Upwelling is an ocean process where dense, cooler water from the deeper ocean rises to replace warmer, less dense surface water. This natural circulation replaces displaced surface water.
How Upwelling Occurs
Upwelling is driven by a combination of wind, the Coriolis effect, and Ekman transport. Prevailing winds blowing across the ocean surface exert a force on the water, causing movement. This wind-driven motion of surface water is not directly in the wind’s direction due to Earth’s rotation.
The Coriolis effect, a force from Earth’s rotation, deflects moving objects like ocean currents. In the Northern Hemisphere, deflection is to the right of the wind direction; in the Southern Hemisphere, it is to the left.
Ekman transport describes the net movement of water in the upper ocean, approximately 90 degrees to the wind’s direction. This occurs as wind drags the surface layer, and deeper layers move more slowly, deflected by the Coriolis effect, creating a spiral down to about 100 meters. When surface water moves offshore due to Ekman transport, deeper water rises to fill the void.
This upward movement of deeper water compensates for the offshore transport of surface water. The combination of persistent winds causing offshore surface water movement and replacement by water from below defines upwelling’s physical mechanism.
Where Upwelling Happens
Upwelling occurs in various oceanic regions. Coastal upwelling is common along coastlines where winds blow parallel to the shore. For example, along the west coasts of continents in the Northern Hemisphere, such as California and Northwest Africa, winds from the north cause surface water to move offshore due to Ekman transport, leading to upwelling. Similarly, along the west coasts of continents in the Southern Hemisphere, like Peru and Chile, winds from the south result in offshore surface water movement and subsequent upwelling.
Equatorial upwelling takes place near the equator, driven by trade winds. These winds cause surface waters to diverge, moving away from the equator due to the Coriolis effect on either side. This divergence pulls deeper, cooler water upwards to replace displaced surface water. This is evident in the Pacific and Atlantic Oceans.
Other types of upwelling, though less common, also contribute to oceanic circulation. These include open ocean upwelling, associated with eddies or seamounts, or large-scale wind-driven upwelling in the ocean interior. These are often linked to specific topographic features or broader wind patterns that create zones of divergence, allowing deeper waters to rise.
Why Upwelling Matters
Upwelling significantly impacts marine ecosystems by bringing nutrient-rich waters from ocean depths to the surface. These deeper waters contain nitrates, phosphates, and silicates. When these nutrients reach the sunlit surface layers, they act as a natural fertilizer.
These nutrients support the rapid growth of phytoplankton, microscopic marine plants. Phytoplankton form the base of the marine food web, converting sunlight and nutrients into organic matter through photosynthesis. This abundance then supports higher trophic levels.
Zooplankton graze on phytoplankton, becoming food for various fish species, marine mammals, and seabirds. Upwelling zones are characterized by high primary productivity and support some of the most productive fisheries globally. While these regions account for only about one percent of the ocean surface, they contribute roughly 50 percent of the world’s fisheries landings.