Observed variability of the East India Coastal Current on the continental slope during 2009–2018

Abstract

We describe the variability of the East India Coastal Current (EICC) during 2009–2018 using data from ADCP (acoustic Doppler current profiler) moorings deployed on the continental slope in the western Bay of Bengal. The four moorings are deployed off Gopalpur ($\sim 19.5^\circ \mathrm{N}$), Visakhapatnam ($\sim 18^\circ \mathrm{N}$), Kakinada ($\sim 16^\circ \mathrm{N}$), and Cuddalore ($\sim 12^\circ \mathrm{N}$) on the Indian east coast. The longer data record allows us to attach a statistically more robust basis to the conclusions drawn by Mukherjee et al. (2014) on the basis of four years (2009–2013) of ADCP data. The data confirm that the seasonal cycle dominates the variability of the EICC. The amplitude of the annual band varies over the time series. In the intra-annual band, the variability switches between the semi-annual and 120-day bands off Gopalpur, Visakhapatnam and Kakinada, but the semi-annual band is stronger than the 120-day band off Cuddalore throughout the time series. Upward phase propagation is common in the seasonal bands, but downward phase propagation is common in the intra-annual band off Cuddalore during the summer and winter monsoons, leading to stronger undercurrents there. Off Cuddalore, even the annual EICC appears as a shallow current. In contrast, the EICC appears as a deep flow off Gopalpur, Visakhapatnam, and Kakinada particularly during the spring inter-monsoon. This deep flow is evident at these locations even in the intraseasonal (30–90-day) band; the longer data set suggests, however, that the intraseasonal variability does not necessarily peak during spring. The annual EICC is coherent along the coast, but it is only the semi-annual band that shows a comparable coherence between Kakinada and Cuddalore: in the 120-day and intraseasonal bands, the EICC decorrelates along the coast. Wavelet analysis suggests significant variability at sub-annual periods. The sub-annual EICC exceeds $20\,\mathrm{cm}\,{\rm s}^{-1}$ on many occasions, but it too decorrelates along the coast. The long ADCP record allows us to confirm the dominance of seasonality in the EICC regime in a robust fashion; the data show that the EICC tends to flow in its canonical poleward (equatorward) direction during spring (winter). This dominance of seasonality enhances the predictability of the EICC.