NCC - TOr-ll " ONE DEGrttE SUMMARIZATION 



NORA 



- NATIONAL MARINE FISHER! 



ES SERVICE 





PACIFIC ENVIRONMENTAL G, 



iOUP 







MONTEREY. CALIFORNIA 







LONGSHORE SURFACE 



STRESS 





( DYNE CM-2 ) 







LONGSHORE 



TIMESERIES GRID 



SECTION I 1 ) 





LONG TERM MEAN 









1B57 - 1972 







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



JAN FEB MAR APR MAY JUN JUl AUG SEP OCT NOV DEC 



April and May off the coast of Baja California to June, 

 July, and August off Cape Mendocino and Cape Blanco. 

 Figure 14 indicates a tendency for the surface wind 

 stress to be directed onshore throughout the year north of 

 lat. 30°N. Off the coast of Baja California, surface stress 

 is characterized by offshore components, except between 

 the months of April and October. Offshore components 

 are also apparent in the vicinity of Cape Mendocino and 

 Point Conception. Near these points, abrupt changes in 

 coastline orientation may influence the direction and 

 magnitude of the surface wind stress. 



WIND STRESS CURL 



The surface wind stress curl is the forcing function for 

 the vertically integrated mass transport of the wind- 

 driven ocean circulation. Under linear, steady-state con- 

 ditions on the "/3-plane," in the absence of friction and 

 interactions with bottom topography, the meridional 

 component of mass transport (M y ) is directly propor- 

 tional to the vertical component of the curl of the wind 

 stress as expressed in Equation (6): 



M, = 



k ■ ( Vx T 







(6) 



where M y is the meridional component of the vertically 

 integrated mass transport, (3 is the meridional derivative 

 of the Coriolis parameter /, and k • ( V X~F) is the vertical 

 component of the wind stress curl. According to the sim- 

 plified model, positive (negative) wind stress curl is 

 associated with northward (southward) meridional 

 transport. Surface Ekman divergence (convergence) cor- 

 responding to positive (negative) wind stress curl is 

 balanced by geostrophic convergence (divergence) in the 

 northward (southward) meridional flow. 



Coastal upwelling occurs only at the ocean boundary. 

 However, wind induced upwelling will occur whenever 

 divergence in the surface wind drift is not balanced by 

 other modes of horizontal surface flow. Figure 15 shows a 

 mechanism by which the wind stress curl determines the 

 divergent or convergent nature of the surface wind drift 

 offshore of the primary coastal upwelling zone. A 

 seaward increase in the equatorward wind stress parallel 

 to the coast (Fig. 15A) is characterized by positive wind 

 stress curl. In this situation, the offshore component of 

 Ekman transport increases in the offshore direction, 

 resulting in continued surface divergence. Upwelling is 

 required to maintain the mass balance. If the equator- 

 ward longshore surface wind stress decreases in the off- 

 shore direction (Fig. 15B), the wind stress curl is 

 negative. Convergence in the surface wind drift will 

 result. Frontal formation and downwelling may occur 

 just offshore of the primary coastal upwelling zone. 



Figure 13. — Seasonal cycle of alongshore surface wind stress near 

 the coast. Means of the alongshore components of wind stress were 

 computed by month for the 1 -degree squares immediately adjacent to 

 the coast. Units are dyne cm. The contour interval is 0.5 dyne cm" . 

 Equatorward alongshore stress is shaded. Numbers along ordinate 

 are degrees north latitude. 



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