The major currents of the oceans are caused by the combined effects of wind action and barometric pressures on the surface, and density differences between different parts of the sea. The density differences exist mainly because of inequalities of heat exchange between atmosphere and water at various parts of the sea surface, and also because of differences of evaporation and dilution. The course taken by currents is influenced by the rotation of the earth and by the shape of the continents and ocean floor. To-and-fro and rotatory oscillations generated by tidal forces are superimposed on the movements resulting from interactions of atmosphere and ocean (see Section 8.1). The flow of ocean currents is consequently meandering rather than steady, complicated by innumerable ever-changing eddies, comparable in some respects to the movements of the atmosphere but generally proceeding much more slowly (Perry and Walker, 1979; McWilliams, 1977).
Mostly, the ocean currents move slowly and irregularly. In the Equatorial currents the surface water usually flows at some 8-14 km/day. The North Atlantic Drift transports water from the region of Nova Scotia towards the British Isles at an average speed of approximately 19 km/day. Parts of the Gulf Stream move exceptionally rapidly, speeds of up to 180 km/day having been recorded. Less is known about flow rates below the surface. Some measurements at deep levels indicate speeds of 2-10 km/day, sometimes much faster.
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