III. CURRENTS 



10. EQUATORIAL CURRENT SYSTEMS 



J. A. Knauss 



1. Methods of Study of Ocean Currents 



A descriptive knowledge of ocean currents has been gained in four general 

 ways, all of which have made substantial contributions to the description of 

 the equatorial circulation. 



A. Ship Drift 



The oldest method is the use of ship-drift observations (this method gives 

 surface currents only, while the others describe sub -surface as well as surface 

 currents). If a ship finds herself off course after making due allowance for the 

 wind, the most likely explanation is that she has been moved by a surface 

 current. A picture of the surface circulation can be made by a careful study of 

 ships' logs. Accuracy is limited by any errors in allowance for the wind and 

 errors in navigation. 



The equatorial currents generally run from east to west, and the detection of 

 such drifts requires a knowledge of longitude. Therefore, discovery of the 

 detailed equatorial circulation depended upon the invention of the chronometer, 

 which was first used by Captain Cook on his voyages of exploration, 1768-1779. 

 The first mention of the Pacific Equatorial Countercurrent was by Captain 

 Freycinet in 1817. By the middle 1850's both the French hydrographer de 

 Kerhallet (1869) and the English geographer Findlay (1853) had published 

 charts of the currents of the different world oceans which were qualitatively 

 correct in most respects. The current charts of the American hydrographer 

 Maury (1859), published during the same period, were not as good as the 

 others. However, it was Maury who organized the systematic collection of ship- 

 drift data which made possible many of the future improvements in these 

 current charts. One of the most notable improvements was that of Puis (1895), 

 who presented charts of surface currents and temperature by months for the 

 entire equatorial Pacific. Few refinements have been made on these charts of 

 Puis. 



B. Distribution of Properties 

 A second method has been to infer the circulation from the distribution of 

 properties. Water-mass analysis, the "Kernschichtmethode" of Wiist (1935) 

 and isentropic analysis of Montgomery (1938) are three examples. These 

 methods are similar in that the circulation is inferred from the gradient of 

 certain properties such as temperature and salinity on possible paths along 



[MS received January, 1961] 235 



