resulting latitudinal temperature distribution is shown to vary from 

 about 28°C at the equator to about -1.7°C at the poles. Much greater 

 differences in temperature between the tropics and poles would be 

 experienced, if it were not for the relatively free circulation of 

 atmosphere and oceans, which compose the working substance of a gigantic 

 heat exchanger. Large-scale heat transport from warmer to colder 

 regions is due to atmospheric winds and ocean currents. 



Air, which acts as a major mechanism for redistributing heat, 

 is driven by unequal heating of the Earth's surface to rise at the 

 equator and sink at the poles. However, air cannot circulate uniformly 

 in a single convection cell, since Earth's rotation, through the 

 deflecting force of the Coriolis effect, prevents a direct north-south 

 path for the air. Earth's rotation and the distribution of land and 

 water cause a three-celled vertical circulation system from the equator 

 to a pole (fig. 5-2). 



In the oceanic environment, moving air produces waves and 

 drives the major surface currents. Once surface waters are set in 

 motion, they create distributions of sea-surface temperatures which 

 in turn alter the winds that drive the currents. There is, however, 

 a mean steady-state condition. 



A wind-driven current does not flow in exactly the same direction 

 as the wind, but is deflected by Earth's rotation (fig. 5-3). This 

 deflecting force, the Coriolis effect, is greater at higher latitudes 

 and is to the right of wind direction in the Northern Hemisphere and to 

 the left in the Southern Hemisphere. Between 10°N and 10°S, the 

 current usually sets downwind. In general, angular direction between 

 wind and the surface current varies from about 10° in shallow coastal 

 areas to as much as 45° in some areas of the open ocean. The angle 

 increases with the depth of current, and at certain depths the current 

 may flow in the opposite direction to that at the surface. 



Ocean current patterns, consisting of huge gyres, are related 

 to wind systems of the world (fig. 5-4). Water motion at the surface, 

 modified by Coriolis effect, responds to: (1) density gradients 

 caused by latitudinal heating effects, and (2) surface winds, which 

 themselves are modified by the Earth's rotation and the distribution 

 of land and water masses. Position of the gyres shifts only slightly 

 with season, except in the northern part of the Indian Ocean and along 

 the China coast, where monsoons cause the currents to flow in opposite 

 directions in winter and summer. 



Ocean currents may attain speeds of 5 or more knots as reached 

 by the Florida current. In middle latitudes, the strongest surface 

 currents rarely reach speeds above 2 knots. At depth, currents are 

 generally slow, usually less than 0.5 knot. However, there are sub- 

 merged high-velocity streams, such as the Cromwell Current in the 

 Pacific Ocean. 



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