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George O. Marmorino,1 Benjamin Holt,2 M. Jeroen Molemaker,3 Paul M. DiGiacomo,4 ...... C. Wackerman, O. M. Johannessen, and P. W. Vachon (1996),.
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, C05010, doi:10.1029/2009JC005863, 2010

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Airborne synthetic aperture radar observations of “spiral eddy” slick patterns in the Southern California Bight George O. Marmorino,1 Benjamin Holt,2 M. Jeroen Molemaker,3 Paul M. DiGiacomo,4 and Mark A. Sletten1 Received 30 September 2009; revised 3 December 2009; accepted 8 December 2009; published 12 May 2010.

[1] Repeat sampling on hourly time scales using an airborne synthetic aperture radar (SAR) is used to investigate the occurrence and evolving characteristics of spiral‐shaped slick patterns, commonly presumed to be indicators of submesoscale ocean eddies, in the area around Santa Catalina Island, California (∼33.4°N, 118.4°W). Simultaneous SAR imagery and boat survey data are examined over two ∼5 h long periods spaced 3 days apart in April 2003. The SAR imagery reveals several spiral‐like patterns, roughly 5 km in diameter, occurring downstream of the western end of Catalina. We believe that the most likely formation mechanism for these patterns is current‐wake instability related to the flow of the Southern California Countercurrent along the north shore of Catalina. In one case, there is an observed cold‐core eddy and vortex sheet attached to the tip of the island, similar to island‐wake simulations done by Dong and McWilliams (2007). In another case, the SAR imagery shows a series of slick patterns that, at least initially, resemble spiral eddies, but the data show no clear evidence of actual ocean eddies being present either at depth or through a rotating surface expression. A speculation is that such features signify island‐wake eddies that are relatively weak and dissipate quickly. An unexpected finding was how quickly a spiral slick pattern could deteriorate, suggesting a time scale for the surface feature of the order of only several hours. An implication of this result is that care is needed when interpreting a single satellite SAR imagery for evidence of active submesoscale eddies. Recommendations are made for future field studies. Citation: Marmorino, G. O., B. Holt, M. J. Molemaker, P. M. DiGiacomo, and M. A. Sletten (2010), Airborne synthetic aperture radar observations of “spiral eddy” slick patterns in the Southern California Bight, J. Geophys. Res., 115, C05010, doi:10.1029/2009JC005863.

1. Introduction [2] High‐resolution imaging radar systems, such as synthetic aperture radar (SAR), can be used to study and monitor a range of ocean phenomena. For an overview of applications see the Synthetic Aperture Radar (SAR) Marine User’s Manual [Jackson and Apel, 2004]. In this paper we report on the use of repeat‐sampling SAR measurements to gain insight into the dynamics of small‐scale ocean eddies, that is, those having diameters ∼O(10 km) and comparatively short lifetimes. Eddies of this size represent a manifestation of a submesoscale oceanography associated with upper‐ocean stirring [Munk et al., 2000], and as such have 1 Remote Sensing Division, Naval Research Laboratory, Washington, D. C., USA. 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA. 3 Department of Earth and Space Science, University of California at Los Angeles, Los Angeles, California, USA. 4 NOAA‐NESDIS Center for Satellite Applications and Research, Camp Springs, Maryland, USA.

Copyright 2010 by the American Geophysical Union. 0148‐0227/10/2009JC005863

important implications for dispersal of pollutants, transport of larvae, distribution of nutrients and plankton, and so on [e.g., DiGiacomo and Holt, 2001; Bassin et al., 2005; Caldeira et al., 2005]. [3] Ocean eddies can be detected in SAR imagery through several mechanisms [Johannessen et al., 1996; Ivanov and Ginzburg, 2002; Lyzenga et al., 2004]. One involves the accumulation of naturally occurring (biogenic) surfactant materials into elongated bands, which arise through surface convergences in either the background current or within an eddy’s evolving flow patterns [Cooper et al., 2005; Johannessen et al., 2005; McWilliams et al., 2009]. A surfactant film creates an elastic sea surface that attenuates short, O(10 cm), surface waves, resulting in slick bands having low radar backscatter. As a result, an eddy can be detected in radar imagery as a set of approximately concentric dark bands, at least under winds favorable for visualization of slicks (e.g., wind speeds of ∼2 to 7 m/s) [DiGiacomo and Holt, 2001]. Not only do slicks indicate regions of convergence, but they also act as passive tracers that can be used to map the surface current field of the eddy [Lyzenga and Marmorino, 1998; DiGiacomo and Holt, 2001]. A common characteristic of such eddies is that the slicks appear to

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to the countercurrent), suggesting that eddies in the area may be created through more than one mechanism. [5] To investigate the physical characteristics of the small‐scale eddies and to explore possible mechanisms for their formation, we conducted a field campaign in April 2003 that combined repeated SAR sampling on hourly time scales with coincident shipboard measurements. Details of the approach are given in the next section, and results are presented in section 3. While the measurements were limited to only a few “good‐visualization” days having joint aircraft‐ boat operations (16 and 19 April), the data do provide new perspectives into the evolution and lifetimes of eddy patterns. The observations are compared with previous studies in section 4; in particular, the “island current wake” hypothesis of Dong and McWilliams [2007] is used to further investigate and assess the physical characteristics found in our data.

2. Methods

Figure 1. Area of the Southern California Bight. The present study focuses on the area around Santa Catalina Island, located in the center of the figure. The two arrows indicate the approximate flow directions of the nearshore Southern California Countercurrent and the offshore California current. SMB, Santa Monica Basin; SPC, San Pedro Channel. spiral inward cyclonically (i.e., anticlockwise in the Northern Hemisphere). The term “spiral eddy” is often applied to such a feature [e.g., Scully Powers, 1986; Stevenson, 1989; Munk et al., 2000]. The spiral pattern is thought to indicate an underlying vortex or eddy created through instability of the larger‐scale flow field [Munk et al., 2000; Eldevik and Dysthe, 2002; Shen and Evans, 2002], although this has yet to be confirmed with actual ocean measurements. [4] In this work, an airborne SAR is used to investigate eddy patterns in the Southern California Bight (Figure 1). A previous study by DiGiacomo and Holt [2001] used satellite SAR imagery (ERS‐1/2 imagery from the period 1992– 1998) and other data as available to describe for the first time in detail the characteristics of small‐scale eddies in this area. Eddies appeared predominantly as cyclonic spirals having diameters of 10 km or less. About half of the eddies occurred in the vicinity of the irregularly shaped island of Santa Catalina (hereafter, Catalina), which suggested some form of topographic generation. Caldeira et al. [2005] and Dong and McWilliams [2007] proposed that the northwestward flowing Southern California Countercurrent induces a wake downstream of Catalina, consisting in part of “spiral current wake eddies.” Some support for this is that DiGiacomo and Holt [2001] found an increased occurrence of eddies around Catalina during winter, when the countercurrent is typically strongest; on the other hand, eddies were detected both upstream and downstream of the island (relative

[6] SAR data were collected using the multifrequency polarimetric Airborne Synthetic Aperture Radar (AIRSAR) system flown aboard a NASA DC‐8 aircraft [van Zyl et al., 1992]. Only data collected at L band (radar wavelength of ∼24 cm) vertical polarization are analyzed in the present paper. The L band images were found to have better signal‐ to‐noise across the range of available incidence angles (22°– 62°). The full polarization L band signatures of slick features in our data set are discussed by Schuler and Lee [2006], and the variation of spiral‐eddy slicks with radar frequency band and sea surface elasticity is examined theoretically by Cooper et al. [2005]. Flight lines, or passes, were done on reciprocal headings of 121°T and 301°T (approximately east‐southeastward and north‐northwestward) to align with the long axis of Catalina (see Figure 1). The coverage area included most of the Santa Monica Basin and San Pedro Channel, with each pass ∼100 km long, extending from south of Catalina to the channel islands of Santa Cruz and Anacapa in the north. Most of the observed eddy features occurred near Catalina (matching results of DiGiacomo and Holt [2001]), and this area is the focus of the present study (Figure 1). Because the width of sea surface (or swath) imaged on each aircraft pass was 11.8 km, it took three adjacent passes (identified as 301–3, 121–4, 301–4) and 46 min to sample the study area. Additional flight lines, using the same orientations but done both closer inshore and farther offshore, were also worked into the sampling to provide larger spatial context. In practice, each of the primary passes was repeated, on average, at an interval of ∼1.5 h, although a ∼2 km wide strip of overlap on adjacent passes was typically resampled in about 25 min. [7] Postflight analysis of AIRSAR data consisted of survey image processing and precision image processing. Survey imagery, which shows entire flight lines processed to a low‐ resolution uncalibrated browse product, are available for all our flights from the Jet Propulsion Laboratory data library (http://airsar.jpl.nasa.gov) under mission name “Southern California Coastal Waters, Ocean Eddies Study” and file names “SanMon” (for Santa Monica Basin) followed by a pass identification number (see Table 1). A subset of the precision‐processed, calibrated AIRSAR data is available from the Alaska Satellite Facility (https://ursa.asfdaac.alaska. edu/cgi‐bin/login/guest/). We found the survey imagery, after

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Table 1. Summary of Synthetic Aperture Radar Imagery Shown in This Papera Order

UT

1 6 7 8 11 13

1549 1744 1808 1830 1949 2026

1 4 5 9 13

1600 1711 1735 1907 2040

Pass ID 16 April 121–4 301–3a 121–4a 301–4a 121–4b 121–4c 19 April 121–4 301–3 121–4a 121–4b 121–4c

Figure Figure 6a Figure 3 Figures 3 and 6b Figure 3 Figures 6c and 7 Figure 6d Figure 12a Figure 10 Figures 10, 12b, and 13 Figure 12c Figure 12d

a Thirteen passes were made on each of the 2 days listed. An a, b, or c in the pass identification (ID) indicates a second, third, or fourth pass done over the same area, respectively.

being converted to ground‐range coordinates, useful both for preliminary analysis and in selecting subscenes for precision processing. An example of the survey imagery is shown in later Figure 3. Subsequent figures show data that has been precision processed. All images are oriented with their horizontal (across‐page) axis aligned along 121°T and 301°T. [8] In situ measurements were made using the R/V UCLA Sea World. Profiles of temperature and salinity were made underway to ∼35 m depth using a “minibat” system (M. J. Molemaker, Preliminary evaluation of spiral eddy experiment April 2003, Inst. of Geophys. and Planet. Phys., Univ. of Calif., Los Angeles, unpublished manuscript, 2003). Additional measurements were made at 0.6 m depth using a semisubmerged platform towed outside the wake of the boat [Marmorino and Trump, 2000]. Current profiles were measured using a hull‐mounted 150 kHz acoustic Doppler current profiler (ADCP) and also a 600 kHz ADCP deployed on the towed platform. Not all measurement systems operated at all times. Typically, the boat made an initial transect from Marina del Rey, near Santa Monica (Figure 1), across San Pedro Channel, and toward the west end of Catalina Island. Coordination between the boat and aircraft relied on a real‐time SAR processor on board the aircraft, which provided a display of the L band imagery. This allowed an assessment of candidate eddy slick patterns, whose locations were then relayed via radio to the research boat. [9] The primary data collection period was 15–19 April 2003. A series of weather systems passed through the study area, resulting in periods of high wind speed (>7 m/s) on each day (Figure 2). One result of the unsettled weather was that advanced very high resolution radiometer imagery and quick‐look RADARSAT products were of little help in guiding the aircraft sampling. Flights made on 16 and 19 April, when the wind speed was relatively low (