Upwelling is a natural process where wind pushes cooler, nutrient-rich water from the deep ocean upward to the surface. This water replaces the warmer, less nutrient-rich water that is usually found near the ocean's surface. The nutrients in the upwelled water help plants and tiny ocean organisms, such as phytoplankton, grow and reproduce. These areas can be recognized by cooler sea surface temperatures and higher levels of chlorophyll a, which indicates the presence of phytoplankton.
The increase in nutrients from upwelling supports high levels of plant growth, which in turn supports fish populations. About 25% of the world's total marine fish catches come from five major upwelling regions. These areas cover only 5% of the ocean's total surface. Upwelling caused by coastal currents or open-ocean movements has the greatest effect on enriching ocean waters and supporting global fish production.
Mechanisms
The three main factors that work together to cause upwelling are wind, Coriolis effect, and Ekman transport. These factors operate differently depending on the type of upwelling, but their overall effects are similar. During upwelling, winds blow across the ocean surface in a specific direction, creating an interaction between the wind and water. This interaction causes the water to move in a net direction that is 90 degrees from the wind's direction due to the Coriolis effect and Ekman transport. Ekman transport causes the top layer of water to move about 45 degrees from the wind's direction. Friction between this layer and the layer below it makes the deeper layers move in the same direction, creating a twisting movement of water downward through the ocean. The Coriolis effect then determines the direction of this movement: in the Northern Hemisphere, water moves to the right of the wind's direction, and in the Southern Hemisphere, water moves to the left. If this movement causes water to spread apart, deep water rises to replace the water that has moved away.
Types
Ocean upwelling occurs when currents move apart, bringing deep, cold, and nutrient-rich water to the surface. There are five main types of upwelling: coastal upwelling, large-scale wind-driven upwelling in the open ocean, upwelling linked to ocean eddies, upwelling caused by underwater features, and broad-diffusive upwelling in the open ocean.
Coastal upwelling is the most well-known type and is closely tied to human activities because it supports some of the world’s most productive fisheries. This happens when wind blows parallel to the coastline, creating currents that move surface water away from the shore. In the Northern Hemisphere, these currents are pushed to the right of the wind direction, while in the Southern Hemisphere, they are pushed to the left. This movement, called Ekman transport, causes deeper water to rise and replace the surface water that has moved away. Upwelling typically happens at a rate of about 5–10 meters per day, but wind strength and distance can affect this rate.
Deep ocean water contains nutrients like nitrate, phosphate, and silicic acid, which come from the breakdown of organic matter from dead plankton. When this water rises to the surface, phytoplankton use these nutrients, along with sunlight and carbon dioxide, to produce organic compounds through photosynthesis. This process leads to high levels of primary production, which is the amount of carbon fixed by phytoplankton. Upwelling areas account for about 50% of the ocean’s total marine productivity. Since phytoplankton form the base of the ocean food chain, this high productivity supports the entire ecosystem.
The ocean food chain begins with phytoplankton and moves upward through zooplankton, small fish, larger fish, and marine mammals. Coastal upwelling occurs year-round in some regions, called major coastal upwelling systems, and only during certain months in other regions, called seasonal systems. Many of these systems are classified as Large Marine Ecosystems due to their high carbon productivity.
Five major coastal currents are linked to upwelling: the Canary Current (off Northwest Africa), the Benguela Current (off southern Africa), the California Current (off California and Oregon), the Humboldt Current (off Peru and Chile), and the Somali Current (off Somalia and Oman). These currents support major fisheries. Four of these are eastern boundary currents where coastal upwelling is most common: the Canary, Benguela, California, and Humboldt currents. The Benguela Current is part of the South Atlantic subtropical gyre and splits into northern and southern systems, with the strongest upwelling near Luderitz, Namibia. The California Current splits at Point Conception, California, due to differences in upwelling strength between its north and south sections. The Canary Current is divided by the Canary Islands, and the Humboldt Current flows west along the coast of South America, extending up to 1,000 kilometers offshore.
Upwelling near the equator is linked to the Intertropical Convergence Zone (ITCZ), a region where trade winds converge. Even though the Coriolis force is absent along the equator, upwelling occurs just north and south of it, creating nutrient-rich waters that support high phytoplankton concentrations detectable from space.
Large-scale upwelling also happens in the Southern Ocean, where strong westerly winds drive water northward. This is a type of coastal upwelling, and in areas near Antarctica with no nearby continents, deep water is drawn upward. This process is critical for bringing deep water to the surface globally. In some Antarctic regions, upwelling near the coast brings warm Circumpolar Deep Water onto the continental shelf, which can melt ice shelves and affect ice stability. Similar upwelling occurs off the west coasts of North and South America, northwest and southwest Africa, and southwest and south Australia, all linked to subtropical high-pressure systems.
Some ocean models suggest that broad-scale upwelling occurs in the tropics, where water flows toward the equator and is warmed from above. However, the required diffusion rates for this process are higher than observed in the real ocean. Despite this, some diffusive upwelling likely occurs.
Variations
Upwelling intensity is influenced by wind strength, seasonal changes, how water layers are arranged, the shape of the ocean floor, and changes in ocean currents.
In some areas, upwelling happens only during certain seasons, causing short periods of increased ocean life, similar to the way plants grow rapidly in spring near the coast. Wind-driven upwelling occurs because warm, light air over land moves toward cooler, denser air over the ocean. In temperate regions, the temperature difference between land and sea changes greatly with the seasons, leading to strong upwelling in spring and summer, and weak or no upwelling in winter. For example, along the Oregon coast, there are four to five strong upwelling events each year, with little to no upwelling between them during the six-month upwelling season. In contrast, tropical regions have a more consistent temperature difference, causing upwelling to happen year-round. The Peruvian upwelling, for instance, occurs most of the year, supporting one of the world’s largest fisheries for sardines and anchovies.
During unusual years when trade winds weaken or change direction, the water brought up from the deep ocean is warmer and has fewer nutrients, which greatly reduces the number of marine life and phytoplankton. This event is called the El Niño-Southern Oscillation (ENSO). The Peruvian upwelling system is especially affected by ENSO, leading to large yearly changes in ocean productivity.
The shape of the ocean floor can also influence upwelling strength. For example, a submarine ridge near the coast can create better upwelling conditions than nearby areas. Upwelling often starts at these ridges and remains strongest there, even if upwelling occurs elsewhere.
High productivity
Upwelling regions are the most productive and fertile areas in the ocean. They are important sources of marine productivity and attract hundreds of species across all trophic levels. These systems' diversity has been an important area of study for marine research. When scientists study the trophic levels and patterns in upwelling regions, they find that these systems often show a wasp-waist richness pattern. In this pattern, the highest and lowest trophic levels have high species diversity. However, the middle trophic level has very few species, usually only one or two. This middle level includes small, pelagic fish and makes up about 3 to 4 percent of all fish species in the area. The lower trophic levels are very diverse, with about 500 species of copepods, 2,500 species of gastropods, and 2,500 species of crustaceans on average. At the highest and near-highest trophic levels, there are usually about 100 species of marine mammals and about 50 species of marine birds. The middle trophic level species are small pelagic fish that typically eat phytoplankton. In most upwelling systems, these species are usually anchovies or sardines, and only one species is often present, though two or three may occasionally be found. These fish are an important food source for predators such as large pelagic fish, marine mammals, and marine birds. Even though they are not at the base of the trophic pyramid, they are essential for connecting the entire marine ecosystem and maintaining the high productivity of upwelling zones.
Threats to upwelling ecosystems
A major danger to the middle position in the food chain and the entire upwelling food system is commercial fishing. Upwelling areas are among the most productive and diverse places on Earth, which draws many commercial fishers and fisheries. This is a benefit of upwelling because it provides food and income for people and nations, as well as for marine animals. However, overfishing can harm both the targeted species and the ecosystem as a whole. In upwelling ecosystems, every species plays an important role in keeping the system balanced. If one species is greatly reduced in number, it can affect all other levels of the food chain. For example, if a common prey species is heavily fished, large numbers of these fish may be removed from the water. This reduces the food supply for predators that rely on them, which can cause those predators to decline in number. This decline can then affect the predators that hunt them, continuing up the food chain and possibly leading to the collapse of the ecosystem. While the ecosystem might recover over time, not all species can fully return to their original numbers. Even if some species adapt, the recovery of the upwelling community may take a long time.
The risk of ecosystem collapse is a serious danger caused by fishing in upwelling regions. Fisheries often target many different species, but they are especially harmful to the intermediate pelagic fish. These fish are central to the food chain in upwelling ecosystems and are found throughout the system, even if only one species exists. Unfortunately, these fish are often the main targets of fisheries, as about 64% of their total catch includes pelagic fish. Of these, the six main species that usually occupy the middle position in the food chain make up more than half of the catch.
Besides causing ecosystem collapse by removing key species, overfishing can also harm the system in other ways. Animals higher in the food chain may not die completely, but a lack of food can still reduce their numbers. If animals do not get enough food, they may reproduce less often or less successfully, leading to fewer offspring. This is especially harmful to species that do not reproduce often or take a long time to reach maturity. Another issue is the loss of genetic diversity within a species due to overfishing. This can reduce biodiversity, making it harder for species to adapt to changes in their environment. If biodiversity drops too much, species may not survive, which could lead to the collapse of the ecosystem.
Another threat to upwelling regions is the El Niño-Southern Oscillation (ENSO) system, particularly during El Niño events. During normal periods and La Niña events, strong easterly trade winds continue the upwelling process. However, during El Niño events, these winds weaken, reducing upwelling in equatorial areas. The movement of water north and south of the equator becomes less strong, and coastal upwelling zones shrink because they depend on wind. As a result, global upwelling decreases, leading to less productivity because nutrient-rich water is no longer brought to the surface. Without these nutrients, the rest of the food chain cannot be supported, and the rich upwelling ecosystem may collapse.
Effect on climate
Coastal upwelling greatly affects the local climate of the area it influences. This effect is stronger when the ocean current is already cool. As cold, nutrient-rich water rises to the surface and sea temperatures drop, the air above it cools and may form water droplets, creating sea fog and stratus clouds. This process also reduces the formation of high clouds, rain, and thunderstorms, causing rainfall over the ocean to leave the land dry. In regions with upwelling that happens all year, such as the western coasts of Southern Africa and South America, temperatures are usually cooler, and rainfall is limited. In areas with seasonal upwelling, like the western coasts of the United States and the Iberian Peninsula, upwelling often occurs alongside downwelling, leading to cooler, drier summers and milder, wetter winters. Areas with permanent upwelling typically have semi-arid or desert climates, while areas with seasonal upwelling usually have Mediterranean, semi-arid, or oceanic climates. Some cities worldwide affected by strong upwelling include San Francisco, Antofagasta, Sines, Essaouira, Walvis Bay, Curaçao, and others.