Synoptic-scale atmospheric cyclones in the South-East Tropical Indian Ocean (SETIO) and their relation to IOD variability

This study focuses on the regional wind variability that controls the intensity of cold-water upwelling off Sumatra – a key feature of the Indian Ocean Dipole (IOD). Our analysis of daily atmospheric data reveals the existence of convectively triggered synoptic-scale atmospheric cyclones in the South-East Tropical Indian Ocean (SETIO). The northern branch of the cyclones corresponds to westerly equatorial wind events, whereas the eastern branch involves north-westerly winds that operate to suppress cold-water upwelling off Sumatra’s west coast. Data for the period 1988–2022 show that 5–9 SETIO cyclones normally form each year during the boreal summer–autumn season, effectively suppressing upwelling in the region. In contrast, there are only few (1–2) cyclone events in years identified as positive phases of the IOD, when the absence of cyclones concurs with the development of strong coastal upwelling off Sumatra. Our findings suggest that the absence or presence of SETIO cyclones contributes to IOD variability.