Microstructures along the Indian west-coast continental shelf: layering and vertical mixing

Abstract

During the month of October, the West Indian Coastal Current (WICC) is in a stage of transition from equatorward to poleward flow. The sea surface salinity near the Indian Coast is lower in the south compared to the north. Whereas the sea surface temperature is higher in the north than in the south. A salinity maximum (SM, Salinity $\ge$ 35.7 psu) exists in the sub-surface between depth ranges of 50–80 m. In situ vertical microstructure profiles collected along the continental shelf off the West Coast of India (WCI) for the first time, are used in this study to characterize vertical mixing under these complex thermodynamic regimes. Estimated sectional mean (median) values of the dissipation rates of turbulent kinetic energy ($\varepsilon$) and thermal variance ($\chi$) are in the range $10^{-9}-10^{-8}$ ($10^{-9}-10^{-8}$) W kg⁻¹ and $10^{-8}-10^{-7}$ ($10^{-9}-10^{-8}$) $°C$ ${}^2$ s⁻¹ respectively. Sectional mean (median) values of the diapycnal diffusivity ($K_{\rho }$) and eddy thermal diffusivity ($K_T$) are in the range $10^{-6}-10^{-5}$ ($10^{-6}-10^{-5}$) m${}^2$ s⁻¹ and $10^{-4}-10^{-2}$ ($10^{-6}-10^{-4}$) m${}^2$ s⁻¹, respectively. The barrier layer (BL) was observed at most of the measurement locations. The layering structures in the salinity profiles observed within BL indicate the phase of transition of halocline reformation. Experiments using a one-dimensional model suggest that this layering is caused by the mixing of surface freshwater (surface freshening caused by heavy rains) with mixed-layer water. Double diffusion with weak staircase structures in temperature and salinity is observed predominantly in the interior layer of the ocean in this region. The presence of SM at most stations, with its thickness decreasing from north to south, created conditions favorable for double-diffusive mixing. Signatures of salt fingering (SF) and diffusive convection (DC) were observed in 40% and 8% of the water column, respectively. Turbulent mixing in the column below the mixed layer (ML) is weak ($K_T\le 10^{-6}$ m${}^2$ s⁻¹) to moderate ($K_T\sim 10^{-4}$ m${}^2$ s⁻¹), with some highly turbulent intermittent regions ($K_T\sim 10^{-2}$ m${}^2$ s⁻¹) conducive to DC. The data presented here show the prevalence of two mixing regimes in the vertical: (1) vertical mixing of freshwater (due to rain) with the ambient mixed layer water, and (2) small-scale double-diffusive mixing due to contrasting temperature–salinity gradients predominantly in the ocean’s interior layer (below the isothermal layer).