Upwelling

Upwelling is the rising of cold, deeper water to the surface where wind and the Earth’s rotation drive surface water away from a coast or apart at the equator. Along an eastern ocean boundary, a wind toward the equator drives the surface layer offshore through Ekman transport, and colder, nutrient-rich water rises to replace it, dropping the sea surface temperature by several degrees. The cold surface water cools the air above and favors advection fog, the practical reason it matters: the Benguela, California, Humboldt, and Canary current coasts are upwelling zones known for persistent sea fog. Weather routing accounts for the colder water and the fog risk over an upwelling region.