Ocean circulation refers to the constant motion of oceanic waters. Winds, the tide-causing gravitational pull of the moon and the sun, and differences in seawater density all affect ocean circulation. Ocean circulation involves both surface currents and movements of deep-water masses.
Surface waters are set in motion by wind. Some of these currents are short-lived and affect only small areas. Such water movements are responses to seasonal influences. Other surface currents are relatively permanent phenomena that extend over large portions of the oceans, such as the Gulf Stream. These major movements are driven by the unequal heating of Earth by the sun.
Unlike the horizontal movements of the surface currents, the deep-ocean circulation has a vertical component that accounts for the thorough mixing of of deep-water masses. These movements are a response to the density differences among water masses and reflect contrasts in temperature, salinity, or both. Currents driven by density contrasts, as determined by differences in temperature and/or salinity, are described as thermohaline circulation.
Where the atmosphere and ocean are in contact, energy is passed from moving air to the water through friction. The drag exerted by the winds blowing steadily across the ocean causes the surface layer of water to move. Winds are the primary driving force of surface currents. Thus there is a relationship between the ocean circulation and the general atmospheric circulation.
Other factors also influence the movement of ocean waters, the most significant of which is the Coriolis effect. Because of Earth's rotation, currents are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. As a consequence, the direction of the surface currents does not coincide precisely with wind direction.
North and south of the equator are two westward-moving currents, known as the equatorial currents. They derive their energy principally from the trade winds that blow from the northeast and southeast, respectively, toward the equator. Because of the Coriolis effect, these currents are deflected poleward to form clockwise gyres in the Northern Hemisphere and counterclockwise gyres in the Southern Hemisphere. These nearly circular patterns are found in each major ocean basin.
Winds can also cause vertical water movements. Upwelling, the rising of cold water from deeper layers to replace warmer surface water, is a common wind-induced vertical movement of water. Coastal upwelling occurs when winds blow toward the equator and parallel to the coast. Owing to the Coriolis effect, the surface water movement is deflected from the shore. As the surface layer moves away from the coast, it is replaced by water that "upwells" from below the surface. This slow upward flow from depths of 50 to 300 meters brings water that is cooler than the original surface water and creates a characteristic zone of lower temperatures near the shore.
Coastal upwelling also brings to the ocean surface greater concentrations of dissolved nutrients, such as nitrates and phosphates. These nutrient-enriched waters from below promote the growth of plankton, which in turn supports extensive populations of fish.