Ocean circulation

This article introduces the main circulation patterns in the upper ocean, which are driven primarily by winds. The article Thermohaline circulation of the oceans discusses ocean circulation driven mainly by density differences as well as related climate impacts. The influence of Earth’s rotation on ocean circulation is explained in the articles Ekman transport and Geostrophic flow.

By redistributing heat around the globe, ocean currents play an important role in regulating Earth’s climate. In the North Atlantic, ocean heat transport contributes to the relatively mild climate of Western Europe, together with the prevailing westerly winds and coupled atmosphere–ocean circulation patterns. Ocean and atmospheric circulation form a strongly interconnected dynamic system in which instabilities, particularly the El Niño–Southern Oscillation (ENSO), can produce major climate fluctuations. Ocean currents also play a key role in marine ecosystems by storing [math]CO_2[/math] and recycling nutrients.

Although tides are the dominant driver of water motion in shallow coastal seas, their influence on large-scale ocean circulation is smaller. Ocean tides are generated mainly in the open ocean by the gravitational forces of the Moon and Sun and are amplified as they propagate onto continental shelves (see Ocean and shelf tides).

Currents in the upper ocean

Fig. 1. Ekman mean velocity profiles measured at 10°N in the western/central tropical North Pacific (adapted from Price and Sundermeyer(1999). The upper 0–20 m were not observed; blue shading schematically indicates increasing density anomaly and the transition toward the upper thermocline. The observed Ekman velocity decays over an e-folding depth scale of about 30 m.

Fig. 2. Representative along-stream velocity profiles adapted from ADCP/LADCP observations in the Gulf Stream (Rossby et al., 2019) and in the Agulhas current (Beal and Bryden, 1999).

Low atmospheric pressure in the tropics (warm ascending air) and high atmospheric pressure in the subtropics (cooled descending air) drive large-scale atmospheric circulation cells under the influence of the Earth’s rotation. The resulting global wind field consists of dominant westerly winds at latitudes between 30° and 60° in both hemispheres (the Westerlies) and dominant easterly winds in the tropical and subtropical regions (the Trade winds).

Wind stress directly drives currents in the upper ocean mixed layer, whose thickness is typically several tens of meters but varies with season and location. This wind-driven Ekman transport should be distinguished from the large-scale boundary currents described under Geostrophic flow. Ekman transport is a shallow, ageostrophic response to local wind stress: turbulent momentum transfer in the upper ocean is balanced mainly by the Coriolis force, producing a current that rotates and decays with depth and a net transport approximately perpendicular to the wind direction. Observed Ekman-layer velocities are typically of order centimeters per second and are concentrated in the upper tens of meters, although they may reach the upper thermocline at low latitudes (Fig. 1).

Due to the Earth's rotation, the trade winds at low latitudes and the westerlies at mid latitudes drive large basin-scale gyres in the upper ocean: clockwise gyres in the subtropical Northern Hemisphere and anticlockwise gyres in the subtropical Southern Hemisphere.

According to Sverdrup theory, the subtropical wind-stress curl generates downwelling and broad equatorward flow in the ocean interior (see Ekman transport). The return flow is concentrated in narrow, intense western boundary currents such as the Gulf Stream, Agulhas and Kuroshio, a process known as western boundary intensification. (see Western Boundary Currents). Western boundary currents are basin-scale geostrophic currents associated with horizontal pressure gradients, sea-surface slope, density structure, and the closure of the wind-driven Sverdrup circulation. Velocities are commonly of order 1 m/s or more, extending hundreds to more than a thousand meters below the surface (see Fig. 2).

There are five subtropical gyres: the North Atlantic, the South Atlantic, the North Pacific, the South Pacific and the Indian Ocean Gyre, see Fig. 3. They are driven by the anticyclonic wind-stress curl generated by the trade winds at low latitudes and the westerlies at mid-latitudes. Convergence associated with Ekman transport generates large downwelling zones within these gyres.

Besides the subtropical gyres, at high latitudes subpolar gyres are driven by a cyclonic wind-stress curl. Large upwelling zones exist in the subpolar gyres. However, in the North Atlantic, dense, salty surface water cooled by evaporation sinks to depth, fueling the Atlantic Meridional Overturning Current (AMOC), see Thermohaline circulation of the oceans.

Fig. 3. Ocean surface currents. Red: warm currents, blue: neutral currents, black: cold currents.

The most famous ocean current, the Gulf Stream, is a strong western boundary current that transports large amounts of heat from the Caribbean toward the North Atlantic. Along the US east coast it forms a narrow, fast jet due to western boundary intensification associated with the Earth’s rotation. Farther offshore, it becomes a meandering current that generates numerous mesoscale eddies. The North Atlantic subtropical gyre is completed by the Canary Current, which carries relatively cool water southward along the eastern Atlantic. The Kuroshio is a similar western boundary current in the North Pacific, forming part of the North Pacific subtropical gyre together with the California Current and the North Equatorial Current. The North Equatorial Current and South Equatorial Current are driven by the tropical trade winds. The South Pacific subtropical gyre is bounded in the east by the cool Peru (Humboldt) Current, while the Indian Ocean subtropical gyre includes the warm Agulhas Current and the relatively cool West Australian Current.

Mesoscale and submesoscale eddies

Large-scale ocean currents are highly unstable and generate rotating vortices known as mesoscale eddies, with typical diameters of tens to hundreds of kilometers and lifetimes of weeks to months. Eddies are especially abundant in strong current systems such as the Gulf Stream, Kuroshio, Agulhas and Antarctic Circumpolar Current. They transport heat, salt, nutrients and other tracers across the ocean and contribute importantly to the redistribution of energy within the ocean circulation. At smaller scales, sub-mesoscale eddies and fronts generate intense vertical motions and mixing in the upper ocean. These processes influence the exchange of heat, gases and nutrients between the ocean surface and deeper layers and play an important role in marine ecosystems and biological productivity.

Mixing and subduction processes in the upper ocean also contribute to the formation of mode waters and intermediate waters, which transport heat, freshwater, oxygen and dissolved carbon from the surface ocean into the ocean interior. Mode waters are relatively thick layers of nearly uniform temperature and salinity formed by deep winter mixing in the subtropical and subpolar oceans. Intermediate waters form at somewhat greater depth, particularly in high-latitude regions such as the Southern Ocean and North Atlantic, and spread over large distances between the upper and deep ocean. These water masses form an important link between the upper-ocean wind-driven circulation discussed here and the deeper overturning circulation described in Thermohaline circulation of the oceans.

Upwelling

Fig. 4. Principle of coastal upwelling, see the article Ekman transport.

Fig. 5. Ekman transport and resulting equatorial upwelling, with rise of the thermocline

In coastal regions where alongshore winds drive offshore Ekman transport, surface water is displaced away from the coast and deeper water rises to replace it, see Fig. 4. This phenomenon, which is called 'upwelling', enriches surface waters with inorganic nutrients that stimulate primary production. Upwelling zones are characterized by a very rich marine life with abundant resources for fishery, see Open ocean habitat. Major coastal upwelling systems occur along eastern ocean boundaries, including the California, Canary, Peru/Humboldt and Benguela systems, where prevailing alongshore winds drive offshore Ekman transport.

Equatorial upwelling occurs because trade-wind stress drives surface Ekman transport away from the equator: northward just north of the equator and southward just south of it. This divergence draws cooler, nutrient-rich water upward, see Fig. 5. Downwelling zones exist north and south of the equator, as explained in the article Ekman transport.

The Antarctic Circumpolar Current (ACC) is situated in the Southern Ocean and continuously circles Antarctica because no continental barriers interrupt the flow. It is an eastward-flowing current driven primarily by the strong westerly winds at these latitudes. Northward Ekman transport at the surface induces upwelling of deep water in the Southern Ocean, including Circumpolar Deep Water and other deep water masses. Because no continental barriers exist at Drake Passage latitudes, large-scale zonal pressure gradients cannot easily be maintained, which strongly constrains geostrophic meridional flow in the upper and mid-depth ocean. Part of the compensating return flow therefore occurs through deep pathways steered by bottom topography, where variations in water-column thickness permit meridional motion consistent with potential-vorticity conservation (see Geostrophic flow#Vorticity). In addition, the ACC is structured by several major fronts associated with strong horizontal density gradients. Instabilities of these fronts generate mesoscale eddies that contribute importantly to meridional exchange and partly compensate the wind-driven Ekman transport (“eddy compensation”), thereby influencing the strength of the Southern Ocean overturning circulation.

El Niño–Southern Oscillation

Variability in the coupled ocean–atmosphere system of the tropical Pacific produces the El Niño–Southern Oscillation (ENSO), the dominant mode of interannual climate variability on Earth. ENSO arises through interactions between trade winds, ocean circulation, sea-surface temperature, and the thermocline in the equatorial Pacific. These interactions influence weather and climate patterns far beyond the Pacific region through large-scale atmospheric teleconnections.

Under normal conditions, strong easterly trade winds drive warm surface water westward, causing warm water to accumulate in the western Pacific and promoting upwelling of relatively cold, nutrient-rich water along the equator and the west coast of South America. The associated tilt of the equatorial thermocline maintains relatively cool sea-surface temperatures in the eastern Pacific.

During an El Niño event, the trade winds weaken. Warm surface waters then spread eastward through enhanced Equatorial Counter Currents and Kelvin-wave adjustment of the thermocline. Upwelling in the eastern Pacific is suppressed, causing sea-surface temperatures there to rise. The warmer eastern Pacific lowers atmospheric surface pressure, while relatively cooler conditions in the western Pacific increase surface pressure there. This pressure difference further weakens the trade winds, allowing even more warm water to move eastward and further reducing upwelling. This self-reinforcing ocean–atmosphere interaction is known as the Bjerknes feedback.

El Niño events strongly affect marine ecosystems and fisheries because reduced upwelling limits the supply of nutrients to surface waters. ENSO also alters rainfall patterns, storm tracks, drought occurrence and tropical cyclone activity in many parts of the world through atmospheric teleconnections.

The opposite phase, La Niña, is characterized by stronger-than-average trade winds, enhanced upwelling in the eastern Pacific, and increased accumulation of warm water in the western Pacific. ENSO events typically recur every 2–7 years, although their timing and intensity are irregular. The detailed mechanisms controlling the onset, growth and termination of ENSO events remain active topics of research.

Related articles

Thermohaline circulation of the oceans

Ekman transport

Geostrophic flow

Open ocean habitat

Shelf sea exchange with the ocean

Ocean and shelf tides

Coriolis acceleration

References

The main authors of this article are Katrien Töpke and Job Dronkers Please note that others may also have edited the contents of this article. Citation: Katrien Töpke; Job Dronkers; (2026): Ocean circulation. Available from http://www.coastalwiki.org/wiki/Ocean_circulation [accessed on 29-05-2026] For other articles by this author see Category:Articles by Katrien Töpke For other articles by this author see Category:Articles by Job Dronkers For an overview of contributions by this author see Special:Contributions/Topke K For an overview of contributions by this author see Special:Contributions/Dronkers J