FEATURE SURFACE CIRCULATION IN THE SANTA BARBARA CHANNEL By M.C. Hendershott and C.D. Winant THE SANTA BARBARA CHANNEL is an elongated serves. Particularly the Santa Maria Basin has the basin -100 km by 40 km lying between the Cali- potential for discovery of important new reserves. fornia coast, where it trends westward from greater The channel is additionally a major transportation Los Angeles to Pt. Conception, and the east-west lane for shipping to and from the greater Los An- trending Channel Island chain of four offshore is- geles area. The Minerals Management Service has lands (San Miguel, Santa Rosa, Santa Cruz, and the responsibility for long-term analysis of the oil Anacapa) -50 km offshore (Fig. 1). A 500-m deep spill risks associated with these activities as well basin occupies the western center of the channel, as participating in planning for rapid response to and narrow continental shelves with depths the individual oil spills. Consequently, in 1991, the order of 100 m border its northern and southern Minerals Management Service entered into a co- boundaries. The channel is separated from the operative agreement with the Scripps Institution of Southern California Bight to the east by a sill Oceanography to carry out a detailed study of the ~200 m deep between Anacapa and Port Huen- surface circulation of the channel and Santa Maria The project consists eme. A sill ~400 m deep between Pt. Conception Basin region. The goals of this study are 1) to de- and San Miguel Island separates the channel from scribe and scientifically understand the complex of a systematic the open ocean to the west. The Santa Maria Basin flow in the channel and Santa Maria Basin region analysis of historical is around the corner, north of Pt. Arguello. The with emphasis on the surface circulation, and 2) to surface waters of the channel are primarily mix- summarize the description and understanding in a data, execution of tures of warm and saline waters from the Southern manner useful to Minerals Management Service new fieldwork, and California Bight with colder and fresher waters analysts, whose responsibility is oil spill risk upwelled near and poleward of Pts. Conception analysis. The second goal focuses the work more data analysis and and Arguello. The California current flows equa- closely on the documentation of surface parcel tra- synthesis . torward past the western mouth of the channel, but jectories than has been usual in previous studies of its greatest speeds and most pronounced water coastal circulation along the California coast. properties (higher temperatures and lower salini- The project consists of a systematic analysis of ties than those of the upwelled water) are usually historical data, execution of new fieldwork, and found several hundred kilometers to the west of data analysis and synthesis in support of the two the channel. Prevailing winds around Pt. Arguello major goals outlined above. An important part of are equatorward, strong and steady in the summer, the data management has been the construction weaker but far more variable in the winter. The and maintenance of an extensive data bank that is channel itself is a relatively sheltered region. The generally accessible over the Internet, and that will transition from sheltered to exposed wind condi- ultimately contain the new data collected during tions occurs over a few kilometers in the western the fieldwork as well as the historical data. channel. These conditions all make the circulation The earliest surveys (Sverdrup and Fleming, in the channel and adjoining regions more com- 1941) outlined the general pattern of 0/500 m cir- plex-and less well understood--than anywhere culation in the greater Southern California Bight. else along the west coast of the United States. The California Current has a strong shoreward At the same time, the Santa Barbara Channel component at about the latitude of the US-Mexi- and the Santa Maria Basin comprise a region of can border; near the coast this flow bifurcates, and intense exploitation of known local oil and gas re- the poleward component flows alongshore toward the eastern mouth of the Santa Barbara Channel. Subsequent California Cooperative Fisheries In- M.C. Hendershott and C.D. Winant, Center for Coastal vestigation (CalCOFI) data both filled in spatial Studies, Scripps Institution of Oceanography, La Jolla, CA detail and delineated a strong seasonal cycle, with 92093-(1209, USA. poleward flow in the vicinity of the eastern mouth 1 14 OCEANOGRAPHY*VoI. 9, No. 2"1996 of the channel year round except for February 35* through April (Reid, 1965; Lynn and Simpson, 1987). But the CalCOFI data stations are too widely spaced to resolve the flow within the channel. Motivated by the 1969 Santa Barbara Channel oil spill, Kolpack (1971) carried out three hydro- graphic cruises during May, August, and Decem- ber, 1969, as well as eight drift card surveys of the surface circulation. The drift card surveys resolved a counterclockwise circulation in the western half of the channel and a northwesterly flowing current in the eastern part. Kolpack (1971) stated that 34* "The zone of interaction between these two cur- rents occurs in the area between Santa Barbara ~anta uruz is. Santa Rosa Is. and Santa Cruz Island." In 1983 the Organization of Persistent Up- • 25 welling Study (OPUS) documented new facts about the wind and flow fields in the vicinity of Pt. Con- , ..... i I i r i i I i I I i I i i i i ception. Aircraft observations (Brink et al., 1984) -121" -120" -119" revealed strong winds off Pt. Conception that were consistently equatorward, but became much more Fig. 1: Place names, mooring locations (lightly shaded solid circles) at gentle in a sheltered area east of Pt. Conception. which currents, temperatures and conductivities at 5 and 45 m (and 100 m Vigorous upwelling occurs between Pts. Arguello at ANMI), temperatures at 1, 25, and 65 m, and pressure, temperature, and and Conception as well as along the coast north of conductivity at anchor depth of 100 m (200 m at ANMI) are recorded; an- Pt. Arguello (Brink et al., 1984; Atkinson et al., chor locations (lightly shaded triangles) at which pressure, temperature, and 1986); hydrographic observations reported by conductivity are recorded at anchor depth (10 m), downward-looking Atkinson et al. (1986) documented variability of acoustic Doppler current profiler mooring locations (lightly shaded the direction of the upwelling plume between squares), NDBC meteorological buoy locations (heavily shaded squares), oil southwestward and southeastward. The plume fre- platform and coastal meteorological station locations (lightly shaded dia- quently entered the Santa Barbara Channel, but its monds), drifter release sites (heavily shaded solid circles). direction was not obviously related to local winds. Time series of currents and temperature were col- correspondingly traced out counterclockwise paths lected at a number of mooring sites around the pe- in the western channel. The correlation between riphery of the channel and near Pt. Conception dur- flow and winds in the channel continued to be ing a 57-day spring and summer period of 1983 much lower than in the CODE experiment. (Brink and Muench, 1986). At 1983 mooring C1, Lagerloef and Bernstein (1988) analyzed a year located roughly halfway along a line between Pt. of Advanced Very High Resolution Radiometer Conception and Pt. Arguello, there were clear sur- (AVHRR) images approximating sea surface tem- face flow counterparts of wind events at NDBC perature in the channel during the 1984 Minerals The fluctuating Buoy 11 moored -30 km north of the latitude of Pt. Management Service Study. The dominant mode Conception along the 200-m isobath, but about of sea surface temperature variation was seasonal western mouth flow midway through the observing period this corre- modulation of the strength of the thermal front be- was dominated by spondence vanished (Brink and Muench, 1986). tween warm water from the Southern California There was no such a correspondence between Bight and cold water along the Channel Islands. transient events of winds and currents at any other moorings, including The images also suggested the presence of coun- the very nearby mooring P2 -10 km south of Pt. terclockwise eddies in the channel, usually persist- enhanced cyclonicity, Conception. These results were in marked contrast ing for many days and drifting slowly westward. and drifters corre- with those of the then recently concluded CODE The present program of fieldwork was designed experiment (Winant et al., 1987). to complement the 1984 study by providing more spondingly traced out In 1984 the Minerals Management Service car- detail in the interior of the channel and near the counterclockwise ried out a year-long study (Gunn et al., 1987) de- surface. Deployment began in 1992, much of the signed primarily to provide boundary conditions moored array shown in Figure 1 has been in place paths in the western for numerical modeling of the circulation within for >3 years. Meteorological observations are vari- channel. the Santa Barbara Channel. The mean flow at the ously acquired from: National Data Buoy Center east mouth was poleward, at the western mouth (NDBC) buoys moored both inside the SBC (buoys there was westward flow past Pt. Conception and 53 and 54) and at either end of the channel (buoys eastward flow past San Miguel Island. The fluctu- 23 and 25), fixed stations on the Channel Islands ating western mouth flow was dominated by tran- and on oil platforms, and land stations maintained sient events of enhanced cyclonicity, and drifters by various air pollution control agencies. Uninter- OCEANOGRAPHY'VoI.9, No. 2"1996 1 15 rupted series of currents, temperature, salinity, and windage drifter design of Davis (1985) was inter- bottom pressure are collected at the mooring loca- nally modified to be tracked by the Argos system tions shown in Figure 1.