Near-inertial currents off the east coast of India

We use data from moorings equipped with Acoustic Doppler Current Profilers (ADCPs) and deployed in the Bay of Bengal off the east coast of India from May 2009 to February 2012 to study the near-inertial currents (NICs) on the continental shelf and slope. The data show that the NICs are much weaker at the shelf break than on the slope. Inertial energy is weak all along the east coast during January–April. It is high during the summer monsoon (May–September) in the northern Bay of Bengal and early during the winter monsoon (October–December) in the southern bay; at locations in the central bay, the inertial energy does not show this seasonality. This difference between the northern and southern bay is due to the seasonality in the occurrence of storms, which tend to occur in the north (south) during the summer (winter) monsoon. Variability across years is evident in the three-year record, with the NICs being weaker during 2010–2011 compared to 2009. Upward phase propagation is evident in the data, indicating downward propagation of energy. During severe cyclones, the data suggest that the strong NICs extend below the thin surface mixed layer in the bay. A comparison of the NICs amplitude with that of the detided (residual) current shows that the NICs make a significant contribution to the observed current on the east-coast slope: the magnitude of the NICs exceeds that of the residual current on the slope in the northern and southern Bay of Bengal on over 10 days in a year.