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An Equatorial Jet in the Indian Ocean
Abstract
At the surface of the Indian Ocean along the equator a narrow, jet-like current flows eastward at high speed during both transition
periods between the two monsoons. The formation of the jet is accompanied by thermocline uplifting at the western origin of
the jet and by sinking at its eastern terminus. This demonstrates that a time-variable current can have profound effects in
changing the mass structure in the ocean.
... The ITF brings relatively low salinity water (S<34.6) originating from the tropical Pacific Ocean and merges with the South Equatorial Current (SEC) on its way to the western boundary (Gordon, 2005;Sprintall et al., 2009;You, 1997). ICW is formed along the subtropical convergence zone by subduction and is carried westward by the SEC (Sprintall and Tomczak, 1993;You, 1997). ...
... originating from the tropical Pacific Ocean and merges with the South Equatorial Current (SEC) on its way to the western boundary (Gordon, 2005;Sprintall et al., 2009;You, 1997). ICW is formed along the subtropical convergence zone by subduction and is carried westward by the SEC (Sprintall and Tomczak, 1993;You, 1997). ICW is characterized by higher salinity waters (S > 35, Fig. 3a) and a quasi-linear relation above 7 °C (Schott and McCreary, 2001;Sverdrup et al., 1942). ...
... ICW is characterized by higher salinity waters (S > 35, Fig. 3a) and a quasi-linear relation above 7 °C (Schott and McCreary, 2001;Sverdrup et al., 1942). During the summer monsoon, both water masses cross the equator and 245 flow into the Arabian Sea and eventually in the BoB (Schott et al., 2009;Tomczak and Godfrey, 1994;You, 1997). ...
Oxygen minimum zones (OMZ) play an important role for the global oceanic nitrogen cycle because they account for 20 to 40 % of the global loss of bioavailable nitrogen despite covering only about 1 % of the global ocean volume. The intermediate waters of the Bay of Bengal (BoB) host one of the most pronounced OMZs with near-anoxic conditions. However, it has not yet been recognized as a site with significant nitrate reduction. In this study, we examined the nitrogen cycling processes in the East Equatorial Indian Ocean (EEIO) and the BoB by measuring water column properties, including temperature, salinity, oxygen and nutrient concentrations, as well as nitrate isotope signatures, collected during the SO305 BIOCAT-IIOE2 cruise in April and May 2024. Potential temperature and salinity profiles showed distinct water masses and limited mixing between BoB and the EEIO at 5° N. Nitrate stable isotope depth profiles varied significantly, driven by water mass distribution below 300 m and in-situ fractionation above 300 m. Phytoplankton uptake acts as a nitrate sink in the surface waters, showing a significant isotopic enrichment and nitrogen deficit. Below, nitrification was observed, primarily through regenerative production using previously assimilated biomass rather than newly fixed nitrogen from N2 fixation. Within the OMZ of the BoB, we identified a persistent nitrogen deficit and slightly enriched nitrate isotopes between 100 and 300 m, indicating a nitrogen loss, which we attributed to anammox as the dominant nitrogen loss pathway in the BoB.
... During these periods, semi-annual zonal jets called "Indian Ocean equatorial jets" or "Wyrtki jets" (WJs) usually appear on the surface of the equatorial Indian Ocean. These jets have a narrow north-south range, move in an eastward direction, and are divided into spring (boreal spring) and autumn branches (boreal fall) according to the seasonal characteristics [3][4][5][6]. According to Wyrtki (1973) [3], these jets are driven by the westerly wind over the equator and mainly flow through the area between 60 • E and 90 • E. Since their discovery, many scholars have studied the spatiotemporal distribution characteristics and formation mechanism of WJs through observations and numerical simulations [7,8]. ...
... These jets have a narrow north-south range, move in an eastward direction, and are divided into spring (boreal spring) and autumn branches (boreal fall) according to the seasonal characteristics [3][4][5][6]. According to Wyrtki (1973) [3], these jets are driven by the westerly wind over the equator and mainly flow through the area between 60 • E and 90 • E. Since their discovery, many scholars have studied the spatiotemporal distribution characteristics and formation mechanism of WJs through observations and numerical simulations [7,8]. ...
... These jets have a narrow north-south range, move in an eastward direction, and are divided into spring (boreal spring) and autumn branches (boreal fall) according to the seasonal characteristics [3][4][5][6]. According to Wyrtki (1973) [3], these jets are driven by the westerly wind over the equator and mainly flow through the area between 60 • E and 90 • E. Since their discovery, many scholars have studied the spatiotemporal distribution characteristics and formation mechanism of WJs through observations and numerical simulations [7,8]. ...
As important components of the equatorial current system in the Indian Ocean, Wyrtki jets (WJs) play a significant role in distributing heat and matter in the East and West Indian Oceans. By dividing the El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) events into several phases, we find that the spring branch exhibits positive (negative) anomalies during the El Niño (La Niña) decaying phase, while the fall branch exhibits negative (positive) anomalies during the El Niño (La Niña) developing phase. The spring and fall branches are characterized by negative (positive) anomalies under the influence of positive (negative) dipole events, and these anomalies are particularly pronounced during fall. This study systematically analyzes the characteristics of WJs under the interactions between the Indo-Pacific ocean and the atmosphere, based on the phase-locking characteristics of ENSO, and reveals the regulatory mechanisms underlying their different response patterns.
... Interannual variations in equatorial winds associated with the IOD and ENSO dominate the currents in the equatorial Indian Ocean (EIO), which could influence the interannual variations in the salinity distribution in the BoB [10]. After the summer monsoon, the zonal winds are mostly westerlies over the EIO, forming strong surface eastward currents, referred to as Wyrtki jets [11]. Wyrtki jets bifurcate into two branches and extend towards the BoB in the northern direction [12]. ...
... Regression and correlation analysis is necessary for understanding the link between the equatorial zonal wind and the distribution of SSSAs in the BoB. During the fall transition between the summer and winter monsoons, the zonal winds are mostly westerlies in the equatorial Indian Ocean [11]. To measure the strength of the westerlies, we defined the westerly index (WI). ...
... As shown in Figure 8a, consistent with previous research, climatological westerlies prevailing over the EIO drive the eastward Wyrtki jet along the equator during OND. Upon reaching the Sumatra coast, a portion of these currents flows into the BoB [11,12]. Thus, high-salinity waters from the western EIO move eastward, and parts of them bifurcate northward into the BoB. ...
In this study, we investigate the connection between planetary equatorial waves, modulated by the Indian Ocean dipole (IOD) and El Niño Southern Oscillation (ENSO), and the interannual variabilities of the salinity distribution in the Bay of Bengal (BoB) in October–December (OND), along with its associated dynamics, using satellite and reanalysis datasets. In OND 2010 and 2016 (1994, 1997, 2006, and 2019), positive (negative) sea surface salinity anomalies (SSSAs) were distributed in the eastern equatorial Indian Ocean (EIO) and Andaman Sea. Moreover, the southward movement of negative (positive) SSSAs along the eastern Indian coast was observed. This phenomenon was caused by large-scale anomalous currents associated with zonal wind over the EIO. During OND 2010 and 2016 (1994, 1997, 2006, and 2019), due to anomalous westerlies (easterlies) over the EIO and anomalous downwelling (upwelling) Kelvin waves, the strengthened (weakened) Wyrtki jet and the basin-scale anomalous cyclonic (anticyclonic) circulation in the BoB gave rise to positive (negative) SSSAs within the eastern EIO and Andaman Sea. In addition, the intensified (weakened) eastern Indian coastal currents led to the southward movement of negative (positive) SSSAs. It is worth noting that downwelling Kelvin waves reached the western coast of India during OND 2010 and 2016, while upwelling Kelvin waves were only confined to the eastern coast of India during OND 1994, 1997, 2006, and 2019. Furthermore, westward salinity signals associated with reflected westward Rossby waves could modulate the spatial pattern of salinity. The distribution of salinity anomalies could potentially influence the formation of the barrier layer, thereby impacting the sea surface temperature variability and local convection.
... The Wyrtki jet (Wyrtki, 1973), monsoon circulation (Peng et al., 2015), and a large amount of diluted water (Shetye et al., 1991) from land strongly affect the salinity level of surface seawaters in the eastern Indian Ocean and cause severe stratification. Therefore, it is often regarded as a typical tropical oligotrophic sea area (Nagura and Mcphaden, 2018). ...
... In the comparison of the variation amplitude of temperature distribution in the longitude horizontal direction and latitude horizontal direction, it can be seen that the variation difference of temperature in the longitude direction is more prominent. This is likely due to the eastward advance of the Wyrtki jets (EJs) that are driven by equatorial west wind drifts during the spring intermonsoon, which mixes high-temperature water in the western Indian Ocean with relatively low-temperature seawater in the east (Wyrtki, 1973). These may indicate that in a short time series, the intrusion of currents has a more severe impact on the temperature distribution pattern in the eastern equatorial Indian Ocean. ...
... This may be mainly due to the periodic occurrence of Wyrtki jets along the eastern Indian Ocean equator during the spring intermonsoon period. Driven by the equatorial west winds drifts, this current originates in the western Indian Ocean and carries the high temperature and high salt seawater of the Arabian Sea eastward to push into the oligotrophic eastern Indian Ocean (Wyrtki, 1973), thus providing better growth conditions for phytoplankton in western seawater in the eastern Indian Ocean and enabling a high value of integrated chlorophyll a concentration to appear in the sea area. The distribution characteristics of chlorophyll a in the eastern equatorial Indian Ocean showed a more significant correlation with the distribution of temperature (Figures 2A, C). ...
Microzooplankton (MZP) are an important part of the microbial food web and play a pivotal role in connecting the classic food chain with the microbial loop in the marine ecosystem. They may play a more important role than mesozooplankton in the lower latitudes and oligotrophic oceans. In this article, we studied the species composition, dominant species, abundance, and carbon biomass of MZP, including the relationship between biological variables and environmental factors in the eastern equatorial Indian Ocean during the spring intermonsoon. We found that the MZP community in this ocean showed a high species diversity, with a total of 340 species. Among these, the heterotrophic dinoflagellates (HDS) (205 species) and ciliates (CTS) (126 species) were found to occupy the most significant advantageous position. In addition, CTS (45.3%) and HDS (39.7%) accounted for a larger proportion of the population abundance, while HDS (47.1%) and copepod nauplii (CNP) (46.4%) made a larger contribution to the carbon biomass. There are significant differences in the ability of different groups of MZP to assimilate organic carbon. In this sea area, MZP are affected by periodic currents, and temperature is the main factor affecting the distribution of the community. The MZP community is dominated by eurytopic species and CNP. CTS are more sensitive to environmental changes than HDS, among which Ascampbelliella armilla may be a better habitat indicator species. In low-latitude and oligotrophic ocean areas, phytoplankton with smaller cell diameters were found to occupy a higher proportion, while there was no significant correlation between the total concentration of integrated chlorophyll a and the biological variables of MZP. Therefore, we propose that the relationship between size-fractionated phytoplankton and MZP deserves further study. In addition, the estimation of the carbon biomass of MZP requires the establishment of more detailed experimental methods to reflect the real situation of organisms. This study provides more comprehensive data for understanding the diversity and community structure of MZP in the eastern equatorial Indian Ocean, which is also of good value for studying the adaptation mechanism and ecological functions of MZP in low-latitude and oligotrophic ocean ecosystems.
... The bi-annually reversing South Java Current (SJC) flows along the Indonesian Islands, Sumatra and Java [28][29][30][31][32]. The SJC is driven by the monsoon winds and variation in freshwater influx from the Indonesian Archipelago [29,33]. From November to June, the SJC flows eastward along the southern coast of Java (between 100 and 120 • E) with peak speeds up to 1.5 ms −1 . ...
... The SJC is driven by the reversing monsoon winds and variation in freshwater influx from the Indonesian Archipelago [29,33]. From November to June, SJC flows eastward along the southern coast of Java; then flows to the west from July to October. ...
The Southeastern Tropical Indian Ocean (SETIO) forms part of the global ocean conveyor belt and thermohaline circulation that has a significant influence in controlling the global climate. This region of the ocean has very few observations using surface drifters, and this study presents, for the first time, paths of satellite tracked drifters released in the Timor Sea (123.3° E, 13.8° S). The drifter data were used to identify the ocean dynamics, forcing mechanisms and connectivity in the SETIO region. The data set has high temporal (~5 min) and spatial (~120 m) resolution and were collected over an 8-month period between 17 September 2020 and 25 May 2021. At the end of 250 days, drifters covered a region separated by ~8000 km (83–137° E, 4–21° S) and transited through several forcing mechanisms including semidiurnal and diurnal tides, submesoscale and mesoscale eddies, channel and headland flows, and inertial currents generated by tropical storms. Initially, all the drifters moved as a single cluster, and at 120° E longitude they entered a region of high eddy kinetic energy defined here as the ‘SETIO Mixing Zone’ (SMZ), and their movement was highly variable. All the drifters remained within the SMZ for periods between 3 and 5 months. Exiting the SMZ, drifters followed the major ocean currents in the system (either South Java or South Equatorial Current). Two of the drifters moved north through Lombok and Sape Straits and travelled to the east as far as Aru Islands. The results of this study have many implications for connectivity and transport of buoyant materials (e.g., plastics), as numerical models do not have the ability to resolve many of the fine-scale physical processes that contribute to surface transport and mixing in the ocean.
... The OSCAR currents display strong eastward currents of magnitude 0.25 ms − 1 between the equator and 5 • S and a strong western boundary current system such as Somalia and Oman currents with a magnitude 0.2 ms − 1 is observed in the north Indian Ocean. Annually, the equatorial surface currents are eastward due to the dominance of semi-annual WJs during the transition months (Wyrtki, 1973). The reanalysis products reveal diverse distribution in representing the equatorial current system. ...
... However, the eastward extension and magnitude of westward currents have large deviations in various datasets. The strong eastward currents of the equatorial Indian Ocean, called the Wyrtki jets are forced by the inter-monsoon equatorial westerlies (Wyrtki, 1973). They are identified during the spring and fall seasons and have distinct characteristics including their magnitude and flow pathways (Han et al. 1999, Zheng et al., 2012. ...
... After its formation in the north, ASHSW spreads southward along the eastern Arabian Sea at the 26-isopycnal surface during the southwest monsoon (June-September) by the clockwise basin-scale monsoon circulation. At the equator, ASHSW, characterized by subsurface salinity maxima at 100 m, is advected eastward into the Bay of Bengal and the eastern equatorial Indian Ocean by the prevailing monsoon circulation 5 and the Wyrtki Jet 6 . ...
In the northern Arabian Sea, high salinity levels are primarily sustained by year-round evaporation, driving the convective formation of Arabian Sea High Salinity Water (ASHSW) during the winter monsoon (November–February). Although precipitation has largely been discounted as a critical controlling mechanism for winter convection, recent years have seen a notable increase in extreme cyclones over the Arabian Sea, particularly in post-monsoon cyclones (September–December) since 2014. However, the extent to which these cyclone-induced freshwater inputs disrupt the region’s freshwater balance (evaporation – precipitation) and impact ASHSW formation remains unclear. Here, we present observational evidence supported by a suite of model simulation experiments, revealing a significant weakening in ASHSW formation triggered and sustained by extreme tropical cyclones. The addition of freshwater reduces the density of high-salinity water, augmenting stratification and disrupting the convective sinking process, ultimately limiting the depth of convective mixing. These findings underscore the profound implications of extreme cyclone-induced freshwater inputs.
... E1-E3, 16°N-10°N) was stratified by a layer of lower-salinity riverine water, with the mixed layer depth (MLD) fluctuating in response to the development of mesoscale eddies. The MLD was substantially deepened by the eastward Wyrtki Jet(Wyrtki 1973) transporting high-salinity water between 5°N and the equator (st. E4 and E5). ...
Heme B is an iron-coordinated porphyrin cofactor that facilitates essential biochemical reactions. As a major iron component in almost all life forms, the abundance of heme B in the ocean provides novel insights into iron biogeochemistry. In this study, we investigated the distribution of heme B in suspended particulate material collected from the surface mixed layer of the eastern Indian Ocean and the western North Pacific Ocean. Within the photic zone of the regions studied, particulate heme B concentrations ranged 1.24–8.39 pmol L ⁻¹ and were positively correlated with particulate organic carbon and chlorophyll a concentrations, consistent with the biologically ubiquitous nature of heme B. Profiles of heme B normalized to particulate organic carbon (heme B/POC) and chlorophyll a (heme B/chl a ) revealed a complex response of the microbial heme B pool to environmental factors. In the eastern Indian Ocean, heme B/POC increased in response to enhanced iron bioavailability. Notably, a sharp increase in heme B/POC, up to 3.04 µmol mol ⁻¹ in the Bay of Bengal, was attributed to the alleviation of iron stress due to substantial iron inputs from monsoonal aeolian dust and riverine sources. Conversely, heme B/POC as low as 0.88 µmol mol ⁻¹ in the South Indian Ocean was consistent with the previous incubation experiments indicating iron limitation. In the western North Pacific Ocean, relatively low heme B/POC values in both the iron-limited subarctic and nitrogen-limited subtropical regions highlighted the influence of factors beyond iron bioavailability. In the subarctic region, an elevation in dissolved iron concentrations due to seasonal deepening of the surface mixed layer was counterbalanced by greater iron investment in photosynthetic proteins to acclimate to low light intensities. On the other hand, microbial communities in the subtropical western North Pacific Ocean were less likely to have experienced iron stress. However, a reduction in the intracellular abundance of heme B-containing photosynthetic proteins and nitrate reductase under nitrogen-limited conditions may have resulted in heme B/POC values comparable to those observed in the iron-limited regions. Based on our particulate heme B measurements, we estimated the global particulate biogenic iron pool in the surface ocean, which showed consistency with model-simulated estimates. This study highlights the utility of heme B as a valuable parameter for understanding iron biogeochemistry, which is critical for elucidating the links between marine iron and carbon cycles.
... The effect of the thermocline feedback ( wT ʹ z ) hinges on the direction of climatological vertical motions. Due to the monsoon transition and the consequent onset of the fall Yoshida-Wyrtki jet (Han et al., 1999;Wyrtki, 1973;Yoshida, 1959), the equatorial mean upwelling shifts to downwelling ( Figure S9 in Supporting Information S1) and thus the thermocline feedback diminishes from September to October, creating a meridional warming-cooling dipole structure with a near-zero net contribution to the westward extension (Figures 2a and 2e). The residuals for both types of EXpIOD events show similar signs and amplitudes, suggesting they behave similarly across event types and contribute minimally to the differences in cooling patterns. ...
Plain Language Summary
The Indian Ocean Dipole (IOD) is the dominant interannual variability of the tropical Indian Ocean (TIO) during boreal fall. In its positive phase, cold sea surface temperature anomalies (SSTAs) develop off the coasts of Sumatra and Java, while warm SSTAs occur in the western basin. Extreme positive IOD events (EXpIODs), characterized by intense eastern cooling, garner scientific community attention due to their substantial impacts on the hydrological and ecological systems of TIO‐rim countries. In this study, we identify that the eastern cold tongue of EXpIODs, as the core of IOD evolution, generally exhibits two shapes—equatorially‐extended and coastally‐concentrated. The equatorially‐extended EXpIOD features far westward reach of the eastern cooling, along with intensified anomalous easterlies. Strong El Niño events, concurrent with these equatorially‐extended EXpIODs but absent in those coastally‐concentrated ones, amplify the central Indian Ocean easterlies through the atmospheric bridge, resulting in pronounced cold nonlinear vertical advection to reinforce the former type. This inter‐basin regulation between climate modes is also detected in century‐long observations and coupled model simulations. Our results suggest that although the co‐occurrence of strong El Niño and EXpIOD is coincidental, once they co‐occur, strong El Niño could effectively shape the structure of EXpIOD and magnify its climate impacts.
... The spring Wyrtki (1973) jet excites an equatorial Kelvin wave, part of which propagates along the periphery of the bay as a coastal Kelvin wave. A Rossby wave radiated by this coastal Kelvin wave is observed south of Sri Lanka during the Southwest Monsoon (SM) season, due to the topographic "bump" near 5°N. ...
During boreal summer, the Southwest Monsoon Current (SMC) turns northeastward, transporting highly saline water into the Bay of Bengal (BOB) and significantly influencing the dynamics of the upper ocean. Previous studies have shown that an anticyclonic semi-geostrophic (SG) eddy forms on the eastern flank of the SMC, this formation associated with the kinetic energy transfer via the barotropic instability (BTI). The presence of such an eddy can attenuate the meridional salinity flux, potentially affecting the development of the circulation within the BOB. Acknowledging the importance of this phenomenon, this study revisits the SG eddy using satellite altimetry data, reanalysis datasets and in-situ observations from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) project. Our results show that a cyclonic eddy-like (CE-like) negative Sea Level Anomaly (SLA), generated in the eastern BOB due to regional anomalous wind stress curl, also contributes to the formation of the SG eddy. During the formation, mean flows on the northern edge of the SG eddy are strengthened, while southeastward currents on the eastern edge are structured influenced by CE-like SLA. Further instability analyses indicate that the anisotropic component of BTI is significantly larger than the isotropic component, which is attributed to the weak nonlinear planetary geostrophic convergence of the SG eddy and the strong horizontal shear in mean flow field induced by CE-like SLA. Additionally, our results point out that anomalies in wind stress curl over the eastern BOB and subsequent formation of negative SLA are likely influenced by the Indian Ocean Dipole. These findings suggest that the coupling between SMC instability and regional wind stress curl may play a pivotal role in the generation of SG eddy on interannual timescale, with important implications for regional ocean dynamics.
... Unlike the Atlantic or the Pacific Ocean, the Indian Ocean landlocked in the north, experiences seasonally variable monsoonal circulations that are characteristic of this region (Figure 1). These circulations lead to upwelling off Somalia and Oman coast, along the west coast of India and in the Arabian Sea (Banse 1959;Madhupratap et al. 1996;Shetye et al. 1991;Smitha et al. 2008;Wyrtki 1973). In addition, winter cooling during the northeast monsoon (Dec-Feb) also adds up to the variability of physical features (Prasanna . ...
This paper presents a systematic review of studies on bacterial communities and the techniques used to investigate them in the northern Indian Ocean. The analysis identifies key research gaps and proposes directions for future research.
... The Wyrtki Jet refers to the eastward surface jet that occurs during the transition periods of the equatorial Indian Ocean (EIO) monsoon (April-May and October-November, spring and fall in the Northern Hemisphere, hereinafter referred to as spring and fall respectively; Wyrtki, 1973), which is a crucial component of the tropical Indian Ocean circulation system. When the Wyrtki Jet occurs, the velocity of surface zonal current in the EIO reaches 1.5m/s, triggering a significant water mass transport that can carry warm and salty seawater in the upper layer of the western Indian Ocean eastward. ...
This paper examines the intraseasonal variabilities (ISVs) of the Wyrtki Jet in boreal spring and fall and their impacts on the oceanic ISVs along the southern coast of Sumatra-Java Island. The results reveal that the Wyrtki Jet ISVs in spring are significantly stronger than those in fall, with the standard deviation of the 0-30m-averaged zonal current reaching up to 0.25 m/s in spring, while the highest value in fall is only 0.2 m/s. The Wyrtki Jet ISVs are significantly correlated with surface zonal wind anomalies and sea level anomalies (SLAs) in the equatorial Indian Ocean (EIO) at intraseasonal timescale, and are modulated by the propagation of equatorial Kelvin waves. The intraseasonal SLAs along the southern coast of the Sumatra-Java Island are significantly correlated with the Wyrtki Jet ISVs, exhibiting similar seasonal fluctuation characteristics. In spring, the Wyrtki Jet intraseasonal signals initially appear near 75°E at the equator, approximately 10 days before the positive peaks of the intraseasonal SLAs, while in fall, the Wyrtki Jet intraseasonal signals first appear about 15 days before the peaks near 60°E at the equator, which is relatively further west compared to signals in spring. In addition, the composite Wyrtki Jet ISVs in spring are approximately 0.2 m/s stronger than those in fall. The enhanced ISVs of sea surface zonal wind forcing and Wyrtki Jet in spring, relative to those in fall, indicate that the seasonality in the intraseasonal SLAs along the southern coast of Sumatra-Java is attributable to the combined effects of surface wind forcing and current fields.
... In the mean state, a clockwise Indian Ocean tropical gyre (IOTG) circulation within the southern TIO is composed of eastward Wyrtki Jets (WJs) at the equator, the westward South Equatorial Current (SEC) at ∼12-13°S, and the western and eastern boundary currents, that all vary on seasonal to interannual time scales (Du et al., 2019;Schott & McCreary, 2001). The equatorial WJs develop twice yearly during the intermonsoon periods in response to the equatorial wind reversals (Qiu et al., 2009;Wyrtki, 1973), forming a high salinity tongue along the equator. The freshwater from the ITF into the southern TIO is transported westward within the SEC (Wijffels et al., 2002), forming a low salinity tongue along 12°S (Gordon et al., 1997;Reppin et al., 1999). ...
The southern tropical Indian Ocean (TIO) displays large mixed layer salinity (MLS) variation. Circulation in this region is governed by the Indian Ocean tropical gyre (IOTG), where the source water proportion and associated mixing remain unclear. Particles integrating into the IOTG and entering the central southern TIO originate from the Bay of Bengal, Malacca Strait, western Indian Ocean, and Indonesian Throughflow. Surprisingly, cross‐equatorial advection is particularly important, implying a significant connection between both the Bay of Bengal and the South China Sea via Malacca Strait into the southern TIO. The anomalous anticlockwise circulation weakens the IOTG during positive Indian Ocean Dipole (IOD). An opposite pattern is observed in the negative IOD. A particle experiment reveals that water masses are modulated by the anomalous circulation that drives the redistribution of MLS by changing the proportion of the different source waters. This represents a potential predictability for the southern TIO MLS variability.
... Thus, the region experiences westward surface currents of weak magnitude during the southwest and northeast monsoon months and much stronger eastward current during the spring and fall intermonsoon months (Han et al., 1999). These narrow eastward surface currents during the intermonsoon months, known as the Wyrtki Jets, are in response to westerly winds (Wyrtki, 1973). The biogeochemical characteristics of the region have only been recently explored with the help of satellite and in situ data (e.g., Prasanna Strutton et al., 2015). ...
Although the northern Indian Ocean (IO) is globally one of the most productive regions and receives dissolved iron (DFe) from multiple sources, there is no comprehensive understanding of how these different sources of DFe can impact upper-ocean biogeochemical dynamics. Using an Earth system model with an ocean biogeochemistry component, this study shows that atmospheric deposition is the most important source of DFe to the upper 100 m of the northern IO, contributing more than 50 % of the annual DFe concentration. Sedimentary sources are locally important in the vicinity of the continental shelves and over the southern tropical IO, away from high atmospheric depositions. While atmospheric depositions contribute more than 10 % (35 %) to 0–100 m (surface-level) chlorophyll concentrations over large parts of the northern IO, sedimentary sources have a similar contribution to chlorophyll concentrations over the southern tropical IO. Such increases in chlorophyll are primarily driven by an increase in diatom population over most of the northern IO. The regions that are susceptible to chlorophyll enhancement following external DFe additions are where low levels of background DFe and high background nitrate-to-iron values are observed. Analysis of the DFe budget over selected biophysical regimes over the northern IO points to vertical mixing as the most important mechanism for DFe supply, while the importance of advection (horizontal and vertical) varies seasonally. Apart from removal of surface DFe by phytoplankton uptake, the subsurface balance between DFe scavenging and regeneration is crucial in replenishing the DFe pool to be made available to the surface layer by physical processes.
... More recently, using linear stratified models (McCreary et al., 1993), several studies have investigated how wind stress forcing over the Arabian Sea, the southern tip of Sri Lanka, and the equatorial Indian Ocean impacts intraseasonal-to-interannual sea level variations along the coast of India (Suresh et al., 2013(Suresh et al., , 2016(Suresh et al., , 2018. Wind variations leading to CTWs can be attributed to semiannual basin-scale wind variability that drives the equatorial jet (Wyrtki, 1973;Yoshida, 1959), intraseasonal anomalies associated with the Madden-Julian Oscillation (MJO; Madden & Julian, 1971), and interannual anomalies associated with the Indian Ocean Dipole (IOD) (Aparna et al., 2012;Han & Webster, 2002;Saji et al., 1999). ...
The equatorial Kelvin waves, remotely excited by basin‐scale climate modes, and subsequent coastal trapped waves significantly influence the intraseasonal variations, their low‐frequency modulations, and the frequency of extreme sea level events along the western coast of India. This study demonstrates that the frequency of extreme events are linked to the phase of the Indian Ocean Dipole mode. The temporal changes in the occurrence frequency of extremes are simulated in an eddy‐resolving ocean model consistently with observations. However, a non‐eddying model significantly underestimate the occurrence frequency of extreme sea level events, suggesting the importance of coastal trapped wave propagations regulated by the horizontal scale with the Rossby radius of deformation. This result implies that many state‐of‐the‐art climate models with a one‐degree ocean horizontal resolution may underestimate future coastal sea level variability and the frequency of extreme events under global warming and potential modulations of major internal climate modes.
... Dengan Massa Air Selama Periode IOD (+), IOD (-), Musim Barat, dan Musim Timur di Selat Mentawai juga dipengaruhi oleh variabilitas monsun, dimana pada saat peralihan musim di dominasi angin baratan yang membentuk arus equatorial dan dikenal sebagai arus Wrykti (Wyrtki K., 1973). Sehingga dapat mempengaruhi sebaran massa air laut, panas dan salinitas pada lapisan permukaan Samudera Hindia bagian equatorial berperan penting dalam perkembangan Indian Ocean Dipole (IOD) (Supriyadi et al., 2019). ...
Selat Mentawai merupakan bagian Perairan Barat Indonesia yang secara geografis dilalui fenomena antar tahunan Indian Ocean Dipole (IOD) dan monsun. Lokasi penelitian ini secara geografis terletak di antara 2,3°–3,3° LU dan 100°–101,5° BT. Data yang digunakan adalah data Conductivity Temperature Depth (CTD) untuk mengukur parameter kedalaman, temperatur, dan salinitas. Analisis data CTD menggunakan diagram T-S menunjukkan hubungan antara suhu dan salinitas di beberapa kedalaman. Data model arus digunakan untuk melihat distribusi arus laut secara horizontal. Hasil analisa diagram T-S menunjukan lapisan termoklin berada pada kedalaman antara 50-150 m. Lapisan termoklin saat IOD (-) berada pada kedalaman 92–155 m, lebih dalam daripada IOD (+) pada kedalaman 77–130 m. Lapisan termoklin saat monsun barat pada kedalaman 92,3-155,8 m lebih dalam daripada monsun timur pada kedalaman 55,7–109,7 m. Arus laut Selat Mentawai dipengaruhi oleh perubahan IOD dan monsun, di mana saat IOD (-) arah arus dominan dari Barat Laut ke Tenggara dan sebaliknya saat IOD (+) arah arus dominan dari Tenggara ke Barat Laut. Pada monsun barat, arah arus dominan dari Barat Laut ke Tenggara, sebaliknya saat monsun timur arah arus dominan dari Tenggara ke Barat laut. Karakteristik massa air di upper water (0–500 m) wilayah Selat Mentawai terdiri dari Benggal Bay Water (BBW), Subtropical Low Water (SLW), South Indian Central Water (SICW), Indonesian Upper Water, dan Indian Equatorial Water. Pada wilayah intermediate water massa air yang paling dominan berasal dari Red-Sea Persian Intermediate Water (RSPIW). Pada kondisi IOD +, IOD -, Monsun Barat, dan Monsun Timur tidak terdapat perbedaan karakteristik massa air. Kata kunci: IOD, massa air, monsun, Selat Mentawai, T-S diagram.
... In typical SON and MAM seasons, a robust eastward oceanic current, driven by prevailing westerly winds, characterizes the equatorial Indian Ocean region. The equatorial westerly winds propagate the warm water from the western to the eastern TIO, known as the Wyrtki jet (Wyrtki 1973), inducing a climatologically shallow thermocline in the west and a deeper thermocline in the east during boreal fall and spring seasons compared to other seasons (e.g., Nagura and McPhaden 2010;Gnanaseelan et al. 2012). This study examines the simulation of the Wyrtki jet in the CMIP6 models using the Wyrtki jet index (Fig. 12a). ...
This study explores the features of leading modes of Subtropical Indian Ocean (SIO) sea surface temperature (SST) variability and their representation in CMIP6 models. The first EOF mode of SIO SST, featured by an SST anomaly elongated from the northwestern to the southeastern SIO region is triggered by the El Niño/Southern Oscillation (ENSO). ENSO-induced wind anomalies over the SIO region weaken the climatological southeasterlies, reduce evaporative cooling, and consequently warm the SST in subtropics. The first SIO mode is closely related to the Indian Ocean Basin Mode. The CMIP6 models’ skill in simulating this mode is attributable to their accurate representation of ENSO impacts in this region. At the same time, the skill of simulation of the second mode, the Subtropical Indian Ocean Dipole (SIOD), is poor in most models due to the misrepresentation of the pure tropical Indian Ocean Dipole (IOD) events. As pure IOD events generate SIOD, underestimation of the occurrence of pure IOD events will affect the development of SIOD in models. The CMIP6 model experiments reveal the absence of ENSO forcing on SIOD. Notably, regardless of the bias in co-occurred and pure IOD events, most of the models overestimate IOD strength due to the easterly surface wind bias and the associated thermocline depth bias. The easterly bias in the equatorial Indian Ocean surface wind weakens the Wyrtki jet, creating a shallow (deep) thermocline bias in the eastern (western) TIO. This induces an overestimation of warm (cold) SST in the western (eastern) TIO during positive IODs.
... The SJC along the southern shore of Sumatra/Java is a semi-annually reversing current influenced by the northwest (southeast) monsoon during the austral summer (winter) (Quadfasel & Cresswell, 1992). During the southeast monsoon, southeasterly winds induce an offshore Ekman transport that generates strong upwelling along the Java west coast (Susanto et al., 2001;Wyrtki, 1973), shoaling the thermocline and changing the thermohaline structure in the subsurface layer (Annamalai et al., 2003;Du et al., 2005). Such instabilities in the background ocean currents and thermocline, resulting from the monsoon transition, may promote more mesoscale eddy generation. ...
Plain Language Summary
The Southeast Tropical Indian Ocean (SETIO) is a typical region of strong mesoscale (∼10–100 km) eddy generation. Eddies are circular currents that are important in moving heat, nutrients, and marine life around the ocean. The SETIO is also dominated by the Indian Ocean monsoon, which is a seasonal weather pattern that typically occurs in two main phases: the southwest monsoon from June to September, and the northeast monsoon from December to March. To date, the impacts of the Indian Ocean monsoon on the mesoscale eddies remain unclear. Based on satellite and reanalysis data sets, we found that there is a natural latitudinal change in the direction of eddies (anticlockwise/clockwise) formed north/south of 12°S in the summer monsoon, and that this pattern switches in the winter monsoon. The monsoon transition and associated changes to the ocean and its currents drives the dual‐pattern. The geographical boundary along 12°S occurs because it aligns with latitudinal changes in the energy stored in the eddies, which delineates a change in the direction of the newly‐formed eddies. This hot spot region, rich in eddy energy properties, promotes eddies formation and endurance during the monsoon periods.
... The tropical Indian Ocean is influenced by various water masses, including tropical surface waters, Arabian Sea highly saline waters, Bay of Bengal waters, and Indonesian throughflow; and is bounded by the Asian landmass to the north and the African landmass to the west (Talley et al., 2011;Emery, 2015;Kim et al., 2021), which affects its current patterns, nutrient redistribution, productivity, and biogeochemical properties (Sardessai et al., 2010;George et al., 2013). The equatorial Indian Ocean is influenced by the Wyrtki jet, a zonal current system characterized by a narrow, fast-flowing eastward current at the surface, which makes the equatorial Indian Ocean less productive than the equatorial Pacific or Atlantic (Wyrtki, 1973;George et al., 2013). The tropical Indian Ocean also includes the Seychelles Chagos Thermocline Ridge (SCTR), an area between 10°S and 5°S characterized by open ocean upwelling that brings nutrient-rich waters to the surface, resulting in high biological productivity (Hermes and Reason, 2008;Dilmahamod et al., 2016;Kang et al., 2021;Kim et al., 2022;Lee et al., 2022). ...
We investigated dual carbon isotopes within the vertical water column at sites 67-1 and 67-2 of the western equatorial Indian Ocean to determine the source and age of particulate organic carbon (POC) and thus evaluated the contributions of modern and fossil (aged) POC. The concentration of POC ranged from 7 to 47.3 μgC L⁻¹, δ¹³CPOC values ranged from –31.8 to –24.4‰, and Δ¹⁴CPOC values ranged from –548 to –111‰. Higher values of δ¹³CPOC and Δ¹⁴CPOC near the surface indicated an influence of autochthonous POC, whereas decreasing trends toward the bottom suggested a contribution of aged OC sources to the total POC pool. The contribution of fossil POC was lower near the surface, accounting for only 12% and 6% of the total POC at sites 67-1 and 67-2, respectively; however, in the deeper layers below 1,000 m, the contribution of fossil POC increased to 52% and 44% of the total POC at the two sites. Mechanisms for the increased contributions of fossil OC within deeper POC include the inflow of aged OC from sediments resuspended near slopes, the adsorption of old dissolved organic carbon in deep water masses, and the impact of aged OC that may originate from hydrothermal sources. This study highlights the importance of aged OC in the carbon cycle of the equatorial Indian Ocean.
... Note that the other major surface currents in the tropic flow are in the same direction as the prevailing winds, while the flow direction of the ECC is in the opposite direction of the surface winds (Wyrtki and Kendall, 1967;Hermes et al., 2019). Moreover, due to the influence of the monsoon, there exists an eastward Equatorial Jet in the Indian Ocean, also known as the Wyrtki Jet, during the boreal spring (e.g., April to June) and fall (e.g., October to December) in the area 10°S-10°N and 60°E-90°E (Wyrtki, 1973;Hermes et al., 2019). To take this Wyrtki Jet into account, an area 10°S-10°N and 60°E-100°E is chosen, see Figure 3. ...
Lewis Fry Richardson proposed his famous picture of turbulent flows in 1922, where the kinetic energy is transferred from large-scale to small-scale structures until the viscosity converts it into heat. This cascade idea, also known as the forward energy cascade, is now widely accepted and is treated as the cornerstone of not only turbulent modeling, but also global circulation models of the ocean and atmosphere. In this work, the Filter-Space-Technique is applied to the oceanic flow field provided by the CMEMS reanalysis model to quantify the scale-to-scale energy flux. A rich dynamical pattern associated with different scales is observed. More precisely, either positive or negative fluxes are observed, indicating the direction of the energy cascade, where the energy is transferred from large-scale structures to small-scale ones or vice versa. High-intensity energy exchange is found mainly in the Western Boundary Current Systems and Equatorial Counter Currents. For the latter case, a wavelike pattern is observed on the westward travel. Moreover, strong seasonal variation is evident for some scales and regions. These results confirm the existence of forward and inverse cascades and rich regional dynamics.
... However, considering the distribution of debris (more in the southern beaches compared to northern beaches) in Great and Little Nicobar Islands, the dominant role of SSW wave in accumulation of debris is apparent. Corroborating to our observation, Kiran (2017) found stronger currents in southern part of the Andaman sea and observed that equatorial Wyrtki jets (Wyrtki 1973) hits the Sumatra coast and re ect back as Rossby and coastal Kelvin waves; and these Kelvin waves propagate into the Andaman sea. As a result, the probability of transport from coutries like Indonesia, Malayasia, Thailand, Myanmar etc. is also more, and which has also been reported in the present study. ...
Debris from four beaches of the Great Nicobar and five beaches of Little Nicobar Islands were collected using transect based approach during March-April 2016 for assessing the status of marine debris. The collected debris were segregated into five types; plastics, fisheries, medical waste, house waste and food packaging and were analysed to estimate their composition, abundance, number and weight per unit area. Percentage contribution (number) of plastic debris (Great Nicobar (GN): 59.95%; Little Nicobar (LN): 53.02%), fisheries (GN: 17.88%; LN: 25.76%), house waste (GN: 11.07%; LN: 8.89%), medical waste (GN: 5.93%; LN: 6.04%), and food packaging (GN: 5.16%; LN: 6.28%) were determined and compared. Debris of foreign origin, mostly plastics of various colours, were reported in all the beaches while medical wastes were of local origin. The study focuses on plastic debris and its deleterious effects on the marine environment and discusses the role of wind, waves and shipping activities on the accumulation/movement of debris in the Andaman sea.
... The current magnitude is relatively stronger in the eastern Indian Ocean flowing eastward and can be found in the strong thermocline layer (pycnocline) below the upper mixing layer [16,17]. The center of the EUC is found on the equator at ± 1° north-south latitude and has a vertical thickness of about 100 m in the thermocline layer at 60° east longitude during the northeast monsoon period [18,19]. ...
Equatorial currents consisting of the North Equatorial Current (NEC) and Equatorial Under Current (EUC) play a significant role in the dynamics of the Indian Ocean with waters west of Sumatra. This article examines the water masses in the eastern Indian Ocean and the dynamics of the surface equatorial current at 90° BT longitude from 2° LS to 2° LU on March 1–3, 2017, as part of the Indonesia Prima 2017 expedition. NEC was detected at a depth of 5–75 m with a speed of 0–0.5 m/s to the west. EUC was detected at a depth of 75–170 m with a speed of 0.25–0.85 m/s to the east. The results of data analysis of temperature, salinity, and density indicate the intake of high-salinity water masses from the southern Indian Ocean, namely South Indian Central Water (SICW), which has a temperature range of 8.0–25.01 °C and salinity values of 34.6–35.2 PSU. SICW is carried by SEC. The movement of SEC during this study has a characteristic tendency to be asymmetrically stronger toward the north of the equator. The heat transported by the NEC is − 0.0215 PW (toward the Indian Ocean) and the EUC is 0.0347 PW (toward the west of Sumatra). There is added heat transport to the west of Sumatra by 0.0132 PW so that the thermocline layer thickens.
... In general, during the summer monsoon (June to September), SW winds prevail bringing precipitation over South Asia and promoting the flow of the Southwest Monsoon Current, whereas during the winter season (November-April), NE winds promote the flow of the North Equatorial Current westwards (Tomczak and Godfrey, 2003). During the intermonsoon seasons, particularly in October-November, intense westerly winds, the Indian Ocean equatorial westerlies (IEW) or Wyrtki jets, develop in the equatorial region (Wyrtki, 1973). Those westerly winds introduce strong currents into the Maldives Inner Sea and produce intense mixing of the upper water column. ...
The Maldives Archipelago (Indian Ocean), composed of two rows of atolls that enclose an inner sea, offers an excellent study site to explore the forcings of carbonate production at platforms. Glacial–interglacial sea-level changes have been claimed to be the main factor controlling the carbonate platform factories; however, climatic factors may also have an impact. In this work we used geochemical compositional records, obtained by X-ray fluorescence (XRF) core-scanning from the International Ocean Discovery Program (IODP) Site U1467 in the Maldives Inner Sea, to analyze the orbitally driven fluctuations on the carbonate production and export from the neritic environment into the Maldives Inner Sea over the last 1.3 million years.
High Sr aragonite-rich carbonates (HSAC) from neritic settings were deposited in the Maldives Inner Sea during sea-level highstand intervals, increasing the Sr/Ca values. In contrast, low Sr/Ca values are observed coincident with sea-level lowstand periods, suggesting that large areas of the atolls were exposed or unable to grow, and therefore, there was a demise in the carbonate production and sediment export to the Maldives Inner Sea. However, comparison of the Sr/Ca values and the sea-level reconstructions for different interglacial periods before and after the mid-Brunhes event (MBE, ∼ 430 ka) indicates that sea level is not the only factor controlling the production of HSAC during sea-level highstands. The study of monsoon and primary productivity proxies (Fe-normalized, Fe/K, and Br-normalized records) from the same site suggests that the intensity of the summer monsoon and the Indian Ocean dipole probably modulated the carbonate production at the atolls. Moreover, Marine Isotope Stage 11 stands out as a period with high sea level and extraordinary carbonate production in the Maldives platform. This outstanding carbonate production in the Maldives atolls (and in other low-latitude carbonate platforms) probably contributed to the mid-Brunhes dissolution event through a strong shelf-to-basin fractionation of carbonate deposition.
... Dynamically, these differences between JJA and DJF are in line with the seasonal variations in the Walker circulation and Hadley circulation (Schwendike et al., 2015). In JJA, the tropical wind can also trigger ocean circulation jet and Kelvin waves (Wyrtki, 1973) to remotely influence AJS. ...
Nitrate is one of the essential variables in the ocean that is a primary control of the upper ocean pelagic ecosystem. Its three-dimensional (3D) structure is vital for understanding the dynamic and ecosystem. Although several gridded nitrate products exist, the possibility of reconstructing the 3D structure of nitrate from surface data has never been exploited. In this study, we employed two advanced artificial intelligence (AI) networks, U-net and Earthformer, to reconstruct nitrate concentration in the Indian Ocean from surface data. Simulation from an ecosystem model was utilized as the labeling data to train and test the AI networks, with wind vectors, wind stress, sea surface temperature, sea surface chlorophyll-a, solar radiation, and precipitation as the input. We compared the performance of two networks and different pre-processing methods. With the input features decomposed into climatology and anomaly components, the Earthformer achieved optimal reconstruction results with a lower normalized mean square error (NRMSE = 0.1591), spatially and temporally, outperforming U-net (NRMSE = 0.2007) and the climatology prediction (NRMSE = 0.2089). Furthermore, Earthformer was more capable of identifying interannual nitrate anomalies. With a network interpretation technique, we quantified the spatio-temporal importance of every input feature in the best case (Earthformer with decomposed inputs). The influence of different input features on nitrate concentration in the adjacent Java Sea exhibited seasonal variation, stronger than the interannual one. The feature importance highlighted the role of dynamic factors, particularly the wind, matching our understanding of the dynamic controls of the ecosystem. Our reconstruction and network interpretation technique can be extended to other ecosystem variables, providing new possibilities in studies of marine environment and ecology from an AI perspective.
... It should be noted that the MJO can also drive Wyrtki jets, which can contribute to zonal advection (e.g., McPhaden & Foltz, 2013). The Wyrtki jets have a strong seasonal dependence typically being more pronounced during fall and spring while the Equatorial Undercurrent is more evident when the winds have an easterly component from February to June (Prerna et al., 2019;Schott et al., 2009;Wyrtki, 1973). As such, removing the seasonal cycle and limiting the composites to just the boreal winter season minimizes the contribution of Wyrtki jets and the Equatorial Undercurrent to zonal advection, but it does not completely remove it. ...
In the equatorial Indian Ocean, the largest subseasonal temperature variations in the upper ocean are observed below the mixed layer. Subsurface processes can influence mixed layer temperature and consequently air‐sea coupling. However, the physical processes driving temperature variability at these depths are not well quantified. During the boreal winter, the Madden–Julian Oscillation (MJO) partly drives upper ocean heat content (OHC) variations. Therefore, to understand processes driving subseasonal OHC variability in the equatorial Indian Ocean, we use an observationally constrained, physically consistent ocean state estimate from the Estimating the Circulation and Climate of the Ocean (ECCO) Consortium. Using a heat budget analysis, we show that the main driver of subseasonal OHC variability in the ECCO ocean state estimate is horizontal advection. Along the equator, OHC variations are driven by zonal advection while the role of meridional advection becomes more important away from the equator. During the active phase of the MJO, net air‐sea heat fluxes damp OHC variability along the equator, while away from the equator net air‐sea heat fluxes partly drive OHC variability. Equatorial OHC variations are found to be associated with processes driven by Kelvin and Rossby waves consistent with previous studies. By quantifying the physical processes, we highlight the important role of ocean dynamics in contributing to the observed variations of subseasonal OHC in the equatorial Indian Ocean.
... The southwest monsoon current's transport in summer is consistently subtle due to the sinking of dense, high-salinity water (Vinayachandran et al., 2012). In the tropical IO, the Wyrtki Jet transports high-salinity water eastward during the monsoon transition periods in spring and autumn (Chi et al., 2021;Subrahmanyam et al., 2011;Wyrtki, 1973). The equatorward freshwater transport mainly occurs in the eastern and southern parts of the Bay of Bengal (BOB) (Han, Lawrence, & Webster, 2001;), while the cross-equator transport predominantly takes place at the eastern edge of the IO (Jensen, 2003;Miyama et al., 2003). ...
The Indian Ocean Dipole (IOD) has been extensively studied for its significant impact on salinity distribution in tropical regions. However, its off‐equatorial influence in the northeastern Indian Ocean (IO) has received limited documentation thus far. In October 2019, a buoy in the central Andaman Sea (AS) observed an extreme freshening event that resulted in intensified upper‐ocean stratification and increased internal wave activities. The salt budget evaluation revealed the dominant role of horizontal advection. Interannual variability in autumn and winter sea surface salinity (SSS) showed a significant correlation with the IOD. Extreme freshening was observed exclusively during strong positive IOD (pIOD) years. This freshening primarily resulted from the outflow of low‐salinity water from the northeastern coast of the AS, driven by an anomalous anti‐cyclonic coastal‐trapped circulation exclusive to strong pIODs in the northeastern IO. This circulation tends to hinder southward freshwater transport in the western region while enhancing it in the eastern region. This circulation pattern is primarily influenced by Kelvin wave forcing, which is triggered by robust equatorial easterly anomalies that are typically more pronounced during strong pIOD events and weaker during weak pIOD events. The anticipated increase in the frequency of extreme freshening events due to the greenhouse warming has the potential to significantly modify the salinity distribution and freshwater transport in the future.
... Although the cause of IODs is still an active area of research (e.g., (Allan et Ocean (off the east African coast). This leads to anomalous westerly winds (as opposed to 37 the typical easterly winds) over the equatorial Indian Ocean, which weakens the easterly 38 equatorial ocean currents (Wyrtki jets: (Wyrtki, 1973), resulting in a shallow thermocline, 39 and a lower (higher) sea level in the eastern (central) equatorial Indian Ocean (Lu et al.,40 2018; Vinayachandran et al., 2009Vinayachandran et al., , 2007. These dynamics brought about by the pIOD can 41 be considered as a near-reversal of the expected conditions for the region. ...
The influence of the Indian Ocean Dipole (IOD) on winds, waves, and water levels has been studied in considerable detail, but the effect of these anomalous IOD-induced met-ocean conditions on coastal zones has so far received limited attention. This study analyses the impact of IOD events on the morphological response of coral reef islands by undertaking a case study of five islands in the Maldivian archipelago during the 2019 extreme positive IOD (pIOD) event. A six-year dataset of shorelines before, during and after the pIOD event was analysed to establish the magnitude of year-on-year variability and seasonal oscillation trends in the shorelines, and thereby identify variability induced by the pIOD event. The results indicate a departure in the shorelines beyond the magnitude of the year-on-year variability during the mature phase of the pIOD event (September to November 2019) and interestingly, a substantial change in the morphodynamics of the shoreline in the months immediately following the end of the pIOD event (November 2019 to April 2020). It was identified that the anomalous met-ocean conditions during the pIOD event caused a change in the seasonally oscillating shoreline buffer around the island, with potential implications on the medium-term island change and overall stability for the vegetated island core. The results showed that it took up to two years after the end of the pIOD event for the shorelines to return to their pre-pIOD state.
... This inhibited near-surface primary production (Roxy et al., 2016). In addition, Wyrtki jets occurred regularly during the boreal spring intermonsoon in the EIO (Wyrtki, 1973). The main consequence of Wyrtki jets was the depression nitracline in the EIO (Wiggert et al., 2006), and the decreased nutrient concentrations resulted in the inhibition of phytoplankton photosynthesis. ...
... Intense equatorial westerlies led upwelling in the western sector of the Indian Ocean and increased the subsidence of the convection cell over WEIO (Hastenrath et al., 1993). In addition, it drives a narrow strong eastward flowing jet (Wyrtki Jet), causing thermocline to shoal (thin mixed layer) in the WEIO, and deepen in the EEIO (Wyrtki, 1973;Hastenrath et al., 1993). Therefore, existence of cool subsurface waters in the WEIO and a strong zonal subsurface δ 18 O m gradient in the equatorial Indian Ocean suggest that equatorial westerlies were strong after 2.3 Ma in the Indian Ocean. ...
Surface and subsurface dynamics of the western equatorial Indian Ocean (WEIO) significantly impacted the East African climate, and played an important role in hominin evolution and dispersal. Planktic foraminiferal abundance and stable isotope data from Site 241, helped to ascertain cool subsurface and thermocline shoaling in the WEIO along with a strong zonal subsurface δ18O gradient in the equatorial Indian Ocean between 2.3 and 0.4 Ma. This east-west contrast in the Indian Ocean triggered an intensification of the equatorial westerlies, which enhanced the subsidence of air convection over the WEIO and hampered moisture transport to East Africa, thus expansion of C4 grassland during the Pleistocene. The intensification of the East African monsoon was governed by positive wind-evaporation feedback during ~ 1.9-1.7 Ma, 1.2 -0.9 Ma, and 0.14-0.09 Ma, where increasing wind intensity of Findlater Jet enhanced the evaporation over WEIO. The wind-evaporation feedback drove short-lived wet-dry phases that influenced hominin speciation, adaptation, and dispersal.
Over the past two decades, numerous countries have actively participated in the International Argo Program, working toward the global “OneArgo” goal. China’s Argo program has deployed over 500 autonomous profiling floats in the Indo-Pacific, with 55 Beidou (BD) floats, equipped with the Beidou satellite communication system, currently operational. During the operation of the BD float network, we found that in addition to the limitation of floats battery, the loss may also be caused by communication loss due to the floats escaping from the Beidou-2’s short message coverage. In this study, float trajectories are simulated using velocity fields from an eddy-resolved resolution Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) model and a Lagrangian particle tracking model programmed to represent the vertical motions of profiling floats. The simulations can help to explore both the representativeness and the predictability of profiling float displacements. By deploying a large number of synthetic floats in the Lagrangian particle tracking system, we construct probability density functions (PDFs) of the simulated-float trajectory among key oceans, for example, a joint region of East Indian–South China Sea–Northwest Pacific Ocean (5°–40°N, 70°–140°E), which is generally similar to the location of the present BD float network. These statistics can help to estimate the chance of floats drifting into shallow seas (such as the East China Sea) and out of the coverage of the Beidou satellite communication. With this knowledge changes to the future China’s Argo observing system could be made.
The Indian Ocean is the third largest of the world's oceans, accounting for ~20 % of the global marine realm. It is geomorphologically complex, hosting a wide variety of ecosystems across basins, trenches, seamounts, ridges, and fracture zones. While modern exploration has contributed significantly to our knowledge of its coastal ecosystems, deeper waters (>1000 m) remain relatively unknown despite accounting for over 90 % of its total area. This study provides the first comprehensive review of the Indian Ocean's diverse deep sea, presenting ecosystem knowledge summaries for each major seafloor feature, contextualised with the broader historical, socioeconomic, geological, and oceanographic conditions. Unsurprisingly, some ecosystems are better characterised than others, from the relatively well-surveyed Java (Sunda) Trench and hydrothermal vents of the Carlsberg, Central and Southwest Indian Ridges, to the unexplored Southeast Indian Ridge and hadal features of the western Indian Ocean. Similarly, there is a large depth discrepancy in available records with a clear bias towards shallower sampling. We identify four outstanding problems to be addressed for the advancement of deep-sea research in the Indian Ocean: 1) inconsistencies in research extent and effort over spatial scales, 2) severe lack of data over temporal scales, 3) unexplored deep pelagic environments, and 4) a need to place the Indian Ocean's deep-sea ecosystems in a global context. By synthesising and championing existing research, identifying knowledge gaps, and presenting the outstanding problems to be addressed, this review provides a platform to ensure this forgotten ocean is prioritised for deep-sea research during the UN Ocean Decade and beyond.
The study examines the mechanisms of Tropical Indian Ocean (TIO) circulation biases in Phase 6 of the Coupled Model Intercomparison Project (CMIP6) historical simulations across four variants of the Community Earth System Model (CESM): CESM2, CESM2-FV2, CESM2-WACCM, and CESM2-WACCM-FV2. The dominant equatorial flow, known as the Wyrtki Jets (WJ), is considerably underestimated due to the predominance of easterly wind bias, with the fall WJs showing the least skill, particularly in CESM2-WACCM-FV2, due to the underestimated westerlies. However, the eastward Equatorial Undercurrent (EUC) is strongly overestimated in all models, with maximum amplitudes observed in CESM2-WACCM-FV2, followed by CESM2-FV2, due to wind-induced westward thermocline tilt through the positive Bjerknes feedback mechanism. The northward intensification and deepening of south equatorial currents are attributed to strong easterly wind stress in the southern TIO in CESM models. The equatorial easterly wind bias in CESM models originates from a southeasterly wind bias in their Atmospheric Model Intercomparison Project (AMIP) counterparts during June-August, with air-sea coupling driving the westerly wind bias over the equatorial region. Overly strong easterlies and the intensification of midlatitude westerlies in AMIP models contribute to the intensification and poleward shift of the subtropical gyre in CESM models. This, in turn, weakens the Agulhas leakage (AL) transport from the south Indian Ocean to the Atlantic, which is partially due to the underestimated Indonesian Throughflow (ITF) in CESM models.
In the northern Arabian Sea, high salinity levels are primarily sustained by year-round evaporation, driving the convective formation of Arabian Sea High Salinity Water (ASHSW) during the winter monsoon (November – February). Although precipitation has largely been discounted as a critical controlling mechanism for winter convection, recent years have seen a notable increase in extreme cyclones over the Arabian Sea, particularly in post-monsoon cyclones (September – December) since 2014. However, the extent to which these cyclone-induced freshwater inputs disrupt the region's freshwater balance (evaporation – precipitation) and impact ASHSW formation remains unclear. Here, we present observational evidence supported by a suite of model simulation experiments, revealing a significant weakening in ASHSW formation triggered and sustained by extreme tropical cyclones. The addition of freshwater reduces the density of high-salinity water, augmenting stratification and disrupting the convective sinking process, ultimately limiting the depth of convective mixing. This strengthened stratification stabilizes the water column, exacerbating warming trends and destabilizing the freshwater balance between the Arabian Sea and the Bay of Bengal. These findings underscore the profound implications of extreme cyclone-induced freshwater inputs.
The Island Mass Effect (IME) around the Maldives is responsible for intense blooms with distinct seasonal patterns. These blooms sustain the fishing industry of the archipelagic nation, a vital source of income that occupies about 30% of the population. Through high resolution ocean simulations, we explore the physical processes responsible for the increased productivity and its observed variability, and their sensitivity to changes in land distribution. Year-round the frictional break of the currents in the presence of shallow bathymetry produces a strong vertical shear in the flow that favors vertical mixing, independently of the currents direction. The impact of this mixing is visible at the ocean surface during March and April, when the mixed-layer is shallow and the ocean currents are generally weak. A different mechanism than observed in spring modulates the IME during the monsoon seasons, in both winter and summer, when the zonal currents forced by the strong winds cross the archipelago: the flow accelerates while squeezing between the atolls, giving rise to intense wakes, and strong upwelling originates in the lees in response to the kilometer-scale flow divergence. This upwelling creates an asymmetric cooling signal in the lee of the islands and obfuscates the effects of the enhanced vertical mixing. The land reclamation efforts being planned on some of the islands may undercut the upwelling that drives the blooms during the monsoon seasons.
The layered structure and seasonal variation of the water exchanges in the Sulawesi Sea areas, as well as the corresponding dynamics, are investigated using HYCOM data. A net inward water mass transport is identified in the upper and intermediate layers in the Sulawesi Sea and it is compensated by the net outward transport in the deep layer. As an important source of this net inward transport, the southward flow through the Sibutu Passage (denoted hereafter as Sibutu Flow) reaches 5.06 Sv in winter, accounting for ∼43% of the net inflow. However, it decreases substantially to 1.79 Sv in summer, accounting for only 12% of the inflow. Further analysis reveals that this Sibutu Flow modulates the seasonal variation of the corresponding outward flow at the Makassar Strait, the major exit of the water mass in the Sulawesi Sea. In winter, the enhancement of the Sibutu Flow associated with the greater intrusion of Kuroshio water into the Luzon Strait produces a stronger eastward pressure gradient force, suppressing the surface intrusion of the Mindanao Current. Conversely, this pressure gradient force weakens and shifts westward in summer, permitting a larger water intrusion from the eastern section. Given that the outward Makassar Flow is the main passage of the Indonesian Throughflow (ITF), our study highlights the essential role of the Sibutu Flow in modulating seasonal variabilities of the ITF via the Makassar Strait and, thus, the transfer of water masses and heat content between the Pacific and Indian Oceans.
This study examines direct current measurements in the central equatorial Indian Ocean during the positive phase of the extreme Indian Ocean Dipole (pIOD) in 2019. Analysis of near-surface zonal current at 77°E and 83°E reveals a notable delay in the onset of fall Wyrtki Jets (WJs), typically occurring in October. The WJs formed in December but persisted for a shorter duration. On the other hand, the subsurface currents display a strong eastward flow persistent from October 2019 to June 2020 which is abnormal to a normal condition. The anomalous subsurface zonal currents exhibited magnitudes three times stronger during pIOD 2019 compared to pIOD composites. Due to the continued sub-surface flow, wavelet analysis revealed a dominant semi-annual cycle near the surface and an annual cycle in the subsurface layers. To explore the underlying processes behind these anomalies, we used the Modular Ocean Model in conjunction with a linear, continuously stratified model. Our experiments revealed that the weakening of fall Wyrtki Jets in 2019 was driven by anomalous easterlies associated with the extreme pIOD phase. However, the persistence of eastward subsurface currents was attributed to anomalous westerlies in the western Indian Ocean. These westerlies generated Kelvin waves propagating eastward, which upon impinging the eastern boundary, reflected and propagated downwards as Rossby waves, ultimately intensifying the subsurface currents. Overall, our study underscores the significance of equatorial wave dynamics in driving currents during the extreme IOD event.
The basic structure and intraseasonal evolution of currents in the southeastern Andaman Sea was analyzed based on data collected in 2017 from two subsurface moorings (C1 and C5). Periodic variation in the upper ocean currents of the Andaman Sea was investigated by combining observational and satellite data. Mooring observations show that rapid changes of current speed and direction occurred in May and June, with a significant increase in current velocity at the C1 mooring. In the second half of the year, southward flow dominated at the C1 mooring, and alternating northward and southward flows were evident at the C5 mooring during the same period but the northward flow prevailed in boreal winter. In addition, analysis of the power spectra of the upper currents revealed that the tidal period at both moorings is primarily semidiurnal with weaker energy than that of the low-frequency currents. The upper ocean currents at the C1 and C5 moorings exhibited intraseasonal variation of 30–60 d and 120 d, while the zonal current at the C1 mooring exhibited a notable period of approximately 180 d. Further analysis indicated that the variability of currents in the Andaman Sea is influenced primarily by equatorial Kelvin waves and Rossby wave packets. Moreover, our results suggest that equatorial Kelvin waves from the eastern Indian Ocean entered the Andaman Sea in the form of Wyrtki Jets and propagated primarily along two distinct pathways during the observation period. In addition to coastal boundary Kelvin waves, it was found that a branch of the Wyrtki Jet that directly enters the Andaman Sea and flows northward along the slope of the continental shelf, and reflected Rossby wave packets by topography.
The influence of the Indian Ocean Dipole (IOD) on winds, waves, and water levels has been studied in considerable detail, but the effect of these anomalous IOD-induced metocean conditions on coastal zones has so far received limited attention. This study analyses the impact of IOD events on the morphological response of coral reef islands by undertaking a case study of five islands in the Maldivian archipelago during the 2019 extreme positive IOD (pIOD) event. A 6-year dataset of shorelines before, during, and after the pIOD event was analysed to establish the magnitude of year-on-year variability and seasonal oscillation trends in the shorelines and thereby identify variability induced by thepIOD event. The results indicate a departure in the shorelines beyond the magnitude of the year-on-year variability during the mature phase of the pIOD event (September–November 2019) and a substantial change in the morphodynamics of the shoreline in the months immediately following the end of the pIOD event (November 2019–April 2020). It was identified that the anomalous metocean conditions during the pIOD event caused a change in the seasonally oscillating shoreline buffer around the island, with potential implications on medium-term island change and overall stability for the vegetated island core. The results showed that it took up to 2 years after the end of the pIOD event for the shorelines to return to their pre-pIOD state.
A prominent ocean region exhibiting depleted oxygen concentration is the northern Indian Ocean, whose projected deoxygenation trend in response to climate change requires a comprehensive understanding of the roles of ocean dynamics. We present newly compiled in situ data across platforms (e.g. cruises, Argo, buoy) in the Indonesian coasts of Sumatra and Java between 2010–2022. Combined with reanalysis products, our data detect oxygen-depleted waters attributed to the eastward advection of the northern Indian Ocean waters and monsoon-driven coastal upwelling. Oxygen limited zones (OLZs, DO < 60 μmol kg-1) occupy various depths off the Sumatra-Java coasts, in which dissolved oxygen (DO) reaches ~40 μmol kg-1 in northwest Sumatra. The eastward propagating Equatorial Counter Current plays a major role in delivering the oxygen-depleted waters during the boreal summer; similarly, the South Equatorial Counter Current in the winter monsoon and the Wyrtki Jet during the transition months. Coastal upwelling regulates DO variations via primary production and the respiration of organic matter at intermediate depths in southern Java as the upwelled waters being advected westward towards Sumatra in the summer monsoon. Indonesian Throughflow with enriched organic matter modifies the oxygenated-depleted waters at its outlets. We observe a trend towards deepened OLZ in western Sumatra, while positive Indian Ocean Dipole events (2006, 2012, 2015, 2019) lower DO in the thermocline depths of southern Java on the interannual timescale. Altogether, high-resolution observational biogeochemical data are key to advance our understanding of dynamical DO changes in the southeastern tropical Indian Ocean under the global deoxygenation trend.
Future sea level rise under global warming poses serious risks of extreme sea level events in coastal regions worldwide. Numerous state-of-the-art climate models, with their relatively coarse horizontal resolution, may not adequately resolve coastal wave dynamics, leading to uncertainties in coastal sea level variability representation. This study compared eddy-resolving and non-eddying ocean models in reproducing sea level variability, focusing on the probability distribution along the western coast of India. The eddy-resolving model can simulate intraseasonal sea level variations associated with coastal waves driven by equatorial wind anomalies. The non-eddying model fails to capture over 81% of observed extreme sea level events, as shown in the probability distribution for intraseasonal time series. Although capable of simulating Indian Ocean Dipole-related low-frequency sea level anomalies, the non-eddying model does not replicate their connection to intraseasonal extreme events. The results suggest that climate model projections may underestimate future changes in extreme sea level events.
The present study adopts an energy-based approach to interpret the negative phase of Indian Ocean dipole (IOD) events. This is accomplished by diagnosing the output of hindcast experiments from 1958 to 2018 based on a linear ocean model. The authors have performed a composite analysis for a set of negative IOD (nIOD) events, distinguishing between independent nIOD events and concurrent nIOD events with El Niño–Southern Oscillation (ENSO). The focus is on investigating the mechanism of nIOD events in terms of wave energy transfer, employing a linear wave theory that considers the group velocity. The proposed diagnostic scheme offers a unified framework for studying the interaction between equatorial and off-equatorial waves. Both the first and third baroclinic modes exhibit interannual variations characterized by a distinct packet of eastward energy flux associated with equatorial Kelvin waves. During October–December, westerly wind anomalies induce the propagation of eastward-moving equatorial waves, leading to thermocline deepening in the central-eastern equatorial Indian Ocean, a feature absent during neutral IOD years. The development of wave energy demonstrates different patterns during nIOD events of various types. In concurrent nIOD–ENSO years, characterized by strong westerly winds, the intense eastward transfer of wave energy becomes prominent as early as October. This differs significantly from the situation manifested in independent nIOD years. The intensity of the energy-flux streamfunction/potential reaches its peak around November and then rapidly diminishes in December during both types of nIOD years.
Significance Statement
The present study provides an interpretation of wave energy transfer episodes in the upper ocean during the negative phase of the Indian Ocean dipole (IOD) based on the diagnosis of hindcast experiments. The results suggest that the reflection of Kelvin and Rossby waves at the eastern and western boundaries of the Indian Ocean (IO), respectively, accompanied by variations in thermocline depth, plays a crucial role in the development process of IOD events. Specifically, during the negative phase of the IOD, the tropical IO exhibits positive signals of energy-flux streamfunction in the Northern Hemisphere, along with positive signals of energy-flux potential associated with westerly wind anomalies occurring in October–December. These findings highlight the significance of these factors in shaping the characteristics of negative IOD events.
