Upwelling is a natural ocean process where deep, cold water rises toward the surface to replace warmer water that has been pushed away by wind. It happens along coastlines and near the equator, and it plays an outsized role in marine life: upwelling regions make up roughly 1% of the ocean’s surface but produce over 50% of the world’s fish catch.
How Upwelling Works
The process starts with wind. When persistent winds blow across the ocean surface, they push the top layer of water along with them. But the water doesn’t move in the same direction as the wind. Earth’s rotation deflects it, a phenomenon known as the Coriolis effect. In the Northern Hemisphere, surface water moves roughly 90 degrees to the right of the wind direction. In the Southern Hemisphere, it moves 90 degrees to the left. This net sideways movement of water driven by wind and Earth’s rotation is called Ekman transport.
Along coastlines, this becomes significant. When winds blow parallel to a western-facing shore in just the right direction, Ekman transport pushes surface water away from the coast. That creates a gap. Cold, nutrient-rich water from depths of 100 to 200 meters or more rises to fill it. The entire cycle is self-sustaining as long as the wind keeps blowing.
In the Northern Hemisphere, this typically happens along west coasts when winds blow from the north. Along the California coast, for example, northerly winds push surface water offshore, drawing deep water up in its place. The same physics apply in the Southern Hemisphere but in mirror image: southerly winds along the coasts of Peru, Chile, and southwest Africa drive surface water westward, triggering upwelling from below.
Where Upwelling Happens
The most productive upwelling zones on Earth are the four major Eastern Boundary Upwelling Systems. These sit along the eastern edges of ocean basins, where continents meet the open sea:
- California Current System along the west coast of North America
- Humboldt Current System off Peru and Chile
- Canary Current System off northwest Africa
- Benguela Current System off southwestern Africa
These four systems share the same basic setup: a north-south coastline, steady along-shore winds, and deep water loaded with nutrients just offshore. But upwelling isn’t limited to these regions. It also occurs along the equator, where trade winds blowing from east to west push surface water away from the equatorial line in both directions, allowing deeper water to rise. Upwelling can even happen in large lakes like the North American Great Lakes, where persistent winds, shallow bottoms, and shoreline geometry combine to pull cold water upward.
Why Deep Water Is So Valuable
Sunlit surface water is warm and pleasant, but it’s often nutrient-poor. Phytoplankton, the tiny organisms at the base of the marine food web, consume available nutrients quickly. The deeper ocean, by contrast, accumulates nutrients over time as organic matter sinks and decomposes. Nitrogen compounds, phosphorus, and silica all concentrate at depth.
When upwelling brings this deep water to the surface, it delivers a pulse of fertilizer to the sunlit zone where photosynthesis happens. Phytoplankton populations bloom, which feeds zooplankton, which feeds small fish, which feeds larger fish, seabirds, and marine mammals. This is why upwelling zones are so disproportionately productive. Research in the Arabian Sea illustrates the scale of this nutrient delivery: during summer upwelling, roughly 50% of the nitrate stored between 100 and 140 meters depth gets transported into the upper water column where organisms can use it.
The nutrient ratio matters too. Nitrogen tends to be the limiting factor for phytoplankton growth. In many surface waters, the ratio of nitrogen to phosphorus falls well below what phytoplankton need for optimal growth. Upwelling corrects this imbalance by injecting nitrogen-rich water from below, unlocking bursts of biological productivity that ripple through the entire food chain.
Effects on Coastal Weather
If you’ve ever visited the California coast in summer and been surprised by how cold and foggy it is, you’ve experienced upwelling’s atmospheric effects firsthand. The deep water that rises to the surface is significantly colder than the water it replaces. Along the southeastern Baltic Sea coast, upwelling lowers average summer sea surface temperatures by about 1°C across the season. During individual upwelling events, coastal air temperatures drop 2 to 4°C compared to conditions just before the event.
This temperature contrast between cold ocean water and warmer air flowing over it creates ideal conditions for advective fog. Warm, moist air passes over the chilled surface, cools below its dew point, and condenses into thick fog banks that can roll miles inland. San Francisco’s famous summer fog is a direct product of strong upwelling along the central California coast.
Downwelling: The Opposite Process
Where upwelling pulls water up, downwelling pushes it down. This happens when winds or ocean currents force surface water to converge in one area, and the only place for it to go is deeper. The driving forces are essentially the reverse of upwelling: negative wind stress curl (wind patterns that push water together rather than apart) and convergence of surface currents.
Downwelling tends to be biologically quieter than upwelling. Instead of delivering nutrients to the surface, it pushes warm, nutrient-depleted water downward. This suppresses productivity rather than fueling it. Downwelling zones are generally less studied precisely because they lack the dramatic biological richness of their upwelling counterparts.
Climate Change and Upwelling’s Future
As the climate warms, scientists are watching upwelling systems closely because even small changes could have enormous consequences for marine ecosystems and fishing economies. The picture is complicated. In some temperate regions, warming is intensifying the temperature difference between land and ocean, which strengthens coastal winds and could boost upwelling. But responses vary enormously by location.
A striking example emerged in early 2025, when Panama’s Pacific upwelling system experienced an unprecedented suppression. The failure highlighted how climate disruption can shut down wind-driven tropical upwelling systems with direct consequences for local ecology and coastal economies. The event also reinforced that regional dynamics matter more than broad global predictions. Some upwelling systems may strengthen while others weaken or fail entirely, depending on local wind patterns, ocean circulation, and how atmospheric warming reshapes both.
Tropical upwelling systems remain particularly vulnerable because they are poorly monitored compared to the major temperate systems. Panama’s 2025 event was a reminder that these smaller, less-studied upwelling zones sustain ecosystems and livelihoods that can’t simply relocate if the deep water stops rising.