Wind stress curl forcing of the coastal ocean near Point Conception, California

Abstract

Near Point Conception, California, the atmospheric flow separates from the coast and a large wind stress curl results. Direct spatial wind field observations from 20 aircraft overflights in the spring of 1983 suggest that Ekman pumping of on average 4 m day-1 contributes to the local dynamics. During strong and persistent upwelling events the curl-driven Ekman pumping reaches up to 20 m day-1. A single complex empirical orthogonal function explains more than 72% of the spatial and temporal wind stress variance. It reveals large wind stress curl at the center of the western Santa Barbara Channel entrance. This dominant mode correlates strongly (r2 = 0.79) with the wind stress observed at a moored buoy. The location of largest Ekman pumping in the ocean indicated by this mode coincides with the location of a laterally sheared, cyclonic flow. Cold upwelled waters enter the Santa Barbara Channel along its southern perimeter while warm, and thus buoyant, waters from the Southern California Bight exit the channel along its northern perimeter. Buoy wind stress and lateral current shear at 30-m depth correlate significantly at periods of about 3.5 and 6 days with phase lags of about 0.5 and 2 days, respectively. Hydrographic observations from 1983 do not, however, indicate effects of Ekman pumping on the internal mass field as winds vary in speed and direction at daily timescales. This contrasts with 1984 observations that do indicate strong doming of isopycnals over the center of the channel at its western entrance. Winds prior to and during hydrographic observations in the spring of 1984 were both stronger and more steady than they were in 1983. Cyclonic shears reach 0.4f at the entrance of the Santa Barbara Channel near Point Conception; here f is the planetary vorticity. The thermal wind balance explains the lateral shear of the alongchannel surface flow rather well.