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Knowledge of the effects of climate modes on the equatorial intermediate current (EIC) remains limited. This paper investigates exceptional events of the EIC in the Indian Ocean and their relationships with climate modes at various time scales by using observations, reanalysis outputs, and a continuously stratified linear ocean model (LOM). A mooring at 80°E from 2015 to 2019 revealed four exceptionally strong EIC events, occurring in 2015 July–August (JA), 2016 January–February (JF), 2016 JA, and 2019 JF. Component analysis revealed that these exceptional events are attributed to the co-occurrence of the seasonal components peaking during JF and JA, as well as the larger current anomalies associated with intraseasonal and interannual components. In the intraseasonal band, the Madden–Julian oscillation (MJO) generates a significant EIC anomaly through a 40–50-day process involving equatorial waves. The MJO exerts a substantial effect when the amplitude of the MJO index exceeds 1 and the oscillation is in phase 4. In the interannual band, El Niño–Southern Oscillation and the Indian Ocean dipole (IOD) can each independently contribute to the EIC anomaly. This contribution initiates during the mature phase and persists throughout the subsequent year. Concurrent occurrences of these effects can lead to larger interannual anomalies. Notably, El Niño–positive IOD (El Niño–pIOD) and La Niña–negative IOD (La Niña–nIOD) synergies are most pronounced in August and in January of the following year. Significance Statement: Four exceptional equatorial intermediate current (EIC) events characterized by strong eastward velocities are observed in the Indian Ocean during 2015–19. This study explored their underlying dynamics and their relationships with climate modes. Climatologically, the EIC has seasonal cycles with peaks in January–February and July–August, which is helpful in producing exceptional EIC events. However, the occurrence of these events is attributed to the intensity of the intraseasonal and interannual variability that occurs around the seasonal cycle peaks. This study revealed that climate modes, including the Madden–Julian oscillation (MJO), El Niño–Southern Oscillation (ENSO), and Indian Ocean dipole (IOD), contribute to exceptional EIC events in the intraseasonal and interannual bands. Notably, their influence is comparable, albeit contingent upon specific conditions. [ABSTRACT FROM AUTHOR]

The Indian Ocean dipole (IOD) is a prominent interannual phenomenon in the tropical Indian Ocean (TIO), influencing weather and climate globally, particularly during extreme IOD events. The IOD shows notable amplitude asymmetry in both observations and historical simulations from the phase 6 of Coupled Model Intercomparison Project (CMIP6), with positive events having a greater magnitude than negative events, mainly due to the negative nonlinear dynamical heating. However, simulations under the shared socioeconomic pathway 5-8.5 (SSP5-8.5) scenario indicate a notable reduction in IOD asymmetry. It shows that this reduction points to an increased frequency of extreme negative IOD events under global warming. The primary cause of this reduced IOD asymmetry is less negative nonlinear dynamical heating in future simulations, especially the nonlinear zonal advection. Under global warming, the increased atmospheric static stability weakens the large-scale atmospheric response to sea surface temperature (SST) anomalies forcing. This leads to reduced strength of nonlinear zonal advection, resulting in a decreased IOD asymmetry. Nevertheless, nonlinear vertical advection, another key factor in IOD asymmetry, remains comparable due to the increased upper-ocean stratification in the eastern TIO. The reduced inhibition of negative nonlinear zonal advection and the increased SST response to deepening thermocline contribute to the increased frequency of extreme negative IOD events. These changes underscore the potential risks associated with negative IOD events in a warming world, emphasizing the importance of understanding IOD dynamics for improved climate impact prediction and future preparedness. [ABSTRACT FROM AUTHOR]

The prediction skill for individual Madden–Julian oscillation (MJO) events is highly variable, but the key factors behind this remain unclear. Using the latest hindcast results from the subseasonal-to-seasonal (S2S) phase II models, this study attempts to understand the diverse prediction skill for the MJO events with an enhanced convective anomaly over the eastern Indian Ocean (IO) at the forecast start date, by investigating the preference of the prediction skill to the MJO-associated convective anomalies and low-frequency background states (LFBS). Compared to the low-skill MJO events, the high-skill events are characterized by a stronger intraseasonal convection–circulation couplet over the IO before the forecast start date, which could result in a longer zonal propagation range during the forecast period, thereby leading to a higher score for assessing the prediction skill. The difference in intraseasonal fields can further be attributed to the LFBS of IO sea surface temperature (SST) and quasi-biannual oscillation (QBO), with the high-skill (low-skill) events corresponding to a warmer (colder) IO and easterly (westerly) QBO phase. The physical link is that a warm IO could increase the low-level convective instability and thus amplify MJO convection over the IO, whereas an easterly QBO phase could weaken the Maritime Continent barrier effect by weakening the static stability near the tropopause, thus favoring eastward propagation of the MJO. It is also found that the combined effects of IO SST and QBO phases are more effective in influencing MJO prediction skill than individual LFBS. Significance Statement: Given the importance of the Madden–Julian oscillation (MJO) in the subseasonal predictability of global weather and climate, how well the MJO itself can be predicted is a matter of concern. This study reveals the critical observational factors that determine the variation in MJO prediction skill across events. Without making any presumptions about what the factors would be, the observed MJO events are separated according to their individual prediction skill, and the difference between the MJO events with higher and lower skill is then investigated. The results show that the low-frequency background states of the Indian Ocean sea surface temperature and the quasi-biannual oscillation are good indicators for MJO prediction skill, for their modulatory role in the MJO propagation range. [ABSTRACT FROM AUTHOR]

We study the near-inertial response of the salinity-stratified north Bay of Bengal to monsoonal wind forcing using 6 years of hourly observations from four moorings. The mean annual energy input from surface winds to near-inertial mixed layer currents is 10–20 kJ m−2, occurring mainly in distinct synoptic "events" from April–September. A total of fifteen events are analyzed: Seven when the ocean is capped by a thin layer of low-salinity river water (fresh) and eight when it is not (salty). The average near-inertial energy input from winds is 40% higher in the fresh cases than in the salty cases. During the fresh events, 1) mixed layer near-inertial motions decay about two times faster and 2) near-inertial kinetic energy below the mixed layer is reduced by at least a factor of three relative to the salty cases. The near-inertial horizontal wavelength was measured for one fresh and one salty event; the fresh was about three times shorter initially. A linear model of near-inertial wave propagation tuned to these data reproduces 2); the thin (10 m) mixed layers during the fresh events excite high modes, which propagate more slowly than the low modes excited by the thicker (40 m) mixed layers in the salty events. The model does not reproduce 1); the rapid decay of the mixed layer inertial motions in the fresh events is not explained by the linear wave propagation at the resolved scales; a different and currently unknown set of processes is likely responsible. [ABSTRACT FROM AUTHOR]

The East African Coastal Current (EACC) is an important western boundary current of the tropical south Indian Ocean and plays an important role in the ocean circulation and biogeochemical cycles in the Indian Ocean. This study investigates the interannual variability of the EACC and its dynamical mechanisms. The result shows that the EACC has interannual variability associated with El Niño–Southern Oscillation (ENSO) during 1993–2017. The EACC shows a significantly positive correlation with the Niño-3.4 index with a correlation coefficient of 0.65, lagging the Niño-3.4 index by 18 months. During the decaying phases of El Niño (La Niña) events, the negative (positive) sea level anomaly (SLA) propagates westward as upwelling (downwelling) Rossby waves from the southeast Indian Ocean to the southwest Indian Ocean and then strengthens (weakens) the EACC due to zonal SLA gradient off the East African coast under geostrophic equilibrium. The SLA gradually weakens in the southeast Indian Ocean during its westward propagation but strengthens in the southwest Indian Ocean promoted by local wind stress curl anomaly. This study can improve our understanding of the relationship between the western boundary current of the tropical south Indian Ocean and large-scale ENSO air–sea processes and is important for managing marine fisheries and ecosystems on the East African coast. [ABSTRACT FROM AUTHOR]

Recent research has shown that there is a projected increase in the frequency of strong positive Indian Ocean dipole (spIOD) events in terms of both sea surface temperature (SST) and precipitation. However, it is not clear whether the spIOD events defined by SST and precipitation, hereafter referred to as SST-spIOD and Pr-spIOD events, respectively, will increase at the same pace under greenhouse warming. Using climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and CMIP6 that can reasonably simulate the spIOD events, here we find that the occurrence of Pr-spIOD events increases much more than that of SST-spIOD events (+217% vs +77%) under the future high-emission scenario. The diverse response stems from a notably large increase in the spIOD events with strong rainfall but moderate SST anomalies (Pr-only spIOD), which is facilitated by a change in the local meridional Hadley circulation induced by land–sea thermal contrast in addition to the background mean-state SST warming. For the spIOD events with prominent anomalies in both rainfall and SST (concurrent spIOD), their occurrences are projected to increase, but their average SST amplitude is projected to weaken. The increased occurrence is associated with a westward-extended structural change induced by mean linear zonal advection feedbacks, while the weakened event-average SST amplitude results from a more stabilized atmosphere and coincides with the projected reduction in IOD SST skewness. Our results suggest that IOD-related mitigation strategies should consider the diverse responses of different kinds of spIOD events to greenhouse warming. [ABSTRACT FROM AUTHOR]

This study identifies two distinct patterns of the summer Indian Ocean dipole (IOD)—the coastal IOD and the offshore IOD—named based on the proximity of their eastern pole to Sumatra. Their spatial characteristics, evolutionary mechanisms, relationships with ENSO, impacts on precipitation, and the factors controlling the simulation performances of climate models are discussed. The coastal IOD has the same eastern pole as the conventional IOD, which is located off the coast of Sumatra, but its western pole is situated in the central region of the tropical Indian Ocean (TIO). In contrast, the offshore IOD shares the same western pole as the conventional IOD, which is located off the coast of Somalia, whereas its eastern pole is positioned in the central TIO. Regarding their evolutions, while they initially develop similarly, their later evolutions differ due to their distinct pole locations: The offshore IOD peaks in summer, while the coastal IOD can be sustained into autumn. The coastal IOD correlates to preceding and late ENSO states, but the offshore IOD does not, making it an independent internal mode of TIO. The two IODs affect climate differently, with only the coastal IOD affecting Australian rainfall. Climate models exhibit varied levels of performance in simulating the two IODs. Specifically, a stronger link between spring TIO rainfall and ENSO, as well as stronger southeasterly monsoon winds in the southern TIO, can enhance the coastal IOD modeling, while a stronger summer Somali jet benefits the simulation of the offshore IOD. Distinguishing these two IODs has implications for accurate diagnosis and prediction of the summer climate surrounding the TIO. Significance Statement: The Indian Ocean dipole (IOD) significantly influences the monsoon climate. Previous studies use a unified IOD index based on all-season sea surface temperature (SST) data of tropical Indian Ocean (TIO), resulting in a representation biased toward the autumn peak season. However, understanding SST patterns during summer is crucial for Asian summer monsoon precipitation, and what are the characteristic patterns of summer IOD remains unclear. This study fundamentally advances the understanding of the IOD's seasonal diversity by identifying two distinct dipole patterns in the summer TIO. These patterns differ from the conventional IOD in terms of spatial characteristics, evolutions, ENSO relationships, and precipitation impacts. Distinguishing between these two IODs is anticipated to improve the diagnosis and prediction of summer monsoon rainfall. [ABSTRACT FROM AUTHOR]

Atmospheric teleconnections from the Pacific El Niño are key to determining the East Asian winter climate. Using the database for policy decision-making for future climate change (d4PDF) large-ensemble simulations, the present study investigates a mechanism for the warm and cold East Asian winters during El Niño with a focus on atmospheric teleconnections triggered by anomalous sea surface temperature (SST) patterns in the tropical Indo-Pacific. Our results show that the western Pacific (WP) teleconnection pattern plays a primary role in the warm winters in East Asia. The WP pattern tends to appear in years when both an early El Niño and the positive phase of the Indian Ocean dipole (IOD) mode develop in boreal autumn. In those years, the tropical Indian Ocean (TIO) strongly warms in the following winter, forming a distinct zonal contrast in precipitation anomalies over the tropical Indo-Pacific through a reduced Walker circulation. The Rossby wave source anomalies indicate that the WP pattern is associated with the weakened Indo-Pacific Walker circulation. By contrast, the WP pattern does not dominate in the cold winters due to the absence of strong TIO warming. The present study proposes a mechanism that promotes the excitation of the WP pattern through the upper-troposphere divergence in East Asia associated with the Walker circulation modulated by the tropical Indo-Pacific interbasin interaction. Significance Statement: The East Asian winter temperature variability is controlled not only by the strong atmospheric internal variability in the midlatitudes and high latitudes but also by remote forcing from the tropical ocean. Our study investigates how El Niño exerts diverse impacts on the East Asian winter temperature, depending on where atmospheric convection intensifies in the tropical Pacific Ocean and the Indian Ocean. Our results show that an intense warming of the tropical Indian Ocean and the early development of El Niño are the major factors for warm winters in East Asia. Given that a precursor of the intense Indian Ocean warming appears in boreal autumn, our findings should contribute to the improvement of seasonal prediction for the East Asian winter climate. [ABSTRACT FROM AUTHOR]

On decadal time scales, a zonal SST dipole dominates the tropical Indian Ocean in boreal late summer and fall, called the decadal Indian Ocean dipole (D-IOD). The D-IOD has a spatial pattern different from the traditional interannual IOD, with its eastern pole located off Java, rather than the whole Sumatra–Java coasts as the latter. Here, we show that the D-IOD is generated by both the remote tropical Pacific decadal variability (TPDV) forcing and the decadal modulation of interannual IODs, but with its distinctive spatial pattern and seasonality mainly shaped by the former. In August–September (AS), due to the seasonal strengthening of trade winds, the descending branch of TPDV-induced Walker circulation moves westward into the eastern Indian Ocean relative to June–July, which stimulates equatorial easterly anomalies and oceanic upwelling Kelvin waves, causing subsurface cooling off Java. The subsurface cooling just occurs within the time window of climatological coastal upwelling so that subsurface cold anomalies are brought into the surface by mean upwelling and further transported offshore by mean flows, forming the D-IOD eastern pole. The subsurface cooling is only generated near Java but not Sumatra, because the former is closer to the exit of the Indonesian Throughflow (ITF). Weakened ITF during positive TPDV inhibits the growth of subsurface warming off Java prior to the establishment of AS equatorial easterly anomalies, whereas this ITF effect is not observed off Sumatra. Moreover, warming of the D-IOD western pole might be associated with off-equatorial Rossby waves induced by TPDV-related wind stress curls. Significance Statement: The decadal Indian Ocean dipole (D-IOD) is one of the leading decadal modes of the tropical Indian Ocean SST variations. Its formation mechanisms, especially those related to its spatiotemporal characteristics, are not well understood. Based on observations and reanalysis, we show that the tropical Pacific decadal variability (TPDV) mainly accounts for the distinctive spatial pattern and seasonality of the D-IOD via the seasonal migration of the anomalous Walker circulation and ensuing oceanic subsurface dynamics. Our results highlight that the TPDV is an important source of D-IOD's predictability, and it might be beneficial for operational decadal predictions for the tropical Indian Ocean. [ABSTRACT FROM AUTHOR]

Indian Ocean (IO) stratification has important effects on the air–sea interaction, ocean dynamics, and ecology. It is, therefore, of significance to investigate the changes in IO stratification. In this study, we use ensemble empirical mode decomposition (EEMD) to extract the nonlinear long-term trend in the upper IO stratification quantified by potential energy anomalies. The results show that the strengthening of the stratification is spatially and temporally nonuniform. Specifically, the trend of stratification intensified gradually before 1996, but accelerated rapidly after 1996. Temperature and salinity changes play a crucial role in the fast enhancement of stratification and its regional differences. Temperature variations dominate the stratification trend in ∼90% of the IO area, while the contributions of salinity changes are mainly in the southeast Indian Ocean (SEIO). Vertically, the rapid enhancement of stratification is caused by the trend of temperature and salt in the upper 400 m. We further perform temperature budget analysis and find that the warming trend in the upper 400 m south of the IO is mainly modulated by vertical advection and meridional advection, while the warming in the north of the IO is mainly induced by air–sea heat fluxes. Salinity budget analysis shows that ocean advection has played a primary role in modulating SEIO salinity over the past 20 years. [ABSTRACT FROM AUTHOR]

Winter precipitation is critical for glacier growth, snow accumulation, and terrestrial storage on the Tibetan Plateau (TP). However, less attention has been paid to the changes in winter precipitation on the TP. In this study, based on the newly developed high-accuracy precipitation dataset, our diagnosis illustrated a distinct precipitation deficit over the southern TP and northern Indian continent (STNI) in December. Together with the ERA5 reanalysis, CESM1.1 large ensemble experiments, and pacemaker experiments, the dynamic diagnosis revealed that the precipitation deficit was attributed to the decrease in dynamic effects. In particular, the enhanced descending motion, accompanied by the strengthened anticyclone and regional meridional circulation over the STNI, resulted in the precipitation deficit. The changes in circulation system related to the precipitation deficit could be attributed to the Pacific and Indian sea surface temperature (SST) variability. With the negative interdecadal Pacific oscillation (IPO), there were circumglobal alternative geopotential height anomalies and an anomalous anticyclone over the southern TP, and descending motion around the STNI associated with the enhanced Walker circulation over the Indo-Pacific, which exhibited a similar pattern to the precipitation deficit over the STNI in the observations. In addition, the synergistic effect of Pacific and Indian SST variability can improve the simulated precipitation deficit over the STNI relative to the Pacific SST variability alone. These results imply a contribution of the tropical SST variability to the precipitation trends over the STNI in early winter and provide an insight into the understanding of climate warming on the winter climate over the TP. [ABSTRACT FROM AUTHOR]

Low-level cross-equatorial flows (CEFs) over the Indian Ocean–western Maritime Continent (IO-wMC) play a crucial role in transporting energy, mass, and water vapor into the Northern Hemisphere during the Asian summer monsoon season (May–September). Utilizing ECMWF reanalysis data (ERA5), we investigate three CEFs over the IO-wMC: the Somali-CEF, Bay of Bengal CEF (BoB-CEF), and South China Sea CEF (SCS-CEF). The statistical independence of the Somali-CEF and BoB-CEF implies distinct formation processes on the interannual time scale. To examine the interannual variability of CEFs, we perform an empirical orthogonal function (EOF) analysis of vertically integrated meridional winds from surface pressure to 850 hPa over the equatorial IO-wMC. The first EOF mode reveals a weakening of the Somali-CEF and strengthening of the BoB-CEF and SCS-CEF, which is associated with the concurrent summer Indian Ocean dipole and quasi-biennial phase transition of El Niño events. The second EOF is related to the Indo-western Pacific Ocean capacitor (IPOC) mode that often emerges in post–El Niño summers. Despite the IPOC's influence on meridional winds over the tropical southwestern Indian Ocean, three CEFs are uncorrelated with the EOF2. On the other hand, the Somali-CEF and BoB-CEF are significantly weakened in the EOF3. The EOF3 is an ENSO-unrelated internal IPOC mode. We further use a multilinear regression model based on preceding sea surface temperature (SST) over the Indo-Pacific and EOF3 to predict regional climate anomalies and compare the prediction skill with the SST-based models. Our results suggest that adding EOF2- and EOF3-related variability to the prediction model can improve Asian summer monsoon predictability. Significance Statement: This study examines the interannual variability of low-level cross-equatorial flows (CEFs) over the Indian Ocean and western Maritime Continent, crucial components of the Asian summer monsoon. We identify three CEFs and investigate their relationships with interannual sea surface temperature modes in the Indo-Pacific region. The leading empirical orthogonal function (EOF) modes of equatorial meridional wind reveal diverse CEF configurations and post–El Niño summer wind variability. Also, the EOFs distinguishes between El Niño and El Niño–unrelated equatorial meridional wind variability. Utilizing a multilinear regression model based on both preceding Indo-Pacific SST modes and configurations of CEFs, we demonstrate the potential to enhance regional climate predictions, contributing to a more comprehensive understanding and forecasting of the Asian summer monsoon and its associated climate impacts. [ABSTRACT FROM AUTHOR]

In the eastern off-equatorial Indian Ocean, deep current intraseasonal variability within a typical period of 10–20 days was revealed by a mooring at 5°N, 90.5°E, accounting for over 50% of the total bottom subtidal velocity variability. The 10–20-day oscillations were more energetic in the cross-isobathic direction (STD = 3.02 cm s−1) than those in the along-isobathic direction (STD = 1.50 cm s−1). The oscillations were interpreted as topographic Rossby waves (TRWs) because they satisfied the TRWs dispersion relation that considered the smaller Coriolis parameter and stronger β effect at low latitude. Further analysis indicated significant vertical coupling between the deep cross-slope oscillations and cross-isobathic 10–20-day perturbations at the depth of 300–950 m. The 10–20-day TRWs were generated by cross-isobathic motions under the potential vorticity conservation adjustment. The Mercator Ocean output reproduced the generation of kinetic energy (KE) of deep current variability. The associated diagnostic analysis of multiscale energetics showed that the KE of TRWs was mainly supplied by vertical pressure work. In the seamount region (2°–10°N, 89°–92°E), vertical and horizontal pressure works were identified to be the dominant energy source (contributing to 94% of the total KE source) and sink (contributing to 98% of the total KE sink) of the deep current variability, transporting energy downward and redistributing energy horizontally, respectively. [ABSTRACT FROM AUTHOR]

The Indian Ocean dipole (IOD) is the dominant mode of interannual variability in the tropical Indian Ocean (TIO), characterized by warming (cooling) in western TIO and cooling (warming) in eastern TIO during its positive (negative) phase. Observed IOD events exhibit distinct amplitude asymmetry in relation to negative nonlinear dynamic heating. Nearly all models in phase 5 of the Coupled Model Intercomparison Project (CMIP) simulate a less-skewed IOD than observed, but 6 out of 20 CMIP6 models can reproduce realistic high skewness. Analysis of less-skewed models indicates that the positive IOD-like biases in the mean state, which can be traced back to their weaker simulations of the preceding Indian summer monsoon, reduce the convective response to positive sea surface temperature anomalies in the western TIO, resulting in a weaker zonal wind response and weaker nonlinear zonal advection during positive IOD events. Besides, ocean stratification in the eastern TIO influences the IOD skewness: stronger stratification leads to larger mixed-layer temperature response to thermocline changes, contributing to larger anomalous vertical temperature gradient, larger nonlinear vertical advection, and thus stronger positive IOD skewness. Our findings underscore the importance of reducing Indian summer monsoon biases and eastern TIO stratification biases, for properly representing the IOD in Earth system models. [ABSTRACT FROM AUTHOR]

This study explores the key differences between single-year (SY) and multiyear (MY) El Niño properties and examines their relative importance in causing the diverse evolution of El Niño. Using a CESM1 simulation, observation/reanalysis data, and pacemaker coupled model experiments, the study suggests that the Indian Ocean plays a crucial role in distinguishing between the two types of El Niño evolution through subtropical ENSO dynamics. These dynamics can produce MY El Niño events if the climatological northeasterly trade winds are weakened or even reversed over the subtropical Pacific when El Niño peaks. However, El Niño and the positive Indian Ocean dipole (IOD) it typically induces both strengthen the climatological northeasterly trades, preventing the subtropical Pacific dynamics from producing MY events. MY events can occur if the El Niño fails to induce a positive IOD, which is more likely when the El Niño is weak or of the central Pacific type. Additionally, this study finds that such a weak correlation between El Niño and the IOD occurs during decades when the Atlantic multidecadal oscillation (AMO) is in its positive phase. Statistical analyses and pacemaker coupled model experiments confirm that the positive AMO phase increases the likelihood of these conditions, resulting in a higher frequency of MY El Niño events. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Droughts are becoming more frequent and severe in southwest China, with Yunnan being the most affected region. Most of the current studies on droughts in Yunnan are limited to individual cases and lack a common determination of the causes of historical drought events. In this study, we analyzed drought characteristics and causes over the last 60 years (1961–2020) in Yunnan Province in two seasons: dry and rainy. There was a clear trend of aridity in Yunnan Province in terms of both temporal and spatial changes, and there was a long-term drought period in approximately 2010; in particular, the frequency of drought in the dry season significantly increased. Due to its unique geographical location, precipitation in Yunnan Province is influenced by various factors, and the main influencing factors differed in different periods. SST variation is the most important factor affecting Yunnan drought. Precipitation in Yunnan Province is affected by the warm pool of the Indian Ocean throughout the year, and thermal anomalies in different locations of the Pacific Ocean have different effects on Yunnan Province and often overlap with the effects of SST anomalies in the Indian Ocean. Significance Statement: In this study, we analyzed drought characteristics and causes over the last 60 years (1961–2020) in Yunnan Province in two seasons: dry and rainy. The causes of historical drought events in Yunnan Province in terms of common characteristics were determined, and SST variation was the most influential factor, with the Indian Ocean and the Pacific Ocean having the greatest influence. Increased temperature may be a more important cause of dry season drought in Yunnan Province, while drought in the rainy season is mainly due to reduced water vapor transport. These results point the way toward better understanding of the occurrence of drought in Yunnan Province. [ABSTRACT FROM AUTHOR]

Previous studies have hypothesized that climatologically thick salinity-stratified barrier layers (BLs) in the north Indian Ocean (NIO) influence the upper ocean heat budget, sea surface temperature (SST), and monsoons. Here, we investigate how state-of-the-art Coupled Model Intercomparison Project phase 6 (CMIP6) climate models simulate the NIO barrier layer thickness (BLT). CMIP6 models generally reproduce the BLT seasonal cycle and spatial distribution, but with shallow November–February (NDJF) biases in regions with thick observed BLT: the eastern equatorial Indian Ocean (EEIO), Bay of Bengal (BoB), and southeastern Arabian Sea (SEAS). We show that the intensity of the CMIP6 equatorial easterly wind bias controls the EEIO shallow isothermal layer depth (ILD) and BLT biases. It also controls the BoB shallow BLT bias, both through the propagation of the EEIO shallow ILD bias into the NIO coastal waveguide and because it is linked to the BoB dry and cold bias through the Bjerknes feedback, hence also controlling the mixed layer depth (MLD) deep bias there. Finally, the SEAS shallow BLT bias is due to a too-deep MLD, in response to subdued monsoonal currents around India, which do not bring enough BoB low-salinity water. The BL insulating effect mentioned in literature does not seem to dominate in CMIP6. Rather, the CMIP6 salinity-related deep MLD biases diminish the BoB cooling rate by winter upward surface heat fluxes, reducing cold SST biases. This suggests that salinity effects alleviate the easterly equatorial wind, cold, and dry BoB biases that develop through the positive Bjerknes feedback loop in CMIP6. [ABSTRACT FROM AUTHOR]

The Indian Ocean basin (IOB) mode is the dominant mode of the interannual sea surface temperature (SST) variability in the Indian Ocean, with the Indian Ocean dipole (IOD) as the second mode. An IOB event normally occurs after an El Niño or a concurrent IOD–El Niño event, the dynamics of which are traditionally believed as forced by ENSO through the Walker circulation anomalies over the tropical Indian Ocean. A strong IOB in 2020 took place after the strongest 2019 IOD on record but independent of El Niño, which challenges the traditional atmospheric bridge dynamics of the IOB event. In this study, the dynamics of the 2020 IOB event are investigated using the numerical seasonal climate prediction system of the National Marine Environmental Forecasting Center of China. It is found that the initialization of the Indian Ocean subsurface temperature during the 2019 IOD event has led to the outburst of the 2020 IOB event successfully, the dynamics of which are the propagation and the western boundary reflection of the equatorial and off-equatorial Rossby waves, inducing heat content recharge over the tropical Indian Ocean upper thermocline. In comparison, experiments of SST initialization over the tropical Indian Ocean, with the subsurface temperature in a climatological state, were unable to reproduce the onset of the 2020 IOB event, suggesting that the local air–sea interaction within the Indian Ocean basin is of secondary importance. The numerical experiments suggest that the thermocline ocean wave dynamics play an important role in forcing the IOB event. The revealed thermocline dynamics are potentially useful in climate prediction associated with IOB events. [ABSTRACT FROM AUTHOR]

Previous studies have indicated that boreal winter-to-spring sea surface temperature anomalies (SSTA) over the tropical Atlantic or Indian Ocean can trigger the central-Pacific (CP) type of ENSO in the following winter due to winds over the western Pacific. Here, with the aid of observational data and CMIP5 model simulations, we demonstrate that the ability of the winter-to-spring north tropical Atlantic (NTA) SSTA or Indian Ocean Basin (IOB) mode to initiate CP ENSO events in the following winter may strongly depend on each other. Most warming events of the IOB and NTA, which are followed by CP La Niña events, are concomitant. The synergistic effect of the IOB and NTA SSTA may produce greater CP ENSO events in the subsequent winter via Walker circulation adjustments. The impacts between warming and cooling events of the IOB and NTA SSTA are asymmetric. IOB and NTA warmings appear to contribute to the subsequent CP La Niña development, which is much greater than IOB and NTA cooling contributing to CP El Niño. Overall, a combination of the IOB and NTA SSTA precursors may improve predictions of La Niña events. Significance Statement: Although boreal winter-to-spring sea surface temperature anomalies over the tropical Atlantic or Indian Ocean can trigger central-Pacific (CP) ENSO in the following winter, it is not yet clear whether the effects of these two basins are independent. The purpose of this study is to better understand the joint effect of these two basins on CP ENSO events. We demonstrate that the ability of the north tropical Atlantic (NTA) SSTA to initiate CP ENSO events in the following winter may strongly depend on the state of the Indian Ocean Basin mode (IOB). The synergistic impact of these two basins may produce stronger CP ENSO events. These results highlight the role of three-ocean interactions in ENSO diversity and prediction. [ABSTRACT FROM AUTHOR]

Air–sea coupling system in the southwestern Indian Ocean (SWIO; 35°–55°S, 40°–75°E) exhibits predominant multidecadal variability that is the strongest during austral summer. It is characterized by an equivalent barotropic atmospheric high (low) pressure over warm (cold) sea surface temperature (SST) anomalies and a poleward (equatorward) shift of the westerlies during the positive (negative) phase. In this study, physical processes of this multidecadal variability are investigated by using observations/reanalysis and CMIP6 model simulations. Results suggest that the multidecadal fluctuation can be explained by the modulation of the Atlantic meridional overturning circulation (AMOC) and the local air–sea positive feedback in the SWIO. In both observations/reanalysis and CMIP6 model simulations, the AMOC fluctuation presents a significantly negative correlation with the multidecadal SST variation in the SWIO when the AMOC is leading by about a decade. The mechanisms are that the preceding AMOC variation can cause an interhemispheric dipolar pattern of SST anomalies in the Atlantic Ocean. Subsequently, the SST anomalies in the midlatitudes of the South Atlantic can propagate to the SWIO by the oceanic Rossby wave under the influence of the Antarctic Circumpolar Current (ACC). Once the SST anomalies reach the SWIO, these SST anomalies in the oceanic front can affect the baroclinicity in the lower troposphere to influence the synoptic transient eddy and then cause the atmospheric circulation anomaly via the eddy–mean flow interaction. Subsequently, the anomalous atmospheric circulation over the SWIO can significantly strengthen the SST anomalies through modifying the oceanic meridional temperature advection and latent and sensible heat flux. [ABSTRACT FROM AUTHOR]

The influence of meridional shift of the oceanic subtropical front (STF) on the Agulhas Current (AC) regime shifts is studied using satellite altimeter data and a 1.5-layer ocean model. The satellite observations suggest the northward shift of the STF leads to the AC leaping across the gap with little Agulhas leakage, and the southward shift of the STF mainly results in the AC intruding into the Atlantic Ocean in the forms of a loop current and an eddy-shedding path, while there are three flow patterns of AC for moderate latitude of the STF. The ocean model results suggest no hysteresis (associated with multiple equilibrium states) exists in the AC system. The model reproduces similar AC regimes depending on different gap widths as in the observations, and model results can be used to explain the observed Agulhas leakage well. We also present the parameter space of the critical AC strength that results in different AC flow patterns as a function of the gap width. The vorticity dynamics of the AC regime shift suggests that the β term is mainly balanced by the viscosity term for the AC in the leaping and loop current paths, while the β and instantaneous vorticity terms are mainly balanced by the advection and viscosity terms for the AC in the eddy-shedding path. These findings help explain the dynamics of the AC flowing across the gateway beyond the tip of Africa affected by the north–south shift of the STF in the leaping regime or penetrating regime. [ABSTRACT FROM AUTHOR]

As one of the most prominent weather systems over the Indian subcontinent, the Indian summer monsoon low pressure systems (MLPSs) have been studied extensively over the past decades. However, the processes that govern the growth of the MLPSs are not well understood. To better understand these processes, we created an MLPS index using bandpass-filtered precipitation data. Lag regression maps and vertical cross sections are used to document the distribution of moisture, moist static energy (MSE), geopotential, and horizontal and vertical motions in these systems. It is shown that moisture governs the distribution of MSE and is in phase with precipitation, vertical motion, and geopotential during the MLPS cycle. Examination of the MSE budget reveals that longwave radiative heating maintains the MSE anomalies against dissipation from vertical MSE advection. These processes nearly cancel one another, and it is variations in horizontal MSE advection that are found to explain the growth and decay of the MSE anomalies. Horizontal MSE advection contributes to the growth of the MSE anomalies in MLPSs prior to the system attaining a maximum amplitude and contributes to decay thereafter. The horizontal MSE advection is largely due to meridional advection of mean state MSE by the anomalous winds, suggesting that the MSE anomalies undergo a moisture–vortex instability (MVI)-like growth. In contrast, perturbation kinetic energy (PKE) is generated through barotropic conversion. The structure, propagation, and energetics of the regressed MLPSs are consistent with both barotropic and moisture–vortex growth. [ABSTRACT FROM AUTHOR]

The relationship between the seasonal Antarctic sea ice concentration (SIC) variability and the extratropical southern Indian Ocean (SIO) sea surface temperature (SST) is explored in this study. It is found that the Antarctic SIC in a wide band of the SIO, Ross Sea, and Weddell Sea is significantly related to an SIO dipole (SIOD) SST anomaly on the interannual time scale during austral spring. This relationship is linearly independent of the effects of El Niño–Southern Oscillation, the Indian Ocean dipole, and the Southern Hemisphere annular mode. The positive phase of the SIOD, with warm SST anomalies off of western Australia and cold SST anomalies centered around 60°E in high latitudes, stimulates a downstream wave train that induces large-scale cyclonic circulations over the SIO and the Ross and Weddell Seas. Subsequently, anomalous horizontal moisture advection causes water vapor divergence, changes the surface energy budget, and cools the underlying ocean, which leads to the increased SIC over the region in the SIO, Ross Sea, and Weddell Sea. This SIOD SST anomaly reached a record low during the austral spring of 2016 and promoted the prominent wave pattern at high latitudes, contributing to the dramatic decline of sea ice in the 2016 spring. In addition, the proportion of the SIC trend that is linearly congruent with the SIOD SST trend during austral spring is quantified. The results indicate that the trend in the SIOD SST may account for a significant component of the 1979–2014 SIC trend in the Ross Sea with the congruency peaking at 60%. [ABSTRACT FROM AUTHOR]

The 1931 Yangtze River flood in eastern China, which had an associated death toll of over 2 million, is regarded as one of the world's deadliest natural disasters on record. However, due to the lack of meteorological data before the 1950s, the causes of this event are rarely investigated. Here, we combine multiple lines of evidence from recently available historical observations, reanalysis datasets, and atmospheric general circulation model simulations driven by historical sea surface temperatures (SSTs) that cover the early twentieth century to reveal the physical mechanisms underlying this flood. We find that the flooding in 1931 along the Yangtze River valley was dominated by July rainfall. Although the rainfall totals in July 1931 were not the largest in history (ranking second over the past century), the totals exceeded those for many other pluvial years between 1951 and 2010 in terms of its persistence, which was associated with a steady western Pacific subtropical high (WPSH). The flooding resulted from the combined effects of tropical El Niño–related SST forcing and extratropical wave activities over the Eurasian continent. On the one hand, warm SST anomalies in the tropical Indian Ocean following an El Niño event led to the southwestward extension of the WPSH. On the other hand, the southward shift of the westerly jet due to extratropical wave activities prevented the normal northward movement of the WPSH typical in July. Additionally, the impact that the preceding springtime precipitation and soil moisture had on the summer flood of 1931 is also discussed. [ABSTRACT FROM AUTHOR]

Marine heatwaves (MHWs) off Western Australia (110°–116°E, 22°–32°S; herein, WA MHWs) can cause devastating ecological impacts, as was evidenced by the 2011 extreme event. Previous studies suggest that La Niña is the major large-scale driver of WA MHWs, while the Indian Ocean dipole (IOD) may also play a role. Here, we investigate historical WA MHWs and their connections to these large-scale climate modes in an ocean model (ACCESS-OM2) simulation driven by a prescribed atmosphere from JRA-55-do over 1959–2018. Rather than analyzing sea surface temperature, the WA MHWs and climate mode indices were characterized and investigated in vertically averaged temperature (VAT) to ∼300-m depth to afford the longer ocean dynamic time scales, including remote oceanic connections. We develop a cyclostationary linear inverse model (CS-LIM; from 35°S to 10°N, across the Indo-Pacific Ocean), to investigate the relative contributions of La Niña VAT and positive IOD VAT to the predictability of WA VAT MHWs. Using a large ensemble of CS-LIM simulations, we found that ∼50% of WA MHWs were preceded about 5 months by La Niña, and 30% of the MHWs by positive IOD about 20 months prior. While precursor La Niña or positive IOD, on their own, were found to correspond with increased WA MHW likelihood in the months following (∼2.7 times or ∼1.5 times more likely than by chance, respectively), in combination these climate mode phases were found to produce the largest enhancement in MHW likelihood (∼3.2 times more likely than by chance). Additionally, we found that stronger and longer La Niña and/or positive IOD tend to lead stronger and longer WA MHWs. Significance Statement: This study examines seasonal-to-interannual time-scale predictability of marine heatwaves off Western Australia. We developed and applied a linear inverse model, informed by numerical model results, to generate a large number of 60-yr temperature simulations across the broader Indian–Pacific Ocean region to quantify this marine heatwave predictability. We found that La Niña typically increases the likelihood of marine heatwaves off Western Australia about 5 months (3–7 months) later, while positive Indian Ocean dipole events increase their likelihood about 20 months (18–22 months) later. Marine heatwaves can severely impact local marine ecosystems and the economy. Our findings are expected to be valuable for marine heatwave prediction system development on time scales that can be beneficial to marine ecosystem conservation and fishery management. [ABSTRACT FROM AUTHOR]

The southwest monsoon delivers over 70% of India's annual rainfall and is crucial to the success of agriculture across much of South Asia. Monsoon precipitation is known to be sensitive to sea surface temperature (SST) in the Bay of Bengal (BoB). Here, we use a configuration of the Unified Model of the Met Office coupled to an ocean mixed layer model to investigate the role of upper-ocean features in the BoB on southwest monsoon precipitation. We focus on the pronounced zonal and meridional SST gradients characteristic of the BoB; the zonal gradient in particular has an as-yet unknown effect on monsoon rainfall. We find that the zonal SST gradient is responsible for a 50% decrease in rainfall over the southern BoB (approximately 5 mm day−1), and a 50% increase in rainfall over Bangladesh and northern India (approximately 1 mm day−1). This increase is remotely forced by a strengthening of the monsoon Hadley circulation. The meridional SST gradient acts to decrease precipitation over the BoB itself, similarly to the zonal SST gradient, but does not have comparable effects over land. The impacts of barrier layers and high-salinity subsurface water are also investigated, but neither has significant effects on monsoon precipitation in this model; the influence of barrier layers on precipitation is felt in the months after the southwest monsoon. Models should accurately represent oceanic processes that directly influence BoB SST, such as the BoB cold pool, in order to faithfully represent monsoon rainfall. [ABSTRACT FROM AUTHOR]

Indonesian Throughflow (ITF) waters move along multiple pathways within the Indian Ocean. The western route is within the thermocline of the South Equatorial Current (SEC), and the southern route is via injection into the Leeuwin Current (LC) along western Australia. We use gridded Argo data to examine heat content anomaly (HCa) within three boxes in the eastern Indian Ocean, one adjacent to the ITF outflow from the Indonesian Seas (ITF box), the second in the eastern portion of the SEC (SEC box), and the third in the LC (LC box). Although interannual HCa variability in the SEC and ITF boxes is well correlated, a large increase in HCa within the ITF box does not appear in the SEC box in 2011 but is evident in the LC box. The 2011 change in the SEC–LC partitioning is investigated using GODAS reanalysis by examining the strength of the SEC and LC during a 2009 HCa increase within the ITF box and the subsequent increase in 2011. During 2009, a strong SEC and weakened LC spread the increased ITF HCa into the central Indian Ocean, whereas a weak SEC and strengthened LC during 2011 transmit the HCa signal to the south. Near-surface winds and mean sea level pressure from NCEP–NCAR reanalysis reveal that Ningaloo Niño events led to shifts in ocean circulation during 2000 and 2011. LC and SEC exports show a high negative correlation at interannual time scales, indicating that a reduction of outflow from one pathway is partially compensated by an increase from the other. [ABSTRACT FROM AUTHOR]

The Indian Ocean dipole (IOD) is one of the leading modes of interannual climate variability in the tropical Indian Ocean (IO). The paleoclimate provides real climate scenarios to examine IOD behaviors and the linkage to basic states. Based on 18 models from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5/6), the IOD change from the preindustrial period to the mid-Holocene is investigated. The multimodel mean reveals that the IOD variability weakens by 14%, as measured by the standard deviation of the dipole mode index, which is defined using the zonal sea surface temperature (SST) difference. Such attenuation is dominated by the spatially consistent suppression in the western-pole SST variability, while the eastern pole contributes little due to the opposite-signed changes in its northwestern and southeastern portions. The primary reason for the aforementioned changes comes from the altered climatic background, which displays a positive IOD-like pattern during IOD growing seasons, with intensified westward currents along the equator and northwestward currents in the southeastern equatorial IO. Such changes in the mean-state currents modulate the strength of the IOD-related anomalous advection and subsequently cause alterations in the IOD variability. Further analyses show that the IOD attenuation in the mid-Holocene is unlikely to be caused by the concurrently subdued El Niño–Southern Oscillation in the tropical Pacific, partially because of the diminished connections between the two oscillations themselves. The above simulated changes in both the IO mean climatology and IOD variability agree well with the available paleo records in literature. Significance Statement: Understanding variations in the Indian Ocean dipole (IOD) and its relationship to the altered background mean state can advance our knowledge of tropical climate dynamics. The paleoclimate provides the opportunity to address this issue under real climate scenarios in the past. Based on multiple models from CMIP5/6, we investigate IOD changes during the mid-Holocene compared to the preindustrial period. The result shows a weakening of the IOD, and the main mechanism lies in the altered anomalous advection modulated by changes in the mean-state currents. The simulated changes in both the mean state and IOD variability are consistent with available paleo data. The present study extends the research scale of IOD dynamics beyond instrumental periods and provides scientific bases for deciphering geological records. [ABSTRACT FROM AUTHOR]

Climate models typically have a cold sea surface temperature (SST) bias for the Arabian Sea, which regulates the Indian monsoon system as a water vapor source. Since the SST for the Arabian Sea is critical for the Indian monsoon, a better understanding of the processes that affect the SST is required. In this study, the effects of mesoscale oceanic variability on simulations of the Arabian Sea SST and Indian summer monsoon precipitation were investigated based on a comparison of climate model experiments in which non-eddying and eddy-permitting ocean components are coupled. In the eddy-permitting model, warm water advection driven by mesoscale variability near the Gulf of Aden and the Gulf of Oman increases the SST in the western Arabian Sea. Lateral eddy heat transport enhances warm water outflows to the Arabian Sea and suppresses surface water cooling by coastal upwelling during the southwestern monsoon season. Furthermore, a sensitivity experiment shows the primary importance of resolving oceanic mesoscale eddies and the secondary importance of resolving the Persian Gulf and Red Sea for the Arabian Sea SST. Also, the summer monsoonal precipitation decreases (increases) over the southeastern Arabian Sea (western and northern India) in the eddy-permitting model due to enhancement of wind convergence in the lower troposphere. Atmospheric general circulation model experiments indicate that the precipitation difference is partly caused by SST changes over the western Arabian Sea. The findings imply that the ocean resolution of climate models is a key factor in efficiently simulating the Indian monsoon. [ABSTRACT FROM AUTHOR]

The relatively weak sea surface temperature bias in the tropical Indian Ocean (TIO) simulated in the coupled general circulation model (CGCM) from the recently released CMIP6 has been found to be important in model simulations of regional and global climate. However, the cause of the bias is debated because the bias is strongly model dependent and shows marked seasonality. In this study, we separate the bias in CGCMs into bias arising from oceanic GCMs (OGCMs) and bias that is independent of OGCMs using a set of CMIP6 and OMIP6 models. We found that OGCMs contribute little to mixed layer bias in the CGCMs. The OGCM-independent bias exhibits a large-scale cold mixed layer bias in the TIO throughout the year, with an unexpectedly high degree of model consistency. By conducting a set of OGCM experiments, we show that the OGCM-independent mixed layer bias is caused mainly by surface wind bias in the utilized CGCMs. About 89% of surface wind bias in the CGCMs is due to the inability of atmospheric GCMs (AGCMs), whereas atmosphere–ocean coupling in the CGCMs has only a minor influence on surface wind bias. The bias in surface wind is also found to be the cause of subsurface temperature bias besides the ocean dynamics such as vertical mixing and vertical shear in currents. Our results indicate that correcting TIO mixed layer bias in CGCMs requires improvement in the capability of AGCM in simulating the climatological surface winds. Significance Statement: We aimed to discover the cause of temperature bias in the Indian Ocean in CMIP6 models. The bias was separated into oceanic model and ocean-model-independent bias to correspond exactly to bias caused by the oceanic model and by the atmospheric model and coupling, respectively. Oceanic bias contributes little to bias in CMIP6, but ocean-model-independent bias explains the CMIP6 bias throughout the year. We ran oceanic model experiments to show that surface wind bias causes ocean-model-independent temperature bias in the entire TIO and subsurface temperature bias in some areas of the Indian Ocean. We further found that 89% of surface wind bias originates from the atmospheric model. The results improve our understanding of the cause of the bias in the Indian Ocean and show that our method of bias separation is effective for attributing the source of bias to different proposed mechanisms. [ABSTRACT FROM AUTHOR]

Frequency spectra of in situ meridional velocity measurements in the central equatorial Indian Ocean show two distinct peaks at "quasi biweekly" periods of 10–30 days. One is near the surface at frequencies of 0.06–0.1 cpd (periods of 10–17 days) and the other is in the pycnocline (∼100-m depth) at lower frequencies of 0.04–0.06 cpd (periods 17–25 days). Analysis of a wind-forced ocean general circulation model shows that variability in the two frequency bands represents wind-driven mixed Rossby–gravity waves. The waves share a similar horizontal structure, but the meridional scale of lower-frequency variability is about one-half of that of higher-frequency variations. Higher-frequency variability has its largest amplitude in the eastern basin while the lower-frequency variability has its largest amplitude in the central basin. The vertical wavelength of lower-frequency variability is smaller by a factor of 3–4 than that of higher-frequency variability. These results are consistent with expectations from linear mixed Rossby–gravity wave theory. Numerical simulations show that the primary driver of these waves is surface wind forcing in the central and eastern Indian Ocean and that dynamical instability does not play a major role in their generation. Significance Statement: Spectra of meridional velocity in the central equatorial Indian Ocean from in situ measurements show two distinct peaks in the biweekly period band with different spatial structures. This study uses an ocean general circulation model to show that variability in these two bands is driven by surface winds that are themselves spatially structured. The variability in both period bands is consistent with linear mixed Rossby–gravity wave theory, but the spatial structures, including meridional trapping scale, vertical wavelength, and zonal distribution of energy, are very different. [ABSTRACT FROM AUTHOR]

Passage of the Indonesian Throughflow (ITF) water through the Indian Ocean constitutes an essential section of the upper limb of the global ocean conveyor belt. Although existing studies have identified a major exit of the ITF water to the Atlantic Ocean through the Agulhas Current system, our knowledge regarding other possible destinations and primary pathways remains limited. This study applies the Connectivity Modeling System (CMS) particle tracking algorithm to seven model-based ocean current datasets. The results reveal a robust return path of the ITF water to the Pacific Ocean. The partition ratio between the Atlantic and Pacific routes is 1.60 ± 0.54 to 1, with the uncertainty representing interdataset spread. The average transit time across the Indian Ocean is 10–20 years to the Atlantic and 15–30 years to the Pacific. The "transit velocity" is devised to describe the three-dimensional pathways in a quantitative sense. Its distribution demonstrates that the recirculation structures in the southwestern subtropical Indian Ocean favor the exit to the Atlantic, while the Antarctic Circumpolar Current in the Southern Ocean serves as the primary corridor to the Pacific. Our analysis also suggests the vital impact of vertical motions. In idealized tracing experiments inhibiting vertical currents and turbulent mixing, more water tends to linger over the Indian Ocean or return to the Pacific. Turbulence mixing also contributes to vertical motions but only slightly affects the destinations and pathways of ITF water. [ABSTRACT FROM AUTHOR]

The study evaluates the performance of the Conformal Cubic Atmospheric Model (CCAM) when simulating an urban heat island (UHI) over the city of eThekwini, located along the southeast coast of South Africa. The CCAM is applied at a grid length of 1 km on the panel with eThekwini, in a stretched-grid mode. The CCAM is coupled to the urban climate model called the Australian Town Energy Budget (ATEB). The ATEB incorporates measured urban parameters including building characteristics, emissions, and albedo. The ATEB incorporates the land-cover boundary conditions obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite. The CCAM configuration applied realistically captured the orientation of the city and land-cover types. Simulations of meteorological variables such as temperatures and longwave radiation reproduced the spatial distribution and intensity of the UHI. Results show that the UHI is stronger during summer and weaker in all other seasons. The UHI developed because of natural factors (e.g., distribution of longwave radiation) and human factors (e.g., urban expansion, an increase in anthropogenic emissions, and additional heating). Because of the city's location along the coast, the UHI simulation could be weakened by atmospheric circulations resulting from land and sea breezes. Mitigation methods such as applying reflective paints and revegetation of the city may increase albedo and latent heat fluxes but reduce the sensible heat fluxes and weaken the UHI. However, the UHI may not be completely eliminated since natural factors and emissions constantly influence its development. Significance Statement: The outcome of this study could be particularly valuable for municipalities in their disaster management planning since the occurrence of UHIs can cause heat-related diseases such as heatstrokes and even fatalities, especially for the elderly, in cities. Increases in temperatures also lead to higher demand for air conditioners, which in the long term lead to higher demand and pressure on the electricity grid system as well as increased costs for the individual. As higher temperatures increase heatwave events, increases in anthropogenic emissions also result in degraded air quality that impacts health. UHIs impact human lives and can cause deterioration in health when individuals experience high temperatures in summer. Warmer temperatures also reduce energy demand (and in the long term assist with global environmental restoration). [ABSTRACT FROM AUTHOR]

As subsystems of the Asian summer monsoon, summer precipitation variations in India and the northern part of eastern China (NEC) are physically connected. This study noted that the connection has been significantly enhanced after 1999 compared to 1979-98, which is due to the strengthened water vapor transportation connection between the two regions. That is associated with interdecadal variations of the combined effects of El Niño-Southern Oscillation (ENSO) and sea surface temperature anomalies (SSTAs) over the tropical Indian Ocean (TIO) on the northwest Pacific subtropical high (NWPSH) and the Indo-Pacific Walker cell. Against the background of La Niña, the strengthened NWPSH and Indo-Pacific Walker cell favor water vapor transport to India and the NEC since 1999. Accordingly, summer precipitation in the two regions increases simultaneously, leading to the enhancement of the summer precipitation teleconnection between them. In addition, the influence of TIO SSTAs and the Indian Ocean dipole (IOD) on Indo-Pacific circulations decreases, which further enhances the relative importance of ENSO on the summer precipitation in the two regions. However, during 1979-98, La Niña SSTAs has weak effects on the NWPSH and Indo-Pacific Walker cell, the negative TIO SSTAs significantly weaken NWPSH, and the negative IOD also obstructs the westward extension of the Indo-Pacific Walker cell. Circulations and water vapor transportation related to the Indian Ocean and NEC summer precipitation are inconsistent, resulting in a weak precipitation teleconnection between them. The above conclusions are also validated by extreme case analysis and CMIP6 models. [ABSTRACT FROM AUTHOR]

The Agulhas Current (AC) is a critical component of global ocean circulation. However, due to a lack of multidecadal observations, it is not clear how the AC has changed in response to anthropogenic forcing. A recent observational study suggests a broadening and slight weakening of the AC in the past few decades, while others suggest a strengthening of the AC during the historical period. In this paper, we find substantial internal variability of the AC on decadal to multidecadal time scales in high-resolution models. We show that the AC consistently exhibits two modes of decadal and multidecadal (i.e., low frequency) variability in a series of high-resolution climate models: a uniform mode that is largely associated with changes in the AC strength and a dipole mode that is mainly related to width changes of the AC. We demonstrate that the uniform mode is mainly forced externally by the decadal variations of the wind field and presents a decline under global warming, suggesting a weakening of the AC in response to anthropogenic forcing. The dipole mode, on the other hand, is mainly due to internal dynamics and does not show a trend during the historical period. Using a quasigeostrophic model that captures the dipole mode, we attribute the dipole mode to low-frequency potential vorticity changes in the western boundary, driven by a divergence of relative potential vorticity due to eddy activity. Thus, our results present further context for the interpretation of the AC responses in a changing climate based on a short observational record. [ABSTRACT FROM AUTHOR]

The heat content in the Indian Ocean has been increasing owing to anthropogenic greenhouse warming. Yet, where and how the anthropogenic heat is stored in the Indian Ocean have not been comprehended. Analysis of various observational and model-based datasets since the 1950s reveals a robust spatial pattern of the 0–700 m ocean heat content trend (ΔOHC), with enhanced warming in the subtropical southern Indian Ocean (SIO) but weak to minimal warming in the tropical Indian Ocean (TIO). The meridional temperature gradient between the TIO and SIO declined by 16.4% ± 7.5% during 1958–2014. The heat redistribution driven by time-varying surface winds plays a crucial role in shaping this ΔOHC pattern. Sensitivity experiments using a simplified ocean dynamical model suggest that changes in surface winds over the Indian Ocean, particularly those of the SIO, caused a convergence trend in the upper SIO and a divergence trend in the upper TIO. These wind changes primarily include the enhancements of westerlies in the Southern Ocean and the subtropical anticyclone in the SIO. Albeit with systematic biases, the ΔOHC pattern and surface wind changes simulated by phase 6 of the Coupled Model Intercomparison Project (CMIP6) models broadly resemble the observation and highlight the essence of external forcing in causing these changes. This heat storage pattern is projected to persist in the model-projected future, potentially impacting future climate. [ABSTRACT FROM AUTHOR]

Previous theoretical studies suggest that the topography along the west coast of Australia plays an important role in strengthening and trapping the Leeuwin Current (LC) at the coast. To isolate and quantify the effect of the continental shelf and slope on the LC and Ningaloo Niño, high-resolution (1/12°) ocean general circulation model experiments with different bottom topographies are performed. The "control" experiment uses a realistic bottom topography along the west coast of Australia, whereas the sensitivity ("no-shelf") experiment uses a modified topography with no continental shelf and slope near the coast. The mean and variability of LC are realistically simulated in the control experiment. Compared to the control experiment, the strength of LC in the no-shelf experiment decreased by about 28%. The continental shelf influences the development of the 2010/11 Ningaloo Niño through modulating the LC variability: in August–October 2010 and January–February 2011, the LC in the control experiment is enhanced much more than that in the no-shelf experiment. As a result, the upper-50-m ocean temperature in the control experiment is about 26% warmer than the no-shelf experiment from September 2010 to March 2011. Different evolution of SST warming is also found in the two experiments. Comparisons of oceanic processes in the two experiments show that the shelf-slope topography can effectively trap the positive sea level anomaly at the coast in the control experiment while more Rossby waves radiate from the coast in the no-shelf experiment, resulting in a weaker LC. [ABSTRACT FROM AUTHOR]

Eastern China (EC) suffered an extreme drought with long-lasting duration and record-breaking intensity in late summer–autumn 2019. Our diagnosed results show that the central Pacific (CP) El Niño, in tandem with warm sea surface temperature anomalies (SSTAs) over the Kuroshio Extension (KE) region, induces the meridionally elongated cyclonic circulation anomalies stretching from the western North Pacific (WNP) to the Yellow Sea. Its western flank corresponds to overwhelming low-level northerly wind anomalies over EC, which result in deficient moisture and anomalous descent over EC and hence cause the extreme drought in 2019. To investigate the relative contributions of SSTAs over different regions, we performed sensitivity experiments, and analyzed the relationship between extreme drought like what occurred in 2019 (a 2019Drought-like event) and the SSTAs in CMIP6 historical simulations. Modeling evidences reveal that both warm SSTAs over the central equatorial Pacific and the KE region are indispensable for shaping the meridionally elongated cyclone anomaly. Specifically, the cyclone anomaly over the WNP induced by CP El Niño aligns with the cyclone anomaly over the Yellow Sea induced by the warm SSTAs over the KE region, merging into a meridionally stretched cyclone anomaly to the east of EC. Consequently, the northerly anomalies stretch across EC, leading to unfavorable atmospheric conditions and the rainfall deficit there. Projection results show that the occurrence probability of a 2019Drought-like event will increase by 20% (decrease by 40%–50%) under a high (medium-low) emission scenario compared to present-day climate, indicating the nonlinear response of extreme drought to different emission scenarios and the urgency of carbon emission reduction. Significance Statement: An extreme drought hit the Eastern China (EC) region in 2019 and caused tremendous losses. This study proposed that both the 2019 CP El Niño and the warm SST anomalies over the Kuroshio Extension (KE) region induce the meridionally elongated circulation anomalies and the resultant extreme drought. We showed that the typical circulation anomalies induced by central Pacific (CP) El Niño cannot totally explain the meridionally elongated circulation anomalies in 2019. Our modeling evidences confirmed the indispensable role of warm SST anomalies over KE region in the 2019 extreme drought's formation. The projection results show that extreme drought like that in 2019 will occur more (less) frequently under a high (medium-low) emission scenario compared to modern-day level, indicating the urgency of carbon emission reduction. [ABSTRACT FROM AUTHOR]

Using the recently developed multiscale window transform and multiscale energy and vorticity analysis methods, this study diagnoses the climatological characteristics of the nonlinear mutual interactions among mesoscale eddies, low-frequency (seasonal to interannual) fluctuations, and the decadally modulating mean flow in the Agulhas Retroflection Current System (ARCS). It is found that mesoscale eddies are generated primarily in the retroflection region by mixed barotropic and baroclinic instabilities. The barotropic instability dominates the generation of eddy kinetic energy (EKE) here, contributing power roughly 10 times larger than the baroclinic one. These locally generated eddies are transported away. In the rings drift and meanders regions, the nonlocal transport serves as an important energy source for the eddy field, making a contribution comparable to that of the baroclinic instability for the EKE production. Contrarily, in the stable region, the EKE is generated mainly due to the baroclinic instability. In most of the ARCS area, the kinetic energy (KE) is further transferred inversely from mesoscale eddies to other lower-frequency motions. In particular, in the retroflection, rings drift, and stable regions, the inverse KE cascade plays a leading role in generating seasonal–interannual fluctuations, providing roughly 3–5 times as much power as the forward KE cascade from the mean flow and the advection effect. In the meanders region, however, the forward cascade contributes 4 times more KE to the low-frequency variabilities than the inverse one. All the results provide a model-based benchmark for future studies on physical processes and dynamics at different scales in the ARCS. [ABSTRACT FROM AUTHOR]

Moist static energy (MSE) and ocean heat content (OHC) in the tropics are inextricably linked. The processes by which sources and sinks of OHC modulate column integrated MSE in the Indian Ocean (IO) are explored through a reformulation of the MSE budget using atmosphere and ocean reanalysis data. In the reframed MSE budget, interfacial air–sea turbulent and radiative fluxes are replaced for information on upper ocean dynamics, thus "mooring" the MSE tendency to the subsurface ocean. On subseasonal time scales, ocean forcing is largely responsible for the amplification of MSE anomalies across the IO, with basin average growth rates of 10% day−1. Local OHC depletion is the leading contributor to anomalous MSE amplification with average rates of 12% day−1. Along the equator, MSE is amplified by OHC vertical advection. Ocean forcing only weakly reduces the propagation tendency of MSE anomalies (−2% day−1), with propagation predominantly resulting from atmosphere forcing (10% day−1). OHC in the IO acts as an MSE reservoir that is expended during periods of enhanced intraseasonal atmosphere convection and recharged during periods of suppressed convection. Because OHC is an MSE source during enhanced intraseasonal convection periods, it largely offsets the negative MSE tendency produced by horizontal advection in the atmosphere. The opposite effect occurs during suppressed convection periods, where OHC is a sink of MSE and counters the positive MSE tendency produced by horizontal advection in the atmosphere. [ABSTRACT FROM AUTHOR]

This study reveals that the relationship between autumn north tropical Atlantic (NTA) sea surface temperature (SST) anomalies and Northeast China's winter snowfall (NECWS) undergoes a remarkable interdecadal enhancement after 2001. Previous research confirmed that the relationship between the NTA SST anomaly and atmospheric circulation experienced interdecadal changes after the 2000s and suggested various reasons for this phenomenon. During 1961–2000, the NTA SST anomaly has a significantly positive correlation with other oceans, especially the tropical Indian Ocean (TIO), and the latter modulates the former's impact on atmospheric circulations over the Eurasian continent with a cancelling effect, which results in a weaker relationship of the NTA SST anomaly and NECWS. In contrast, the warm NTA SST anomaly is relatively independent from other oceans during 2001–20, and it proves to be the forcing factor for NECWS since its solo influence on the winter atmospheric circulations initiated from the North Atlantic to East Asia is more robust, featuring the negative phase of the North Atlantic Oscillation and a downstream quasi-barotropic Rossby wave train over the mid- to high latitudes of Eurasian continent. Accordingly, together with the deepened East Asian trough and the strongly northward transported humid and warm air from the western Pacific, the local significantly enhanced ascending motions with cooling temperature favor much more NECWS. The linear baroclinic model simulates the effects of NTA and TIO SST anomalies on winter atmospheric circulations, corroborating the aforementioned results. Such results can be used for the prediction of NECWS with respect to the precursor of the autumn NTA SST anomaly. [ABSTRACT FROM AUTHOR]

This study aims to better understand the ENSO impacts on climate anomalies over East Asia in early winter (November–December) and late winter (January–February). In particular, the possible mechanisms during early winter are investigated. The results show that ENSO is associated with a Rossby wave train emanating from the tropical Indian Ocean toward East Asia (denoted as tIO-EA) in early winter. This tIO-EA wave train in El Niño (La Niña) is closely related to a weakening (strengthening) of the East Asian trough, and thereby a weakened (strengthened) East Asian winter monsoon and warm (cold) temperature anomalies over northeastern China and Japan. By using partial regression analysis and numerical experiments, we identify that the formation of tIO-EA wave train is closely related to precipitation anomalies in the tropical eastern Indian Ocean and western Pacific (denoted as eIO/wP). In addition, the ENSO-induced North Atlantic anomalies may also contribute to formation of the tIO-EA wave train in conjunction with the eIO/wP precipitation. The response of eIO/wP precipitation to ENSO is stronger in early winter than in late winter. This can be attributed to the stronger anomalous Walker circulation over the Indian Ocean, which in turn is caused by higher climatological SST and stronger mean precipitation state in the Indian Ocean during early winter. [ABSTRACT FROM AUTHOR]

Leading modes of interannual variability in upper-ocean salinity in the tropical Indian Ocean (TIO) and their connections were studied based on 17 years (2002–18) of oceanic historical and reanalysis data. Empirical orthogonal function (EOF) analysis depicted the dominant roles of the first two leading modes in salinity variability in the TIO over a wide range of interannual time scales. Among the rich oscillations of the leading EOF modes, a coherent near-biennial band was identified with basinwide loading of sea surface salinity anomalies (SSSa) (EOF1) leading/lagging the northeast–southwest dipolar mode of SSSa (EOF2) by around 4 months across the TIO, with southwestward migration of SSSa center. The spatial loadings of the SSSa leading modes in the TIO were strongly shaped by sea surface temperature–related freshwater fluxes and wind-driven regional ocean circulation on a near-biennial time scale. Composite analysis of the mixed layer salinity budget reflected characteristic features of basin-scale ocean–atmosphere coupling, both temporally and regionally during the life cycle of the near-biennial fluctuation in anomalous salinity in the TIO. Consistent with the intrinsic oscillation paradigm in the observed Indian Ocean dipole (IOD) variation, the dynamic and thermodynamic feedbacks associated with switches from the positive to negative IOD modes provided the phase-connection mechanisms for the SSSa leading-mode displacement over the TIO. Significance Statement: This study investigates the leading modes of interannual variability in upper-ocean salinity in the tropical Indian Ocean (TIO). The intrinsic oscillation and associated dynamic and thermodynamic feedbacks over the TIO drive the basinwide connections of upper-ocean salinity variability. Our results show that a coherent near-biennial band is identifiable within the leading modes of sea surface salinity anomalies (SSSa), in which the wind-induced horizontal advections and evaporation-minus-precipitation anomalies associated with the switches from positive to negative Indian Ocean dipole modes mainly provide the phase-transition mechanism of SSSa. This research illustrates substantial evidence for the displacement of basin-scale sea surface temperature anomalies modulating the structures of SSSa and inducing the dynamical connections of leading modes of SSSa on the near-biennial time scale. [ABSTRACT FROM AUTHOR]

We present an internationally coordinated plan to consolidate and enhance the Indian Ocean Observing System (IndOOS) to meet future societal needs for climate information and prediction. The Indian Ocean Observing [...]

ON THE COVER: The study site of the Inner-Shelf Dynamics Experiment near Point Sal, California, reveals various processes that were studied, including flows around headlands, internal waves, and the offshore [...]

Precipitation variability over the Tibetan Plateau (TP) depends largely on the atmospheric circulation pattern associated with remote oceanic forcing, while the contribution of sea surface temperature anomalies (SSTAs) in different ocean basins, especially the tropical North Atlantic (TNA), remains unclear. By using multisource data and atmospheric general circulation model, this study reveals the individual and combined effects of the Indian Ocean basin mode (IOBM) and TNA SSTA on the interannual variability of TP precipitation in late summer (August). During the positive phase of the IOBM, warm SSTAs in the Indian Ocean induce an anomalous anticyclone over the Bay of Bengal (BOBAC) via the Kelvin wave–induced Ekman divergence and a resulting positive precipitation anomaly over the southeastern TP. Simultaneously, an eastward-extending Kelvin wave triggered by the positive TNA SSTA overlaps with that caused by the IOBM, further strengthening the BOBAC. In addition, the Kelvin wave triggered by the TNA SSTA induces anomalous easterlies over the tropical Indo-Pacific, which contribute to the warm Indian Ocean SSTA and thus amplify the IOBM affecting TP precipitation. Moreover, the positive TNA SSTA generates a westward-extending Walker circulation anomaly that is responsible for the suppressed convection over the central Pacific, which in turn triggers a Rossby wave response and further strengthens the BOBAC. As a result, the positive precipitation anomaly over the southeastern TP is strengthened significantly. Particularly, considering the 2–3-month lead time of the IOBM and TNA SSTA, the tropical SSTA can be used as a predictor for the TP precipitation anomaly. [ABSTRACT FROM AUTHOR]

Prominent interannual-to-decadal variations were observed in both heat content and mesoscale eddy activity in the southeast Indian Ocean (SEIO) during 1993–2020. The 2000–01 and 2008–14 periods stand out with increased 0–700-m ocean heat content (OHC) by ∼4.0 × 1021 J and enhanced surface eddy kinetic energy (EKE) by 12.5% over 85°–115°E, 35°–12°S. This study provides insights into the key dynamical processes conducive to these variations by analyzing observational datasets and high-resolution regional ocean model simulations. The strengthening of the Indonesian Throughflow (ITF) and anomalous cyclonic winds over the SEIO region during the two periods are demonstrated to be the most influential. While the ITF caused prevailing warming of the upper SEIO, the cyclonic winds cooled the South Equatorial Current and attenuated the warming in the subtropical SEIO by evoking upwelling Rossby waves. The EKE increase exerts significant influence on OHC only in the Leeuwin Current system. Dynamical instabilities of the Leeuwin Current give rise to high EKEs and westward eddy heat transport in climatology. As the Leeuwin Current was enhanced by both the ITF and local winds, the elevated EKEs drove anomalous heat convergence on its offshore flank. This process considerably contributes to the OHC increase in the subtropical SEIO and erases the wind-driven cooling during the two warm periods. This work highlights the vital role of eddies in regional heat redistribution, with implications for understanding time-varying ocean heat storage in a changing climate. [ABSTRACT FROM AUTHOR]

The global freshwater cycle is intensifying: wet regions are prone to more rainfall, while dry regions experience more drought. Indian Ocean rim countries are especially vulnerable to these changes, but its oceanic freshwater budget—which records the basinwide balance between evaporation, precipitation, and runoff—has only been quantified at three points in time (1987, 2002, 2009). Due to this paucity of observations and large model biases, we cannot yet be sure how the Indian Ocean's freshwater cycle has responded to climate change, nor by how much it varies at seasonal and monthly time scales. To bridge this gap, we estimate the magnitude and variability of the Indian Ocean's freshwater budget using monthly varying oceanic data from May 2016 through April 2018. Freshwater converged into the basin with a mean rate and standard error of 0.35 ± 0.07 Sv (1 Sv ≡ 106 m3 s−1), indicating that basinwide air–sea fluxes are net evaporative. This balance is maintained by salty waters leaving the basin via the Agulhas Current and fresher waters entering northward across the southern boundary and via the Indonesian Throughflow. For the first time, we quantify seasonal and monthly variability in Indian Ocean freshwater convergence to find amplitudes of 0.33 and 0.16 Sv, respectively, where monthly changes reflect variability in oceanic, rather than air–sea, fluxes. Compared with the range of previous estimates plus independent measurements from a reanalysis product, we conclude that the Indian Ocean has remained net evaporative since the 1980s, in contrast to long-term changes in its heat budget. When disentangling anthropogenic-driven changes, these observations of decadal and intra-annual natural variability should be taken into account. [ABSTRACT FROM AUTHOR]

We present high-resolution sustained, persistent observations of the ocean around Sri Lanka from autonomous gliders collected over several years, a region with complex, variable circulation patterns connecting the Bay of Bengal and the Arabian Sea to each other and the rest of the Indian Ocean. The Seaglider surveys resolve seasonal to interannual variability in vertical and horizontal structure, allowing quantification of volume, heat, and freshwater fluxes, as well as the transformations and transports of key water mass classes across sections normal to the east (2014–15) and south (2016–19) coasts of Sri Lanka. The resulting transports point to the importance of both surface and subsurface flows and show that the direct pathway along the Sri Lankan coast plays a significant role in the exchanges of waters between the Arabian Sea and the Bay of Bengal. Significant section-to-section variability highlights the need for sustained, long-term observations to quantify the circulation pathways and dynamics associated with exchange between the Bay of Bengal and Arabian Sea and provides context for interpreting observations collected as "snapshots" of more limited duration. Significance Statement: The strong seasonal variations of the wind in the Indian Ocean create large and rapid changes in the ocean's properties near Sri Lanka. This variable and poorly observed circulation is very important for how temperature and salinity are distributed across the northern Indian Ocean, both at the surface and at depths. Long-term and repeated surveys from autonomous Seagliders allow us to understand how freshwater inflow, atmospheric forcing, and underlying ocean variability act to produce observed contrasts (spatial and seasonal) in upper-ocean structure of the Bay of Bengal and Arabian Sea. [ABSTRACT FROM AUTHOR]