Downwelling is the sinking of surface water to deeper levels in the ocean. It happens when winds, currents, or changes in water density push surface water downward, carrying dissolved oxygen and other properties from the surface into the deep. While less famous than its counterpart, upwelling, downwelling plays a critical role in ocean circulation, climate regulation, and the distribution of life in the sea.
How Downwelling Works
Two main forces drive downwelling: wind and water density. In wind-driven downwelling, steady winds push surface water toward a coastline or into a converging zone. When that water piles up with nowhere to go horizontally, it’s forced downward. The Coriolis effect, which deflects moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, determines exactly how wind translates into water movement. This deflected flow is called Ekman transport, and it’s the mechanical engine behind most coastal downwelling.
In the open ocean, downwelling also occurs inside large rotating current systems called gyres. In anticyclonic gyres (which rotate clockwise in the Northern Hemisphere), Ekman transport pushes surface water inward toward the center. That convergence forces water downward, a process sometimes called Ekman pumping.
Density-driven downwelling works differently. When surface water becomes denser than the water below it, gravity pulls it down. Water gets denser by getting colder or saltier, or both. In polar regions, frigid air temperatures chill the ocean surface, and sea ice formation leaves extra salt behind, making the remaining water especially heavy. This dense water sinks rapidly, sometimes plunging thousands of meters to the ocean floor.
Where Downwelling Happens
The most important downwelling zones on Earth are in the high latitudes, where cold, salty water sinks to form the deep water masses that drive global ocean circulation.
- The Norwegian Sea: Surface water cooled in the northern North Atlantic seeps over the ridge running between Greenland, Iceland, and Scotland, then sinks to form North Atlantic Deep Water (NADW), one of the ocean’s three major deep water masses.
- The Labrador Sea: Another production zone for NADW, where cold winter air chills the surface and triggers deep convection.
- The Antarctic coast and Ross Ice Shelf: Evaporative cooling and ice formation produce Antarctic Bottom Water, the densest and coldest water mass in the global ocean. It fills the deepest layers of every ocean basin.
- The Antarctic Convergence (Polar Front): Where southward-flowing surface currents meet northward-flowing Antarctic water, the collision forces surface water downward, creating Antarctic Intermediate Water.
Wind-driven coastal downwelling happens in many regions seasonally. In the Great Australian Bight, for example, wintertime eastward winds push water onshore, creating downwelling along the southern Australian coast. Similar seasonal patterns occur along the west coasts of continents, where summer upwelling can weaken or reverse into downwelling during winter as wind patterns shift.
Effects on Marine Life and Nutrients
Downwelling has long been considered biologically quiet compared to upwelling. The logic is straightforward: upwelling brings nutrient-rich deep water to the sunlit surface where photosynthesis happens, fueling massive blooms of phytoplankton that support entire food chains. The four major eastern boundary upwelling systems cover less than 1% of the ocean’s surface but produce up to 7% of global marine primary production and 20% of the world’s wild fish catch.
Downwelling does the opposite. It pushes nutrient-depleted surface water downward, away from the light. Surface waters in downwelling zones tend to be low in the dissolved nitrogen and other nutrients that phytoplankton need, which limits biological productivity at the surface.
But recent research has complicated this picture. A study in the northeastern South China Sea found that downwelling can transport nutrient-rich coastal water downslope into deeper offshore areas. Nearshore waters, enriched by river runoff and bottom sediments, were carried along the coast by downwelling currents and then pushed into deeper water, where the nutrients accumulated and supported biological productivity at depth. Surface nutrient concentrations at study stations were limited to around 0.1 micromoles per liter, but concentrations increased significantly with depth, reflecting this downslope delivery of nutrients. The upper water column was well mixed, while the bottom was stratified, with a clear buildup of both nutrients and chlorophyll (a marker of living phytoplankton) at greater depths.
In other words, downwelling doesn’t just shut biology down. It redistributes nutrients and organic material to deeper layers, where they can be stored and potentially used later when conditions change.
Downwelling’s Role in Ocean Circulation
Density-driven downwelling in the polar regions is the pump that drives thermohaline circulation, often called the ocean’s “conveyor belt.” Cold, dense water sinking in the North Atlantic and around Antarctica fills the deep ocean and slowly spreads across all ocean basins. This deep flow is balanced by the gradual return of water to the surface elsewhere, creating a global loop that moves heat, carbon, and dissolved gases around the planet. The entire cycle takes roughly 1,000 years to complete.
One of the most important things downwelling does is carry oxygen from the atmosphere into the deep ocean. Surface water absorbs oxygen from the air, and when that water sinks, it ventilates the deep layers. Without this process, the deep ocean would gradually become oxygen-depleted, with major consequences for deep-sea life.
Downwelling also pulls dissolved carbon dioxide into the deep ocean. Carbon absorbed at the surface gets transported to depth, where it can remain locked away for centuries. This makes downwelling zones important components of the ocean’s carbon sink, the mechanism by which the ocean absorbs roughly a quarter of the carbon dioxide humans emit.
How Climate Change Threatens Downwelling
Rising temperatures and melting ice sheets are directly threatening the density-driven downwelling that powers global circulation. The mechanism is simple: warmer air means warmer surface water, and melting glaciers pour enormous volumes of freshwater into the ocean. Both changes make surface water lighter and less likely to sink. Increased precipitation at high latitudes, driven by a warmer and wetter atmosphere, adds even more freshwater, further reducing surface salinity and density.
Climate models have shown that this combination can weaken or even collapse the North Atlantic branch of thermohaline circulation. In one modeling study, a doubling of freshwater input over 1,000 years led to a full collapse of North Atlantic deep water formation after just a few centuries. The consequences would be dramatic: northern Europe could cool significantly even as the rest of the planet warms, because the ocean conveyor belt currently transports a large amount of tropical heat northward. Perhaps more concerning, such a shift could happen abruptly rather than gradually, making it far harder for ecosystems and human societies to adapt.
Seasonal wind-driven downwelling is also shifting. In several major coastal systems, the timing and intensity of upwelling and downwelling seasons are changing as wind patterns respond to atmospheric warming. Regions that currently experience winter downwelling may see that season lengthen or shorten depending on how regional wind fields evolve, with cascading effects on local fisheries and nutrient cycling.
How Downwelling Differs From Upwelling
Downwelling and upwelling are mirror processes. Upwelling brings deep, cold, nutrient-loaded water to the surface. Downwelling sends warm, oxygen-rich, nutrient-poor surface water to depth. Upwelling zones are biologically explosive, supporting dense concentrations of fish, seabirds, and marine mammals. Downwelling zones are typically quieter at the surface but perform the essential work of ventilating and nourishing the deep ocean.
The two processes are also physically linked. In many coastal systems, they alternate seasonally. Summer winds may favor upwelling along a coastline, bringing productive conditions, while winter winds reverse direction and drive downwelling instead. In the large subtropical ocean gyres, downwelling at the center is balanced by upwelling at the edges. Globally, every parcel of water that sinks in a downwelling zone must eventually return to the surface somewhere else, meaning these two processes are really two halves of the same circulation.