Currents
Currents
Deep water (or density) currents
Currents are steady, smooth movements of water following a specific course; they proceed either in a cyclical pattern or as a continuous stream. In the Northern Hemisphere, currents generally move in a clockwise direction, while in the Southern Hemisphere they move counterclockwise. There are three basic types of ocean currents: surface currents; currents produced by long wave movements or tides; and deep-water currents. Furthermore, turbidity currents play a role in shaping underwater topography. Measured in a variety of ways, currents are responsible for absorbing solar heat and redistributing it throughout the world.
Surface currents
Perhaps the most obvious type of current, surface currents are responsible for the major surface circulation patterns in the world’s oceans. They are the result of the friction caused by the movements of the atmosphere over water; they owe their existence to the winds that form as a result of the warming of air masses at the sea surface near the equator and in temperate areas. When wind blows across the water’s surface, it sets the water in motion. If the wind is constant and strong enough, the currents may persist and become permanent components of the ocean’s circulation pattern; if not, they may be merely temporary. Surface currents can extend to depths of about 656 feet (200 m). They circle the ocean basins on both sides of the equator in elliptical rotations.
There are several forces that affect and sustain surface currents, including the location of land masses, wind patterns, and the Coriolis effect. Located on either side of the major oceans (including the Atlantic, Indian, and Pacific), land masses affect currents because they act as barriers to their natural paths. Without land masses, there would be a uniform ocean movement from west to east at intermediate latitudes and from east to west near the equator and at the poles. The Antarctic Circumpolar Current can illustrate the west to east movement. Because no land barriers obstruct the prevailing current traveling between the southern tips of South America and Africa and the northern coast of Antarctica, the Antarctic Circumpolar Current consistently circles the globe in a west to east direction. Interestingly, this current is the world’s greatest, flowing at one point at a rate of 9.5 billion cubic feet per second.
In addition to the presence of land barriers, two other factors work together to affect the surface currents—wind patterns and the Coriolis effect. The basic wind patterns that drive the currents in both hemispheres are the trade winds and the westerly winds. The Coriolis effect is a force that displaces particles, such as water, traveling on a rotating sphere, such as Earth. Thus, currents develop as water is deflected by the turning of Earth. At the equator, the effect is nonexistent, but at greater latitudes the Coriolis effect has a stronger influence. As the trades and the west-erlies combine with the Coriolis effect, elliptical circulating currents, called gyres, are formed. There are two large subtropical gyres dominating each side of the equator. In the Northern Hemisphere, the gyre rotates in a clockwise direction; in the Southern Hemisphere, it rotates counterclockwise. At the lower latitudes of each hemisphere, there are smaller, tropical gyres which move in the opposite direction of the subtropical gyres.
A good illustration of a surface current is the Gulf Stream, also called the Gulf Current. This current is moved by the trade winds in the Atlantic Ocean near the equator flowing in a northwesterly direction. Moving along the coasts of South and North America, the Gulf Stream circles the entire Atlantic Ocean north of the equator. Currents similar to this exist in the Pacific Ocean and in the Atlantic south of the equator.
One of the major consequences of surface currents is their ability to help moderate Earth’s temperatures. As surface currents move, they absorb heat in the tropical regions and release it in colder environments. This process is referred to as a net poleward energy transfer because it moves the solar radiation from the equator to the poles. As a result, places like Alaska and Great Britain are warmer than they otherwise would be.
Tidal currents
Tidal currents are horizontal water motions associated with the sea’s changing tides. Thus, in the ocean, wave tides cause continuous currents that change direction 360 degrees every tidal cycle, which typically lasts from six to twelve hours. These tides can be very strong—reaching speeds of 6 inches (15 cm) per second and moving sediment long distances—or they can be weak and slow. Of interest to swimmers, rip currents are outward-flowing tidal currents, moving in narrow paths out to sea. The flow is swift in order to balance the consistent flow of water toward the beach brought by waves. In general, tidal currents are of minimal effect beyond the continental shelf.
Deep water (or density) currents
Deep water currents move very slowly, usually around 0.8-1.2 inches (2-3 cm) per second. They dominate approximately 90% of the oceans’ circulation. Water circulation of this type is called thermohaline circulation. Basically, these currents are caused by variations in water density, which is directly related to temperature and salt level, or salinity. Colder and saltier water is heavier than warmer, fresher water. Water gets denser in higher latitudes due to (1) the cooling of the atmosphere and (2) the increased salt levels, which result from the freezing of surface water. (Frozen water normally contains mostly freshwater, leaving higher concentrations of salt in the water that remains liquid.) Differences in water density generate slow moving currents, due to the sinking of the colder, saltier water into deeper parts of the oceans’ basins and the displacement of lighter, fresher currents.
Turbidity currents
Turbidity currents are local, rapid-moving currents that travel along the ocean floor and are responsible for shaping its landscape. These currents result from water, heavy with suspended sediment, mixing with lighter, clearer water. Causes of turbidity currents are earthquakes or when too much sediment piles up on a steep underwater slope. They can move like avalanches. Turbidity currents often obscure the visibility of the ocean floor.
Measuring currents
Oceanographers measure currents in a variety of ways using a variety of equipment, yielding results that range from crude to sophisticated. Currents can be measured directly, by clocking the water movement itself, or indirectly, by looking at some characteristic closely related to water movement. Two common direct ways to measure currents are the Lagrangian and the Eulerian methods. Lagrangian measurements monitor the movement of water by watching objects that are released into the current. These objects are monitored and recollected at a later time. Eulerian measurements look at the movement of water past a designated fixed location and usually include an anchored current meter.
Ocean currents and climate
The oceans cover over 70% of Earth’s surface. In the tropics, ocean water absorbs heat from the atmosphere. As the warmed water is carried north by surface currents, immense amounts of stored energy, in the form of heat, are transferred from one part of the world to another, contributing to weather and climate patterns. For example, the Gulf Stream carries warm water far up the eastern coast of North America, and then swings east, towards Europe. The warm water of the Gulf Stream heats the air above it, creating a warmer climate for Iceland and western Europe than would otherwise exist. Thermohaline currents also carry stored heat from the tropics to the mid-latitudes.
Oceanographers and climatologists are still exploring the important relationships between the oceans and their currents and ongoing global climate
KEY TERMS
Coriolis effect— Generically, this force affects particles traveling on a rotating sphere. As it pertains to currents, it is a deflection of water caused by the turning of Earth. At the equator, the effect is nonexistent but it gets stronger toward the poles. Water tends to swirl to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Gyre— Typically elliptical in shape, a gyre is a surface ocean current that results from a combination of factors, including: the Coriolis effect, Earth’s rotation, and surface winds.
Rip currents— Narrow areas in the ocean where water flows rapidly out to sea. The flow is swift in order to balance the consistent flow of water toward the beach brought by waves.
Thermohaline circulation— The flow of water caused by variations in water density rather than caused by the wind. In certain situations, colder water from the sea floor mixes upward with the warmer water. As it does this, it rotates faster, moving toward the two poles.
Turbidity currents— Local, rapid-moving currents that result from water heavy with suspended sediment mixing with lighter, clearer water. Causes of turbidity currents are Earthquakes or when too much sediment piles up on a steep underwater slope. They can move like avalanches.
change due to greeenhouse gases. Much of the heat resulting from global warming is being stored in the oceans, according to scientists, thus delaying part of the surface warming global climate change theorists have expected to see as the result of human-induced increases in greenhouse gases in the atmosphere. The heat being stored by ocean waters will contribute to warming trends throughout the world as the water is circulated by oceanic currents.
Resources
BOOKS
Davis, Richard A., Jr. Oceanography, An Introduction to the Marine Environment. Dubuque, IA: William C. Brown Publishers, 1991.
Goudie, Andrew, ed. The Encyclopaedic Dictionary of Physical Geography. New York: Blackwell Reference, 1985.
Hendrickson, Robert. The Ocean Almanac. Garden City, New York: Doubleday and Company, 1984.
Pinet, Paul. Invitation to Oceanography. 2nd ed. Boston: Jones & Bartlett Publishing, 1999.
Thurman, Harold V., and Alan P. Trujillo. Essentials of Oceanography. 7th ed. Englewood Cliffs, NJ: Prentice Hall, 2001.
OTHER
National Aeronautics and Space Administration. ”Ocean Currents“<http://seawifs.gsfc.nasa.gov/OCEAN_PLANET/HTML/oceanography_currents_1.html> (accessed November 16, 2006).
University Corporation for Atmospheric Research. ”Currents of the Ocean“<http://www.windows.ucar.edu/cgi-bin/tour_def/earth/Water/ocean_currents.html> (accessed November 16, 2006).