Coastal and Equatorial Upwelling

node · earth · Eastern ocean margins, the equator and the Southern Ocean

Where surface water is pushed away and deeper water rises to replace it. Along an eastern ocean margin, an equatorward alongshore wind drives Ekman transport offshore and pulls water up from one or two hundred metres — cold, and loaded with the nitrate and phosphate that sank out of the sunlit layer elsewhere. The four eastern boundary upwelling systems — California, Canary, Benguela and Humboldt — occupy about one per cent of the ocean surface and produce on the order of twenty per cent of the global fish catch. Upwelling also happens at the equator, where the Coriolis deflection changes sign and Ekman transport diverges on either side, and around Antarctica, where the westerlies pull deep water back to the surface and close the overturning circuit. Upwelling is the return half of the circulation: without it, what sinks would never come back.