In view of the fact that the prevaihng winds are almost invariably 

 easterlies with a weak southerly component (U.S.W.B., 1938) it may be 

 necessary to seek an explanation of the advance and retreat of the dry 

 tongue in the conditions of the water surface at different times of the year. 



In winter the southern Pacific is under the control of two great anti- 

 cyclones, one centred about 100° W. and 25° S., the other one about 

 180° E. and 25° S. (Map 7b). The eastern anticyclone is generally recog- 

 nized (Haurwitz and Austin, 1944), the western one links up with the 

 Australian winter anticyclone and loses its identity. It is the eastern 

 anticyclone that gives rise to the steady easterlies of winter between 

 latitudes 2° and 12° S. These easterlies blow over a water surface which 

 is as cool as 16° and 17° c. near the South American shore, and warms 

 up progressively until it reaches 28° c. at about 177° e. along the Equator 

 {Sverdrup and others, 1942). The air mass finds itself constantly warmer 

 than the underlying surface, and its relative humidity decreases, 

 notwithstanding the continuous intake of more moisture from the 

 ocean surface. 



In summer the same air mass travels over a water surface which 

 varies in temperature between 21° and 28° c. over the same distance. 

 The rise in temperature is only 7° c, as against 12° c. in winter. 



The moist tongue reaches from the Solomons to Samoa, but its 

 eastward extension varies with the seasons. It always reaches as far as 

 Pukapuka in the Danger Group, where summer is the wettest season, 



95 



