Tide-current-eddy interaction: A seasonal study using high frequency radar observations along the western Bay of Bengal near 16°N

Abstract

The seasonally reversing boundary currents along the western Bay of Bengal (BoB) provide a background for studying the rich and complex relationship between tidally-driven currents, mean current structure and mesoscale eddies. A year-long high-resolution surface current data set from high-frequency radars (HFR) during 2015 along the Andhra Pradesh coast (near 82°E, 16°N) has been utilized to investigate the interaction of tidal currents with bathymetry and eddies during three seasons: January, March–April and November–December. The seasonal variability shows the reversibility of boundary currents with speeds of ~1.58 (1.24) m s−1 during poleward (equatorward) flow associated with an offshore anticyclonic (inshore cyclonic) eddy in this region. The amplitudes of the tidal constituents from the HFR currents are highly correlated (R > 0.80) with those from the sea level observations from a nearby tide gauge. Among the semi-diurnal and diurnal tidal constituents, M2 (5.25–6.51 cm s−1) and K1 (2.36–4.32 cm s−1) are the dominant two respectively, in all three seasons. The tidal variance of 40–70% is observed throughout the domain, with highest contribution from the M2 tidal currents. The M2 tidal ellipse patterns match well with the mean circulation pattern, i.e., cyclonic (anticyclonic) during November–December (March–April). In the nearshore regions, the tidal currents amplify mainly through shallow-water tidal constituents (M4 and MS4) interacting with bathymetry; whereas in the offshore regions, they are dominated by M2, which co-vary with the mean seasonal circulation pattern. The highest amplitude of M2 tidal currents during November–December is predominantly due to high stratification along the western BoB. In the presence of stratification, the loss of tidal kinetic energy into turbulent processes is reduced, resulting in the amplification of M2 tidal currents. The shallow water constituents are also observed with significant amplitudes of 0.50–3.50 cm s−1 in the nearshore region. The amplitudes of overtides (M3, M4 and S4) associated with an offshore anti-cyclonic eddy are higher during March–April, whereas those for the compound tides (MK3, SK3, MS4, 2SK5, 2MN6 and 2SM6) are associated with an inshore cyclonic eddy dominate during November–December.