Structure and dynamics of undercurrents in the western boundary current of the Bay of Bengal

Abstract

The structure and variability of undercurrents in the East India Coastal Current (EICC), which is the western boundary current system in the Bay of Bengal (BoB), and the mechanisms of their formation are examined in this study. We used current data collected by Acoustic Doppler current profilers (ADCP) moored off Cuddalore (~ 12oN), Kakinada (~ 16.5oN), Visakhapatnam (~ 17.7oN), and Gopalpur (~ 19.4oN) and simulations for the period 2013–2014 from a high-resolution model configured for the BoB. The undercurrents were observed at all these locations, mainly during summer (June–August) and winter (October–December). Undercurrents were seen at relatively shallow depths (75 m), and their occurrences were more frequent off Cuddalore, whereas they were deep (100–150 m) and less frequent in the northern part of the east coast (off Visakhapatnam and Gopalpur). Numerical simulations showed that the interaction of the westward propagating anticyclonic eddies with the equatorward EICC weakened the strong surface flow and reversed the weak subsurface flow in the northern part of the western BoB. This interaction resulted in the formation of the poleward undercurrent here. Once these mesoscale eddies dissipated due to the interaction with the continental slope, the poleward undercurrents vanished and equatorward flow in the subsurface reappeared. The observed undercurrents near the shelf break region (75–200 m) in the southern part of the coast (off Cuddalore) were associated with small subsurface eddies (diameter of about 20–30 km), which developed due to large zonal gradient in the alongshore component of EICC. Subsurface anticyclonic circulations of larger spatial extent (diameter > 200 km) were responsible for the observed undercurrents in the deeper levels (deeper than 250 m) off Cuddalore. We further show that intraseasonal variability of undercurrents near the shelf break off Cuddalore was directly linked to intraseasonal variability in the strength of surface EICC itself. Results from this study suggest that the undercurrents observed below the EICC were not continuous poleward flow, but they were part of distinct anticyclonic eddies.