Characteristic patterns of the circulation in the Santa Barbara

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. C2, PAGES 3041-3065, FEBRUARY 15, 1998 Characteristic patterns of the circulation in the Santa Barbara Channel Sabine Harms and Clinton D. Winant Center for Coastal Studies, Scripps Institution of Oceanography, La Jolla, California Abstract. The subtidal near-surface circulation in the Santa Barbara Channel (SBC) and on the shelf north of Point Conception is described based on observations obtained during the 3-year period from 1993 to 1995. Near-surface currents in the channel are a superposition of a largerthan-sbc scale flow and a cyclonic circulation of variable intensity located inside the channel. On seasonal timescales the larger-than-sbc scale flow near the surface is equatorward in spring and poleward from summer through winter. The increase in equatorward flow in spring occurs concurrently with the increase in equatorward wind stress and the decrease in near-surface temperatures and synthetic subsurface pressures (SSPs). The flow reverses in late spring, simultaneously with the increase in the along-channel SSP difference and months before wind stress has reached its peak. The period during which the cyclonic circulation within the SBC is strongest coincides with the period of strongest poleward flow through the eastern entrance. A synoptic description of the circulation in the SBC is presented in terms of six characteristic patterns, labeled Upwelling, Relaxation, Cyclonic, Propagating Cyclones, Flood East, and Flood West. An analysis of the 5- and 45-m currents into empirical orthogonal functions (EOFs) isolates 50% (53%) of the 5-m (45-m) low-frequency current variance into three (two) modes. Combining these modes with the mean current fields, the modes have spatial patterns that correspond to the characteristic flow patternsubjectively deduced from inspection of daily averages of the near-surface currents. From late spring through fall the two largest 5-m current modes produce a repeating pattern of circulation in which the sequence of four states (namely, Upwelling, Cyclonic, Relaxation, and Quiescent) is traversed roughly every 16 days. In addition to the large-scale cyclonicity in the central SBC, smaller cyclonic eddies form frequently in the eastern channel and travel toward the west with an average speed of 0.06 m s -. The patterns described here develop in response to the wind stress and the along-channel SSP gradient. to regions which are characterized by simple topography, 1. Introduction by limited alongshore variability, and by strong upwelling- Conducted to study the response of continental shelf wafavorable winds. Thus Bray and Greengrove [ 1993], Largier ters to strong wind forcing, the Coastal Ocean Dynamics Exet al. [1993], and Magnell et al. [1990] show that on the periment (CODE) in the upwelling seasons of 1981 and 1982 northern California shelf near Cape Mendocino, equatoron the straight section of coastline between Point Arena and ward winds can be balanced by the alongshelf pressure gra- Point Reyes led to an improved understanding of the dynamdient set up by spatial gradients in the alongshore wind field, ics of the upwelling process and of the response of shelf resulting in convergence of the shelf currents and offshore waters to local and remote wind forcing [Davis and Bogflow at the cape. Send et al. [ 1987] show that during periods den, 1989; Huyer, 1984; Lentz, 1992; Winant et al., 1987]. of weak winds, spatial variations in the upwelling intensity Temperatures and alongshelf currents over the shelf were obcause poleward density-driven currents near the coast north served to be well correlated with local winds. During strong of Point Reyes. Hickey [1992] and Lentz and W nant [1986] upwelling-favorable wind events, alongshelf currents were find that in the Southern California Bight, where winds are directed equatorward, and the direction of the cross-shelf generally weak, currents over the shelf are primarily driven flow was generally consistent with that expected in twoby local gradients in adjusted sea level. dimensional upwelling systems. Observations obtaine dur- In the Santa Barbara Channel (SBC, Figure 1) atmospheric ing larger-scalexperiments, such as the Northern California and oceanic conditions change significantly over a few tens Coastal Circulation Study (NCCCS) suggest, however, that of kilometers. Strong and persistent equatorward winds, these CODE results may be restricted in their application which occur along the central California coast and off Point Conception, separate from the coast in the vicinity of Point Conception, leaving the winds in the channel and in the Copyright 1998 by the American Geophysical Union. Southern California Bight weak and highly variable in time Paper number 97JC02393. and space [Brink et al., 1984; Caldwell et al., 1986; W nant 0148-0227/98/97JC-02393 $09.00 and Dorman, 1997]. Oceanic forcing is complex because 3041

3042 HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 35. OøN 34. 5øN $4. OøN San 1 Point Arguello,. :acd;t.a..l$, ', 2 Jalama Beach "-., ]rs, ",..,,,...? $ Point Conception '" ta ß "' i "-.. $ El Capitan...?. :,,, :'? 6 4 C.m= "ZS 7 Hermosa PlaOrorm 8 Hondo PlaOrorm ', ': '--2." b 9 Gail PlaOrorm... 10 Santa Cruz Island ' ".33.5ON I ' ' ' I ". -,... I. 121. Oø[g ' 120. 5o[g ' 120. OO[g ' H9. 5o[g ' H 9. OO[g ' H 8. 5o[g ' Figure 1. Santa Barbara Channel - Santa Maria Basin Coastal Circulation Study (SBC-SMB CCS) moored and meteorological instrument locati. ons. the SBC is located in the transiti. on zone where cold waters in the SBC, while known to be present, were too poorly samupwelled around and north of Points Arguello and Concep- pled to be fully appreciated. tion meet warm Southern California Bight waters of sub- In early 1992 an extensive multiyear observational protropical origin [Chelton, 1984; Lynn and Simpson, 1987]. A gram, the Santa Barbara Channel - Santa Maria Basin Coastal first attempt to describe the circulati. on around Point Con- Circulation Study (SBC-SMB CCS) sponsored by the Minception and in the SBC was made during the biological and erals Management Service (MMS), was initiated to yield a oceanography survey of the Santa Barbara Channel oil spill more detailed picture of the circulation. The experiment inin 1969 [Kolpack, 1971]. Results obtained from three hydro- cludes a basin-wide array of shelf moorings instrumented graphi cruises, conducted in May, August, and December with current meters, temperature, conductivity, and bottom 1969, and eight drifter card surveys indicate that the near- pressure sensors. Meteorological observations in this area surface circulati.on in the SBC consists of a counterclock- are available from a number of Nati onal Data Buoy Center wise cell in the western half of the channel and northwest- (NDBC) buoys, meteorological stations, and air pollution ward flow in the eastern part. A complex pattern of small stations. Using the combined meteorological and moored eddies was observed in the convergence zone of these two data sets, the near-surface circulation in the SBC and the flows. In 1983 and 1984, two experiments, the Organizati. on Santa Maria Basin is described on both seasonal and synof Persistent Upwelling (OPUS) experiment and the Santa optic timescales. Particular emphasis placed on develop- Barbara Channel Circulation Study (SBCCS), were designed ing the characteristicirculati.on patterns of the SBC. The to study the response of the shelf circulation in this area to next section will briefly describe the moored array and the local wind forcing. Resultshow that off Point Conception, data-processing procedures applied to the raw data to yield shelf currents and water properties respond to the strong and hourly, lowpass-filtered ti. me series. An overview of the mapersistent upwelling-favorable winds in the classical sense jor circulation features in the SBC is given in secti.on 3. The [Brink, 1983; Huyer, 1983], yet flows offshore and in the mean and seasonal characteristics of wind stress, water tem- SBC consist of eddies, jets, and fronts which show no persis- perature, synthetic subsurface pressure (SSP), and currents tent correlati. on with the local winds [Atkinson et al., 1986; are discussed in sections 4 and 5. Characteristi. c patterns Barth and Brink, 1987; Brink and Muench, 1986; Gunn et of the 5-m and 45-m circulation are identified by means of al., 1987; Lagerloef and Bernstein, 1988]. The effects of empirical orthogonal functions (EOFs) in section 6. The wind stress gradients, buoyancy gradients, topography, and transition between the dominant flow regimes is examined the larger-scale California Current system on the circulation in detail. The relation between the different flow regimes

HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 3043 and wind stress and pressure gradient forcing is examined by wind records are flagged, indicating missing data. Gaps in sorting hourly averaged flow maps according to the strength the temperature and bottom pressure time series are filled by of the wind stress and the SSP difference through the chan- multiple linear regression between records of neighboring nel. These results are discussed in section 7. Section 8 sum- stations. Anchor shifts are removed from the bottom presmarizes the main results. Throughouthe paper, "poleward" sure records before averaging, following the procedure deand "equatorward" are used to denote the current which is scribed by Harms and Winant [1994]. Because of the diffiparallel to the coast directed either up or down the coast. culty in determining an absolute reference level for the pressure observations the mean pressures are not meaningful, and they are subtracted from the bottom pressure time se- 2. Observations ries. Tidal fluctuations are removed from the resulting pressure signal by subtracting the tides predicted from the major The SBC basin, located at the northern edge of the Southharmonic constituents calculated from the bottom pressure ern California Bight, is bounded to the north by the south records. The amplitudes and phases of the major tidal concentral California coast and to the south by the four channel stituents are calculated via a least squares fit method [Godin, islands: San Miguel, Santa Rosa, Santa Cruz, and Anacapa 1972] for only those constituents that can be resolved over (Figure 1). The coastline at Point Conceptio near the westthe record length. This method was chosen because it is ern edge of the channel is markedly curved, directed northadaptable to time series of arbitrary length and it can also south north of Point Conception and east-west south of the be used for series containing short gaps. In order to com- Point. The channel, which has a general east-west orientapare bottom pressure observations different depths, bottion, is 100 km long and 40 km wide with a central basin tom pressure is converted into synthetic subsurface presdepth of about 500 m and has a narrow shelf on both sides sure (SSP); that is, the contribution of density fluctuations to ranging in width between 3 and 10 km. The sills at the eastpressure, here approximated linearly by temperature, is subern and western entrances of the channel are 220 and 430 tracted from the bottom pressure measurements [Harms and m deep, respectively. The passages between the islands are Winant, 1994]. The atmospheric observations are adjusted about 40 m deep. to a standard height of 10 m above sea level, as described by The moored array consists of a total of 10 moorings de- Dorman and Winant [1995]. Since wind stress rather than ployed over the shelf along the 100-m isobath and at 200 m wind speed is responsible for the flux of momentum between depth in the center of the eastern entrance of the SBC (Figthe atmosphere and the ocean, winds are converted to wind ure 1). NDBC buoys 53 and 54 are equipped with a downstress following the method described by Large and Pond ward looking acoustic Doppler current profiler (ADCP). The [1981] for a neutral atmosphere. The time series analyzed observations are augmented by measurements obtained durhere have additionally been lowpass-filtered by convolution ing the California Monitoring Program (CAMP) off Point with a symmetric filter with a half-power period at 38 hours Arguello [Hamilton et al., 1994]. Instruments deployed at [Limeburner, 1985]. The filter does not introduce any phase each mooring include current meters, temperature, conducshift. The 95% and 99% significance levels for the corretivity, and bottom pressure sensors. Meteorological obserlations presented in this report are computed following the vations are available from NDBC buoys off Point Concepmethod described by $ciremammano [1979]. tion (NDBC 23) and off Los Angeles (NDBC 25), from two meteorological buoys within the channel (NDBC 53 and 54), from Scripps Institution of Oceanography (SIO) oper- 3. Overview ated meteorological stations, and from air pollution stations maintained by the Santa Barbara County Air Pollution Con- Yearlong time series of wind stress and currents are illustrol District. All meteorological buoys are instrumented to trated in Figure 2 for the year 1994. North of San Miguel measure wind speed and direction, atmospheric pressure, air Island, at SMOF (Figure 2b), currents (especially the northtemperature, and sea temperature I meter beneath the sur- south component) respond to upwelling-favorable wind stress face. The meteorological and air pollution stations mea- fluctuations NDBC buoy 54 (Figure 2a) year-round. About sure wind speed and direction, atmospheric pressure, and 25 km to the north, near Point Conception, the relationship air temperature. The exact locations and depths of the in- between coastal currents and wind stress varies. From Janstruments are listed in Table 1. The field program further uary until the end of April the north-south component of includes surface drifter surveys conducted every 2 months, near-surface currents at SMIN (Figure 2c) responds rapidly biannual ADCP, conductivity-temperature-depth (CTD) pro- to wind stress fluctuations at NDBC buoy 54 in a manner filer, and expendable bathythermograph (XBT) surveys, and consistent with other observations of upwelling. At the bedaily archiving of advanced very high resolution radiome- ginning of May this correlation breaks down and remains ter (AVHRR) satellite imagery [Hendershott and Winant, low during the rest of the year. Brink and Muench [1986] 1996]. analyzed current observations in the same area over a period The analyses described subsequently are all based on hour- extending from April 1 to July 25, 1983, and found a similar ly averages of the original moored measurements. Gaps in relationship between currents and wind stress. During April the time series shorter than 24 hours are filled by linear in- and May 1983, currents were well correlated with the local terpolation. Gaps longer than 1 day in the current meter and wind stress. After late May the relation to winds Was much

3044 HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL Table 1. Santa Barbara Channel - Santa Maria Basin Moored Instrument Locations and Equipment Instrument Acronym Station Name Latitude, N Longitude, W Deployed Recovered Depth, m Depth, m Type PAIN Point Arguello Inshore 34048'29" SMIN San Miguel Inshore 34023'57" SMOF San Miguel Offshore 34ø09'17" ROIN Rosa Inshore 34ø25'04" ROOF Rosa Offshore 34ø06'16" GOIN Goleta Inshore 34ø21'28" GOOF Goleta Offshore 34ø07'05" CAIN Carpinteria Inshore 34 ø 13'52" BARB Santa Monica Bay 33ø55'00" ANMI* Anacapa Middle 34ø03'13" CAMP Cal. Monitoring Progr. 34ø30'07" NDBC 23 NOAA NDBC 46023 NDBC 25 NOAA NDBC 46025 NDBC 54tt NOAA NDBC 46054 NDBC 53** NOAA NDBC 46053 Moorings 120 ø46'58" Dec. 9, 1993 * 100 120027'00" April 16, 1992 * 100 120ø27'16" April 24, 1992 * 100 120ø09'16" Dec. 15, 1993 Jan. 12, 1996 100 120ø09'06" Dec. 16,!993 Jan. 12, 1996 100 119050'24" Oct. 27, 1992 Jan. 13, 1996 100 119050'58" Oct. 26, 1992 Jan. 13, 1996 100 119034'34" Oct. 27, 1992 Jan. 13, 1996 100 118ø33'17" Dec. 9, 1993 March 12, 1995 100 119ø18'16" Oct. 26, 1992 * 200 120 ø43'05" April 4, 1992 * 180 All All 1, 25, 65 T 5, 45 (100t) VMCM, T, C 100 (200*) BP, T, C 14, 54, 126 VMCM, T, C NDBC Buoys 34ø18'00" 120042'00" April 7, 1982 May 18, 1996 640 All All 33ø42'00" 119ø06'00" April 20, 1982 * 380 10+ W, AT, AP 34ø16'12" 120025'59" Aug. 25, 1993 * 430 1 T, ADCP** 34013'59" 119049'59" Aug. 6 1993 * 400... Air Pollution and Meteorolo9ical Stations "1" (PA) Point Arguello 34034'37" 120038'24" Jan. 1, 1994 * NA 52+... "2" (JA) Jalama Beach 34030'42" 120029'56" Jan. 1, 1994 * NA 6+... "3" (PC) Point Conception 34ø27'07" 120027'28" Jan. 1, 1994 * NA 55+... "4" (GE) Gaviota Odor East 34028'20" 120010'35" Jan. 1, 1994 * NA 34+... "5" (EC) E1 Capitan 34027'45" 120001'28" Jan. 1, 1994 * NA 39+ All "6" (WC) Exxon 4 West Campus 34024'55" 119052'43" Jan. 1, 1994 * NA 9+ W, AT, AP "7" (HE) Hermosa Platform 34ø27'21" 120038'53" June 16, 1994 * NA 40+... "8" (HO) Hondo Platform 34023'25" 120ø07'17" June 16, 1994 * NA 44+... "9" (GA) Gail Platform 34ø07'31" 119023'59" June 17, 1994 * NA 30+... "10" (SC) Santa Cruz Island 34003'35" 119055'37"Feb. 16, 1994 * NA 30+... VMCM, vector measuring current meter; T, temperature logger; C, conductivity cell; BP, bottom pressure sensor; W, anemometer; AT, air temperature sensor; AP, atmospheric pressure sensor; ADCP, Acoustic Doppler Current Profiler; NA, not applicable. All depths are in meters below the sea surface except for the meteorological observations where the plus refers to the height of the instrument above the sea surface or land. Deployment and recovery dates of the instruments are listed in columns 5 and 6. * Still operating. *ANMI is equipped with an additional VMCM at 100 m and a bottom pressure sensor at 200 m. *tadcps are mounted only on National Data Buoy Center (NDBC) buoys 53 and 54. less clear. These wind-current correlations are not signifi- ticorrelated fluctuations of alongshelf currents on opposite cant at 45 m depth. shelves recur regularly from summer through early winter Alongshelf currents on the northern shelf of the SBC are when the mean shear on opposite shelves is large. Anticoroften in opposition to currents on the southern shelf, both related fluctuations are absent in late winter and early spring in terms of the mean and the fluctuations, a situation first when the mean cross-shelf shear is small. The opposition of described by Brink and Muench [1986] and by Gunnet al. currents on opposite shelves of the channel is also observed [ 1987]. An example of this north-south shear is illustrated in at the ROIN-ROOF and GOIN-GOOF lines. Together these Figures 2d and 2e. Near-surface currents at SMIN frequently opposing flows lead to a cyclonic circulation of variable inincrease toward the west at the same time as 5-m currents tensity concentrated in the central and western SBC. at $MOF increase toward the east. On seasonal timescales Currents at ANMI, located over the eastern sill, are domthe magnitude of the cross-shelf shear between alongshelf inated by a seasonal cycle (Figure 20. From February until currents on opposite shelves is greatest during summer and mid-may, surface waters flow southeastward out of the SBC. early fall and weakest in late winter and early spring. Su- In summer, fall, and early winter the current flows northperposed on the seasonally varying north-south shear are westward over the sill. These seasonal trends can be reversed fluctuations with periods of the order of weeks. These an- for periods of several days. Near-surface currents at ANMI

HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 3045 +0.2 Pa 1..,.I,.I..,..., +0.2ms" 1. -0.2 m.1-0.2m, '1 j,,--.?,v-,j I -..,,,rl,,,,- ß -0.2 m +0.2 m s (e) SMIN -0. a a a,..,, '.. -0.2 m ' I,,,,,,,I,,,,,,,I,,,,,,,I, 1,,,,,I,,,,,,,I,,, Jan 1 Feb 26 1994 Tic are 1 week apart Figure 2. (a) Time series of wind stress at National Data Buoy Center (NDBC) buoy 54. Wind stress observations are presented along principal axes. Time series of 5-m currents at SMOF and SMIN. These current observations are presented in a coordinate system aligned with the local coastline, i.e., (b, c) across the shelf, a direction in which vector correlations between wind stress and currents near the western entrance are at a maximum, and (d,e) along the shelf, a direction closely resembling that of the principal axes. (f) Time series of principal axes 5-m currents at ANMI. Superposed is the cross-shelf synthetic subsurface pressure (SSP) difference between SMIN and SMOF (dashed line). (g) Time series of principal axes 5-m currents at PAIN. For each time series the arrow, sketched on the right-hand side, points toward the north. All time series are lowpass filtered with a symmetric filter with a half-power period at 38 hours. are significantly correlated with 5-m currents at PAIN north of Point Arguello (0.68 at zero lag; Figure 2g). The correlation is best in late winter and spring. In summer and fall the magnitude of the poleward excursions in the eastern part of the SBC is generally larger than off central California. Correlated at scales greater than the horizontal extent of the SBC, in this reporthe currents at PAIN and ANMI are taken to represent the large-scale circulation. It has been shown that there is an anticorrelation between currents at SMIN and SMOF in the direction parallel to the

3046 HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL... :..:&.' 33.8'N 1 33.4.Nt (!nd. tr.,s. [pa.] Figure 3. Averaged fields from January 1993 to December 1995. For instruments deployed after January 1993 or recovered before December 1995 the statistics were calculated over the available record length (Table 1). (a) Mean and principal axis wind stress at the NDBC buoys and at selected meteorological and air pollution stations. Arrow sizes and the magnitude of the variancellipses are proportional to the wind stress magnitude in pascals. (b) Mean (solid line) and standardeviation (dashed line) 1-m temperatures in degrees Celsius. (c) Standardeviation of SSP in kilopascals. (d) Mean and principal axis 5-m currents at the moorings. Arrow sizes and the magnitude of the variance ellipses are proportional to' the current magnitude in m s-. (e) Same as Figure 3d but for 45-m currents. (f) Vertical structure of the mean currents at NDBC buoys 53 and 54. The measurements extend from 24 to 328 m depth, with a vertical resolution of 16 m. Arrows are proportional to the current magnitude in m s -x. Note thathe scaling arrow is different from Figures 3d and 3e. channel axis. When anticorrelated events occur, the flow at The mean wind stress is greatest south of Point Conception ANMI is generally directed poleward. When the value of at the site of the maximum observeduring aircraft surveys the negative correlation is low, the flow at ANMI tends to be in 1981 [Brink et al., 1984; Eddington, 1985] and decreases equatorward. An independent estimate of the surface trans- sharply toward the east. The coastal winds between Point port in the SBC can be obtained from the cross-shelf SSP dif- Conception and Santa Barbara are weak. The amplitude of ference, assuming that the momentum balance in the cross- wind stress fluctuations is similar to the mean in the westchannel direction is in geostrophic balance. Variations in the ern channel and exceeds the mean in the eastern channel and cross-shelf SSP difference (Figure 2f, dashed line) are fre- in the Southern California Bight. The ellipticity of the variquently associated with fluctuations in the alongshelf com- ance ellipses (the ratio of minor to major axes) varies from ponent of currents at ANMI and PAIN, suggesting that there 0.2 to 0.5, with higher values in the eastern SBC and lower is a relationship between the large-scale flow and the surface values over the western channel. Average surface tempertransport through the channel. From these observations we atures (Figure 3b) increase from the northwest toward the conclude that the near-surface flow in the SBC is a superpo- southeast. Within the channel, average temperatures on the sition of alongshore currents with scales larger than the SBC northern shelf exceed those on the southern shelf by typiand a cyclonic circulation which is specific to the channel cally 1 øc. Standardeviations of SSP (Figure 3c) are largest interior on the northern shelf of the SBC between Point Conception and Santa Barbara and smallest at the eastern entrance of the channel and off south central California. 4. Mean Fields The mean flow at 5 m depth in the SBC consists of a Mean wind stress vectors and principal axes variance el- concentrated jet flowing westward on the northern shelf and lipses (Figure 3a; Table 2) show that the mean wind stress a weaker eastward return flow on the southern shelf (Figis directed equatorward off Points Arguello and Conception ure 3d). Together these.opposing flows lead to a mean cyand turns toward the east in the SBC and in the Southern clonic circulation concentrated in the western part of the California Bight. The Santa Ynez mountain range, rising to channel, consistent with that described by Brink and Muench more than 1000 m, may steer the winds parallel to the coast, [1986]. The direction of the mean current vectors over the explaining the direction of wind stress within the channel. central basin (NDBC buoys 53 and 54) is consistent with a

,,,.,, *** ***,,, HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 3047 Table 2. Mean and Standard Deviation Statistics Station Wind Stress, Pa Temperature, øc SSP, kpa 5-m Currents, m s -1 45-m Currents, m s- Mean Mean Mean Mean Mean Speed Dir Dir Major Minor Speed Dir Dir Major Minor Speed Dir Dir Major Minor PAIN... 13.7 SMIN... 14.8 ROIN... 15.6 GOIN... 16.1 CAIN... 16.3 ANMI... 16.3 GOOF... 15.4 ROOF... 14.3 SMOF... 14.4 BARB... 16.5 CAMP... 13.3 NC 23 0.093 139 319 0.093 0.021 14.5 NC 54 0.113 122 304 0.121 0.023 14.5 NC 53 0.036 89 285 0.066 0.020 15.6 NC 25 0.015 112 293 0.037 0.016 17.0 PA 0.062 177 344 0.074 0.021... PC 0.036 137 307 0.056 0.014... GE 0.009 165 325 0.031 0.014... WC 0.005 91 280 0.016 0.005... GA 0.017 93 275 0.056 0.016... 1.4 2.0 2.1 2.0 2.0 2.1 2.0 1.7 1.7 2.4 1.6 1.7 1.7 2.0 2.2 0.50 0.068 213 356 0.146 0.090 0.048 72 357 0.110 0.044 0.70 0.202 265 278 0.183 0.109 0.172 284 281 0.166 0.053 0.65 0.180 278 278 0.146 0.053 0.169 279 276 0.133 0.032 0.64 0.125 282 282 0.127 0.051 0.148 284 278 0.127 0.032 0.58 0.072 305 316 0.117 0.074 0.147 305 301 0.123 0.051 0.50 0.051 350 319 0.192 0.089 0.140 328 312 0.155 0.051 0.50 0.077 107 274 0.146 0.082 0.027 72 280 0.108 0.061 0.52 0.147 99 264 0.129 0.076 0.093 75 271 0.085 0.036 0.54 0.126 147 244 0.135 0.115 0.111 94 269 0.120 0.052 0.55 0.040 121 327 0.085 0.045 0.015 94 328 0.050 0.024... 0.047 287 327 0.149 0.069 0.078 318 318 0.127 0.048... 0.041 209 244 0.143 0.085 0.027 182 241 0.138 0.084... 0.037 313 230 0.106 0.080 0.037 302 236 0.109 0.081, standard deviation; Dir, direction. ADCP currents at National Data Buoy Center (NDBC) buoys 54 and 53 at 24 and 40 m depth are included. Directions are relative to true north. The statistics are calculated over the 3-year period from January 1993 to December 1995. If instruments were deployed later than January 1993 or recovered earlier than December 1995, the statistics are calculated over the available record length (Table 1). cyclonic gym-like pattern in the SBC. Mean westward cur- The vertical structure of the mean current field in the cenrents on the northern shelf strengthen between the eastern tral and western SBC is described using the averaged ADCP entrance and Point Conception. West of Point Conception, velocities at NDBC buoys 53 and 54 (Figure 3f; Table 3). mean westward currents drop sharply as the flow out of the The measurements extend from 24 to 328 m, with a verchannel encounters equatorward flow along the south central tical resolution of 16 m. The vertical distribution in the California coast. Mean eastward flow on the southern shelf central SBC (NDBC 53) suggests a three-layered structure; decreases in strength from the west toward the east. The between 24 and 80 m the mean flow is uniform in magmajor axis of current fluctuations 5 m is generally par- nitude, and current vectors rotate counterclockwise with inallel to the mean current, except over the central basin and creasing depth from the northwest toward the west. Between in the southern portion of the western entrance (Figure 3d). 80 and 200 m, mean currents are directed westward and de- The magnitude of the fluctuations frequently exceeds that crease gradually in strength (vertical shear of 4 x 10-4 s -1). of the mean by a factor of 2 or more. The variability is Below 200 m the mean velocities are small, and the current greatest at the eastern entrance and around Points Concep- vectors rotate counterclockwise with increasing depth from tion and Arguello. The ellipticity of the variance ellipses west to east. South of Point Conception (NDBC 54), the varies from 0.4 to 0.9, with highest values occurring in the mean currents are largest near the surface (at 24 m). At this southern portion of the western entrance. The mean flow at depth the mean current vector is directed toward the south- 45 rn depth (Figure 3e) is similar to the flow at 5 m, with west, to the right of the wind stress. Between 24 and 240 m, three exceptions. Mean westward flow on the northern shelf mean velocities decrease gradually at 4 x 10-4 s -1, while is continuous and uniform in magnitude between Port Huen- rotating counterclockwise with increasing depth from the eme and Point Conception. Westward flow on the northern southwestoward the northeast. The mean velocity profile shelf appears to continue poleward north of Points Concep- has a zero crossing at about 160 m. Below 240 m, mean tion and Arguello. Mean currents north of San Miguel Island currents are directed toward the northeast and are uniform in are directed eastward rather than southeastward. The differ- magnitude. ences in the mean flows at 5 and 45 m are consistent with the idea that wind forcing is diminished beneath the thennocline. Compared to the surface, the standar deviations at 45 m are generally smaller, and the flow over the shelf is more polarized in the alongshelf direction. 5. Seasonal Cycle The seasonal variability of the coastal wind field in the SBC region is determined by two large-scale atmospheric

3048 HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL Table 3. Mean and Standard Deviation Statistics of the ADCP Currents NDBC 54 NDBC 53 Depth Mean o' Mean o' Speed Dir Dir Major Minor Speed Dir Dir Major Minor 24 0.041 209 64 0.143 0.085 0.037 313 50 0.106 0.080 40 0.027 182 61 0.138 0.084 0.037 302 56 0.109 0.081 56 0.025 179 55 0.127 0.084 0.040 287 59 0.111 0.078 72 0.022 175 56 0.120 0.081 0.042 276 60 0.113 0.078 88 0.021 168 57 0.111 0.078 0.043 269 61 0.112 0.080 104 0.016 156 60 0.099 0.077 0.041 266 61 0.106 0.079 120 0.013 146 62 0.087 0.074 0.038 265 58 0.097 0.074 136 0.008 148 69 0.078 0.069 0.034 264 57 0.088 0.067 152 0.003 119 68 0.070 0.062 0.030 263 56 0.079 0.060 168 0.003 70 81 0.061 0.058 0.026 264 53 0.069 0.053 184 0.004 50 69 0.056 0.053 0.023 263 49 0.060 0.046 200 0.006 57 95 0.053 0.049 0.018 260 45 0.054 0.040 216 0.010 50 106 0.050 0.043 0.015 250 43 0.048 0.036 232 0.011 58 111 0.048 0.038 0.012 240 39 0.043 0.034 248 0.013 64 111 0.048 0.035 0.009 227 35 0.039 0.032 264 0.013 65 114 0.047 0.034 0.006 200 24 0.035 0.033 280 0.016 62 111 0.046 0.032 0.006 167 337 0.033 0.030 296 0.018 60 118 0.050 0.031 0.005 140 324 0.033 0.027 312 0.017 59 118 0.049 0.031 0.005 116 322 0.033 0.024 328 0.016 62 120 0.053 0.030 0.005 100 319 0.034 0.023 a, standard deviation; Dir, direction. Directions are relative to true north. Depths are in meters. The statistics are calculated from the time of the deployment of the instruments (August 1993) until December 1995. The ADCP currents are in m s -. pressure patterns: the subtropical high over the eastern North interrupted by equally long calm periods. The seasonal vari- Pacific and the low over southwestern North America [Reid ability of wind stress in the SBC is illustrated in Figure 4. et al., 1958]. In the winter the most energetic atmospheric Seasonal cycle amplitudes decrease in magnitude from Point fluctuations along the south and central California coast re- Conception toward the east and are smallest at the coast and sult from propagating storm tracks [Halliwell and Allen, in the Southern California Bight. The maximum wind stress 1987]. These storm systems pass over the SBC in 2-4 days occurs in the summer at NDBC buoy 54 south of Point Conand are large in size compared to the size of the channel. ception. In the eastern SBC and in the Southern Califor- At the beginning of spring the North Pacific high strength- nia Bight, wind stress is strongest in the spring. Minimum ens, and its center moves toward the north. At the same monthly means occur everywhere in February. The weak time the low over the southwestern United States deepens. monthly averages at the coast are consistent with the year- The interaction between these two pressure systems results round sheltering by the coastal mountain range described by in strong and persistent southeastward winds off Point Con- Dotman and Winant [1995]. ception. Winds in the Southern California Bight are signif- The seasonal variability of currents is described in two icantly weaker than offshore. In the summer the gradient parts. First, the horizontal structure of currents at 5 rn depth between the North Pacific high and the thermalow reaches is analyzed, and large-scale and local characteristics of the a maximum. Compared to the spring, the subtropical high flow field are identified. Then the vertical structure of curis displaced farther toward the north. This displacement rents is described. The seasonal cycle of 5-m currents at the only slight, but the resulting change in the direction of the eastern entrance (ANMI) is in phase with the seasonal cycles geostrophic wind along the central California coast (from of currents south of Point Arguello (CAMP) and off Point southeastward in the spring to southward in the summer) Sal (PAIN) (Figure 5). Monthly mean currents at ANMI and cause gradients in the wind field near Point Conception that CAMP are of similar magnitude and are directed equatorare stronger and occur over a shorter distance than in the ward in the spring and poleward from summer through winspring. In the fall the North Pacific high weakens and moves ter. At PAIN, equatorward flow persists through the sumtoward the south, and early storm systems penetrate the area. mer. The sense of flow depicted by large-scale patterns of The most energetic wind fluctuations off Point Conception seasonally averaged geostrophicurrents in this area [Chelare comparable in magnitude to those in the summer, yet ton, 1984; Lynn and Simpson, 1987] agrees with the directhese fluctuations rarely last longer than a few days and are tion of flow at ANMI, CAMP, and PAIN. The seasonal cy-

HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 3049 $5.0oN - 34. 34. OøN 33.5oN...,..., 121.0øW 120.5øW 120.0øW 119.5øW 119.0øW 118.5øW Figure 4. Seasonal cycle of wind stress in the Santa Barbara Channel region. The monthly mean time series are rotated for convenience. The direction of each arrow is the actual direction of the mean wind stress for that particular month. Arrows are proportional to the wind stress magnitude during individual months in pascals. Solid circles next to the time serie show the actualocation of the measurement sites. The beginning of the year is indicated by "J"(January). c!es of alongshelf flow at these three stations are further in to that of wind stress at NDBC buoy 54. Currents on the phase with the seasonal cycles of long-term averaged along- northern shelf (SMIN, ROIN, and GOIN) are directed westshelf relative geostrophic velocities computed from Cali- ward anytime during the year and have maximum monthly fornia Cooperative Oceanic Fisheries Investigations (Cal- means in the summer and early fall. COFI) hydrographic observations obtained outside the chan- Seasonal fluctuations in the vertical structure of currents nel between 1949 and 1995 (Figure 5, open arrows). These in the western (NDBC 54) and eastern (NDBC 53) channel comparisons suggest that on seasonal timescales the flow are illustrated in Figure 6. The monthly mean vertical velocat ANMI, CAMP, and PAIN is representative of the larger- ity profiles in the western and eastern channel are similar to than-sbc scale flow. each other in late winter and early spring. During this time The seasonal variability of near-surface currents in the of the year the flow is strongest near the surface and con- SBC appears to be a superposition of alongshelf currents siderably sheared in the vertical from the surface to a depth with scales larger than the SBC (e.g., the seasonal cycles of about! 00 m. Minimum velocities occur at middepth beof monthly means at ANMI and PAIN have the same phase) tween 150 and 250 m. From late spring through fall the verand a cycloni circulation which is specific to the channel tical structures of currents in the western and eastern SBC interion Monthly averaged currents at ROOF and GOOF on differ. In the western channel (NDBC 54), strongest curthe southern shelf are eastward year-round. Maximum east- rents occur near the surface. The near-surface flow rotates ward currents occur in the spring when the flow on scales clockwise from eastward in the spring to westward as the larger than the SBC is equatorward, and smallest eastward year progresses. The vertical shear in the upper 100 m of the currents occur in the winter when the larger-than-sbc scale water column is much less than in the spring. In the eastern flow has its maximum poleward velocities. North of San channel (NDBC 53), currents are directed westward from Miguel Island, at SMOF, monthly mean currents are directed late spring through fall from the surface to a depth of about toward the southeast, and the seasonal cycle phase is similar - 250 m. In late spring the flow exhibits a subsurface maxi-

3050 HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 35. OøN 34.5øN 34.0øN 33.5oN.,.,. 121.0øW 120. 5øW 120. OøW 119. 5øW 118. 5øW Figure 5. Seasonal cycle of 5-m currents in the Santa Barbara Channel region. The monthly mean time series are rotated for convenience. The direction of each arrow is the actual direction of the mean current for that particular month. Arrows are proportional to the current magnitude during individual months in m s-. The locations of the time series correspond to the buoy locations. The beginning of the year is indicated by "J". Monthly averaged alongshelf geostrophic velocities (open arrows) at the surface relative to 500 dbar computed from California Cooperative Oceanic Fisheries Investigations (CalCOFI) hydrographic observations obtained between 1949 and 199 are shown at two locations. mum at a depth of about 140 m. From summer through fall, westward flow near the surface increasesteadily until the near-surface currents exceed the currents at middepth. Even though the deep flows at NDBC buoys 54 and 53 change direction seasonally, the flow at ANMI at 100 m depth (not shown) is always into the $BC (poleward). Contour maps of monthly averaged temperatures 1 m beern portion of the channel until midsummer. Temperatures on the northern shelf start to increase at the beginning of the summer. The increase occurs concurrently with the flow reversal at ANMI (from equatorward poleward) and with the strengthening of westward flow on the northern shelf. Temperature gradients between colder waters in the southwestern channel and warmer waters in the northeastern channel are neath the surface are illustrated in Figure 7. The warmest greatest from June through September. Cross-channel temtemperatures are found in the Southern California Bight during the summer, as could be expected from the yearly and perature gradient start to decrease in fall as colder waters around Points Conception and Arguello and off the south latitudinal cycles of heating and cooling in combination with central California coast mix with warmer waters of eastern the year-round sheltering from strong wind forcing. Min- origin. imum temperatures occur off the south central California coast and around Point Conception in the spring, as a result of the upwelling processes driven by equatorward winds. The seasonal variability of SSP at selected mooring locations is illustrated in Figure 8. S$P is computed from bottom pressure (Pro0) by the addition of the baroclinic pressure In the channel, temperatures increase from the southwest correction, which is derived from temperature measurements toward the northeast. Horizontal gradients in near-surface [Harms and Winant, 1994]. The computed SSP signal is temperature are smallest in the winter when winds are weak. In early spring, colder waters appear off Points Arguello and Conception and spread eastward over the southern shelf of compared to seasonal fluctuations in measured bottom p.ressure, computed baroclinic pressure (SSP-P 00), and wind stress. Minimum S$Ps occur in the spring, and maximum the $BC. These colder waters are apparent in the southwest- SSPs occur in the winter. There is a secondary maximum in

HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 3051 January March Ma O. OS m. July September November Figure 6. Seasonal fluctuations in the vertical structure of currents at NDBC buoys 53 and 54. Monthly averaged ADCP velocities between 24 and 328 m depth are shown.for eve other month. The vertical resolution is 16 m. Arrows are proportional to the current magnitude n m s TM. late summer and early fall. In early spring, SSPs decrease (May) is associated with two processes: (1) a decrease in more or less simultaneously with the onset of equatorward SSP at PAIN in response to persistent upwelling-favorable wind stress. The decrease in SSP is due to a decrease in wind stress north of Point Conception and (2) an increase both bottom pressure and baroclinic pressure. Starting late in SSP at BARB in the absence of strong wind forcing in spring, SSPs rise steadily until fall, despite the fact that wind the Southern California Bight. The SSP difference is fully stress off Point Conception is still strong and upwelling fa- established by early summer, when equatorward wind stress vorable. The rise in SSP is associated with a rise in both bot- off Point Conception has maximum monthly mean values, tom pressure and baroclinic pressure. The increase bottom and remains strong through August. Starting in early fall, pressure occur simultaneously at all stations. The increase the SSP difference decreases concurrently with a weakening in baroclinic pressure occurs first at the southernmost station of wind stress off Point Conception. (BARB) and about 1 month later at the northernmost station The flow reversal through the eastern entrance in late (PAIN). spring and the increase in westward flow on the northern The increase in equatorward 5-m flow at PAIN, CAMP, shelf in late spring and summer appear simultaneously with and ANMI in early spring occurs concurrently with the in- the strengthening of the BARB-PAIN SSP difference. The crease in equatorward wind stress and the decrease in near- agreement of the seasonal cycle phases of the along-channel surface temperatures and SSPs and might be interpreted as SSP difference and currents on the northern shelf (Figure 5) the result of wind-generated coastal upwelling. In contrast, and the existence of a subsurface westward velocity maxthe flow reversal at the eastern entrance and the strengthen- imum in the eastern channel (NDBC 53) in summer and ing of westward flow on the northern shelf in the summer and fall when the pressure difference is large (Figure 6) suggest early fall occurs at a time when wind stress off Point Concep- that the subsurface pressure gradient along the channel can tion is still strong and upwelling favorable. Why do currents be important in driving the flow poleward in regions where and associated SSP values reverse befor equatorward wind wind forcing is weak. stress has reached its peak? The mean along-channel SSP difference between BARB and PAIN (Figure 9) is positive 6. Characteristic Patterns of the Circulation throughout the year, corresponding to a poleward pressure gradient, and is due mostly to alongshelf differences in baro- 6.1. Synoptic Views clinic pressure. The BARB-PAIN SSP difference opposes A synoptic description of the circulation in the SBC, simthe wind stress year around, and on seasonal timescales it is ilar to the synoptic views identified by meteorological forenearly a mirror image of wind stress at NDBC buoy 54 (Fig- casters, is first presented based on daily averages of the 5- ures 8 and 9). The SSP difference increases late spring and 45-m current fields. These were computed for each and summer, concurrently with an increase the alongshelf mooring, and sequences of daily averaged horizontal maps wind stress. The setup of the SSP difference in late spring were examined to identify characteristic flow patterns in the

3052 HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL $$.0'N $4.$ N ß $4.0 N - $$.$øn' ]a.nua,ry... { F. eb.r.a,ry... ø $$.0'N $4.5*N- $4.0oN ß 35.$*N- $$.0'N. $4.$*N' 34.0'N' $3. $*N' 121'W! November 12low 120øVe ' 119øW 1210W!........, :: :: :: :, December 120*IF ' 1190W' Figure 7. Contour maps of monthly averaged 1-m temperatures. Temperature units are degrees Celsius. The contour interval is 0.5øC. channel. The different observations of circulation in the SBC, spanning the period from October 1992 through January 1996, can be sorted into six different states or synoptic views: Upwelling, Relaxation, Cyclonic, Propagating Cyclones, Flood East, and Flood West. About 60% of the observations can be categorized in terms of these six flow regimes. The remaining observations cannot be clearly identified with one flow regime or another and either mark transition periods between synoptic states or involve flow features of a scale smaller than the moored array can resolve. The circulation in the SBC always tends to be cyclonic. The cyclonic component of flow is strongest in summer and weakest in winter. The Upwelling, Relaxation, Cyclonic, and Propagating Cyclones flow regimes occur pre-' dominantly from spring through fall. The Upwelling synoptic pattern (Figure 10a) describes a condition in which the circulation is composed of large equatorward (alongshelf) currents at either end of the channel and along its southern boundary, while the flow along the northern boundary is weakly toward the west. The Relaxation pattern (Figure 10b) consists of a strong westward jet from the eastern entrance and continuing along the northern side of the channel past Point Conception, while the flow along the southwestern shelf is weakly toward the east. The Cyclonic pat-

HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 3053 tern (Figure 10c) describes a condition which consists of currents with opposite orientation and similar speeds on opposite sides of the channel. Propagating Cyclones (Figure 10d) describes recirculating flow trajectories involving smaller cyclonic eddies which appear to slowly drift toward the west. During the Cyclonic and Propagating Cyclones flow regimes a cold squirt (the"santa Rosa cold squirt" [Hendershott and Winant, 1996]) is frequently observed on the southeastern side of the cyclonic eddy, as colder water is drawn eastward on the southern shelf. In the winter, two different flow regimes occur. Flood East (Figure 10e) describes a condition in which the flow is directed everywhereastward. Flood West (Figure 10f) is the converse of the Flood East and exists when the flow is directed everywhere westward. Eastward flow during a Flood East tends to be stronger on the southern shelf than on the northern shelf, and westward flow 1.01 ' ' " ' ' ' ' ' ' ' I 0.3 t nm.pnm a L... 0.1 0.0 0.0-0.1 -O. 5 -O.2-1.0,,,,,,,,,,, -0.3 0.5 I I I I I I I I I I v BARB.. v PAIN 0.0 -....*"'"','"' -0.5' -.... z',,,, 1 ß $$p o P oo v $$p. P oo. -,....._0..-'0"-0---0........ I (a) PAIN -2 i i i I I I I I i I I 1-2 -- F,C.--O.....0'-...... --.. I I I I I I I I I I ß BARB 0.5 o PAIN 0.0 '...,. - -0.5 Ploo I ' I I I '1 I [ I i I 1 2 3 4 5 6 7' 8 9 101112 Month Figure 9. (a) SSP, bottom pressure (P oo), and biaroclinic pressure (SSP- P o0) differences between BARB and PAIN. A positive pressure difference corresponds to a poleward pressure gradient. Units are kilopascals. The shaded time series representseasonal variations in the alongshelf (304øN) wind stress at NDBC buoy 54 in units Pa. Note that the along-channel SSP difference opposes the wind stre s year-round. (b) Baroclinic pressures (SSP- Pmo) at BARB and PAIN. (c) Bottom pressures (P oo) at BARB and PAIN. 1-1 (c) BARB.2 --, i i i I I i '1 I I I 0.3 0.1 0.2 * s4 ø xs m xs4- xs -0.2 during a Flood West is generally stronger on the northern shelf than on the southern shelf. Flood East and Flood West differ from the Upwelling and Relaxation patterns in that the flow described by the former two regimes is unidirectional, while the flow described by the latter two patterns is opposite in direction on opposite sides of the channel. Flood East and Flood West events are typically of shorter duration than the more persistent Upwelling and Relaxation flow regimes. Drifter studies conducted as part of the experiment confirm the existence of each of the six flow regimes. 6.2. EOF Patterns of Near-Surface Currents Empirical orthogonal functions (EOFs) provide an objective means to synthesize the moored current measurements 1 2 3 4 5 6 7 8 9 101112 and to establish the principal patterns of current variability Month in the SBC. The 5-m current field was analyzed according to Figure 8. (a)-(c) Seasonal variability of SSP, bottom pres- the methods described in the appendix. Our discussion fosure (J:'zoo), and baroclinic pressure (SSP- J:'zoo) at selected cuses on the year 1994 because it includes current observastations. Mean values are relative to the 3-year mean. Units tions in the Santa Maria and Santa Monica Basins. The 1994 are kilopascals. (d) Seasonal fluctuations in alongshelf wind stress at NDBC buoys 54 and 53 in pascals. The difference results were compared to results obtained during 1993 and between both time series is shaded. 1995 and found to be qualitatively and quantitatively simi-

3054 HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL UPWELLING/RELAXATION CYCLONIC FLOODS :'"' ' :': "'"'"'., ß,... "-* (a) Figure 10. Schematic diagram of the six synoptic views of circulation in the Santa Barbara Channel. (a) Upwelling, (b) Relaxation, (c) Cyclonic, (d) Propagating Cyclones, (e) Flood East, and (f) Flood West. lat. EOF analyses applied to each season separately produce the same significant modes but swap the order of the modes, indicating that individual characteristic patterns of circulation have a preferred season of occurrence. The matrix of covariance coefficients computed between the low-frequency components of 5-m currents at each measurement site is formed for the common time period from January 1 to December 31, 1994. The first three modes are resolved (appendix), and together account for 50% of the low-frequency 5-m current variance in 1994. The other half of the variance is presumably due to flow features of a scale smaller than the array can resolve. The first three current modes account for 23%, 16%, and 11% of the total lowfrequency variance over the 1994 year. The spatial pattern of each of the first three modes is illustrated in Figure 11. In order to reconcile the EOF description (which excludes the mean flow) with the subjective analysis (section 6.1) the spatial distribution of the amplitude is represented in three ways. Figures 1 l a, 1 l d, and 1 l g representhe standar deviation of the mode, i.e., the eigenvector components of the mode scaled by the square root of the corresponding eigenvalue. Figures 1 lb, 1 le, and 1 lh show the vector sum of the mean velocity plus the standardeviation of the mode. Figures 1 lc, 1 lf, and 1 li show the vector sum of the mean velocity minus the standar deviation of the mode. The mean circulation consists mainly of a cyclonic eddy centered in the western portion of the SBC and is essentially identical to the mean circulation computed over the 3-year period (Figure 3d). The first 5-m current mode corresponds to a spatial pattern of current fluctuation which is uniform in sign and primarily oriented in the alongshelf direction (Figure 11 a; Table 4). The largest variability occurs in the upwelling regions off south central California and off Point Conception and at the eastern entrance of the SBC. Current fluctuations over the central northern shelf are small in this mode. Adding the standar deviation of mode 1 to the mean (Figure 1 lb) results in a flow pattern that is similar to the Upwelling synoptic pattern (Figure 10a). Subtracting the standar deviation of mode 1 from the mean (Figure 1 l c) results in a circulation pattern that is similar to the Relaxation synoptic pattern (Figure 10b). The amplitude of mode 1 (Figure 12a)is persistently positive from March through mid-may. Starting in mid-may, the mode frequently changesign and the temporal evolution of the mode is dominated by fluctuations with periods of the order of 1-2 weeks. Fluctuations appear more energetic in summer and early fall and less energetic in late fall and winter. The second mode consists of currents which are opposite in direction on opposite sides of the channel (Figure 1 ld). In the middle of the channel the amplitude and direction of this mode are such as to close the circulation pattern. Cur- rent fluctuations at the eastern entrance and in Santa Monica Bay are small in this mode. The spatial pattern of mode 2 resembles closely that of the mean currents. When the amplitude of mode 2 is added to the mean (Figure 1 l e), the circulation pattern is similar to the Cyclonic synoptic pattern (Figure 10c). When the amplitude of mode 2 is subtracted from the mean (Figure 1 If), the circulation represents a state in which velocities are everywhere weak. On seasonal timescales the intensity of the cyclonic circulation described by this mode (Figure 12b) increases from March through May, is largest from June through September, and decreases after September. In mid-january and February the amplitude of mode 2 is negative, in accordance with the observation that the cyclonic flow is weakest during winter. Flood East and Flood West (Figures 10e and 10f) occur typically during the winter when the cyclonic component of the

o.. HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 3055 1. Mode: 23% j 2. Mode: 16% 3. Mode: 11% Mean + 1. Mode Mean + 2. Mode ß Mean + 3. Mode. 35.0W 33.4W 121.0'W Figure 11. Spatial structures of the first three empirical orthogonal eigenfunctions of the low-frequency 5-m currents during 1994. The spatial structures are shown in conjunction with the mean 5-m current field. Figures i 1 a, 11 d, and 1 lg represent the standardeviation of the mode, i.e., the eigenvector components of the mode scaled by the square root of the corresponding eigenvalue. Figures 1 lb, 1 le, and 1 lh show the vector sum of the mean velocity plus the standar deviation of the mode. Figures 1 lc, 1 lf, and 1 li show the vector sum of the mean velocity minus the standardeviation of the mode. The 1994 mean circulation is similar to the 3-year mean displayed in Figure 3d. The first three modes describe 23%, 16%, and 11% of the total low-frequency variance in 1994. flow is smallest. Superposed on the seasonal cycle are low- northern shelf and eastward on the southern shelf) north of frequency fluctuations with periods of several weeks which Santa Rosa Island and southward flow near the western endominate mode 2 from summer thi'ough winter. Energetic trance. Currents in the eastern SBC are directed everywhere fluctuations higher frequencies (periods of the order of 1 westward. Mode 3 is interpreted to describe cyclonic eddies week) occur in spring. of limited extent at different locations in the SBC, consistent The third 5-m current mode describes circulation features with the synoptic state Propagating Cyclones. Events dewith smaller spatial scales than the first two modes. The scribed by the third current mode (Figure 12c) are typically third mode consists of current fluctuations which, across of shorter duration (periods of about 1 week) than events deany given north-south.section, are opposite in direction (Fig- scribed by modes 1 and 2. Fluctuations are most energetic ure 11 g). Current fluctuations at nearby stations on the same from summer through midwinter and less energetic in late shelf also oppos each other. Mode 3 has maximum ampli- winter and spring. tude on the northwestern shelf and north of Santa Rosa and Santa Cruz Islands. Adding the standardeviation of mode 3 to the mean (Figure 1 lh) results in a circulation cohsisting of strong opposing flows (westward on the northern shelf and eastward on the southern shelf) north of San Miguel Island and north of Santa Cruz Island, while the flow north of Santa Rosa Island is weaken Subtracting the standardeviation of mode 3 from the mean (Figure 1 l i) produces a circulation pattern consisting of strong opposing flows (westward on the 6.3. Sequence of Synoptic States During the summer and fall seasons (May through October) the first two 5-m current modes have a maximum correlation of 0.58, with the second mode lagging the first by 102 hours or about 4 days (Figure 12d), suggesting that in combination with the mean flow these modes describe a re- peating pattern of circulation with an approximate period of

3056 HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL Table 4. Empirical Orthogonal Function Analysis of the 5-m Currents Station Mean Mode 1, 23 % Mode 2, 16 % Mode 3, 11% East North East North East North East North Aij % Aij % Aij % Aij % Aij % Aij % PAIN -0.06-0.07 0.01 i -0.16 44 0.02 3-0.05 3 0.02 2-0.02 0 SMIN -0.21-0.07 0.12 26-0.07 18-0.07 7-0.01 0-0.15 30 0.05 7 ROIN -0.20 0.02 0.02 I -0.02 6-0.15 55 0.02 6 0.05 4 0.01 1 GOIN -0.13 0.03 0.00 0-0.01 I -0.15 54 0.05 17-0.07 9 0.01 0 CAIN -0.07 0.05 0.07 28-0.04 8-0.02 3 0.05 8 0.01 0 0.03 3 ANMI 0.00 0.06 0.16 68-0.16 55 0.00 0 0.04 3 0.00 0-0.02 1 GOOF 0.08-0.02 0.11 29 0.00 0 0.02 I 0.03 7 0.14 36 0.02 1 ROOF 0.15-0.03 0.06 12-0.04 15 0.10 35 0.02 3-0.01 0-0.05 16 SMOF 0.07-0.12-0.03 3-0.05 11 0.05 6-0.09 29 0.02 I 0.00 0 BARB 0.04-0.02 0.03 18-0.03 12-0.01 I 0.02 4 0.01 I -0.01 1 NC54-0.03-0.04 0.02 I -0.02 i -0.08 19-0.08 27 0.13 44 0.06 17 NC53-0.03 0.03 0.06 19 0.01 0 0.04 8 0.03 5-0.04 7 0.01 0 Mean 5-m currents at the mooring sites during 1994, amplitude, and the percent variance in each time series explained by the first three 5-m current eigenfunctions in 1994. Units are m s -. 16 days. From May through October both modes are dominated by fluctuations in the 10-to-25-day band. When mode 1 has maximum temporal values, the circulation is composed of large equatorward (alongshore) currents at either end of the channel and along its southern boundary, while the flow along the northern boundary is weakly toward the west. About 4 days later, mode 2 reaches maximum positive values, and the amplitude of mode 1 is near zero. At... I,,,,,,, I,,,,,,, I,,,,,,, I,,,,,,, I,,:',::,,,, I,,,,,,, I,,, Jan l Feb26 0r23 Jun18 Aug13 Oct8 Dec3 1994 Ticks are 1 week arart Figure 12. (a)-(c) Temporal amplitudes of the three largest 5-m current eigenfunctions for 1994. The eigenfunctions are normalized so that their variance is 1. From May to October the two largest modes are significantly correlated (Cmax = 0.58) when the second mode lags the first by 4 days. The 95% and 99% significance levels are 0.33 and 0.43, respectively. (d) The temporal amplitudes of the first and second 5-m current eigenfunctions, where the second mode is lagged relative to the first by about 4 days. The vertical dashed lines mark the times of the events illustrated in Figure 13.

HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 3057 this time the current north of the $BC and north of the chan- 6.4. Propagating Cyclones nel islands is still equatorward, but westward flow along the northern boundary has increased. After 4 more days, when Drifter trajectories and sequences of AVHRR satellite imthe first mode reaches maximum negative values and the am- ages frequently show the presence of smaller cyclonic eddies plitude of mode 2 is near zero, the flow is everywhere weak in the SBC. These structures tend to drift slowly toward the with the exception of a strong poleward jet which begins at west. The maximum lagged correlations between the alongthe eastern entrance and continues along the northern side shelf component of the 5-m currents, computed for the 10- of the channel to Point Conception. Another 4 days later, to-25-day band, are summarized in Figure 14. Alongshelf when mode 2 reaches maximum negative values and the am- currents on opposite shelves are significantly anticorrelated. plitude of mode 1 is near zero, the flow described by these This anticorrelation between alongshelf currents on opposite shelves is maximum when currents on the southern shelf two modes is everywhere weak. After about 16 days this circulation pattern repeats itself. lead currents on the northern shelf by 1 to 3 days, i.e., an This sequence of synoptic states is illustrated by an event increase in eastward flow at GOOF (ROOF, SMOF) is genwhich lasted from August 28 through September 8, 1994 erally succeeded by an increase in westward flow at GOIN (Figure 13, solid arrows). On August 28 the flow is very (ROIN, SMIN) shortly afterward. Alongshelf currents on similar to the Upwelling pattern depicted by the mean-plus- the same shelf are significantly correlated when currents at stations in the east lead currents at the nearest station to the mode 1 flow field. In the course of the following few days the flow at the eastern entrance shifts from equatorward west by 4-5 days. On the northern shelf an increase westpoleward. On September 1, the 5-m circulation resembles ward flow at ROIN (SMIN) is often preceded by an increase the Cyclonic pattern depicted by the mean-plus-mode 2 flow in westward flow at GOIN (ROIN). Similarly, on the southfield. From September 1 to September 5 the magnitude of ern shelf an increase eastward flow at ROOF (SMOF) is the poleward flow along the northern shelf increases. On generally preceded by an increase in eastward flow at GOOF September 5 the flow is dominated by a poleward jet which (ROOF). These results are consistent with the notion that the characterizes the Relaxation pattern depicted by the mean- circulation in the SBC results at times from a field of cyminus-mode 1 flow field. The flow field reconstructed from clonic eddies of limited horizontal extent which move from modes 1 and 2, and the mean field (open arrows) captures the easto west with a velocity of about 0.06 rn s -x. dominant flow features presented in these daily snapshots of A sequence of AVHRR satellite images, representing the the 5-m circulation. On September 8 the 5-m circulation sea surface temperature for each day from August 7 through consists of flow features of a scale smaller than those de- 14, 1994, show cyclonic motion in the Santa Barbara Chanscribed by the two largest 5-m current EOFs. nel (Plate 1 a). On August 7 an eddy is observed in the central $$.0øN $4.6øN $4.2ø2V Figure 13. Sequence of daily averaged maps of 5-m currents (solid arrows) for every fourth day between August 28 and September 8, 1994. These daily snapshots of the 5-m circulation are compared to daily averages of the 5-m current field reconstructed from EOF modes 1 and 2 and the mean field (open arrows). The times of the events illustrated in this figure are marked in Figure 12 by the vertical dashed lines.

3058 HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL. maximum lagged correlation. --- regression coefficient x.4.0. 0.64/ lag [days] -0.65 I O. Io. I o. I ' " I X Because of instrument failure the analysis performed over a different and shorter time block than the analysis of the 5-m currents. The period analyzed is March 15 to October 31, 1995. To allow comparison between the 5- and 45- m current fields, the EOF analysis for the 5-m current field is repeated for the same time block. The 5-m results (Table 5b) Figure 14. Maximum lagged correlation, regression coefficient, and lag between alongshelf 5-m currents during summer and fall. Displayed are results for the bandpass-filtered time series (10d < T < 25d). Solid arrows point from the leading to the lagging station. Shaded arrows point in the direction of the flow. The 95% and 99% significance levels are 0.43 and 0.56, respectively. The correlations between alongshelf currents on the same shelf are not significant at zero lag. SBC north of Santa Rosa Island. Over the next few days this eddy moves westward. On August 10 the eddy is located north of San Miguel Island, and on August 11 it is located west of San Miguel Island. As this eddy leaves the channel through the western mouth, a second eddy appears in the eastern channel north of the western tip of Santa Cruz Island. The second eddy also travels westward during the next few days. The slope of the lines, connecting the circles which subjectively define the center of the eddies at each time of the sequence, suggests an average traveling speed of 0.06 m s -, consistent with the results obtained from the lagged correlation analysis. The features of the eddy field may be illustrated with a partial field reconstruction based on mode 3 and the mean flow. Shown along with the sequence of AVHRR images are daily averaged maps of the reconstructed mode 3- plus-mean flow field for each day from August 7 through 14, 1994 (Plate lb). At each time of the sequence the regions of maximum cross-shelf shear between the alongshelf currents match the positions of the eddies identified in the corresponding AVHRR image. 6.5. Currents at 45 m Depth The EOF analysis of the 45-m current field is based on subtidal current observations from nine moorings (Table 5a). Plate 1. (a) Sequence of AVHRR satellite images show- ing the sea surface temperature for each day from August 7 through 14, 1994. Red circles define the center of the eddies at each time of the sequence. Verticalines connecting maps indicate subjective "best" fits to the centers of the eddies. (b) Sequence of daily averages of the reconstructed mode 3-plus-mean flow field for each day between August 7 and 14. Red circles define the location of maximum shear between currents on opposite shelves at each time of the sequence. Verticalines connecting maps indicate subjective "best" fits to the locations of maximum hori- zontal shear. 08 Aug 9 09Aug 94 10 A 9 11 Aug 94 12Aug94 13Aug94 14 AUg 94

HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 3059 Table 5a. Empirical Orthogonal Function Analysis of the 45-m Currents Station Mean Mode 1, 38 % Mode 2, 15 % East North East North East North Aij % Aij % Aij % Aij % PAIN 0.05 0.00 0.01 2-0.05 7 0.01 I -0.03 3 SMIN -0.15 0.03 0.17 52-0.03 9-0.13 22 0.02 6 ROIN -0.19 0.04 0.10 26-0.02 18 0.16 52-0.01 4 GOIN -0.16 0.04 0.11 42-0.01 4 0.02 I -0.01 2 CAIN -0.12 0.10 0.11 49-0.08 45 0.02 I -0.02 2 ANMI -0.06 0.11 0.12 35-0.11 33 0.03 2-0.02 1 GOOF 0.04 0.03 0.01 0 0.01 0 0.05 11 0.01 2 ROOF 0.11 0.03 0.01 I -0.01 3-0.05 10-0.01 5 SMOF 0.15-0.02-0.10 36-0.01 2 0.08 15 0.04 14 Mean 45-m currents at nine mooring sites from March 15 to October 31, 1995, amplitude, and the percent variance in each time series explained by the first two 45-m current eigenfunctions during the same time period. Units are m s-. are consistent with the results described in the previous three sections. northern boundary of the channel, between Port Hueneme and Point Conception. Current fluctuations north of Santa The first two 45-rn current modes are resolved (appendix) Rosa and Santa Cruz Islands and north of Point Concepand account for 38% and 15% of the low-frequency 45-m tion are small in this mode. When the amplitude of mode 1 current variance. The first mode of the 45-m current field is added to the mean (Figure 15b), the circulation is weak accounts for nearly as much of the variance as the first two everywhere. When the amplitude of mode 1 is subtracted 5-m current modes combined. The March through October from the mean (Figure 15c), the circulation is composed of a mean 45-m circulation consists of a strong and continuous strong poleward jet along the northern boundary that begins westward jet on the northern shelf between Port Hueneme at the eastern entrance and continues along the northern shelf and Point Conception and weaker eastward return flow over the southwestern shelf. This mean circulation is similar to the mean 45-rn circulation computed over the 3-year period (Figure 3e). The first 45-rn current mode corresponds to a spatial pattern of current fluctuation which is oriented in of the SBC to Point Conception. Eastward flow is observed north of San Miguel Island. The flow on the southern shelf east of San Miguel Island is weak. This pattern is very similar to the Relaxation synoptic pattern (Figure 10b). From June through Augusthe amplitude of the first 45-rn current the alongshelf direction and, with the exception of currents mode is persistently negative, indicating that the poleward north of San Miguel Island, is uniform in sign (Figure 15a). The variability is largest and uniform in amplitude along the jet is strongest during this period. The first 45-rn current mode is significantly related to the first current mode at 5 rn Table 5b. Empirical Orthogonal Function Analysis of the 5-m Currents Station Mean Mode 1, 26 % Mode 2, 14 % Mode 3, 13 % East North East North East North East North Aij % Ai.i % Ai.i % Ai.i % Ai.i % Aij % PAIN -0.04-0.09-0.02 1-0.03 3-0.04 10-0.05 7 0.01 1-0.02 1 SMIN -0.20-0.11 0.23 57-0.02 2-0.01 0-0.07 20-0.09 7-0.04 5 ROIN -0.22 0.01 0.12 32-0.05 24-0.10 19 0.01 I 0.14 32 0.00 0 GOIN -0.11 0.02 0.07 13-0.02 6-0.12 38 0.02 4-0.02 I 0.00 0 CAIN -0.01-0.01 0.03 10-0.02 3-0.01 I 0.02 4 0.02 3 0.00 0 ANMI 0.05-0.01 0.10 32-0.15 47 0.02 I -0.07 10 0.02 I 0.00 0 GOOF 0.12-0.03 0.06 8 0.02 2 0.14 34 0.02 3 0.13 27 0.02 2 ROOF 0.21-0.03-0.01 0-0.04 14 0.11 34-0.01 I -0.09 20-0.05 17 SMOF 0.11-0.12-0.07 13-0.02 2 0.01 0-0.07 11 0.12 33 0.05 7 To allow comparison between the 5- and 45-m current fields, the EOF analysis for the 5-m current field is repeated for the period from March 15 to October 31, 1995. Units are m s -.

3060 HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 1. Mode: 38% ß 33.4W ean. 1. Mode... ß... 121. O'W 118.5'W Mean. 2. Mode. Figure 15. Spatial structures of the first two empirical orthogonal eigenfunctions of the low-frequency 45-m currents. The analysis period is March 15 to October 31, 1995. Figures 15a and 15d represent the standardeviation of the mode, i.e., the eigenvector components of the mode scaled by the square root of the corresponding eigenvalue. Figures 15b and 15e show the vector sum of the mean velocity plus the standardeviation of the mode. Figures 15c and 15f show the vector sum of the mean velocity minus the standardeviation of the mode. The mean 45-m circulation for this period is similar to the 3-year mean illustrated in Figure 3e. The first two modes describe 38% and 15% of the total low-frequency variance. depth. The time series of these two modes has a zero lag 7. Discussion correlation of 0.76. The principal difference between the There is general agreement that wind stress is the immode 1 current patterns 5 and 45 m depth is that the con- portant forcing agent for the surface flow in the southwesttinuousequatorwardjetnorth of Point Conception and along ern portion of the SBC, an area exposed to strong wind the southern boundary of the channel, which dominates the forcing throughout the year. It is impossible, however, to 5-m circulation during Upwelling events (Figure 1 lb), is di- rationalizeither the seasonal cycle of the circulation or minished at 45 m depth. shorter period variability within the channel, where winds The second 45-m current mode consists of current fiuctua- are generally weak, solely on the basis of wind stress fluetions which, across any given north-south section, are oppo- tuations. Observations suggest that horizontal differences site in direction (Figure 15d). Current fluctuations nearby in near-surface temperature and related pressure over scales ß stations on the same shelf tend to oppos each other. The somewhat larger than the SBC constitute an additional foremode has maximum amplitude on the northwestern shelf. ing agent for the surface flow at timescales from seasonal The spatial distributions of amplitude of the second 45-m to days. At seasonal timescales the equatorward wind stress and the third 5-m current modes are similar, and their time and poleward SSP difference oppos each other. At subtidal dependencies are significantly correlated zero lag (0.81). frequencies the time series of wind stress off Point Concep- Interpreting these two modes to describe cyclonic eddies of tion and along-channel SSP difference have a maximum ½orlimited extent at different locations in the SBC, these results relation of -0.71, with the SSP difference lagging the wind suggest that the smaller cyclonic eddies, which were shown stress by 1 day. to travel toward the west at a speed of about 0.06 m s -t, ex- The most direct demonstration that both the wind stress tend from the surface to some depth below the thermocline. and the pressure gradient are important in driving the flow

HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 3061 in the Santa Barbara Channel comes from sorting the current data by the strength of the wind stress and the SSP difference through the channel (Figure 16). The component of the wind stress toward 124 ø N at NDBC buoy 54 is chosen to represent the strength of wind forcing over the wind stress results in an increase of equatorward flow north of Point Conception and along the island boundary and results in a decrease in poleward flow through the eastern entrance and along the northern shelf of the SBC. An increase in the poleward SSP gradient is associated with a strengthwestern channel. The SSP difference between GOIN and ening of poleward flow through the eastern entrance and PAIN is chosen to approximate the alongshore pressure gradient. The 5-m circulation changes systematically as the relative strengths of NDBC 54 wind stress and the GOIN- PAIN SSP difference change. The flow in the SBC always along the northern boundary of the SBC and is associated with a decrease of equatorward flow north of Point Conception and along the southern side of the channel. The Upwelling regime typically prevails when upwelling-favorable has a cyclonic tendency. An increase upwelling-favorable wind stress is strong (> 0.1 Pa) and when the SSP difference Equatorward Component of NDBC 54 Wind Stress [Pa] along 124øN 0.5 > x > 0.3 0.3 > x > 0.1 0.1 > x >.0.1.0.1 > x > -0.5 A,,,,..... :.... Figure 16. Averaged 5-m current velocity (m s - ) as a function of equatorward wind stress (in pascals) at NDBC 54 along 124 ø N and the along-channel SSP difference (102 Pa) between GOIN and PAIN. The analysis period is January 1, 1994, to December 31, 1995. The ranges for the wind stress and the SSP difference, displayed along the top for wind stress and along the left for the SSP difference, span all values observed during the years 1994 and 1995. The number of realizations (i.e., hours) of each combination of forcing is shown in the upperight corner of each panel.

3062 HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL through the channel is small (< 2 x 102 Pa). The Relaxation mean shear is small, i.e., from late winter to early spring. regime occurs when upwelling-favorable wind stress is weak Towed ADCP and hydrographic surveys in the channel doc- (< 0.1 Pa) and when the along-channel SSP difference is umenthat this cyclonicirculation extends frequently to the large (> 2 x 102 Pa). Cyclonic flow is strongest when both bottom. the upwelling-favorable wind stress and the SSP difference Empirical orthogonal functions (EOFs) are used to estabare large (wind stress > 0.1 Pa; SSP difference > 2 x 102 lish the characteristic patterns of the circulation in the SBC. Pa). The two synoptic states Flood East and Flood West are The Upwelling, Cyclonic, and Relaxation synoptic patterns not clearly evident in Figure 16. Examination of daily aver- are described by the first two 5-m current EOFs. Upwelling aged maps of the 5-m current suggest that Flood East occurs represents a circulation pattern which is composed of large in response to an equatorward wind stress when gradients in equatorward (alongshore) currents at either end of the chanthe wind stress field are small and when SSP gradients are nel and along its southern boundary, while the flow along the in the same direction as the wind stress. Flood West occurs northern boundary is weakly toward the west. The Cyclonic in response to a poleward wind stress when gradients in the pattern consists of a singular cell cyclonic gyre centered in wind stress field are small and when SSP gradients are also the central and western SBC. The Relaxation pattern is domdirected poleward. inated by a strong poleward (alongshore)jet which begins at Combining these observations, it is concluded that the the eastern entrance and continues along the northern side characteristicirculation patterns in the SBC result from the of the channel to Point Conception, while the flow in the balance between the wind stress and the pressure gradient southwestern channel is weakly toward the east. Upwelling through the channel. These balances will be explored in dedominates the circulation at 5 m depth in spring. During tail in a subsequent paper. this season a cyclonic gyre of variable strength may be superposed on the Upwelling circulation. In summer and fall 8. Summary the 5-m circulation fluctuates between the Upwelling, Cy- This study has investigated the subtidal near-surface cir- clonic, and Relaxation patterns. The first two 5-m current culation in the SBC and on the shelf north of Point Con- modes are significantly correlate during this time when the ception. The observationsuggest that currents in the chan- second mode lags the first by about 4 days, suggesting that nel are a superposition of a larger-than-sbc scale flow and over a period of 2-3 weeks the circulation in the channel a cyclonic circulation which is specific to the channel in- switches from the Upwelling to the Cyclonic to the Relaxterior. Near the surface the flow on scales larger than the ation regime. Superposed on these three flow regimes are SBC is equatorward in spring and poleward from summer small cyclonic eddies of limited extent which originate in through winter. These seasonal trends can be reversed for the eastern SBC and travel westward with a speed of about periods of several days. The increase in equatorward flow 0.06 m s -. The flow regime describing theseddies is lain spring occurs concurrently with the increase in equator- beled Propagating Cyclones. The eddies extend from the ward wind stress and the decrease in near-surface tempera- surface to some depth beneath the thermocline and are detures and SSPs. The flow reverses in late spring, simultane- scribed by the third (second) 5-m (45-m) current EOF. Two ously with the increase in the along-channel SSP difference different flow regimes, labeled Flood East and Flood West, and months before wind stress has reached its peak. Most of occur predominantly the winter when the cyclonic comthe observed SSP difference variability is accounted for by ponent of flow is very small. Flood East exists when the trafluctuations in the density difference between cold, upwelled jectories of surface parcels are everywhereastward. Flood West is the converse of the Flood East and exists when the waters along the south central California coast and around Points Arguello and Conception and warmer waters in the circulation is everywhere westward. At 45 m depth, equatornortheastern channel and the Southern California Bight. The ward flow north of Point Conception and along the southern density and related SSP differences may result from spatial shelf of the SBC is significantly weaker than at 5 m depth, gradients in upwelling intensity, established by spatial gra- consistent with diminished wind forcing at depths beneath dients in the wind stress field. the thermocline. The time-averaged circulation within the SBC consists While no attempt has been made to develop a momenmainly of a cyclonic eddy occupying the central and west- tum budget in this paper, the circulation is shown to reern channel. The strength of the cyclonic eddy fluctuates spond to both the wind stress and the pressure gradient seasonally. It is strongest in summer and early fall and weak- through the channel which, in this report, is represented by est in late winter and early spring. In each year, from 1993 the along-channel SSP difference. The wind stress is generto 1995, the period during which the cross-shelf shear of ally upwelling favorable; the along-channel pressure gradithe along-channel flow is greatest coincides with the period ent is directed poleward most of the year. At subtidal freof strongest poleward flow through the eastern entrance of quencies the time series of wind stress at Point Concepthe SBC. Superposed on the seasonally varying north-south tion and along-channel pressure gradient are significantly shear are anticorrelated fluctuations with periods of the or- anticorrelated, with the pressure gradient lagging the wind der of weeks. These fluctuations are most energetic when stress by about 1 day. The relative strengths of the wind the mean shear on opposite shelves is large, i.e., from late stress and the pressure gradient determine the overall sense spring to early winter, and they are virtually absent when the of flow in the channel. When wind stress overwhelms the

HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL 3063 pressure gradient, the Upwelling circulation prevails and the flow is equatorward everywherexcept along the north coast of the SBC. When the pressure gradient dominates, the flow is strongly poleward everywher except along the channel islands. Because this pattern is most common when upwelling-favorable winds have subsided, it is labeled Relaxation. When both the pressure gradient and the wind stress are strong, the flow is poleward along the north coast of the SBC and equatorward along the channel islands, with less vigorous flow at the eastern entrance and north of Point Conception. This pattern is labeled Cyclonic. Flood East and Flood West occur at times when the wind stress is ho- mogeneous and largely unidirectional over the entire SBC and when the along-channel pressure gradient is in the same direction as the wind stress. Detailed consideration of the...,., 1-,,,An. -,,,h;t-h results in the rqrr,,,lnt r n patt.rnq described here is reserved for a future report. Appendix: Analysis Method Empirical orthogonal function (EOF) analysis was first introduced to atmospheric sciences by Lorenz [1956] and is now widely used to describe geophysical fields in diverse geophysical sciences [Barnett and Hasselmann, 1979; Davis, 1976; Kutzbach, 1967; Preisendorfer, 1988; Wilks, 1995]. The EOFs are the eigenvectors of the data covariance matrix. Each eigenvector pattern is associated with a Usually, a large portion of the total variance can be repseries of time coefficients that describe the time evolution resented by a small number of modes. The question arises of a particular spatial mode. The contribution of any comas to how few modes can be retained without discarding imponent to the total variance in the field is given by the asportant information carried in the original data. A number of sociated eigenvalue which provides a measure of its relative tests known as "principal-component selection rules" have importance. Two important properties of EOFs are thathe been designed to determine which modes are statistically spatial distributions are orthogonal and thatheir time series significant and which are not [Preisendorfer et al., 1981]. are uncorrelated over the data set. Thus the EOFs are uncor- The significant modes may be physically meaningful as well. related modes of variability. The scree graph (eigenvalues are plotted linearly versus the xlo 2 corresponding mode number [Cattell, 1966] and the logeigenvalue (LEV) diagram (eigenvalues are plotted logarithmically versus the corresponding mode number) are used to determine how many modes should be retained in the analstandard error ysis. Both tests yield the same results and suggest that the first three (two) eigenmodes, which explain 50% (53%) of the low-frequency 5-m (45-m) current variance (Figure A1; 5 Table A1), should be retained. In other words, 50% (53%) of the low-frequency variance contained in the 5-m (45-m) eigenvalue current field can be classified as "signal," the rest being at- 45m \ IN, "break" "break" i,i I I I I I I I i 1 2 3 4 5 6 7 8 9 10 Mode Number Figure A1. Scree graph displaying the 10 largest eigenvalues (A) as a function of mode number for the EOF analysis of the 5- and 45-m current fields. The error bars represent the standard error (6A) for each eigenvalue. The first three (two) eigenmodes of the 5-m (45-m) current fields are resolved and statistically significant. Table A1. Values Explained by the 10 Largest Eigenmodes of the 5- and 45-m Currents Mode Number 5 m 45 m 1 669 113 23 521 99 38 2 460 78 16 210 40 15 3 322 54 11 129 25 9 4 208 35 7 113 22 8 5 198 33 7 84 16 6 6 155 26 5 75 14 5 7 135 23 5 59 11 4 8 100 17 3 38 7 3 9 95 16 3 29 6 2 10 84 14 3 24 5 2 Eigenvalue, A; standard error, 5A; and percentage of variance, %. % has units cm2s -2. The standard error is computed according to % = %( )«[North et al., 1982]. The EOF analysis of the 5-m currents is performed from January I to December 31, 1994; the EOF analysis of the 45-m currents is performed from March 15 to October 31, 1995. The degrees of freedom are N - 70 and N = 55 for the 5- and 45-m current fields, respectively. tributed to noise. Solutions may suffer from sampling errors, particularly if the eigenvalue spectrum is nearly white, i.e., if there are no dominant modes. In this case the information from the spatial patterns can become mixed up when modes are discussed with eigenvalueseparated by less than the sampling error. In this paper the sampling error is estimated following the method described by North et al. [1982], 6Ai - Ai, (A 1) where 6Ai refers to the sampling error of the ith mode, %i

3064 HARMS AND WINANT: CIRCULATION IN THE SANTA BARBARA CHANNEL is the ith eigenvalue, and N is the number of independent Chelton, D. B., Seasonal variability of alongshore geostrophic vemeasurements or degrees of freedom. N is calculated after locity off central California, J. Geophys. Res., 89(C3), 3473-3486, 1984. Davis [1976], according to Davis, R. E., Predictability of sea surface temperature and sea level pressure anomalies over the North Pacific Ocean, J. Phys. nat N =. (A2) Oceanogr., 6, 249-266, 1976. Davis, R. E., and P.S. Bogden, Variability on the California shelf forced by local and remote winds during the Coastal Ocean Dy- Here At is the sampling interval, nat the record length, and namics Experiment, J. Geophys. Res., 94(C4), 4763-4783, 1989. r an integral timescale Dorman, C. E., and C. D. Winant, Buoy observations of the atmosphere along the west coast of the United States, 1981-1990, J. Geophys. Res., 100(C8), 16029-16044, 1995. Eddington, L. 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