Wind Stress and Wind Stress Curl 

 Over the California Current 



CRAIG S.NELSON 1 



ABSTRACT 



Historical surface marine wind observations are summarized by 1-degree square areas and 

 months to describe the seasonal distribution of wind stress over the California Current. Off the coasts 

 of southern California and Baja California, an alongshore equatorward component of surface wind 

 stress is present throughout the year. The distributions of wind stress north of Cape Mendocino are 

 characterized by marked changes in direction and magnitude between summer and winter. The 

 predominant wind stress maximum shifts northward coherently from off Point Conception in March 

 to south of Cape Blanco in September, and extends approximately 500 km in the offshore direction and 

 1,000 km in the alongshore direction. Maximum values of surface wind stress occur during July near 

 Cape Mendocino. 



The wind stress curl is positive near the coast and negative in the region offshore. A line of zero 

 wind stress curl parallels the coast 200 to 300 km offshore, except off central Baja California. The pat- 

 terns of wind stress curl are consistent with the existence of an equatorward Sverdrup transport off- 

 shore and a poleward transport near the coast. 



INTRODUCTION 



Spatial variation of surface wind stress has long been 

 recognized as a fundamental quantity in discussions of 

 the wind-driven ocean circulation. Sverdrup's (1947) 

 transport balance relates the vertically integrated 

 meridional mass transport in the interior ocean to the 

 open ocean wind stress curl. Munk (1950) extended the 

 theory to a closed basin and estimated the mass trans- 

 ports for several oceanic circulations from a knowledge of 

 the wind stress alone. Recent theoretical developments 

 have attemped to explain the well-known westward in- 

 tensification, but have largely ignored the wind-driven 

 circulation in eastern boundary currents, such as the 

 California Current. 



The California Current flows southeastward along the 

 west coast of the United States as one branch of the large 

 anticyclonic gyre in the North Pacific Ocean. The 

 predominant surface current occurs between a cell of 

 high atmospheric pressure to the west and a continental 

 thermal low situated over California. Seasonal variations 

 in the direction and strength of the surface wind are 

 related to shifts in location of the high pressure system 

 and intensification of the semipermanent thermal low. 



The basic flow of the California Current system may 

 be described in terms of the local wind stress (Reid et al. 

 1958). The equatorward surface current and poleward 

 undercurrent are typical of eastern boundary currents 

 (Wooster and Reid 1963). Munk (1950) has suggested 

 that the average wind distribution off California is con- 

 sistent with the existence of an equatorward surface cur- 

 rent offshore and a poleward current inshore. However, 



Pacific Environmental Group, National Marine Fisheries Service, 

 NOAA, Monterey, CA 93940. 



lack of an adequate data base has precluded a detailed 

 analysis of the relationships among the distributions of 

 surface wind stress, the southward flowing California 

 Current, and the northward flowing countercurrent 

 which appears seasonally off the coasts of California, 

 Oregon, and Washington. 



A related phenomenon in eastern boundary currents is 

 the process of coastal upwelling. The role of upwelling in 

 bringing nutrients into the surface layers where they are 

 available for organic production is widely recognized. Ac- 

 cording to the simplified model (Sverdrup 1938), 

 equatorward wind stress parallel to the coast induces 

 flow in the surface Ekman layer of the ocean which is 

 deflected offshore by the earth's rotation (Ekman 1905). 

 When this occurs over an expanse of coast where horizon- 

 tal surface flow cannot compensate for that driven off- 

 shore, the balance is maintained by upwelling of subsur- 

 face water. Smith (1968) suggested that wind stress, 

 being the driving force in this mechanism, may be the 

 most important single variable in coastal upwelling. 



Previous estimates of the surface wind stress over the 

 oceans have been based primarily on wind data taken 

 from the U.S. Hydrographic Office Pilot Chart wind 

 roses (Hidaka 1958; Hellerman 1967). The large-scale 

 features evident in these distributions are consistent 

 with the occurrence of the major cyclonic and anticy- 

 clonic circulations in the oceans. However, the coarse 

 spatial and temporal resolution of these data (5-degree 

 quadrilateral space average and 3-mo time average) are 

 not adequate to describe the locally driven processes evi- 

 dent in the California Current system. 



Wooster and Reid (1963) used Hidaka's estimates of 

 surface wind stress to calculate the offshore Ekman 

 transport and described the locations and seasonal tim- 

 ing of the major coastal upwelling regimes in terms of 



