Mixing in the surface layers in association with internal waves during winter in the northwestern Bay of Bengal

Abstract

The upper ocean has complex and variable temperature stratification, and the surface layers in the northwest Bay of Bengal in winter indicate the presence of transient thermal inversions that wane with the advancement of the season. During winter, the sea surface loses heat and the surface waters of the coastal regions of the east coast of India are fairly stratified with the residual freshwater atop from the preceding southwest monsoonal discharge. The vertical stability favors the formation and sustenance of temperature inversions. To investigate the mechanism and the influence of ubiquitous internal waves that thrive on stability, a three-dimensional Princeton Ocean Model is configured for the east coast of India and is applied to study the process in the surface layers in association with the internal waves. The model domain constitutes a variable curvilinear grid, and the input fields comprise bathymetry, initial temperature and salinity, wind stress, air-sea heat fluxes and tidal forcing at the open boundaries. The numerical experiments demonstrate that vertical stability alone cannot cause, support or augment the internal wave oscillations, if the stratification is attributed to salinity only. Internal waves may therefore be perceived in stable layers, essentially from temperature-induced stratification. Despite stratification and enough vertical density gradient in the upper ocean, the conditions may not suit for the occurrence of internal waves due to thermal diffusive processes that overpower the salinity gradients. The vertical spreading of heat due to double diffusion is believed to be transparent to tidal forcing as the generation of internal waves is subdued even under density stratification. The model simulations indicate that the horizontal convergence/divergence motions, required for the manifestation of internal waves at the surface are inhibited in the presence of temperature inversion. The available SAR imageries in winter endorse the model simulations to this effect.