What is "downwelling"

n. 1 The sinking of a higher density medium through a lower density one 2 a current caused by the clash of two currents causing one to dive down

Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the sinking limb of a convection cell. Upwelling is the opposite process and together these two forces are responsible in the oceans for the thermohaline circulation. The sinking of cold lithosphere at subduction zones is another example of downwelling in plate tectonics.