Current Dynamics over the Northern California Inner Shelf

Abstract

Subtidal current dynamics at a northern California inner-shelf site are analysed using moored current observations in 30 m of water, in conjunction with wind and bottom pressure measurements acquired during the summer of 1981 as part of the first Coastal Ocean Dynamics Experiment. The subtidal flow is driven locally by both an alongshelf wind stress and an alongshelf pressure gradient, which tend to be similar in magnitude but opposite in direction. Model depth-average alongshelf currents are about twice as large as observed. Analyses suggest that this discrepancy is due to a larger drag on the inner-shelf currents than suggested by bottom tripod measurements at the site, due to the presence of large rock outcrops over the inner shelf in this region. A notable characteristic of the observations is the weakness of the alongshelf flow over the inner shelf; alongshelf current standard deviations are a factor of 4 smaller than at midshelf. Analyses suggest this is due to a decrease in the wind stress toward the coast, shallower water resulting in a weaker body force due to the alongshelf pressure gradient and the increased drag on the flow over the inner shelf noted above. The moored current observations reveal a simple cross-shelf circulation pattern. The near-surface flow was typically offshore in response to equatorward winds with an onshore flow in the lower water column driven by the opposing alongshelf pressure gradient. The depth-average cross-shelf velocity was consistently zero to the accuracy of the observations, suggesting a two-dimensional circulation. A simple two-dimensional eddy viscosity model reproduced the basic features of the observed flow, including the vertical structure, orientation, and temporal variability. The model results showed persistent, substantial vertical stress divergence throughout the water column supporting the notion that the moored observations were at an inner-shelf site where the surface and bottom boundary layers merge.