Currents 



103 



An alternate concept is that of entrain- 

 ment of water by the CaUfornia Current. 

 Studies of the Gulf Stream in the Atlantic 

 Ocean show that along its course it en- 

 trains vast quantities of water from the open- 

 sea side (Sverdrup, Johnson, and Fleming, 

 1942, pp. 676-677); presumably the Cali- 

 fornia Current is capable of similar entrain- 

 ment. During its passage southeasterly 

 along the northern California coast the Cali- 

 fornia Current is unable to entrain water 

 on its left because little is available. How- 

 ever, when the California Current passes 

 Point Conception it encounters a large body 

 of water on its left. Because the Santa 

 Rosa-Cortes Ridge rises to a depth of less 

 than 200 meters along most of its length, 

 only water shallower than its crest can be 

 entrained. When this water is carried away, 

 it is replaced partly by surface water from 

 farther east and partly by colder subsurface 

 water. The bringing up of water from depths 

 greater than the crest of the Santa Rosa- 

 Cortes Ridge should produce much the same 

 changes in the surface water as ordinary up- 

 welling induced by wind, but without re- 

 quiring the presence of high wind velocities. 

 South of Cortes Bank the ridge is continued 

 as a series of widely separated topographic 

 high areas between which entrainment can 

 occur at all depths so that there is no neces- 

 sity for surface water to be replaced. Be- 

 cause mixing of cold water with the surface 

 of the California Current occurs only north 

 of Cortes Bank, it must depress the dynamic 

 topography only in that area and cause some 

 of the water at the east side of the Cah- 

 fornia Current to become deflected toward 

 the coast and around the south end of the 

 area of colder water near Cortes Bank. On 

 passage of a temperature-stratified mass of 

 water across a ridge, even a deep one, the 

 current and its isotherms are deflected to 

 the right in the Northern Hemisphere 

 (Sverdrup, Johnson, and Fleming, 1942, pp. 

 466, 672). Such a deflection probably adds 

 to the area of cold water around which the 

 countercurrent must flow until the configu- 

 ration of the coast causes it to turn south- 

 ward again and flow out of the area. Until 

 a more rigorous study is made, it is impos- 

 sible to determine the. relative roles of wind 



and entrainment in producing upwelling 

 along the ridge; however, both processes 

 may act more or less in unison to produce 

 the characteristic surface current pattern off" 

 southern California. 



Intermediate Depths 



At intermediate depths off southern CaU- 

 fornia there occurs water distinctly diff'erent 

 from that at the surface. Sverdrup and 

 Fleming (1941) showed that the water at 

 depth has a southern origin in contrast to 

 the northern origin of most of the water 

 near the surface. The bottom of the inter- 

 mediate water, or Southern Water, is taken 

 as the depth of basin sills, a depth which 

 varies between about 500 and 1900 meters 

 in the continental borderland. Nearer the 

 surface is a broad zone of mixing of the two 

 water types. Pure examples of the two types 

 have quite diff'erent salinities for given temp- 

 eratures, the Southern Water being the more 

 saline. A plot of temperature against salinity 

 (a T-S diagram — Fig. 90) clearly defines 

 each of the two types. The proportions of 

 the two types present in a water of mixed 

 origin can be determined by the position of 

 its temperature-salinity curve with respect 

 to those of the pure Southern and Northern 

 Water types. At most stations the waters 

 are mostly of northern origin at the surface 

 and of southern origin at depth. Inspection 

 of plots of such mixed waters in a T-S dia- 

 gram readily indicates the depth at which 

 the mixed water is 50 per cent Southern and 

 50 per cent Northern Water. The depth 

 over the continental borderland at which 

 this particular mixture occurs is variable 

 but is mostly between 200 and 300 meters, 

 locally as deep as 500 meters; the mixture 

 is commonly absent or occurs deeper than 

 500 meters beyond the continental slope 

 (Fig. 91). A reasonable average depth 

 throughout the continental borderland of 

 Chart I is 300 meters; for this reason the 

 surface currents of Figure 87 discussed in the 

 preceding section were based on the dynamic 

 topography of the sea surface relative to the 

 300-decibar level. 



The chief depths of mixing of the two 

 water types are those immediately above 



