ocean circulation

(oceanography) Water current flow in a closed circular pattern within an ocean. Large-scale horizontal water motion within an ocean.

The general circulation of the ocean. The term is usually understood to include large-scale, nearly steady features, such as the Gulf Stream, as well as current systems that change seasonally but are persistent from one year to the next, such as the Davidson Current, off the northwestern United States coast and the equatorial currents in the Indian Ocean. A great number of energetic motions have periods of a month or two and horizontal scales of a few hundred kilometers—a very low-frequency turbulence, collectively called eddies. Energetic motions are also concentrated near the local inertial period (24 h, at 30° latitude) and at the periods associated with tides (primarily diurnal and semidiurnal).

The greatest single driving force for currents, as for waves, is the wind. Furthermore, the ocean absorbs heat at low latitudes and loses it at high latitudes. The resultant effect on the density distribution is coupled into the large-scale wind-driven circulation. Some subsurface flows are caused by the sinking of surface waters made dense by cooling or high evaporation. See also Ocean waves.

Except in western boundary currents, and in the Antarctic Circumpolar Current, the system of strong surface currents is restricted mainly to the upper 330–660 ft (100–200 m) of the sea. The mid-latitude anticyclonic gyres, however, are coherent in the mean well below 3300 ft (1000 m). The average speeds of the open-ocean surface currents remain mostly below 0.4 knot (20 cm/s). Exceptions to this are found in the western boundary currents, such as the Gulf Stream, and in the Equatorial Currents of the three oceans, all of which have velocities of 2–4 knots (1–2 m/s).

The deep circulation results in part from the wind stress and in part from the internal pressure forces which are maintained by the budgets of heat, salt, and water. Both groups of forces are dependent upon atmospheric influences. Apart from Coriolis and frictional forces, the topography of the sea bottom exercises a decisive influence on the course of deep circulation.

The deep circulation in marginal seas depends largely on the climate of the region, whether arid or humid. Under the influence of an arid climate, evaporation is greater than precipitation. The marginal sea is therefore filled with relatively salty water of a high density. Its surface lies at a lower level than that of the neighboring ocean. Examples of this type are the Mediterranean Sea, Red Sea, and Persian Gulf. The deep circulation of marginal seas in humid climates shows a different pattern. The level of the sea is higher than in the neighboring ocean. Therefore, the surface water with its lower density and accordingly its lower salinity flows outward, and the relatively salty ocean water of higher density flows over the sill into the marginal sea. Examples of this circulation are the Baltic Sea with the shallow Darsser and Drogden rises, the Norwegian and Greenland fiords, and the Black Sea with its entrance through the Bosporus. See also Black Sea; Fiord; Mediterranean Sea.

The deep circulation in the oceans is more difficult to perceive than the circulation in the marginal seas. In addition to the internal pressure forces, determined by the distribution of density and the piling up of water by the wind, there are also the influences of Coriolis forces and large-scale turbulence. There are areas in tropical latitudes in which the surface water, as a result of strong evaporation, has a relatively high density. In thermohaline convection, the water sinks while flowing horizontally until it reaches a density corresponding to its own, and then spreads out horizontally. In this way the colder and deeper levels of the oceans take on a layered structure consisting of the so-called bottom water, deep water, and intermediate water. See also Atlantic Ocean; Pacific Ocean.

Wherever oceanographers have made long-term current and temperature measurements, they have found energetic fluctuations with periods of several weeks to several months. These low-frequency fluctuations (compared to tides) are caused by oceanic mesoscale eddies which are in many respects analogous to the atmospheric mesoscale pressure systems that form weather. Like the weather, mesoscale eddies often dominate the instantaneous current, and are thought to be an integral part of the ocean's general circulation.

Eddies occur in virtually all oceans and seas, but their amplitude varies greatly from place to place. The largest amplitudes are found on the western sides of the oceans in conjunction with the strongest ocean currents (the Gulf Stream in the North Atlantic, the Kuroshio in the North Pacific) and near the Equator. Much weaker eddies are found in the ocean interior, distant from major currents. This consistent pattern of eddy amplitude suggests that instabilities of western boundary currents are an important source of eddy energy. Atmospheric forcing by variable winds can also generate eddies, and is probably most important at low latitudes where the horizontal scales of the oceanic eddies best match the scales of the atmospheric forcing.