Oceanic Rossby waves induced two types of ocean-atmosphere response and opposite Indian Ocean Dipole phases

This study analyzed the downwelling Rossby waves in the south Indian Ocean induced spring asymmetric mode and the relationship with the Indian Ocean Dipole (IOD) event based on observations and reanalysis data sets. 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 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 weak asymmetric mode and positive IOD event, usually associated with preceding weak El Niño or anomalous anti-cyclonic atmospheric circulation in the southeastern IO.