SMALL SCALE DYNAMICS OF OCEAN BOUNDARY LAYER IN SHALLOW WATERS IN EAST CHINA SEA AND BAY OF  BENGAL 

The small-scale dynamical processes such as internal waves and turbulence play a key role in understanding and quantifying ocean mixing that influences bio geochemical transports in ocean boundary layers and water interior. This research describes the analyzes of  turbulence and internal wave (IW) data acquired from the bottom and surface boundary layers (BBL) in the seasonal pycnoclines of the East China Sea (ECS) and Bay of Bengal (BoB) using a suite of modern instrumentation.  The ECS is a shallow sea, where episodes of strong, high frequency velocity oscillations are recorded over the entire water column during the period of Acoustic Doppler Current  Profiler (ADCP) measurements. In this study, we explain the link between IW episodes and  different barotropic tidal phases. Also, the dynamics of BBL is explored during the investigations. The velocity profiles in the BBL contains segments of different length that can be well-fitted to the classic log layer formula. The friction velocity is so deduced,  however, appears to be unreasonably high, leading to anomalously high drag coefficients. Similar inferences are made by fitting velocity data to Coles’ log-wake model, and in some cases the model-data fit is excellent. The measurements of turbulent kinetic energy dissipation rate show a significant disagreement with those predicted by the log-layer  formulation. A more acceptable friction velocity magnitude could be obtained via a modified log layer model that accounts for density stratification in BBL. The measurements taken in the BoB and around Sri Lanka in 2013, 2014 and 2019 are used to study turbulence in the upper oceanic layer influenced by monsoon wind. Filaments and lenses of diluted surface low-saline water led to active frontogenesis. Below low salinity lenses, the turbulence is readily damped, due to high local stratification that suppresses vertical mixing. Turbulence in the surface layer quickly reacted to the changes of the wind stress. Stirring of horizontal gradients of salinity/density affected entrainment at the  pycnocline, and hence the deepening of the mixed layer. Statistical analysis of turbulence and mixing is focused on waters around Sri Lanka (along the east-west and north south transects). Results show that in strongly startified pycnocline, which is dynamically detached from the low saline surface mixed layer, the probability  distributions of the logarithm of the kinetic energy dissipation rate 
log ₁₀ ε follow generalized  extreme value (GEV) model.  This could be associated with rare and highly energetic turbulent inttermittent patches in the  pycnocline are formed due to the breaking of internal wave and sporadic shear induced instabilities in the region. The mixing Reynolds number (a nomalized eddy diffusivity) is found to be a function of the gradient Richardson number, and followed the parameterization of Lozovatsky and Fernando (2013).