Role of internal tide mixing in keeping the deep Andaman Sea warmer than the Bay of Bengal.

Vertical profiles of temperature obtained from various hydrographic datasets show that deep waters (below 1,200 m) in the Andaman Sea are warmer (about 2 °C) than that of the Bay of Bengal. As a result, the biochemical properties in the deep waters also exhibit significant differences between these two basins. Higher temperature in the deep waters of Andaman Sea compared to the BoB had been widely attributed to the enclosed nature of the Andaman Sea. In this study, we show that strong tidal energy dissipation in the Andaman Sea also plays an important role in maintaining the higher temperatures in the deep waters. Dissipation rates inferred from the hydrographic data and internal tide energy budget suggests that the rate of vertical mixing in the Andaman Sea is about two-orders of magnitude larger than that in the Bay of Bengal. This elevated internal tide induced vertical mixing results in the efficient transfer of heat into the deeper layers, which keeps the deep Andaman Sea warm. Numerical experiments conducted using a high-resolution setup of Regional Ocean Modelling System (ROMS) further confirm the effect of tidal mixing in the Andaman Sea.

Intensification of tidally generated internal waves in the north-central Bay of Bengal.

Flow of barotropic tidal currents over topographic features, such as continental slopes and submarine ridges, generates internal gravity waves at tidal periods known as internal tides. Amplitude of these waves are generally large near the generation regions. Analysis of Sea Surface Height (SSH) data, derived from satellite altimeter revealed the amplification of internal tides in the semidiurnal period in the north-central Bay of Bengal (BoB) (around 89[Formula: see text]E, 16[Formula: see text]N), which is about 450 km away from their generation sites. SSH signals found in the north-central BoB ([Formula: see text]3 cm) were comparable to the maximum amplitudes (2.5 to 3.5 cm) observed near their potential generation sites in the BoB such as continental slopes in the head of the bay and Andaman-Nicobar (AN) Ridge. Simulations from a high-resolution regional ocean model also confirmed the presence of large internal tide amplitude in the north-central BoB. Our study revealed that convergence of internal tides, which were generated along the concave-shaped source (continental slopes in the head of the bay and the northern parts of AN Ridge), into its focal region caused their amplification in the north-central BoB. It was also found that internal tide energy dissipation rates in this focal region were about 10 times larger than those in other open ocean regions.