Sub-Mesoscale Circulation Features along the Andhra Pradesh Coast, Bay of Bengal: Observations from HF Radars

This work presents the coastal circulation variability along the Andhra coast in western Bay of Bengal (BoB) using hourly high frequency radar (HFR, 4.4 MHz) derived ocean surface currents during 2015. The comparison of daily averaged HFR surface currents with satellite derived currents shows high correlations (0.86, 0.70) and low errors (0.28 m s(-1), 0.25 m s(-1)) for the zonal and meridional components. The HFRs captured and quantified the dominant circulation features associated with the reversal of boundary currents. The northward (southward) currents are observed from mid -March till May end (October to November with highest current speed of similar to 1.58 (1.24) m s(-1) along the Andhra coast (between 15-17 degrees N). In addition, the sub-mesoscale features associated with dominant circulation features are observed, like, the propagation of an anticyclonic eddy (81.80 degrees E, 15.25 degrees N) during March and cyclonic eddy (82.25 degrees E, 15.40 degrees N) during November. The mean vorticity pattern from the HFR currents shows the dipole nature during April - May to indicate the near coast upwelling signatures which are also supported by chlorophyll observations from satellite. The tidal driven currents, extracted from the HFRs, show that the semi -diurnal tidal constituent, M2 dominates during both the seasons with less intensity in March than in November. The distributions of the M2 tidal ellipses match well with the broad -scale current pattern in the offshore region. They follow the bathymetry in the nearshore region to indicate that it is driven by tides resulting in the generation of the shallow water tidal constituents (M3, M4, MS4 and M8) with significant amount of amplitudes.