UC San Diego

Coastal circulation studies have focused largely on alongshore uniform sandy coastlines and some coral reef regions. Circulation off of rocky shorelines has yet to be studied. Rocky shorelines have significantly more bathymetric variability across a range of scales. Here we analyze the inner-shelf depth-averaged circulation at China Rock on the Monterey Peninsula CA with an array of 15 ADCPs deployed for a six week duration between 24~m and 3~m mean water depth, spanning 800~m in the cross-shore and 600~m in the alongshore together with coupled surface gravity wave and wind stress observations. The depth-average circulation is dominated by the subtidal, diurnal, and semidiurnal frequency bands with no preferred sense of circulation rotation. Counter to that on a sandy inner-shelf, the principal-axes ellipses decay onshore indicating the effect of strong bottom friction and there is variability in the axis orientations indicating steering by the larger-scale bathymetric variations. Analysis is focused on three regions: the offshore, midshore, and near-bay regions. In the offshore and midshore regions, a subtidal balance between alongshore wind stress and linear bottom friction has skill consistent with previous studies. The linear drag coefficient in 8--12~m water depth is much larger than that on sandy inner-shelves which is due to the large bathymetric roughness present at China Rock. In the embayment but not in regions of wave breaking, the cross-shore subtidal flow is linearly related to the onshore Stokes-drift transport, indicating it is wave driven. However, the return flow is stronger than expected for an open coast inner-shelf. The ratio of return flow magnitude to Stokes-drift transport is consistent with the variations in the embayment geometry. In the tidal bands, the cross-shore velocity decay indicate the strong effects of bottom stress, far larger than those observed across the sandy continental shelf of North Carolina or on a fringing coral reef shelf on the western coast of O'ahu, Hawai'i. The depth-averaged circulation along rocky shorelines are analogous to sandy shorelines but with much elevated bottom stress effect.