Evolution of freshwater plumes and salinity fronts in the northern Bay of Bengal

The evolution of freshwater plumes and the associated salinity fronts in the northern Bay of Bengal (henceforth the bay) is studied using rotated empirical orthogonal function (REOF) analysis and extended associate pattern analysis (EAPA). The results show that sea surface salinity distribution is featured by eastern‐bay and western‐bay plumes in the northern bay during different seasons. The western‐bay plume begins in early July, peaks in late August, and then turns into a bay‐shaped plume with the two plumes in either side of the bay, which peaks in late October. The southward extension of the western‐bay plume can be explained by the southwestward geostrophic flow associated with the cyclonic gyre in the northern bay, which counters the northeastward Ekman drift driven by wind stress. The offshore expansion of the western‐bay plume is induced by the offshore Ekman drift which also produces a salinity front near the east coast of India. The bay‐shaped plume appears when the cyclonic gyre shifts westward and a weak anticyclonic gyre occupies the northeastern bay. As the season advances, the western part of the bay‐shaped plume decays while the eastern part persists until the following June, which is believed to be associated with the anticyclonic gyre in the northern bay. The evolution of the plumes except the eastern part of the bay‐shaped plume in fall can be partly explained by the seasonal variation of mass transport associated with the Sverdrup balance. The fact that the western‐bay (eastern‐bay) plume appears when surface freshwater flux in the northeastern bay increases (decreases) dramatically suggests that the plumes are not produced directly by surface freshwater flux. River discharge seems to be the freshwater source for the plumes and has little to do with the evolution of the plumes.