Downwelling

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.

Oceanic downwelling

Downwelling occurs at anti-cyclonic places within the ocean where warm rings are spinning clockwise creating surface convergence. When these surface waters converge, they push the surface water downwards. Another way that downwelling can occur is by the wind driving the sea towards the coastline. Regions that have downwelling have low productivity because the nutrients in the water column are utilized but are not continuously resupplied by the cold, nutrient-rich water from below the surface.