What Is Upwelling and Why Does It Matter?

Published: May 13, 2026 · 7 min read

Upwelling: The Ocean's Conveyor Belt

Upwelling is the process by which deep, cold, nutrient-rich water rises to the ocean surface, replacing warmer, nutrient-depleted surface water. It is one of the most important physical mechanisms in marine science, because it directly controls the fertility of the world's oceans. Without upwelling, much of the tropical and subtropical ocean would be a biological desert — clear blue water with almost no life in it. With it, some of the most productive fisheries on Earth become possible.

The basic mechanism is surprisingly simple. When persistent winds blow parallel to a coastline, the Coriolis effect — the deflection of moving water by Earth's rotation — pushes surface water away from the coast. This water is replaced by cold water that rises from depths of 100 to 300 meters. The result is a narrow band of cold, green, life-filled water hugging the coast, visible from space as a chlorophyll-rich ribbon against the deep blue of the open ocean.

Where Upwelling Happens

The world's four major coastal upwelling zones are all on the eastern boundaries of ocean basins: the California Current off the U.S. West Coast, the Humboldt (or Peru) Current off South America, the Benguela Current off southwestern Africa, and the Canary Current off northwestern Africa. These regions cover less than 1 percent of the ocean surface but produce roughly 20 percent of the world's fish catch. They are the marine equivalent of rainforests — small in area but disproportionately productive.

Upwelling also occurs in the equatorial ocean, where the trade winds blow from east to west, pushing surface water away from the equator on both sides. The Coriolis effect then deflects this water north and south, drawing cold water up from below. This equatorial upwelling is what creates the cold tongue of sea surface temperatures that extends along the equator in the eastern and central Pacific — the same cold tongue that weakens or disappears during an El Niño event.

The Nutrient Factory

Why is upwelled water so rich? In the sunlit surface layer, phytoplankton consume nutrients — nitrate, phosphate, silicate — as they grow. When they die, their remains sink into the deep ocean, where bacteria break them down, releasing the nutrients back into the water. Over time, the deep ocean accumulates a reservoir of nutrients that far exceeds what is available at the surface. Upwelling acts as a pump, bringing these nutrients back to the surface where they can re-enter the food web.

The first beneficiaries are phytoplankton, microscopic algae that form the base of the marine food web. When nutrient-rich water reaches the surface, phytoplankton bloom explosively, turning the water green across hundreds of kilometers. These blooms are consumed by zooplankton, which are eaten by small fish, which are eaten by larger fish, seabirds, and marine mammals. A single upwelling cell can support a food chain that ends with tuna, sharks, and whales.

El Niño: The Upwelling Killer

El Niño directly disrupts upwelling. During an El Niño event, the trade winds that drive both coastal and equatorial upwelling weaken — sometimes dramatically. The easterly winds that normally push surface water away from the coast relax, and in some years, reverse direction entirely. Without these winds, the engine of upwelling stalls. The cold, nutrient-rich water stays trapped in the depths, and the surface warms rapidly.

The results are felt first and most severely in the Peru Current system. Normally, the Humboldt Current off Peru and Chile supports one of the largest fisheries in the world — primarily anchoveta (Engraulis ringens), a small anchovy that feeds directly on phytoplankton. During the strong El Niño of 1997–1998, the anchoveta catch collapsed from roughly 8 million tons to near zero. The fishing fleet sat idle, and the entire economy of Peru's coastal region felt the shock.

The collapse is not just economic. Seabirds that depend on anchoveta — guanay cormorants, Peruvian boobies, and pelicans — starve in enormous numbers during El Niño events. Their breeding colonies, which can number in the hundreds of thousands, fail completely. The guano deposits that have been harvested as fertilizer for centuries do not accumulate in El Niño years. The ecological chain reaction is almost instantaneous.

Beyond Fish: Climate Feedbacks

Upwelling also plays a critical role in the climate system. The cold water that reaches the surface absorbs carbon dioxide from the atmosphere, making upwelling zones important sinks for anthropogenic CO2. When upwelling weakens during El Niño, this carbon uptake is reduced, and in some regions, the ocean can even become a net source of CO2 to the atmosphere.

Upwelling also affects coastal climate. The cold water it brings to the surface cools the overlying air, creating stable marine layers and frequent fog. This is why San Francisco, for example, is notorious for its chilly summers despite its southerly latitude. During El Niño, this cooling effect weakens, and coastal temperatures rise — a phenomenon sometimes called "coastal El Niño."

Long-Term Changes

Climate change is altering upwelling patterns in complex ways. Some studies suggest that the temperature gradient between land and ocean will strengthen coastal upwelling in certain regions (a process called "upwelling-favorable wind intensification"), while other research points to increased ocean stratification — a warmer, lighter surface layer that resists mixing with deeper water — that could reduce the nutrient content of upwelled water even if the physical upwelling itself continues.

This distinction between "more upwelling" and "more nutrient-rich upwelling" is crucial. If stronger winds bring up water that is already warmer and poorer in nutrients because stratification has deepened the thermocline, the biological benefit may not materialize. Understanding how these competing effects play out in each upwelling zone is a high priority for fisheries management and conservation planning.

Monitoring Upwelling

Satellites are the primary tool for monitoring upwelling at global scale. Sea surface temperature measurements reveal the cold filaments and plumes characteristic of active upwelling. Ocean color sensors detect chlorophyll, providing a direct measure of phytoplankton response. Argo floats and moorings measure subsurface temperature and nutrient distributions, giving scientists the vertical picture that satellites cannot see. Together, these observing systems make upwelling one of the best-monitored ocean processes — and one of the clearest indicators of El Niño's impact on the marine world.

Explore more at the El Niño Guide — comprehensive climate science explained.