated with ENSO at a few months lag, and from anomalous oceanic poleward heat export during the La Niña events in 1999 and 2008. Regression analysis reveals that atmospheric energy transport and radiation at the top of the atmosphere (RadTOA) almost perfectly balance the OHC changes and ITF variability associated with ENSO. Only a small fraction of El Niño–related heat lost by the Pacific Ocean through anomalous air–sea fluxes is radiated to space immediately, whereas the major part of the energy is transported away by the atmosphere. Ample changes in tropical atmospheric circulation lead to enhanced surface fluxes and, consequently, to an increase of OHC in the tropical Atlantic and Indian Ocean that almost fully compensates for tropical Pacific OHC loss. This signature of energy redistribution is robust across the employed datasets for all three tropical ocean basins and explains the small ENSO signal in global mean RadTOA.

The Indonesian Throughflow response to Indo‐Pacific climate variability

AbstractThe 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 Niño‐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 Niño and positive IOD episodes consistently show contradictory results from those composites constructed for purely El Niño 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.

Indonesian Throughflow and monsoon activity records in the Timor Sea since the last glacial maximum

Abstract Indonesian Throughflow (ITF) is known to play an important role in the heat exchange between the Pacific and the Indian Oceans. However, our understanding of the long-term evolution of the ITF and, in particular, the mechanism of heat transport is limited. Here, we present a high-resolution foraminifera-based multi-proxy study in the main ITF outflow area of the Timor Sea, to reconstruct the ITF variability and to understand the relationship between the ITF changes and monsoon activity from the last glacial maximum (LGM) to the Holocene. Our results show that when the strong surface water ITF occurs, high productivity is related to the mixing of the upper water column owing to the wind-driven upwelling rather than the shoaling of the depth of thermocline (DOT). By contrast, the DOT is affected more strongly by the ITF than by the monsoonal wind-driven upwelling in the Indonesian Seas. During the LGM (23–19 ka) and middle Holocene (8–6 ka), warm surface water ITF was dominated owing to the lowered sea level and (or) the higher steric height difference between the western Pacific and eastern Indian Oceans as a result of the strong southeast monsoon. During the early Holocene (11–8 ka) and late Holocene (last ~ 6 ka), because of the postglacial high sea level, the strong northwest monsoon and heavy rains, large amounts of freshwater flowed into the Java Sea from the South China Sea (SCS). The freshwater plug at the southern tip of the Makassar Strait blocked the warm surface flow, thus initiating the enhanced thermocline ITF. In the Timor Sea, the changes in the vertical profile of the ITF were influenced by the glacio-eustatic sea-level changes that have modified the geometry of the pathways within the Indonesian Seas, as well as by the monsoon activity which was modulated by the changes in the insolation with a precessional cyclicity.

Variability of Indonesian throughflow within Makassar Strait, 2004–2009

In contrast to earlier measurements, January 2004 through May 2009 Makassar Strait velocities within the main Pacific inflow pathway of the Indonesian throughflow (ITF) are larger with a clear signal of the Asian‐Australian monsoon overriding the relatively weak 2006/2007 El Niño and 2007/2008 La Niña. The Makassar flow is thermocline intensified with maximum along‐channel velocity of −0.8 m/s near 120 m during the southeast monsoon, July to September, decreasing to −0.6 m/s from October to December, during the transition to the northwest monsoon. The temperature variability is highly correlated to ENSO, and the salinity variability reveals low‐salinity surface water inputs to the ITF, possibly from the Java and Sulu seas. Empirical Orthogonal Function (EOF) analysis of the velocity profile reveals that the first mode (45%) is dominated by the intrusions of Kelvin waves from the south, the second mode (30%) reflects ENSO modulation, and the third mode (17%) is associated with regional monsoon winds. The strength of the northward intrusions of Kelvin waves plays an important role in the total transport. The 2004–2009 average seasonal transport varied from −15.5 Sv (Sv = 106 m3/s) during the northwest monsoon (January to March) to −9.6 Sv during the monsoon transition (October to December). The annual mean transport is southward at 13.3 ± 3.6 Sv, with small year‐to‐year range from 12.5 to 14.0 Sv, substantially higher than measurements from 1997 when El Niño suppressed the transport (9.2 Sv).