A Hindcast of Barotropic Response over the Oregon-Washington Continental Shelf during the Summer of 1972

Abstract

Hindeasts of bottom currents on the southern Washington continental shelf are made on the basis of a long-wave approximation to the vorticity equation for a barotropic flow. Resistance of the shelf bottom to the flow is incorporated with the specification of a bottom stress that is proportional to the bottom geostrophic velocity. Observed longshore bottom currents on the Oregon continental shelf off Newport and winds at a Newport jetty from 21 July to 26 August 1972 enter as forcing functions. Comparison of the hindcast results with the observation of two current meter mooring sites off the southern Washington coast shows that more than half of the observed along-isobath variance in bottom velocity components parallel to local depth contours becomes accountable when a bottom resistance coefficient on the order of 0. 10 g cm?2 s?1 is used. The accountability of similar variance in cross-isobath components is nearly as high but suffers from relatively large absolute errors in the computed flow field. Coherence analyses also show that the hindcast generates along-isobath velocity time series whose contents at low frequencies (?0.14 cpd) are nearly statistically identical to those observed. An examination of hindcast results with and without the Newport jetty wind indicates that the wind-forced low-frequency response contained in alongshore currents is in phase with the wind. This result has been attributed to the fact that a significant amount of the onshore transport that compensates for the surface wind drift at the coast is carried by the bottom boundary layer.