Internal Tides in the Western Bay of Bengal : Role of Local and Remote Generation

Generation, propagation and energetics of internal tides in the continental margins of western Bay of Bengal (BoB) are studied using measurements from Acoustic Doppler Current Profilers and simulations from a high-resolution model. Internal tides in the western BoB mainly occur in the semidiurnal frequencies. Baroclinic velocity in the semidiurnal band (5-15 cm/s) is stronger than their barotropic counter-part (4-8 cm/s) on the shelf. Observations show phase difference between spring-neap variability of barotropic and internal tides in the southern and northern part of the western BoB, where the latter lag by about 5-7 day compared to the local barotropic tides. Our analysis using the numerical simulations revealed that a significant part (~ 80 %) of the internal tides observed in the western BoB come from remote sources and only a small part get generated locally by the barotropic tidal flow over the continental slope. Most of these remotely generated internal tides originate from Andaman and Nicobar (A & N) Islands ridge and propagate westward across the BoB about 1000-1500 km and reach the continental slopes off the east coast of India after 5-7 days. Energetic internal tides observed during the neap phase of the barotropic tides in the western BoB are essentially due to the arrival of remotely generated waves. Depending upon the slope criticality, semidiurnal internal tides that reach in the western BoB either get reflected back into the deep Bay or transmitted further onto the shelf. Reflections of remotely generated waves on the steep continental slopes result in partly standing internal tides in this region. Our estimates show that baroclinic energy dissipation in the western BoB is about 3-4 times larger than local energy conversion, which suggests that this region is a sink for internal tides generated at the A & N Islands.