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To investigate the South Equatorial Current (SEC) in the Indian Ocean, 18 drifters were deployed off Sumatra‐Java during boreal spring 2019. These drifter observations revealed a large SEC meander that swung southward about 1,100 km during 40 days along ∼100°E. The southward motion occurred on the eastern edge of the Seychelles‐Chagos thermocline ridge, accompanied by an anticyclonic eddy/Rossby wave from the east, indicated by the geostrophic velocity induced by the zonal pressure gradient. Meanwhile, the ageostrophic velocity induced by the southeasterly wind also contributes to this southward motion. This is verified by a Lagrangian Trajectory Model in which drifter trajectories are well reproduced using Chinese‐French Oceanic SATellite winds products. Further analysis of the historical drifters with similar trajectories reveals that the geostrophic and ageostrophic currents jointly drive drifters to move southward. Ageostrophic component could contribute up to 45% of the southward movement of the SEC meander. Plain Language Summary: The South Equatorial Current (SEC) is westward in the tropical Indian Ocean. During boreal spring 2019, 18 drifters were deployed off Sumatra‐Java, and their trajectories show a large SEC meander, with a typical drifter moved southward over 1,100 km during 40 days along ∼100°E. The geostrophic component, associated with the Seychelles‐Chagos thermocline ridge and an anticyclonic eddy/Rossby wave, and ageostrophic component, induced by southeasterly wind, jointly drive drifters to move southward. Further diagnoses of historical drifters show that the ageostrophic component could take up to 45% contribution in the southward movement in the SEC meander. Key Points: South Equatorial Current in the Indian Ocean advected drifters southward more than 1,100 km within 40 days (∼32 cm s−1) in 2019Combined effects of the zonal pressure gradient and the southeasterly wind contribute to the southward movementSouthward movement usually occurs in spring and summer, contributing from ageostrophic current up to ∼45% [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 2019 positive Indian Ocean Dipole (IOD) was the strongest event since the 1960s which developed independently without coinciding El Niño. The dynamics is not fully understood. Here we show that in March–May, westward propagating oceanic Rossby waves, a remnant consequence of the weak 2018 Pacific warm condition, led to anomalous sea surface temperature warming in the southwest tropical Indian Ocean (TIO), inducing deep convection and anomalous easterly winds along the equator, which triggered the initial cooling in the east. In June–August, the easterly wind anomalies continued to evolve through ocean‐atmosphere coupling involving Bjerknes feedback and equatorial nonlinear ocean advection, until its maturity in September–November. This study clarifies the contribution of oceanic Rossby waves in the south TIO in different dynamic settings and reveals a new triggering mechanism for extreme IOD events that will help to understand IOD diversity. Plain Language Summary: The Indian Ocean Dipole (IOD) is an ocean‐atmosphere coupled climatic phenomenon which can cause severe social and economic losses in the surrounding regions such as drought in the Maritime Continent/Australia and flooding in East Africa. The IOD features a see‐saw structure accompanied by an anomalous sea surface temperature gradient, winds, and oceanic adjustments. The El Niño–Southern Oscillation in the Pacific is an important trigger to a strong IOD event. However, an extreme positive IOD event occurred in 2019 without a concurrent or ensuing El Niño. We show that the thermocline warming associated with anomalous ocean downwelling in the southwest tropical Indian Ocean triggered atmospheric convection, inducing anomalous easterly winds along the equator and hence, positive feedbacks associated with an IOD event. This study may help to understand the evolution of extreme IOD and improve IOD predictability. Key Points: The oceanic downwelling Rossby waves in the south tropical Indian Ocean is key to the 2019 extreme positive Indian Ocean Dipole (IOD)The Rossby waves induced thermocline warming, triggering wind‐evaporation‐SST feedback thus anomalous easterly winds along the equatorThe easterly wind anomalies further triggered the Bjerknes feedback and other positive feedbacks and established an extreme IOD [ABSTRACT FROM AUTHOR]