Background: To enhance our understanding of the circulation of the Gulf of the Farallones, a 13-month field program was sponsored by the EPA and US Navy. Five cruises (February, May, August, October 1991 and February 1992) were made to determine the likely dispersal pattern of dredge spoils proposed to be deposited periodically in the area. This area coincides with the region where the Tiburon Laboratory conducts its juvenile rockfish surveys each spring.
Purpose of Activity/Goal of Project: To describe the pattern of the circulation in the Gulf of the Farallones, and suggest what mechanisms are responsible for this pattern and its variability.
Description of Accomplishment and Significant Results: The currents over the slope and shelf in the Gulf are dominated by a pervasive, largely barotropic flow, with a complex mesoscale flow field superimposed. A Slope Countercurrent (SCC) was present in all seasons and transports 1-3 Sv poleward, much greater than previous estimates for the California Current. 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. Mesoscale eddy-like circulation features often dominated the general flow field. The centers of these features typically have a distinct water type associated with either the SCC or the southward-flowing California Current. Despite a seasonal cycle in regional wind and SST time series, there is no obvious seasonal pattern in the circulation. Most of the temporal variability in the currents 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 west coast locales. This has important implications for recruitment of rockfish and other marine populations.
Significance of Accomplishment (e.g., to the Center, to Management, and to NMFS Strategic plan Goals): The results improve our understanding of the coastal ocean's response to atmospheric forcing, coastal and bathymetric features and the California Current. Because this area is a key habitat for a number of commercially important fish species, including the rockfish complex, circulation features such as fronts and eddies are thought to be critical in controlling the dispersal and distribution of larvae and juveniles. Variations in these features on seasonal and interannual time scales may play an important role in recruitment variability. The insight gained from this study on the region's circulation will allow us to develop new hypotheses on how environmental factors, specifically coastal ocean currents and water mass features, may impact fish populations.
Key Contact: Frank Schwing (831-648-9034, fschwing@pfeg.noaa.gov)