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This study analyzed the downwelling Rossby waves in the south Indian Ocean (IO)-induced spring asymmetric mode and the relationship with the Indian Ocean dipole (IOD) event based on observations and reanalysis datasets. The westward downwelling Rossby waves favor significant sea surface temperature (SST) warming in the Seychelles thermocline dome that triggers atmosphere response and the asymmetric mode in spring. The zonal sea level pressure gradient causes anomalous easterly winds in the central and eastern equatorial IO, cooling the SST off Sumatra–Java. Meanwhile, the remainder of the downwelling Rossby waves reach the west coast, transform to northward coastal-trapped waves, and then reflect as eastward downwelling Kelvin waves along the equator. The downwelling Kelvin waves reach the Sumatra–Java coast during late spring to early summer, favoring SST warming in the southeastern tropical Indian Ocean. Thus, there are two types of ocean–atmosphere response almost at the same time along the equator. The final SST status depends on which process is stronger and, as a consequence, triggers a negative or a positive phase of the IOD event in the fall season. The results show four positive and three negative IOD events related to the above processes from 1960 to 2019. The strong downwelling Rossby waves are easier to induce an intense asymmetric mode and negative IOD event, usually associated with preceding strong El Niño in the Pacific. In contrast, the weak downwelling Rossby waves tend to induce a weak asymmetric mode and positive IOD event, usually associated with preceding weak El Niño or anomalous anticyclonic atmospheric circulation in the southeastern IO. [ABSTRACT FROM AUTHOR]

Marine heatwaves (MHWs) in the tropical Indian Ocean (TIO) showed remarkable increases in duration and frequency during the satellite observing era, responding to rising sea surface temperature. Long‐lasting MHWs were found in three upwelling regions of the TIO in 2015–2016 and 2019–2020, closely related to persistent downwelling oceanic planetary waves. In 2015, a prolonged MHW (149 days) in the western TIO was initiated by the downwelling Rossby waves associated with the co‐occurring super El Niño and positive Indian Ocean dipole (IOD) events. In the following year, the negative IOD sustained the longest MHW (372 days) in the southeastern TIO, prompted by the eastward‐propagating equatorial Kelvin waves. In 2019–2020, the two longest MHWs recorded in the southwestern TIO (275 days in 2019 and 149 days in 2020) were maintained by the downwelling Rossby waves associated with the 2019 extreme IOD. This study revealed the importance of ocean dynamics in long‐lasting MHWs in the TIO. Plain Language Summary: Marine heatwaves (MHWs) are extreme sea surface warming events, which can have destructive impacts on marine ecosystems and socio‐economy. The tropical Indian Ocean (TIO) has experienced the most significant sea surface temperature warming among the global oceans in the satellite observing era since the 1980s. The increases in duration and frequency of MHWs were found statistically significant in the TIO, and further increases can be expected under global warming. Several long‐lasting MHWs occurred in recent years, associated with extreme climate modes. Internal ocean waves triggered by anomalous winds deepened the thermocline through the convergence of warm upper‐ocean water and reduced upwelling of subsurface cold water, resulting in a surface warm that facilitated the MHW formation. Thus, the consecutive propagating waves during the extreme climate events sustained the prolonged MHWs in the TIO. These results expand our understanding of the physical mechanisms responsible for MHWs in the TIO under the Indo‐Pacific climate variability and global warming. Key Points: Long‐lasting marine heatwaves (MHWs) occurred in the upwelling regions of the tropical Indian Ocean during 2015–2016 and 2019–2020The recent long‐lasting MHWs are related to the super El Niño in 2015–2016 and extreme Indian Ocean dipole in 2019Oceanic downwelling waves induced‐thermocline warming plays a crucial role in the long‐lasting MHWs [ABSTRACT FROM AUTHOR]

The south tropical Indian Ocean (TIO) warms following a strong El Niño, affecting Indo-Pacific climate in early boreal summer. While much attention has been given to the southwest TIO where the mean thermocline is shallow, this study focuses on the subsequent warming in the southeast TIO, where the mean sea surface temperature (SST) is high and deep convection is strong in early summer. The southeast TIO warming induces an anomalous meridional circulation with descending (ascending) motion over the northeast (southeast) TIO. It further anchors a "C-shaped" surface wind anomaly pattern with easterlies (westerlies) in the northeast (southeast) TIO, causing a persistent northeast TIO warming via wind–evaporation–SST feedback. The southeast TIO warming lags the southwest TIO warming by about one season. Ocean wave dynamics play a key role in linking the southwest and southeast TIO warming. South of the equator, the El Niño–forced oceanic Rossby waves, which contribute to the southwest TIO warming, are reflected as eastward-propagating oceanic Kelvin waves along the equator on the western boundary. The Kelvin waves subsequently depress the thermocline and develop the southeast TIO warming. [ABSTRACT FROM AUTHOR]

Argo float trajectory data has been used to estimate the velocities of mid-depth (1000 db) currents in the North Indian Ocean (NIO). The structure of the upper ocean absolute geostrophic currents can be derived more accurately from the Argo temperature and salinity profiles by referring to the estimated velocities rather than by assuming a level of "no motion". The derived flow field reveals that the eastward zonal velocities generate striation-like structures in the Arabian Sea, which is most prominent above the layer of 500 db layer, with a meridional scale of ~300 km; however, such structures are barely observed in the Bay of Bengal. This meridional scale as well as the distribution of the mid-depth striations are unique to the NIO, as compared with the other ocean basins. The nonlinear 1 1 2 -layer reduced-gravity model combined with the triad baroclinic Rossby wave interaction theory have been used to determine the essential factors controlling the characteristics of the quasi-zonal striation structure. Compared with the North Pacific Ocean, the meridional scale in the NIO is narrower, and is caused by the smaller basin size in the equatorial zone, rather than by the semiannual wind stress forcing period or the eastern boundary angle. Furthermore, the coastal trapped Kelvin waves significantly contribute to the generation of the zonal striations in the Arabian Sea. • Mid-depth striations in the North Indian Ocean have been observed by the Argo trajectory data and T/S profiles. • The striations are more prominent in the Arabian Sea while barely observed in the Bay of Bengal. • The meridional scale of those striations is 300 km, which is because of the smaller basin scale in the equatorial zone. • Coastal trapped Kelvin waves contribute significantly to the generation of the zonal striation in the Arabian Sea. [ABSTRACT FROM AUTHOR]