The vertical structure of currents over the Continental Shelf off Point Sur, CA, during Spring 1990

Abstract

A 300 kHz acoustic Doppler current profiler (ADCP) was deployed off Point Sur, CA (36°17.8′N, 121°59.5′W) from 28 February to 11 May 1990 to study the current response to local and remote forcing over a narrow continental shelf near a coastal promontory. The unit was moored 4 m off the bottom in water 84 m deep, and it profiled the currents in the water column in 4-m depth bins to within 12 m of the surface. The flow can be described in terms of the response to equatorward wind events versus the response to poleward wind events and wind relaxations. During equatorward wind events, the current vectors were directed equatorward and offshore near the surface at about 35 cm s -1, then backed counterclockwise and decreased in magnitude with depth to about 10 cm s -1, 4 m off the bottom. The vertical current structure during these events was well reproduced using a simple model in which the bottom Ekman layer was added to the unstratified model of Csanady (1982) for the wind-forced setup near a coast. The counterclockwise turning (as opposed to the open-ocean, clockwise sense) resulted from the interior geostrophic component, which overwhelmed the surface Ekman spiral and combined with the bottom Ekman layer to produce the observed vertical structure. The best agreement between the modelled and observed current vectors was obtained using a vertical mixing coefficient of about 12×10 -3 m 2 s -1. During poleward winds and wind relaxations, the currents were northward at about 20 cm s -1 over most of the water column, then also backed counterclockwise as the bottom was approached. The more barotropic response during the weak poleward winds and wind relaxations showed no evidence for local wind forcing, which suggests forcing due to an alongshore pressure gradient arising from an alongshore surface slope of about 2×10 -7. The across-shore mass balance from the best-fit modelled flow field during upwelling events showed that the onshore transport in the lower water column was 2–3 times the offshore transport in the upper water column. This may be due to three-dimensional effects or to the across-shore adjustment drift (Csanady, 1982), which feeds the alongshore geostrophic flow.