An analysis of HF radar measured surface currents to determine tidal, wind-forced, and seasonal circulation in the Gulf of the Farallones, California, United States

摘要

A complete year of hourly 3 km resolution high-frequency radar measured surfacecurrents covering the Gulf of the Farallones were analyzed with the following threeprimary objectives: (1) describe the seasonal surface circulation, (2) identify tidalcurrents, and (3) determine the influence of wind forcing. Three predominant seasonalcirculation regimes were identified: relaxation, storm, and upwelling. The relaxationperiod exhibited mean poleward flow over the slope, variable equatorward flow over theshelf, and cyclonic and anticyclonic eddies south of Pt. Reyes. The storm period meanflow was variable and exhibited evidence of coastally trapped buoyancy flow from themouth of the San Francisco Bay. The upwelling period exhibited equatorward flowthroughout the gulf with regions of intensified flow at the northern and southern regionsover the slope. The tidal variance ranged from 4 to 60%, was highest around the mouthof the Bay, decreased past the shelf, and its spatial pattern reflected the combinedinfluence of the K_1 andM_2tidal current amplitudes. K_1ellipses typically rotatedclockwise throughout the gulf and decreased in amplitude past the shelf. M2 ellipseswere comparably more variable in orientation and magnitude. Harmonic "tidal" analysesof buoy wind data over 4 month time periods showed significant K1 amplitudes whichdid not appear when the analysis was done for the entire year. This indicates thatharmonic tidal analyses on surface currents are probably more effective at disassociatingdiurnal sea breeze driven currents when performed over long periods of time such as ayear.