World Ocean Circulation Experiment

The World Ocean Circulation Experiment is an ambitious international project designed to increase knowledge about the movement of water, heat, and various substances in the sea. The data obtained are expected to be of major importance in predicting future long-term changes in climate.

Movement of Seawater

The World Ocean Circulation Experiment (WOCE), which began in 1990, brought together scientists from around the globe to study the way water moves in all parts of the sea. The WOCE was also intended to obtain information on the movement of heat in the ocean and substances such as salt and oxygen.

The movement of seawater, known as ocean circulation, consists of horizontal and vertical motion. Horizontal movements are known as currents. Vertical motions are known as upwellings and downwellings. Currents vary widely in speed, ranging from a few centimeters per second to as much as four meters per second. Surface currents typically move between five and fifty centimeters per second, with deeper currents generally moving more slowly. Vertical movement of seawater is much slower, with a typical speed of only a few meters per month.

Ocean circulation is primarily caused by two major factors. Wind-driven circulation is caused by air moving across the surface of the sea. This induces friction, known as wind stress, between the water and the air, thus applying a directional force and setting the water in motion. Thermohaline circulation is caused by differences in temperature and salt concentration. These differences cause variations in the density of seawater, leading to differences in pressure and resulting in motion. Surface currents are mostly caused by wind-driven circulation. Deeper currents and vertical movements are mostly the result of thermohaline circulation.

Several factors are involved in determining the size, shape, and speed of ocean circulation patterns. A significant influence is the Coriolis effect, named for the French scientist Gustave-Gaspard de Coriolis. This effect, caused by the planet's rotation, causes the actual path of a moving object to be curved away from the straight line path that it would otherwise follow. The Coriolis effect causes ocean currents to bend to the right in the Northern Hemisphere and the left in the Southern Hemisphere.

Friction also influences the nature of ocean circulation. Layers of water moving at different speeds produce friction where they meet, forming an intermediate zone of turbulent eddy currents as the two layers interact. This transfers energy from one layer to the other and causes the faster layer to move more slowly and the slower layer to move more quickly. Friction also occurs between moving water and the continents and between currents at the bottom of the sea and the ocean floor. This friction, with the attendant resulting turbulence zones, tends to slow the motion of seawater.

Important effects on ocean circulation are seen in the sea region known as the Ekman layer, named for the Swedish scientist Vagn Walfrid Ekman. The Ekman layer extends from the ocean's surface to a depth of about 100 meters. In this layer, the wind has a direct effect on water movement. Wind stress, the Coriolis effect, and friction between layers of water combine to move the Ekman layer in complex ways.

Water at the ocean's surface tends to move at an angle of about 45 degrees to the direction of the wind because of the Coriolis effect. This angle is bent to the right in the Northern Hemisphere and the left in the Southern Hemisphere. With increasing depth, the water moves more slowly, increasing the angle. At a depth where the speed of the water is about 4.3 percent of the surface speed, the water moves in the opposite direction to the wind. The overall effect is that the average movement of the water is at about 90 degrees to the wind.

This movement in the Ekman layer, combined with differences in wind stress, creates areas on the ocean's surface where water converges or diverges. Where it converges, water sinks in downwellings. Where it diverges, the water rises in upwellings. Downwellings and upwellings also occur where the wind blows parallel to the coast of a landmass. They also occur because of differences in temperature and salt concentration, which alter the seawater's density. Cold water and salty water tend to sink, while warm water and less salty water tend to rise.

Worldwide Patterns of Ocean Circulation

The numerous factors involved in ocean circulation, combined with the irregular shapes of the continents, result in complex patterns of water movement. Although major surface currents have been known since the earliest days of ocean travel, much less is known about deeper currents. Similarly, less is also known about the Southern Hemisphere than the Northern Hemisphere. The WOCE was designed to fill these gaps in scientific knowledge.

Before the WOCE, the basic pattern of surface currents was fairly well understood. In the Northern Hemisphere, strong currents tend to move northward along the eastern coasts of the continents. These include the Gulf Stream-North Atlantic- Norway Current, along North America, and the Kuroshio-North Pacific Current, along Asia. In the Southern Hemisphere, strong currents tend to move northward along the western coasts of the continents. These include the Peru Current, along South America; the Benguela Current, along Africa; and the Western Australia Current, along Australia.

In the regions north and south of the equator, major surface currents move westward. These are the Pacific North Equatorial Current and the Pacific South Equatorial Current, between South America and Asia; the Atlantic North Equatorial Current and the Atlantic South Equatorial Current, between Africa and South America, and the Indian South Equatorial Current, between Australia and Africa.

At the equator, narrow eastward currents occur between the wider westward currents. These are the Pacific Equatorial Countercurrent between Asia and South America; the Atlantic Equatorial Countercurrent between South America and Africa; and the Indian Equatorial Countercurrent between Africa and Asia. Another major surface current is the Antarctic Current, moving eastward around Antarctica.

Although scientists know less about deep currents, certain broad movement patterns are understood from WOCE data. Cold water in the northern Atlantic Ocean sinks in downwellings. This deep, cold water tends to move southward along the eastern coasts of North and South America to join the deep, cold water that sinks in downwellings near Antarctica. This water then tends to flow eastward in a deep current around Antarctica. Some of this water then moves northward along the coasts of Asia and Africa, rising in upwellings as it warms. Overall, deep ocean currents function as a continuous conveyor belt system, moving thermal energy in ocean water and distributing it throughout the oceans. Cold water upwelling in the Indian and North Pacific Oceans circulates westward between Australia and Asia and around the southern tip of Africa to reach the North Atlantic. There, it sinks again to depths and enters the deep current that takes it back eastward, where it splits and rises again in the Indian and northern Pacific Oceans.

Studying Ocean Circulation

Scientific studies of surface currents began in the eighteenth century to aid navigation. Later studies concentrated on the effect of changes in ocean circulation on the weather. The need for a major effort to increase the amount of information known about the movement of seawater became clear in the 1980s. Based on the available data, the best ocean circulation models failed to describe the conditions observed in the sea with complete accuracy.

A major determining factor in the establishment of the WOCE was the development of new techniques for studying ocean circulation. Temperature measurements could be made from a ship without stopping the movement of the vessel using an instrument known as a bathythermograph. Devices designed to drift in the ocean, both on the surface and at specific depths, were developed that could be tracked for months or years. Advanced methods of accurately measuring the concentration of substances present in seawater in very low concentrations were also developed. In addition, computers able to handle the enormous amount of data that the WOCE would generate were made possible through advances in electronic manufacturing methods.

Among the most important new instruments available for the WOCE were satellites capable of obtaining data on ocean circulation. In 1979, the Seasat satellite mission, lasting one hundred days, demonstrated that the detection of radar echoes and microwave radiation from the sea could be used to produce detailed information. After years of planning, the WOCE project was ready to begin collecting data in 1990.

Scientists from more than thirty nations participated in the many studies involved in the WOCE. In the United States, the headquarters of the WOCE is located at the Department of Oceanography at Texas Agricultural and Mechanical University. Data collection ended in 1998, but information analysis was expected to last until at least 2002, and many scientists expected data processing to last until at least 2005.

The first WOCE study began with the launching of the German research ships Polarstern and Meteor in 1990. These ships collected data in the southern part of the Atlantic Ocean between Antarctica and South Africa. Other early WOCE studies also concentrated on the Southern Hemisphere because this area had been studied in less detail before the WOCE than the Northern Hemisphere. Later, WOCE studies moved into the Indian Ocean, the North Atlantic Ocean, and the Pacific Ocean.

Satellites used by the WOCE included the ERS series launched by the European Space Agency, the TOPEX/POSEIDON, a joint project of France and the United States, and the Japanese ADEOS. More than one thousand drifting instruments, designed to remain at specific depths far below the sea's surface, were also used. The movement of these instruments was measured by satellites or by sonic equipment. Tens of thousands of measurements were made at the ocean's surface.

Significance

The WOCE project is one of the most important sources of oceanographic data in the early twenty-first century. The official goals of the WOCE included providing a complete description of the general circulation of the ocean, creating a numerical model of ocean circulation for use in advanced computers, accounting for seasonal changes in ocean circulation, obtaining data on the exchange of substances between layers of water in the ocean, providing detailed information on the interaction between the ocean and the atmosphere, and obtaining data on the movement of heat within the ocean.

The most important application of WOCE data is the study of the effect of ocean circulation on climate change. This information is expected to aid scientists in predicting the effect of various human activities, such as the increase in carbon dioxide in the atmosphere, on long-term weather patterns. Such data will also be useful in predicting natural climate changes that occur over years or decades.

Several examples of the interaction between ocean conditions and weather changes are well documented. The Sahel, a region of Africa along the southern fringe of the Sahara Desert, experienced severe droughts in the 1970s and 1980s after having experienced much wetter conditions in the 1950s. These droughts were associated with higher-than-normal surface temperatures in the South Atlantic Ocean, Indian Ocean, and southeast Pacific Ocean, as well as lower-than-normal temperatures in the North Atlantic and most of the Pacific Ocean. Ocean temperature is also important in forming tropical cyclones and other powerful storms.

Currents have a powerful effect on weather patterns. The Gulf Stream-North Atlantic-Norway Current brings relatively warm tropical water northward, moderating the climates of eastern North America, Ireland, the British Isles, and the coast of Norway. The Kuroshio-North Pacific Current does the same for Japan and western North America. These warm currents also encourage increased water evaporation, resulting in increased rainfall in these areas. The Peru Current brings cold polar water northward along the western coast of South America, decreasing water evaporation and creating deserts in Peru and Chile. The Benguela Current, running northward along the western coast of Africa, has the same effect in Namibia.

Perhaps the best-known example of the effect of changes in ocean circulation on weather is the El Niño phenomenon. This situation occurs at irregular intervals in the eastern Pacific Ocean. Increased water temperatures, typically 2 to 8 degrees Celsius higher than normal, are associated with changes in climate. Typical effects seen during an El Niño condition are droughts in Australia, northeastern Brazil, and southern Peru; excessive summer rainfall in Ecuador and northern Peru; severe winter storms in Chile; and warm winter conditions in North America.

The El Niño effect is also associated with large reductions in fish populations along the western coast of South America. During normal conditions, the water near the coast consists of a thin layer of warm, nutrient-poor water above a thick layer of cooler, nutrient-rich water. The top layer is thin enough to allow coastal upwellings to bring nutrients to the surface, supporting marine life. During El Niño conditions, the top layer of warm water is much thicker, preventing nutrients from reaching the surface. Data from the WOCE project aimed to aid in predicting climate change, such as El Niño, and have a major impact on human activities.

Principal Terms

Coriolis effect: the phenomenon, resulting from Earth’s rotation, that causes the path of a moving object to curve away from a straight line

current: the horizontal movement of ocean water at a generally uniform depth

downwelling: the sinking of ocean water to a lower depth

Ekman layer: the region of the sea, from the surface to about 100 meters down, in which the wind directly affects water movement

ocean circulation: the worldwide movement of water in the sea

thermohaline circulation: movement of ocean water caused by differences in temperature and salt concentration

upwelling: the rising of ocean water from a depth toward the surface

wind-driven circulation: movement of ocean water caused by frictional interaction with moving air

wind stress: the frictional interaction between moving air and the surface of the ocean

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