120 THE OCEAN 



north-westerly winds of the southern hemis- 

 phere. 



Subsidiary causes powerfully influencing 

 oceanic circulation are : — (1) abnormally heavy 

 rainfall such as occurs in the West Pacific, 

 (2) undercurrents of high temperature and 

 density from the Mediterranean and Red 

 Sea, (3) the causes leading to the extensive 

 upwelling seen in the eastern Pacific and in 

 analogous positions in the Atlantic and 

 Indian Oceans, which are closely connected 

 with the supply of a portion of the water of 

 the great surface currents from the deeper 

 waters of these oceans, and (4) the intertropical 

 position of the line of lowest mean barometric 

 pressure, resulting in a temperature much 

 higher in the North Atlantic than in the South 

 Atlantic and much higher in the South-west 

 Pacific than in the North Pacific. 



The winds are in turn dependent upon 

 differences of atmospheric pressure, blowing 

 from areas of high pressure towards areas of 

 low pressure, and it seems desirable here to 

 direct attention to this aspect of the subject. 

 Over the equator the air, which contains much 

 water-vapour, is heated, therefore expands 

 and ascends, so that a belt of permanently 

 low pressure occurs over this area all the year 

 round. In the temperate regions both 

 north and south of the tropics there are large 



