Evolution of a Coastal Mesoscale Cyclonic Eddy along the Western Bay of Bengal using HF Radar Observations

Evolution of a coastal cyclonic eddy has been investigated using surface current observations from high-frequency radar (HFR) along the western Bay of Bengal (BoB) near Andhra Pradesh coast during October-December 2015. The HFRs tracked the genesis of the cyclonic eddy from early October, which persisted throughout November and dissipated after mid-December within the shelf-slope region. The eddy propagated with a mean speed of 23.36 km day^-1. It is asymmetric with average radius of ~90 (80) km along the eastern and western (northern and southern) sides of the eddy aligned along-shelf. The eddy has been characterized with Eulerian parameters: normalized vorticity (~0.75), divergence (~0.20), strain (~0.25), and Okubo-Weiss (OW) parameter (-0.7×10^-9 s^-2). Positive vorticity and divergence, along with lower strain at the eddy center, justify the cyclonic eddy. Negative values of OW parameter show good agreement with the eddy-cores detected by an independent vector geometry-based technique. Kinematics show that the Rossby number (R₀) varies within 0.6–1.2 confirming the mesoscale nature of the eddy and its cyclostrophic balance. Eddy-induced upwelling signatures are observed from the subsurface temperature and salinity structures, and are supported by positive wind stress curl during November. This study reveals that the eddy genesis is dominated by baroclinic instability, indicated by positive rate of conversion of mean potential energy to eddy potential energy and lower values of Brunt-Vaisala Frequency, whereas the growth and intensification of the eddy are attributed to barotropic instability, supported by positive rate of conversion of mean kinetic energy to eddy kinetic energy.