Observational evidence from direct current measurements for propagation of remotely forced waves on the shelf off the west coast of India

Abstract

We use data from six Acoustic Doppler Current Profiler (ADCP) moorings deployed during March–September 2008 on the continental shelf and slope off Bhatkal, Goa, and Jaigarh on the central west coast of India to present evidence for poleward propagation of shelf or coastal-trapped waves (CTWs). Wave propagation is seen on the shelf in the 20–40-day, 10–14-day, and 3–5-day-period bands. The lag from south to north indicates that remote forcing is important even at periods as short as 4 days. Using QuikSCAT wind data, we show that the contribution of remote forcing to the shelf West Indian Coastal Current (WICC) is significant even when the local alongshore wind is strong, as during the summer-monsoon onset during May–June, and forces a strong local response that masks the effect of remote forcing. Forced wave calculations using CTW theory show that remote forcing of the WICC is present at all times, but is most striking when the local winds are weak, as during March–April. The CTW calculations show that the source region for the remote forcing may extend beyond the west coast into the Gulf of Mannar between India and Sri Lanka. On the slope, propagation is seen only at the 4-day period. At higher periods, the slope WICC decorrelates rapidly along the coast, but upward phase propagation, implying downward propagation of energy associated with poleward propagation, is evident even at these higher periods.