Ocean dynamics

The movement of water in the Earth’s oceans is the primary process that transports heat from the tropics to the polar regions, keeps the oceans uniform in chemistry, and transports heat between the ocean’s surface and its floor.

Background

Motions in the Earth’s oceans include tides, which are caused by the gravity of the Sun and Moon; surface currents, which are driven by wind; and thermohaline circulation, which is driven by density differences in seawater. All of these motions move large amounts of water from place to place. In so doing, they also transport heat from equator to pole and from the surface to the deep ocean, and they also transport dissolved chemicals, including greenhouse gases. Although the saltiness of ocean water varies from place to place, the relative proportions of dissolved materials—say, the ratio of sodium to chlorine—are extremely uniform. Ocean dynamics keep the oceans thoroughly mixed and profoundly affect Earth’s temperature; hence, they are important in climate modeling. Although waves are the most obvious water motion to most people, the water in waves merely oscillates back and forth; that is, waves do not transport water long distances and thus are not discussed in this article.

Tides

Tides are the result of the gravitational attraction of the Moon and Sun. Although the Sun is far more massive than the Moon, its much greater distance means that its tidal effect is only about half that of the Moon. Nevertheless, if the Earth lacked a Moon, it would still have appreciable tides.

The continents prevent water from moving freely, so the actual movement of the tides is very complex. In most the high and low tides revolve like the spokes of a wheel or the wave in a drinking glass oscillating in a circle. Tides move water through the connections between oceans and are important in keeping ocean water uniformly mixed. In small bodies of water it can take hours for the tides to progress from one end to the other, so tide predictions have to be based on local observations as well as the positions of the Sun and Moon.

Solar and lunar tides affect each other appreciably. Solar and lunar high and low tides can reinforce each other or partially cancel each other out. When the Earth, Sun, and Moon are in a straight line, at new or full moon, solar and lunar tides reinforce each other. The range between low and high tide is large, a condition called spring tide. When the Sun and Moon are 90° apart, as at first or last quarter moon, solar and lunar tides partially cancel each other out. The range between low and high tide is unusually small, a condition called neap tide.

Ocean Currents

Surface ocean currents are driven by the winds. Generally, water near the equator is pushed west by the trade winds until it strikes a continent. Most of it then is diverted poleward, where it encounters the prevailing westerlies and is pushed east. Once it reaches the eastern side of the ocean, most of the water is diverted toward the equator. Thus, in all the ocean basins there is a large loop, or gyre, rotating clockwise in the Northern Hemisphere and counterclockwise in the Southern.

Around the Antarctic is a unique geography, a belt of latitude consisting entirely of ocean. With no topography to hinder them, the westerly winds in the Southern Hemisphere, called the Roaring Forties, create some of the roughest seas in the world. They also create a globe-girdling current, the Circum-Antarctic Current, that continuously circles Antarctica and is the principal mechanism for transferring water from one ocean to another.

Thermohaline Circulation

The densest seawater on Earth is found around the Antarctic. The water is dense because it is both cold and salty. The water is salty because freezing of leaves dissolved salt concentrated in the remaining liquid. This cold, dense water sinks to the bottom and flows northward as a dense layer called Antarctic bottom water. The largest amount of Antarctic bottom water flows beneath the Pacific until it reaches Alaska, where it rises and merges into the surface circulation. It then flows around the North Pacific until it reaches the southwest Pacific, where it has warmed to over 30° Celsius. Some warm water circulates through Indonesia to the Indian Ocean, and even though the amount of water involved is fairly small, the amount of heat transferred is large. Warm Indian Ocean water rounds Africa, cooling somewhat, then warms again in the South Atlantic. Some warm water crosses into the North Atlantic, travels up the eastern coast of North America as the Gulf Stream, then crosses to Europe. Finally, in the Arctic, the water cools and sinks. It then begins traveling south as North Atlantic deep water. Antarctic bottom water is also creeping northward in the Atlantic, colder and denser than North Atlantic deep water. Thus, in the North Atlantic, water is moving northward on the surface, Antarctic bottom water is moving north along the bottom, and North Atlantic deep water is moving south just above the Antarctic bottom water. Because much of the water movement is northward or southward, is sometimes called meridional overturning circulation.

Context

Large-scale movements of ocean water affect local and global climate by transporting heat. They also affect climate change by transporting dissolved greenhouse gases into the deep ocean for storage and back to the surface for release. One aspect of thermohaline circulation has received particular attention. Many scientists are convinced that a sudden release of freshwater from glacial lakes in North America covered the North Atlantic with a layer of freshwater that prevented the exchange of heat between the ocean and the atmosphere and caused a sharp cooling event, called the Younger Dryas, about twelve thousand years ago. Some have suggested that melting of the Greenland might have a similar effect, so that warming of the climate might, paradoxically, produce a cooling episode.

Key Concepts

• gyres: large, rotating loops of ocean current found in all major oceans; they are driven westward by the trade winds near the equator and eastward by the at high latitudes
• meridional: referring to the motion of air or water in a generally north-south direction, that is, along meridians
• thermohaline circulation: a vertical circulation in the oceans that is mostly driven by water-density differences, which in turn are governed by temperature and salinity
• trade winds: twin wind belts on either side of the equator that generally blow westward
• westerlies: belts of wind in midlatitudes that generally blow from west to east

Bibliography

Denny, Mark. How the Ocean Works: An Introduction to Oceanography. Princeton, N.J.: Princeton University Press, 2008.

Garrison, Tom S. Essentials of Oceanography. 4th ed. Belmont, Calif.: Brooks/Cole, 2006.

Gozdz, Olivia, et al. "The Impact of Interactive Ocean Dynamics on Atlantic Sea Surface Temperature Variability." American Meteorological Society, 15 May 2024, doi.org/10.1175/JCLI-D-23-0609.1. Accessed 21 Dec. 2024.

Pinet, Paul R. Invitation to Oceanography. Sudbury, Mass.: Jones & Bartlett, 2009.