IOD-induced Gill-pattern winds determine the interannual variability of southwestern monsoon current axis in the southern Bay of Bengal

Abstract

The southwestern monsoon current (SMC) flowing from the Arabian Sea (AS) into the Bay of Bengal (BoB), driven by summer monsoon, can significantly affect the thermohaline structure of the BoB and further influence summer monsoon variability through air-sea interaction. The SMC exhibited significant interannual variability, especially in its axis position during 1993–2022. Analysis reveals that the interannual variability of the SMC axis position is mainly related to the Indian Ocean Dipole (IOD) and its induced Ekman effect, which is forced by local wind stress curl (WSC) in the southern BoB. A positive (negative) IOD triggers a Gill-pattern response in the eastern Indian Ocean, as well as an anomalous anticyclonic (cyclonic) circulation over the BoB and an easterly (westerly) wind anomaly across the equatorial Indian Ocean from June to September. The SMC axis migrates northward (southward) when there is a negative (positive) local WSC in the southern BoB during positive (negative) IOD, which leads to positive (negative) sea surface height anomalies (SSHA), and hence the maximum meridional SSHA gradient shifts northward (southward). The influence of the equatorial wind anomaly on the SMC axis is weaker than the Ekman effects of the local wind anomaly in the southern BoB, but opposite in direction. The counteraction between the eastern boundary-reflected Rossby waves and the local negative (positive) wind stress curl forcing induced by a positive (negative) IOD is quantitatively confirmed by analyzing vorticity balance and wind forcing over southern BoB using the vorticity budget and the 1.5-layer reduced-gravity model.