Impacts of strong positive Indian Ocean Dipole on the generation of the Great Whirl

The Great Whirl (GW) is an anticyclonic mesoscale eddy, forming periodically in the tropical western Indian Ocean. The lifespan of the GW exhibits a marked interannual variability, which has a late generation time and short lifetime due to the abnormal local wind field in a strong positive IOD year. The anomalous northeasterly wind inputs positive wind stress curl into the ocean, which is not conducive to establishing a stable anticyclonic circulation at the early stage of GW formation. In addition, oceanic Rossby waves exert impacts on the GW generation. The abnormal Rossby waves account for the earlier generation time of the GW in 2019 despite the unfavorable local wind field. This study identifies the atypical lifespan of the GW associated with a positive IOD event and reveals the contribution of local Ekman pumping and Rossby wave propagation.