Characteristics and evolution of a coastal mesoscale eddy in the Western Bay of Bengal monitored by high-frequency radars

Abstract 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 regions of HFR domain along the western boundary of BoB. A vector-based technique has been adapted to detect and track the mesoscale cyclonic eddy. The eddy is observed to propagate with a mean speed of ∼0.27 m s−1 (23.36 km day−1). It is asymmetric in nature with an average radius of ∼90 (80) km along the eastern and western (northern and southern) sides of the cyclonic eddy aligned along-shelf. The eddy has been characterized based on the Eulerian parameters; normalized vorticity (∼0.75), divergence (∼0.20), strain (∼0.25), and Okubu-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. The negative values of OW parameter show good agreement with the eddy-cores detected. Kinematics show that the Rossby number (R0) varies in the range 0.6–1.2, depicting that the cyclonic eddy is associated with mesoscale dynamical features and matches perfectly with the geostrophic balance. The eddy-induced upwelling signatures are observed from the subsurface temperature and salinity structures. The upwelling is well supported by positive (∼11 × 10−7 N m−3) wind stress curl during November. The sea surface temperature (SST), surface chlorophyll-a concentration as well as sea surface salinity (SSS) associated with the cyclonic eddy, show the advection of warm waters from the open ocean and low saline cold waters from the coastline. This study reveals that the eddy evolved due to baroclinic instability, well indicated by positive values of T2 (rate of conversion of mean potential energy to eddy potential energy) and lower values of Brunt–Vaisala Frequency (N2), whereas growth and intensification of the eddy are attributed to barotropic instability, supported by the positive values of T4 (the conversion of mean kinetic energy to eddy kinetic energy).