Eastern Tropical Indian Ocean Research Articles

Variability in Makassar Strait Heat Flux and Its Effect on the Eastern Tropical Indian Ocean

Variability in Makassar Strait Heat Flux and Its Effect on the Eastern Tropical Indian Ocean

Evidence of solar insolation and internal forcing of sea surface temperature changes in the eastern tropical Indian Ocean during the Holocene

The forcing and mechanisms governing sea surface temperature (SST) variations in the Indo-Pacific Warm Pool (IPWP) are complex. Insight into the full spectrum of IPWP climate dynamics, however, is limited by the spatial and temporal coverage of the climate data. In particular, the relationships among the changes in the SSTs, the precipitation patterns mainly associated with rainfalls from Asian Monsoons (AM), and the Indian Ocean Dipole (IOD) are poorly understood. To help us further assess the climate linkages, we have reconstructed a more spatial SST pattern during the Holocene by using multiple SST proxies (alkenone unsaturation index U37k' and Mg/Ca of planktic foraminifer) in the Eastern Tropical Indian Ocean (ETIO), the western margin of the IPWP based on three sedimentary cores from NW offshore of Sumatra, and offshore of Sumatra and Java (BS24, SO139-74KL, and SO184-100430). Stable hydrogen and carbon isotope records of terrestrial plant waxes from a nearby marine sediment core SO189-144KL and geochemical tracers measured from the coral reefs within the Mentawai Islands in the ETIO are used here as AM driven precipitation and IOD records in our data synthesis. Not surprisingly, our synthesis suggests that insolation plays a major role that has been responsible for the increased SSTs in ETIO since the early Holocene, while other mechanisms remain effective in determining the timing of our reconstructed SST variations. In particular, our SST pattern shares less similarity with that of coral Sr/Ca SST and is decoupled from the coral IOD events in the mid-Holocene. We interpret that our reconstructed ETIO SSTs are driven dominantly by the solar forcing, but are also affected by other internal climate mechanisms such as the local shifts in AM-controlled upwelling and precipitation, episodic reductions in the flow of warm western Pacific surface water into the Indian Ocean due to increased precipitation over the Indonesian archipelago, and long-term ENSO or IOD-like climate change.

Geography and island geomorphology shape fish assemblage structure on isolated coral reef systems.

We quantify the relative importance of multi‐scale drivers of reef fish assemblage structure on isolated coral reefs at the intersection of the Indian and Indo‐Pacific biogeographical provinces. Large (>30 cm), functionally‐important and commonly targeted species of fish, were surveyed on the outer reef crest/front at 38 coral reef sites spread across three oceanic coral reef systems (i.e. Christmas Island, Cocos (Keeling) Islands and the Rowley Shoals), in the tropical Indian Ocean (c. 1.126 x 106 km2). The effects of coral cover, exposure, fishing pressure, lagoon size and geographical context, on observed patterns of fish assemblage structure were modelled using Multivariate Regression Trees. Reef fish assemblages were clearly separated in space with geographical location explaining ~53 % of the observed variation. Lagoon size, within each isolated reef system was an equally effective proxy for explaining fish assemblage structure. Among local‐scale variables, ‘distance from port’, a proxy for the influence of fishing, explained 5.2% of total variation and separated the four most isolated reefs from Cocos (Keeling) Island, from reefs with closer boating access. Other factors were not significant. Major divisions in assemblage structure were driven by sister taxa that displayed little geographical overlap between reef systems and low abundances of several species on Christmas Island corresponding to small lagoon habitats. Exclusion of geographical context from the analysis resulted in local processes explaining 47.3% of the variation, highlighting the importance of controlling for spatial correlation to understand the drivers of fish assemblage structure. Our results suggest reef fish assemblage structure on remote coral reef systems in the tropical eastern Indian Ocean reflects a biogeographical legacy of isolation between Indian and Pacific fish faunas and geomorphological variation within the region, more than local fishing pressure or reef condition. Our findings re‐emphasise the importance that historical processes play in structuring contemporary biotic communities.

Cold Season Southwest Asia Precipitation Sensitivity to El Niño–Southern Oscillation Events

Abstract The sensitivity of southwest Asia (25°–40°N, 40°–70°E) precipitation during the November–April rainy season to four types of El Niño–Southern Oscillation (ENSO) events, eastern Pacific (EP) and central Pacific (CP) El Niño and La Niña, is assessed using an ensemble of atmospheric model simulations forced by 1979–2015 boundary conditions. Sensitivity is assessed in terms of 1) the spread of precipitation across the ensemble members around the ensemble mean, 2) the probability of precipitation falling into the upper and lower terciles of the historical distribution, and 3) the relationship between the tropical atmosphere and southwest Asia precipitation during ENSO. During CP La Niña, the magnitude of the below-average mean precipitation exceeds the magnitude of the precipitation spread, thereby conditioning the probability of lower-tercile southwest Asia precipitation to greater than 70%. By contrast, EP La Niña does not alter the odds of southwest Asia precipitation terciles, as the magnitude of the near-zero mean precipitation is overwhelmed by the magnitude of the precipitation spread. EP and CP El Niño similarly result in above-average mean precipitation whose magnitude approaches the magnitude of the precipitation spread, thereby conditioning the probability of upper-tercile southwest Asia precipitation to around 50% region-wide. However, the notable effect of the precipitation spread during El Niño allows for a 20%–30% probability that the regional precipitation falls into the lower tercile. ENSO types simultaneously modify the probability of eastern Indian Ocean precipitation and southwest Asia precipitation, supporting the hypothesis that the tropical eastern Indian Ocean atmosphere serves as the medium by which ENSO forcing is communicated to southwest Asia.

Factors controlling the interannual variation of 30–60-day boreal summer intraseasonal oscillation over the Asian summer monsoon region

The 30–60-day boreal summer intraseasonal oscillation (BSISO) is a dominant variability of the Asian summer monsoon (ASM), with its intensity being quantified by intraseasonal standard deviations based on OLR data. The spatial and interannual variations of the BSISO intensity are identified via empirical orthogonal function (EOF) analysis for the period 1981–2014. The first EOF mode (EOF1) shows a spatially coherent enhancement or suppression of BSISO activity over the entire ASM region, and the interannual variability of this mode is related to the sea surface temperature anomaly (SSTA) contrast between the central–eastern North Pacific (CNP) and tropical Indian Ocean. In contrast, the second mode (EOF2) exhibits a seesaw pattern between the southeastern equatorial Indian Ocean (EIO) and equatorial western Pacific (EWP), with the interannual fluctuation linked with developing ENSO events. During strong years of EOF1 mode, the enhanced low-level westerlies induced by the summer-mean SSTA contrast between the warmer CNP and cooler tropical Indian Ocean tend to form a wetter moisture background over the eastern EIO, which interacts with intraseasonal low-level convergent flows, leading to stronger equatorial eastward propagation. The intensified easterly shear favors stronger northward propagation over the South Asian and Eastern Asian/Western North Pacific sectors, respectively. Opposite situation is for weak years. For interannual variations of EOF2 mode, the seesaw patterns with enhanced BSISO activity over the southeastern EIO while weakened activity over the EWP mostly occur in the La Nina developing summers, but inverse patterns appear in the El Nino developing summers.

Tropical Indo-Pacific hydroclimate response to North Atlantic forcing during the last deglaciation as recorded by a speleothem from Sumatra, Indonesia

Abrupt changes in Atlantic Meridional Overturning Circulation are known to have affected the strength of the Indian and Asian Monsoons during glacial and deglacial climate states. However, there is still much uncertainty around the hydroclimate response of the Indo-Pacific Warm Pool (IPWP) region to abrupt climate changes in the North Atlantic. Many studies suggest a mean southward shift in the intertropical convergence zone (ITCZ) in the IPWP region during phases of reduced Atlantic meridional overturning, however, existing proxies have seasonal biases and conflicting responses, making it difficult to determine the true extent of North Atlantic forcing in this climatically important region.Here we present a precisely-dated, high-resolution record of eastern Indian Ocean hydroclimate variability spanning the last 16 ky (thousand years) from δ18O measurements in an aragonite–calcite speleothem from central Sumatra. This represents the western-most speleothem record from the IPWP region. Precipitation arrives year-round at this site, with the majority sourced from the local tropical eastern Indian Ocean and two additional long-range seasonal sources associated with the boreal and austral summer monsoons. The Sumatran speleothem demonstrates a clear deglacial structure that includes 18O enrichment during the Younger Dryas and 18O depletion during the Bølling–Allerød, similar to the pattern seen in speleothems of the Asian and Indian monsoon realms. The speleothem δ18O changes at this site are best explained by changes in rainfall amount and changes in the contributions from different moisture pathways. Reduced rainfall in Sumatra during the Younger Dryas is most likely driven by reductions in moisture transport along the northern or southern monsoon transport pathways to Sumatra. Considered with other regional proxies, the record from Sumatra suggests the response of the IPWP to North Atlantic freshwater forcing is not solely driven by southward shifts of the ITCZ, but also a reduction in moisture transport along both monsoon pathways.

Severe Drought Event in Indonesia Following 2015/16 El Niño/positive Indian Dipole Events

During boreal fall and winter 2015/16, Indonesia experienced catastrophic drought event causing many environmental problems. This study explored dynamical evolution of drought event in Indonesia associated with those two climate modes. Based on the Niño3.4 index, the evolution of the El Niño has started in April 2015, reached its peak in January 2016 and terminated in April 2016. Meanwhile, the Dipole Mode Index (DMI) revealed that the evolution of positive Indian Ocean Dipole has started in August, reached its peak in September and terminated in November 2015. It is shown that during those two events, Indonesia experienced severe drought in which the precipitation was extremely decreased. During the peak drought condition co-occurring with the dry season, the anomalous of precipitation reached −450 mm/month in September 2015. The peak of the drought was associated with the El Niño and positive Indian Ocean Dipole sea surface temperature anomaly (SSTA) patterns, in which negative SSTA covered the eastern tropical Indian Ocean and the western Pacific Ocean including Indonesia seas. Meanwhile, positive SSTA observed in the western tropical Indian Ocean and Eastern Pacific Ocean.

Structuring of larval fish assemblages along a coastal-oceanic gradient in the macro-tidal, tropical Eastern Indian Ocean

In the eastern Indian Ocean, the Kimberley region off north-western Australia is characterised by a wide continental shelf, complex coastal topography and very large tides (>10 m). Diversity of fishes is high although little is known about the distribution of their larvae in this dynamic region. This study investigated spatial variation in ichthyoplankton assemblages along four coastal-oceanic transects and related the results to tidal cycles and other environmental variables. Larvae representing 92 neritic and 21 mesopelagic teleost families were collected; this far exceeds the diversity described for other comparable tropical continental shelf and oceanic systems. Distinct ichthyoplankton assemblages characterised inner shelf (Engraulidae and Gobiidae were dominant families), outer shelf / slope (mixed neritic and mesopelagic taxa) and oceanic waters (mesopelagic taxa, particularly Myctophidae, Gonostomatidae and Phosichthyidae), and were correlated with changes in environmental variables (water depth, seawater density, mixed layer depth and zooplankton). The spring-neap tidal cycle and subsequent tides (same stations sampled 12 h apart) had no significant influence on larval fish assemblages. Coastal waters were delineated by a density front and larval fish assemblages were significantly different from offshore assemblages. This study provides new insights into the pelagic ecosystem in the tropical eastern Indian Ocean and contributes to the understanding of the role of environmental variables and physical forcing in the structuring of larval fish assemblages.

Effect of Aerosols on Ocean Parameters in India by Using Satellite Data

The rapid industrial and economic development in India leads to high level of pollution in environment. Due to increased level of aerosols oceans are warming. This paper highlights on aerosols effect on sea surface wind and sea surface temperature by using remote sensing data. The windplays important role in Aerosol Optical Depth (AOD) and radiative forcing and is analyzed using National Centers for Environmental Prediction (NCEP) monthly wind data. The radiative forcing observed is much higher and up to 60% of total AOD during summer monsoon. SeaSurface Temperature (SST) is another important parameter in ocean atmosphere system and a key variable in coupling the atmosphere and ocean. The SST has changed during the change of atmospheric pattern and it plays an important role in aerosols mechanism. The study observed that during summer monsoon SST over the South Eastern Tropical Indian Ocean (SETIO) much higher than Western Tropical Indian Ocean (WTIO) and it plays important role in aerosols mechanism.

Enhancement of the spring East China precipitation response to tropical sea surface temperature variability

The boreal spring relationship between variabilities of East China precipitation (ECP) and tropical Ocean sea surface temperature (SST) during the period 1951–2014 is investigated in this study. The results show that the leading mode of the ECP variability exhibits an enhanced response to the anomalous El Nino–Southern Oscillation (ENSO)-like SST after the late 1970s, when the SST underwent a decadal change, with two positive centers over the eastern tropical Pacific (ETP) and tropical Indian Ocean (TIO). To further understand the relative roles of the ETP and TIO SST anomalies (SSTAs) in the variability of ECP after the late 1970s, partial regression and correlation methods are used. It is found that, without the contribution of the TIO, ETP SSTA plays a limited role in the variability of ECP after the late 1970s; comparatively, a significant correlation between TIO SST and ECP is identified during the same period, when the ETP signal is linearly removed. Physical analyses show that, after the late 1970s, the TIO SSTA affects East Asian atmospheric circulation in two ways: by exciting a zonal wave-train pattern over the mid-latitude Eurasian Continent and by inducing anomalous convection over the Maritime Continent. Via these two mechanisms, the TIO SST variability results in an anomalous East Asian trough and vertical motion over East China and consequently leads to anomalous precipitation over the region. The physical processes linking the ECP and TIO SST are confirmed by an atmospheric general circulation model experiment forced with idealized TIO warming.

Assessing the Reliability of Planktic Foraminifera Ba/Ca as a Proxy for Salinity off the Sunda Strait

The Ba/Ca ratio of planktic foraminifera carbonate serves as a new geochemical proxy for seawater Ba/Ca and thus providing information on modern and past salinity and freshwater discharge. In this study the applicability of Ba/Ca ratio of core-top Globigerinoides sacculifer collected off the eastern tropical Indian Ocean (ETIO) for paleoceanographic reconstructions was investigated. In doing so, we conducted a series of cleaning experiments for Ba/Ca extraction by using different reductive solutions. Our new results suggest that the Ba/Ca ratio of G. sacculifer cannot be utilized as a tracer for modern and past salinity changes in the ETIO region off the Sunda Strait. We suggest that the existence of seasonal upwelling adds an additional signal to the seawater Ba/Ca in the ETIO, and thus complicates the interpretation of G. sacculifer Ba/Ca as a freshwater tracer. Moreover, our cleaning experiment results show that the cleaning protocol of Mg/Ca, DTPA, and hydroxylamine can be used to extract valuable Ba/Ca ratios from planktic foraminifera tests. Keywords: G. sacculifer Ba/Ca, foraminifera cleaning experiments, the Sunda Strait.

The tropical Pacific–Indian Ocean associated mode simulated by LICOM2.0

Oceanic general circulation models have become an important tool for the study of marine status and change. This paper reports a numerical simulation carried out using LICOM2.0 and the forcing field from CORE. When compared with SODA reanalysis data and ERSST.v3b data, the patterns and variability of the tropical Pacific–Indian Ocean associated mode (PIOAM) are reproduced very well in this experiment. This indicates that, when the tropical central–western Indian Ocean and central–eastern Pacific are abnormally warmer/colder, the tropical eastern Indian Ocean and western Pacific are correspondingly colder/warmer. This further confirms that the tropical PIOAM is an important mode that is not only significant in the SST anomaly field, but also more obviously in the subsurface ocean temperature anomaly field. The surface associated mode index (SAMI) and the thermocline (i.e., subsurface) associated mode index (TAMI) calculated using the model output data are both consistent with the values of these indices derived from observation and reanalysis data. However, the model SAMI and TAMI are more closely and synchronously related to each other.

Causes of Interannual and Interdecadal Variations of the Summertime Pacific–Japan-Like Pattern over East Asia

A Pacific–Japan (PJ) pattern index is defined based on the singular value decomposition (SVD) analysis of summertime 500-hPa height in East Asia and precipitation in the tropical western North Pacific (WNP). The time series of this PJ index shows clearly the interannual and interdecadal variations since 1948. Idealized atmospheric general circulation model (AGCM) experiments were carried out to understand the remote and local SST forcing in causing the interannual variations of the PJ pattern and interdecadal variations of the PJ-like pattern. It is found that the PJ interannual variation is closely related to El Niño–Southern Oscillation (ENSO). A basinwide warming occurs in the tropical Indian Ocean (TIO) during El Niño mature winter. The TIO warming persists from the El Niño peak winter to the succeeding summer. Meanwhile, a cold SST anomaly (SSTA) appears in the eastern WNP and persists from the El Niño mature winter to the succeeding summer. Idealized AGCM experiments that separate the TIO and WNP SSTA forcing effects show that both the remote eastern TIO forcing and local WNP SSTA forcing are important in affecting atmospheric heating anomaly in the WNP monsoon region, which further impacts the PJ interannual teleconnection pattern over East Asia. In contrast to the interannual variation, the interdecadal change of the PJ-like pattern is primarily affected by the interdecadal change of SST in the TIO rather than by the local SSTA in the WNP.

Late Holocene slowdown of the Indian Ocean Walker circulation

Changes in tropical zonal atmospheric (Walker) circulation induce shifts in rainfall patterns along with devastating floods and severe droughts that dramatically impact the lives of millions of people. Historical records and observations of the Walker circulation over the 20th century disagree on the sign of change and therefore, longer climate records are necessary to better project tropical circulation changes in response to global warming. Here we examine proxies for thermocline depth and rainfall in the eastern tropical Indian Ocean during the globally colder Last Glacial Maximum (19–23 thousand years ago) and for the past 3000 years. We show that increased thermocline depth and rainfall indicate a stronger-than-today Walker circulation during the Last Glacial Maximum, which is supported by an ensemble of climate simulations. Our findings underscore the sensitivity of tropical circulation to temperature change and provide evidence for a further weakening of the Walker circulation in response to greenhouse warming.

Indo-Pacific climate during the decaying phase of the 2015/16 El Niño: role of southeast tropical Indian Ocean warming

This study investigates the influence of southeast tropical Indian Ocean (SETIO) sea surface temperature (SST) warming on Indo-Pacific climate during the decaying phase of the 2015/16 El Nino by using observations and model experiments. The results show that the SETIO SST warming in spring 2016 enhanced local convection and forced a “C-shape” wind anomaly pattern in the lower troposphere. The “C-shape” wind anomaly pattern over the eastern tropical Indian Ocean consists of anomalous westerly flow south of the equator and anomalous easterly flow north of the equator. The anomalous easterly flow then extended eastward into the western North Pacific (WNP) and facilitates the development or the maintenance of an anomalous anticyclone over the South China Sea (SCS). Correspondingly, the eastern part of the Bay of Bengal, the SCS and the WNP suffered less rainfall. Such precipitation features and the associated “C-shape” wind anomaly pattern shifted northward about five latitudes in summer 2016. Additionally, the SETIO warming can induce local meridional circulation anomalies, which directly affect Indo-Pacific climate. Numerical model experiments further confirm that the SETIO SST warming plays an important role in modulating Indo-Pacific climate.

The 2015/2016 El Niño Event in Context of the MERRA-2 Reanalysis: A Comparison of the Tropical Pacific with 1982/1983 and 1997/1998.

The 2015/2016 El Niño is analyzed using atmospheric/oceanic analysis produced using the Goddard Earth Observing System (GEOS) data assimilation systems. As well as describing the structure of the event, a theme of the work is to compare and contrast it with two other strong El Niños, in 1982/1983 and 1997/1998. These three El Niño events are included in the Modern-Era Retrospective analysis for Research and Applications (MERRA) and in the more recent MERRA-2 reanalyses. MERRA-2 allows a comparison of fields derived from the underlying GEOS model, facilitating a more detailed comparison of physical forcing mechanisms in the El Niño events. Various atmospheric/oceanic structures indicate that the 2015/2016 El Niño maximized in the Niño3.4 region, with the large region of warming over most of the Pacific and Indian Ocean. The eastern tropical Indian Ocean, Maritime Continent, and western tropical Pacific are found to be less dry in boreal winter, compared to the earlier two strong events. While the 2015/2016 El Niño had an earlier occurrence of the equatorial Pacific warming and was the strongest event on record in the central Pacific, the 1997/1998 event exhibited a more rapid growth due to stronger westerly wind bursts and Madden-Julian Oscillation during spring, making it the strongest El Niño in the eastern Pacific. Compared to 1982/1983 and 1997/1998, the 2015/2016 event has a shallower thermocline over the eastern Pacific with a weaker zonal contrast of sub-surface water temperatures along the equatorial Pacific. While the three major ENSO events have similarities, each are unique when looking at the atmosphere and ocean surface and sub-surface.

Large‐scale ocean‐atmospheric processes and seasonal rainfall variability in South Australia: accounting for non‐linearity and establishing the hierarchy of influence

ABSTRACTAn important step in developing climate forecasts is the identification of an appropriate set of predictors. Seasonal rainfall forecasts for South Australia (SA) currently have low predictive skill. It is hypothesised that this is because the large‐scale climate processes influencing SA's rainfall variability have yet to be completely identified and therefore are not adequately represented in forecast models. Here, relationships between large‐scale climate influences and rainfall in SA are assessed with a focus on methods that account for non‐linearities inherent in the climate system. A threshold method and also a novel method for climate predictor selection, Partial Mutual Information, are used to establish the hierarchy of importance of the key influences. From a large suite of potential predictors, variability in the Subtropical Ridge intensity, blocking and the gradient of sea surface temperatures (SSTs) in the tropical central and eastern Indian Ocean are found to be key indicators of seasonal rainfall variability in SA. Interactions between processes are shown to increase the amount of variability accounted for and therefore need to be better considered when producing seasonal forecasts. Notably, the view of which climate process(es) is(are) important changes depending on the input variables used, suggesting that it is not wise to make a priori assumptions about what is or is not important. Furthermore, even a perfect forecast of El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD), the focus of forecasting efforts in Australia, will not necessarily lead to an improvement to forecast skill in SA given that these processes are not key for the region. The results of this analysis can inform the development of existing and new statistical and dynamical seasonal forecasting systems for SA, and for other regions where the impacts of climate variability are significant but not necessarily dominated by ENSO and IOD.

The interannual relationship between anomalous precipitation over southern China and the south eastern tropical Indian Ocean sea surface temperature anomalies during boreal summer

AbstractThe relationship between anomalous precipitation in the Guangxi Autonomous Region and Guangdong Province (GG) and sea surface temperature anomalies (SSTA) over the south eastern tropical Indian Ocean (SETIO) during the boreal summer is investigated using observational data, reanalysis data and the atmospheric linear baroclinic model. It is demonstrated that the anomalous precipitation in GG (PGG) has a significantly negative correlation with the SSTA in SETIO. The warm SSTA in SETIO could induce below normal PGG in at least three ways. First, the warming over SETIO can cause two cyclonic circulations along the tropical Indian Ocean and easterlies from the tropical central Pacific though Matsuno‐Gill responses. The easterlies have the advantage of maintaining the anomalous negative vorticity in GG. Second, the water vapour diverges from SETIO and converges in GG. Finally, the anomalous ascent motion in SETIO is enhanced, which favours the strengthening of the anomalous local Hadley circulation over East Asia. The anomalous local Hadley circulation causes the anomalous descent motion in GG. All these anomalous circulations could favour below normal rainfall in GG and vice versa. This suggests the SSTA over SETIO could be a monitor of PGG.

Relative importance of the processes contributing to the development of SST anomalies in the eastern pole of the Indian Ocean Dipole and its implication for predictability

Using outputs from an ocean general circulation model, the relative importance of the processes contributing to the development of the Indian Ocean Dipole (IOD) is examined systematically based on two metrics. One metric quantifies the relative importance of the surface heat flux term against the oceanic terms, while the other metric compares the contribution from the horizontal advection and vertical diabatic terms. It is revealed that the anomalous cooling in the eastern tropical Indian Ocean associated with the positive IOD varies with events and four representative events are investigated in more detail. During the 1991 IOD, the horizontal advection term made the largest contribution to the eastern cooling, and the vertical diabatic term was equally important in the early stage of the development. In the 1994 event, negative SST anomalies were generated by the surface heat flux term at first, and then matured by the vertical diabatic term. Anomalous cooling during the 1997 IOD was mainly produced by the vertical diabatic term. In 2012, anomalous surface heat flux and horizontal advection played the crucial role in the development of the eastern pole, but the vertical diabatic term opposed to the anomalous cooling. Furthermore, the dependence of the seasonal prediction skill by a global ocean–atmosphere coupled general circulation model on the generation mechanisms was examined. It is demonstrated that events with the vertical diabatic term playing a more important role in the development of the eastern pole are better predicted than those with the vertical diabatic term making relatively small contribution or opposing the occurrence.

Late Quaternary fingerprints of precession and sea level variation over the past 35 kyr as revealed by sea surface temperature and upwelling records from the Indian Ocean near southernmost Sumatra

Abstract We report high-resolution paleoclimatic reconstructions using isotope, alkenone sea surface temperature (SST), productivity, and terrigenous proxy variations over the past 35 kyr from core SO184-10043 (07°18.57′S, 105°03.53′E) in the eastern tropical Indian Ocean offshore of southernmost (S) Sumatra. The core was retrieved at 2171 m water depth, from the monsoon driven seasonal upwelling area off southernmost Sumatra. Our paleoclimatic reconstructions show that an enhanced marine productivity was closely linked to strengthening of coastal upwelling during the increases of boreal summer insolation and associated southeastern (SE) monsoon strength, with pronounced semi-precession cycles (∼11 kyr). In contrast, our record of alkenone SSTs shows glacial to interglacial, a sea-level variation, but this fluctuates at dominant precession cycles (∼21 kyr). We also observed four SST “plateaus” with relatively warmer hydrographic stability in our records over the past 35 kyr: 32-27 ka, 24-20 ka, 17-12 ka, and 9-4 ka. The time scale of each plateau is ∼4-6 kyr. Our study indicates that since the last glacial maximum (LGM), the sea level rise has been responsible for the Sunda shelf flooding and the Indonesian Throughflow (ITF) from the South China Sea (SCS) to the eastern Indian Ocean, leaving a dominant fingerprint of a glacial to interglacial increase in SSTs since ∼9.5 ka We further argue that the a most recent centennial to millennial scale cooling at ∼3.5 ka observed in our SST record is a widespread event in the tropical Pacific, which has implications for advancing our understanding of the climate dynamics of the tropical oceans at these time scales.