Coastal upwelling along the southwestern coast of Sumatra is a seasonal upwelling that occurs in areas of high sea surface temperature and abundant precipitation at the southeastern edge of the Indian Ocean warm pool. Based on observations from two Argo floats that drifted and stayed around Sumatra, we investigated ocean temperature and salinity variations during several coastal upwelling events observed in 2013--2017. The Argo floats observed the vertical structure of temperature and salinity every 10 days within 100 km from the southwestern coast of Sumatra. The observation data show intraseasonal-scale subsurface temperature cooling events with significant upward displacements of the thermocline and high-salinity water, led by anomalous local southwesterly winds and equatorial easterly winds. During the coastal upwelling events, salinity stratification and a thick barrier layer related to local precipitation were also observed. Surface mixed layer temperature cooling were relatively small in contrast to the significant subsurface anomalies. It was found that during the coastal upwelling events, subsurface cold-water upwelling signals did not necessarily reach the mixed layer when salinity stratification and a thick barrier layer were present. The implications of these observational results for understanding the local atmosphere ocean interaction, and hence the occurrence of the Indian Ocean Dipole, are discussed.

Coastal upwelling along the southern coast of Java brings cold and nutrient-rich subsurface water to the surface. We explored whether the upwelling could trigger the onset of the Indian Ocean Dipole (IOD) by supplying cold water to the southeastern tropical Indian Ocean. We used satellite-based daily chlorophyll-a concentration (Chl-a) data during 2003-2020 as a proxy of the coastal upwelling. We focused on first Ch-a blooming that occurred in April–June, the onset phase of the positive IOD (pIOD). We found that the timing and strength of the upwelling signals were significantly correlated with the subsequent IOD peaks. We diagnosed processes associated with the upwelling affecting sea surface temperature (SST) in the southeastern Indian Ocean. Results indicate that after the cold-water upwelling south of Java, westward surface temperature advection plays a role in anomalously cooling the SST in the southeastern Indian Ocean and setting a favorable condition for the subsequent pIOD development.