Characteristics of mesoscale eddies and their evolution in the north Indian ocean

Mesoscale eddies are ubiquitous in the oceans and play a significant role in setting the turbulence, transporting momentum, heat, salinity, and nutrients from their formation site to the place of dissipation, and controlling biogeochemical processes and air-sea exchanges. Using satellite-measured daily sea level anomaly from 1993 to 2021, we studied statistical characteristics of mesoscale eddies in the north Indian Ocean. The procedure uses information on the vorticity vector, Okubo-Weiss velocity gradient tensor and its threshold, and Lagrange transport. The continental margin of the Arabian Sea (AS) with its western and northeastern flanks, the mouth of the Gulf of Aden, the Lakshadweep Sea, the western margin of the Bay of Bengal (BOB), and the Andaman Sea have been depicted as the hotspot regions. The occurrence of cyclonic eddies (CEs) and anticyclonic eddies (AEs) are comparable in numbers with similar probability distribution as a function of their lifetime in AS and BOB at the basin scale and differ significantly in the regional domains. CEs dominate over AEs during monsoon and postmonsoon periods in the AS and during January-August in the BOB. In contrast, AEs are prominent in the great whirl (GW) during the monsoon period, and the EICC exhibits their peaks in March, July and November. Among the total observed eddies during the study period, 87 % of which are short-lived (<30 days), 10 % are moderately lived (30-60 days), and 3 % are long-lived (>60 days). Eddies are smaller and less energetic, with longer life in the higher latitudes than the lower latitudes. The frequency of eddies exhibits distinct seasonal variability with preferred periods during May-August and November-March, respectively, for their occurrence in AS and BOB. They also show significant inter-annual oscillations and decreasing trends on the background of the weak boundary currents.