Spatiotemporal Variation in Cross-Shelf Exchange across the Inner Shelf of Monterey Bay, California

Abstract

ross-shelf exchange resulting from wind- and wave-driven flows across the inner shelf has been the focus of a considerable body of work. This contribution extends recent analyses to the central California coastline using 5-yr of moored current observations. Acoustic Doppler Current Profiler (ADCP) data from stations across the Monterey Bay (two in the northern bay and one in the southern bay), in water depths of ~20 m, showed net offshore transport throughout the year. For the northern bay sites, cross-shelf exchange was dominated by Ekman transport driven by along-shelf diurnal sea breezes during the upwelling season. Intense stratification in the northern bay leads to very shallow observed Ekman layers (~5?8 m), and consequently no overlap between bottom and surface Ekman layers within a few hundred meters of the coast. The total transport is less than predicted by theory consistent with models of shallow-water Ekman transport. The observed transport (~42% of full Ekman transport) is shown to be caused by the influence of a positive vorticity that effectively increases the Coriolis parameter. Wave-driven return flow estimated from an offshore buoy was strongly correlated with observed transport during nonupwelling conditions for the northern, outer bay site, but not for the two inner bay sites (northern and southern). In the southern bay, winds and waves have a significantly reduced effect on the cross-shelf exchange. Internal tidal bores are believed to contribute most of the observed cross-shelf exchange in this region.