On the Effect of Bottom Friction on Barotropic Motion Over the Continental Shelf

Observations of the velocity fields over the continental shelf and slope off Oregon and off Peru have shown that there is a phase difference in the onshore-offshore direction, with the velocity fluctuations nearshore leading those offshore in time. It is shown here that the effects of bottom Ekman layer friction and cross-shelf depth variation combine to result in such a phase lag in a model for forced or free long barotropic continental shelf waves. The model also shows that bottom friction results in a smaller phase lag between the alongshore components of velocity and wind stress than that predicted by a frictionless model, a feature found in the observations off Oregon.