A relationship between wind stress curl and the 

 California Current system has been suggested. The data 

 indicate that the poleward undercurrent observed along 

 the west coast of North America may be driven locally by 

 positive wind stress curl. However, a possibility does ex- 

 ist that the boundary current is not entirely the result of 

 local forcing. Coastal areas may respond to wavelike dis- 

 turbances propagating from other regions of the ocean 

 (Wyrtki 1975), or to other nonlocal processes (Pedlosky 

 1974b). 



Biological Implications 



Relationships among patterns of coastal and 

 equatorial upwelling and distributions of primary 

 production have been discussed by Cushing (1969). 

 Favorable conditions for phytoplankton growth are 

 maintained within surface photic layers by upwelling of 

 nutrient-rich subsurface water. Offshore divergence 

 related to the curl of the wind stress effectively extends 

 the width of the biological upwelling zone. Fronts may 

 form in areas of negative wind stress curl just offshore of 

 the primary coastal convergence, such as near Punta 

 Eugenia. These fronts would tend to concentrate both 

 the available food and the grazers within the same areas. 

 This process may be important to survival of fish stocks 

 which spawn in this coastal region. 



Seasonal changes in surface circulation may also 

 provide mechanisms for survival and possible separation 

 of stocks along the coast. The current-countercurrent 

 system characteristic of southern California suggests a 

 mechanism whereby pelagic fish could migrate seasonal- 

 ly from primary feeding grounds within the coastal up- 

 welling regime off northern California to spawning 

 grounds off Baja California. 



A general requirement for vertically integrated north- 

 ward Sverdrup transport along the coast apparently 

 breaks down near Punta Eugenia where negative wind 

 stress curl is observed (Fig. 18A). In this region, there 

 must be southward vertically integrated Sverdrup trans- 

 port. 



With this consideration in mind, it is interesting to 

 note the winter distribution of surface currents depicted 

 in Figure 18B. The large-scale pattern suggests two 

 separate cyclonic gyres associated with positive wind 

 stress curl in the Southern California Bight, and south of 

 Punta Eugenia. In the region of negative wind stress curl, 

 the indicated surface flow is toward the south. 



The importance of coastal upwelling, offshore diver- 

 gence, and patterns of surface circulation to the early 

 survival of larvae in the California Current region has 

 been recognized (Anonymous 1952). The cor- 

 respondences of features in the patterns of wind stress 

 curl, surface currents, and winter distributions of the 

 northern anchovy, Engraulis mordax, (Vrooman and 

 Smith, 1971), shown in Figure 18C, are highly suggestive 

 of larval transport mechanisms which could lead to for- 

 mation of subpopulations of pelagic fishes in the Califor- 

 nia Current. These mechanisms have been described by 



I28W I26W I24WI22W I20W II8W II6W II4W II2W HOW 



34N- 



32N 

 30N 

 28N 

 26 N 

 24N 

 22N 







- 5 



-> 



5 



-10 



— > 



10 



-15 



— » 



15 



25 



* 



Figure 18. — Distributions of: A. wind stress curl, B. surface cur- 

 rents, and C. anchovy subpopulations. Wind stress curl is shown for 

 September. Units are dyne cm " 2 per 100 km. The contour interval is 

 0.25 dyne cm -2 per 100 km. Negative values are shaded. The winter 

 distribution of surface currents is depicted in terms of 2-degree sum- 

 marizations of ship drift data. Vector symbols are scaled according to 

 the key on the chart. Units are cm s . Large arrows suggest the split 

 cyclonic circulation which develops off southern California and Baja 

 California. The winter distributions of the three subpopulations of 

 northern anchovy are shown in the bottom figure. Figure C is after 

 Vrooman and Smith (1971). 



24 



