Mesoscale variability in the Alboran Sea: Synthetic aperture radar imaging of frontal eddies

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. C6, 3059, /2001JC000835, 2002 Mesoscale variability in the Alboran Sea: Synthetic aperture radar imaging of frontal eddies Jordi Font, 1 Stephan Rousseau, 1 Bernardo Shirasago, 2 Elisa García-Górriz, 1,3 and Robert L. Haney 4 Received 2 February 2001; revised 23 September 2001; accepted 30 October 2001; published 27 June [1] In autumn 1992 the entire Alboran Sea (western Mediterranean) was sampled by the Spanish R/V García del Cid. The incoming jet of Atlantic water, its associated meandering front, and the two big anticyclonic gyres were described from conductivity-temperaturedepth (CTD) and acoustic Doppler current profiler (ADCP) data. Smaller-scale eddies were also observed. Additionally, 36 ERS-1 synthetic aperture radar (SAR) scenes were obtained from mid-september to mid-october. The SAR images capture these features when wind conditions are suitable. The current shear is mainly depicted as narrow lines of low backscatter because of the damping of waves by natural surface films. These lines delineate the northern border of both gyres and the beginning of the alongslope Algerian current at the eastern limit of the Alboran Sea. ADCP observations confirm that lines on SAR imagery follow the direction of the surface currents. The two gyres present high backscatter values in their center, while their frontal boundaries appear modulated by the variation of the marine atmospheric boundary layer stratification due to the surface thermal front. SAR has observed small spiral eddies that were not evidenced by the almost contemporaneous but too coarse CTD in situ sampling. Good spatial correspondence between radar-detected and in situ-measured structures occurs when comparing SAR images to the surface dynamic topography, rather than strictly surface water characteristics. INDEX TERMS: 0933 Exploration Geophysics: Remote sensing; 4223 Oceanography: General: Descriptive and regional oceanography; 4520 Oceanography: Physical: Eddies and mesoscale processes; 4528 Oceanography: Physical: Fronts and jets; 4572 Oceanography: Physical: Upper ocean processes; KEYWORDS: ocean mesoscale, SAR, Alboran Sea, western Mediterranean Sea, surface layer circulation 1. Introduction [2] The Alboran Sea is the region of the western Mediterranean in contact with the Atlantic Ocean through the strait of Gibraltar (Figure 1). As a consequence of the Mediterranean s being like a negative estuary (concentration basin: annual evaporation is higher than freshwater input), a twolayer exchange takes place with an inflow of fresh Atlantic water (S < 36.5) in the upper layer ( m) and an outflow of saltier Mediterranean water (S > 38.4) below. The incoming Atlantic water mixes with the surface water present in the region and gives rise to the modified Atlantic Water (AW) [Gascard and Richez, 1985] that fills the southern Alboran Sea and then flows eastward to the whole Mediterranean basin. The surface inflow at Gibraltar occurs as a 1 Institut de Ciències del Mar, CSIC, Barcelona, Spain. 2 Interdisciplinary Center of Marine Sciences CICIMAR-IPN, La Paz, Baja California, Mexico. 3 Now at Inland and Marine Waters Unit, Institute for Environment and Sustainability, Joint Research Centre, Ispra, Italy. 4 Department of Meteorology, Naval Postgraduate School, Monterey, California, USA. Copyright 2002 by the American Geophysical Union /02/2001JC narrow northeastward jet (25 30 km wide) that later forms a meandering front, usually coupled to one or two large anticyclonic gyres (100 km diameter), as commonly observed in satellite infrared imagery: the Western Alboran Gyre (WAG) and the Eastern Alboran Gyre (EAG). The Alboran Sea presents the most intense horizontal density gradients and current speeds in the Mediterranean basin. Downstream from the EAG, an intense front (the Almeria-Oran front) can develop in the upper 300 m between the incoming AW and the resident surface Mediterranean water [Tintoré et al., 1988; Prieur and Sournia, 1994]. Near Oran this frontal jet forms the alongslope Algerian current that flows eastward, becomes unstable, and produces the main dynamic characteristics of the southern basin of the western Mediterranean [Font et al., 1998; Millot, 1999]. Most of the studies have revealed that the Alboran Sea large-scale structure presents high temporal variability. Diagnostic analysis of experimental data have shown that mesoscale eddies are observed along the edges of the large gyres and are associated with intense vertical motion [Tintoré etal., 1991]. To investigate the threedimensional structure and the mesoscale variability of the Alboran Sea gyres, a high-resolution in situ and remote sensing program was organized for autumn [3] The frequent periods of minimal cloud cover in the Mediterranean region have made it possible to detect by 12-1

2 12-2 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA Figure 1. The 10 dbar dynamic topography in the Alboran Sea derived from the 134 CTD profiles in September October 1992 shows the meandering front associated with the incoming jet of Atlantic water and the two big anticyclonic gyres. The reference level is 400 dbar. G is the Strait of Gibraltar; A is Almería; O is Oran; and the station locations are indicated. temperature differences in satellite infrared imagery the existence of considerable mesoscale phenomena (meanders, eddies, and filaments) originating near frontal boundaries [e.g., López-García et al., 1994]. Because the internal Rossby radius of deformation is usually smaller in the Mediterranean Sea (close to 10 km) than in the open ocean, mesoscale structures are small and sometimes only marginally resolved in images with a pixel size of the order of 1 km. In addition, Mediterranean fronts and mesoscale structures do not always have a clear thermal signature but are dominated by salinity gradients. Therefore synthetic aperture radar (SAR) can be a more efficient satellite sensor than infrared radiometers in detecting some mesoscale circulation structures in this basin. The imaging capability of SAR has been used to detect structures in the sea surface that can be related to circulation in frontal areas [Liu et al., 1994; Nilsson and Tildesley, 1995]. Variations in the Marine Atmospheric Boundary Layer (MABL) stratification in the imaged area induce variations of backscatter that lead to the appearance of ocean frontal structures in SAR images [Wu, 1991; Beal et al., 1997]. The effect is associated with variations of the thermal difference between ocean and atmosphere, mainly due to changes in sea surface temperature (SST), as the horizontal scales of ocean temperature variability are smaller than those of the lower atmosphere. Moreover, horizontal current shear can be identified as radar cross-section perturbations: imparted by small gravity waves-current interaction (bright or dark features) or by damping by surfactants (natural films and algae) trapped within lines of convergence or confluence associated with the local current (dark lines) [Johannessen et al., 1996; Beal et al., 1997; Lyzenga and Marmorino, 1998]. The damping of surface roughness that can reach 9 10 db is strongly dependent on wind speed as well as slick concentration and slick origin [Espedal et al., 1998; Gade et al., 1998]. Marmorino et al. [1997] have shown that the width and position of current signatures on SAR images agree with measured currents associated with a density front of order of a few hundred meters. In the western Mediterranean, preliminary studies [Martínez et al., 1992; Font et al., 1993] indicated that mesoscale structures can also be observed there in SAR images. However, in general, SAR cannot image ocean structures with winds below 2 3 m s 1, when resonant waves are not fully developed, or above m s 1, when the radar backscatter from short gravity waves is effectively saturated by the wind stress field [Johannessen et al., 1996]. [4] Some authors [Lyzenga, 1991; Johannessen et al., 1996] have modeled the expression of density fronts in SAR images of the sea surface as a function of the radar look and

3 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA 12-3 wind directions with respect to the front, as well as the shear and convergence intensity. In film-free water, surface convergence will lead to increased roughness in the vicinity of the convergence region. This is found for internal waves, in shallow water regions, as well as in many open ocean fronts such as the Gulf Stream. In the Alboran Sea, in spite of having diagnosed significant convergence from in situ data along the gyres [Ruiz, 2000], wave-current interactions with increased roughness are not a dominant feature, as we will see in the present study. It is also known that in the presence of surface film, converging fronts may cause the opposite negative backscatter anomaly because of accumulation of the film material in the frontal region. Computations with different data sets in the Alboran Sea [Tintoré et al., 1991; Viúdez et al., 1996b; Ruiz, 2000; Allen et al., 2001] have shown that the unstable mesoscale fronts induce intense upward motions in the vicinity of the frontal area. This upwelling, which results in low SSTs, can also give a low backscatter in the presence of weak winds. [5] In this paper we present a comparative analysis of in situ, infrared, and SAR data collected during the Alboran Sea 1992 experiment. The aim of the study is to assess whether, under certain environmental conditions, SAR imagery can provide additional, and higher-resolution, information on mesoscale eddies generated in the frontal areas of the Mediterranean Sea. In section 2 we describe the in situ experiment and the satellite data used for this study. In section 3 we include a general description of the main features imaged in the whole SAR scenes set and an analysis of specific mesoscale structures obtained by comparing SAR information with other data sources. In sections 4 and 5 we present a discussion and the conclusions, respectively. 2. Autumn 1992 Observations [6] An oceanographic cruise (FE92) was carried out in autumn 1992 onboard the Spanish R/V García del Cid. From 22 September to 3 October the entire Alboran Sea was sampled in a grid of 117 regularly spaced (10 0 latitude, 20 0 longitude) conductivity-temperature-depth (CTD) casts (SeaBird SBE25 probe), while a vessel-mounted acoustic Doppler current profiler (ADCP) (RD Instruments 150 khz) was continuously recording the water motion in the upper 400 m. After a short interruption caused by bad weather conditions, on 6 and 7 October the region east of 1 W was sampled with 17 extra stations. This quite synoptic sampling allowed a detailed study of the Alboran three-dimensional hydrographic structure and dynamic characteristics [Viúdez et al., 1996a, 1996b; Viúdez and Haney, 1997], as well as an accurate analysis and spectral decomposition of the recorded velocity field (E. García-Górriz et al., Nearinertial and tidal currents detected with a vessel mounted acoustic Doppler current profiler in the western Mediterranean Sea, submitted to Journal of Geophysical Research, 2001). The ADCP velocity profiles were recorded under carefully controlled conditions [García-Górriz et al., 1997], which achieved 1 hour averaged profiles with an accuracy of 1 cm s 1 and showed a divergence of the currents in the gyres. An intensive ERS-1 SAR coverage of the Alboran Sea was accomplished for the period mid-september to mid-october, during the satellite 35 day repeat cycle phase. A total of 36 SAR scenes (100 km 100 km squares) were obtained in eight ascending and six descending satellite passes. For a first qualitative analysis, SAR Fast Delivery Copy products (FDC) were used to produce a global mosaic at 200 m resolution [Shirasago et al., 1994]. Here we focus our analysis on specific features, using precision SAR images (PRI) (around 25 m resolution) in conjunction with CTD, ADCP, wind, and infrared information, to identify the signature of mesoscale frontal eddies. These features were selected from the global mosaic in cases where wind conditions were favorable for visualization on the sea surface. [7] Calibrated radar cross-section coefficients have been obtained using the SARTOOLBOX developed by European Space Agency (ESA)/European Space Research Institute (ESRIN) following Laur et al. [1998]. SAR cross-section lines s 0, extracted from calibrated images, are presented, overlaid with the estimated backscatter for 3 and 5 m s 1 winds calculated using CMOD-4. The CMOD-4 wind retrieval model was developed for ERS scatterometer [Stoffelen and Anderson, 1993] but has been successfully used for ERS-1 and ERS-2 SAR. It provides an estimate of s 0 as a function of the wind direction (relative to the radar look angle) and the wind speed at 10 m. The model is tuned for a mean thermal atmospheric stratification. Influence of variations of the MABL due to nonneutral stratification can be estimated with Wu s [1991] model: s=s 0 ¼ expð at=u 10 Þ; ð1þ where T = T atm T sea and a =0.3ms 1 K 1 for a stable atmospheric stratification (T > 0) and 1.55 for an unstable stratification (T < 0). Here s is the radar cross section for nonneutral conditions, while s 0 is the radar cross-section for neutral conditions. [8] A problem appears in the joint analysis of structures in SAR images and in the in situ data: the lack of synopticity (up to 10 days lag) between SAR scenes and the data collected during the cruise at the same geographical location. This is important when comparing both kinds of information since the surface geostrophic jet in the Alboran Sea is about 1 m s 1, and the mesoscale structures generated along the borders of the big gyres are not steady even for a few days. The time mismatch was unavoidable since the typical revisit interval of the ERS-1 SAR is longer than 2 weeks. With this limitation in mind we present the results of an analysis of several specific frontal mesoscale features. 3. Results [9] Figure 1 shows the dynamic topography of the surface of 10 dbar referred to 400 dbar (with extrapolation to the narrow continental shelf) in the Alboran Sea as computed from the 134 CTD stations in September October The meandering front and its associated jet are clearly identified, as are the two big anticyclonic gyres and the formation of the alongslope Algerian current in the southeastern boundary of the sampled region. The same features are traced in the ERS-1 Along Track Scanning Radiometer (ATSR) SST image of 28 September (Figure 2), where the highest temperatures correspond to the centers of the anticyclonic gyres. CTD data indicate that high temperatures

4 12-4 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA Figure 2. Sea surface temperature in the Alboran Sea measured by the ERS-1 Along Track Scanning Radiometer (ATSR) on 28 September 1992, 1031 UT. coincide with low salinities throughout the upper part of the water column at both these locations, thus giving rise to the intense density front and frontal jet observed in Figure 1 (see Viúdez et al. [1996b] for details). Figure 3 presents three temperature and salinity profiles in the upper 200 m, typical of the center of the WAG, the trough between both gyres, and the center of the EAG. The difference between the hydrographic structure inside and outside the gyres is especially strong in the layer m. The intensity of the front can be characterized by the slope of the 28.0s q isopycnal, an indicator of the interface between Mediterranean and Atlantic waters, as shown in Figure 4 for a transect along N. The 28.0s q isopycnal is found at 170 m in the center of the gyres and at 50 m in the trough between them (Figure 4); that is, it has a slope of more than 1 m km 1. This means an horizontal density gradient up to kg m 4 at 100 m. The salinity minima in the upper layers are found in the inflow core near Gibraltar at all depths, but light water accumulates in the center of both gyres. This indicates that ageostrophic mechanisms have to act to transport recent Atlantic water across the quasi-geostrophic frontal jet, as has been revealed by the comparison between dynamic topographies (Figure 1) and ADCP data (Figure 5). [10] The dynamic topographies (e.g., Figure 1), as well as the horizontal temperature and salinity distributions at several depths (not shown; see Viúdez et al. [1996b]) reveal the presence of small cyclonic eddies in the upper ocean along the borders of the big anticyclonic gyres. In spite of the sampling being too coarse for a correct resolution of all such structures, in some cases they are clearly identified, such as the two eddies km in diameter observed west and southwest of both gyres in Figure 1. The hydrographic data also reveal that the large-scale structure and the mesoscale features can be distorted in the top m by the seasonal mixed layer: The surface distributions, as well as SST maps obtained from satellite radiometers, are noisier than the distributions at deeper levels or the surface dynamic topographies, which show more clearly the circulation patterns and their effects on the water masses distribution. [11] Figure 6 is a composition of the 7 SAR.PRI scenes used for the analysis, together with the position of the CTD stations for the area N and W. One can broadly identify the position of the two big anticyclonic gyres and their associated meandering front. The ship track was always following increasing station numbers, except for interruptions due to bad weather between stations 54 and 55, 56 and 57, and 117 and 118. The ERS-1 SAR images were recorded on 15 and 28 September and 1 and 2 October Meandering Border of EAG and Formation of the Algerian Current: 28 September and 2 October 1992 [12] An ascending ERS-1 pass (orbit 06031; frames 0729 and 0711) at the eastern exit of the Alboran Sea (Figure 7) occurred the same day (28 September) as the infrared image shown in Figure 2. When examining both kinds of information, an excellent correspondence is found between the cold water trace and the frontal shear lines along the northern border of the EAG. This shows that when both sensors are observing the same area with good environmental conditions (no clouds and a thermally differentiated water vein for ATSR and adequate wind stress for SAR images, in this case, 4 5 m s 1 ), they locate the frontal jet

5 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA 12-5 Figure 3. Potential temperature (thin) and salinity (thick) profiles from three CTD stations: 32 in the center of the WAG, 58 in the Mediterranean water between the WAG and the EAG, and 99 in the center of the EAG. Figure 4. West-east vertical distribution of density (s q,kgm 3 ) in the top 200 m along across the two anticyclonic gyres of the Alboran Sea in September October 1992.

6 12-6 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA station grid or the along-track surface measurements. Smallscale variations in surface in situ variables (continuous thermosalinometer record from 4 m below sea level) were observed 3 days later between the coast and the strong density gradient of the EAG border (Figure 9). [13] Around 1 W the jet associated with the EAG reaches the African coast almost perpendicularly, and then, while part of it recirculates to the west to close the EAG, most of the flow is directed to the east and forms the Algerian current along the narrow continental slope (Figures 1 and 5). On 2 October, the same day that the ship was sampling this area, ERS-1 SAR imaged it (Figure 10: orbit 06351; frames 2889 and 2871) under winds from 1 to 8 m s 1. A strip <10 km wide of narrow low backscatter lines, similar to those observed on the SAR image of 28 September, clearly indicates the position and direction of the EAG eastern border, in coherence with the situation 4 days earlier (see composition in Figure 6). Some 11 km before reaching the coast, this strip of dark lines turns to the east and indicates the beginning of the Algerian current. The in situ data (ADCP, Figure 10, and thermosalinometer (not shown)) also show a good correspondence with this feature in the two locations where it was intersected (stations and ), even though the easternmost crossing occurred on 6 October (a time lag during which it appears to have shifted slightly to the location of station 122). The singularity is situated in the fresher side of the front (Atlantic water) and is marked by a temperature minimum. It is important to stress that in this area, unlike in the northern sides of the gyres imaged on other satellite passes, the slicks detected by SAR are concentrated only along the front. Previous authors [e.g., Tintore et al., 1988], and also our own visual observations just between stations 106 and 107, have noted the accumulation of several kinds of materials (algae and debris) in the convergence zone along the Almerı a-oran front. Figure 5. ADCP data recorded at 16, 48, and 96 m depth during FE92 cruise, interpolated to a regular grid of at the same place; in this case, with a precision of 3 km. Similar results have been obtained in other oceanic regions (e.g., Nilsson and Tildesley [1995] in the East Australian Current, Johannessen et al. [1996] in the Norwegian Coastal Current, and Beal et al. [1997] in the Gulf Stream). Figure 8 shows the geostrophic jet centered between stations 97 and 96 and the density front intersecting the surface between stations 96 and 95. This is in agreement with the SAR image in spite of the 3 day time lag between the two data sources. The ADCP data, recorded on 1 October (Figure 7), confirms some of this circulation pattern, as well as its southeastward continuation along the border of the gyre, which appears also in the SAR and infrared images. Some meandering (40 km wavelength) of the shear lines is observed by SAR (Figure 7) along the northern border of the EAG, and also smaller dark structures are depicted in the generally bright area north of the jet. These could be signatures of mesoscale motion generated in the front, but they are too small to be detected by the CTD 3.2. Coastal Dipole North of the WAG: 15 September 1992 [14] SAR images of the WAG were acquired on 15 September (Figure 11, strip of two consecutive scenes: orbit 06115; frames 0729 and 0711) with southwesterly winds of 5 6 m s 1 and abundant cloud cover, which prevented infrared sensing of the sea surface. Several surface film features delineate the current shear along the northern side of the meandering front and also indicate the flow splitting between the current lines that turn cyclonically to the eastern Alboran Sea and those that close the WAG to the west along the African coast, as is also seen by ADCP (Figure 11) in the in situ survey on September. The north-south meandering observed by SAR near 3 30 W can be related to the undulations seen in the dynamic topography (Figure 1). The shear lines also depict a dipole (mushroom-like) eddy in the northern part, near the Spanish coast: a small coastal anticyclonic eddy and a bigger cyclonic one in contact with the current lines along the frontal jet. This dipole structure was not revealed by the hydrographic sampling taken 10 days after the SAR image. Although the time elapsed could be enough for the disappearance of this mesoscale structure (the e-folding timescale for the WAG has been evaluated to be of the order of 10 days [Perkins et al., 1990; Garcı a Lafuente et al., 1998]),

7 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA 12-7 Figure 6. Alboran Sea: mosaic of seven ERS-1 SAR images contemporaneous to the FE92 campaign in autumn 1992 plus positions of the CTD stations. (#ESA )

8 12-8 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA Figure 7. The EAG identified by ERS-1 SAR as contoured by shear lines on 28 September 1992 plus overlaid ADCP velocity vectors at 16 m averaged every 3.5 km along the ship track from stations 85 to 94 (30 September to 1 October) and 94 to 102 (1 2 October 1992). Some gaps are due to interruptions in data acquisition. (#ESA )

9 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA 12-9 Figure 8. Vertical section of s q (lines, kg m 3 ) in the top 200 m across the EAG border (stations 94 99) on 1 October 1992 plus cross-section geostrophic velocity (graytones) referred to 400 dbar. it is evident that the spatial resolution of the station grid would never allow its detection. We cannot tell whether this dipole is directly related to the frontal circulation or not, but it certainly affects the surface Mediterranean water trapped between the coast and the WAG. The SAR image presents the WAG as an area having backscatter values higher than its surroundings. North and east of the WAG, slicks are observed on the SAR images, while no slicks are found inside the WAG. [15] The front between the WAG and the water to its north is clearly visible with SAR, and its width, about km according to the backscatter gradient (Figure 12), agrees with the one observed with the in situ data. Allowing for a possible southward shift of the frontal location during the 10 day lag between the SAR and in situ data, there is a good correspondence between the observed signals. According to Wu s [1991] model (equation (1)) and assuming neutral atmospheric stratification in the center of the WAG and the same air temperature on both sides of the surface density front, the measured 2.5 C increase in SST when crossing from north to south (Figure 12) and the 4 m s 1 wind imply an increase of 2 db in surface backscatter. The SAR image indicates a backscatter increase of 3 db on 15 September along the line from station 36 to 35, in good agreement with our estimated value. North of the front, the in situ SST also increases, but this was not reflected in the SAR backscatter 10 days earlier because of the high mesoscale variability. The location of the front is more precise in SAR backscatter than in thermosalinometer records Cyclonic Eddy Between WAG and EAG: 1 October 1992 [16] On 1 October the central Alboran Sea was imaged by ERS-1 SAR, when the winds there were light and variable. An enlargement of the area of the trough between the two anticyclonic gyres (Figure 13: orbit 06344; frame 0711) shows the presence of a very well defined cyclonic eddy, 20 km in diameter, that was not revealed by CTD data. This spiralling eddy is superimposed on a more diffuse signature of what almost certainly is the effect of atmospheric internal waves [Vachon et al., 1995] traveling toward the northeast. The eddy is in contact (Figure 6) with the lines that delimit the eastern border of the WAG. Southeast of the eddy, one can observe the signal of the jet flowing to the eastern Alboran and initiating the EAG, also revealed by the ADCP records on 26 September (Figure 5). The high-resolution thermosalinometer data (not shown) indicate a relative peak in water density in the eddy location (between stations 50 and 51) but do not display a difference between the dense Mediterranean water to the north and lighter water to the south. The unfiltered ADCP along-track data indicate weak velocities in the eddy location (not shown; see Figure 5), although a cyclonic structure is observed at a larger scale (stations 49 52), related to the positive vorticity in the area due to the meandering frontal jet that links the WAG to the

10 12-10 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA EAG and not to the presence of this mesoscale eddy, whose vorticity should be more than just the background positive vorticity. The closest uncloudy ATSR image (28 September, Figure 2) reveals a roughly complicated SST structure east of the WAG. In any case a mesoscale cyclonic eddy, as observed by SAR on 1 October, was not detected by either in situ or infrared data when the vessel was sampling the area. [17] The vorticity field derived from ADCP data (Figure 14) shows an area of positive vorticity at 35.9 N, 3.7 W, some 45 km upstream from the location where the cyclonic eddy was imaged by SAR 5 days later. This clearly suggests this eddy s being formed along the northern border of the WAG, possibly by an instability of the jet, and then being propagated downstream. Viúdez et al. [1996b] computed the vertical shear in the jet stream from the CTD data recorded in this cruise and found a pronounced maximum at 36.1 N, 4.3 W [Viúdez et al., 1996b, Figure 19], so that north of the WAG might be a place for baroclinic instability processes. [18] Besides the signature of the slicks and the atmospheric internal wave features, the backscatter intensity displays a clear decrease while crossing the eddy (not shown). The minimum is observed in the center of the eddy and reaches an average decrease of 4 db. Meteorological charts of the day present a complicated pattern in this area. Several atmospheric fronts are crossing it, and it is not possible to obtain a correct estimation of the wind direction and thus of the wind speed using the CMOD-4 model [Stoffelen and Anderson, 1993]. Nevertheless, using it for a large range of wind direction, we found that wind speed should lie between 2 and 3 m s 1 in the surroundings of the eddy. The backscatter decrease inside the gyre is in accordance with a SST decrease and its cyclonic nature. Assuming a neutral MABL stratification outside the eddy, and using Wu s [1991] model (equation (1)), we simulate the radar cross-section variation due to MABL stratification while crossing the front for the measured SST difference of 1.5 C. The decrease of s 0 for a 3 m s 1 speed is about 3 db, which is similar to the backscatter variation of 4 db observed on the SAR image. Moreover, such conditions (low wind speed and cold eddy) are the most favorable for inducing a decrease of radar cross section [Beal et al., 1997]. Figure 9. Surface temperature, salinity, and density recorded on 1 October 1992 along stations track plus SAR-calibrated backscatter intensity along the same line on 28 September. 4. Discussion [19] Although in this specific experiment in autumn 1992 the salinity difference (between Atlantic and Mediterranean waters) and the seasonal temperature gradients were acting in the same sense on the density gradients in the Alboran Sea, they do not always play the same role in the upper ocean dynamics. Viúdez et al. [1996b], after a detailed analysis of the hydrographic data collected during this cruise, concluded that the temperature gradients at 100 m mainly control the density gradients in the WAG, whereas salinity gradients control the density gradients in the front. In the area of the EAG we obtained the best simultaneity between SAR, infrared, and in situ information, and the three data sets located coherently the surface layer front. Our results indicate that even if a cold water vein was not entrained by the frontal jet, the salinity-controlled frontal circulation would have been similar, and the SAR would have been able to image it. As a result, SAR imagery is a much better indicator of such salinity-controlled frontal dynamics than is satellite SST. [20] In our set of SAR images the main features of the circulation are identified by the presence of surface films, in the shearing zones or near the coast, always on the denserwater side of the front. We also found that the core of the WAG presents high values of backscatter that seem to be associated with the influence of variations of the stratification of the atmospheric boundary layer caused by the elevated SSTs in the gyre [Wu, 1991]. Unlike other studies with currents of similar characteristics (e.g., Johannessen et al. [1996]: Norwegian current with V max = 1.25 m s 1 and core width = 25 km), no current-wave interaction features appeared clearly on our SAR images. Beal et al. [1997] pointed out that SAR images may not display this kind of hydrodynamic interaction because of short wave damping by surface-floating algae (sargassum). For higher (intermediate) wind speed, when surfactants start to be dispersed, the hydrodynamic interactions should appear. In the same way, if the concentration of surfactants reduces

11 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA Figure 10. The EAG eastern limit and formation of the Algerian current identifed by ERS-1 SAR on 2 October 1992 plus ADCP velocity vectors at 16 m averaged every 3.5 km along the ship track from stations 102 to 107 (2 October) and 118 to 123 (6 October 1992). (#ESA )

12 12-12 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA

13 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA Figure 12. Surface temperature, salinity, and density recorded on 25 September 1992 along the station track plus SAR-calibrated backscatter intensity along the same line on 15 September. along the current shear, hydrodynamic features should also appear. Such a pattern was found in October 1996 in the WAG area with a similar density gradient as in 1992 [Rousseau and Font, 2000]. In early autumn 1992 the conditions (low winds and enhanced biological activity by summer stratification) were favorable for damping in the whole Alboran basin and especially close to the northern side of the frontal jet and near the Spanish coast because of the presence of upwelling. [21] The northeastern border of the EAG was imaged twice in our SAR data set: 28 September (ascending pass) and 2 October (descending pass). Its location and orientation is consistent between the two images 4 days apart, but the surface roughness signature is significantly different. On 2 October (Figure 10) we observe a band of slicks that mark the convergence zone where the front intersects the surface, just on the dense-water side of the frontal geostrophic jet (Figure 15). The simultaneity between SAR and in situ data is complete, as ERS-1 passed at 1045 UT when the vessel was steaming from station 105 to station 106 over the core of the jet. The wind recorded onboard at stations 106 and 107 indicated 3 m s 1 from the north. One should note (Figure 15) that the geostrophic velocity perpendicular to the ship track is lower and spatially smoother than in Figure 8 since we were crossing the jet at a small angle (20 ). The ADCP data (Figure 10) reveal also a flow directed toward the jet between stations 106 and 107. This is an indication of convergence in the area of the surface slicks both imaged by SAR and observed visually from onboard. [22] On 28 September (Figure 7) the SAR look angle with respect to the front was very similar to that on 2 October. However, the frontal area appears quite different in the image. One reason could be the different wind conditions. We do not have in situ wind records near the front for 28 September, and the available analysis for 29 September at 0 UT (2 hours after ERS-1 overpass) indicates a small wind variability over the Alboran Sea with 4 5 m s 1 from SE at that point. However, the image clearly indicates the effect of very low winds (dark area) south of the region near N. This implies the presence of a wind front not resolved by the analysis. In fact, at that moment the vessel was at station 60 (36 N, 3 W) it recorded no wind, while the diagnosis indicated 6 7 m s 1 from SE there. As the characteristics of the front were similar along the EAG border (Figure 8) and the beginning of the Algerian current (not shown), we have to conclude that the different signature in SAR images should be due to atmospheric variability. In spite of this different visualization the location of the front was always correct. [23] Let us now focus on the small eddy identified in the trough between the two gyres. García Lafuente et al. [1998] sampled in July 1993 a high-salinity cyclonic eddy of similar size in a position just south of ours. On that occasion the size and position of the WAG was very similar to what we observed in September 1992, but the jet in the trough was slightly farther north. Their cyclonic eddy, which originated in the northern area of Mediterranean waters as revealed by its salinity and fish larvae content, was located on the southern side of the jet. García Lafuente et al. [1998], by analyzing a series of satellite SST images and sea level data at Gibraltar, draw a hypothesis of temporal evolution of the WAG structure related to the flow characteristics at the strait. The cyclonic eddy should be the leading edge of a vein of Mediterranean water dragged southward by the jet and later trapped in its southern location by a reconstructed WAG-jet structure caused by an enhanced intrusion of cold Atlantic water at Gibraltar. This interpretation is supported by similar sequences of images presented by other authors [Heburn and La Violette, 1990; Viúdez et al., 1998] and could be a possible scenario for the evolution of the situation we found in September Figure 11. (oppposite) The WAG and mesoscale eddies near the Spanish coast viewed by ERS-1 SAR on 15 September 1992 plus ADCP velocity vectors at 16 m averaged every 3.5 km along the ship track from stations 21 to 47 (24 26 September 1992). (#ESA )

14 12-14 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA Figure 13. SAR detection of a cyclonic ocean eddy in the central Alboran Sea on 1 October The low backscatter (dark) spiral lines are intersected by parallel bands corresponding to atmospheric signals. (#ESA ) In our case the horizontal distribution of density in the upper layers (Figure 16) shows a leading edge of dense (Mediterranean) water near the area where the eddy appears in the SAR image. [24] This eddy, as well as the cyclonic one in Figure 11, has a size and aspect similar to the spirals presented and discussed recently by Munk et al. [2000] and Munk [2001]. Their study of 400 observations, mainly from Sun glint photographs, indicates that ocean spirals mainly occur in autumn and that the Mediterranean Sea is one of the preferential areas of observation. Munk et al. [2000] and Munk [2001] explain the formation of spirals by baroclinic processes in a frontal jet associated with the concentration of surfactants along a converging line. The mechanisms they present for frontal shear evolution, convergence lines twisting into a cyclonic spiral, and spiralling line visualization can be responsible for the existence of the small eddies we have observed by SAR in the Alboran Sea. 5. Conclusions [25] The presence of mesoscale structures along the border of the frontal jet in the Alboran Sea has been detected in a series of ERS-1 SAR images in September October 1992, mainly by means of low radar backscatter slicks in the areas occupied by Mediterranean water. The structures include meanders and eddies. The visualization of these structures in the sea surface roughness is, as previ-

15 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA Figure 14. Horizontal distribution of vorticity at 16 m computed from ADCP data. Shading indicates positive values, and white indicates negative values. ously known, strongly dependent on the prevailing wind conditions. We have observed small eddies that were not evidenced by the almost contemporaneous CTD in situ sampling. The field survey, in spite of being the most complete and highest-resolution coverage ever made in the Alboran Sea, was still too coarse to resolve some mesoscale structures. These small structures, of the order of 2 internal Rossby radii of deformation, were not always seen in infrared satellite images. It is important to stress that good spatial correspondence between radar-detected and in situ-measured structures occurs when comparing SAR images to the surface dynamic topography (a vertical integral of the entire upper layers where the Atlantic water circulates) and not to strictly surface water characteristics. From this we can conclude that baroclinic currents, including frontal circulations, have a significant effect on the sea surface roughness properties detected by SAR. SST maps, on the other hand, will not always reveal this information since the SST will not always be an adequate tracer of the upper ocean circulation, especially in the Mediterranean when the summer upper mixed layer is developed and prevents the density fronts from reaching the surface. [26] The 1992 Alboran Sea experiment was a unique opportunity to compare sea surface roughness structures observed in SAR images with several kinds of in situ information under different hydrographic and meteorological conditions. Although the cruise was not planned to achieve a maximum spatial and temporal coincidence between field and satellite measurements, it has provided important information to increase the understanding of the SAR imaging capabilities and mechanisms in the Mediterranean. The threedimensional description of the density and velocity fields, at high resolution along specific directions and within a few hours or days of the satellite passage, has allowed a detailed comparison of the location and characteristics of hydrodynamic features to their signature in the radar image. The SAR measurements were able to add precision to the location and description of features such as fronts and eddies that were Figure 15. Vertical distribution of salinity (lines) and geostrophic velocity (shading) along the ship track on 2 October (stations ) in the Almería-Oran front.

16 12-16 FONT ET AL.: SAR IMAGING OF FRONTAL EDDIES IN THE ALBORAN SEA Figure 16. Horizontal distribution of potential density (s q,kgm 3 ) at 5, 50 and 100 dbar from the CTD stations (26 29 Sept.) in the vicinity of the eddy observed in the SAR image on 1 Oct. (indicated by a dashed circle). detected by the in situ data and to identify additional features not resolved by such data. The weak points of our study are the lack of repeated and strictly simultaneous in situ measurements to investigate in depth the different imaging mechanisms of specific features as a function of radar look direction, wind direction, intensity, etc. The availability of simultaneously recorded wind data is a big help in this regard. [27] To advance effectively in the study of SAR imaging of mesoscale frontal features in the Mediterranean Sea, it is crucial to perform in situ measurements with adequate temporal and spatial simultaneity. Higher horizontal resolution sampling is needed in the whole upper ocean, not only near the sea surface. In the Alboran Sea 1992 experiment it was not possible to have the SAR images until several weeks after acquisition, so they could not be used to direct the sampling in relation to imaged structures. The real-time imagery reception, now a current tool for infrared data with portable satellite reception stations, is much more difficult with SAR since it implies a huge data processing, although some tests have been done in the Mediterranean [Chic et al., 1997]. In future experiments a better explanation and interpretation of SAR images will only be obtained with such a simultaneity and the use of additional instruments: surface drifting floats, ship-mounted scatterometer, radiometer, atmospheric sensor package for mean and turbulent wind and flux observations at several heights, and ocean color from satellite. [28] Acknowledgments. This is a contribution to the ESA Earth Observation Program (project AO E1) that provided all the ERS-1 data and to the OMEGA project (MAS3-CT95-001) of the European Union Marine Sciences and Technology (MAST) program. The Spanish National Program on Marine Resources (project MAR ) funded the field work. Thanks are given to the crew of R/V García del Cid and to all the colleagues that participated in the oceanographic cruise, data processing, and discussions on the results obtained. Stephan Rousseau received a Marie Curie grant from MAST (MAS3-CT ). Bernardo Shirasago did his thesis work in Barcelona with a scholarship from the Universidad Nacional Autónoma de México. Elisa García-Górriz acknowledges a predoctoral scholarship from Generalitat de Catalunya and a postdoc fellowship from Spanish Ministery of Education and Science. M. Emelianov and O. Chic assisted in preparing several figures. 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