Southeastern Indian Ocean Research Articles

The 1994 Positive Indian Ocean Dipole Event as Investigated by the Transfer Routes of Oceanic Wave Energy

Abstract The present study investigates the interannual variability of the tropical Indian Ocean (IO) based on the transfer routes of wave energy in a set of 61-yr hindcast experiments using a linear ocean model. To understand the basic feature of the IO dipole mode, this paper focuses on the 1994 pure positive event. Two sets of westward transfer episodes in the energy flux associated with Rossby waves (RWs) are identified along the equator during 1994. One set represents the same phase speed as the linear theory of equatorial RWs, while the other set is slightly slower than the theoretical phase speed. The first set originates from the reflection of equatorial Kelvin waves at the eastern boundary of the IO. On the other hand, the second set is found to be associated with off-equatorial RWs generated by southeasterly winds in the southeastern IO, which may account for the appearance of the slower group velocity. A combined empirical orthogonal function (EOF) analysis of energy-flux streamfunction and potential reveals the intense westward signals of energy flux are attributed to off-equatorial RWs associated with predominant wind input in the southeastern IO corresponding to the positive IO dipole event. Significance Statement The present study gains a new insight into the mechanism of the Indian Ocean dipole events using a new diagnostic scheme for wave energy based on 61-yr hindcast experiments. The results have shown the existence of two sets of westward transfer of wave energy at the equator during 1994. One set of westward signals shows the same group velocity with theoretical equatorial Rossby waves that appear reasonably along the equator. The other set of westward signals at the equator represents a slightly slower group velocity than the theoretical equatorial Rossby waves, which is associated with abnormally extended southeasterly winds during the Indian Ocean dipole event.

Latitudinal variation in diversity and abundance of mesopelagic fishes associated with change in oceanographic variables along 110°E, south-east Indian Ocean

As part of the second International Indian Ocean Expedition, an oceanic transect from 39.5°S to 11.5°S along 110°E in the south-eastern Indian Ocean enabled concurrent oceanographic measurements and sampling of early stages of mesopelagic fishes within the upper 500 m of the water column. Notable oceanographic features along the generally oligotrophic transect included an increase in surface water temperature from 12 to 28 °C from south to north and low salinities at both ends of the transect. High salinity Subtropical Surface Water was evident over the central part of the transect and low dissolved oxygen occurred at depth at the northern end where tropical waters were influenced by the Indonesian Throughflow. Throughout the study region larval fish stages were generally concentrated in the epipelagic zone. Species richness, abundance and diversity of fishes (n = 166 species) increased towards the tropical waters. The families Myctophidae (lanternfishes; 60 species), Phosichthyidae (lightfishes; 2 species) and Gonostomatidae (bristlemouths; 5 species) were numerically dominant. Clustering of integrated abundance data (0–500 m depth) revealed four major fish assemblages along the transect. When linked to oceanographic variables, multivariate multiple regression indicated that surface temperature, surface salinity and dissolved oxygen at 400 m depth provided the best fitting model explaining fish distribution patterns. Where possible, comparisons have been made with early data collected along this 110° E line in the 1960s during the first International Indian Ocean Expedition.

The dominant modes of the long rains interannual variability over Tanzania and their oceanic drivers

AbstractMitigating the impacts of droughts and floods over East Africa (EA) may largely be possible through accurate predictions of the long rains (March–May). Though the close interannual variability between the long rains, Indian and Pacific Oceans has been widely studied, Tanzania's long rain variability remains unclear. This paper examines the Tanzanian long rains dominant modes through an empirical orthogonal function (EOF) analysis, and their relationship with winter sea surface temperature (SST) for the period 1951–2018. Correlation and composite analyses were applied to explore the associated mechanisms. The first EOF (EOF1) mode is characterized by negative anomalies over Tanzania. The second EOF (EOF2) mode depicts positive (negative) anomalies over northern (southern) Tanzania. The third EOF (EOF3) mode reveals positive (negative) anomalies over eastern (western) Tanzania. EOF1 is closely related to the SST anomalies (SSTAs) in the southeastern Indian Ocean (SEIO). When the SSTAs are positive, they subsequently linked to upper (lower) level convergence (southeasterly winds) over Tanzania. Associated with this convergence entails a sinking motion, leading to a negative rainfall anomaly. The southeasterly winds indicate an early onset of the southeast monsoon, leading to normal rainfall been recorded. EOF2 is linked to the SSTAs in the central Pacific (CP). Positive SSTAs are related to reversed Walker circulation (WC) over northern Tanzania, while the upper‐level convergence was observed around southern Tanzania. This meridional dipole vertical motion pattern enhances (suppresses) rainfall anomalies. EOF3 is associated with mega El Niño–Southern Oscillation (ENSO)‐like SSTAs. The SSTAs are related to eastward shift of WC over the Indian Ocean, and zonal cell over the western Indian Ocean with rising limb is located east of Tanzania. The ascending motion enhances rainfall over eastern Tanzania. Predictions based on the SEIO, CP, and mega ENSO index may significantly support long rains prediction efforts and provide early warning.

Late Miocene‐Pliocene Vigorous Deep‐Sea Circulation in the Southeast Indian Ocean: Paleoceanographic and Tectonic Implications

AbstractDeep ocean circulation in the Southern Hemisphere plays a central role in global ocean overturning circulation and determines ocean carbon sink variability on multimillion‐year timescales. For this reason, it is important to understand how deep currents that originate in the Southern Ocean responded to past climate fluctuations and changing temperature patterns in the Southern Hemisphere oceans. To investigate this feedback mechanism, we analyze deep‐sea sediment cores formed under the influence of bottom currents from the Mentelle Basin (International Ocean Discovery Program Sites U1513, U1514, and U1516), in the Southeastern Indian Ocean. We show that bottom current activity intensified in the Australian‐Antarctic Basin during the cooling intervals of the late Miocene (ca. 7.8–4.8 Ma) and late Pliocene (ca. 3.59–3.2 Ma). At those intervals, bulk grain‐sizes and terrigenous input increased, via current winnowing. Ferromanganese nodules and stratigraphic hiatuses are also observed within these periods, indicative of intense sediment erosion at the seafloor. Sediment accumulation and preservation in the basin was particularly low at the deep Site U1514 (3,838 m water depth), suggesting that enhanced current transport and sediment erosion occurred preferentially within the deepest parts of the basin. We suggest that bottom water fluxes in the Australian‐Antarctic Basin intensified during late Miocene and Pliocene cooling intervals, likely boosted by renewed Australian‐Antarctic Discordance spreading since the Miocene.

How Does El Niño Affect Predictability Barrier of Its Accompanied Positive Indian Ocean Dipole Event?

The effects of El Niño on the predictability of positive Indian Ocean dipole (pIOD) events are investigated by using the GFDL CM2p1 coupled model from the perspective of error growth. The results show that, under the influence of El Niño, the summer predictability barrier (SPB) for pIOD tends to intensify and the winter predictability barrier (WPB) is weakened. Since the reason for the weakening of WPB has been explained in a previous study, the present study attempts to explore why the SPB is enhanced. The results demonstrate that the initial sea temperature errors, which are most likely to induce SPB for pIOD with El Niño, possess patterns similar to those for pIOD without El Niño, whose dominant errors concentrate in the tropical Pacific Ocean (PO), with a pattern of negative SST errors occurring in the eastern and central PO and subsurface sea temperature errors being negative in the eastern PO and positive in the western PO. By tracking the development of such initial errors, it is found that the initial errors over PO lead to anomalous westerlies in the southeastern Indian Ocean (IO) through the effect of double-cell Walker circulation. Such westerly anomalies are inhibited by the strongest climatological easterly wind and the southeasterlies related to the pIOD event itself in summer, while they are enhanced by El Niño. This competing effect causes the intensified seasonal variation in latent heat flux, with much less loss in summer under the effect of El Niño. The greater suppression of the loss of latent heat flux favors the positive sea surface temperature (SST) errors developing much faster in the eastern Indian Ocean in summer, and eventually induces an enhanced SPB for pIOD due to El Niño.

Cross-oceanic distribution and origin of microplastics in the subsurface water of the South China Sea and Eastern Indian Ocean

Marine microplastic (MP) pollution is a global environmental problem that has received attention from scientific researchers and the public for the past several decades. However, without a suitably large-volume sampling method, the presence of MPs in subsurface water (< 5 m) is poorly understood. Here, MP content in subsurface water was determined using a pump-underway ship intake system along the cross-oceanic transect from the Pearl River Estuary to the Indian Ocean. The study regions have always been considered as one of the major MPs hotspots in the global oceans and still lack of study. Generally, MP abundance ranged between 0 and 4.97 items m-3, with an overall mean value of 0.40 ± 0.62 items m-3. A total of 679 MP particles were identified using μ-FT-IR. These collections identified polyethylene (PE), polypropylene (PP), and polyester (PET) as the major polymers represented (73.14–88.81%). The presence of MPs in coastal regions was significantly higher than that in the open ocean, revealing the contribution of land-based sources to marine MPs and the ocean dynamics. Therefore, an effective and feasible way to retard the penetration of MPs into the marine environment is to exhibit controls at the source. No significant correlation was found between the MP abundance and the physical and chemical properties of water. The results of the analysis of similarities (ANOSIM) and non-metric multidimensional scaling (NMDS) also showed that MP communities in different environments were significantly greater than the differences in different sites within the same environment. These findings of this study provide reliable information on MP distribution and characterization in cross-oceanic region of South China Sea and Eastern Indian Ocean, which will help to improve our understanding about the fate of MPs in the ocean.

Review of speleothem records of the late Holocene: Indian summer monsoon variability & interplay between the solar and oceanic forcing

Indian summer monsoon (ISM) variability in the late Holocene (last 4000 years) is of great interest as the arid phase of ISM and cultivation of kharif (rainy season) crops started in this period. The present study aims to revisit the available speleothem records to study ISM rainfall variations and possible controlling factors in different Indian regions viz., core monsoon zone (CMZ), peninsular, the Himalaya, and the Bay of Bengal in the last 4000 years. The late Holocene encompasses notable global warm periods viz., Medieval Climate Anomaly, Roman Warm Period (RWP) & Minoan Warm Period, and cold periods viz., Dark Age cold period and Little Ice Age (LIA). ISM rainfall declines until the middle of RWP in all the regions. The decline is linked with the southward migration of the Intertropical convergence zone caused by the northern Hemisphere summer insolation. Though the Solar insolation follows the same trend, all the records show large variability with the disappearance of the declining trend around the middle of RWP. This change coincided with the reversal in the north Atlantic Sea surface temperature (SST) trend, northern hemisphere temperature, and abrupt changes in the southeastern Indian Ocean SST. This period also witnessed high El-Nino Southern Oscillation activities. This synergy of ISM variations in the characterized zones falls apart during the pre-LIA with a slight increasing (decreasing) trend in CMZ and Bay of Bengal (peninsular and Himalayan). The spatially variable response of ISM rainfall in recent centuries might be caused by the interaction between internal variability and external forcing and also by anthropogenic-induced warming and aerosol forcing.

Proximity to Undersaturation and the Influences on G. bulloides Area‐Density in Southern Indian Ocean Marine Sediments

AbstractThe area density proxy of foraminiferal shell thickness and calcification intensity has the potential to provide information about past ocean CO2 content and has the benefit of small sample requirements, simple analytical techniques, and the ability to re‐use the analyzed foraminifera for other paleo‐proxies. Using a series of multicore core‐tops collected from the southeastern Indian Ocean (1.8–3.8 km water depth), we evaluate the reliability of utilizing area density values of Globigerina bulloides from sediment cores to estimate surface ocean carbonate parameters. Because foraminifera in marine sediments can rarely be considered “pristine” (or “glassy”), we grouped area density measurements of shells to designate various stages of diagenesis. Visual signs of alteration were apparent at area density values as low as ∼0.122 × 104 µg/µm2, with deviations from the “pristine” endmember beginning at area density values of ∼0.087 × 104 µg/µm2. We find that increases in area density overprint the surface ocean carbonate signature in thicker (&gt;0.122 × 104 µg/µm2 shells), but small increases associated with marine sedimentary burial and diagenesis can be accounted for, allowing this proxy to be applied back in time. Reconstructing the distribution of area density values in a given sample has the potential to provide valuable information on overall sample preservation by estimating the percent of well‐preserved shells (&lt;0.122 × 104 µg/µm2; %wp) in a given sample. Our %wp metric has the potential for use as a proxy for lysocline variability in addition to assessing the suitability of marine sediment samples for surface ocean reconstructions.

Observed Extreme Air–Sea Heat Flux Variations during Three Tropical Cyclones in the Tropical Southeastern Indian Ocean

AbstractSix-month buoy-based heat flux observations from the poorly sampled tropical southeastern Indian Ocean are examined to document the extremes during three tropical cyclones (TCs) from December 2018 to May 2019. The most striking feature at the mooring site (16.9°S, 115.2°E) during the TCs is the extensively suppressed diurnal cycle of the net surface flux (Qnet), with a mean daytime (nighttime) reduction of 470 (131) W m−2, a peak decrease at approximately noon of 695 W m−2 and an extreme drop during TC Riley of 800 W m−2. The mean surface cooling in the daytime is primarily contributed by the 370 W m−2 decrease in shortwave radiation associated with the increased cloudiness. The air–sea turbulent heat fluxes increase by approximately 151 W m−2 in response to the enhanced wind speed under near-neutral boundary conditions. The daily mean rainfall-induced cooling is 8 W m−2, with a maximum magnitude of 90 W m−2. The mean values, seasonal variation, and synoptic variability of the characteristic heat fluxes are used to assess the new reanalysis data from ERA5 and MERRA2 and the analyzed OAFlux. The overall performance of the high-frequency net heat flux estimates at the synoptic scale is satisfactory, but the four flux components exhibit different quality levels. A serious error is that ERA5 and MERRA2 poorly represent TCs, and they show significant daily mean Qnet biases with opposite directions, −59 W m−2 (largely due to the overestimated latent heat with a bias of −76 W m−2) and 50 W m−2 (largely due to the overestimated shortwave radiation with a bias of 41 W m−2), respectively.

Observational estimates of turbulent mixing in the southeast Indian Ocean

AbstractThis study investigates the spatio-temporal variability of turbulent mixing in the eastern South Indian Ocean using a collection of data from EM-APEX profiling floats, shipboard CTD and microstructure profilers. The floats collected 1566 profiles of temperature, salinity and horizontal velocity data down to 1200 m over a period of about four months. A fine-scale parameterization is applied to the float and CTD data to estimate turbulent mixing. Elevated mixing is observed in the upper ocean, over bottom topography and in mesoscale eddies. Mixing is enhanced in the anticyclonic eddies due to trapped near-inertial waves within the eddy. We found that cyclonic eddies contribute to turbulent mixing in the depth range of 500 – 1000 m, which is associated with downward propagating internal waves. The mean diapycnal diffusivity over 250 – 500 m depth is O(10−6) m2 s−1 and it increases to O(10−5) m2 s−1 in 500 – 1000 m in cyclonic eddies. The turbulent mixing in this region has implications for watermass transformation and large-scale circulation. Higher diffusivity (O(10−5) m2 s−1) is observed in the Antarctic Intermediate Water (AAIW) layer in cyclonic eddies whereas weak diffusivity is observed in the Subantarctic Mode Water (SAMW) layer (O(10−6) m2 s−1). Counter-intuitively, then, the SAMW watermass properties are strongly affected in cyclonic eddies whereas the AAIW layer is less affected. Comparatively high diffusivity at the location of the South Indian Countercurrent (SICC) jets suggests there are wave-mean flow interactions in addition to the wave-eddy interactions that warrant further investigation.

Evolution of the Oceanic 13C Suess Effect in the Southeastern Indian Ocean Between 1994 and 2018

AbstractThe decrease in δ13C of dissolved inorganic carbon (δ13CDIC) owing to uptake of anthropogenic CO2 (the oceanic 13C Suess effect) in the Southeastern Indian Ocean over the last decade was calculated using an extended multiparameter linear regression technique. Samples collected on the CROCCA‐2S (Coring to Reconstruct Ocean Circulation and Carbon Dioxide Across 2 Seas) cruise in November–December 2018 were compared to samples from the CLIVAR (Climate and Ocean: Variability, Predictability, and Change) and OISO (Océan Indien Service d'Observation) programs from 2007 to 2009. Surface ocean δ13CDIC decreased by an average of −0.53 ± 0.04‰ across this period, at an average rate of −0.053 ± 0.004‰ per year. This rate of δ13CDIC change is an increase from −0.021 ± 0.024‰ per year between 1994 and 2008. We find that the interior water mass most impacted by the oceanic 13C Suess effect between 2008 and 2018 was Subantarctic Mode Water (SAMW), within which δ13CDIC decreased by −0.044 ± 0.002‰ per year. Using previously published relationships between the oceanic 13C Suess effect and anthropogenic carbon, we estimate the annual storage of anthropogenic carbon within SAMW in the southeastern Indian Ocean has increased from ∼2.0 ± 0.2 μmol/kg per year between 1994 and 2008 to 5.5 ± 0.6 μmol/kg per year between 2008 and 2018.

Enhanced Tropical Eastern Indian Ocean Rainfall Breaks Down the Tropical Easterly Jet–Indian Rainfall Relationship

AbstractPrevious studies found a tight connection between the tropical easterly jet (TEJ) and Indian summer monsoon rainfall (ISMR). Here we show that the TEJ–ISMR relationship is nonstationary and breaks down from 1994 to 2003 (epoch P2), in contrast to the significant positive correlation during epochs P1 (1979–93) and P3 (2004–16). The breakdown of the TEJ–ISMR relationship concurs with the increased rainfall variability over the tropical eastern Indian Ocean (TEIO). The enhanced TEIO rainfall anomalies excite a significant lower-level cyclonic circulation that reduces the ISMR and meanwhile strengthen the upper-level divergence and excite a pair of upper-level anticyclones to the west of the TEIO as Rossby wave responses, both accelerating the TEJ. Thus, the TEIO rainfall plays a more important role than the ISMR in TEJ variability during P2, causing the breakdown of the TEJ–ISMR relationship. In contrast, a relatively weak amplitude of the TEIO rainfall during P1 and P3 was unable to change the positive TEJ–ISMR relationship. The changes in the TEIO rainfall variability are mainly attributed to the increased SST variability over the tropical southeastern Indian Ocean, but their cause remains elusive.

The pollen record from marine core MD03-2607 from offshore Kangaroo Island spanning the last 125 ka; implications for vegetation changes across the Murray-Darling Basin

A palynological record spanning the last glacial–interglacial period was derived from high-resolution, deep-sea core MD03-2607, located near Kangaroo Island in South Australia. The core site lies opposite the mouth of the River Murray that, together with the Darling River, drains the extensive (∼1.6 × 106 km2) Murray-Darling Basin (MDB). The record comprises 120 samples and is compared with detailed records of sea-surface temperature (SST), the C3/C4 plant ratio obtained from the δ13C of n-alkanes from leaf waxes, the fluvial clay fraction and its neodymium isotopic composition, airborne dust and the biomass-burning component levuglosan. The chronology of the core is robust; it is built on 24 radiocarbon dates derived from planktic foraminifera, 16 optically stimulated luminescence dates, plus 12 tie points linked to the astronomically tuned marine isotopic record. Algal remains are found in nearly all samples supporting our postulation that the palynoflora is predominantly waterborne. Major findings are that the gymnosperm Callitris, together with high percentages of herb pollen (mostly C3 plants), is predominant during cold, arid phases, whereas Eucalyptus, is predominant during warmer and wetter periods. High charcoal concentration coincides with high percentages of Eucalyptus, mostly during wet and warm periods. Using the geochemistry of the core’s fluvial sediments, it has been possible to identify when water-transported palynoflora and charcoal originated from the Murray sub-basin (consisting of the River Murray and its main tributaries but not from central or western South Australia). During those periods, rainfall principally originated from the southeastern Indian Ocean. When the Darling sub-basin was the main source of the palynoflora, rainfall must have instead originated from northern Australia. The eolian dust record from the core shows that the dust signal generally coincides with the increased values in herb pollen, in particular during the Last Glacial Maximum (LGM) when, in addition to high herb percentages, Callitris representation also increased. This dry landscape taxon likely colonised the then-exposed Lacepede Shelf during this period of extreme low sea-level. There is a good correspondence between SST and mean annual precipitation reconstructed from the pollen counts. During warm phases in the ocean, Eucalyptus was the dominant tree taxon, especially for the entirety of Marine Isotope Stages (MIS) 5, plus MIS 3 and MIS 1. Charcoal levels were particularly low during the dry phases MIS 4 and 2, and even more so during the LGM. KEY POINTS We provide a very detailed pollen diagram from a deep-sea core taken offshore from the mouth of the River Murray. This record details vegetation changes in the Murray-Darling Basin spanning the last glacial/interglacial cycle. The pollen record is compared with other proxies obtained from the marine core, such as sea-surface temperature, changes in C3/C4 plants percentages, geochemical analyses of terrigenous sediments that inform on the origin of the sediments reaching the ocean and by implication the origin of the pollen. Charcoal counts obtained from the core are compared with the pollen spectra and clearly indicate the large incidence of fires during wet and warm periods when Eucalyptus pollen was abundant. Low charcoal counts occur during dry and cold phases when Callitris pollen were predominant over Eucalyptus.

Synoptic mode of Antarctic summer sea ice superimposed on interannual and decadal variability

In contrast to decreased Arctic sea ice extent, Antarctic sea ice extent shows a somewhat increased trend. There is a large interannual variability of Antarctic sea ice, especially in the Pacific sector of the Southern Ocean. The change and variability of Antarctic sea ice in synoptic timescales in the recent decades remain unclear. We identify synoptic modes of variability of Antarctic summer sea ice by applying the Self Organizing Map (SOM) technique to daily sea ice concentration data for the period 1979–2018. Nearly 40% of the variability is characterized by opposite changes between sea ice cover in the Bellingshausen, Amundsen and western Ross Seas and in the rest of the Antarctic seas, and another 30% by meridional asymmetry in the Weddell, Amundsen, and Ross Seas. Most of these spatial patterns may be explained by the dynamics and thermodynamic processes associated with anomalous atmospheric circulations related to the Southern Annular Mode (SAM) with a structure of strong zonal asymmetry. The interannual variability of the sea ice modes appears to have little connection to SAM, and only a weak relation to ENSO. The annual frequencies of SOM node occurrences also show a great decadal variability. Node 9 appears mainly prior to 1990; while node 1 occurs mainly after 1990. The decadal variability of nodes 1 and 9 is associated with the asymmetrical SAM, which results from two wavetrains excited over northern Australia and the southeastern Indian Ocean. These results further highlight the importance of understanding the role of southern mid-to-high latitude atmospheric intrinsic variability in predicting Antarctic summer sea ice variations from synoptic to decadal timescales.

Impact of the April\u2013May SAM on Central Pacific Ocean sea temperature over the following three seasons

Although the impact of the extratropical Pacific signal on the El Niño–Southern Oscillation has attracted increasing concern, the impact of Southern Hemisphere Annular Mode (SAM)-related signals from outside the southern Pacific Basin on the equatorial sea temperature has received less attention. This study explores the lead correlation between the April–May (AM) SAM and central tropical Pacific sea temperature variability over the following three seasons. For the positive AM SAM case, the related simultaneous warm SST anomalies in the southeastern Indian Ocean favor significant regulation of vertical circulation in the Indian Ocean with anomalous ascending motion in the tropics. This can further enhance convection over the Marine Continent, which induces a significant horizontal Kelvin response and regulates the vertical Walker circulation. These two processes both result in the anomalous easterlies east of 130° E in the equatorial Pacific during AM. These easterly anomalies favor oceanic upwelling and eastward propagation of the cold water into the central Pacific. The cold water in turn amplifies the development of the easterly wind and further maintains the cold water into the boreal winter. The results presented here not only provide a possible link between extratropical climate variability in the Indian Ocean and climate variation in the equatorial Pacific, but also shed new light on the short-term prediction of tropical central Pacific sea temperature.

Paleoceanographic turnovers during the Plio-Pleistocene in the southeastern Indian Ocean: Linkages with Northern Hemisphere glaciation and Indian Monsoon variability

Earth experienced warm temperatures during the Pliocene followed by a substantial cooling that culminated in an ice age climate at the beginning of the Pleistocene. This warmth had a significant impact on global environments and ocean circulation. The collision of the Indian and Eurasian plates further amplified this climatic pattern. The southern Indian Ocean was severely impacted by these changes in ocean circulation and climatic pattern during the Plio-Pleistocene. We reconstruct paleoceanographic turnovers in the southeastern Indian Ocean during the Plio-Pleistocene using benthic foraminiferal assemblages from ODP Sites 752 and 757, combined with relative abundances of selected planktic foraminiferal species and stable isotope ratios (δ18O and δ13C) of surface-dwelling and thermocline planktic foraminifera from Site 757. The influence of southern source Antarctic Intermediate Water (AAIW) since 4.5 Ma is marked by fluctuations in productivity till the intensification of the Northern Hemisphere glaciation (NHG) at ~3 Ma. Our isotopic data provide no evidence for cooling during the 4–3 Ma interval either for upper (Gs. ruber) or lower (M. menardii) mixed-layer waters, at a time when a switch in the source of the Indonesian Throughflow may have occurred. The increased mixed-layer species abundances since 4 Ma suggest oligotrophic conditions at low-latitude Site 757 due to thick mixed-layer coinciding with the constriction of the Indonesian Gateway and intensification of the NHG. The intensification of the NHG and monsoon seasonality caused an enhanced inflow of AAIW in the southeastern Indian Ocean since ~2.5 Ma.

Weakened ENSO‐Ningaloo Niño/Niña Teleconnection Under Greenhouse Warming

AbstractNingaloo Niño/Niña is a mode of climate variability in the southeastern Indian Ocean with huge impacts on Australian climate. El Niño‐Southern Oscillation (ENSO), as the dominant remote forcing, triggers Ningaloo Niño/Niña. However, how this teleconnection will respond to greenhouse warming is unclear. Using Coupled Model Intercomparison Project phase 5 (CMIP5) multimodel simulations, we find a weakened ENSO‐Ningaloo Niño/Niña teleconnection under greenhouse warming, which manifests as weakened atmospheric teleconnection from La Niña to Ningaloo Niño. Such weakened teleconnection can be linked to the tropical Pacific mean state changes including an El Niño‐like warming pattern and more stable atmosphere in the future climate, both suppressing the atmospheric convection in the western tropical Pacific, leading to a weaker Matsuno‐Gill response in the southeastern Indian Ocean. Our results suggest that Ningaloo Niño/Niña becomes more challenging to predict as greenhouse warming continues.

Diversity of the Madden–Julian Oscillation: Initiation Region Modulated by the Interaction between the Intraseasonal and Interannual Variabilities

AbstractAs one of the aspects of the diversity of the Madden–Julian oscillation (MJO), the modulation of initiation regions of the boreal-winter MJO is studied in terms of the relationship between intraseasonal and interannual variabilities. MJOs are categorized as those initiating in the Indian Ocean (IO), Maritime Continent (MC), and western Pacific (WP), referred to herein as IO-MJOs, MC-MJOs, and WP-MJOs, respectively. The composite analyses for each MJO category using observational data reveal that the diversity of MJO initiation regions directly results from the modulation of areas where horizontal advective premoistening efficiently occurs via intraseasonal/synoptic-scale winds. This is supported by the difference in the zonal location of equatorial intraseasonal circulations established before MJO initiation, which is related to a spatial change in background convection and associated Walker circulations forced by interannual sea surface temperature (SST) variability. Compared to IO-MJOs (favored in the climatological background on average), MC-MJOs tend to be realized under the eastern-Pacific El Niño–like condition, as a result of eastward-shifted intraseasonal convection and circulation patterns induced by background suppressed convection in the eastern MC. WP-MJOs are frequently initiated under the central-Pacific El Niño–like and positive IO dipole–like conditions, in which the WP is selectively moistened with the aid of background enhanced (suppressed) convection over the WP (the southeastern IO and the central-to-eastern Pacific). This major tendency derived from sample-limited observations is verified by a set of 15-yr numerical experiments with a global nonhydrostatic MJO-permitting model under a perpetual boreal-winter condition where observation-based SSTs are prescribed.

Impact of the South China Sea Summer Monsoon on the Indian Ocean Dipole in CMIP5 Models

AbstractThe impact of the South China Sea summer monsoon (SCSSM) on the Indian Ocean dipole (IOD) has been systematically investigated in observations. This study focuses on the ability of climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) to reproduce the observed relationship between the SCSSM and IOD and the relevant physical mechanisms. All 23 models reproduce significant correlations between the SCSSM and IOD during boreal summer [June–August (JJA)], whereas the influence of the SCSSM on the IOD varies considerably across the CMIP5 models. To explore the causes, all models are divided into two groups. Models that successfully simulated both the correlations between the SCSSM and JJA IOD and of the SCSSM and JJA IOD with precipitation over the western North Pacific and Maritime Continent are classified as Type I, and these produce stronger low-level wind anomalies over the tropical southeastern Indian Ocean. The stronger low-level wind anomalies enhance local sea surface temperature (SST) anomalies via positive wind–evaporation–SST (WES) and wind–thermocline–SST (Bjerknes) feedbacks. This corresponds to a strengthening of IOD events due to the increased zonal gradient of SST anomalies over the tropical Indian Ocean. In contrast, Type II models perform poorly in representing the relationship between the SCSSM and JJA IOD or relevant physical processes, corresponding to weaker WES and Bjerknes feedbacks, and produce weaker IOD events. These results demonstrate that the better the model simulation of the critical physical processes, the larger contribution of the SCSSM to the IOD.

Long-term variability of Sea Surface Temperature in the Tropical Indian Ocean in relation to climate change and variability

Abstract The long-term change in the sea surface temperature (SST) of the Tropical Indian Ocean (TIO) and the underlying mechanisms are examined in detail by analysing multiple reanalysis datasets and historical simulations of a global climate model. A robust basin-wide warming signal was found in SST, with the highest long-term warming trend (~0.09 ± 0.015 °C decade−1) seen over the north-western and equatorial Indian Ocean regions. The robust warming trend of TIO SST is a dominant warming signal of the global tropical oceans and is largely in phase with similar warming in western Pacific and Atlantic Oceans. The central equatorial Indian Ocean and south TIO regions went through significant warming during recent decades. The weakened cross-equatorial flow and the associated cyclonic winds over the Arabian Sea region have resulted in equatorial westerlies with an enhanced loss of latent heat over the central and eastern equatorial Indian Ocean. The anomalous cyclonic wind stress curl, in turn, caused mixed layer stratifications in the western Indian Ocean. The advective heat transfer from the south-eastern Indian Ocean to the western Indian Ocean and reduced oceanic cooling by vertical processes, overcome the cooling by the net loss of surface heat fluxes, and favor surface warming the TIO.