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Circulation and transport near Bodega Bay, California derived from modal decomposition of HF-radar currents


David M. Kaplan
Institute for Marine Sciences, Earth & Marine Sciences A147, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA, 95064


HF radar data has been used to study in detail the circulation patterns near Bodega Bay, California, just north of Pt Reyes. Surface flow patterns reveal a strong bimodality in flow, with equatorward flow dominating during periods of upwelling-favorable winds, particularly over the shelf edge, and considerable poleward flow occurring over the inner- and mid-shelf during relaxation. Here, we use modal current decomposition to study patterns of transport, divergence and convergence in the area. Modal current decomposition, also known as normal mode analysis, is advantageous because it interpolates over spatial gaps and produces currents with a desired level of spatial aggregation without loss of temporal variability. Here we apply this technique directly to the radial current data that HF radars measure, producing total current fields that are potentially more reliable than those from the standard process of locally combining current measurements from several instruments. We show that the current fields produced by this method robustly describe dynamics in the area, though there are limitations to the method, particularly in areas where only data from a single radial site is available. Interpolated current fields are used to study spatial patterns of upwelling and relaxation flows, as well as tides. Current fields tend to flow around Cordell Bank, a submerged depth-minimum 50 km from shore, dividing the upwelling jet into two pieces at this latitude. These results are in general agreement with oceanographic model results.

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