Time series of temperature and salinity in the upper ocean, measured at 17 degrees 30'N, 89 degrees E in the northern Bay of Bengal, from 27 July to 6 August 1999 captured an event of upper layer freshening. Initially, the upper layer that is homogeneous in both temperature and salinity was about 30 m deep. Subsequently, the arrival of a freshwater plume caused the depth of the mixed layer to decrease to about 10 m and the salinity in the surface layer by about 4 psu. The plume led to the formation of a new halocline and hence a barrier layer within the upper 30 m of the water column. The ensuing ocean-atmosphere interaction was restricted to the new thinner mixed layer. The cooling that was restricted to the mixed layer led to an inversion in temperature amounting to 0.5 degrees C just below the mixed layer. The source of the plume is traced to freshwater from river discharge and rainfall that was advected by Ekman flow as a 15 m thick layer. This study suggests that wind-driven circulation is crucial in determining the path of freshwater in the Bay of Bengal. The fresh water affects the sea surface temperature and ocean- atmosphere coupling through the dependence of the depth of the mixed layer on salinity.