Freshwater transport by eddies within the Bay of Bengal's central axis

Abstract

The excess of freshwater from rivers and precipitation delivered to the Bay of Bengal (BoB) must be exported to the neighboring ocean in order to establish a quasi-steady state salinity field. The BoB's sea surface salinity is approximately 3 PSU less than that of the Arabian Sea, which experiences net evaporation, a difference reflecting the efficiency of exchange between the two embayments (Gordon et al., 2019). Using Argo salinity profiles from 2003 to 2018 along the BoB's central axis, we investigate the significance of eddies in the removal of excess freshwater injected into the northern BoB by rivers. Southwestward freshwater volume flux is estimated by combining horizontal freshwater (salinity) gradients, as a function of latitude, depth, and season, with a realistic estimate of the horizontal mixing coefficient. The ratio of these fluxes for 0–50 m: 50–100 m: 100–300 m depth slabs is 5.68 : 1.75: 1 in the 15 ± 2° N (northern) study box and 4.61 : 2.24: 1 in the 10 ± 2° N (southern) study box, highlighting the importance of the upper 50 m in the removal of river runoff. In the northern BoB, boreal winter experiences the highest freshwater eddy flux, a consequence of the accumulated freshwater from the previous summer monsoon season. Lower latitudes reach their peak freshwater flux in the summer, a delayed response due to the time required to transport the freshwater southward. Southwestward eddy freshwater flux in the top 300 m can account for almost all of the export of the 0.094 Sv (Sv = 106 m3/s) river runoff into the northern BoB.