upwelled

Upwelling is the process by which deeper, cooler, nutrient-rich water rises to the ocean surface to replace warmer water that has moved away or cooled. It occurs when Ekman transport, driven by persistent winds, moves surface waters offshore or away from coastlines, causing deeper water to upwell. Along the equator, diverging surface currents also permit upwelling.

Coastal upwelling is common along the western margins of continents, including California, Peru-Chile, Namibia, and Northwest Africa; equatorial upwelling occurs where trade winds drive surface currents away from the equator. Upwelled water brings nutrients such as nitrate and phosphate into the photic zone, fueling phytoplankton growth and high primary productivity.

This productivity supports robust marine ecosystems and large fisheries, including sardine, anchovy, and hake populations in various regions. Seasonal and interannual variability in wind strength and stratification means that the intensity of upwelling can change, producing strong but finite blooms.

Climate variability, such as the El Niño-Southern Oscillation, can suppress upwelling by weakening winds or altering currents, reducing productivity and catches. Measurements combine satellite data on sea surface temperature and chlorophyll with in situ ocean profiles and nutrient analyses. In the context of climate change, shifts in wind patterns and ocean warming may alter the intensity and geography of upwelling, with wide-ranging effects on ecosystems and coastal economies.