When surface waters converge or are made denser by cooling or evaporation, they sink. This downward movement is downwelling. It carries oxygen, heat, and dissolved carbon into the interior ocean and helps close the global circulation by returning water that earlier upwelled elsewhere.

Coastal downwelling occurs when winds drive surface water toward the shore, piling it up and forcing it downward. Open-ocean downwelling is common in the subtropical gyres where Ekman transport converges. In polar and subpolar seas, intense winter cooling produces dense water that sinks to great depth, feeding the deep limb of the overturning circulation.

Downwelling regions are often less productive at the surface because nutrients are not resupplied from below. They are, however, critical for ventilating the deep ocean and for sequestering anthropogenic carbon. Observing downwelling directly is difficult; its presence is inferred from density structure, tracer distributions, and the divergence of horizontal currents.

A parcel of water that downwells in the North Atlantic may not return to the surface for centuries.