M3 Upwelling and Sinking

Topic

Coastal upwelling is the rising of cold, nutrient-rich deep water to the surface, driven primarily by Ekman transport along the coast: when the wind blows parallel to the coast—keeping the land to its left in the Northern Hemisphere—this transport moves surface water away from the shore, creating a deficit near the coast that is subsequently compensated for by the upwelling of subsurface water. The planet's major coastal upwelling systems—located off the coasts of Peru and Chile, California, Benguela, and the Canary Islands—are all situated on the eastern boundaries of the oceans; thanks to this constant supply of nutrients from the depths, they are among the most productive marine ecosystems on Earth.

Equatorial upwelling, meanwhile, occurs because the Coriolis parameter changes sign exactly at the equator, causing Ekman transport to point northward in the Northern Hemisphere and southward in the Southern Hemisphere; this creates a divergence that draws deep water to the surface. In the central and eastern equatorial Pacific, this upwelling keeps surface waters notably cooler—around 24°C, compared to the nearly 29°C observed in the western Pacific—and it is precisely this temperature gradient that drives the trade winds and initiates the El Niño-Southern Oscillation cycle; in turn, the strengthening of these trade winds intensifies the Ekman transport divergence that originally triggered the upwelling, thereby reinforcing the very equatorial upwelling mechanism that started the process. Deep convection is the sinking mechanism at work in polar latitudes: when the cooling of surface water overcomes the forces that normally keep it stable, that surface water becomes denser than the water beneath it and sinks in convective columns—roughly one hundred meters in diameter—reaching depths of between five hundred and two thousand meters; this forms the deep water that ultimately ventilates the rest of the ocean.

The expected behavior associated with these upwelling and sinking processes includes coastal upwelling speeds of barely one meter per day, surface temperatures two to eight degrees Celsius cooler than the surrounding ambient water, and biological productivity rates five to twenty times higher than those observed in the open ocean.

Thus, from the coastal upwelling that opens this document to the biological productivity rates that conclude it—and encompassing the equatorial upwelling that fuels the El Niño-Southern Oscillation cycle as well as the deep convection that drives water downward at the poles—upwelling and sinking cause the ocean to constantly exchange water between the surface and the depths, redistributing cold, nutrients, and biological productivity across the planet.

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