The depth range, where it exists, between the bottom of the oceanic surface mixed layer and the thermocline, usually at a depth between 30 and 80 m. The barrier layer is found in tropical regions where the mixed layer contains water of lower salinity than but identical in temperature to that of the water below. Its significance is that it acts as a barrier to the vertical penetration of heat into the ocean because, without a temperature gradient at the bottom of the mixed layer, entrainment of water from below does not remove any heat from the mixed layer.