You may have seen cold, foggy coasts or heard about rich fishing grounds and wondered what is upwelling in the ocean. It is wind pushing surface water aside so cold, nutrient-rich deep water rises to replace it.
This page is one part of the wider marine science section on this site.
| Key fact | Value |
|---|---|
| Share of ocean area covered by the major upwelling zones | About 5% (five zones) |
| Share of global marine fish catch from those zones | About 25% |
| Angle between wind direction and net surface water movement | About 90 degrees |
| Principal eastern boundary currents with coastal upwelling | 4 currents (Canary, California, Benguela, Humboldt) |
| Peak upwelling season in the California Current | Spring and summer |
The figures come from WorldAtlas and Ocean Tracks. They are estimates and can change as new research appears, so check the sources before you rely on them.
How wind lifts deep water to the surface
Upwelling begins with wind. When wind blows steadily across the sea, it drags the top layer of water, and the rotation of the Earth bends that motion away from the wind’s path. This sideways push is called Ekman transport, and WorldAtlas describes it as moving water about 90 degrees away from the direction of the wind.
The side it bends toward depends on the hemisphere. In the Northern Hemisphere the water moves to the right of the wind, and in the Southern Hemisphere it moves to the left, as WorldAtlas explains. NOAA researchers describe the same pattern off the US West Coast: the ocean develops currents to the right of the wind, which pushes surface water offshore (NOAA Physical Sciences Laboratory).
On a coast, the sequence runs like this:
- Wind blows along the coastline, parallel to the shore.
- Earth’s rotation turns the moving surface water so that it heads away from the land.
- The departing surface water leaves a gap near the coast.
- Cooler, denser water from below rises to fill that gap.
The water that arrives is nutrient-rich. According to WorldAtlas, deep water holds nitrate, silicic acid and phosphate released as dead organic matter sinks and decomposes. Upwelling returns those nutrients to the sunlit layer where life can use them.
Main types of upwelling
Oceanographers sort upwelling by what drives it. WorldAtlas lists coastal, equatorial, large-scale wind-driven, broad-diffusive, eddy-related and topography-related types. These are the best known:
- Coastal upwelling: wind runs parallel to the coastline, water is carried away from shore, and deeper, colder water rises to replace it.
- Equatorial upwelling: trade winds create currents that are diverted, and upwelling takes place just north and south of the Equator. It has been detected in the equatorial Pacific, where phytoplankton concentrations are high.
- Southern Ocean upwelling: strong westerly winds circling Antarctica move a large amount of water northward. With no continental landmass between the tip of the Antarctic Peninsula and South America, water is drawn up from greater depths.
Coastal upwelling is the type most closely tied to human activity. It is mainly seen where the four principal eastern boundary currents flow: the Canary, California, Benguela and Humboldt currents (WorldAtlas). In some places it runs all year, and in others it appears only in certain months as a seasonal system.
Why upwelling matters for life and fisheries
Upwelling delivers deep, nutrient-rich water to the sunlit surface, and those nutrients stimulate the growth of phytoplankton, which fuels the whole marine ecosystem, according to NOAA Physical Sciences Laboratory. Phytoplankton are the base of the food chain. Fish, seabirds and marine mammals depend on them directly or indirectly.
The payoff is large for the space involved. WorldAtlas reports that the five major upwelling zones occupy a mere 5% of the total ocean area but account for approximately 25% of the global marine fish catch. Estimates of such shares vary with how the zones are defined, so treat them as a rough guide.
The location of these zones is not random. Ocean Tracks explains that cool, nutrient-rich water sits closer to the surface in eastern boundary currents than in western boundary currents, which is why rich upwelling zones form along eastern boundaries such as the California Current.
Upwelling also changes the weather at the coast:
- It cools the sea-surface temperature, which regulates air-sea interactions and low-level stratus clouds (NOAA Physical Sciences Laboratory).
- The cool air above the water condenses into low-level clouds and sea fog, as WorldAtlas notes.
- Cities such as San Francisco, Walvis Bay and Antofagasta are affected by strong upwelling regimes, according to the same WorldAtlas page.
The California Current as a worked example
The California Current is the eastern boundary current of the North Pacific Gyre. It runs southward from British Columbia, Canada to Baja California, Mexico, and it draws cool, nutrient-rich water from the Alaska current down the western coast of North America (Ocean Tracks).
Strong northwesterly winds blow mostly along the shore. Through Ekman transport they push surface water offshore, and cooler, nutrient-rich water is upwelled over the narrow continental shelf to the surface, as Ocean Tracks describes.
NOAA scientists add a second ingredient. Geostrophic transport, which arises from horizontal differences in sea level, is an important part of coastal upwelling near the West Coast, and combined with Ekman transport it gives a very good estimate of surface currents (NOAA Physical Sciences Laboratory).
Shape matters too. Where the continental shelf is narrow and drops steeply to deep water, upwelling is more intense, because less water has to be displaced offshore before cool water can rise (Ocean Tracks).
What strengthens or weakens upwelling?
Several factors change how strongly water rises at a given coast:
- Season: wind strength changes through the year.
- Coastline and shelf shape: the angle of the shore to the wind and the steepness of the seafloor both play a role.
- Long-term climate patterns: cycles in the Pacific can strengthen or weaken the whole system.
In California, the upwelling season is most pronounced in spring and summer, when northwesterly winds are at their highest of the year. It is reduced in fall and winter, when winds relax and become more variable (Ocean Tracks).
Longer cycles matter as well. Ocean Tracks names the Pacific Decadal Oscillation and the El Nino Southern Oscillation as influences, and says that El Nino is associated with a warming of the Pacific that weakens coastal upwelling in California. Warm and cool phases of the Pacific Decadal Oscillation weaken and intensify upwelling respectively.
Climate change adds another question. A study published in February 2021 in Geophysical Research Letters by CIRES and NOAA researchers found that greenhouse gas warming changed Ekman-driven upwelling by season and location: more upwelling north of San Francisco in April and less off southern California in summer (NOAA Physical Sciences Laboratory).
The same study found that changes in geostrophic transport matched or even exceeded the Ekman-driven changes, and that many earlier studies had neglected them. NOAA says better projections are essential for managing marine resources in US coastal regions (NOAA Physical Sciences Laboratory).
Readers who want the chemistry side of ocean change can continue with the guide to ocean acidification causes. For how warming is measured through the water column, see the explainer on ocean heat content, which is a different measure from the surface cooling that upwelling causes.
Frequently asked questions
Why is upwelled water cold?
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It comes from deeper in the ocean, where water is cool and dense. WorldAtlas describes upwelling as the movement of cool, nutrient-rich, dense water toward the surface to replace warmer, nutrient-depleted surface water.
Where does upwelling happen?
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Coastal upwelling is mainly seen along the Canary, California, Benguela and Humboldt eastern boundary currents. It also occurs near the Equator and in the Southern Ocean, according to WorldAtlas.
Does El Nino affect upwelling?
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Yes, along the California coast. Ocean Tracks says El Nino is associated with a warming of the Pacific that weakens coastal upwelling in California.
Why do upwelling coasts get so much fog?
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The cold water cools the air just above it, and that air condenses into low-level clouds and sea fog, as WorldAtlas explains.
Could climate change alter upwelling?
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Possibly, and the effect may differ by season and place. NOAA Physical Sciences Laboratory calls possible changes a major concern for ecosystem health, fishers’ livelihoods and food security.
