The circulation and water masses in the Gulf of the Farallones

Abstract

Six ADCP and CTD ship surveys of the continental shelf and slope in the vicinity of the Gulf of the Farallones, CA, were conducted in 1990}1992. ADCP data provide much more detail on the structure of the currents over the slope and shelf in the Gulf and reveal a persistent, largely barotropic poleward #ow with a complex mesoscale #ow "eld superimposed. The directly measured currents are not well represented by the geostrophic velocity "elds derived from hydrographic casts. Important upper-ocean circulation features include: a Slope Countercurrent (SCC), variable shelf circulation, and submesoscale eddy-like features. The SCC was present in all seasons and is believed due to a strong year-round positive wind-stress curl enhanced by Point Reyes. Its #ow was poleward throughout the upper 300 m, and often surface intensi"ed. Poleward transport in the upper 400 m was 1}3 Sv, much greater than previous geostrophic estimates for the California Current System constrained to a 500 dbar reference level. The shelf circulation was much more variable than the SCC and generally exhibited a pattern consistent with classic Ekman dynamics, responding to synoptic wind forcing. Submesoscale vortices, or eddies, often dominated the general #ow "eld. These eddies are thought to be generated by the frictional torque associated with current}topography interactions. Their centers typically have a distinct water type associated with either the SCC or the southward-#owing California Current. Higher spiciness anomalies, representing a higher percentage of Paci"c Equatorial Water (PEW), were typically found in the core of the SCC or within anticyclonic eddies. Lower (bland) spiciness anomalies, characteristic of a higher percentage of Paci"c Subarctic Water (PSAW), were associated with cyclonic eddies. While these circulation features were largely barotropic, the #ow also adjusted baroclinically to changes in the density "eld, as di!erent water types were advected by the general #ow "eld or by mesoscale instabilities in the large-scale boundary currents as they interacted with topography. Despite a seasonal cycle in regional wind and ocean temperature time series, there is no obvious seasonal pattern in the circulation. Most of the temporal variability in the current appears to be due to synoptic and interannual variations in atmospheric forcing. Because of the very dynamic three-dimensional nature of the regional circulation, the Gulf of the Farallones is likely to be a center for active mixing and exchange between the coastal and California Current waters, relative to most US west coast locales.