Weak interannual variability of mixed layer depth in Bay of Bengal.

Abstract:

Bay of Bengal (BoB) is a semi-enclosed oceanic basin of tropical Indian Ocean having immense influence on the evolution of Asian monsoon. Warm SSTs and extremely low salinity in the basin lead to strong stratification. The oceanic mixed layer depth (MLD) of the bay is generally shallow and requires substantial amount of energy from atmosphere to accelerate the mixing. Here, using a long run(60 years) from simple ocean model, the interannual variability of mixed layer depth is studied.

The dominant modes of IAV in the BoB mixing are governed by the correspondingly varying surface momentum, heat, and fresh water fluxes with very little contribution from entrainment of heat and/or salt at the base of the mixed layer. Further, these fluxes are controlled by ENSO variability with very little influence from Indian Ocean Dipole. A stability analysis revealed that the turbulent kinetic energy (TKE) and the stability function (SH) are negatively correlated when ENSO is the dominant forcing. This is expected since unstable stratification conditions exist during positive ENSO, where the kinetic energy production is enhanced by the unstable buoyancy forcing leading to an increased TKE. During the negative phase of ENSO, stably stratified conditions exist, where the kinetic energy production is offset by the stable buoyancy force, reducing the TKE. This is indicative of high (low) turbulent kinetic energy production, low (high) flux Richardson number based stability function (SH), and low (high) dominance of buoyancy-driven mixing during positive (negative) phases of ENSO. The results highlight that the counteracting influence of TKE and SH is a plausible reason for relatively weaker amplitude of IAV of BoB mixing compared to its normal seasonal cycle.