4 Ocean Circulation

How ocean circulation works

Ocean circulation moves seawater through the surface and deep ocean. It redistributes heat, salt, oxygen, and nutrients, links ocean basins, and influences regional and global climate. Large-scale circulation is shaped by two interacting systems: wind-driven currents near the surface and density-driven flows at depth. Neither system follows a simple, fixed loop; currents vary and mix as they encounter winds, coastlines, and seafloor topography.

Forces that move ocean water

Several forces interact to move and steer ocean water. Prevailing winds transfer momentum to the sea surface and drive much of the upper-ocean circulation. The winds themselves are powered mainly by unequal solar heating of Earth.

Earth’s rotation causes the Coriolis effect: moving water is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This effect is weakest at the equator.

Wind can pile water up in parts of a basin. Gravity helps push water away from areas of higher sea level toward areas of lower sea level, and pressure differences also help drive currents. Temperature and salinity affect seawater density: colder or saltier water is generally denser than warmer or fresher water, so density contrasts can drive sinking, spreading, and rising flows.

Coasts, ocean-basin shape, ridges, and other underwater features can steer, channel, or redirect currents. For example, wind may start a surface flow while Earth’s rotation and a continent help organize and redirect it.

Surface currents and gyres

Wind-driven currents move water through the upper ocean. On a global scale, their patterns form enormous circular systems called gyres. The five major subtropical gyres are in the North and South Atlantic, North and South Pacific, and Indian Ocean. Subtropical gyres generally rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

A gyre’s western boundary current is typically narrow and fast, while its eastern boundary current is broader and slower. In the North Atlantic, the Gulf Stream is a warm western boundary current, and the Canary Current is a cooler eastern boundary current. In the North Pacific, the Kuroshio and California Current form comparable western and eastern currents. These currents transport heat as they carry water between low and higher latitudes.

Not all surface currents fit neatly into subtropical gyres. The Antarctic Circumpolar Current flows eastward around Antarctica, connecting the Atlantic, Indian, and Pacific sectors of the Southern Ocean. Equatorial currents, including eastward countercurrents, are also shaped by winds and sea-level differences, as well as by the weaker Coriolis effect near the equator.

Deep circulation and water masses

Deep circulation is driven mainly by differences in water density. In cold regions, surface water can become dense enough to sink. When sea ice forms, much of the salt is excluded from the ice and remains in the surrounding water, increasing that water’s salinity and density. Sinking water spreads through the ocean at depth; elsewhere, deep water gradually rises through upwelling and mixing.

A water mass is a body of seawater with recognizable properties, especially temperature and salinity, that help identify where it formed and how it has moved. Major examples include:

• North Atlantic Deep Water (NADW): Dense water formed in the North Atlantic that flows southward at depth.

• Antarctic Bottom Water (AABW): Very dense water formed near Antarctica that spreads along the seafloor and into ocean basins.

• Antarctic Intermediate Water (AAIW): A cooler, relatively fresh water mass that spreads northward at intermediate depths.

These water masses mix and change along their pathways. Their circulation is better understood as a complex, connected overturning system than as a single conveyor belt following one unchanging route.

Overturning circulation and its effects

The Atlantic Meridional Overturning Circulation (AMOC) is one part of the broader overturning system. In the Atlantic, relatively warm upper waters flow northward while deep water, including NADW, flows southward. Antarctic waters also contribute to deep- and bottom-water circulation. Winds and the Antarctic Circumpolar Current influence how water rises, sinks, and moves between layers, so deep circulation is not driven by temperature and salinity alone.

Circulation transports heat between regions, redistributes dissolved gases and nutrients, and connects surface waters with the deep sea. Surface currents carry heat away from the tropics, while upwelling can bring nutrient-rich deep water toward the sunlit surface. These exchanges influence climate and marine environments.