How does upwelling bring cold water and nutrients to the ocean surface?

Upwelling is a physical process in the ocean that brings cold, nutrient-rich subsurface water to the sunlit surface layer. This phenomenon occurs primarily through two mechanisms:12

Wind-Driven Upwelling

Along coastlines, winds blowing parallel to the shore interact with the Earth's rotation—a phenomenon known as the Coriolis force—to move surface waters away from the coast. This offshore movement of surface water creates a divergence zone, which draws deeper, colder water upward to replace it. This process is also driven by Ekman transport, where the average movement of surface water is directed at a 90° angle to the wind. Similar divergence zones occur at the Equator and around Antarctica, where wind patterns force surface waters apart, allowing deeper water to rise.12

Thermohaline Circulation

Upwelling also occurs as part of the global thermohaline circulation, which is driven by differences in water density caused by temperature and salinity. As dense, cold water sinks in polar regions, it spreads across the ocean floor. To balance this sinking, deep water gradually rises or upwells in other regions, such as the Southern Ocean and the North Pacific, replenishing the upper layers of the ocean.3

These processes are vital for marine life, as the influx of nutrients fuels the rapid reproduction of phytoplankton, supporting highly productive marine ecosystems and major fisheries.41