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The eastern boundary current of the tropical Indian Ocean (IO) connects the equatorial/south tropical IO, contributing to the meridional heat and salinity exchange. As a part of the IO tropical gyre, the meridional current in the eastern IO tropical gyre (East Sumatra Current, ESC) flows southward in the mean state. However, the route of this current varies with months. Drifter trajectories display an exceptional route that turns southeastward before reaching Sumatra in the austral winter and spring, considered the interior ESC. This study uses observations and reanalysis data sets to investigate the seasonal structure and forcing mechanisms behind the interior ESC. The velocity is decomposed into geostrophic and wind‐driven components. The results suggest that the geostrophic component plays a dominant role in the ESC recirculating early from April to June. The southward geostrophic currents are mainly caused by positive zonal temperature gradients, which are induced by both the cold anomaly of the Chagos Dome and the warm water carried by the Indonesian Throughflow, particularly in April and May. Meanwhile, the annual Rossby wave contributes to the temperature anomaly of the Chagos Dome. During the monsoon that prevails after July, the wind‐driven component increases and becomes the primary forcing, accompanying the slowdown of geostrophic velocity. Therefore, the local wind stress, wave propagation, and water mass transport account for the formation of the seasonal interior ESC. Plain Language Summary: The eastern boundary current of the tropical Indian Ocean (ESC) is an important ocean current that contributes to the heat‐salinity exchange of the Indian Ocean (IO) tropical gyre. By using drifters and reanalysis data sets, the routes and seasonal evolution of ESC are investigated. The drifter trajectories reveal an exceptional route, which is closer to the interior IO. Indonesian Throughflow brings warm water into the southeast tropical Indian Ocean and induces the positive gradient together with the cold anomaly of the Chagos Dome. The zonal gradient of potential temperature accounts for the formation of the interior ESC. Furthermore, the formation of ESC cannot be fully explained by geostrophic currents. The wind‐driven currents indicate an enhancement effect of the ESC during the austral winter, together forcing the interior ESC. Key Points: Drifter trajectories display an exceptional route of the meridional current in the eastern Indian Ocean tropical gyreThe positive zonal gradient of potential temperature dominates the southward vertical shear in the southeast tropical Indian OceanThe formation of the interior East Sumatra Current is due to the prevailing monsoon, wave propagation, and water mass transport [ABSTRACT FROM AUTHOR]

Based on the Argo and multi‐satellite observations, this study investigates the asymmetric response of sea surface salinity (SSS) to two extreme positive and negative Indian Ocean Dipole (IOD) events in the southern tropical Indian Ocean (TIO). During positive IOD events, the easterly wind anomalies trigger zonal freshwater advection along the equator, decreasing SSS, while cold sea surface temperature (SST) depress the convective precipitation in the southeastern TIO (SETIO), increasing the SSS. Contrast SSS anomalies along the equator and in the SETIO show opposite phases during positive and negative IOD events. These SSS anomalies have enhanced in recent years, reaching the strongest in the extreme negative IOD in 2016 and positive IOD in 2019. The case study shows that positive SSS anomalies occurred in the southern TIO during both positive and negative IOD events in 2016 and 2019, suggesting significant asymmetry in SSS. Analysis of the mixed layer salinity budget shows that the local precipitation and the entrainment associated with the upwelling Rossby waves played an essential role in the SSS increase in 2016, while the decreased precipitation and horizontal advection caused by the westward South Equatorial Current contributed to the SSS increase in 2019. Further analyses suggest that SSS response is more sensitive to thermocline shoaling than deepening, and SSS response to precipitation is greater at shallower mixed layer than at deeper ones, which is the main reason for the asymmetric response of the SSS to negative and positive IOD events. Plain Language Summary: Indian Ocean Dipole (IOD) is one of the leading climate modes in the Indian Ocean, which impacts changes in temperature, precipitation, and salinity. This study reveals significant asymmetric responses in SSS during the decay phase of positive and negative IOD events. During the mature phase of positive and negative IOD events, there are opposite changes in sea surface temperature, atmospheric circulation, rainfall, and SSS in the equatorial and southeastern tropical Indian Ocean (SETIO). However, SSS increases in the SETIO during the decay phase of both positive and negative IOD events, revealing an asymmetric feature. This asymmetric structure of SSS anomalies has strengthened recently and reached extremes in the 2016 negative IOD event and the 2019 positive IOD event. The upwelling (downwelling) Rossby waves play an important role in the asymmetric response of SSS anomalies in the southern TIO since the SSS response is more sensitive to thermocline shoaling than deepening, and the shallower mixed layer may imply the SSS response to precipitation anomaly. Key Points: Contrast sea surface salinity (SSS) anomalies occur in the equatorial and southeastern tropical Indian Ocean (TIO) during the mature phases of Indian Ocean Dipole (IOD)SSS anomalies are out of phase during positive and negative IOD events but show an asymmetric change in the southern TIOThe asymmetry is due to different SSS responses to precipitation and thermocline changes associated with the ocean Rossby waves [ABSTRACT FROM AUTHOR]

The spatio‐temporal characteristics of the interannual variability and long‐term trend of the marine heatwaves (MHWs) and related dynamic mechanisms in the western equatorial Indian Ocean (WEIO) are investigated using satellite observations. A prominent MHW hot spot is found in a region of the WEIO (48°E−54°E, 2°S–2°N), with a mean MHWs' intensity, duration, and frequency of 1.54°C, 13.33 days, and 1.97 times, respectively. MHWs in the hot spot region have significant interannual variability after removing the long‐term trend, associated with Indo‐Pacific major climate modes. In 1982/1983, 1983/1984, 1987/1988, 1997/1998, 2006/2007, 2009/2010, 2011/2012, 2012/2013, 2014/2015, 2015/2016, and 2019/2020, the MHWs occurred with longer duration, higher frequency, and more total days. These years correspond to a positive Indian Ocean Dipole, or an El Niño event, or both. The occurrence of MHWs accompanied by anomalous positive sea surface height suggests that oceanic planetary wave processes modulate MHWs in the WEIO. Westward‐propagating downwelling equatorial Rossby waves triggered by anomalous equatorial easterly winds drive the convergence of warm upper‐ocean water and weaken the upwelling of cool subsurface water, which favor anomalously warm sea surface temperature (SST) and the occurrence of MHWs. In addition, the westward‐propagating off‐equatorial downwelling Rossby waves in the southern tropical Indian Ocean also affect MHWs in the WEIO through the propagation and reflection of waves. The annual MHW frequency, duration, and total days in the hot spot region increase up to 1.56 times, 4.95 days, and 31.72 days per decade, respectively, related to the significant increase in mean SST under global warming. Plain Language Summary: Marine heatwaves (MHWs) are anomalous warming events in the ocean that can be disastrous for the global marine ecosystem and lead to local socioeconomic losses. The western equatorial Indian Ocean (WEIO), as the western pole of the Indian Ocean Dipole (IOD) and the upwelling region where the Indian Ocean monsoon prevails, experiences strong MHWs. This study shows the characteristics and dynamic mechanisms of MHWs in the WEIO. The MHWs had a significant interannual variability during 1982–2020, associated with the positive IOD and El Niño events. It was found that oceanic planetary wave processes cause the occurrence of MHWs in the WEIO. Westward‐propagating downwelling equatorial Rossby waves and off‐equatorial downwelling Rossby waves in the southern tropical Indian Ocean increase sea surface temperature in the WEIO by suppressing upwelling and entrainment. Under global warming, increasing frequency, duration, and annual days of MHWs are found in the WEIO. Global warming is consistently one of the main causes of extreme warm events. Understanding the characteristics and mechanisms that drive the occurrence of MHWs in the WEIO is a step toward predicting these extreme events. Key Points: Marine heatwaves in the western equatorial Indian Ocean (WEIO) have significant interannual variability related to the Indo‐Pacific climateMarine heatwaves in the WEIO are mainly driven by westward‐propagating downwelling equatorial Rossby waves and off‐equatorial Rossby wavesThe frequency and duration of marine heatwaves exhibited significant increasing trends in the WEIO from 1982 to 2020 [ABSTRACT FROM AUTHOR]