What Is Upwelling, and Why Does It Hold Fish?

Upwelling is wind pushing surface water away from the coast, which pulls colder, nutrient-rich water up from below to replace it. That nutrient injection fuels plankton blooms, and plankton blooms are the base of the entire food chain, which is why cold upwelled water is often the most productive water on the chart, not the water to avoid.

The mechanism

NOAA's own explanation is direct: “conditions are optimal for upwelling along the coast when winds blow along the shore”. The wind drags surface water offshore, and water from depth rises to replace it. That deep water carries nutrients that were never depleted by sunlit-zone plankton, so when it reaches the surface and gets light, phytoplankton growth explodes.

Why upwelling zones matter disproportionately

NOAA's figures on this are striking: coastal upwelling regions account for only about one percent of the ocean's surface, yet contribute roughly 50 percent of the world's fisheries landings. The U.S. West Coast's California Current is one of the major coastal upwelling systems worldwide, alongside the Benguela Current off southern Africa and the Humboldt Current off South America.

How upwelling looks on Chlorofishy's charts

On SST, upwelling shows up as a band of noticeably colder water hugging the coastline, often with a sharp temperature break where it meets warmer offshore water. On chlorophyll, the same zone typically shows elevated readings within days, as the nutrient pulse turns into a visible plankton bloom. See chlorophyll numbers that mean something for why this green is a different kind of signal than river-runoff turbidity.

Check today's coastal upwelling on the Central California region page or the chlorophyll map. Definition only? Glossary entry. Back to Learn.