The circulation and water masses in the Gulf of the Farallones
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Authors
Steger, John M.
Schwing, Franklin B.
Collins, Curtis A.
Rosenfeld, Leslie K.
Garfield, Newell
Gezgin, Erhan
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Date of Issue
2000
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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.
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Oceanography
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Citation
Deep-Sea Research II, Volume 47, (2000), pp. 907-946
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This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.