Internal Tidal Beams and Their Scattering in Nonuniform Stratification—Observations at a Continental Slope in the Northwest Bay of Bengal

Satellite altimetry and numerical ocean modeling have revealed significant internal tide activity within the northwest Bay of Bengal, likely originating from the continental shelf break region nearby. This study presents vertical profile measurements of conductivity, temperature, and depth (CTD); currents; and turbulence microstructure along a 20-km transect at this continental shelf slope region. Performed during the postmonsoon period, when the region experiences weak winds and is abundant with freshwater, a 4-day observational campaign was designed to study internal tides in an upper ocean with a shallow mixed layer and multiple strongly stratified layers beneath. CTD measurements reveal the presence of a dominant pycnocline at around 40-m depth, below which nonuniform stratification layers of $\sim$15–20-m width occur upto around 200-m depth. The vertical profiles of currents at shallow stations near the shelf break show distinct signals of both upward- and downward-propagating primary internal tidal beams (ITBs) generated from the continental shelf break. The deeper stations away from the shelf break show a more complex spatial structure in the upper 200 m, suggesting that significant scattering of ITBs occurs in the nonuniformly stratified upper ocean. Vertical microstructure profiles show enhanced turbulent kinetic energy dissipation rates (${10}^{-8}$-${10}^{-7}$ W/kg) associated with ITB scattering, with corresponding signatures in the Richardson number only at depths below 200 m. Our observations, corroborated with high-resolution numerical modeling, suggest that internal tide-driven turbulence in the nonuniformly stratified upper ocean at continental slope sites could be an important factor in understanding vertical mixing in the global oceans.