24" 



N. 



22'- 



20- 



MEXICO 



SAN BENEDICTO I 



II4°W. 



(12° 



IIO» 



I08» 



I06» 



Figure 40. — Horizontal distribution of the standing crop of zooplankton as standardized volume (milliliters per 1,000 m.-' 

 of water strained) from oblique hauls to about 300-m. depth. The contourinterval between full lines is 50 ml./l, 000 m.^; 

 the 75 ml./ 1,000 m.-' contour has been added as a dashed line. For further explanation see text. 



DISCUSSION 



The data of cruise TO-60-1 show that the 

 Cape San Lucas frontal system is horizontal 

 as well as vertical. The system is usually 

 thought of as a sharp boundary, at or near 

 Cape San Lucas, between California Current 

 Water and Gulf or Subtropical Water. Ap- 

 parently, the frontal system, in spring, is 

 comparatively vertical only close to Cape 

 San Lucas. Farther away the low-salinity 

 California Current Surface Water spreads 

 extensively, at depths between about 50 and 

 100 m., well into the high- salinity Gulf and 

 Subtropical Surface Waters. Thus the frontal 

 system becomes horizontal and has an upper 

 and a lower boundary. These boundaries, 

 especially the upper one, are about as strong 

 as the vertical one at the Cape, where hori- 

 zontal spreading is restricted. At the leading 

 edge of the intrusion of California Current 



Water the boundary is weakest, mainly be- 

 cause nnixing is occurring along roughly hori- 

 zontal density surfaces . Although the horizontal 

 density gradient is greatest at the Cape, the 

 actual density difference at a given depth 

 between the warm, saline water and the cool, 

 low-salinity water is not particularly great. 



The facts revealed by cruise TO-60-1 show, 

 nevertheless, that rapid mixing between Cali- 

 fornia Current and Gulf Water is impeded. 

 Presumably the geography of the area helps 

 to account for this impediment. The Lower 

 California peninsula separates the flow of 

 California Current Water and the outflow from 

 the Gulf. The transport off the western coast 

 of the peninsula is much greater than that 

 from the Gulf, and for most of the year the 

 prevailing winds favor the southward flow 

 of the California Current, whereas the trans- 

 port from the Gulf is more passive. The 

 smallness of the Gulf outflow explains why 



45 



