less than 1°C. Details of the calculations will be reported 

 elsewhere. 



The effect of atmospheric stability was computed as a 

 percentage increase (decrease) in the magnitude of the 

 monthly mean surface wind stress above (below) the ap- 

 propriate neutral values. The following ratio was cal- 

 culated: 



R = 



FL 



-i 



x 100 



where I t | val is the magnitude of the monthly mean sur- 

 face wind stress calculated using a drag coefficient which 

 varies with stability, and | T*| neu is the magnitude of the 

 monthly mean surface wind stress computed with a con- 

 stant (neutral) coefficient. A value of R = 0.0 cor- 

 responds to neutrally stable conditions. 



The ratio defined by Equation (5) is displayed in 

 Figures 5 and 6 for the months of June and December, 

 respectively. The average percentage difference is 

 between 2 and 5%. Values greater than 10% are rare. In 

 December, the values are positive, indicating unstable 

 conditions. Negative values occur in June. These are 

 related to the stable stratification induced by positive 

 air-sea temperature differences within the coastal up- 

 welling zone. The effects of atmospheric stability deter- 

 mined in this study are less than the values of 6 to 15% 

 reported by Husby and Seckel (1975) for Ocean Weather 

 Station "Victor," but consistent with the values of less 

 than 5% reported by Dorman et al. (1974) for Ocean 

 Weather Station "November." 



WIND STRESS DISTRIBUTIONS 



The long-term composite monthly mean fields of sur- 

 face wind stress are displayed in Appendix I as resultant 

 vectors (Charts 1 to 12), east components (Charts 13 to 

 24), and north components (Charts 25 to 36). These data 

 indicate two characteristic features. The mean stress 

 tends to be directed equatorward along the coast from 

 Cape Mendocino to Baja California throughout the year. 

 Off the coasts of Oregon and Washington, the wind stress 

 exhibits marked seasonal variability in both magnitude 

 and direction. 



The distributions of surface wind stress off Baja 

 California have been previously discussed by Bakun and 

 Nelson (1975). 4 They concluded that most of the seasonal 

 variability is in magnitude rather than direction. The 

 mean surface wind stress tends to have an alongshore, 

 equatorward component during all months, implying 

 conditions generally favorable for coastal upwelling 

 throughout the year. This is contrasted with the situa- 

 tion to the north. Off the coasts of Oregon and Washing- 

 ton, patterns of surface wind stress change seasonally. 

 From December to February, the stress impinges on the 



'Bakun, A. and C. S. Nelson, 1975. Climatology of upwelling-related 

 processes off Baja California. Unpubl. manuscr., 41 p. On file at Pacific 

 Environmental Group, National Marine Fisheries Service, NOAA, Mon- 

 terey. CA 93940. 



coast at Cape Blanco, forming the southern boundary of 

 the low pressure system which develops in the Gulf of 

 Alaska during the winter. From May through Septem- 

 ber, the surface wind stress veers toward the south. The 

 components are directed onshore and equatorward. The 

 months of March-April and October-November appear 

 as transition periods. The surface wind stress is directed 

 primarily onshore off Washington and Oregon during 

 these months. 



The most evident feature in these distributions is the 

 position and extent of the predominant wind stress max- 

 imum. For purposes of discussion, this maximum is 

 defined by the 1.0 dyne cm -2 contour and is highlighted 

 by light shading in Charts 1 to 12. Large values of stress 

 occurring between lat. 45°N and 50 C N during winter are 

 probably associated with either high winds or sparse 

 data, and will be ignored in this discussion. 



The maximum is first evident in March, south of Point 

 Conception. During succeeding months, the maximum 

 strengthens, increases in size, and shifts northward. In 

 April, values exceeding 1.0 dyne cm -2 cover an area from 

 Point Conception to San Francisco. Winds off Oregon 

 have veered, suggesting an alongshore component and 

 implying conditions generally favorable for coastal up- 

 welling. The pattern of surface wind stress is repeated in 

 May. The northern boundary of the central maximum 

 reaches the coast south of Cape Blanco. 



The mean wind stress reaches maximum intensity dur- 

 ing June and July off Cape Mendocino, where charac- 

 teristic values exceed 1.5 dynes cm -2 . The large-scale 

 maximum extends approximately 500 km in the offshore 

 direction and 1,000 km in the longshore direction. A more 

 intense coastal maximum (|~f* |^1.5 dynes cm - ") is 

 coherent over an area approximately one-fifth this size. 

 Maximum values of mean surface wind stress typically 

 occur 200 to 300 km off the coast, rather than adjacent to 

 the coast. This feature was previously observed by Munk 

 (1950) and Reid et al. (1958). Mean distributions for 

 August and September suggest relaxing conditions; the 

 maximum is reduced in extent and intensity. These 

 months still exhibit a degree of equatorward stress along 

 the coasts of Oregon and Washington, but in October, 

 the direction backs to the north. The poleward com- 

 ponents correspond to onshore Ekman transport and 

 subsequent downwelling at the coast. The distributions 

 for November, December, January, and February in- 

 dicate characteristic winter conditions. Mean values are 

 typically less than 0.5 dyne cm" . High winds as- 

 sociated with intense storm activity in the North Pacific 

 Ocean result in regions of mean surface wind stress ex- 

 ceeding 1.0 dyne cm" off the coasts of Oregon and 

 Washington. 



Seasonality of the surface wind stress and its as- 

 sociation with coastal upwelling are easily observed along 

 the northern section of the grid. Large changes in mean 

 direction are apparent. Maximum magnitudes occur 

 during the winter. Winds favorable for coastal upwelling 

 occur from April to September. During the remaining 

 months, the direction of the mean surface wind stress 

 corresponds to downwelling at the coast. 



