Wind stress and offshore water transport
Coastal upwelling occurs when alongshore winds push warm surface water offshore through Ekman transport. As wind drags the surface layer, the Coriolis effect deflects it at an angle. Successive layers below are deflected further, creating a spiral pattern. The integrated effect is that surface water moves at right angles to the wind direction. When northerly winds blow along a west-facing coast in the Northern Hemisphere, surface water moves offshore, leaving a void that must be filled.
Cold, nutrient-rich water from below rises to replace the water pushed offshore. This upwelled water is typically 10-15 degrees Celsius cooler than the surface water it replaces and is enriched in nitrogen, phosphorus, and silica from decomposition at depth. These nutrients trigger phytoplankton blooms visible from space as turquoise or green discoloration, forming the base of one of Earth's most productive food webs.
Fisheries and global productivity
Just five upwelling regions (Peru-Chile, California, Canary, Benguela, and parts of the Arabian Sea) produce nearly 50% of the world's fish catch despite covering less than 1% of the ocean surface. The Peru-Chile upwelling alone supports over 10 million tonnes of anchoveta annually in normal years, more than any other single fish species. These productive zones persist because consistent seasonal wind patterns reliably drive upwelling.
Upwelling regions are vulnerable to climate change because they depend on specific wind patterns. If wind patterns weaken or shift, upwelling intensity declines and productivity crashes. El Nino events disrupt upwelling off Peru, causing fishery collapse. Models project that some upwelling regions may weaken with future climate change, while others may strengthen. These uncertainties make upwelling regions important focal points for monitoring ocean ecosystem change.