Ocean Currents: Geography, Types, and the Global Conveyor Belt

Earth is a water planet, and the world’s oceans are always in large-scale motion, slowly rearranging their waters over large extents of latitude and longitude. Whereas tides and waves are vertical motions, ocean currents are horizontal. Currents can be directly forced by surface wind or occur more slowly and deeply because of gradients of temperature, pressure, and salinity (the saltiness of the water). Humans have long been knowledgeable about currents because they are large and readily associated with fishing conditions and navigation. More recently, science has connected changes in currents with global changes in weather and long-term changes in climate.

THE GEOGRAPHY OF CURRENTS. There is a known geography to the major currents of the world. Winds emanate outward from subtropical highs that sit over the world’s oceans. This means that air blows from known directions over large stretches of oceans for days and weeks at a time. The ocean water starts to move in response, though not as fast or in exactly the same direction because of the increased force of friction and the correspondingly lesser Coriolis effect (the effect of the Earth’s rotation).

The main geographic regularity that can be observed is the presence of oceanic gyres, immense circulations around ocean basins. These gyres have a clockwise flow of water in the Northern Hemisphere and a counterclockwise flow of water in the Southern Hemisphere. This means that there is considerable water being transported from tropical to polar latitudes and vice versa. The main subtropical gyres are joined by equatorial countercurrents in the Pacific and Indian Oceans. The strong trade winds on the equatorward side of the gyres cause the westward motion of the water and the piling up of water levels in the eastern parts of these ocean basins. The higher water in the east literally runs downhill into the linear area of light and variable Intertropical Convergence Zone winds, and it is this eastward-moving water that is the equatorial countercurrent.

When water temperatures are studied, it is apparent that east coast currents are “warm” and west coast currents are “cold” compared to the average of the ocean temperatures at each latitude. These temperature adjectives do not have absolute meaning. For instance, the Gulf Stream leaves the vicinity of Florida with surface temperatures in excess of 80° F in the summer, and the temperature decreases as it travels northward. By the time the remnants reach Ireland, a dip of the human hand would confirm that it is cold to the touch at 55° F. Yet the current is still considered “warm” because it is warm relative to the rest of the water at that latitude.

NOTABLE OCEAN CURRENTS. There are many notable ocean currents in the world. In general, ocean currents are set up wherever wind is strong and prolonged from one direction, either seasonally or yearly. The largest flow of water on the planet is the Antarctic Circumpolar Current. It girdles the globe in the region of the Southern Hemisphere’s westerly winds and does not experience significant interference from landmasses. So, too, there are many turns and splits of currents because of the particular shapes of the various ocean basins. The currents have undoubtedly changed with the drift of continents caused by plate tectonics.

The Gulf Stream is a warm current supplying water to eastern North America. It was discovered in the early 1500s and thereafter used as a boost to sailing ships returning to Europe. The water has tropical origins in the Atlantic and the Gulf of Mexico and is tightly focused as it passes through the straits between Florida and Cuba. It has an average speed of about 1.5 to 2.5 miles per hour. It has been estimated that the Gulf Stream near Newfoundland transports 196 million cubic yards per second, making it much larger than the combined flow of all the river discharges into the Atlantic Ocean. As it travels across the North Atlantic, it becomes known as the North Atlantic Drift and comes against the European continent to split north and south. The northern extension, the Norwegian Current, keeps the Barents Sea ice-free year-round, which played a pivotal role in keeping the Soviet Union port of Murmansk supplied with arms during the Second World War. Although there is scientific controversy regarding the nature of the climatic influences, there is no doubt that these waters of tropical origin play an important role in the mildness of the climate in Western Europe.

The Peru Current, also known as the Humboldt Current after the geographer Humboldt, is a huge flow of cold water off the western coast of South America, especially pronounced in the latitudes of Peru and northern Chile. Its origins are at the latitudes of southern Chile (43°S) proceeding into tropical waters, where it turns westward at about 5°S to feed the South Equatorial Current. The current can be as much as 621 miles wide and a couple of hundred meters deep, flowing at an average rate of about 0.2 miles per hour. Compared with the remainder of the Pacific Ocean at the latitudes of Peru, the Peru Current represents a swath of water cooler by 46° F. This pronounced coolness stems not only from the arrival of water from the higher latitudes but also from coastal and continental shelf configurations encouraging upwelling of deep, cold waters. The coolness helps to stabilize the lower atmosphere and is a partial cause of the extreme aridity of the Atacama Desert in the narrow coastal strip west of the Andes Mountains. More importantly, the upwelling waters transport nutrients and life upward, making them available to an unusually diverse chain of life near the surface. The result is that the Peru Current is associated with about one-fifth of the fish tonnage caught in the world’s oceans. At times, the Peru Current weakens, causing rains in the Atacama Desert and catastrophic die-offs of ocean life; this phenomenon is known as El Nino.

THERMOHALINE CIRCULATION. A thermohaline circulation connecting the ocean currents of the top waters with the currents’ deep waters has become better known in the last couple of decades. Whereas the surface ocean currents are driven by winds, deep currents are driven by differences in density. There are two ways in which the density differences are maintained: the first is by salinity differences, and the second is by temperature differences. Mainly in the North Atlantic and near the Antarctic periphery, cold surface water sinks into the deep waters. The ocean bottom geography is key as the dense flow seeks the lowest ocean bottoms. The deep currents are prevented from flowing to certain locations because of the relatively high ocean bottom, like mid-oceanic ridges, blocking the way. The rate of travel of these waters is a mere fraction of surface currents. At these tortoise-like rates, water sinking in the North Atlantic may take on the order of 1,000 years to become surface water again in the North Pacific and the Indian Ocean. The thermohaline is sometimes likened to a conveyor belt. Scientists speculate that long-term variations in the thermohaline circulation play a role in climate change.

FURTHER READING: Burns, Loree Griffin. 2007. Tracking Trash: Flotsam, Jetsam, and the Science of Ocean Motion. Scientists in the Field. Boston: Houghton Mifflin.

Colling, Angela. 2001. Ocean Circulation, 2nd ed. Burlington, VT: Elsevier Science.

Long, John A. and David S. Wells. 2009. Ocean Circulation and El Nino: New Research. New York: Nova Science Publishers.

Ulanski, Stan. 2008. The Gulf Stream: Tiny Plankton, Giant Bluefin, and the Amazing Story of the Powerful River in the Atlantic. Chapel Hill: University of North Carolina Press.