102 



Water 



VIII-49 



■■■'^ 



m. 



iiisi ^<^:<:<^, 



FROM 



lOO 



Figure 88. Generalized patterns of theoretical surface 

 currents relative to those at a depth of 500 meters for 49 

 Scripps cruises between 1937 and 1952. Correction for 

 tidal effect has been made only for the 1949 and later 

 cruises. Cross-hatched areas indicate northward currents, 

 and plain white areas show southward currents or areas 

 where data are lacking, mostly near shore. Note the great 

 similarity of most of the maps. 



of surface currents based on the 300-decibar 

 level and that based on the 500- or even the 

 1000-decibar level, indicating that currents 

 at depth are relatively sluggish. Support 

 for the general pattern of surface currents is 

 provided by experiments with drift bottles 

 (Tibby, 1939) and in a general fashion by 

 limited measurements with the shipboard 

 geomagnetic electrokinetograph (von Arx, 

 1950). Interesting checks can be made in 

 the future through electropotential measure- 

 ments between the mainland and offshore 

 islands, as previously done for the Florida 

 Current by Wertheim (1954). 



The frequency of the current pattern and 

 its important bearing on the distribution of 

 organisms and sediments require some 

 speculation about its origin. Sverdrup and 

 Fleming (1941) supposed that the area of 

 cold water and its related eddy is a product 

 of upwelling caused by winds from the 

 northwest near Point Conception blowing 

 most strongly during the spring and early 

 summer months. They recognized a sub- 

 surface current flowing northwesterly during 

 all times of the year but flowing at the sur- 

 face only during the fall when upwelling was 

 less intense. Their studies were based on 

 cruises of 1937 through 1940, but inspection 

 of current patterns for later cruises fails to 

 show such limited areas of countercurrent 

 as apparently occurred during the springs of 

 1937 and 1940. Data from later cruises 

 show general similarities of the current pat- 

 tern throughout the entire year and a lack 

 of correspondence of average wind velocity 

 and direction to average water temperature 

 (Fig. 89), such as to cast doubt on the con- 

 cept of seasonal upwelling due to winds, as 

 a full explanation at least. 



JFMAMJ JASOND 



Oi4 



LjJ40 

 2 



o\; 



WIND > 13 KNOTS 6 M/SEC 

 FROM N, NNW, NW, WNW 



JFMAMJJASOND 



Figure 89. Comparison of annual surface temperature 

 curves of Figure 84 with average annual air temperature 

 and wind at San Nicolas Island for the period April 1945 

 through December 1945 and March 1947 through June 

 1953. Meteorological data from records of the U. S. 

 Weather Bureau. 



