A region of negative wind stress curl (Ekman conver- 

 gence) reaches the coast of Baja California between Punta 

 Eugenia and Punta Baja to the north. This feature ap- 

 pears consistently throughout the year. A partial break- 

 down in this system occurs in August, October, and 

 November. However, considering the probable uncer- 

 tainties in these derived data, one might reasonably con- 

 clude that the coastal region near Punta Eugenia can be 

 characterized in the mean by convergence in the surface 

 wind drift. The distributions of wind stress curl in this 

 area imply favorable conditions for formation of fronts 

 and convergent patches of recently upwelled water. 



Coastal Time Series 



The mean annual cycle of wind stress curl near the 

 coast is shown in Figure 17. Positive wind stress curl oc- 

 curs along most of the coast throughout the year. Excep- 

 tions to this generalization are found in the region south 

 of lat. 30°N and between lat. 40°N and 47°N. The tem- 

 poral persistence of the negative wind stress curl near 

 Punta Eugenia (lat. 29°N) is clearly seen in this time 

 series. A similar feature occurs near the tip of Baja 

 California. Seasonal variation between positive and 

 negative curl is apparent from Cape Mendocino to the 

 Columbia River. 



Yoshida and Mao (1957) presented evidence indicating 

 that open-ocean upwelling is related to wind stress curl. 

 This process is distinct from coastal upwelling which is 

 primarily a boundary phenomenon. However, the two 

 mechanisms are not totally independent. In regions 

 where positive wind stress curl occurs, such as between 

 lat. 30° N and 40° N throughout the year, and near the 

 coasts of Oregon and Washington during the summer, as- 

 cending motion related to the wind-induced divergence 

 offshore enhances the upwelling associated with the more 

 dominant coastal divergence. 



DISCUSSION 



Monthly distributions of surface wind stress show an 

 offshore maximum which progressively develops over a 

 large extent of the California Current and shifts north- 

 ward and intensifies from winter to summer. This 

 process is a primary forcing function for coastal up- 

 welling. Off Baja California, wind-induced surface 

 divergence occurs on average throughout the year. Down- 

 welling occurs near the coast of Vancouver Island, except 

 during the restricted period between May and August 

 when a small equatorward component of surface wind 

 stress is observed. 



The major seasonal variations in the magnitude and 

 direction of the surface wind stress can be described 



Figure 17. — Seasonal cycle of wind stress curl near the coast. The 

 wind stress curl is shown by month for the 1 -degree squares im- 

 mediately adjacent to the coast. The calculations were based on the 

 monthly mean surface wind stress distributions. Units are dyne cm" 2 

 per 100 km. The contour interval is 0.25 dyne cm- 2 per 100 km. 

 Negative values are shaded. Numbers along ordinate are degrees 

 north latitude. 



NCC - TDF-11 - ONE DEGREE SUMMARIZATION 



NOAA - NATIONAL MARINE FISHERIES SERVICE 



PACIFIC ENVIRONMENTAL GROUP 



MONTEREY, CALIFORNIA 



WIND STRESS CURL 



JAN FEB MAR APR MAY JUN JUL BUG SEP OCT NOV DEC 



22 



