Effects of Wind Stress and Wind Stress Curl Variability on Coastal Upwelling

Abstract

Aircraft measurements during the winter 1989 Shelf Mixed Layer Experiment (SMILE) and summer 1982 Coastal Ocean Dynamics Experiment (CODE) were used to characterize the spatial variation of the low-level wind and wind stress over the northern California shelf. The curl of the wind stress was calculated from directly measured turbulent stress components. The accuracy of the computed curl was estimated to be adequate to map the spatial structure. Wintertime measurements showed a concentration of large positive curl [over 1 Pa (100 km)?1] west of Point Arena, regardless of wind direction, due to the effects of the coastal topography on the wind fields. Results from summertime measurements showed a similar local maximum of positive curl west of Point Arena. Larger curl values [over 3.5 Pa (100 km)?1], however, were observed across a hydraulic jump propagating from Stewarts Point for highly supercritical marine boundary-layer flow. A two-layer, vertically integrated numerical model of coastal upwelling was used to assess the relative importance of the stress curl to the stress-driven transport. The nonzero stress curl altered the thickness of the upper layer considerably after a day of integration, expanding the horizontal extent of upwelling offshore. The greatest effects were around areas of high positive curl, increasing coastal upwelling for downcoast winds and decreasing downwelling for upcoast winds. The effect of the stress curl, however, was attenuated near the coast as compared to the maximum possible deep water values. The validity of the numerical model was verified by comparison with an analytical solution of a simplified set of one-dimensional, frictionless equations of motion.