Upwelling happens when wind pushes surface water away from an area, and deeper, colder water rises to replace it. This process is one of the ocean’s most important circulation patterns, fueling enormous bursts of marine life in regions that cover less than 2 percent of the ocean surface but produce roughly 20 percent of global fish catches. The mechanics behind it involve wind, Earth’s rotation, and the simple fact that water displaced from one place has to be replaced from somewhere else.
Wind, Rotation, and Ekman Transport
The engine behind most upwelling is a phenomenon called Ekman transport. When wind blows steadily across the ocean surface, friction drags the top layer of water along. But instead of moving in the same direction as the wind, that water veers to the side because of the Coriolis effect, a consequence of Earth’s spinning on its axis. In the Northern Hemisphere, surface water deflects to the right of the wind direction. In the Southern Hemisphere, it deflects to the left.
The deflection isn’t uniform through the water column. Right at the surface, water moves at about 45 degrees from the wind direction. Deeper layers get pushed progressively further off course, creating a spiraling pattern that weakens with depth. When you average the movement of all these layers together, the net transport of water ends up at a full 90 degrees from the wind direction. This net sideways movement of water is the critical piece: it’s what creates the gap that deeper water fills.
Coastal Upwelling Along Continents
The most productive upwelling zones sit along the western edges of continents, where winds blow roughly parallel to the shoreline. These are called Eastern Boundary Upwelling Systems, named for their position on the eastern side of ocean basins. The four major ones are the Humboldt system off Peru and Chile, the Benguela system off southwestern Africa, the California system along the U.S. and Mexican west coast, and the Iberian/Canary system off northwestern Africa and the Iberian Peninsula. Each features a narrow coastal band, roughly 50 to 150 kilometers wide, where surface temperatures run noticeably cooler than the open ocean nearby.
Here’s the step-by-step process in these regions. Winds blow toward the equator along the coast. Ekman transport pushes surface water 90 degrees to the right (in the Northern Hemisphere), which means offshore, away from shore. Since the coastline acts as a wall, no surface water can flow in from land to replace what’s been pushed away. The only source of replacement water is below. Cold, nutrient-dense water from depths of roughly 100 to 300 meters rises to fill the gap. This vertical movement is upwelling.
The strength of upwelling depends on wind speed, wind consistency, and the shape of the coastline and seafloor. A straight, north-south coastline with steady equatorward winds produces the strongest, most reliable upwelling. Points and capes where the coastline juts out can amplify the effect locally.
Equatorial Upwelling
Upwelling also occurs along the equator through a slightly different mechanism. The easterly trade winds blow steadily westward across the tropical Pacific and Atlantic. Ekman transport pushes surface water to the right of the wind in the Northern Hemisphere and to the left in the Southern Hemisphere. Since the equator is the dividing line, water on the north side gets pushed northward and water on the south side gets pushed southward. This creates a divergence, a zone where surface water splits apart, right along the equator.
Deep water rises to replace it. In the equatorial Pacific, peak upwelling velocities have been measured at about 50 meters depth, with an estimated 10 million cubic meters of thermocline water per second being drawn up from the Southern Hemisphere side alone. This equatorial upwelling is a major reason the eastern tropical Pacific stays relatively cool despite sitting in the tropics.
Seamounts and Islands
Not all upwelling requires wind-driven surface divergence. When ocean currents flow into underwater obstacles like seamounts, ridges, or gently sloping island shelves, the current gets physically pushed upward. This topographic upwelling forces cold, nutrient-rich water from depth into the sunlit upper ocean. The Galápagos Islands provide a clear example: the Equatorial Undercurrent, a deep eastward-flowing current, collides with the islands’ western slope and lifts nutrient-rich water to the surface, creating a visible cold-water signature and unusually high biological productivity around the archipelago.
Even small features like shallow sand banks and limestone plateaus can create detectable cooling and productivity spikes in the surrounding water. Satellite imagery of sea surface temperature often reveals these patterns as cold patches shaped by the underlying seafloor topography.
What Upwelling Brings to the Surface
The water that rises during upwelling is cold, but its real importance is chemical. Surface waters in most of the ocean are relatively starved of the nutrients that phytoplankton need to grow, because organisms in the sunlit zone consume those nutrients faster than they’re replenished. Deep water, by contrast, accumulates nutrients from the steady rain of decomposing organic material sinking from above.
The contrast can be dramatic. In the Sea of Oman during summer, nitrate concentrations at depth exceed 34 micromoles per liter at depths below 1,000 meters, while surface concentrations sit around 0.39 micromoles per liter. That’s roughly a 90-fold difference. Silicate, another nutrient essential for diatoms (a key type of phytoplankton), can reach 120 micromoles per liter in deep water versus just 2 to 3 at the surface. When upwelling delivers this nutrient-loaded water into the zone where sunlight penetrates, it triggers explosive growth.
How Quickly Life Responds
The physical response to upwelling-favorable winds happens fast, within 5 to 10 days. But the biological response takes longer. Phytoplankton blooms typically develop on timescales of 30 to 60 days after an upwelling event begins. This lag exists because the tiny organisms need time to absorb nutrients, divide, and build population density to bloom levels. Once phytoplankton populations boom, zooplankton follow, then small fish, then larger predators. This is why upwelling regions support such dense food webs, from anchovies and sardines to seabirds, marine mammals, and commercial fisheries.
Despite covering a tiny fraction of ocean area, upwelling systems generate about 7 percent of all marine primary production and support roughly a fifth of the world’s fish catch.
Fog and Coastal Weather
Upwelling has a pronounced effect on coastal weather that anyone living near San Francisco or the coast of Namibia knows well. When cold water from hundreds of meters deep reaches the surface, it chills the air directly above it. Warm, humid air blowing in from farther offshore passes over this cold surface and cools below its dew point, causing moisture to condense into fog. This is advection fog, and it’s the reason California’s coast is often shrouded in a thick marine layer during summer, precisely when upwelling is strongest.
The cold surface water also stabilizes the lower atmosphere, suppressing the formation of rain clouds. Coastal deserts like the Atacama in Chile and the Namib in southwestern Africa owe part of their extreme aridity to persistent upwelling offshore. The ocean delivers fog but blocks rain.
How Climate Change Affects Upwelling
Climate projections point to a weakening of the equatorial Pacific upwelling cycle over the coming decades. Analysis of 19 climate models shows a consistent decline in upwelling intensity across the equatorial Pacific through the year 2100, driven by weaker surface wind divergence and a gradual flattening of the thermocline, the boundary between warm surface water and cold deep water. When that boundary levels out, the wave-driven component of upwelling loses strength, particularly in the eastern Pacific.
The picture isn’t uniform everywhere, though. While equatorial Pacific upwelling appears to be weakening, equatorial Atlantic upwelling has shown signs of strengthening. Coastal upwelling systems present an even more complex picture, with some models projecting that stronger land-sea temperature contrasts could intensify coastal winds and boost upwelling in certain regions. The consequences for fisheries remain difficult to predict, because productivity depends not just on how much water rises but on how warm it is and what nutrients it carries.