Southern California Bight

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. C10, PAGES 22,521-22,543, OCTOBER 15, 2001 Satellite observations of small coastal ocean eddies in the Southern California Bight Paul M. DiGiacomo and Benjamin Holt Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California Abstract. This study describes the characteristics of extensive small-scale coastal ocean eddies in the Southern California Bight. These surface features were primarily detected by using ERS-1 and ERS-2 synthetic aperture radar (SAR) satellite imagery from 1992 to The eddies, predominantly cyclonic in their rotation, appeared to result from several forcing mechanisms. They were mainly observed within the Santa Barbara Channel and the Santa Monica-San Pedro Basin regions and appeared to be seasonal in their distribution. Observed eddy diameters were all less than 50 km, with over 70% less than 10 km. The SAR data were complemented by sea surface temperature measurements derived from advanced very high resolution radiometer satellite imagery, as well as in situ data from moorings and drifters that provided substantial verification of the small-scal eddies. These findings are significant in that the eddies were, in general, smaller in size and more abundanthan previously reported. Additionally, these results provide further evidence of the complex near-surface circulation within the Southern California Bight, with important implications for nutrient flux, productivity, plankton patchiness, larval transport and recruitment, and dispersal of pollutants. 1. Introduction Circulation patterns within the Southern California Bight (SCB), the coastal ocean from Point Conception to just south of San Diego and inshore of San Nicolas Island and the adjacent Santa Rosa Ridge (Figure 1), are more complex than elsewhere off the west coast of the United States [Tsuchiya, 1980; Jackson, 1986; Hickey, 1992; Hendershott and Winant, 1996]. The equatorward California Current, a well-described eastern boundary current [e.g., Hickey, 1979, 1998; Lynn and Simpson, 1987], dominates flow offshore of the SCB and is strongest during summer, at which time it moves closer to shore and becomes increasingly jet-like [Bray et al., 1999]. Within the SCB, surface flow is predominantly poleward except during spring [Hickey, 1979; Bray et al., 1999], when the strongest equatorward winds are found along the southern California coast [Hickey, 1998]. Otherwise, the Southern California Countercurrent (surface) and the California Undercurrent (subsurface) exhibit poleward flow over the nearshore basins that is strongest in summer [Hickey, 1993, 1998; Bray et al., 1999]. Winds within the SCB are generally weaker and highly variable in comparison with offshore waters and the rest of the California coast [Hickey, 1992; Winant and Dorman, 1997]. Relatedly, upwelling events within the bight tend to be limited to winter and spring; local upwelling during summer, while strong elsewhere along the California coast, is minimal in the SCB owing to a large reduction in wind stress [Hickey, 1992, 1998]. Local forcing, for example, associated with coastal winds, islands, promontories, submarine canyons, basins, and ridges introduces some variability to the largescale SCB circulation [Owen, 1980; Hickey, 1993]; remote Copyright 2001 by the American Geophysical Union. Paper number 2000JC /01/2000JC ,521 forcing, for example, coastal trapped waves, is also a significant contributor (B. M. Hickey et al., Currents and water properties of Santa Monica Bay and nearby basins, submitted to Journal of Geophysical Research, 2001) (hereinafter B. M. Hickey et al., submitted manuscript, 2001). There are two key coastal regions within the SCB that have interrelated but distinct circulation and topographic characteristics. The Santa Barbara Channel (SBCH), bounded to the south by the four northernmost Channel Islands (San Miguel, Santa Rosa, Santa Cruz, and Anacapa), is approximately 100 km long by 40 km wide with a narrow (-3-10 km wide) shelf on both the mainland and island sides of the channel and a central basin that extends to a depth of 500 m [Harms and Winant, 1998]. The SBCH has been the site of intensive research efforts in the past [e.g., Kolpack, 1971; Atkinson et al., 1986; Brink and Muench, 1986; Lagerloef and Bernstein, 1988] and more recently as a result of the Santa Barbara Channel-Santa Mafia Basin Coastal Circulation Study (SBCH-SMB CCS) [Hendershott and Winant, 1996]. The impetus behind much of this work was a desire for predictive capabilities of channel circulation patterns in the event of an oil spill related to offshore drilling activities. The SBCH exhibits much of the same general near-surface flow characteristics (Figure 2) as in the rest of the SCB, with poleward flow in the eastern portion of the channel in all seasons except spring [Harms and Winant, 1998; Dever et al., 1998; Bray et al., 1999]. However, in the western extent of the channel, cyclonic circulation commonly occurs that is independent of larger-scale SCB circulation. Harms and Winant [1998] noted that this cyclonic flow seemed most evident when both a strong alongshelf pressure gradient (poleward) and strong wind stress forcing (equatorward) were present. Relatedly, Munchow [2000] and Oey et al. [2001], among others, have described the importance of wind stress curl forcing in this region.

2 22,522 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT W W o 35 N 35øN B23e.B54 Santa Miguel Santa Cruz Rosa * B25 SMB/B 34 ø Santa SPC/B...,, Barbara San Nicolas Santa Catalina 33 ø San Clemente o km '50' ' N 32 N 121 ø W 120 o 119 ø 118 ø 117øW Figure 1. Southern California Bight (SCB): prevailing near-surface circulation and buoy locations (BXX = B460XX) [after Hickey, 1998; Bray et al., 1999]. Solid arrows indicate the general SCB pattern of poleward flow nearshore and equatorward flow offshore (i.e., California Current, migrates closer to shore in spring and summer); dashed arrow signify a shift to bight-widequatorward flow during spring. SBC, Santa Barbara Channel; SMB/B, Santa Monica Bay (inshore) and Santa Monica Basin (offshore) regions; SPC/B, San Pedro Channel and San Pedro Basin region. See text for further discussion of the variable flow regimes in the Santa Barbara Channel and Santa Monica Bay. o Figure 2. Schematic diagram of the six synoptic views of circulation in the Santa Barbara Channel. After Harms and Winant [ 1998].

3 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT 22,523 The Santa Monica-San Pedro Basin (SM-SPB) region to the south, seaward of a continental shelf that is generally wider (-20 km) than elsewhere off the southern California coast [Hickey, 1993], is bounded offshore by two of the four scale eddies to distinguish them from SCVs. More often than not, the surface manifestations of small-scale eddies take the form of a spiral; hence the term "spiral eddies" has often been applied to such features [Scully-Power, 1986; Stevenson, 1989, southern Channel Islands' Santa Catalina and Santa Barbara. 1998, 1999; Munk et al., 2000]. Small-scale eddies, The basins themselves are deeper (-900 m) than the one found in the Santa Barbara Channel and are separated near the northwestern entrance of the San Pedro Channel (Figure 1) by a narrow (-40 m) seaward sill at 860-m depth, as well as a particularly spirals, have been detected with various remote sensors, including Sun glint photography on numerous platforms [Soules, 1970; Maul et al., 1974; Scully-Power, 1986; Johannessen et al., 1987; Stevenson, 1989; Wakatsuchi shallower nearshore passage [Hickey, 1991]. Several and Ohshima, 1990; Munk et al., 2000] and, as will be submarine canyons are also found in this region, as well as prominent points and headlands. Like the SBCH, the SM-SPB region exhibits similar seasonal circulation patterns as in the rest of the SCB, including predominantly poleward flow along described below, on SAR imagery. These remote observations have found small-scale eddies to be global in distribution, predominantly cyclonic, generally less than 30 km in diameter, and fairly short-lived, though comparatively little is known the slope, except in spring (B. M. Hickey et al., submitted about this latter issue. manuscript, 2001). However, it has a generally weaker wind field compared with the SBCH and the offshore SCB [Hickey, 1992; Winant and Dotman, 1997; Harms and Winant, 1998], As indicated, small-scale eddies have been seen extensively in SAR imagery [e.g., Fu and Holt, 1982, 1983; Johannessen et al., 1987, 1993, 1996; Liu et al., 1994; Nilsson and and its complex topography introduce significant variability Tildesley, 1995' Gade et al., 1998]. These eddies are to the local current field [Hickey, 1993]. The SM-SPB region has long attracted attention owing to chronic pollution concerns that arise from its proximity to the Los Angeles megalopolis and associated urban and storm water runoff, visualized by SAR in several ways [see Liu et al., 1994; Johannessen et al., 1996; Munk et al., 2000], but the most common morphology is as a series of dark (i.e., radar backscatter is reduced in relation to adjacent waters), narrow, particularly into Santa Monica Bay. The complex current curvilinear bands arranged within the eddy into a characteristic structure of the bay has only recently been described in detail spiral appearance. This general pattern is similar to that (B. M. Hickey et al., submitted manuscript, 2001); flow in the observed for eddies in Sun glint photography, albeit inner bay is variable (poleward and/or equatorward) and often occasionally in reverse as the aforementioned dark bands can runs counter to the flow observed over the adjacent slope. appear bright depending upon the viewing angle [Munk et al., While examining various satellite data sets for use in 2000; Stofan et al., 1995]. A number of explanations have describing the general near-surface circulation patterns of the been offered as to the mechanism responsible for the radar SCB and their potential impact on plankton patchiness, we signature (applicable in theory to the optical signature as well). obtained a collection of imagery from the European Space Most likely capillary and short gravity waves are suppressed Agency's (ESA) ERS-1 (El) and ERS-2 (E2) synthetic by sea surface slicks that result from convergence of aperture radar (SAR) instruments. Upon inspection, a rather surfactants oriented in the direction of the eddy's rotational remarkable assemblage of small-scal eddies was clearly sense, resulting in the narrow parallel bands that appear dark evident in the radar imagery throughout the SCB, especially on the radar imagery compared with the radar bright the SBCH and SM-SPB regions. There have been a number of reports of mesoscale (>50-kin diameter) eddies and, to a lesser extent, submesoscale (<50 km) eddies in the SCB and other regions of the California Current System [e.g., Burkov and Pavlova, 1980; Owen, 1980; Lynn and Simpson, 1987; Poulain and Niiler, 1989' Hickey, 1992; Swenson and Niiler, 1996; Munk et al., 2000] and particularly in the SBCH [Harms and Winant, 1998]. In this study we describeddies ranging from surrounding waters where wind-generated short waves are generally present [Johannessen et al., 1993; Nilsson and Tildesley, 1995; Gade et al., 1998' Munk et al., 2000]. Thus both SAR and Sun glint photography provide synoptic, high spatial resolution (30 m for El/E2 SAR) imagery capable of resolving small-scaleddies. Additionally, SAR has the advantage of operating regardless of cloud or light conditions and, on satellite platforms, can provide routine 1 to 50 km in diameter. However, the vast majority (over (monthly or better) regional coverage. However, despite the 70%) had diameters of 10 km or less, representing a size class compelling surface appearance of such small eddies on SAR that has been largely undetected by in situ measurements and Sun glint imagery, to date there have been only limited in well as satellite observations and hence are unreported in the situ measurements or modeling of eddy properties coincident SCB. The eddies were predominantly cyclonic in rotation, and with such high-resolution remote observations [e.g., Johannessen et al., 1987, 1993; Nilsson and Tiidesiey, 1995; on radar imagery (as dark thin concentric slicks composed of biogenic surfactants) suggests that the eddies may have an important role with regard to biological processes. Liu et al., 1994] and no extended regional or temporal studies. This study examines the size and distribution characteristics of the small-scaleddy field in the SCB as seen primarily on Eddies less than 50 km in diameter can be placed into two El/E2 SAR imagery obtained between June 1992 and April general categories for purposes of this discussion Section 2 describes the data and methods used in the Submesoscale coherent vortices (SCV) [McWilliams, 1985] SAR analysis. Near-coincident sea surface temperature (SST) are long-lived, subsurface, remotely generated eddies with data from the advanced very high resolution radiometer minimal surface expression [D'Asaro, 1988]. Conversely, (AVHRR) and surface drifter and moored buoy data were short-lived, near-surface, locally generated eddies that compared with the SAR data where available. In section 3 we typically have a distinct surface signature have also been identified [e.g., Lee, 1975; Pingtee, 1979; Marullo et al., 1985' Ka:::'min and Ku:::'mina, 1990; Bogtad et al., 1994; Bruce, 1995; Munk et al., 2000]. Herein we refer to these as smallexamine the regional characteristics of these small-scale eddies first within the SBCH and then the SM-SPB regions. In the SBCH, small eddies in the larger size range (30-40 km) have been well documented by the SBCH-SMB CCS using current

4 22,524 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT data from moorings and surface drifters [Harms and Winant, generally before onset of the characteristic afternoon sea 1998; Dever et al., 1998; Winant et al., 1999]. Several radar breeze found in the SCB [Dorman, 1982]. Winds are images that were part of this study were coincident with these generally calm to moderate in the SCB, typically of the order mooring data and nearly coincident with drifter tracks, of 2.5 m s - nearshore and 5 m s -1 offshore but also are highly providing significant verification of a subset of the SAR- variable [Dailey et al., 1993; Hickey, 1992; Winant and detected eddies. Fewer field measurements of suitable Dorman, 1997]. Higher winds tend to reduce the appearance temporal and spatial resolution were available in the SM-SPB of coastal sea surface features such as slicks [Romano and region, where previous studies using in situ and/or satellite Marquet, 1991; Romano and Garabetian, 1996], especially in data have indicated that small eddies ( km) may SAR imagery. In general, sea surface features are best likewise be common. However, these features and even visualized on SAR images when winds are between about 2 smaller ones have largely gone unreported, as might be and 7 m s - [e.g., Fu and Holt, 1982, 1984; Johannessen et al., expected given their size, ephemerality, and associated 1996], a range commonly observed in the SCB. Sea surface sampling limitations. In section 4 we describe the overall characteristics of the SCB eddies in terms of their size, location, and seasonal distributions. In section 5 we discuss possible forcing mechanisms of these eddies and potential slicks (natural films) with enriched organic content (surfactants) dampen small waves that provide the necessary Bragg scatter. This leads to reductions in radar backscatter of between 2 and 17 db within slicks (depending on surfactant impact of the eddies on a wide variety of coastal phenomena source, concentration, and wind speed) in C band ERS SAR including nutrient concentrations, plankton patchiness, larval transport and recruitment, and dispersal of pollutants. Section 6 contains a summary and brief discussion on future directions. data compared with returns from surrounding nonslick ocean surfaces [Espedal et al., 1996, 1998; Johannessen et al., 1996]. Slicks are generally associated with numerous coastal ocean phenomena, including small-scaleddies, fronts, and internal waves [e.g. Ewing, 1950; LaFond and LaFond, 1972; 2. Data and Methods Kingsford, 1990; Ermakov et al., 1992] and help make these near-surface phenomena visible in SAR images [e.g., Fu and SAR imagery was obtained from ESA's ERS-1 and ERS-2 Holt, 1982; Gower, 1993; Johannessen et al., 1996]. In missions; data acquisition was at the Prince Albert, Saskatchewan, receiving station. The C band (5.3 GHz) data were received in strips with a swath width of 100 km and then processed into frames of 100 by 100 km. For this study, we addition to slicks, other sea surface features such as swell and atmospheric expressions were often visible in the SAR imagery of the SCB. All the ERS SAR frames were examined independently by examined 119 SAR frames spanning the years 1992 to 1998 each author for evidence of eddies and other oceanic and (Table 1); the vast majority originated from 1994/1995, and all but 11 frames are E1 products. The SAR imagery was obtained when the satellites were in 35-day exact repeat orbits, which precludes short-term temporal sampling. However, several pairs of images from 1995/1996 were obtained when both E1 and E2 were operating in the so-called tandem phase, atmospheric phenomena. Only those features clearly identified as eddies per the following criteria were included for subsequent analyses. The visual identification process was guided by numerous assessments of ocean feature (e.g., eddies, internal waves) detection from SAR imagery and shuttle photography found in atlases [Beal et al., 1981; Fu and Holt, with the satellites still within the 35-day repeat cycle but 1982; Stevenson, 1989; Johannessen et al., 1994] and spaced 1 day apart. This enabled some evidence of short-term temporal variability (see section 3.2). The El/E2 products utilized here were not specifically acquired for this study. Instead, they were selected nearly randomly over time from the published studies [e.g., Vesecky and Stewart, 1982; Fu and Holt, 1983; Johannessen et al., 199!, 1993, 1996; Liu et al., 1994; Alpers, 1995; Nilsson and Tildesley, 1995; Munk et al., 2000], minimizing the potential for subjective errors. In SAR more extensive ESA SAR archive. The frames are located imagery found in these studies, most small-scal eddies were throughout the SCB, though they are predominantly primarily identified through the characteristic assemblage of concentrated near the coast with some bias toward the Santa dark (i.e., low backscatter), narrow, curvilinear, concentric Monica Basin region. Most of the data utilized here were processed at the Canada Centre for Remote Sensing, which at bands (slicks) that appeared to spiral inward. Larger eddies (e.g., Gulf Stream, East Australia) were typically identified by the time did not produce calibrated image products and thus limited the scope of the radar analysis. The SAR data were all acquired (except one pass) during the descending (southward) portion of the Sun-synchronous orbit at midmorning local time (-1830 UTC), which is Table 1. Monthly Distribution of ERS-1 and ERS-2 SAR Frames a a narrow band of enhanced brightness, usually associated with current shear. In this study, all eddies were of the former (i.e., dark banded) variety. We only measured completely manifested eddies, that is, intact, coherent dark-banded structures that were clearly and Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec. Total (1)... 1(0) 1(0) 1(0) 1(1) 5(2) (0)... 1(1)... 1(1) 3(1)... 6(3) (1) 7(3) 5(2) 3(2) 3(1) 5(2) 4(1) 7(3) 2(2) 37(17) (0) 6(1) 7(1) 10(3) 4(1) 2(1)... 2(1) 4(2) 4(2) 7(5) 8(5) 59(22) (1) 2(2)... 2(0)... 6(3) ( 1 ) 1 (0)... 1 (0) 3(1 ) (1) 1(1)... 1(0)... 3(2) Total 8(2) 9(4) 9(2) 15(4) 11(4) 8(4) 3(2) 6(3) 10(4) 10(4) 18(9) 12(8) a Number in parentheses indicates how many of these frames contained small-scale eddies.

5 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT 22,525 distinctly identifiable and where the outermost ring was not obscured by other oceanic or atmospheric features. This outermost dark curvilinear band demarcated the boundary of an eddy. Eddy diameters were calculated by taking the geometric average of major and minor axis lengths [e.g., Mattcoda and Glenn, 1996] measured through each eddy's center (see later example in Figure 5 area A). It is possible that these measurements might not reflect the true size of an eddy; that is, the eddy may be larger than the manifestation of the slick pattern. However, there were numerous instances of good spatial coherence between eddies in coincident SAR and AVHRR images; so overall we feel that our measurement approach provided a reasonably accurate representation of the true size of these eddies. Cooperative Oceanic Fisheries Investigations (CalCOFI) drift bottle and drogue studies [Schwartzlose, 1963' Crowe and Schwartzlose, 1972' Schwartzlose and Reid, 1972; Dewees and Strange, 1984]. Finally, we have found similar small-scale spiral eddies throughout the SCB in numerous space shuttle Sun glint photographs from archives at the Johnson Space Center in Houston, Texas. The strong correspondence we nhcprx7prl hptxxtppn c r'h fpstllrpc in rm ltiplp inrlpnpnrlpnt nrl often coincident data sets, complemented by their similarities in appearance and size with published reports of small-scale eddies found elsewhere, provides considerable confidence that our observations of the SCB eddy field are sound and repeatable. 3. Regional Analyses and Verification In this section, we describe a representative set of eddy observations within the Santa Barbara Channel and Santa Monica-San Pedro Basin regions, comparing the radar observations with SST fields and near-coincident in situ measurements where available Santa Barbara Channel The Santa Barbara Channel has complex circulation patterns [Harms and Winant, 1998; Winant et al., 1999; Bray et al., 1999], in general reflective of the characteristic flow of the SCB but also distinct, particularly in terms of its eddy field. The most comprehensive findings to date for this region were from an extensive field program, the Santa Barbara Channel-Santa Mafia Basin Coastal Circulation Study. The SBCH-SMB CCS primarily took place in the SBCH from 1992 to 1996 and the Santa Mafia Basin from 1996 to 1999 (some measurements will continue through 2004) and included a suite of current moorings as well as the regular release of surface drifters [Hendershott and Winant, 1996' Harms and Winant, 1998' Dever et al., 1998' Winant et al., 1999' Bray et al., 1999]. It was designed to describe SBCH flow and its dynamical relationship with local and remote (i.e. SCB and Coincident AVHRR sea surface temperature data for the SCB were obtained from the NOAA CoastWatch Program. As indicated, the AVHRR data generally supported and enhanced our interpretations of the SCB SAR images. Unfortunately, beyond) forcing mechanisms, to improve the physical basis for the lower resolution of the AVHRR images (-1 km) makes developing regional numerical models and, ultimately, to aid resolving the smaller eddies difficult, as does the circumstance in the prediction of oil dispersion in the event of a spill. that SAR coincident AVHRR data were often unavailable Harms and Winant [1998] summarized SBCH-SMB CCS owing to complete or partial cloud cover. To help with mooring data for the SBCH, describing six near-surface verification of satellite (SAR, AVHRR) image analysis, synoptic flow regimes (Figure 2) that are forced by changes in coincident in situ data were obtained from moored buoys the relative strengths and/or direction of wind stress and the maintained in the SCB by the National Data Buoy Center pressure gradient. Oey et al. [2001] remarked that the above 1 :œ1 _'... ' _1_._1...._1 (NDBC) (Figure 1). The measured parameters included hourly cia Hmauon m gnt be euunuant, as they all possibly share a wind speed (5-m elevation for buoys 46011, 46025, 46045, common dynamical theme. For purposes of this study, 46051, and 46053' 10-m elevation for buoys and however, the original classifications are of primary interest, as 46054), wind direction, and sea surface temperature (-1-m we are trying to identify representative patterns' forcing depth). Surface drifter and mooring data were also referenced subtleties are a secondary concern. From Harms and Winant from the SBCH-SMB CCS [Hendershott and [1998], spring is dominated by the "upwelling" circulation Winant, 1996]. These moorings measured currents, pattern (Figure 2a). Summer and fall are marked by the temperature, salinity, and bottom pressure [Harms and Winant, repeated progression from upwelling to "cyclonic"(figure 2c) 1998]. In several instances, coincident current data from these to "relaxation"(figure 2b) and quiescent states over a period moorings were used to validate SAR-interpreted surface flow of a few weeks. Found concurrently with these synoptic states fields, generally confirming the existence of coastal eddies are "propagating cyclones" (Figure 2d), referring to small- (and their rotational sense) and jets. Additionally, the drifter scale (30-km diameter) cyclonic eddies that are generated in data [Dever et al., 1998' Winant et al., 1999], while generally the eastern end of the SBCH and propagate westward with an not coincident, supported our conclusions about the approximate translational speed of 0.06 m s -. These recurring widespread presence of small-scale coastal ocean eddies in the cyclones appear to have an average duration upward of 2 SCB by indicating their presence throughout numerous drifter weeks, at least half of which is spent in the channel [Harms, tracks. Similar features were revealed in California 1996]. They are among the largest and longest lived of the SCB eddies described here. Finally, solely eastward (i.e., "flood east", Figure 2e) and westward (i.e., "flood west", Figure 2f), circulation patterns are primarily observed in winter, a time when the cyclonic component of flow is minimal [Harms and Winant, 1998]. Coincident SAR (Figure 3) and AVHRR (Plate la) images from July 1-2, 1994, detected a cyclonic eddy in the SBCH, comparable to the aforementioned features described by Harms and Winant [1998]. This eddy was visualized on the SAR image by distinct, narrow surface slicks, with a larger dark band to the north likely enhanced by oil from the wellknown natural oil seeps off Coal Oil Point to the west of Santa Barbara [Hornafius et al., 1999]. Entrained in shear zones, the oil seepage helped improve the definition of the eddy flow pattern despite wind gusts in the area up to 8 m s -. The eddy was approximately 35 km in diameter, consistent with the approximate size of propagating cyclones reported by Harms [1996]. Its rotation, as outlined by the SAR image (Figure 3, area E), appeared cyclonic, with northwest flow emanating from the channel between Santa Cruz and Santa Rosa Islands toward the mainland mainland in the western SBCH. and southeastward return flow from the These patterns were

6 22,526 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT PC A) July 2, 1994 SST B) September 30, 1995 SST _21øC Pc cop SB... OP SB 20. :. A " B C 21øC II 15 SC 14 'E 16 C) December 19, 1994 SST 16.2 C D) April 21, 1994 SST 15 o C ,.,"., 17.5 II , A c 16.5 CAT 14.2 CAT E) October 26, 1997 Chlorophyll-a 2 mg m ' F) October 26, 1997 SST 22 C 21 2O E O. lo E. 19 a Plate 1. AVHRR sea surface temperature (SST) images. (a) July 2, 1994, at 0022 UTC; notation as in Figure 3. (b) September 30, 1995, at 2045 UTC; notation as in Figure 5. (c) December 19, 1994, at 0259 UTC; notation as in Figure 8. (d) April 21, 1994, at 2348 UTC; notation as in Figure 9. (e) SeaWiFS chlorophyll a image of the Santa Barbara Channel from October 26, 1997, at 2040 UTC; E, approximate location of smallscale eddy. (f) AVHRR SST image from October 26, 1997, at 2223 UTC; notation as in Plate l e.

7 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT 22, A) NøV 23'i:1994, B) N øv 25, 1994 C) Nov 27, 1994 D) Nov 29, 1994 E E) Nov 30, 1994 F) Dec 01, 1994 E SST øc Plate 2. AVHRR sea surface temperature time series depicting track of November 29, 1994, eddy on (a) November 23 at 1622 UTC, (b) November 25 at 1539 UTC, (c) November 27 at 0236 UTC, (d) November 29 at 1553 UTC, (e) November 30 at 1531 UTC, and (t) December 1 at 1509 UTC. Approximate location of eddy denoted by area E.

8 22,528 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT Figure 3. ERS-1 SAR image of the Santa Barbara Channel from July 1, 1994, at 1839 UTC. E, approximate location of eddy; SB, Santa Barbara; COP, Coal Oil Point; SC, Santa Cruz Island; PC, Point Conception. Copyright European Space Agency supported by the coincident SST image (Plate l a) which showed warm water from the southeast extending noah and then west along the coast, bifurcated by colder waters from north of Point Conception headed southeast along either side of the westward Channel Islands, both mixed within the eddy. Corroborating these satellite data interpretations were coincident current measurements from moorings in the SBCH (Figure 4a) that showed near-surface cyclonic flow. Drifters that were released June 15, 1994, also showed the general cyclonic circulation patterns expected in the SBCH for this time of year [Dever et al., 1998; Winant et al., 1999]. An intriguing question which a SAR image alone cannot answer is whether these eddies are shallow surface phenomena. In answer to this, Harms and Winant [1998] indicated that propagating cyclones have a subsurface componenthat extends below the thermocline (to at least 250 m), and Harms [1996] suggested that these eddies are in geostrophic balance. Given this, one would expect to see coldcore signatures, indicative of isotherm doming, for these SBCH cyclonic eddies in SST images. A cold core was indeed visible in Plate l a, marking the center of the eddy. Harms cautioned, however, that thermal signatures for these eddies can be misleading as warmer "mainland" waters are mixed with colder "island" waters inside the eddy; this results in variable core temperatures. For the eddy detected on the above SAR image (Figure 3), a sequence of SST images (not shown) seemingly indicated a lifespan for the eddy of at least 14 days with a westward propagation. Heavy offshore cloud cover made it difficult to track the eddy once it reached the terminus of the channel. Harms [1996] speculated that these eddies occur when alongshelf flow becomes unstable in the eastern SBCH. In the next SAR example (Figure 5), an apparent northwestward jet, seen as a band of comparatively uniform brightness extending northwest-southeast between two regions of highly varying backscatter, was clearly visible off the mainland coast on September 30, SAR flow patterns were again corroborated by satellite SST data (Plate lb) that revealed a strong, warm, westward coastal jet; current measurements (Figure 4b) likewise indicated sharp, poleward, near-surface flow along the mainland. These patterns are characteristic of the relaxation circulation state (Figure 2b), occurring when upwelling favorable wind stress is weak and the alongshelf pressure gradient is strong and poleward. Again, thicker dark bands on the mainland nearshore part of the jet likely resulted from a convergence of oil from nearby natural seeps, which may also account for the extensive dark region found adjacent to the coast, with oil entrained north of the prevailing northwestward flow. Of interest were the five small (<20-km diameter) eddies found in the SAR image (Figure 5), particularly the four within the channel itself. Of these, eddies A, B, and C appeared to be cyclonic and associated with the jet; eddy D, the smallest of the features, appeared to have anticyclonic rotation and was found near Santa Cruz Island. Eddies A, B, C, and D had estimated diameters of 17, 5, 8, and 3 km, respectively. They were too

9 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT 22,529 Figure 4. Coastal Circulation Study mooring data from the Santa Barbara Channel showing daily-averaged 5-m currents and wind stress from (a) July 1, 1994, (b) September 30, 1995, (c) November 8, 1994, and (d) November 18, small to be resolved by the SBCH-SMB CCS mooring data, and whether they possessedistinct thermal signatures is questionable (Plate lb). Other small-scale features were prevalent in the Santa Barbara Channel. An enlarged subscene from a November 9, 1994, SAR frame (Figure 6a) appeared to show a northward jet that bifurcated as it approached the coast, with two very small (-5-km diameter) eddies to the north of the easternmost arm of the bifurcation. As in Figures 3 and 5, flow visualization seemed to be enhanced by oil from local seeps. These eddies appeared to occur in association with the terminus of the "Santa Rosa cold squirt" [Hendershott and Winant, 1996], a condition where cold water flows northeastward from Santa Rosa Island toward the mainland as a cold squirt, bifurcating near the coast. This results in westward flow that joins warm water traveling toward Point Conception as well as flow toward the southeast. Nearly coincident drifter data (Figure 6b) supported the existence of onshore flow, with a subsequent northwestward track from point a (November 7) to point b (November 8) and onward. Approximately 10 hours elapsed between these two readings, with flow toward the coast at an average speed of 0.4 m s -1 during this time period, consistent with mean values reported by Harms [1996]. Figure 4c similarly supports these SAR and drifter-revealed flow patterns, as moored buoy data from November 8 indicated onshore flow, with a speed comparable to that derived from the drifter data. AVHRR images from the days preceding the SAR image were generally cloud covered, but open patches appeared to indicate the presence of a cold squirt Santa Monica Bay and Santa Monica-San Pedro Basins The circulation patterns in the Santa Monica-San Pedro Basin region are also quite complex. Previous studies, including the most comprehensive to date [Hickey, 1992; B. M. Hickey et al., submitted manuscript, 2001], described flow patterns in the SM-SPB region that are largely characteristic of the SCB as a whole, albeit with some variability closer to shore, particularly in Santa Monica Bay. Superimposed on these regional patterns is a seemingly energetic mesoscale and small-scaleddy field [e.g., Schwartzlose, 1963; Wyllie, 1966; Crowe and Schwartzlose, 1972; Schwartzlose and Reid, 1972; Burkov and Pavlova, 1980; Dewees and Strange, 1984]. Hickey [1992, 1993] indicated that these features could be locally generated via current instabilities or by the complex regional bottom topography impacting the mean flow. B.M. Hickey et al. (submitted manuscript, 2001) discussed the formation of eddies through flow separation at the sharp upqtream cnmers of,qanta Monica Bay, with cyclonic r.,,;...,.,;,...aa;... *' "when forced by...,,uu,,turw,,ru a (poleward) flow. This study also noted the importance of lateral advection in this region, potentially enhancing the transport of small-scaleddies. Despite the apparent prevalence of small eddies in this region, comparatively little is known about them. This is likely the result of the spatial (and temporal) constraints typically associated with in situ sampling and perhaps even moderate resolution (-1 km), cloud-affected satellite data (e.g., AVHRR). Here we illustrate several representative examples of small-scale eddies detected in SAR imagery, which are

10 22,530 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT Figure 5. ERS-1 SAR image of the Santa Barbara Channel from September 30, 1995, at 1840 UTC. Areas A, B, C, and E denote four apparent cyclonic eddies; area D denotes an apparent anticyclone. SB, Santa Barbara; COP, Coal Oil Point; SC, Santa Cruz Island; PC, Point Conception. Solid white arrows near area B mark the westward jet discussed in the text. The dashed white arrows near area A illustrate how eddies were measured, with /T/1 the major axis (---20 kin) and rn 2 the minor axis (---15 km) as discussed in the text. Copyright European Space Agency compared with AVHRR data and more limited in situ The next SAR example (Figure 8a) covered the Santa measurements than those available in the SBCH. Monica Bay and the SM-SPB region and contained at least six An example from SAR of an apparently recurrent small- eddies. These eddies all appeared to have cyclonic rotation, scale eddy in.the SM-SPB region is shown in Figure 7, located and most of their diameters were less than 10 km. Figure 8b between Anacapa and Santa Barbara Islands on November 29, enlarges the approximately 6-km eddy adjacent to (Santa) This eddy appeared to be cyclonic and was Catalina Island (i.e., eddy F), where the dark slicks were approximately 25 km in diameter. Coincident SST imagery approximately 50 to 250 m wide and separated by roughly 1 (Plate 2D) supported the likelihood of its counterclockwise km near the outer edge of the eddy, narrowing to 150 m at the rotation and revealed a core temperature 2øC cooler than core. The smaller eddies seen in this SAR image (i.e., eddies surrounding waters. A sequence of SST images (Plate 2) C, D, E, and F) were not readily apparent in a near-coincident suggests that the eddy formed sometime around November 23 SST image (Plate l c), but the larger eddies (eddies A and B) and translated southwestward over 9 days at an approximate appeared to be associated with patches of colder water in the speed of 0.14 m s -. The eddy could not be tracked on AVHRRimage. AVHRR after December 1 owing to persistent cloud cover. The San Pedro Channel, between Catalina and the The eddy seemingly formed during an upwelling event off mainland, exhibits dynamic circulation properties that result in Point Dume at the northwest edge of Santa Monica Bay. Mooring data (Figure 4d) indicated the presence of upwelling favorable winds in the days prior to this feature's first appearance on AVHRR images. This eddy appears to be a recurring feature since eddies have been observed in this particular region in several other SAR images, but only during part from the constriction of flow that occurs there [Hickey, 1992]. Evidence of this is provided by SAR images in Figures 9 and 10. In Figure 9, the SAR image showed two eddies that seemed to converge in the channel. The eddy toward the west (eddy A) was apparently anticyclonic, and the one toward the east (eddy B) was apparently cyclonic. Another apparent winter and spring, when local wind-driven upwelling typically cyclone (eddy C) was observed further to the east. These three occurs in the SCB [Hickey, 1993]. Meteorological buoy eddies were all less than 15 km in diameter. The cyclones (B records have indicated upwelling favorable conditions prior to the appearance of each such feature observed in this area. and C) appeared to be marked by colder waters on the corresponding SST i nage (Plate l d), while a distinct thermal

11 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT 22,531 signature for the anticyclone (A) was not readily apparent. In Figure 10, a 1-day repeat sequence from the El-E2 tandem mission is shown for this region. An apparent cyclonic eddy approximately 15 km in diameter is shown, propagating in a west-northwest direction at an average speed of 0.12 m s -. This represents the only instance from this data set where we were able to observe an individual eddy in SAR for more than (a) (b) o o ' ' ' 34ø30, i )!ii -w... ;." ':... :--';:,,,:'::' *"" '": -.* : ;... " '" '-'": :- :':" ' ::5'f"::' " "?::) :::*:.. ;"'":'::. " :?/: } /': 34 ø30' Nov8 x... 34ø15, o15,.././5 /' :.t... '" No.v 1 34ø00,... - ;:.... Ooo, _20ø00,,.9ø30, Figure 6. Near-coincident satellite and drifter data off the coast of Santa Barbara. (a) ERS-1 SAR image from November 9, 1994, at 1839 UTC, extending off the coast of Santa Barbara (SB) approximately 20 km into the Santa Barbara Channel. The areas marked E denote the presence of two small eddies. Copyright European Space Agency (b) Coastal Circulation Study drifter track following release on November 1, 1994, through November 8, Point a indicates last position recorded on November 7; point b indicates first position recorded on November 8. SB, Santa Barbara; SC, Santa Cruz Island. 1 day. More importantly, this pair of images 1 day apart indicates that these eddies are coherent and persistent features. 4. Eddy Characteristics In all, 107 small-scale eddies were detected on 119 SAR images of the SCB. This number is likely a conservative estimate, as only distinctly recognizable, completely manifested eddies were included in this count. We note that essentially all the SAR images shown here included slicks aligned by near-surface current patterns that did not exhibit a full, inward spiral and hence were not identified as eddies. SAR images were also acquired when local winds were outside the general wind speed range (-2 to 7 m s -1) considered most favorable for SAR ocean feature detection (Figure 11). The size distribution of eddies observed is presented in Figure 12. It is striking that small eddies were the most common, with nearly 75% under 10 km in diameter and 94% under 20 km, making prior lack of observations of these features understandable. Also, observations of larger eddies may be effectively limited by the size of the ERS-1 SAR images (100 x 100 km), potentially skewing the relative abundance toward these smaller size classes. However, the fact that such a large number of small eddies were observed at all is the important finding, regardless of what fraction of SCB eddies they ultimately comprise. Most (-94%) of the eddies observed in this study appeared to have cyclonic rotation. Previous findings for the California Current region [e.g., Burkov and Pavlova, 1980; Owen, 1980; Poulain and Niiler, 1989] indicate that eddies there tend to be predominantly cyclonic, though not necessarily at smaller (<50 km) scales or in all locations. Munk et al. [2000] in their global analyses of Sun glint photography found spiral eddies to be overwhelmingly cyclonic, consistent with our findings. Swenson and Niiler [1996] speculated that the convergent nature of cyclonic eddies might have led them to be preferentially sampled by drifters in their study. This point is of related interest here, as Munk et al. [2000] indicated that surfactants might also preferentially concentrate within cyclonic convergence zones, thereby potentially allowing these eddies to be visualized more readily than anticyclonic eddies (e.g., in SAR images). However, they also noted several other likely physical explanations for this seeming predominance of cyclones; we will discuss their findings further in section 5.1. Figure 13a maps the location and size of all eddies detected in this study. Though this plot and the following seasonal subplots (Figures 13b-13e) are not entirely free of sampling biases, for example, variable regional frame distributions (section 2) and wind aliasing (discussed below), a number of robust eddy distribution patterns seem to be evident. Near obstacles to flow (e.g., islands, headlands), there were clusters of eddies less than 10 'km in diameter, suggestive of topographic influence and thus possible recurrence in time and/or space. Numerous eddies were found in the Santa Monica Basin away from such topographic features, potentially indicative of their local generation [e.g., Munk et al., 2000], if not advection from source regions elsewhere [e.g., Hickey, 1993]. Eddies were also abundant in the Santa Barbara and San Pedro Channels, regions of constricted and/or converging flow (e.g., cross-shelf shear). The issues of

12 22,532 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT physical forcing, advection, and other related phenomena will be considered in more detail below. An interesting nearshore region where small-scale eddies were absent begins at Point Dume and continues to the northwest. It is possible that no eddies were seen here because high/low wind speeds prevented them from being detected by SAR, but also there may have been few features to detect. Wind speed records from local NDBC buoys (Figure 1) indicate that this "dead" zone was likely not an artifact of wind. Buoys (B53) and (B25), closest to this zone, revealed moderate winds suitable for eddy detection, though wind speeds were generally somewhat higher at B53. As SAR detected numerous eddies in the vicinity of B53, between Santa Cruz Island and the mainland, and near B25, in the Santa Monica Basin, it is unlikely that high or low winds alone were responsible for the absence of features northwest of Point Dume. Dever et al. [1998] provide support for an alternative explanation, that there were few features to detect. Their study summarizes surface drifter data and indicates that the shelf waters between Port Hueneme (just northwest of Point Dume) and Santa Barbara have low values of fluctuating kinetic energy. Weak flow may then be responsible for the lack of features in the dead zone described above. The eddy deficient region found offshore south of the islands of Santa Cruz and Santa Rosa and near San Nicolas Island (Figure 13a) was the result of fewer SAR frame retrievals for these waters, but even when such images were examined, few eddies were observed. This was likely due, at least in part, to stronger winds characteristic of that region. Along the coast, winds are typically strongest in the western SBCH and off Point Conception [Caldwell et al., 1986; Dorman and Winant, 2000]; fewer eddies were similarly observed in that region (Figure 13a). The potential for wind "aliasing" is an important consideration when trying to interpret both geographical (Figure 13a) and seasonal (Figures 13b-13e; Table 2) distributions of these SCB eddies. Regarding the latter, fall and winter considered together exhibited approximately twice as much small-scale eddy activity in the SCB than did the corresponding spring and summer period (Table 2). If stronger winds were present during spring and summer, this might serve to diminish ocean feature detection during these seasons as a function of wind aliasing, possibly leading to the observed seasonality in eddy prevalence. Using CalCOFI (ship) observations, Winant and Dorman [1997] found that seasonally averaged wind stresses over the entire SCB domain are stronger during spring and summer and weaker in fall and winter. Closer to shore, however, this was not necessarily the case, especially during summer. Instantaneous wind data (primarily nearshere) associated with the SAR frames used in this study (Table 2) are largely consistent with these findings. While the large-scale seasonal trend of stronger winds in spring and summer is marginally observed for SAR frames that contain eddies, it does not hold true for SAR frames that Figure 7. ERS-1 SAR image of the Santa Monica Basin area from November 29, 1994, at 1837 UTC Area E denotes approximate location of eddy. PD, Point Dume; SC, Santa Cruz Island. Copyright European Space Agency 1994.

13 22,533 A Figure 8. (a) ERS-1 SAR image of the Santa Monica Bay/Santa Monica Basin region from December 19, 1994, at 1834 UTC. Areas A and B denote the two largest eddies present; areas C, D, E, and F denote smaller eddies. CAT, Catalina Island; boxed area off Catalina is enlarged in Figure 8b. (b) Enlargement of boxed area off Catalina Island from Figure 8a. Copyright European Space Agency 1994.

14 22,534 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT Figure 9. ERS-1 SAR image of the San Pedro Channel from April 21, 1994, at 1833 UTC. Areas A, B, and C denote three eddies. CAT, Catalina Island. Copyright European Space Agency lack eddies, the more important category. In particular, of the SAR frames at somewhat lower than expected wind speeds four seasons, summer exhibits the lowest mean wind speed by (i.e., below 2 m s-l). However, as discussed above, there are far with regard to SAR frames without eddies. Additionally, clearly some important trends evident in Figure 11 and Table 2 when considering mean wind speed for all SAR frames, while that can help interprethe small-scaleddy field observed in spring is nominally highest, summer is lowest. These results the SCB. In particular, it seems apparent that some non-wind suggest that wind aliasing alone is not responsible for the aliased eddy distribution patterns (seasonal and otherwise) are observed seasonal differences in eddy distribution. likely discernible, as will be discussed further below. This is not meant to diminish the potential role played by The seasonal eddy distribution throughouthe SCB is wind aliasing, however. As alluded to earlier, many of these illustrated in Figures 13b-13e. During winter (Figure 13b), we SCB S AR frames were acquired under wind conditions less see that eddies were primarily concentrated in the SM-SPB than favorable for SAR ocean feature detection. For example, region, with a cluster of eddies near the southern end of San regardless of season (Table 2), mean wind speeds are Clemente Island and numerous eddies broadly distributed uniformly higher for SAR frames without eddies versus SAR around Catalina Island. The Santa Monica Basin region in frames with eddies (though the differences are marginal for particular exhibited a higher number of eddies during these summer). A variation of this theme is illustrated by the winter months than in any other season. Characteristic of this observed relationship between wind speed and the number of period is a second seasonal maximum in the Southern eddy-containing SAR frames (Figure 11). Eddy observations California Countercurrent, that is, strong coastal poleward are generally high at low to moderate wind speeds but taper off flow, as well as strong local upwelling [Hickey, 1992, 1993, with winds between 4 and 6 m s -l. At winds higher than 6 m s- 1998]. These conditions might be expected to foster eddy, none of the SAR frames contain eddies. As discussed pattern similar to those described here, including through earlier, this trend might partly explain the observed lack of possible offshore and/or northward advection of features into features in the SBCH throughout much of the year, as well as Santa Monica Basin. Conversely, the general absence of in offshore waters, where generally higher winds exist. features in the SBCH may partly result from weak cyclonic However, low to moderate wind speeds alone do not ensure circulation. During winter, channel flow tends to be more the presence/detection of eddies. This is evidenced by the fact unidirectional, particularly the flood east (Figure 2e) and flood that below 6 m s -l, frames are equally divided between those west (Figure 2f) conditions described by Harms and Winant that contain eddies and those that do not. Further, there is little [ 1998]. These states occur when gradients in the equatorward difference in mean wind speed between summer SAR frames (flood east) or poleward (flood west) wind stress fields are with eddies and without them (Table 2). It is important to note small and in the same direction as the alongshelf pressure that these wind relationships are merely approximations gradient. because the buoy measurements used to derive them were not During spring (Figure 13c), eddies were generally found in always close to the features of interest nor necessarily accurate central south SCB waters, in a pattern similar to that of winter (+1 m s' ). This perhaps explains the presence of eddies in (Figure 13b). Fewer eddies were observed in the Santa

15 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT 22,535 Figure 10. (a) ERS-1 SAR image of the San Pedro Channel from February 14, 1996, at 1834 UTC. (b) ERS- 2 SAR image of the San Pedro Channel from February 15, 1996, at 1834 UTC. Area E denotesame eddy. CAT, Catalina Island; PV, Palos Verdes Peninsula. Copyright European Space Agency 1996.

16 22,536 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT Frames Frames with Eddies without Eddies 10 i i i i i i Wind Speed (m S -1) 8-' Figure 11. Distribution of ERS-1 and ERS-2 SAR frames (1992 to 1998) with and without small-scaleddies relative to wind speed in the Southern California Bight (SCB). Based on a subset (~75%) of all SCB SAR frames that had nearly instantaneous buoy data available in moderately close (- 50 km) proximity. Monica Basin region, and those found toward the southeast in the San Pedro Channel and the channel between the islands of Catalina and San Clemente were more broadly dispersed. SCB surface flow during spring tends to be reversed in relation to winter conditions, that is predominantly equatorward [Bray et al., 1999]. This reversal could partly account for differences observed in the spring eddy field, possibly coupled with fewer local upwelling events as summer approaches. Eddies in summer (Figure 13d) and fall (Figure 13e) were distributed throughout the SCB but were fewer in number during summer as discussed earlier. During these two time periods, strong poleward flow is found nearshore in the SCB, and strong equatorward flow is found offshore; a seasonal maximum has also been described for large-scale cyclonic recirculation within the SCB, that is, the Southern California Eddy [Hickey, 1998]. Further, cyclonic flow within the Santa Barbara Channel also experiences a seasonal maximum in summer and early fall [Harms and Winant, 1998]. These circulation 50 I % % % 5.6% 6.5% 0 t r 1' -i 1to5 5to10 10to15 15to20 20 to 50 Eddy Size Class (kilometers) Figure 12. Size distribution of eddies detected in the Southern California Bight via ERS-1 and ERS-2 SAR imagery from 1992 to 1998; percentages out of total number (107) of eddies observed.

17 DIG!ACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT 22, 'W 120' 119' 118' 117øW 35 ø 35'N C B aa a C C B a 34'.', - a A '"::: :::" '- a.. AA A A A B AA A CABA B.'- 33' B B :...A / ::!!ii ;};:. A B A A 33 ø A AB A B All Seasons 32øN 35'N 32'N 35'N A C 34" 34" A b 33" A 33 ø 32'N 35'N lwinter: December to FebruaryJ Spring: March to May J 32"N 35"N c B C 34' 34' C A A Summer: June to August " Fall: September to November 32øN 32'N 121'W 120' 119' 118' 117'W121"W 120' 119" 118' 117'W Figure 13. Total and seasonal ((a) total, (b) winter, (c) spring, (d) summer, and (e) fall) distribution map of eddies detected in the Southern California Bight via ERS-1 and ERS-2 SAR images from 1992 to Size of eddies indicated by the following: A, 1-10 km; B, km; C, 20 km and greater. Capital letters indicate cyclonic eddies, and lowercase letters indicate anticyclonic eddies. : :

18 22,538 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT Table 2. Average Number of Detected Eddies on ERS-1 and ERS-2 SAR Frames by Season and With Associated Wind Speeds Win. a Spr. a Sum. a Fa. Average number of eddies per standard SAR frame b Wind speed for SAR frames with eddiesc Mean, m s ' Standard deviation Total number of frames Wind speed for SAR frames without eddies c Mean, m s ' Standard deviation Total number of frames Wind speed for all SAR frames c Mean, m s ' Standard deviation Total number of frames a Win., December to February; Spr., March to May; Sum., June to August; and Fa., September to November. b Based on all SCB SAR frames (119 total); values were derived by dividing the total number of eddies observed in each season by the number of standardized SAR image equivalents for that season (i.e., total ocean area imaged by all SAR frames for that season scaled in relation to a single 10,000 km 2 "ocean" SAR frame). c Based on a subset (-75%) of all SCB SAR frames that had nearly instantaneous buoy data available in moderately close (-50 km) proximity. 121 'W 120' 119' 118' 117' 1:winter 2=spring ' r... 2B ZA 2( 3=summer 1 33' 4=fall "5, 2 A 33' B: km C: km 32'N 32' 35'N 35' 3C. '.'...:r 3A4A4B 33' : : :: " 3A 33ø patterns likely contributed to the widespread distribution of eddies throughouthe SCB. Further characterizations of SCB eddy distributions are possible via comparisons with circulation patterns reported by Winant et al. [1999] (hereinafter referred to as W99). W99 (Table 1 and Figure 2 therein) described synoptic, drifterrevealed SBCH circulation patterns, that is, upwelling, relaxation, and cyclonic which correspond to the upwelling/flood east, relaxation/flood west, and cyclonic/propagating cyclones states, respectively, of Harms and Winant [1998]. W99 also showed tracks of the drifters after exiting the SBCH (Plate 1 of W99), characterizing the observed SCB flow as a spring, summer, or winter pattern; these generally correspond with the earlier synoptic SBCH patterns identified by W99, that is, upwelling, cyclonic, and relaxation, respectively. Though the W99 SCB flow patterns have characteristic seasonal appellations, they are not necessarily solely inclusive of that particular season (see Table 1 and Plate 1 of W99). We subset from Figure 13a those SCB eddies that were nearly coincident with the W99 drifter releases, up to 15 days prior or 30 days after a release. This sampling interval reflects an attempt to remain within a relevant temporal context of patterns revealed by the drifters. W99 note that the mean drifter residence time in the SBCH was about 8 days, with the drifters tracked up to 40 days following their release. The extracted eddies were grouped (Figure 14) according to the synoptic channel circulation patterns identified in W99 (as 32'N 32' 35'N 35' 34' 33' 4C 4A 4B 4A 4C 4A4c 4A 4B 4A o 4B 4B 4 A 32'N 32' 121'W 120' 119' 118' 117' Figure 14. Subset of eddy locations from Figure 13a coincident with Santa Barbara Channel drifter releases reported by Winant et al. [1999] and grouped according to circulation patterns identified in the latter; see text for further details. Capitaletters indicate cyclonic eddies, and lowercase letters indicate anticyclonic eddies. 34' 33'

19 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT 22,539 described above). Relative to flow throughouthe SCB, the W99 drifter releases were obviously somewhat biased as they were only released in the SBCH. Further, the extracted eddies recirculation. As with Figure 13e, distribution of the eddies appears generally consistent with strong poleward flow throughouthe nearshore region (Figure 14c). represent only a subsampling of the entire SCB SAR/eddy data set, with further limitations imposed by wind and geographic 5. Discussion biases as previously discussed. However, several interesting trends are still evident. During the W99 upwelling (so-called spring) pattern 5.1. Physical Forcing of Small-Scale Eddies (Figure 14a), eddies were found in the southern half of the The SCB eddies discussed here were informally SCB, especially nearshore in the SM-SPB region, with only one eddy found in the SBCH. This pattern is similar to what was observed in the spring plot (Figure 13c) but now includes data from three seasons (winter, spring, and summer). The W99 upwelling pattern (most common in early spring) is characterized by persistent equatorward winds, and Plate 1 in W99 revealed drifters exiting the eastern side of the SBCH and then flowing broadly equatorward at this time. This pattern may be reflected in the likewise broad distribution of the SAR eddies in the SM-SPB areas, perhaps also indicating some equatorward advection of the eddies by this mean flow along the coast. W99 noted that the most common grounding region characterized, primarily on the basis of their size (i.e., <50-km diameter), as belonging to a class of features identified as "small-scale" eddies. While an examination of the physical mechanisms responsible for genesis of such features is beyond the scope of this paper, some limited discussion and speculation seems warranted. All the SCB eddies documented in this study generally appeared as spirals. Munk et al. [2000], in a significant recent contribution, proposed an analytical model addressing the formation and observed physical attributes of spiral eddies, albeit of a particular type. Briefly, two frontal models [after Margules, 1904' Hoskins and Bretherton, 1972] were considered, part of a preconditioning for the drifters outside of the SBCH was the Santa Monica phase whereby frontal convergences produce and/or concentrate shear and surfactants. These initial conditions Basin, occurring primarily during spring conditions. The W99 cyclonic (so-called summer) pattern (Figure 14b) showed two clusters of eddies, in the vicinity of the northern Channel Islands and in the southeast near the islands of Catalina and San Clemente, and now includes data from both summer and fall periods (Figures 13d-13e). This cyclonic pattern is most common from late spring to early fall, when winds are still persistently equatorward but are countered by a strong poleward alongshelf pressure gradient. Relative to this pattern, Plate 1 in W99 revealed cyclonic drifter circulation in the SBCH, as well as southward flow between and to the west of the northern Channel Islands. Further, Plate 1 (W99) shows some evidence for poleward flow (e.g., via cyclonic recirculation) from the drifters in the vicinity of the engender the subsequent formation and visualization of spiral or "cat's-eye" eddy circulation patterns associated with horizontal shear instability, modified by rotation. The study noted that for these eddies cyclonic rotation is favored for numerous reasons, including shear, static, centrifugal, and inertial instabilities that limit anticyclonic development; we found a similar predominance of cyclones in our study as discussed earlier. Therefore given the ease with which they form, these overwhelmingly cyclonic spiral eddies have been observed in coastal and open ocean waters throughout the world [Munk et al., 2000, Figure 14], though inconsistently given poor spatial and temporal resolution sampling as well as wind aliasing. southeastern islands. This same plate also includes SST fields averaged over 30-day periods and indicates warmer nearshore waters advecting northward from the south. The location of the eddies in Figure 14b addear 2enerallv consistent with these processes. During the W99 relaxation (so-called winter) pattern (Figure 14c), eddies were broadly distributed throughouthe SCB. This pattern is in fact a duplicate of Figure 13e, since the subset of eddies in Figure 14c were in fact all derived from the fall period. This relaxation pattern is most common from Thus despite this detailed treatment by Munk et al. [2000], there is still much that is not known about spiral eddy dynamics. For example, their temporal evolution and overall r ersistence. of the order c f ctnvq ren ireq fi r hpr et,,ay the lack of adequately resolved time series, either remotely (satellite/aircraft) or in situ derived. Similarly, while spiral eddies are thought to extend through the mixed layer, associated with some amount of localized upwelling [e.g., Munk et al., 2000], relatively little is known about their subsurface properties. This point is particularly important late fall through winter. Winds are no longer persistently given their potential for nutrient pumping into surface waters equatorward at this time, but a strong poleward alongshelf [e.g., Falkowski et al., 1991' Siegel et al., 1999], with its pressure gradient still exists. From Plate 1 in W99, during these periods there is evidence for cyclonic activity in the SBCH and strong poleward flow (especially along the mainland coast) from drifters exiting the SBCH to the west but not equatorward flow in the eastern channel as seen during attendant biological ramifications (discussed below). The work by Munk et al. [2000] likely acco nt.q fc r runny, though not all, of the observed SCB eddies. As indicated earlier, many eddies are regularly found in the close proximity of the numerous SCB islands, headlands, etc., thus suggestive upwelling. Off the western channel entrance and further of being topographically generated (e.. separation vortices). offshore of the bight, equatorward flow is observed, with some apparent nearshore poleward flow in the south near the coast. The SSTs in Plate 1 of W99 also indicate warmer nearshore waters extending poleward from the south in two of the Observational data and modeling results from numerous coastal and island studies [e.g., Pattiaratchi et al., 1987; Wolanski and Hamnet, 1988; Signell and Geyer, 1991; Davies et al., 1995; Denniss et al., 1995; Heywood et el., 1996; B. M. identified relaxation periods, possibly providing additional Hickey et al., submitted manuscript, 2001] demonstrated the evidence for some manner of large-scale SCB cyclonic formation and common occurrence of such features, which

20 22,540 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT often occur in relation to tidal flow. There are certainly other factors contributing to the genesis, development, and distribution of SCB eddies. Among them, wind likely plays an important role, at least indirectly: as previou studies [e.g., Sverdrup, 1938; Owen, 1980] have noted the formation of Perissinotto and Rae, 1990; Ari'stegui et al., 1997]. Further, since many SCB eddies are likely not just shallow surface features, instead quite possibly extending beneath the mixed layer, there is also the potential for vertical pumping of colder, nutrient-rich water to the surface, for example, isopycnal coastal eddies in this region in association with upwelling doming in cyclones [Owen, 1980; Heywood et al., 1990; events. Hickey [1992, 1993] indicated that many small (~20-40 km) SCB features were seemingly locally generated via vertical or horizontal current instabilities and advected with the Arœstegui et al., 1997]. Accordingly, small-scale eddies are capable of impacting phytoplankton patchiness and primary productivity in the SCB, as illustrated here by a SeaWiFS mean flow, extending to at least the depth of the seasonal ocean color image of the Santa Barbara Channel (Plate l e). thermocline. Elsewhere, but of a related nature, Marullo et al. [1985] used in situ and satellite measurements to describe a Note the correspondence between the high chlorophyll locations (Plate l e) with the cold areas of the coincident SST small-scale (~6-km radius) baroclinic eddy in the Ligurian Sea. image (Plate l f). While the potential for synergy between This cyclonic feature exhibited strong isothermal doming and extende down into the surface mixed layer approximately 50 to 100 m. Building on the findings of these and other studies, we intend to address the issue of physical forcing of smallscale SCB eddies in more detail in a future investigation Implications for Coastal Biology and Pollution SAR and ocean color imagery is quite high, unfortunately most of the SAR data utilized here predate the availability of coincident daily ocean color data (i.e., late 1997 onward). The biological impact of these eddies extends beyond the primary producers to higher trophic levels. Nishimoto et al. [1999] documented significantly higher concentrations of juvenile fish in the core of this recurring Santa Barbara Channel cyclonic eddy relative to surrounding waters. Small-scale eddies may be exceptionally important for the biology of coastal waters, potentially affecting nutrient concentrations, plankton and nekton distributions (e.g., 6. Summary and Future Directions patchiness), productivity, larval transport and recruitment, and dispersal of pollutants [e.g., Uda and Ishino, 1958; Murdoch, As documented from a combination of satellite and field 1989; Schumacher et al., 1993; Liu et al., 1994; Pinot et al., measurements, small-scale eddies (<50-km diameter) were 1995; Heywood et al., 1996; Ar[stegui et al., 1997; Limouzy- found to be common in the Southern California Bight, Paris et al., 1997]. However, these biogeophysical impacts, particularly within the Santa Barbara Channel and the Santa with corresponding vertical scales of the order of meters to hundreds of meters and horizontal scales of tens of meters to tens of kilometers, in general are poorly characterized. As seen in the previous SAR imagery (e.g., Figure 8b), Monica-San Pedro Basin regions. The SCB eddies observed here appeared to be predominantly cyclonic in their rotation, were possibly seasonal in their distribution, and may have had distributions characteristic of prevailing large-scale circulation surfactants can converge within small-scale eddies, delineating patterns. Though some artifacts were likely introduced from sea surface slicks with widths of tens of meters to hundreds of wind aliasing and other sampling biases, the trends and meters and lengths upward of 10 km. Slicks are often patterns described here appear robust. Features of this type associated with convergence zones found in fronts, internal waves, and Langmuir cells and typically signify areas of intense biological activity, particularly in coastal waters at were previously known for the SCB but to a limited degree; these eddies were far more prevalent and smaller in size than initially indicated. This is presumably because their small size small spatial scales. For example, these regions have been and ephemeral nature made them difficult to detect with found to be sites of organism aggregations [e.g., Hamnet and Schneider, 1986; Kingsford and Croat, 1986; Wolanski and Hamnet, 1988] and pollutant accumulation [e.g., Szekielda et al., 1972; Shanks, 1987; Tanabe et al., 1991; Garabetian et al., 1993] and similarly can play an important role in larval recruitment [e.g., Kingsford, 1990; Shanks et al., 2000]. previous techniques. High spatial resolution SAR imagery is consistently effective in detecting even the smallest of eddies, while larger eddies are sometimes also observed in AVHRR thermal and SeaWiFS ocean color imagery as seen here, as well as in space shuttle Sun glint photography. Small-scale eddies increase the complexity of flow in the SCB, In the SCB, pollutants have both natural and anthropogenic demonstrating the need for more detailed characterizations of sources. The Santa Barbara Channel contains numerous coastal oceans in this and other regions. In particular, further natural hydrocarbon seeps, and both liquid and gaseous investigations are warranted as they likely affect a variety of seepage is commonly observed at the sea surface [Hornafius et coastal zone phenomena, including nutrient flux, plankton al., 1999]. As evidenced by several of the SAR images patchiness, productivity, larval transport and recruitment, and presented here (e.g., Figures 3, 5, and 6), small-scale eddies are able to entrain this liquid oil seepage, potentially dispersal of pollutants. Key questions remain about small-scale eddies. We have transporting it shoreward. Elsewhere in the SCB (e.g., Santa limited knowledge about their life histories, including Monica Bay), SAR imagery (not shown here) has revealed generation, persistence, and recurrence in space and time, not extensive pollutant laden urban storm water runoff plumes to mention other prope.n.es such as rotational and translational entering the coastal zone. The mixing and dispersal of these speeds, vertical extent, etc. A high temporal resolution, plumes is also likely impacted by the activity of these small- region-wide SAR time series using the wide swath modes of scale eddies and may account for oils contained in some of Canada's RADARSAT and the ASAR instrument on ESA's their slicks. Further sludy is thus required to adequately Envisat mission should help to address many of these issues. characterize small-scale eddies and the content of their slicks However, these satellite data are best analyzed with coincident given their potential for impacting various coastal in situ physical measurements to provide information that biogeophysical phenomena of importance as described above. Eddies are also able to laterally entrain nutrient and/or pigment rich water [e.g., Heywood and Priddle, 1987; cannot be derived via remote sensing (e.g., properties at depth). Further, shore-based HF radar (e.g., CODAR) can provide coincident surface velocity fields for ground truthing

21 DIGIACOMO AND HOLT: SMALL EDDIES IN SOUTHERN CALIFORNIA BIGHT 22,541 and interpreting SAR-derived coastal ocean features. These physical data sets should be complemented by ocean color data Dailey, M.D., J. W. Anderson, D. J. Reish, and D. S. Gorsline, The Southern California Bight: Background and setting, in Ecology of (e.g., SeaWiFS, MODIS, Envisat's MERIS, the latter having the Southern California Bight, edited by M.D. Dailey, D. J. Reish, and J. W. Anderson, pp. 1-18, Univ. of Calif. Press, significantly collocated coverage with ASAR) and in situ Berkeley, nutrient and plankton (phytoplankton, zooplankton, and D'Asaro, E. A., Observations of small eddies in the Beaufort Sea, J. ichthyoplankton) analyses. Such synergies would allow for Geophys. Res., 93, , comprehensive biogeophysical characterizations of these Davies, P. A., J. M. Dakin, and R. A. Falconer, Eddy formation eddies and their associated slicks, at scales of the order of behind a coastal headland, J. Coastal Res., 11, , Denniss, T., J. H. Middleton, and R. Manasseh, Recirculation in the kilometers (i.e., the eddy itself) to meters (e.g., slicks). Along lee of complicated headlands: A case study of Bass Point, J. these lines, fine-resolution airborne and/or satellite Geophys. Res., 100, 16,087-16,101, hyperspectral data would add ocean color data of the Dever, E. P., M. C. Hendershott, and C. D. Winant, Statistical aspects of surface drifter observations of circulation in the Santa Barbara appropriate scale (meters) to complement the high-resolution SAR data. Channel, J. Geophys. Res., 103, 24,781-24,797, Dewees, C. M., and E. M. Strange, Drift bottle observations of the nearshore surface circulation off California, , Calif. Coop. Oceanic Fish. Invest. (CalCOFI) Rep., 25, 68-73, Acknowledgments. The authors would like to thank Bill Hamner Dorman, C. E., Winds between San Diego and San Clemente Island, and Jim McWilliams of the University of California, Los Angeles, J. Geophys. Res., 87, , and Frank Carsey of the Jet Propulsion Laboratory (JPL) for Dorman, C. E., and C. D. Winant, The structure and variability of the providing the initial resources for this effort and Bill Patzert of JPL marine atmosphere around the Santa Barbara Channel, Mon. and Walter Munk and Larry Armi of the Scripps Institution of Weather Rev., 128, , Oceanography (SIO) for valuable discussions. We appreciate Ermakov, S. A., S. G. Salashin, and A. R. Panchenko, Film slicks on comments from several anonymous reviewers that greatly improved the sea surface and some mechanisms of their formation, Dyn. the manuscript. Clint Winant, Myrl Hendershott, Sabine Harms, Ed Atmos. Oceans, 16, , Dever, and Doug Alden, also of SIO, generously made available their Espedal, H. A., O. M. Johannessen, and J. Knulst, Satellite detection in situ measurements from the Santa Barbara Channel. The ERS-1 of natural films on the ocean surface, Geophys. Res. Lett., 23, and ERS-2 SAR data were supplied by the European Space Agency; , AVHRR data were supplied by NOAA's CoastWatch Program. The Espedal, H. A., O. M. Johannessen, J. A. Johannessen, E. Dano, D. authors would also like to thank the SeaWiFS Project (code 970.2) R. Lyzenga, and J. C. Knulst, CoastWatch'95: ERS 1/2 SAR and the Distributed Active Archive Center (code 902) at NASA detection of natural film on the ocean surface, J. Geophys. Res., Goddard Space Flight Center, Greenbelt, Maryland, for the 103, 24,969-24,982, production and distribution of the SeaWiFS data, respectively. P. Ewing, G., Slicks, surface films, and internal waves, J. Mar. Res., 9, DiGiacomo was supported by a predoctoral fellowship from the , NASA Graduate Student Researchers Program and a postdoctoral Falkowski, P. G., D. Ziemann, Z. Kolber, and P. K. 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