Convinced that the wind and current systems 

 of the South Pacific Ocean would carry a 

 raft from the west coast of South America 

 to the Polynesian Islands, Thar Heyerdahl 

 and five companions set out in 1947 in the 

 raft Kon-Tiki. They made a voyage of 4300 

 miles in 101 days, and proved that it was 

 possible for South Americans of prehistoric 

 times to have populated the Pacific islands. 



to the water from the air drags the water along as a current. Despite 

 the tremendous power of storms and large waves, the forces acting 

 on a particular stretch of water and creating a current are quite 

 small, perhaps only one millionth of the weight of that particular 

 piece of water. 



When something is moving under the action of such weak forces, 

 it turns out that an effect due to the Earth's rotation has to be taken 

 into account. In our ordinary everyday experience — walking about 

 or driving in a car — we can ignore the fact that all our activities 

 take place on a rotating body. But with the sea it is different. 

 Because the forces that create a current are so small, the effect of 

 the Earth's rotation cannot be ignored. This effect is known as the 

 "Coriolis force" and varies with latitude, being zero at the Equator 

 and maximum at the poles. At first sight its effect is rather sur- 

 prising : a surface current set up by the wind does not flow directly 

 downwind, but is deflected to the right in the Northern Hemisphere, 

 and to the left in the Southern Hemisphere. 



The winds, as well as producing currents by their drag on the 

 surface of the sea, have another effect on the water. They set up 

 slopes in the sea surface by piling up water against some coasts and 

 in places where converging currents are driven by different wind 

 systems. Then other currents flow in consequence of the pressure 

 gradients in the water which arise from these slopes of the sea 

 surface. When a whole ocean basin such as the North Atlantic is 

 considered — taking into account the regional variations of the 

 stress of the wind, the Coriolis force varying with latitude, and the 

 presence of continental boundaries — it is possible to calculate a 

 pattern of currents that is a fair approximation to the observed 

 pattern. In particular, it can be shown that the currents should be 

 stronger and narrower on the western sides of ocean basins, and as 

 it turns out, this is just what observations reveal. For instance, the 

 three strongest of the major ocean currents are all on the western 

 sides of their oceans — the Gulf Stream on the western side of the 

 North Atlantic; theKuro Shio on the western side of the northern 

 Pacific Ocean; and the Agulhas Current on the western side of the 

 Indian Ocean. 



One of the most striking features of the surface current chart is 

 the equatorial circulation of the oceans. In each of the three oceans 

 extending across the Equator - the Atlantic, Pacific, and Indian 

 oceans — two westward-flowing currents are found, and sandwiched 

 between them is an eastward-flowing countercurrent. It is natural 

 to suppose that the North and South Equatorial currents, as they 

 are called, are drift currents driven by the Trade winds; and to 

 regard the middle countercurrent as a compensating flow resulting 

 from the piling up of water at the western boundaries of these three 

 oceans. However, we now know that in the case of the Pacific 

 Ocean, at least, this simple explanation seems inadequate. The 

 observed currents can be accounted for better by a careful consider- 

 ation of the varying wind stress and the Coriolis effect. 



The equatorial currents are not symmetrically arranged about 

 the geographical Equator, but follow the wind systems. In the 

 Indian Ocean, for instance, the equatorial currents are complicated 

 by the seasonal changes of the monsoon. When the southwest 

 monsoon is blowing from August to October, the westward- 

 flowing North Equatorial Current disappears and is replaced by 



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