A barrier layer in the upper ocean forms in regions of strong freshwater input (such as the Bay of Bengal, which receives large riverine discharge and monsoon rainfall), where a shallow, low-salinity surface layer creates a strong halocline (isohaline layer boundary) at a shallower depth than the thermocline (isothermal layer boundary); the barrier layer is the region between the base of this shallow isohaline (mixed) layer and the deeper isothermal layer, within which salinity is stratified but temperature remains nearly uniform, insulating (barrier-ing) the deeper warm water from surface mixing processes.