Downwelling

Downwelling is the downward movement of a fluid parcel and its properties (e.g., salinity, temperature, pH) within a larger fluid.  It is closely related to upwelling, the upward movement of fluid.

While downwelling is most commonly used to describe an oceanic process, it's also used to describe a variety of Earth phenomena. This includes mantle dynamics, air movement, and movement in freshwater systems (e.g., large lakes). This article will focus on oceanic downwelling and its important implications for ocean circulation and biogeochemical cycles. Two primary mechanisms transport water downward: buoyancy forcing and wind-driven Ekman transport (i.e., Ekman pumping).^[1][2]

Downwelling has important implications for marine life. Surface water generally has a lower nutrient content compared to deep water due to primary production using nutrients in the photic zone. Surface water is, however, high in oxygen compared to the deep ocean due to photosynthesis and air-sea gas exchange. When water is moved downwards, oxygen is pumped below the surface, where it is used by decaying organisms.^[3] Downwelling events are accompanied by low primary production in the surface ocean due to a lack of nutrient supply from below.^[3]