Diapycnal mixing induced by salt finger and internal tides on the northwest coast of India

Abstract

Microstructure measurements of velocity shear from the continental slope of the northwest coast of India (NWCI) in the eastern Arabian Sea are used to quantify the relative importance of double diffusion and internal tides induced diapycnal mixing in the different depth layers. It is found that the hydrographic conditions in the NWCI are conducive to the formation of moderately strong salt fingering (Turner angle between 55° and 72°). However, salt finger-induced vertical mixing dominates only in the upper 180 m of the water column, below which intense shear-driven turbulent mixing due to internal tide reduces its significance. As a result of this, the staircase structures, a measure of salt finger dominance in the water column, are frequent, and the mean temperature change across the interface (DT_IH; 0.33 °C) is relatively larger in the upper 180 m compared to sporadic occurrence of steps with a small magnitude of DT_IH (0.18 °C) below 180 m. It is also found that in the upper 180 m of the water column in the NWCI, mean diapycnal diffusivity (K_ρ) is approximately a factor of eight larger (8.3 ± 1.3 × 10⁻⁵ m²s⁻¹) than the estimation in the open ocean region of the eastern Arabian sea (5.4 ± 1.1 × 10⁻⁶ m² s⁻¹). However, due to internal tides, the magnitude of K_ρ reaches as large as O (10⁻²) m² s⁻¹ below 180 m in the NWCI. The mean downward heat flux estimated in the salt finger-dominated (upper 180 m) layers is ∼ -6.1 Wm^-2, and the shear-driven mixing-dominated layers (below 180 m) is ∼ -10.2 Wm^-2.