Lagrangian tracking of the East India Coastal Current during winter 2024–2025: modulation by mesoscale eddies and tropical cyclone in the Bay of Bengal

Coastal currents along the Indian coastline mediate the seasonal exchange of water masses between the Bay of Bengal (BoB) and the Arabian Sea, yet the processes governing their offshore extent and interaction with mesoscale eddies remain sparse. Using in-situ and satellite observations, this study examines the winter-time evolution of the East India Coastal Current (EICC) in the southwestern BoB. An Argos GPS drifter, originally deployed with a moored buoy off Chennai, got detached during Cyclone Fengal in November 2024. Subsequently, it entrained into the southward flowing EICC, traversed two eddies and ultimately merged with the North Equatorial Current off Sri Lanka. Concurrent moored buoy measurements reveal surface-intensified cooling and pronounced subsurface warming below the mixed layer during the eddy passage. Satellite-derived sea level anomaly and surface current fields independently confirm the eddy’s southward propagation from the northern to the southwestern BoB and its associated offshore transport of cooler, fresher waters. Together, these opportunistic in-situ and satellite observations provide new insights on the offshore reach and southward intensification of the EICC, demonstrating its influence well beyond the continental shelf break over depths exceeding ~ 1000 m. They also offer supportive evidence of wintertime export of low-salinity Bay of Bengal waters.