Resonant Forcing, Salinity Stratification and Near-Inertial Oscillations in the North Bay of Bengal

Wind-generated near-inertial (NI) currents in the ocean mixed layer give rise to downward propagating near-inertial waves,

which play a significant role in shear-induced turbulent mixing in the ocean. We study the response of near-inertial currents

in the upper ocean to surface wind stress with the help of hourly observations from several moorings in the north Bay of Bengal

(18º N, 89º E), and one-dimensional models. The annual total near-inertial energy flux (F) from the winds to the mixed layer (ML)

in the north Bay of Bengal is 10-20 kJ/m². The flux has a pronounced seasonal cycle - 80% of annual total wind work is in

April-September; October-February is tranquil, but for one or two energetic events during the passage of tropical cyclones.

The resonant effect of surface winds associated with travelling atmospheric disturbances is the primary driver of near-inertial

mixed-layer currents, contributing a significant amount of the annual energy input. A new finding is that near-surface (5-15 m depth)

salinity-dominated density stratification is an important factor in determining the downward propagation of near-inertial (NI) waves

from the mixed layer. The observations show that in the presence of low-salinity surface water from rivers, the proportion of NI energy

radiated to the subsurface ocean is reduced by a factor of two to three. The data suggests that the lowest-order vertical modes, which are responsible

for quick radiation of NI energy to subsurface, contain little energy in the case of fresh, shallow mixed layers.