Upwelling shadows as nearshore retention sites: the example of northern Monterey Bay

Periods of elevated temperatures in northern Monterey Bay suggest that this is a region of increased residence time during periods of active upwelling. Nearshore increases in near-surface temperature often coincide with offshore decreases in temperature as cold, upwelled water is advected into the center of the bay—the juxtaposition of warm and cold water enhancing the thermal signature of a feature described as an ‘upwelling shadow’. We present a variety of data collected from 1988 to 1993 invoking a shallow, stratified, cyclonic circulation in northern Monterey Bay to explain the upwelling shadow as a dynamic response to upwelling north of the bay. Residence times associated with this warm recirculation are of the order of 8 days which is comparable to semi-enclosed embayments whereas Monterey Bay is an open embayment. We estimate a continuous replacement of upwelling shadow water with offshore water at a rate of 0.083 day−1, and we suggest that a circulation velocity of 0.1 m · s −1, is needed to produce the thermal signature of the feature given published values of surface heat flux in the central California coastal region. The thermal signature of this upwelling shadow persists through much of the upwelling season, surviving brief relaxations in upwelling but breaking down during prolonged relaxations of the order of a week or longer. The stratification and coherent recirculation of this feature is likely to be very important for such biological processes as productivity and the dispersal and recruitment of larvae.