Basin-scale Diapycnal Mixing Rates in the Bay of Bengal Inferred from Fresh water Balance

The annual net input of fresh water to the Bay of Bengal from monsoon rain and river discharge minus evaporation is nearly 1.6 m, or 4000 km³. Low surface salinity and a shallow halocline (1-15 m depth) gives rise to very stable near-surface density stratification in the northern bay after the summer monsoon season. Using daily eddy-permitting ocean reanalysis, continental runoff, precipitation and evaporation estimates, we construct a fresh water balance for waters lighter than ρ₁ = 18 kg/m³ and ρ₂ = 19 kg/m³ for 2011-2015. The total fresh water content (FW) within the control volumes defined by the ρ₁ and ρ₂ isopycnal surfaces is maximum in October-November (mean depths 15 m and 19 m), and minimum in March-April. The difference between FW and the time integral of net freshwater input into the control volume (transport across the southern boundary of the bay is estimated from reanalysis) yields a “deficit curve”. The slope of the deficit curve gives the rate of diapycnal mixing of fresh water across the ρ₁ and ρ₂ surfaces. We find moderate mixing rates in August-October, except under the occasional tropical cyclone. However, in winter (November-February), near-surface fresh water is mixed away at 0.1-0.3 m/day. We propose that surface buoyancy loss due to cool, dry northeasterly winter monsoon winds drives enhanced mixing across the shallow pycnocline.