The role of the fresh surface layer in governing air-sea interaction in the northern Bay of Bengal

A fourteen-month record collected from a well-instrumented surface mooring in the northern Bay of Bengal spans the summer and winter monsoon seasons and the two intermonsoon periods. It also includes the passage of cyclone Komen to the north of the mooring. A combination of rain and riverine inflow created a shallow, fresh surface layer. The role of that layer in governing air-sea interaction is explored using an updated version of the Price-Weller-Pinkel 1-D upper ocean model. The updates include changes to how incoming and penetrating shortwave radiation are treated, both in the albedo and in the absorption with depth of penetrating radiation. Freshwater from cyclone Komen and the subsequent riverine inputs dramatically shoal the surface mixed layer. The shallowness of the mixed layer subsequently reduces the heating of the surface layer by allowing significant penetrating shortwave radiation through its base. The presence of the shallow, capping layer thus constrains SST. At the same time, it allows heating of the water below the surface layer. Under weak to moderate wind-forcing, where the mixing does not reach below the surface layer, the heat available to the atmosphere is limited. However, if mixing entrains fluid from below, additional heat is available to the atmosphere.