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translatory movements of masses of water in the seas and oceans. On the surface of the ocean they spread out into a broad band and encompass a layer of water of a certain depth. At great depths and on the bottom there are significantly slower movements of water particles in a certain general direction, usually opposite that of the surface current, that constitute part of the general circulation of water in the world’s oceans.
Ocean currents are caused by the force of friction between the water and the air moving over the surface of the sea, by pressure gradients arising in the water, and by the tide-forming forces of the moon and sun. The force of the earth’s rotation considerably affects the direction of currents, causing them to be deflected to the right in the northern hemisphere and to the left in the southern hemisphere.
Ocean currents are distinguished by origin, location, physical characteristics, and stability. Wind currents, or drift currents, are the main form of movement of the surface layer of water in oceans and seas and frequently lead to the development of gradient currents. Convection currents, or density currents, are a variety of gradient currents. Runoff and compensation currents are also distinguished. Surface, subsurface, intermediate, deep, and bottom currents are distinguished according to location.
According to their physicochemical characteristics, a distinction is made between warm and cold currents and between salt-water and freshwater currents. The subdivision according to physical (thermal) characteristics is to some degree arbitrary. If the water temperature of a current is higher than the temperature of the surrounding waters, the current is called warm; if the temperature is lower, the current is called cold. Warm currents move from low latitudes to high latitudes, and cold currents move in the opposite direction. In terms of variability, ocean currents may be permanent (stable), seasonal, and periodic (of tidal origin). Permanent and seasonal currents may be of the drift, runoff, density, or pressure-gradient types. Examples of permanent currents are the northern and southern equatorial currents and the Gulf Stream; the monsoon currents of the northern part of the Indian Ocean, which change direction with the summer and winter monsoons, are examples of seasonal currents. Currents entirely caused by any one of the factors mentioned above are rarely observed in the oceans; the most clearly marked currents in the world’s oceans are caused by a combination of several factors. For example, the Gulf Stream is a density, wind, and runoff current.
Modern research on ocean currents is aimed at direct measurement at various depths using automatic recording instruments, neutral-buoyancy floats, and parachute floats. Researchers from a number of countries, among them the USSR, the USA, and Japan, have carried out many long-term observations of currents in the world’s oceans, including observation of deep currents using moored buoy stations. The observations have made it possible to estimate the variability of currents over time and to determine the basic principles of such variation. Direct instrument measurements of currents have led to the discovery of powerful, previously unknown currents in the ocean (the Cromwell Current in the Pacific Ocean, the Lomonosov Current in the Atlantic, and the deep countercurrents under the Gulf Stream and the Japan Current, or Kuroshio). Indirect methods of analysis (by distribution of temperature, salinity, and density) and numerical methods, which make it possible to calculate the direction and velocity of currents on the basis of the wind field or by distribution of water density at the surface and at the depths of the ocean, play a large role in the study of currents.
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A. M. MUROMTSEV