Wind-driven inner-shelf circulation off central Oregon during summer

Velocity measurements from 17 deployments of moored acoustic Doppler current profilers obtained during four summer upwelling seasons are used to describe the cross-shelf divergence of Ekman transport in the inner shelf off Oregon. For each deployment the measured surface and bottom cross-shelf transports were compared with estimates of the theoretical Ekman transports to find the fraction of full theoretical Ekman transport present. In general, in 15 m of water at 1-2 km offshore, measured transport was 25% of the full Ekman transport. Measured transports reached full Ekman transport 5-6 km offshore in 50 m of water. This result indicates that the region of active upwelling marked by the divergence of Ekman transport was limited to a narrow region along the coast. With small wind stress curl and no major headlands in the region, no along-shelf trends in the transport fractions were observed. Average transport fractions at each station were similar from year to year with one exception. The interannual variability seen at this particular site was most likely a result of local along-shelf bathymetric features. In addition, a weak linear relationship was found between the ambient stratification and the fraction of full Ekman transport. Reduced cross-shelf transport occurred at times of decreased stratification. This type of "shutdown'' of the inner-shelf cross-shelf circulation has significant biological implications, sequestering production in the nearshore and reducing larval cross-shelf transport.