On the Sub-mesoscale and Mesoscale variability of the Coastal Currents along the western Bay of Bengal (BoB)

The coastal ocean circulation is primarily driven by the surface currents observed due to winds and buoyancy forcing at different temporal and spatial scales. Studying these coastal ocean processes in precise requires a system that can depict the ocean currents with higher temporal and spatial resolutions. In this regard, observations from a pair of long-range HFRs, operational at the Tamil Nadu coast have been considered from (2010-2021) to understand the structure and temporal variations of East India Coastal currents (EICC) during the Post-monsoon (Oct-Nov-Dec) season. Robust comparison has been performed in two stages: first, the HFR datasets are compared with the Drifting Buoys datasets, and second, the same are compared with the satellite-derived surface currents. In both cases, the comparison of the HFR surface currents fields with other datasets depicts a higher correlation (> 0.84) and lower errors (< 0.15 ms^-1) for both the zonal and meridional components. The lateral displacement of EICC-jet with time (meandering) is defined using a new coordinate frame (known as Jet-coordinate frame) firmly aligned with EICC without impacting the key variables (core-width, core intensity, core-speed, and surface transport). In the Jet coordinate frame (JCF), core-speed (CS) has a median value of (~0.6 m.s^-1) over 1500 m isobath depicting a narrower and more intense core as compared to cartesian coordinate system (CCF). Meandering accounts for ~52% of eddy kinetic energy (EKE) computed in the fixed cartesian frame. As the EICC menders onshore during (Oct), shelf temperature and along-stream velocity varies linearly with jet movement and make the thermal gradient stronger whereas during (Nov-Dec) the shelf gets cooler by (~1.45 °C) with a significant increase of along-flow wind stress (~0.015 N/m²). Temperature and velocity fluctuations at 70 (100 m) isobath are predominantly influenced by wind (EICC onshore meander), with the strongest response when downwelling favorable winds and EICC meandering acts constructively.

 

Keywords: Tamil-Nadu, High-Frequency Radar, Jet-Coordinate, Meandering, East India Coastal Currents (EICC)