Indonesian Throughflow Region Research Articles

Assessment of Indonesian Throughflow transports from ocean reanalyses with mooring-based observations

Abstract. The transport of heat and freshwater from the Pacific Ocean to the Indian Ocean via the Indonesian seas is commonly referred to as the Indonesian Throughflow (ITF). The interaction between the ITF and large-scale phenomena occurring from intraseasonal to decadal timescales reflects its connection to the global ocean and the climate system, indicating the need for monitoring the ITF region. In situ observations in this region are highly valuable, but they are temporally and spatially insufficient for near-real-time monitoring. Ocean reanalyses have the potential to serve as near-real-time monitoring tools and to extend time series backward in time, yet a comprehensive quality assessment of their realism in this region with challenging bathymetry has been lacking so far. We focus on oceanic transports diagnosed from the Copernicus Marine Service (CMEMS) Global Reanalysis Ensemble Product (GREP) and the higher-resolution product GLORYS12V1, totaling six reanalysis products. They are validated against in situ observations taken from two different monitoring programs, namely International Nusantara Stratification and Transport (INSTANT 2004–2006) and Monitoring the Indonesian Throughflow (MITF 2006–2011 and 2013–2017), resulting in a total time series of about 11.5 years in the major inflow passage of the Makassar Strait and shorter sampled time series in the Lombok Strait, the Ombai Strait, and the Timor Passage. Results show that there is reasonable agreement between reanalysis-based transports and observations in terms of means, seasonal cycles, and variability, although some shortcomings stand out. The lower-resolution products do not represent the spatial structure of the flow accurately. They also tend to underestimate the integrated net flow in the narrower straits of Lombok and Ombai, an aspect that is improved in GLORYS12V1. Reanalyses tend to underestimate the effect of seasonal Kelvin waves on the transports, which leads to errors in the mean seasonal cycle. Interannual variations of reanalyzed transports agree well with observations, but uncertainties are much larger on sub-annual variability. Finally, as an application of physically consistent and observationally constrained fields provided by ocean reanalyses, we study the impact of the vertically varying pressure gradient on the vertical structure of the ITF to better understand an apparent two-layer regime of the flow.

The MJO-driven Indo-Pacific barotropic see-saw

Recent discovery of a see-saw in intraseasonal barotropic sea level driven by boreal winter Madden-Julian Oscillation winds in the tropical Indo-Pacific basin has renewed interest in the barotropic dynamics in the tropics, which was otherwise known to be prominently a region of baroclinic dynamics at intraseasonal timescales. In this study, using a reference model simulation and several sensitivity experiments, we provide comprehensive details of the dynamics associated with this see-saw. It is found that the narrow Indonesian straits are instrumental in relaying barotropic excitations from one basin to the other. The intraseasonal barotropic circulation associated with this see-saw dynamics appears well organized at basin scale, both in the Indian Ocean and in the Pacific Ocean. During the positive phase of the see-saw, the Madden-Julian Oscillation winds over the Maritime Continent drive a basin-wide anti-clockwise circulation of ∼2 Sv around the Australian Continent with the eastern arm of this circulation located along the west African coast. Concurrently, it also drives a clockwise circulation in the southern Pacific Ocean. This circulation pattern reverses during the negative phase of the see-saw. In the Indonesian Throughflow region, the relatively narrow Ombai Strait and Lombok Strait facilitate this circulation in comparison to the wider Timor Passage. We discuss and point out the significant differences in this flow pattern with the mean flow across the Indonesian seas. We also address how some regions in the northern Pacific Ocean are decoupled from this large-scale dynamics. We show that it takes about a week for both the basins to adjust and to establish the see-saw, once the Madden-Julian Oscillation winds reach the Maritime Continent.

Trend of annual and semiannual harmonics of sea surface temperature in the tropical Indian Ocean, 1979–2018

In the Indian Ocean (IO), the annual and semiannual oscillations of the sea surface temperature (SST) contribute more than 80% of the total variance. It is known that the SST in the tropical IO has warmed at a rate of 0.15 °C/decade since 1979. However, such an estimate of the decadal trend of the annual and semiannual harmonics of SST remains unknown, despite being a strong component of the SST signal. Here we use a widely accepted data product (TropFlux) to quantify the annual and semiannual harmonics in the tropical IO for the first time. The northern IO has a distinctly strong semiannual cycle (1–1.8 °C), which is showing an amplifying trend (∼ 0.04 °C/decade) since 1979. Furthermore, a damping trend in annual amplitude exists over much of the IO, except along the northwestern Bay of Bengal, the Seychelles-Chagos thermocline ridge, the Persian Gulf, and the Indonesian throughflow region. The estimated damping of the annual amplitude is highest over the Arabian Sea. In contrast, the southern tropical IO has a predominant annual cycle, the amplitude of which has weakened during 1979–2018. In the north IO, net air-sea heat flux and vertical processes show a dominant semiannual harmonic oscillation similar to SST. Additionally, the annual amplitude is influential over southern IO. Horizontal advection contributes significantly along the boundary-current regions and the western equatorial IO. Our analysis of combined annual and semiannual cycles shows significant warming in the tropical IO during October–November and cooling during July–August owing to the amplification of semiannual oscillation. These changes in the SST can influence air-sea interaction processes such as cyclogenesis and monsoon.

Plio-Pleistocene Planktic Foraminiferal Biochronology of ODP Site 762B, Exmouth Plateau, Southeast Indian Ocean

ABSTRACTThe Indonesian Throughflow region connects the tropical Pacific Ocean with the eastern Indian Ocean, and variability of the Throughflow during the Plio-Pleistocene has been related to major climate shifts at the global level. Planktic foraminiferal biostratigraphy integrated with magnetochronology provides a robust time framework for assigning age control to major paleoceanographic events. Understanding of the coupled histories of the El Niño Southern Oscillation, Western Pacific Warm Pool, Indonesian Throughflow, and the eastern Indian Ocean has greatly benefitted from the chronological framework provided by planktic foraminiferal biostratigraphies from these regions. Ocean Drilling Program (ODP) Hole 762 lies under the influence of the Leeuwin Current, originating from the Indonesian Throughflow. Multiproxy data have been collected from Hole 762B and other nearby sites for paleoceanographic interpretation. However, a detailed planktic foraminiferal biostratigraphy integrated with magnetochronology is not available from this site. We provide here the sequential order of planktic foraminiferal First Occurrence (FO) and Last Occurrence (LO) events, which allowed us to divide the examined section into seven biostratigraphic zones. The biostratigraphy was integrated with magnetostratigraphy using revised ages of magnetochrons, which yielded biochronological age estimates for planktic foraminiferal events. A major planktonic faunal turnover between 3.4 and 2.7 Ma is probably related to the onset of the Northern Hemisphere glaciation and related changes in the Indonesian Throughflow. We have compared our numerical age estimates with published ages from other parts of the world. The diachronism observed probably is related to the local oceanographic setting of ODP Hole 762B, which has been alternatively influenced by changing strengths of the warm Leeuwin Current and the cold west Australian Current. The biochronology established here will be useful for correlating paleoceanographic events in the region.

Climatic and tectonic constraints on the Plio–Pleistocene evolution of the Indonesian Throughflow intermediate water recorded by benthic δ18O from IODP site U1482

The Indonesian Throughflow (ITF) is the unique low-latitude connector between the Pacific and Indian oceans. During the Plio–Pleistocene, the ITF may have played a significant role in the global climate associated with the tectonic constriction of the Indonesian gateway, which was possibly one of the factors spurring the Northern Hemisphere glaciation. Persistent constriction of the gateway would have restricted the ITF intermediate water (ITF-IW) with a high priority over the ITF upper waters. Due to limited availability of sediment archives, however, no investigations have been carried out on the Plio–Pleistocene evolution of the ITF-IW. In this study, we present our work on a total of 1368 sediment samples spanning the past ∼3.17–1.07 Ma from Site U1482, which was drilled during the International Ocean Discovery Program Expedition 363 in the Timor Passage, the main exit of the ITF to the Indian Ocean. Site U1482 is bathed by the ITF-IW, the evolution of which could thus be monitored by benthic foraminiferal shells preserved in the sediments of the site. Based on establishment of the first high-resolution (∼1.53 kyr) Plio–Pleistocene benthic δ18O stratigraphy within the ITF region, we recognized that, superimposed over the prominent 41-kyr glacial–interglacial cycles, the long-term change of Site U1482 benthic δ18O was punctuated at ∼2.51–2.43 Ma (Marine oxygen isotope stage (MIS)100/99–96/95) and at ∼1.6 Ma (MIS 56/55), followed by an interval of decreased glacial–interglacial amplitudes in the duration of ∼1.6–1.2 Ma. We propose that Site U1482 benthic δ18O was mainly driven by the global ice-sheet growth prior to ∼1.6 Ma, in addition to possibly influenced by the tectonic constriction of the Indonesian gateway. In contrast, the decreased glacial–interglacial amplitudes in the benthic δ18O during the interval of ∼1.6–1.2 Ma more likely indicate the regional signal carried by the ITF-IW, which is attributed to a freshening of the ITF surface waters that was subsequently transmitted to the intermediate depths via vertical mixing in the Banda Sea, caused by enhanced regional precipitation related to monsoonal and western Pacific warm pool conditions.

Part I: Hydrological properties within the eastern Indonesian throughflow region during the INDOMIX experiment

The Indonesian Mixing (INDOMIX) cruise of July 2010, resolves the Pacific Ocean water masses spreading from the Halmahera Sea through the Seram Sea, Manipa Strait, Banda Sea, to the Ombai Strait, and along the southern margin of Lesser Sunda Arc across the northern Savu Sea, Sumba Strait, and south of Lombok Strait. This paper focuses on the characteristics and stratification of the water masses, along this path and Part II of this study discusses the biogeochemical aspects. Two companion papers on quantification of turbulent mixing have been published elsewhere. We find: a) A marked transformation of South Pacific (SP) thermocline water within a sharp salinity front between the Halmahera Seram Seas. Tidally induced vertical mixing is the main process weakening the SP stratification, as suggested by a simple 1-dimensional diffusion model forced by vertical diffusivity of INDOMIX vertical microstructure profiler dataset. The transformation of SP water by vertical mixing can occur after 3 days, which is in good agreement with previously reported water mass residence times. Lateral advection plays a minor role; b) Interleaving salinity structure within the thermocline of the central Halmahera Sea and two deeper inflow channels. The interleaving salinity features within the central Halmahera Sea thermocline occurs where vertical diffusivity is relatively weak, compared to the vertical mixing at entry/exit portal that removes the interleaving salinity features. Two deeper inflow channels (about 950 m and 740 m depth) in the entry portal of the Halmahera Sea are mapped from the multi-beam echo sounder measurement, where both channels allow the renewal of SP thermocline water and salinity interleaving processes; c) Northward flow of thermocline water (100–200 m depth) in the Manipa Strait that injects Banda homogeneous salinity water into Seram Sea that erode Halmahera salty water via strong diapycnal mixing; d) Cyclonic upper layer circulation in Banda where eastward (westward) flow occurs in the northern (southern) Banda, confirmed by recent modeling study; and e) Relative salty Indian Ocean intermediate water flowing along southern margin of the Lesser Sunda Arc, within the South Java Undercurrent. Two distinct upper thermocline water, due to different source of Indonesian Throughflow (ITF) water, are found in the Banda Sea: fresher and colder water contrasting to saltier and warmer water that converge in this confluence region. Beneath it, Banda intermediate homogeneous salinity water and low dissolved oxygen water is dominant.

EFFECT OF MONSOON PHENOMENON ON SEA SURFACE TEMPERATURES IN INDONESIAN THROUGHFLOW REGION AND SOUTHEAST INDIAN OCEAN

Indonesia is influenced by two types of monsoons, namely, the Asian and Australian monsoons. The differences in conditions occurring during these monsoon phenomena can affect sea surface temperatures (SSTs). This study aims to determine the effect of these monsoons on the SSTs in the southeast Indian Ocean and the Indonesian throughflow (ITF) region. SST and geostrophic current data obtained from Copernicus Marine Service and surface wind speed and direction data from the European Center for Medium-Range Weather Forecasts (ECMWF) from March 2019 to February 2020 were statistically and descriptively analyzed. Observational conductivity–temperature–depth (CTD) data obtained in December 2019 were used to identify statistical errors in the Copernicus Marine Service SST data. The results of the SST data verification show a 0.85°C RMSE and 0.6°C MAE; they are significantly correlated at 0.82 with a 95% confidence level. The results of this study generally show that geostrophic currents move to the east, and SST tends to be warmer during the Asian monsoon period than during the Australian monsoon period, which has a cooler SST (with geostrophic currents moving to the northwest). Specifically, the SST conditions in the ITF region and southeast Indian Ocean cool from the MAM period. This cooling period intensifies during the JJA period and subsides in the SON period. The Australian monsoon, which is dominant during the DJF period, causes warmer-than-average SST conditions in the northern part of Indonesia, particularly the northern part of the ITF.

Seasonal variation of the Sunda Shelf Throughflow

The Sunda Shelf Throughflow (SSTF) is defined as a seasonal reversal shallow current system flowing from the South China Sea to the Indonesian Throughflow (ITF) region via the Natuna and Java Seas. This current system plays an important role on transferring heat and freshwater fluxes into the ITF system and vice versa. This paper aims to investigate seasonal changes of transport volume in three sections within the Sunda Shelf region (namely Natuna, Karimata and Java section). We used the validated daily ocean circulation model outputs from INDESO configuration between 2008 and 2014. It is shown that mean transport volume of the SSTF is relatively small on the order of -0.5Sv toward the ITF region (1Sv=106m3/s) due to a strong seasonal reversal flow of the SSTF forced by the monsoonal winds. However, on seasonal scale this transport volume strengthens drastically to about 2-3Sv (1-2 Sv) during the northwest (southeast) monsoon period, which is in good agreement with previous studies. Transport estimate in Java section is reduced to about -0.1Sv may be due to a leakage of the flow via Sunda Strait and also a strong recirculation of Makassar ITF into Java Sea.

Indonesian throughflow controlled the westward extent of the Indo-Pacific Warm Pool during glacial-interglacial intervals

Abstract The tropical atmospheric circulation is strongly influenced by the sea surface temperature (SST) of the warmest region of the world ocean, the Indo-Pacific Warm Pool (IPWP). The effect of recent anomalous Indian Ocean warming on the extent of the IPWP remains uncertain. Past SST records from the tropical Indian Ocean, associated with different temporal boundary conditions, can help shed light on how climate change impacts the IPWP and associated monsoonal processes. Here, we reconstruct the seawater temperature and precipitation changes (from Mg/Ca and δ18O of surface-dwelling planktic foraminifera Globigerinoides ruber) of the last ~184 kyr BP from the westernmost margin of the IPWP. The data is then used to reconstruct the spatial extent and intensity of the IPWP structure as well as its relationship with other regional phenomena. A consistently warmer (>28 °C) central equatorial Pacific Ocean and significantly cooler equatorial western Pacific and the Indian Ocean suggests that the warm pool extent reduced considerably during MIS6 as well as the last glacial interval, leading to a strong La-Nina like condition. The most enriched seawater δ18O (δ18Osw) during MIS6, was in the westernmost IPWP, suggesting weakened precipitation, despite La-Lina like conditions. The westernmost part of the IPWP became warmer than the threshold 28 °C during MIS5e and the early Holocene, suggesting the westward extension of the IPWP. The eastern equatorial Pacific Ocean also warmed considerably during MIS5e, suggesting a weaker Walker Circulation and a stronger El-Nino. The difference in δ18Osw decreased considerably during MIS5e and is attributed to increased precipitation/freshwater runoff in the central equatorial Indian Ocean. The increase in δ18Osw (~1.5‰) between ~55 kyr until the last glacial maximum, in a majority of the Indian Ocean records, suggests progressively weakened precipitation. A majority of the records have a uniform SST anomaly throughout the Holocene. A warmer equatorial Indian Ocean during interglacials suggests a strong modulation of the IPWP extent by the transport of warm water through the Indonesian throughflow region. The large drop in δ18Osw despite the comparable change in SST during the last glacial interval suggests that monsoon forcing has been much stronger in altering the surface hydrography in the Indian Ocean than the central Pacific Ocean.

Three-ocean interactions and climate variability: a review and perspective

Interactions among the Pacific, Atlantic and Indian Oceans through ocean–atmosphere coupling can initiate and/or modulate climate variability. The Pacific Ocean is home to ENSO which affects other oceans through atmospheric bridges and the oceanic Indonesian throughflow (ITF). A warm Indian Ocean can produce atmospheric Kelvin waves that propagate eastward and increase equatorial easterly wind anomalies in the western Pacific and thus cool eastern Pacific sea surface temperature (SST). A positive Indian Ocean dipole establishes a southwestward pressure gradient force in the ITF region which increases the ITF transport and decreases ocean heat content in the western Pacific and may cool eastern Pacific SST. The Indian Ocean can also influence the Atlantic by atmospheric bridge and the oceanic Agulhas leakage south of Africa. Midlatitude North Atlantic SSTs may affect Pacific climate variability: (1) The Atlantic multidecadal oscillation (AMO) influences North Pacific variability; (2) The warm AMO phase increases the occurrence of central Pacific (CP)-type El Niño; (3) The warm AMO phase helps induce anomalous cyclonic circulation in the tropical western North Pacific; and (4) A cold midlatitude North Atlantic Ocean in the summer may initiate an El Niño in subsequent year via the East Atlantic/West Russia teleconnection. A warm tropical North Atlantic in the spring can induce a CP-type La Niña in the subsequent winter, via two pathways of the tropical eastern North and South Pacific. Finally, the Atlantic Niño (Niña) in the summer, through the Walker circulation and ocean dynamics, helps induce an eastern Pacific-type La Niña (El Niño) in the subsequent winter. The Atlantic Niño can also warm the tropical western Indian Ocean and weaken Indian monsoon rainfall.

Analysis of turbulent mixing in the Eastern Path of Indonesian Throughflow

Several studies confirmed that strong transformation of Indonesian Throughflow (ITF) water appears along the eastern ITF pathway from Maluku to Ceram to Banda Seas. This is, presumably, caused by energetic internal tidal activities that generate turbulent mixing. This study was aimed to analysis the turbulent mixing using Thorpe method from CTD data set acquired during the STOKAS 2015 cruise onboard R.V. Baruna Jaya 7 in the eastern ITF region. The study area was divided into four regions: Sulawesi Sea (SUL), Maluku Sea (MAL), Ceram Sea (CER), and Banda Sea (BAN). Thorpe method provides an approach to quantify the overturn of water mass using Thorpe displacement – validated using Galbraith and Kelley (GK) test – and to calculate the kinetic energy dissipation (ε and vertical diffusivity (Kρ). From diagram T-S, the transformation of ARLINDO waters was identified in the gateway. This is supported by a current pattern in which the ARLINDO water mass recirculated in SUL, then moved southward to MAL. The vertical diffusivity eddy (Kρ) varied in the order of 10−5 to 10−2 m2s−1 and its kinetic energy dissipation (ε) varied in the order 10−11 to 10−5 W/Kg in all regions. This mixing was indicated to be driven by internal tide.

Water mass characteristic in the outflow region of the Indonesian throughflow during and post 2016 negative Indian ocean dipole event

Strong El Niño and positive Indian Ocean Dipole (pIOD) events in 2015/2016 followed by relatively strong negative Indian Ocean Dipole (nIOD) and weak La Niña in 2016 events have affected hydrography conditions in the Indonesian Throughflow (ITF) region. Two research cruises were conducted using RV Baruna Jaya VIII in August and November 2016. These cruises aim to evaluate possible impact of those two climate mode events on the water mass characteristic in the outflow region of the ITF. Hydrographic data from those two cruises were combined with the sea surface temperature (SST) from the Advanced Very High Resolution Radiometer (AVHRR) and surface wind data from the European Centre for Medium-Range Weather Forecasts (ECMWF). The results showed that in the 2016 anomaly year, the cooler sea surface temperature was observed during the negative IOD (nIOD) event while the warmer temperature was found in the post of nIOD event. The observed water mass characteristics in the outflow region of the ITF revealed that the upper layer was dominated by the Indian Ocean water mass, while the Pacific Ocean water mass was observed in the deeper layer. The observed current data across the Sumba Strait showed that the South Java Coastal Current (SJCC) was observed in the upper layer, propagating eastward toward the Savu Sea. A few days later, the observed currents in the upper layer of the Ombai Strait revealed the ITF flow towards the Indian Ocean. Meanwhile, the lower layer showed an eastward flow towards the Ombai Strait.

Geochemical characteristics from tests of four modern planktonic foraminiferal species in the Indonesian Throughflow region and their implications

Test geochemistry of planktonic foraminifera is an indispensable tool in reconstructing past ocean hydrological changes. It is essential to investigate region-specific implications of test geochemistry, although those established from other regions can be broadly applied. In this study, characteristics of δ18O and Mg/Ca from tests of four planktonic foraminiferal species, Globigerinoides ruber sensu stricto (s.s.), Globigerinoides sacculifer, Pulleniatina obliquiloculata and Neogloboquadrina dutertrei, from 60 core-top sediment samples retrieved from the Indonesian Throughflow (ITF) region were studied. These geochemical data were compared with modern hydrographic profiles in order to assess their relations and to investigate potential implications of test geochemical parameters in reconstructing past oceanographic change in the ITF region. Calcification depths of these four species were first estimated based on comparison between measured test δ18O and predicted calcite δ18O that was calculated from modern temperature and salinity. The results indicate that G. ruber s.s. and G. sacculifer calcify within the mixed-layer at 0–50 m and 20–75 m, respectively, whereas P. obliquiloculata and N. dutertrei calcify within the thermocline at around 75 to 125 m. A combined study of excess Mg/Ca (difference between measured and predicted Mg/Ca) and salinity suggests that salinity exerts a negligible impact on test Mg/Ca of these foraminiferal species in the ITF region. Comparison of test Mg/Ca-derived temperatures with temperature profiles of the upper 200 m of the water column from the seas of the ITF region also indicate calcification depths of these species, which match well with the above estimations using test δ18O. It further indicates that G. sacculifer may be more sensitive in reflecting changes in the depth of the mixed-layer, highlighting a potential use of Mg/Ca temperature difference between G. ruber s.s. and G. sacculifer in reconstructing the depth of the mixed-layer in the ITF region.

SPCZ zonal events and downstream influence on surface ocean conditions in the Indonesian Throughflow region

AbstractSeasonal 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.

Stable carbon and oxygen isotopes of four planktonic foraminiferal species from core-top sediments of the Indonesian throughflow region and their significance

Horizontal and vertical distributions of δ18O and δ13C were investigated in shells of four planktonic foraminiferal species, Globigerinoides ruber, Globigerinoides sacculifer, Pulleniatina obliquiloculata and Neogloboquedrina dutertrei, from a total of 62 core-top sediment samples from the Indonesian throughflow region. Results were compared to modern hydrologic conditions in order to explore potential of proxies in reconstructing fluvial discharge and upper ocean water column characteristics in this region. Our results show that, in the Makassar Strait, both of depleted δ18O and δ13C of these four species were linked to freshwater input. In the Bali Sea, however, depleted δ18O and δ13C for these species may be due to different reasons. Depleted δ18O was a result of freshwater input and as well influenced by along-shore currents while depleted δ13C was more likely due to the Java-Sumatra upwelling. Comparison of shell δ18O records and hydrographic data of World Ocean Atlas 2005 suggests that G. ruber and G. sacculifer calcify within the mixed-layer, respectively at 0–50 m and 20–75 m water depth, and P. obliquiloculata and N. dutertrei within the upper thermocline, both at 75–125 m water depth. N. dutertrei calcifies at slightly deeper water depth than P. obliquiloculata does. In general, δ13C values of both G. ruber and G. sacculifer are larger than those of P. obliquiloculata and N. dutertrei at all sites, possibly related to depth habitats of these species and vertical distribution of nutrients in the Indonesian throughflow region.

Pathways of intraseasonal Kelvin waves in the Indonesian Throughflow regions derived from satellite altimeter observation

AbstractThe gridded sea level anomaly (SLA) data-set provided by AVISO is used to track the propagation of intraseasonal Kelvin waves in the Indonesian Throughflow (ITF) region. The large root mean square of intraseasonal SLA along the Sumatra and Java coast is closely related to the propagation of intraseasonal Kelvin waves that derive from the equatorial Indian Ocean. These Kelvin waves are further found to propagate following different pathways at the Lombok Strait. Pathway A propagates eastward throughout the Sumba Strait and Savu Sea to reach the Ombai Strait. Pathway B penetrates into Lombok and propagates northward to reach the Makassar Strait. Pathway C propagates southeastward along the southwest coast of the Sumba Island. The equatorial Kelvin waves take around 15 days to travel from the equatorial Indian Ocean to Lombok Strait, and around 5 days to penetrate into the Makassar and Ombai straits. The Kelvin wave-induced SLA persists in the ITF region for an additional 5 days and then diminishes s...

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.

A model for quantifying oceanic transport and mesoscale variability in the Coral Triangle of the Indonesian/Philippines Archipelago

[1] The Indonesian Throughflow region (ITF) continues to pose significant research challenges with respect to its role in the global ocean circulation, the climate system, and the ecosystem sustainability in this region of maximum marine biodiversity. Complex geography and circulation features imply difficulties in both observational and numerical studies. In this work, results are presented from a newly developed high-resolution model for the Coral Triangle (CT) of the Indonesian/Philippines Archipelago specifically designed to address regional physical and ecological questions. Here, the model is used to quantify the transport through the various passages, sea surface temperature and mesoscale variability in the CT. Beyond extensive skill assessment exhibiting the model ability to represent many conspicuous features of the ITF, the high-resolution simulation is used to describe the mesoscale and submesoscale circulation through the application of Finite Size Lyapunov Exponents (FSLEs). The distribution of FSLEs is used to quantify the spatiotemporal variability in the regional mixing characteristics. The modeled seasonal and interannual variability of mixing suggests a link to large-scale climate signals such as ENSO and the Asian-Australian monsoon system.

Imprint of eastern Indian Ocean surface oceanography on modern organic-walled dinoflagellate cyst assemblages

Abstract Assemblages of organic-walled dinoflagellate cysts (dinocysts) from 116 marine surface samples have been analysed to assess the relationship between the spatial distribution of dinocysts and modern local environmental conditions [e.g. sea surface temperature (SST), sea surface salinity (SSS), productivity] in the eastern Indian Ocean. Results from the percentage analysis and statistical methods such as multivariate ordination analysis and end-member modelling, indicate the existence of three distinct environmental and oceanographic regions in the study area. Region 1 is located in western and eastern Indonesia and controlled by high SSTs and a low nutrient content of the surface waters. The Indonesian Throughflow (ITF) region (Region 2) is dominated by heterotrophic dinocyst species reflecting the region's high productivity. Region 3 is encompassing the area offshore north-west and west Australia which is characterised by the water masses of the Leeuwin Current, a saline and nutrient depleted southward current featuring energetic eddies.

Pacific Ocean Contribution to the Asymmetry in Eastern Indian Ocean Variability

Abstract Variations in eastern Indian Ocean upper-ocean thermal properties are assessed for the period 1970–2004, with a particular focus on asymmetric features related to opposite phases of Indian Ocean dipole events, using high-resolution ocean model hindcasts. Sensitivity experiments, where interannual atmospheric forcing variability is restricted to the Indian or Pacific Ocean only, support the interpretation of forcing mechanisms for large-scale asymmetric behavior in eastern Indian Ocean variability. Years are classified according to eastern Indian Ocean subsurface heat content (HC) as proxy of thermocline variations. Years characterized by an anomalous low HC feature a zonal gradient in upper-ocean properties near the equator, while high events have a meridional gradient from the tropics into the subtropics. The spatial and temporal characteristics of the seasonal evolution of HC anomalies for the two cases is distinct, as is the relative contribution from Indian Ocean atmospheric forcing versus remote influences from Pacific wind forcing: low events develop rapidly during austral winter/spring in response to Indian Ocean wind forcing associated with an enhanced southeasterly monsoon driving coastal upwelling and a shoaling thermocline in the east; in contrast, formation of an anomalous high eastern Indian Ocean HC is more gradual, with anomalies earlier in the year expanding from the Indonesian Throughflow (ITF) region, initiated by remote Pacific wind forcing, and transmitted through the ITF via coastal wave dynamics. Implications for seasonal predictions arise with high HC events offering extended lead times for predicting thermocline variations and upper-ocean properties across the eastern Indian Ocean.