Observation of wind forced circulation on the continental shelf off Point Sur, California from a self-contained acoustic doppler current profiler.
Abbott, Christopher Lynn
Ramp, Steven R.
Chu, Peter C.
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To study the current structure of the California Current as it manifests itself on the continental shelf a subsurface mooring, P1, was anchored 5km west of Point Sur at 36(o), 17' N, 121(o), 59' W from 28 February through 11 May 1990. the P1 mooring, placed on the 84 m isobaths, consisted of a self-contained acoustic Doppler current profiler (SC-ADCP) housed in a syntactic foam sphere and secured to an anchor. The mooring geometry placed the transducer heads at 80 m depth. The instrument operated at 307 kHz with each beam inclined 30(o) from the vertical. An ensemble average of 170 one-second pings were recorded every 15 minutes for 70 days. Good velocity data to within 15 m of the surface were obtained at 4 m intervals throughout the record. Low-pass filtered data indicate high coherence between the observed currents and the local wind forcing measured at the Monterey Bay NOAA buoy located 53 km to the northwest. During equatorward wind events the current response was vertically sheared and time lagged at depth with near surface speeds up to 35 cm s. During wind relaxations (periods of no wind) and poleward winds a pressure induced barotropic poleward flow, on the order of 10 cm s, developed throughout the water column. These poleward currents became more frequent and better developed later in the record. Analysis of adjusted sea level (ASL) differences between stations located to the north and south of mooring P1 suggest an increasing poleward pressure gradient during the span of the record. Alongshore differences in the alongshore wind stress component contributed to the increasing difference in ASL. A simple wind forced theoretical model adapted from Ekman (1905) and Csanady (1982), was used to describe the flow in the coastal regime. The model consists of surface and bottom boundary layers superimposed on a mostly geostrophic interior flow driven by set-up and set-down of the sea surface near the coast. The magnitude, direction and sense of rotation of the current vectors with depth were reproduced well by the model when it was forced by the actual magnitude of the surface wind stress and a realistic vertical eddy coefficient (Az). Difference between the model and the observations can be accounted for by other elements of the coastal circulation not included in the model.
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