Depth-Averaged Subtidal and Tidal Circulation off of a Rocky Shore

Abstract

Inner shelf circulation studies have focused mainly on alongshore uniform sandy coasts and coral reefs in subtidal and tidal bands, with far less attention given to rocky shores. This study examines depth-averaged circulation at China Rock, a rocky shore on the Monterey Peninsula, CA, with 15 ADCPs deployed for about a month. The bathymetry varies strongly on multiple lengthscales. Large-scale bathymetric features include an embayment and two headlands, whereas smaller-scale features consist of a large variety of rocks extending from the inter-tidal zone to offshore. Circulation variability encompasses subtidal, diurnal, and semidiurnal frequency bands. Velocity principal-axes ellipses decay onshore in all frequency bands indicating strong bottom friction, and have orientation variability attributable to nearby large-scale bathymetric features. Alongshore subtidal currents are reasonably well described by a wind stress and bottom friction balance, with skill similar to previous studies, but with larger linear drag coefficients, particularly in shallower waters. Cross-shore subtidal currents near the embayment are directed offshore as a bathymetrically controlled rip current strengthened by feeder currents from the headlands, with magnitude related to the incident waves. In the diurnal and semidiurnal bands, alongshore currents are attenuated onshore and the tidal phase (relative to an offshore location) decreases onshore both due to enhanced bottom friction. The attenuation is greater than on a comparable sandy shelf or coral reef, with larger phase shifts more resembling the coral reef observations. The increased linear drag friction can be related to directly measured bottom roughness.