Mixing of Rain and River Water in the Bay of Bengal From Basin-Scale Freshwater Balance

We construct freshwater balance in the Bay of Bengal (BoB) within a control volume (CV) bounded by 1,018 kg/m(3) isopycnal surface using observations and ocean reanalysis during 2011-2015. Freshwater in CV is maximum in October-November due to monsoonal rain and river inflows, and minimum in April-May. Water lighter than 1,018 kg/m(3) is not transported out of BoB, implying that freshwater lost from CV is mixed away entirely within the basin. From freshwater budget, we infer moderate diapycnal mixing rates (similar to 0.8 x 10-5 m(2)/s) in boreal spring and summer; in winter (December-January), the rate of freshwater loss to subsurface ocean is 0.015 m/day, corresponding to a median turbulent diffusivity of 4.2 x 10(-5) m(2)/s, with standard error of 25%. We show that enhanced winter mixing across the shallow pycnocline is due to reduced shortwave radiation and subseasonal episodes of surface buoyancy loss when cool, dry northeast monsoon winds blow over BoB. Plain Language Summary The freshwater from monsoonal rivers and rain discharged into the Bay of Bengal (BoB) forms a shallow (<10 m deep) salinity-stratified layer which has implications for the regional air-sea interaction. There is very limited understanding of the mixing of the freshwater owing to the absence of near-surface turbulence measurements in BoB. It is important to understand the disappearance of the shallow fresh layer in the Bay due to it's linkages to the regional hydrological cycle and the near-surface stratification. In this study, we infer the seasonality of basin-scale mixing beneath the surface layer of BoB from a freshwater volume balance using a combination of observations and ocean analysis. We find that the highest rates of freshwater mixing (mean and median values of about 5.5 x 10(-5) m(2)/s and 4.2 x 10(-5) m(2)/s) occur during winter (December-January), mainly driven by enhanced surface buoyancy loss thereby reducing the freshwater content at a mean rate of 0.015 m/day. This study has implications for improvement of ocean and climate models, which generally have too-high mixing rates and poor representation of the salinity-stratified near-surface layer in BoB.