572 Basic Principles of the General Oceanic Circulation 



coast as far as 5° S. is thus a one-sided convergence line and as a consequence up- 

 welling occurs along its entire length. The other extreme of seasonal variation is at 

 the end of the Southern Hemisphere summer (Chart b; Feb.-Mar.). Conditions in the 

 equatorial region at this period are very complex and unstable and are subject to the 

 influence the more or less pronounced development of the Equatorial Counter 

 Current and the North Equatorial Current. The Humboldt Current is now weaker 

 and about 4° C warmer at the coast. The unstable character of the current is due to 

 simultaneous instability in meteorological conditions in the entire area between the 

 Cocos Islands, the Galapagos and the coast of Ecuador and Peru. In many years the 

 thermal equator and the associated zone of minimum atmospheric pressure are dis- 

 placed into the Southern Hemisphere, so that the south-eastern trades along the 

 Peruvian coast are then disturbed and rainy north and north-west winds occur in 

 northern Peru. These disturbances of atmospheric and oceanic conditions are, how- 

 ever, usually not too powerful, but in general conditions are so unstable in northern 

 Peru that abnormal developments frequently occur. The warm weakly saline water of 

 the Equatorial County Current can then easily advance into the area of the Humboldt 

 Current. This warm water is then carried southward by the northern and north- 

 western winds (most often at Christmas time). This current in contrast to the Peru 

 Current is regarded as a "counter current"; it is called "El Nino". Normally the 

 changes are not very great but occasionally when the disturbances are particularly 

 well developed there may be torrential rains followed by flood catastrophes in coastal 

 areas of northern Peru which are adapted to a dry climate. The simultaneous change 

 in the character of the water masses off" the coast in addition has disastrous conse- 

 quences for the guano birds which are suddenly deprived of food. Detailed descriptions 

 have been given for years when these disturbances have been particularly well marked, 

 for 1925 by Zorell (1928); Murphy (1926) and Schott and for 1891 by Schott 

 (1931). 



The wide area of the Pacific covered by the essentially eastwards flowing West 

 Wind Drift extends south of the subtropical convergence which is more a "con- 

 vergence region" than a line. The available data on this current, especially in the 

 thirties and forties, is rather uncertain. Near 40° S., off" the South American coast there 

 exists a zone of remarkably low salinity (34%o) apparently originating from western 

 Patagonia (Schott, 1934). Corresponding to this distribution the West Wind Drift 

 must swing sharply north to north-westward, that is, to the left. The Antarctic Con- 

 vergence runs through the West Wind Drift at about 55° S. It was encountered in 

 every profile recorded by the "Discovery" Expedition and is the only convergence 

 line circling the entire earth in the Antarctic region. 



3. Currents Caused by Excess of Precipitation and Run-off Over Evaporation 



The possibility of the direct formation of ocean currents due to the flow of excess 

 water from the precipitation areas and those with run-off" from rivers into evaporation 

 regions, was first investigated in detail by Ekman (1926) using his classical theory of 

 deep and bottom currents. For a circular oceanic region he obtained after considerable 

 simplifications a final equation of the form 



277 



curl K -^{P- E), (XVIII.l) 



