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Marine heatwaves (MHWs) are anomalously warm events that profoundly affect climate change and local ecosystem. During the summer of 2012 (June-September), intense MHWs occurred in the tropical Indian Ocean (TIO) concurrently with an unseasonable positive Indian Ocean Dipole (pIOD) event. The MHW metrics (duration, frequency, cumulative intensity and maximum intensity) were characterized by northwestward-slanted patterns from west Australia to the Somalia coast. The analysis confirmed that these MHWs were closely associated with the unseasonable pIOD 2012. The weakening of Western North Pacific Subtropical High and strengthening of Australian High in spring induced an interhemispheric pressure gradient that drove two anticyclonic circulation patterns over the eastern TIO. The first anticyclonic circulation featured cross-equatorial wind anomalies from south of Java to the South China Sea/Philippine Sea, which led to strong upwelling off Sumatra-Java during the subsequent summer. The second anticyclonic circulation excited downwelling Rossby waves that propagated from the southeastern TIO to the western TIO. Thus, downwelling in the western pole and upwelling in the eastern pole led to a strong pIOD event peaking in summer, namely, the unseasonable pIOD 2012. These downwelling Rossby waves deepened the thermocline by more than 60 m and caused anomalous surface warming, thereby contributing to the occurrences of MHWs. With the development and peak of the unseasonable pIOD 2012, anomalous atmospheric circulation transported moisture from the TIO to the subtropical Western North Pacific (WNP), favoring a strong cyclonic anomaly that profoundly affected the summer monsoon rainfall over the subtropical WNP. This study provides some perspectives on the role of pIOD events in summer climate over the Indo-Northwest Pacific region. [ABSTRACT FROM AUTHOR]

The phytoplankton size structure exerts a significant influence on ecological processes and biogeochemical cycles. In this study, the interannual variations in remotely sensed phytoplankton size structure in the southern Tropical Indian Ocean (TIO) and the underlying physical mechanisms were investigated. Significant interannual fluctuations in phytoplankton size structure occur in the southeastern TIO and central southern TIO and are very sensitive to Indian Ocean Dipole (IOD) events. During positive IOD events, the southeast wind anomalies reinforce coastal upwelling off of Java and Sumatra, leading to a shift toward a larger phytoplankton structure in the southeastern TIO. The anomalous anticyclonic circulation deepened the thermocline and triggered the oceanic downwelling Rossby waves, resulting in a smaller phytoplankton structure in the southwestern TIO. During the decay phase of the strong positive IOD events, the sustained warming in the southwestern TIO induced basin-wide warming, thereby maintaining such an anomalous phytoplankton size structure into the following spring. The response of phytoplankton size structure and ocean dynamics displayed inverse patterns during the negative IOD events, with an anomalous larger phytoplankton structure in the central southern TIO. These findings enhance our understanding of phytoplankton responses to climate events, with serious implications for ecosystem changes in a warming climate. [ABSTRACT FROM AUTHOR]

Above‐normal all‐India summer monsoon (ISM) rainfall was received in 2020 with 109% of its long‐period average. According to the India Meteorological Department (IMD), ISM 2020 experienced robust month‐to‐month rainfall variations; in particular, the peak monsoon month of July 2020 witnessed deficit rainfall. The present study investigated the possible mechanisms that are responsible for the July 2020 deficit rainfall. Analysis revealed that the strong low‐level moisture divergence caused by the westward‐propagating atmospheric Rossby wave, induced by suppressed western North Pacific (WNP) convection, is primarily responsible for the July rainfall deficit. The WNP suppressed convection is linked to anomalous low‐level anticyclonic circulation. It is noted that the intense WNP anticyclone is maintained by the tropical Indian Ocean (TIO) warming‐induced atmospheric Kelvin waves and strong low‐level convergence over the Meiyu‐Baiu rainband. Unlike July, the strength of the WNP anticyclone and TIO warming are weaker in June and August. In addition to the observational analysis, the skill of the Climate Forecast System version‐2 (CFSv2) model in predicting the July rainfall at a lead of about two months is examined. Strong positive rainfall anomalies over India are predicted by the model in July, which is in contrast to the observations. Despite predicting the TIO warming, the model failed to capture the westward extension of the WNP anticyclone, which impacted the July rainfall prediction. To understand the importance of the WNP anticyclone on ISM rainfall, a CFSv2 sensitivity experiment is carried out by imposing strong negative sea surface temperature (SST) anomalies over the WNP region to excite the anticyclone. Both the westward extension of the WNP anticyclone and suppressed rainfall over the core monsoon region in July are captured by the model when low‐level circulation is imposed. The model sensitivity experiment strongly supports the role of the WNP anticyclone and its westward extension in inducing reduced rainfall in July 2020 over the monsoon trough region. [ABSTRACT FROM AUTHOR]

The barrier layer (BL) significantly impacts the upper ocean circulation and thermodynamic structure by inhibiting the heat and momentum exchange between the mixed layer (ML) and the subsurface layer. There exist sea surface temperature and salinity dipole modes in the tropical Indian Ocean, however, a BL dipole mode has not yet been identified. Using the latest observations and ocean reanalysis, here we show a robust BL dipole mode in the central and eastern equatorial Indian Ocean, which is highly correlated with the Indian Ocean Dipole (IOD) events. Composite analysis shows that the BL thickness anomalies peak in autumn and are much larger during positive IOD events than during negative IOD events. We show that a positive BL dipole phase is characterized by positive BL thickness anomalies in the central equatorial Indian Ocean and negative BL thickness anomalies in the eastern equatorial Indian Ocean, and vice versa for a negative BL dipole phase. During positive IOD events, negative surface salinity anomalies slightly affect the ML depth along the equatorial Indian Ocean. Positive subsurface temperature anomalies deepen the isothermal layer (IL) in the central equatorial Indian Ocean and strong negative subsurface temperature anomalies significantly lift the IL in the eastern equatorial Indian Ocean, controlling the BL thickness anomalies and forming a positive BL dipole pattern. This operates in an opposite direction during negative IOD events. Our study shows a close relationship between the BL dipole and the IOD and has far‐reaching implications for better understanding and predicting the IOD events. Plain Language Summary: The ocean barrier layer (BL) is an intermediate layer of water between the base of the ML and the base of the isothermal layer (IL). The BL acts as a "barrier" to entrainment cooling and vertical mixing between the surface ML and subsurface layer. In the tropical Indian Ocean, whether there is a BL dipole mode co‐existing with the Indian Ocean Dipole (IOD) is still unknown. This study identified a BL dipole mode in the central and eastern equatorial Indian Ocean. During the IOD events, the anomalous zonal currents cause surface salinity anomalies and further impact the ML depth along the equatorial Indian Ocean. Subsurface temperature anomalies associated with equatorial waves and anomalous currents significantly change the IL depth in the central and eastern equatorial Indian Ocean. We show that temperature and salinity anomalies can induce a BL dipole, which is highly correlated and co‐varies with the IOD. This BL can potentially enhance the positive feedback of the IOD by changing the ocean circulation and thermodynamic structure in the upper ocean. This study will be beneficial to better understand the evolution of the IOD. Key Points: A barrier layer (BL) dipole is identified in the central equatorial Indian Ocean and eastern equatorial Indian OceanThe BL dipole is highly correlated and co‐occurs with the Indian Ocean Dipole event, which peaks in autumnThe isothermal layer depth dominates the evolution of the BL thickness along the equatorial Indian Ocean [ABSTRACT FROM AUTHOR]

Insolation is the engine of monsoon and Walker circulations over the tropical Indian Ocean. Here, we present Holocene coccolith‐related net primary productivity (NPP) signals from two sediment cores retrieved in the wind‐driven coastal upwelling systems off southern India and southern Sumatra. Upwelling‐induced NPP is enhanced during summer and autumn and is a powerful tool to reconstruct atmospheric features at a seasonal scale. Our records indicate that during summer and autumn, westerly winds off southern India strengthened from the early‐Holocene (EH) to late‐Holocene (LH), while southeasterly winds off southern Sumatra strengthened from the EH to mid‐Holocene (MH) and weakened from the MH to LH. Comparisons with previous paleoclimate records and simulations, allow us to confirm such wind patterns at a regional scale and identify distinct atmospheric features associated to insolation before and after the MH. From the EH to MH, as the insolation in the Northern Hemisphere weakens during summer and strengthens during autumn, the equatorial Indian Ocean is characterized by more vigorous Walker and monsoon circulations in summer and autumn, respectively. From the MH to LH, as the insolation weakens in the Northern Hemisphere during summer and over the equator during autumn, the equatorial Indian Ocean is influenced by a general reinforcement of the Walker circulation during both seasons, a feature that we relate to a modern negative IOD‐like mode. The changes in wind result in increasing precipitation over Indonesia and India from EH to MH and over Indonesia from MH to LH as India is getting dryer. Plain Language Summary: The aim of this study is to examine how monsoon and Walker circulations from the tropical Indian Ocean responded to changing insolation over the Holocene. Understanding their dynamics under varying boundary conditions is important for predicting future climate, as they shape precipitation of a region that sustains billions of people. First, we focus on reconstructing changes in seasonal surface wind intensity, based on net coccolith‐related primary productivity signal from two sediment cores retrieved in the coastal upwelling systems off southern India and off southern Sumatra. When these upwellings occur during summer and autumn, nutrient‐rich deep waters surface and enhance the primary productivity. Second, we use paleoclimate records and simulations to provide broad‐scale atmospheric changes and test our assumptions. Our results show that westerly winds off southern India strengthened from early to late Holocene, while southeasterly winds off southern Sumatra strengthened from early to mid‐Holocene and weakened from mid to late Holocene, thus implying distinct wind and precipitation dynamics patterns before and after mid‐Holocene. These features appear to result in weaker and stronger precipitation over Africa and Indonesia from early‐ to mid‐Holocene, respectively, and in a precipitation maximum over India during the mid‐Holocene. Key Points: Stronger zonal Walker circulation during summer from early‐ to late‐HoloceneStronger meridional monsoon circulation during autumn from early‐ to mid‐HoloceneStronger (weaker) precipitation over Indonesia (Africa) from early‐ to mid‐Holocene and precipitation maximum over India in mid‐Holocene [ABSTRACT FROM AUTHOR]

We investigated benthic foraminifera in cores GPC03 and GPC04 in the northeast tropical Indian Ocean (TIO) over the past ∼450 ka to evaluate the ballasting effect of particulate organic matter (POM) and the long‐term zonal change during the mid‐Brunhes dissolution interval (MBDI). Today, interannual climate and oceanographic variability in the TIO is governed by the Indian Ocean Dipole (IOD), which manifests as asymmetric zonal oceanographic change. Previous studies have been conducted to uncover such zonal paleoceanographic change and have referred to their climatic pattern as an "IOD‐like mode." In the northeast TIO, after ∼390 ka, contributions of lithogenic matter and %Nuttallides umbonifer were unusually high under the better carbonate preservation conditions during glacial periods. Our findings suggest that the effect of lithogenic matter from the Ganga‐Brahmaputra‐Meghna River system increased significantly under glacial low sea‐level conditions, possibly by lateral transport along the continental slope. As a result, ballasting of POM seemed more efficient during glacial periods by abundant riverine lithogenic particles. In addition, the long‐term (∼320–200 ka) trend of high benthic foraminiferal accumulation rate (BFAR) and several short‐term fluctuations of high BFAR at ∼310, ∼280, ∼260, and ∼240 ka were discernible. We attribute these changes to increased paleoproductivity driven by upwelling, which may be related to the Indian Ocean equatorial westerlies and Indian summer monsoon dynamics via variations arising from precession. We propose that a long‐term mean‐state transition of IOD‐like mode might have occurred during the MBDI, similar to other hypotheses invoking concomitant changes in the El Niño‐Southern Oscillation system. Key Points: Nuttallides umbonifer were commonly observed in the glacial sediments at the Ninetyeast Ridge of the northeast tropical Indian OceanBallasting of particulate organic matter by the riverine lithogenic particles was more efficient during the glacial periods at the Ninetyeast RidgeWe found evidence for a long‐term mean‐state transition in northeast tropical Indian oceanography since the mid‐Brunhes period [ABSTRACT FROM AUTHOR]

The unprecedented increase in the Sea Surface Temperature (SST) in the warming climate yield stress to the system and pose severe threats to the marine ecosystem. Marine Heatwaves (MHWs) and Marine Cold Spells (MCSs) are two extreme events related to SST variability. For better management of ocean productivity, marine ecosystem, marine services, and fisheries, the understanding of seasonal discrepancies rather than annual documentation of MHWs and MCSs metrics is more utilitarian. This study documents the decadal changes in the MHWs and MCSs over the Tropical Indian Ocean (TIO) for all seasons. Additionally, highly intense events (based on intensity and duration) are identified and demonstrate the associated drivers. During the past two decades (1982–1990, 1991–2000), the MCSs were more frequent than MHWs in every season. However, in the recent two decades (2001–2010, 2011–2020), TIO become more prone to MHWs with considerably more frequent and prolonged events in JJAS months. Moreover, MCSs are disappearing from the TIO. It was noted that the choice of baseline period has an impact on the magnitude of MHWs and MCSs changes, but the spatial pattern (regions with high/low magnitude MHWs and MCSs) stays fairly constant in all baseline period sensitivity checks. The investigation of highly intense events reveals that MHWs and MCSs are produced and sustained by the same drivers when they are at their opposing edges. In general, the coherence effort from winds, net heat fluxes (shortwave radiation, longwave radiation, latent heat flux, and sensible heat flux), mixed level depth, and mean sea level pressure contribute to the genesis of seasonal MHWs or MCSs events. Additionally, in some cases, a single driver (e.g., wind) may also play a crucial role in these extreme events. The remote climate modes of variability, such as El Niño–Southern Oscillation, also contribute significantly to the MHWs and MCSs. El Niño (La Niña) events not only increase the spatial coverage of MHWs (MCSs) but also increases the intensity. [ABSTRACT FROM AUTHOR]

Based on the 30-year global hourly sea surface temperature (SST) dataset (MLSST) produced by the National Marine Environmental Forecasting Center, Ministry of Natural Resources of China, we analyzed the variability of diurnal sea surface temperature amplitude (DSST) of the tropical Indian Ocean at multiple time scales, as well as its influencing factors. The results show that the DSST in the Arabian Sea, Bay of Bengal, and equatorial Indian Ocean exhibits a bimodal seasonal variation with a semi-annual cycle, while the DSST in the southern Indian Ocean shows an annual cycle. The seasonal variation of DSST is mainly influenced by factors such as sea surface wind speed, shortwave solar radiation, and precipitation. The DSST in the equatorial Indian Ocean is generally higher during El Niño years compared to La Niña years. At the intraseasonal scale, the large standard deviation of DSST in boreal winter is mainly distributed in the southern hemisphere, while the large standard deviation of DSST in boreal summer shifts northward. The intraseasonal variation amplitude of DSST in boreal winter of the tropical Indian Ocean is greater than that in boreal summer. The DSST in the tropical Indian Ocean exhibits significant variation characteristics at multi-time scales. This study provides reference for numerical simulation of air-sea interaction patterns in the tropical Indian Ocean, as well as improvement of short-term climate prediction. [ABSTRACT FROM AUTHOR]

Based on the reanalysis data of monthly mean sea surface temperature (SST) from British Hadley Center and ozone mass mixing ratio from National Aeronautics and Space Administration (NASA) during 1980 -2015, two indexes IOBI and IODI of the main modes characterizing SST changes in the tropical Indian Ocean--Indian Ocean Basin (IOB) and Indian Ocean Dipole (IOD) were calculated firstly, and then the correlation of SST anomaly (SSTA) in the tropical Indian Ocean and ozone mass mixing ratio in the stratosphere over East Asia from 1980 to 2015 was analyzed. Besides, the impact of SST changes in the tropical Indian Ocean on the distribution of ozone layer in East Asia was discussed. The results show that SST changes in the tropical Indian Ocean had significant effects on stratospheric ozone distribution in East Asia, and it was consistent with the temporal changes of IOB and IOD. IOBI and IODI had a certain correlation with stratospheric ozone changes in East Asia, with a particularly significant correlation in the lower stratosphere (70 hPa) and middle stratosphere (40 hPa) especially during spring and autumn. [ABSTRACT FROM AUTHOR]

Record-breaking extreme precipitation occurred over the Yangtze River Valley (YRV) in June–July 2020. The impact of climate change on this event is examined from the perspectives of key physical processes including synoptic-scale circulation pattern and tropical sea surface temperature (SST) forcing based on self‐organizing map approach and the regularized optimal fingerprinting method. The results indicate that the increase in extreme precipitation over the YRV has accounted for 69% of the increase in total precipitation since 1961. Both the amount and frequency of extreme rainfall during the Meiyu period in 2020 broke the historical record since 1961. The SST warming in tropical Indian Ocean (TIO) and tropical Atlantic Ocean (TAO) lead to the anomalous anticyclone in the Northwest Pacific, which further aggravate the occurrence of extreme precipitation in the YRV. Human influence is the primary cause for the observed SST rise in TIO and TAO since 1961, to which anthropogenic greenhouse gas emissions contribute the most. Quantitatively, the changes of TIO and TAO SST caused by anthropogenic forcing are 0.12 °C/10a and 0.08 °C/10a, respectively. Such SST warming drives the frequency change of synoptic-scale circulation patterns through air sea interaction. The higher occurrence likelihood of circulation patterns conducive to precipitation in all forcing experiment than that in natural forcing proves that the extreme precipitation in the YRV in 2020 is attributable to human-induced climate change. [ABSTRACT FROM AUTHOR]

The phytoplankton size structure exerts a significant influence on ecological processes and biogeochemical cycles. In this study, the interannual variations in remotely sensed phytoplankton size structure in the southern Tropical Indian Ocean (TIO) and the underlying physical mechanisms were investigated. Significant interannual fluctuations in phytoplankton size structure occur in the southeastern TIO and central southern TIO and are very sensitive to Indian Ocean Dipole (IOD) events. During positive IOD events, the southeast wind anomalies reinforce coastal upwelling off of Java and Sumatra, leading to a shift toward a larger phytoplankton structure in the southeastern TIO. The anomalous anticyclonic circulation deepened the thermocline and triggered the oceanic downwelling Rossby waves, resulting in a smaller phytoplankton structure in the southwestern TIO. During the decay phase of the strong positive IOD events, the sustained warming in the southwestern TIO induced basin-wide warming, thereby maintaining such an anomalous phytoplankton size structure into the following spring. The response of phytoplankton size structure and ocean dynamics displayed inverse patterns during the negative IOD events, with an anomalous larger phytoplankton structure in the central southern TIO. These findings enhance our understanding of phytoplankton responses to climate events, with serious implications for ecosystem changes in a warming climate.

In this study, the global ocean analysis and forecast system is evaluated against the observations in the Indian Ocean for the 2017–2018 period. A comparison of temperature and salinity analysis with seven moored buoy observations showed an excellent mutual agreement with systematically underestimating the upper ocean variability. A similar comparison of ocean analysis with the ARGO dataset average over the different oceanic regions also shows an underestimation of temperature and salinity up to 300 m depth. Further, the temperature RMSE gradually increases from surface to thermocline depth for all regions. The sea surface temperature (SST) forecast is reasonably good at all buoy locations with RMSE less than 0.5°C and a correlation of more than 0.7 up to the day-7 forecasts for the 2017–2018 period. Similarly, the sea surface salinity (SSS) forecast is also good except northern Bay of Bengal (BoB) region where the high variability leads to poor correlation and large RMSE in this shallow and freshwater region. This study helps to understand how observations fit into the model through data assimilation and also quantifies the model forecast error. Further, the study is also vital for the development/research activity related to the model and data assimilation in the Tropical Indian Ocean (TIO) region. [ABSTRACT FROM AUTHOR]

This study investigates the inter-annual changes in tropical cyclone season onset (TCSO) over the western North Pacific (WNP) in neutral El Niño-Southern Oscillation (ENSO) conditions during 1979–2019. The average TCSO during early TCSO years (~ March 27) is significantly earlier than during late TCSO years (~ June 18). In response to inter-annual changes of TCSO in neutral ENSO years, TC activity shows distinct seasonal features. Although there are no significant differences in total TC counts, more TCs occur in March–May while fewer TCs occur in September–November during early TCSO years. Additionally, TC genesis locations tend to move eastward and equatorward during early TCSO years. These TCs are also found to have stronger lifetime maximum intensities. Changes in seasonal TC activity are closely associated with changes in the large-scale environmental pattern changes, which are driven by the seasonal evolution of sea surface temperature anomalies (SSTAs). Low-level vorticity and mid-level humidity changes are found likely to be the two primary factors inducing the seasonal feature differences between early and late TCSO years. Further analyses suggested that the seasonal evolution of sea surface temperature anomaly patterns over the tropical Indian Ocean and tropical Pacific accompanied by TCSO in neutral years is closely linked to changes in the spring Pacific Meridional Mode. Observational analyses and numerical simulations suggest that the Pacific Meridional Mode mainly causes changes in the low-level large-scale circulation over the tropical WNP via a Rossby wave Gill-type response, thus affects inter-annual variability of TCSO during neutral ENSO years. This response is found to be distinct from the previously documented strong relationship between TCSO and strong ENSO events. [ABSTRACT FROM AUTHOR]

The role of sea surface temperature (SST) variability in the boreal winter intraseasonal oscillation (ISO) over the tropical Indian Ocean (TIO) is investigated in this study using the Community Earth System Model Version 2. An atmospheric general coupled model (AGCM) run forced by daily SSTs derived from a parallel coupled circulation model (CGCM) run is compared with reanalysis and the original coupled simulation. The comparison of atmospheric-coupled GCMs reveals the frequency and spatial tendency of the SST effect on the atmospheric ISO. The 50–80 d component of the ISO over the TIO is significantly intensified in the AGCM run. The SST-to-atmosphere effect is most prominent over the southeastern TIO. In the AGCM run, low-level wind anomalies related to SST appear west of the ISO convection center, contrasting with the previous findings that the SST effect appears east of the convection center.

Previous studies suggested that fast‐decay El Niño events are more favorable in generating the western North Pacific anticyclone (WNPAC) in the decaying summer. However, we found that this is not the case for all fast‐decay El Niño events. By comparing two groups of fast‐decay El Niño events with significant and insignificant WNPAC in the following summer, we found that the westward extension of the equatorial Pacific cold sea surface temperature anomalies (SSTA) and the subtropical central‐north Pacific cold SSTA play important roles in the generation and intensification of the WNPAC during decaying summer. Further analyses indicated that the internal atmospheric mode—North Pacific Oscillation during boreal spring can affect the formation of the cold SSTA over the subtropical central‐north Pacific and the westward extension of the equatorial Pacific cold SSTA during summer. Additional effects of tropical Indian and Atlantic forcing on the maintenance of the WNPAC are also shown. Plain Language Summary: The boreal summer western North Pacific anticyclone (WNPAC) is an important low‐level circulation over the western Pacific, which can have great impacts on the East Asia climate. Previous studies pointed out that the fast‐decay El Niño events are favorable in inducing the WNPAC in the decaying summer. However, our observational and modeling results suggested that not all the fast‐decay El Niño events can generate WNPAC in the decaying summer. By dividing fast‐decay El Niño events into two groups, one with significant WNPAC in the following summer and the other without, we found that the difference in the equatorial Pacific cold sea surface temperature anomalies (SSTA) westward extension, the subtropical central‐northern Pacific cold SSTA and the tropical Indian/Atlantic warm SSTA are important in causing the diverse response of the WNPAC to the fast‐decay El Niño events during decaying summer. Further analyses indicated that the internal atmospheric mode—North Pacific Oscillation during boreal spring can affect the cold SSTA over the subtropical central‐north Pacific and the westward extension of the equatorial Pacific cold SSTA during boreal summer. The tropical Indian and Atlantic warming, can also directly influence the WNPAC intensity or feedback to the equatorial and subtropical Pacific cooling to influence the WNPAC. Key Points: Fast‐decay El Niño is more favorable in generating western North Pacific anticyclone (WNPAC) in the decaying summer but this is not the case for all fast‐decay El NiñoEquatorial Pacific, subtropical Pacific cold sea surface temperature anomalies (SSTA) and tropical Indian/Atlantic warm SSTA in summer are key to the diverse response of WNPACThe spring North Pacific Oscillation is important in causing the diverse equatorial and subtropical Pacific cold SSTA forcing in summer [ABSTRACT FROM AUTHOR]

We evaluate the impact of air‐sea coupling in simulating the characteristics of the Indian summer monsoon (ISM) and its intraseasonal oscillations (ISOs) by using a coupled ocean‐atmosphere regional climate model (CRSM) and an atmosphere‐only regional climate model (URSM). These 20‐km resolution regional climate models (RCMs) were driven by the boundary conditions from Community Climate System Model version 4 (CCSM4) global model. The mean ISM rainfall simulated by URSM and CRSM is comparable with observations and shows an improvement over the CCSM4. The systematic error in the spatial distribution of the mean June–September low‐level and upper‐level winds, sea surface temperature, and latent heat flux shows that CRSM performs better than URSM, suggesting the benefits of air‐sea coupling in the RCM simulations of the mean ISM. Both the CRSM and the URSM perform reasonably and comparably in capturing the spatiotemporal evolution of 10–20‐day high‐frequency and 20–70‐day low‐frequency ISOs of the ISM rainfall. These RCMs simulated the observed features of the ISO‐filtered rainfall anomalies that propagate at a faster rate across the Arabian Sea compared to that over the Bay of Bengal. Furthermore, the difference in the track density of the monsoon low‐pressure systems between the wet and the dry phases of ISOs shows insignificant differences between CRSM and URSM but is found to be superior to the CCSM4 simulation. This study suggests a relative insensitivity of air‐sea coupling in the RCM for simulating ISO and LPS features of ISM, which is likely abetted by the realistic ISO signals in the parent CCSM4. Plain Language Summary: We generated two different high‐resolution regional climate simulations: an atmosphere‐only regional climate model (RCM) and a coupled ocean‐atmosphere RCM forced with coarse‐resolution global fields. By using these high‐resolution regional climate simulations, we investigate how air‐sea coupling affects the observed characteristics of the Indian summer monsoon (ISM) and its intraseasonal oscillations (ISOs). The results of the 20‐year regional climate simulation from each of the RCMs reveal that they reasonably capture the observed mean climatology and ISOs of the ISM. The benefit of air‐sea coupling in simulating the mean ISM is clear from the comparisons between the coupled and uncoupled RCM simulations. Nonetheless, the differences in the simulated characteristics of the ISO and the monsoon low‐pressure systems between the RCMs were trivial. On the other hand, both the RCMs considerably outperform the global model in simulating the monsoon low‐pressure systems and their association with the ISOs of the ISM. This study concludes that ISO simulation in the RCM could be insensitive to air‐sea coupling because of realistic ISO signals in the driving global climate model. Key Points: Air‐sea coupling in dynamically downscaled simulation improves the mean Indian summer monsoon relative to the parent general circulation models (GCM) simulationCoupled ocean‐atmosphere and atmosphere‐only regional climate models (RCMs) simulate intraseasonal oscillations (ISOs) of the monsoon with only subtle differencesRCMs add value to downscaling by simulating realistic intraseasonal variability of the monsoon low‐pressure systems absent in the parent GCM [ABSTRACT FROM AUTHOR]

Based on 12 years (2007–2018) of salinity data from the Array for Real-Time Geostrophic Oceanography (Argo) dataset, we found significant positive salinity anomalies in the upper layer of the central tropical Indian Ocean from autumn 2010 to spring 2011 and from autumn 2016 to spring 2017. We used wind, precipitation, outgoing longwave radiation and ocean current data from satellites and reanalysis datasets to analyze the atmospheric conditions, ocean dynamic processes and salinity budget associated with these high salinity events. Our results suggest that surface buoyancy fluxes are not the dominant factor affecting the positive salinity anomalies and that ocean dynamic processes have a more important role. Under the influence of the La Niña events and strong negative Indian Ocean dipole in 2010 and 2016, positive salinity anomalies appeared in the eastern Indian Ocean at the end of 2010 and 2016 as a result of strong westerlies and positive zonal currents. However, because the La Niña event in 2010 was stronger than that in 2016, the salinity anomalies in 2010 were also stronger and the decrease in the following year was both stronger and lasted longer, meaning that the salinity anomalies weakened only gradually. The maximum value of the salinity anomalies in 2011 therefore appeared in January, whereas in 2017 the salinity anomalies first decreased and then increased, with the largest anomalies in March. Salinity budget analyses showed that ocean advection was the main factor leading to the variations in the salinity anomaly during these two periods and that the changes in the zonal velocity in the zonal advection anomalies had the greatest impact. Zonal advection was positive and strongest at the end of 2010 and negative in early 2011, but weakly positive at the end of 2016. In early 2017, the zonal advection was first negative, then became positive and strengthened in spring, so the salinity anomalies in spring 2017 were higher than those in 2011. The freshwater flux had a small, negative effect on the positive salinity anomalies for these two events. The mutual effects of the horizontal advection and the freshwater flux led to high salinity anomalies. The high salinity anomalies reflect the response of the upper ocean to climate events and may influence regional air–sea interactions and large-scale processes. [ABSTRACT FROM AUTHOR]

The connection between the meridional structure of tropical sea surface temperature (SST) and the Hadley circulation (HC) under the effect of ENSO (El Niño Southern Oscillation) from 1950 to 1977 is studied. We decompose the HC and zonal mean SST into equatorially symmetric (HES for HC, SES for SST) and asymmetric variations (HEA for HC, SEA for SST) to discuss the modulation of their connection by ENSO. During El Niño events from 1950 to 1977, the HC is less sensitive to the different SST meridional structures and expressed by response ratio. The ratio in La Niña and neutral events is around 4, which is equivalent to the result in the climatology. The reason for the decreased ratio during El Niño events is explored. The interdecadal variation in the linkage between the HC and tropical SST is due to a clear interdecadal shift in the impacts of ENSO on the tropical Indian Ocean (TIO) SST. For the period 1950–1977, when El Niño events occur, larger SST warming amplitude is observed over the northern TIO (0°–15°N, 50°–100°E). However, the southern TIO (15°S–0°, 50°–100°E) shows greater warming amplitude during 1980–2016. The anomalous SST variation over the TIO linked to El Niño events alters the meridional SST distribution, inducing anomalies in the meridional circulation. These results can help us understand the interdecadal modulation by ENSO on the relationship between tropical SST and HC. [ABSTRACT FROM AUTHOR]

The significant multi-decadal mode (MDM) of the Indian summer monsoon rainfall (ISMR) during the past two millennia provides a basis for decadal predictability of the ISMR and has a strong association with the North-Atlantic (NA) variability with the Atlantic Multi-decadal Oscillation (AMO) as a potential external driver. It is also known that the annual cycles and interannual variability of ISMR and sea surface temperatures (SST) over the tropical Indian Ocean (IO) are strongly coupled. However, the role of local air–sea interactions in maintaining or modifying the ISMR MDM remains unknown. A related puzzle we identify is that the IO SST has an increasing trend during two opposite phases of the ISMR MDM, namely during an increasing phase of ISMR (1901–1957) as well as a decreasing phase of ISMR (1958–2007). Here, using a twentieth-century reanalysis (20CR), we examine the role of air-sea interactions in maintaining two opposite phases of the ISMR MDM and unravel that the Bjerknes feedback is at the heart of maintaining the ISMR MDM but cannot explain the increasing trend of SST in the tropical IO during the opposite phases. Large-scale low-level vorticity influence on SST and net heat flux changes through circulation and cloudiness changes associated with the two phases of the ISMR MDM together contribute to the SST trends. The decreasing trend of low-level wind convergence during the period between 1958 and 2007 is a determining factor for the decreasing trend of ISMR in the backdrop of an increasing trend of atmospheric moisture content. Consistent with the lead of the AMO with respect to ISMR by about a decade, the AMO drives the transition from one phase of ISMR MDM to another by changing its phase first and setting up low-level equatorial zonal winds conducive for the transition. [ABSTRACT FROM AUTHOR]

Near‐equatorial zonal currents along 80.5°E in the upper Indian Ocean are key to tropical inter‐basin hydrographic exchanges. However, their meridional‐width variability and underlying causes have remained elusive. In this study, we use satellite‐derived ocean current data to reexamine the latitudinal structure of Wyrtki jets (WJs), Monsoon Circulation (MC), and South Equatorial Countercurrent (SECC) along 80.5°E on seasonal to interannual timescales. The WJs are characterized by a transient semiannual feature within the latitudinal range of 2.5°S–3°N, whereas the MC and the SECC have a dominant annual period within the latitudinal range of 3°–5.5°N and 2.5°–6°S, respectively. Empirical orthogonal function (EOF) analysis separates the seasonal latitudinal boundaries of the WJs, the MC, and the SECC, and reveals their interannual variability in response to anomalous wind forcing. EOFs1–3 represent the WJ, MC, and SECC modes accounting for 54%, 19%, and 14% of the total variance, respectively. The near‐equatorial zonal currents are asymmetrically responsive to the Indian Ocean Dipole (IOD), with an abnormally strong westward current resulting from the latitudinal superposition of the WJ and SECC modes in the autumn of 1997, 2006, and 2015 during strong positive IOD events. However, the abnormally enhanced eastward WJs occurred only during weak IOD events in the autumn of 1995, 1999, and 2005, accompanied by latitudinal destructive interference of strong westward SECC. Model sensitivity experiments show that the asymmetric response of near‐equatorial zonal currents to climate modes is closely related to the spatial differences in large‐scale wind anomalies associated with wind‐driven and eastern‐boundary‐generated waves. Plain Language Summary: This study examines and quantifies the meridional‐width variability of near‐equatorial zonal currents on seasonal to interannual timescales and diagnoses the associated forcing scenarios by using remote observations, ocean reanalysis, and a continuously stratified linear longwave ocean model. The frequency and phase decompositions reveal the seasonal latitudinal boundaries of the Wyrtki jets, Monsoon Circulation, and South Equatorial Countercurrent along 80.5°E. The interannual variability of the near‐equatorial zonal currents show an asymmetric response to the mature phases of strong positive and negative Indian Ocean Dipole events. The underlying mechanisms of these asymmetric processes are related to the spatial differences in the large‐scale wind anomalies and the associated wind‐driven and eastern‐boundary‐generated waves. Key Points: Frequency and phase decompositions of near‐equatorial zonal currents show the seasonal latitudinal boundaries of WJs, MC, and SECC along 80.5°EMeridional‐width variability of the near‐equatorial zonal currents shows an asymmetric response to strong pIOD and nIOD on interannual timescalesUnderlying mechanisms are related to spatial differences in large‐scale wind anomalies associated with wind/eastern‐boundary generated waves [ABSTRACT FROM AUTHOR]

Abstract Previous studies suggested that fast‐decay El Niño events are more favorable in generating the western North Pacific anticyclone (WNPAC) in the decaying summer. However, we found that this is not the case for all fast‐decay El Niño events. By comparing two groups of fast‐decay El Niño events with significant and insignificant WNPAC in the following summer, we found that the westward extension of the equatorial Pacific cold sea surface temperature anomalies (SSTA) and the subtropical central‐north Pacific cold SSTA play important roles in the generation and intensification of the WNPAC during decaying summer. Further analyses indicated that the internal atmospheric mode—North Pacific Oscillation during boreal spring can affect the formation of the cold SSTA over the subtropical central‐north Pacific and the westward extension of the equatorial Pacific cold SSTA during summer. Additional effects of tropical Indian and Atlantic forcing on the maintenance of the WNPAC are also shown.

Abstract We evaluate the impact of air‐sea coupling in simulating the characteristics of the Indian summer monsoon (ISM) and its intraseasonal oscillations (ISOs) by using a coupled ocean‐atmosphere regional climate model (CRSM) and an atmosphere‐only regional climate model (URSM). These 20‐km resolution regional climate models (RCMs) were driven by the boundary conditions from Community Climate System Model version 4 (CCSM4) global model. The mean ISM rainfall simulated by URSM and CRSM is comparable with observations and shows an improvement over the CCSM4. The systematic error in the spatial distribution of the mean June–September low‐level and upper‐level winds, sea surface temperature, and latent heat flux shows that CRSM performs better than URSM, suggesting the benefits of air‐sea coupling in the RCM simulations of the mean ISM. Both the CRSM and the URSM perform reasonably and comparably in capturing the spatiotemporal evolution of 10–20‐day high‐frequency and 20–70‐day low‐frequency ISOs of the ISM rainfall. These RCMs simulated the observed features of the ISO‐filtered rainfall anomalies that propagate at a faster rate across the Arabian Sea compared to that over the Bay of Bengal. Furthermore, the difference in the track density of the monsoon low‐pressure systems between the wet and the dry phases of ISOs shows insignificant differences between CRSM and URSM but is found to be superior to the CCSM4 simulation. This study suggests a relative insensitivity of air‐sea coupling in the RCM for simulating ISO and LPS features of ISM, which is likely abetted by the realistic ISO signals in the parent CCSM4.

Accurately estimating the ocean’s subsurface thermohaline structure is essential for advancing our understanding of regional and global ocean dynamics. In this study, we propose a novel neural network model based on Convolutional Block Attention Module-Convolutional Neural Network (CBAM-CNN) to simultaneously estimate the ocean subsurface thermal structure (OSTS) and ocean subsurface salinity structure (OSSS) in the tropical Indian Ocean using satellite observations. The input variables include sea surface temperature (SST), sea surface salinity (SSS), sea surface height anomaly (SSHA), eastward component of sea surface wind (ESSW), northward component of sea surface wind (NSSW), longitude (LON), and latitude (LAT). We train and validate the model using Argo data, and compare its accuracy with that of the original Convolutional Neural Network (CNN) model using root mean square error (RMSE), normalized root mean square error (NRMSE), and determination coefficient (R²). Our results show that the CBAM-CNN model outperforms the CNN model, exhibiting superior performance in estimating thermohaline structures in the tropical Indian Ocean. Furthermore, we evaluate the model’s accuracy by comparing its estimated OSTS and OSSS at different depths with Argo-derived data, demonstrating that the model effectively captures most observed features using sea surface data. Additionally, the CBAM-CNN model demonstrates good seasonal applicability for OSTS and OSSS estimation. Our study highlights the benefits of using CBAM-CNN for estimating thermohaline structure and offers an efficient and effective method for estimating thermohaline structure in the tropical Indian Ocean.

Based on the 30-year global hourly sea surface temperature (SST) dataset (MLSST) produced by the National Marine Environmental Forecasting Center, Ministry of Natural Resources of China, we analyzed the variability of diurnal sea surface temperature amplitude (DSST) of the tropical Indian Ocean at multiple time scales, as well as its influencing factors. The results show that the DSST in the Arabian Sea, Bay of Bengal, and equatorial Indian Ocean exhibits a bimodal seasonal variation with a semi-annual cycle, while the DSST in the southern Indian Ocean shows an annual cycle. The seasonal variation of DSST is mainly influenced by factors such as sea surface wind speed, shortwave solar radiation, and precipitation. The DSST in the equatorial Indian Ocean is generally higher during El Niño years compared to La Niña years. At the intraseasonal scale, the large standard deviation of DSST in boreal winter is mainly distributed in the southern hemisphere, while the large standard deviation of DSST in boreal summer shifts northward. The intraseasonal variation amplitude of DSST in boreal winter of the tropical Indian Ocean is greater than that in boreal summer. The DSST in the tropical Indian Ocean exhibits significant variation characteristics at multi-time scales. This study provides reference for numerical simulation of air-sea interaction patterns in the tropical Indian Ocean, as well as improvement of short-term climate prediction.

Understanding climate variability requires good quality high resolution spatially and temporally varying ocean fields due to its decisive role in regulating the region's climate, including the Indian summer monsoon. In this regard, we employed a new high‐resolution regional earth system model (RESM), namely ROM over CORDEX South Asia. We demonstrated the performance of the RESM and its added value over the global earth system model, namely MPI‐ESM, in simulating the hydrographic characteristics and associated mechanism over the tropical Indian Ocean (TIO). ROM shows better skill than MPI‐ESM in simulating the near‐surface and subsurface characteristics of the ocean. However, larger added values are noticed for subsurface (>350 m) thermal structure. MPI‐ESM's cold sea surface temperature (SST) bias is reduced in ROM which is associated with weaker winds, anomalous cyclonic wind stress curl, enhanced stratifications, and a shallower mixed layer, resulting in greater upper‐ocean heating. The reduced SST bias is also consistent with improved ocean meridional heat transport (OMHT) in ROM. For example, reduced southward export of OMHT over the Arabian Sea increases the surface warming by ~4%, reducing the RMSE by ~0.2–0.6°C and becoming closer to observation. The anomalous cyclonic wind stress curl, in turn, caused mixed layer stratifications in the western Indian Ocean. The advective heat transfer from the south‐eastern Indian Ocean to the western Indian Ocean reduced oceanic cooling by vertical processes, overcame the cooling by the net loss of surface heat fluxes, and favoured the TIO's surface warming. [ABSTRACT FROM AUTHOR]

Positive/negative Indian Ocean Dipole (IOD) tends to co‐occur with El Niño/La Niña. Five positive IODs, however, are observed under La Niña condition in 1961, 1966, 1967, 2007, and 2011. This study uses the monthly ocean temperature and atmospheric reanalysis datasets from ECMWF to investigate how these positive IOD events occurred. It is found that the five positive IODs are consistently accompanied by a moderately strong positive subsurface IOD (Sub‐IOD) event that initiated earlier. The Sub‐IOD‐related oceanic dynamical processes including oceanic Kelvin and Rossby waves help to promote the development of the positive IODs. More specifically, the equatorial easterly anomalies in the Tropical Indian Ocean (TIO) that are crucial for the development of the positive Sub‐IODs and IODs in 1961/2011 are triggered by an anticyclonic curl over South Indian Ocean that is induced by a Southern Hemispheric Indian Ocean Subtropical Dipole mode. The crucial equatorial easterly anomalies in 1967 are triggered by a thermocline warming in the southwestern TIO induced by westward downwelling oceanic Rossby waves, while in 1966/2007, the equatorial easterly anomalies are connected with the anomalous Walker circulation resulting from the certain sea surface temperature anomaly pattern over the equatorial Pacific. On the other hand, the remote effects of La Niña on TIO are relatively weaker, being overwhelmed by the above triggering processes. [ABSTRACT FROM AUTHOR]

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Using mooring observations and reanalysis, we show that anomalously strong westward Equatorial Undercurrent (wEUC) developed in June–July in 2016 and 1998 in the Indian Ocean, which coincided with extreme Indian Ocean Dipole (IOD) and El Niño events. Simulations show that equatorial Kelvin and Rossby waves were excited by winds associated with El Niño and positive IOD events during 2015 and 1997, and their negative phases during 2016 and 1998. The constructive relationship between the delayed‐time contributions of eastern‐boundary‐reflected‐waves that excited by the easterlies in 2015 and 1997 and the direct contributions of wind‐forced‐waves that excited by the westerlies in 2016 and 1998 resulted in the intensified wEUC. Slow intermediate‐order baroclinic‐modes, rather than fast low‐order baroclinic‐modes, dominated the strong wEUC. The eastern‐boundary‐reflected‐waves dominated in 1997–1998 and directly wind‐forced‐waves dominated in 2015–2016. Our results emphasize the importance of constructive interactions of the directly‐wind‐forced and boundary‐reflected waves in driving the interannual variability of Indian Ocean wEUC. Plain Language Summary: This study investigated the unique westward Equatorial Undercurrent (wEUC) and its associated dynamics in the Indian Ocean. Mooring observations at (80°E, 2.5°N) reveal the occurrence of an anomalously strong wEUC between 100 and 200 m depth in June‐July (JJ) 2016. Simulations show that the constructive relationship between the directly forced and boundary‐reflected waves resulted in the unprecedentedly strong wEUC in JJ 2016. Equatorial Kelvin and Rossby waves were excited by winds associated with El Niño and a positive Indian Ocean Dipole (IOD) during 2015, and their negative phases during 2016. Waves excited in 2015 hit the meridional boundary, and boundary‐generated waves propagated into the interior Indian Ocean in 2016, which were superposed onto the directly forced waves. A similar wave‐induced enhanced wEUC also occurred during the 1997–1998 strong El Niño and IOD events but with the dominant contribution coming from the eastern‐boundary‐reflected waves. Key Points: An anomalously strong westward Equatorial Undercurrent (wEUC) was observed in June–July (JJ) 1998 and 2016The intensified wEUC in JJ 1998 and 2016 was driven by the co‐occurrence of strong IOD and ENSO events during 1997–1998 and 2015–2016The dynamics are related to both directly wind‐driven and eastern‐boundary‐reflected intermediate‐order baroclinic waves [ABSTRACT FROM AUTHOR]

The barrier layer (BL) is part of the surface ocean isolated from the local air‐sea exchange processes despite it's great potential to store the heat beneath the mixed layer. The tropical oceans are known for the presence of BL. The interannual variability of the BL of the tropical Indian Ocean is examined in this study with a concomitant analysis of observations and outputs from an ocean general circulation model. The model faithfully reproduces the tropical variability of the dynamics and thermodynamics of the Indian Ocean when compared with observations and other studies. The BL thins (thickens) in the eastern (central‐to‐western) tropical Indian Ocean during the positive phase of the Indian Ocean Dipole Mode (IOD). The individual roles of five major forcing mechanisms on BL‐IOD coupling are examined. The thinning in the east is caused by upward heaving thermocline due to upwelling Kelvin waves during the positive phase of the IOD, causing the isothermal layer to shoal steeper than the mixed layer. The surface freshwater fluxes substantiate the thinning of BL in the east during this time, while the net heat fluxes thin the BL in the entire tropical Indian Ocean. The basin‐wide salinity (temperature) dynamics have widespread and complementary impacts, a BL thickening (thinning) in the equatorial and eastern tropical Indian Ocean during positive IOD years, with the former overwhelming the latter. The thickening of BL in the central‐to‐western tropical Indian Ocean during positive IOD is due to a combined action of salinity and freshwater forcing. The thermocline heaving, temperature and salinity dynamics and buoyancy forcing could explain the net variability of BL in the tropical Indian Ocean almost entirely in the model. The ocean‐only response shows that a BL weakens the shoaling (deepening) of the ocean mixed layer during positive (negative) IOD years in the eastern tropical Indian Ocean. As barrier layers store heat beneath the mixed layer, their role in Indian Ocean weather and climate needs to be examined with coupled model experiments. Plain Language Summary: Barrier layers (BL) are the depth between the isothermal layer and the ocean's mixed layer. The eastern tropical Indian Ocean has a barrier layer for the most part of the year. The year‐to‐year variability of barrier layer thickness (BLT) of the tropical Indian Ocean is studied with the help of an ocean general circulation model. BLT of the eastern tropical Indian Ocean thins by ∼10 m during the positive Indian Ocean Dipole Mode (IOD) years and thickens by a few meters in the western tropical Indian Ocean. The role of ocean dynamics and thermodynamics involving the thermocline, entire Indian Ocean domain salinity & temperature, surface heat and freshwater fluxes were separately identified in the BLT variability. The eastern Indian Ocean thinning of BLT during positive IOD years is caused by the combined action of ocean thermocline dynamics, surface heat fluxes, and freshwater fluxes. From the central to the western tropical Indian Ocean, the thickening of BLT during positive IOD years is due to the combined action of salinity dynamics and freshwater fluxes. The BL weakens MLD's shoaling (deepening) during positive (negative) IOD years in the eastern tropical Indian Ocean. Key Points: Tropical Indian Ocean barrier layer (BL) thickness variability is intimately tied to the Indian Ocean Dipole Mode (IOD)The BL thins (thickens) at eastern (central‐to‐western) tropical Indian Ocean during positive IODThe thermocline heaving, basin‐scale Temperature & Salinity dynamics, and buoyancy forcing explains net variability of BL almost entirely [ABSTRACT FROM AUTHOR]

The Tropical Indian Ocean (TIO) is one of the most vulnerable regions to climate change due to its unique ocean-atmosphere interactions. The region is characterized by warm temperatures, high evaporation rates, and strong convection, making it particularly vulnerable to greenhouse gases. This susceptibility leads to an overall trend of warming across the Earth's surface and atmosphere, intensifying sea surface temperatures (SST) and increasing evaporation rates, consequently influencing Surface Latent Heat Flux (SLHF). The strong influence of both SST and SLHF on atmospheric circulation and precipitation patterns makes them critical factors in determining the region's climate, particularly in monsoon-dominated regions. Analysis reveals a consistent upward trend in both SST and SLHF in the central region of TIO. To model this, standard Logistic Curve Model (LCM) has been applied to these parameters averaged over the central region of the TIO. The model is run for a period of 20 years from 2001 to 2020, grouped into four lustrums. The LCM-derived SST and SLHF are in good consistent with observed datasets during the above periods with, correlation coefficients ranging from 0.92 to 0.96 for SST and 0.86–0.90 for SLHF. Extending the model spatially across the entire TIO region explains the ability to project fluctuations in SST and SLHF values across different seasons. These findings highlight the model's relevance for capturing short-term, long-term, and seasonal variability of the parameters, providing important insights into regional climate dynamics. • The central TIO shows an upward trend in both SST and SLHF. • SST and SLHF has a positive and negative seasonal relationship in the TIO. • LCM is found to efficiently capture monthly SST and SLHF trends over the TIO region. [ABSTRACT FROM AUTHOR]

In this study, the joint effects of the El Niño-Southern Oscillation (ENSO) and Arctic Oscillation (AO) on winter net surface heat flux (Q net) anomalies over the tropical Indian Ocean (TIO) are investigated for the 1979/1980–2017/2018 period. The results show that, in multisource datasets, the Q net pattern for El Niño plus positive AO cases is the most robust. The major feature of Q net is dominated by a significant dipole pattern, with oceanic heat gain anomalies appearing over the southeastern TIO and oceanic heat loss near the western TIO. Analysis of the flux components shows that the Q net anomalies are mainly contributed by changes in latent heat flux and shortwave radiation (Q LHF and Q SWR), which are tightly associated with the regional atmospheric and oceanic conditions. In association with positive AO, northerly wind anomalies on the eastern flanks of a low-level anomalous anticyclone in the Arabian Sea enhance the intertropical convergence zone (ITCZ) in the western TIO. Meanwhile, the El Niño-related SST warming occurs in the western TIO. Both enhanced ITCZ and anomalous SST warming favour in situ increased low and middle cloud cover and a reduced Q SWR. Furthermore, under El Niño's effect, a low-level anomalous anticyclone located in the southeastern TIO leads to a positive Q LHF through decreased near-surface wind speed and increased near-surface air humidity. As a result, a dipole Q net pattern tends to be observed for the combination of El Niño and positive AO. • In multisource datasets, the Q net dipole pattern for El Niño plus positive AO cases is the most robust. • The Q net anomalies are contributed by Q LHF in the southeastern TIO and Q SWR in the western TIO. • The El Niño-related anticyclone and SST warming anomalies and positive AO-related enhanced ITCZ affect Q net changes. [ABSTRACT FROM AUTHOR]

Turbulence within the upper ocean mixed layer plays a key role in various physical, biological, and chemical processes. Between September and November 2011, a dataset of 570 vertical profiles of the turbulent kinetic energy (TKE) dissipation rate, as well as conventional hydrological and meteorological data, were collected in the upper layer of the tropical Indian Ocean. These data were used to statistically analyze the vertical distribution of the TKE dissipation rate in the mixed layer. The arithmetic-mean method made the statistical TKE dissipation rate profile more scattered than the median and geometric-mean methods. The statistical TKE dissipation rate were respectively scaled by the surface buoyancy flux and the TKE dissipation rate at the mixed-layer base. It was found that the TKE dissipation rate scaled by that at the mixed-layer base exhibited better similarity characteristics than that scaled by the surface buoyancy flux, whether the stability parameter D/|LMO| was greater or less than 10, indicating that the TKE dissipation rate at the mixed-layer base is a better characteristic scaling parameter for reflecting the intrinsic structure of the TKE dissipation rate in the mixed layer, where D and LMO are respectively the mixed-layer thickness and the Monin-Obukhov length scale. The parameterization of the TKE dissipation rate at the mixed-layer base on the shear-driven dissipation rate and the surface buoyancy flux was further explored. It was found that the TKE dissipation rate at the mixed-layer base could be well fitted by a linear combination of three terms: the wind-shear-driven dissipation rate, the surface buoyancy flux, and a simple nonlinear coupling term of these two .

In this study, daily outgoing longwave radiation (OLR) product is used to detect the atmospheric intraseasonal oscillation (ISO) in the eastern tropical Indian Ocean (TIO). A 50–80-day ISO is identified south of the equator, peaking in boreal winter and propagating eastward. The mechanisms underneath are investigated using observational data and reanalysis products. The results suggest that the 50–80-day atmospheric ISO is enhanced by ocean dynamic processes during December–January. Monsoon transition in October–November causes large wind variability along the equator. Equatorial sea surface height/thermocline anomalies appear off Sumatra due to the accumulative effects of the wind variability, leading the atmospheric 50–80-day ISO by ~ 5–6 weeks. The wind-driven ocean equatorial dynamics are reflected from the Sumatra coast as downwelling oceanic Rossby waves, which deepen the thermocline and contribute to the SST warming in the southeastern TIO, affecting local atmospheric conditions. It offers insights into the role of ocean dynamics in the intensification of 50–80-day atmospheric ISOs over the eastern TIO and explains the seasonal peak of the eastward-propagating ISO during boreal winter. These results have implications for intraseasonal predictability. [ABSTRACT FROM AUTHOR]

The respective influences of the Indian Ocean dipole (IOD) and El Niño‐Southern Oscillation (ENSO) on the thermocline depth (represented by the 20°C isotherm depth, D20) variability in the south‐central Indian Ocean (SCIO) are investigated for the period of 1960–2020. The results show that pronounced interannual variations in D20 mainly occur in the SCIO and display a close relationship with the two tropical climate modes, with positive (negative) phase being associated with SCIO D20 deepening (shoaling). The D20 anomalies (D20A) induced by IOD and ENSO events have the same intensities but present different spatial–temporal features, as demonstrated by partial regression analysis. The IOD‐related D20A occurs during the mature phase of the event and behaves as a local response to anomalous wind stress curl induced by IOD, while the ENSO signature, dominated by the westward‐propagating oceanic Rossby waves, lasts much longer and extends further west. [ABSTRACT FROM AUTHOR]

The southwestern shelf water of India (eastern Arabian Sea) experiences high seasonality. This area is one of the understudied regions in terms of phytoplankton response to the projected ocean acidification, particularly, during the summer monsoon when phytoplankton abundance is high. Here we present the results of a short-term simulated ocean acidification experiment (ambient CO2 424 µatm; high CO2, 843, 1138 µatm) on the natural phytoplankton assemblages conducted onboard (R. V. Sindhu Sadhana) during the summer monsoon (Aug 2017). Among the dissolved inorganic nutrients, dissolved silicate (DSi) and nitrate + nitrite levels were quite low (< 2 µM). Phytoplankton biomass did not show any net enhancement after the incubation in any treatment. Both marker pigment analysis and microscopy revealed the dominance of diatoms in the phytoplankton community, and a significant restructuring was noticed over the experimental period. Divinyl chlorophylla (DVChla) containing picocyanobacteria and 19'-hexanoyloxyfucoxanthin (19′HF) containing prymnesiophytes did not show any noticeable change in response to CO2 enrichment. A CO2-induced positive growth response was noticed in some diatoms (Guinardia flaccida, Cylindrotheca closterium, and Pseudo-nitzschia sp.) and dinoflagellates (Protoperidinium sp. and Peridinium sp.) indicating their efficiency to quickly acclimatize at elevated CO2 levels. This is important to note that the positive growth response of toxigenic pennate diatoms like Pseudo-nitzschia as well as a few dinoflagellates at elevated CO2 levels can be expected in the future-ocean scenario. The proliferation of such non-palatable phytoplankton may impact grazing, the food chain, and carbon cycling in this region. [ABSTRACT FROM AUTHOR]

A new framework of complex networks based on event synchronization that can be applied to the development of temperature anomalies associated with climate variation phenomena is developed to reveal propagation characteristics of ocean temperature anomalies and their asymmetry between positive and negative phases of the Indian Ocean Dipole (IOD). When a complex network based on sea surface temperature (SST) is constructed, it is revealed that SST anomalies in the eastern pole associated with both phases of the IOD tend to be preceded by those in the eastern equatorial region and from the south. When a complex network is constructed based on both surface and subsurface temperatures, it is found that the positive IOD tends to experience extreme subsurface temperature anomalies over more vast areas before extreme SST anomalies compared to the negative IOD. Plain Language Summary: The Indian Ocean Dipole (IOD) is an interannual climate phenomenon of the tropical Indian Ocean. During positive IOD events, anomalous cooling of sea surface temperature (SST) is observed in the southeastern tropical Indian Ocean, whereas anomalous warming occurs in the western tropical Indian Ocean. Anomalies with the opposite signs occur during negative IOD events. To investigate how such anomalous SST in the southeastern tropical Indian Ocean propagates during the IOD, a new framework of complex networks is proposed. When the complex network is constructed based only on SST, it is shown that both anomalously warm SST associated with the negative IOD and anomalously cool SST associated with the positive IOD tend to be preceded by those in the eastern equatorial region and to the south. When a complex network is constructed based on both surface and subsurface temperatures, it is found that the positive IOD tends to experience extreme subsurface temperature anomalies over more vast areas compared to the negative IOD. This suggests that the positive IOD may be more predictable than the negative IOD. Key Points: Complex network based on event synchronization is developed to examine temperature anomalies associated with the Indian Ocean Dipole (IOD)Sea surface temperature (SST) anomalies in the eastern pole tend to originate from the equator and to the south for both IOD phasesSST anomalies associated with the positive IOD tend to be preceded by subsurface temperature anomalies over more vast areas [ABSTRACT FROM AUTHOR]

The oceanic intraseasonal variabilities (ISVs) are pronounced over the tropical Indian Ocean. Recently, a Central Indian Ocean (CIO) mode was proposed as an ocean–atmosphere coupled mode at intraseasonal timescales. It has a close relation with northward-propagating ISVs and intraseasonal precipitation during the Indian summer monsoon. In this study, the dynamics of tropical oceanic ISVs associated with the CIO mode are analyzed using reanalysis products and observations. A complete heat budget analysis shows that intraseasonal SST anomalies which propagate westward from the eastern to the central tropical Indian Ocean during the CIO mode are mainly attributable to zonal thermal advection. Surface heat flux is the second largest contributor. This is distinct from the traditional tropical oceanic ISVs as a response to the Madden–Julian Oscillation (MJO) in the atmosphere, in which surface heat flux is usually the dominant component. Current results along with the previously reported atmosphere dynamics during the CIO mode depict a framework for the ocean–atmosphere coupled mode over the tropical Indian Ocean. This represents a more comprehensive understanding of tropical ISVs and will ultimately contribute to the improvement in process understanding, simulations, and forecasts of the Indian summer monsoon. [ABSTRACT FROM AUTHOR]

The present study investigated the long-term change in the interannual relationship between the boreal winter Arctic Oscillation (AO) and tropical Indian Ocean (TIO) climate during 1979–2019 and found that their linkage experienced a decadal change in 2001/2002. The 19-yr sliding correlation coefficient between the January–February–March (JFM) AO index and central TIO (0–10°S, 65°–80°E) precipitation was significant, with values of approximately 0.50–0.75 during 1979–2001, but abruptly decreased to 0.35 in 2002 and 0.10 in 2010. Meanwhile, the spatial patterns of the AO-related atmospheric circulation anomalies also displayed different features before and after 2001. During 2002–2019, the anomalous anticyclone in the middle troposphere over the Arabian Sea moved northwestward and strengthened, and the JFM AO was more closely correlated to the anticyclone, with correlation coefficient changed from −0.38 before 2001 to −0.63 after 2001; correspondingly, strong cross-equator air flows were observed over the western TIO (40°–50°E), but no significant anomalies of precipitation in the central TIO were observed. During 1979–2001, however, significant southward cross-equator air flows appeared over the central TIO (65°–75°E), which enhanced the intertropical convergence zone and upward air motions, leading to more precipitation in central TIO. An analysis shows that the AO may modulate the Arabian anticyclone through two Rossby wave paths in the upper troposphere: a midlatitude (50°–60°N) path during 1979–2001 from North Atlantic southeastward to the Middle East and the neighboring Arabian Sea; and a subtropical (20°–30°N) path during 2002–2019 from North Atlantic eastward to the Middle East and Arabian Sea. Large wave activity fluxes induced by AO were concentrated along the two paths before and after 2001, and the location of the cross-equator flows depends on the location of the anticyclone. Causes of the decadal changes in the AO-associated wave trains need further investigation. [ABSTRACT FROM AUTHOR]

Common biases of climate models in simulating sea surface temperature (SST) greatly limit our understanding of future climate change. The cold SST bias in the northwestern Pacific (NWP), one of the most distinct and persistent biases in all generations of climate models, was previously attributed to the local and remote influences of the Pacific and Atlantic Oceans, but has never been related to the Indian Ocean. By analyzing simulations from 52 state‐of‐the‐art CMIP6 models and a series of model experiments, we find the strong cold bias in the NWP is primarily caused by the weak cold bias in the tropical Indian Ocean (TIO). Eliminating the TIO bias induces a prominent pressure high over the NWP, which favors a SST warming there through the enhancement of warm oceanic Ekman advection. Our results indicate that the large NWP cold bias could be reduced by ∼66% once the weak TIO bias is corrected. Plain Language Summary: Understanding future climate largely depends on climate model predictions. However, common sea surface temperature (SST) bias of climate models, resulting in large prediction uncertainties, limiting our understanding of future climate change. One of the most distinct and persistent cold biases is in the northwestern Pacific (NWP). Previous studies attributed the large NWP bias to the local influences of the Pacific and remote influences of large Atlantic Oceans bias, but has never been related to the Indian Ocean. By analyzing 52 state‐of‐the‐art CMIP6 models and a series of model experiments, we confirm a robust magnifying effect of the weak cold bias in the tropical Indian Ocean (TIO). We find the strong cold bias in the NWP is primarily caused by the weak cold bias in the TIO. Correcting the TIO bias induces a largest pressure high over the NWP, the accompanied anticyclone enhances warm oceanic transports from lower latitude, which reduces ∼66% huge NWP cold bias. Our results indicate the importance of correcting the relatively weak cold bias in the TIO. Key Points: Strong cold sea surface temperature (SST) bias in northwestern Pacific (NWP) is closely related to weak cold bias in tropical Indian Ocean (TIO)Correcting TIO bias leads to a basinwide warming in NWP by 0.67°CMeridional gradient of absolute vorticity determines bias pattern in NWP [ABSTRACT FROM AUTHOR]

Initial errors in the tropical Indian Ocean (IO-related initial errors) that are most likely to yield the Spring Prediction Barrier (SPB) for La Niña forecasts are explored by using the CESM model. These initial errors can be classified into two types. Type-1 initial error consists of positive sea temperature errors in the western Indian Ocean and negative sea temperature errors in the eastern Indian Ocean, while the spatial structure of Type-2 initial error is nearly opposite. Both kinds of IO-related initial errors induce positive prediction errors of sea temperature in the Pacific Ocean, leading to under-prediction of La Niña events. Type-1 initial error in the tropical Indian Ocean mainly influences the SSTA in the tropical Pacific Ocean via atmospheric bridge, leading to the development of localized sea temperature errors in the eastern Pacific Ocean. However, for Type-2 initial error, its positive sea temperature errors in the eastern Indian Ocean can induce downwelling error and influence La Niña predictions through an oceanic channel called Indonesian Throughflow. Based on the location of largest SPB-related initial errors, the sensitive area in the tropical Indian Ocean for La Niña predictions is identified. Furthermore, sensitivity experiments show that applying targeted observations in this sensitive area is very useful in decreasing prediction errors of La Niña. Therefore, adopting a targeted observation strategy in the tropical Indian Ocean is a promising approach toward increasing ENSO prediction skill. [ABSTRACT FROM AUTHOR]

Under the background of global warming, the summer land-sea thermal contrasts at the upper troposphere exists great discrepancies in radiosonde data (IUK, RICH, and RAOBCORE), reanalysis data (JRA-55, NCEP/DOE, and ERA5) and CMIP6 models results (MPI, FGOALS, and CESM2) for the period of 1979-2014. It can be found that the descriptive statistical indicators (i.e., maximum, minimum, and skewness) of the summer land-sea thermal contrasts index (TTI) between the Tibetan Plateau (TP) and the Tropical Indian Ocean (TIO) vary greatly. The ERA5 and JRA-55 data have the best correlation with radiosonde data. The linear trend and running linear trend (RTL) of the radiosonde data are significantly correlated with the reanalysis data, and both show that the land-sea thermal contrast rapidly increasing are in 1990s and the late 2000s, and the period of rapid weakening was early 2000s. This interannual variation may modulated by the decadal signals such as Pacific Decadal Oscillation (PDO). Except for the NCEP/DOE and IUK, other data show that the most significant warming in the TP-TIO region is at the upper troposphere, and the vertical profiles of the summer temperature trend are quite different in different data, and CMIP6 shows an obvious warm bias in the upper troposphere.

The date of the first tropical cyclone (TC) landfall (DFTCL) is important for TC disaster prevention. This study analyzes the variation in the characteristics of the DFTCL in continental East Asia and investigates the possible effect of the Pacific Decadal Oscillation (PDO) and the El Niño‐Southern Oscillation (ENSO) on the DFTCL. Results show that the combined effect of the PDO and ENSO plays an important role in whether TC landfall occurs relatively earlier or later. During negative PDO phases, the DFTCL is impacted significantly by ENSO events. TCs tend to make landfall later (earlier) in the years following El Niño (La Niña). However, little influence is noticeable for the ENSO events during positive PDO phases. The combined effect is possibly associated with the anomalous anticyclone (cyclone) in the western North Pacific (South China Sea) which is modulated by different PDO phases asymmetrically. During negative PDO phases, the tropical Indian Ocean (TIO) has a strong response to El Niño events in winter. The warm anomaly persists in the following months until early summer. This induces the anomalous anticyclone in the western North Pacific which is enhanced by the anomalous meridional circulation. The anomalous anticyclone can suppress TC genesis and landfall. On the other hand, the TIO has a significant cooling response to La Niña events and induces the anomalous cyclone near the South China Sea. The anomalous cyclone favors earlier than usual TC genesis and landfall. However, this relationship cannot be detected in the positive PDO phase. Key Points: The relationship between the date of the first tropical cyclone landfall (DFTCL) and the El Niño‐Southern Oscillation (ENSO) is unstable and has decadal change which may be modulated by the Pacific Decadal Oscillation (PDO)The DFTCL is later (earlier) in years following El Niño (La Niña) during negative PDO phasesAnomalous anticyclone (cyclone) in the western North Pacific (South China Sea) plays an important role in the combined effect of the PDO and ENSO [ABSTRACT FROM AUTHOR]

The present study examines the ability of Coupled Model Inter‐comparison Project phase 6 (CMIP6) models in representing the dominant modes of tropical Indian Ocean (TIO) sea surface temperature (SST) variability on the interannual and decadal time scale. Historical simulations from 27 CMIP6 models are assessed against Extended Reconstructed SST over the period of 1854 to 2014. Spectrum analysis reveals that many models reproduce interannual and decadal variability of TIO SST but underestimate the amplitude of variability with some disparity in the periodicity. All models can reproduce the dominant basin‐wide mode of interannual and decadal variability of TIO SST reasonably well. Skill score analysis of TIO SST variability reveals that KACE‐1‐0‐G has highest skill, followed by FGOALS‐f3‐L, EC‐Earth3‐Veg‐LR, ACCESS‐ESM1‐5, CanESM5‐CanOE on the interannual timescale and FGOALS‐f3‐L, CanESM5‐CanOE, KACE‐1‐0‐G and CanESM5, respectively, showed highest skills for decadal variability. It is found that variations in radiation and latent heat flux are primarily responsible for interannual variability in TIO SST, the basin‐wide warming, in the observations. Taylor diagram analysis reveals that all the models exhibit better skill for the radiative flux; however, skill for the latent heat and momentum flux varies from model to model. It is important to note that the models in which the latent heat flux and zonal wind are better represented have produced better TIO SST variability compared to other models. A higher discrepancy in latent heat and zonal momentum flux leads to improper wind‐evaporation‐SST and wind‐circulation‐SST feedback, which in turn restricts the model skill. Besides, model that has realistic central and eastern Pacific SST variability show better skill for TIO SST variability in both interannual and decadal time scales. The present study advocates that better representation of latent heat flux and zonal wind in coupled models is important for the accurate simulation of interannual and decadal variability in TIO SST. [ABSTRACT FROM AUTHOR]

The relationship of the tropical Indo‐Pacific upper ocean heat content (HC) and the South China Sea (SCS) summer monsoon (SCSSM) onset during 1980–2020 was investigated. It was found that when HC anomalies in the preceding winter–spring are positive (negative) in the eastern Indian–western Pacific Ocean or negative (positive) in the southwestern Indian Ocean and eastern Pacific Ocean, SCSSM onset is usually early (late). Empirical orthogonal function (EOF) analysis suggested that the primary EOF modes of HC anomaly in both tropical Pacific and Indian Oceans are associated with El Niño–Southern Oscillation (ENSO). It is through modulating the westerly over the north Indian Ocean–SCS and the western north Pacific subtropical high (WNPSH) via the Walker circulation over the Indo‐Pacific basin, monsoon circulation in the Indian Ocean that the ENSO related Indo‐Pacific HC variation connects with the SCSSM onset. Both the second EOF modes for the tropical Pacific and Indian Ocean also show significant correlations with the SCSSM onset. The former related vertical motion over the SCS–far northwestern Pacific can influence the WNPSH and convection, thus occurrence of the SCSSM. The latter represents the intrinsic Indian Ocean HC variability; the air temperature variation associated with it can modulate the atmospheric circulation in the Indian Ocean–SCS through thermal wind balance, and thus the SCSSM onset by acting on the westerly over the SCS and the WNPSH. A western Pacific index and an Indian Ocean dipole index were defined to represent the leading HC modes in the tropical Pacific and Indian Ocean, respectively. A binary linear regression model was constructed using these two indices to improve SCSSM onset prediction. [ABSTRACT FROM AUTHOR]

The present study examines the relationship between the tropical Indian Ocean (TIO) significant wave height (SWH) and Indian Ocean dipole (IOD) during boreal summer season (June through August) in the latest version of European Centre for Medium‐Range Weather Forecasts reanalysis (ERA5) and wave simulations forced with surface winds and sea‐ice fields from the Coupled Model Intercomparison Project version‐5 (CMIP5) models. The interannual variability of SWH shows a significant negative correlation with the IOD over TIO. SWH anomalies display meridional tripole pattern with significant negative (positive) anomalies over eastern equatorial Indian Ocean caused by anomalous easterlies (westerlies), and positive (negative) anomalies over southeastern TIO and the north Bay of Bengal during positive (negative) phase of IOD. The strong wave heights along the east coast of India during positive IOD and the south and southwest coast of India during negative IOD are noticed. CMIP5 models GFDL‐CM3, MRI‐CGCM3 and the multi‐model mean display considerable skill in capturing these teleconnections with substantial magnitude differences. A thorough understanding of the teleconnections between IOD and TIO wave heights is a significant prerequisite for the accurate forecast of surface waves in the Indian Ocean. Hence, this study advocates the importance evaluating the ability of models in representing the SWH and IOD interactions and its implications on Indian coastal regions in the form of inundation, coastal flooding and other vulnerabilities in a changing climate scenario. [ABSTRACT FROM AUTHOR]

Investigating the temporal and spatial distributions of the atmospheric heat sources (AHS) over the Tibetan Plateau-Tropical Indian Ocean (TP-TIO) region is of great importance for the understanding of the evolution and development of the South Asian summer monsoon (SASM). This study used the Japanese 55-year Reanalysis (JRA-55) data from 1979 to 2016 and adopted statistical methods to study the characteristics of the AHS between the TP and TIO, and theirs link to the SASM on an interannual scale. The results indicated that the monthly variations of the AHS in the two regions were basically anti-phase, and that the summer AHS in the TP was obviously stronger than that in the TIO. There were strong AHS and atmospheric moisture sink (AMS) centers in both the eastern and western TP in summer. The AHS center in the east was stronger than that in the west, and the AMS centers showed the opposite pattern. In the TIO, a strong AHS center in the northwest-southeast direction was located near 10°S, 90°E. Trend analysis showed that summer AHS in the TIO was increasing significantly, especially before 1998, whereas there was a weakening trend in the TP. The difference of the summer AHS between the TP and TIO (hereafter IQ) was used to measure the thermal contrast between the TP and the TIO. The IQ showed an obvious decreasing trend. After 1998, there was a weak thermal contrast between the TP and the TIO, which mainly resulted from the enhanced AHS in the TIO. The land-sea thermal contrast, the TIO Hadley circulation in the southern hemisphere and the SASM circulation all weakened, resulting in abnormal circulation and abnormal precipitation in the Bay of Bengal (BOB). [ABSTRACT FROM AUTHOR]

The impact of land-sea thermal contrast on the South Asian summer monsoon (SASM) was investigated by calculating the atmospheric heat sources (AHS) and baroclinic component with ERA5 data for the period 1979–2019. Using diagnostic and statistical methods, it was found that the thermal contrast between the Tibetan Plateau (TP) and the tropical Indian Ocean (TIO) affects the South Asian monsoon circulation through the meridional temperature gradient in the upper troposphere. The seasonal changes of the AHS of the TP and TIO are reversed. In summer, the TP is the strongest at the same latitude whereas the TIO is the weakest, and the thermal contrast is the most obvious. The heat sources of the TP and TIO are located on the north and south side of the strong baroclinic area of the SASM region, respectively, and both of which are dominated by deep convective heating in the upper troposphere. The TP-TIO regional meridional thermal contrast index (QI) based on the AHS, and the SASM index (MI) based on baroclinicity were found to be strongly positively correlated. In years of abnormally high QI, the thermal contrast between the TP and TIO is strong in summer, which warms the upper troposphere over Eurasia and cools it over the TIO. The stronger temperature gradient enhances the baroclinicity in the troposphere, which results in a strengthening of the low-level westerly airflow and the upper-level easterly airflow. The anomalous winds strengthen the South Asian high (SAH), with the warmer center in the upper troposphere, and the enhanced Walker circulation over the equatorial Indian Ocean. Finally, the anomalous circulation leads to much more precipitation over the SASM region. The influence of abnormally low QI is almost the opposite. [ABSTRACT FROM AUTHOR]

Assessment of mass-induced sea level (MISL) variability in the Tropical Indian Ocean (TIO) was studied using observations from the Gravity Recovery and Climate Experiment (GRACE) during 2003–2017 in conjunction with the steric effects in the sea level anomaly as measured by satellite altimeters. Two steric sea levels were estimated from the ocean model analysis and Argo gridded temperature and salinity fields. These datasets were consistent with each other and to the altimeter measured sea level records. They exhibited a coherent seasonal cycle with unique spatial patterns of amplitude maxima associated with annual and semi-annual harmonics. Steric component remained as a major contributor to the sea level variability at all the time scales. Addition of the GRACE measured MISL to the steric sea level improved the estimation of sea level (as measured by satellite altimeter) over most part of the TIO except over the northern part of the Arabian Sea. It was observed that the MISL had a significant contribution to the sea level variability at intra-seasonal and seasonal time scales and a minor contribution to the sea level inter-annual variability. During all the El Niño years, sea level underwent a large fluctuation coherent to the steric component. A linear barotropic vortex conservation model driven by ocean surface winds explained a major part of the observed MISL high-frequency variability in the Equatorial and southern TIO, and overestimated the observation in the northern TIO. [ABSTRACT FROM AUTHOR]