Multiple thermal fronts near the Patagonian shelf break

Transcription

1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L02607, doi: /2007gl032066, 2008 Multiple thermal fronts near the Patagonian shelf break Bárbara C. Franco, 1,2 Alberto R. Piola, 1,2 Andrés L. Rivas, 3,4 Ana Baldoni, 5 and Juan P. Pisoni 2,3 Received 17 September 2007; revised 9 November 2007; accepted 3 December 2007; published 22 January [1] Eighteen year ( ) sea surface temperature (SST) data are used to study the intraseasonal variability of the Patagonian shelf break front (SBF) in the SW South Atlantic Ocean between 39 and 44 S. The cross-shelf break SST gradients reveal distinct, previously undocumented thermal fronts located both, offshore and inshore of the SBF. Throughout the year the main SBF, identified as a band of negative SST gradient maxima (relatively strong offshore temperature decrease), forms a persistent feature located closed to the 200 m isobath, while two distinct negative gradient maxima are located inshore and offshore of this location. Daily SST images reveal the presence of three branches of cold waters whose edges delineate the above mentioned fronts. The two offshore branches closely follow lines of constant potential vorticity ( f/h) and appear to be associated with the Malvinas Current, while a third branch, located further onshore, is not steered by the bottom topography. South of 40 S the onshore branch forms a quasi permanent front parallel to the SBF. Citation: Franco, B. C., A. R. Piola, A. L. Rivas, A. Baldoni, and J. P. Pisoni (2008), Multiple thermal fronts near the Patagonian shelf break, Geophys. Res. Lett., 35, L02607, doi: /2007gl Introduction 1 Departamento Oceanografía, Servicio de Hidrografía Naval (SHN), Buenos Aires, Argentina. 2 Also at Departamento de Ciencias de la Atmósfera y los Océanos, FCEyN, Universidad de Buenos Aires, Buenos Aires, Argentina. 3 Centro Nacional Patagónico (CENPAT-CONICET), Puerto Madryn, Argentina. 4 Also at Facultad de Ciencias Naturales, Universidad Nacional de la Patagonia, Puerto Madryn, Argentina. 5 Instituto Nacional de Investigación y Desarrollo Pesquero (INIDEP), Mar del Plata, Argentina. Copyright 2008 by the American Geophysical Union /08/2007GL032066$05.00 [2] Downstream of Drake Passage, the northernmost branch of the Antarctic Circumpolar Current, describes a sharp anticyclonic turn and penetrates about 1800 km into the western Argentine Basin forming the Malvinas Current [e.g., Peterson and Whitworth, 1989]. This is the only location in the Southern Hemisphere where a permanent injection of cold, nutrient-rich subpolar waters extends beyond 40 S [Orsi et al., 1995, Figure 11]. The intrusion creates unique oceanographic and environmental conditions in the southwest South Atlantic. Near 38 S the Malvinas Current (MC) collides with the southward flowing Brazil Current creating one of the most energetic regions of the world ocean [Gordon, 1981; Chelton et al., 1990]. The region is characterized by numerous oceanographic fronts, which are generally associated with high biological productivity [Longhurst, 1998; Saraceno et al., 2005], enhanced exchanges between the ocean and the atmosphere, and intense vertical circulation. The focus of this work is on the shelf break front (SBF) which marks the transition between the Patagonia continental shelf and the Malvinas Current. [3] North of about 50 S the cross-shelf hydrographic structure at the outer Patagonian shelf presents a persistent, surface-to-bottom front intersecting the sea bottom near the shelf break and the surface some tens of km offshore [Romero et al., 2006]. The SBF represents the transition between diluted subantarctic shelf waters and cold, salty, and relatively nutrient-rich waters of the MC. The SBF inner boundary is located between the 90 and 100 m isobaths with a W E extension of around 80 km at the surface and 40 km at the bottom [Bogazzi et al., 2005]. In summer the SBF presents strong thermal gradients (>0.08 C/km) [e.g., Martos and Piccolo, 1988; Saraceno et al., 2005], and weak salinity gradients (0.002 psu/km) [e.g., Romero et al., 2006]. Around S the front location varies seasonally, displacing offshore during summer and onshore during spring and autumn [Carreto et al., 1995]. [4] In situ [Hubold, 1980a, 1980b; Lutz and Carreto, 1991; Carreto et al., 1995], and remote sensing measurements [Saraceno et al., 2005; Romero et al., 2006] show that the SBF is associated with a band of high chlorophyll-a (chl-a), which is indicative of high phytoplankton concentration. The region of high chl-a forms a quasi-continuous band located close to the shelf break during austral spring and summer [Podestá, 1988; Longhurst, 1998]. According to Saraceno et al. [2005], local SST gradient and chl-a maxima correspond in time and space and are located at the shelf break, emphasizing the strong topographic control on the frontal system. In addition, other studies suggest that surface chl-a blooms at the shelf break are located inshore of the thermal front [Romero et al., 2006]. The SBF coincides spatially with aggregations of zooplankton, scallops, fishes and mammals [Brunetti et al., 1998; Thomson et al., 2001; Acha et al., 2004; Bogazzi et al., 2005; Ciocco et al., 2006; Campagna et al., 2007]. High phytoplankton biomass associated with the SBF is attributed to nutrient input by upwelling processes along the shelf break [Carreto et al., 1995; Romero et al., 2006; R. Matano and E. Palma, personal communication, 2007]. [5] The thermal expression of the SBF extends approximately between 39 and 44 S and its intensity presents sharp seasonal and somewhat lower interannual variability [Saraceno et al., 2004]. The study also found the SBF closely follows the 300 m isobath. Despite the strong topographic control of the front, zonal displacements have been reported near its northern boundary. Carreto et al. [1995] report zonal displacements of the SBF near 38 L of6

2 39 S. Coastal-trapped waves were suggested as a possible mechanism leading to the peaks in SST and chl-a variability observed at intraseasonal frequencies along the SBF [Saraceno et al., 2005]. Given the ecological importance of the SBF and the evidence of its seasonal zonal displacements near 39 S, in this article we discuss the seasonal and intraseasonal frontal variability based on the analysis of 18 years of monthly mean SST data. 2. Data and Methods [6] Satellite-derived SST data were used to locate the SBF climatological monthly mean position and its variability. As the SBF develops at the transition from warm shelf waters to colder MC waters, it is associated with negative maximum cross-shelf break SST gradients near the shelf break. Zonal (g x ) and meridional (g y ) SST gradients were computed using a centered difference scheme based on 18 years ( ) of monthly mean satellite data from the NOAA/NASA Pathfinder SST [Vazquez et al., 1998]. Cross-shelf break gradients (gsst) were then calculated projecting those components in a direction 125 from true north. The rotation is based on the mean shelf break direction between 39 and 44 S in an Equidistant Cylindrical projection. Thus, gsst = g x. cos(35 ) g y. sin(35 ). [7] We use Pathfinder best-sst values daytime data with 9.28 km resolution. The estimated average accuracy of the Pathfinder SST from most daytime matchups is 0.00 ± 0.24 C [Kearns et al., 2000]. Because in austral summer the MC advects waters much colder than those present on the continental shelf [Rivas and Piola, 2002], the highest SST gradient intensity is reached in that season. In contrast, in winter the temperature difference between the continental shelf and the MC decreases, therefore the SST gradients are significantly lower than in summer. Therefore, to analyze the intraseasonal variability of the SBF, the minimum jgsstj observed between 39 and 44 S during the climatological winter was selected as a threshold value (A. Rivas and J. P. Pisoni, unpublished data, 2007). Although the measurement accuracy is larger than the frontal threshold adopted, fronts can be effectively detected based on gradient algorithms when applied on monthly mean SSTs [Ullman and Cornillon, 1999; Hickox et al., 2000]. 3. Results and Discussion [8] Climatological ( ) monthly means of satellite-derived gsst were analyzed in the area between 39 and 44 S near the shelf break at 1 latitude intervals (Figure 1). In this region the shelf break is located between 110 and 165 m depth [Parker et al., 1997]. The analysis shows that the SBF is characterized by a negative gsst maximum located near the 200 m isobath. The front is persistent throughout the year. The only exception is at 40 S, where the SBF is displaced inshore, close to the 100 m isobath. Similar inshore locations of the SBF near 40 S are apparent in the analysis of Saraceno et al. [2004]. Negative gsst along the front reach values < 0.05 C/km mainly from November to May; while gradients slightly lower than 0.02 C/km are persistent along the front during austral winter (July September). Regardless of the high gradient variability, depicted by the gsst standard deviations, mainly during austral autumn (April June) and spring (October December), the strongest gradients are located around the 200 m isobath (Figure 1). Maximum (negative) gsst are located away from the 200 m isobath mainly in the northern (39 S) and southern (42 44 S) regions, where stronger thermal fronts are apparent at other locations. At 39 S, a negative gsst maximum is observed onshore (offshore) from the mean position of the SBF mainly during May (January) (Figure 1). Previous studies [e.g., Carreto et al., 1995; C. Mauna, personal communication, 2007] have reported similar seasonal displacements of the SBF at this location. However, our analysis of gsst suggests that these gradient maxima are not associated with the main thermal band of the SBF around this latitude (Figure 2). In autumn, between 39 and 44 S, relatively intense gradients (gsst < 0.02 C/km) are observed 40 km onshore from the SBF (Figure 1). A persistent and continuous front parallel to the SBF appears from spring to autumn south of 40 S, while in winter a distinct thermal front develops offshore from the SBF. During December, January, and February at 44 S this feature appears to be displaced further east (Figure 1). This offshore front is associated with well-defined gsst peak at 43 S and 44 S during July and August (Figure 1), and is also apparent as a distinct band of negative gsst separated from the SBF by non-significant (jgsstj < C/km) or weakly positive gradients during austral spring and summer (November February) (Figure 2). [9] The analysis of cross-shelf break gradients (gsst), rather than the SST gradient magnitude, (g x 2 +g y 2 ) 1/2, used in previous studies, reveals distinct thermal fronts around the SBF that have not been previously described in the literature. The core of the MC closely follows the 1000 m isobath [Vivier and Provost, 1999a], its equivalent-barotropic structure [Vivier and Provost, 1999b] is likely responsible for locking the frontal structure to lines of constant potential vorticity (f/h). For instance, Saraceno et al. [2004] found that the SBF approximately follows the line where f/h = m 1.s 1, which lies close to the 300 m isobath. Other studies suggested that the MC bifurcates in the western Argentine Basin [Piola and Gordon, 1989]. Their quasi-synoptic density distribution from winter 1980 suggests that at about 43 S the MC upper layer splits in two branches: an offshore branch describes a sharp cyclonic loop and returns southward, while the westernmost branch continues northward along the continental slope [Piola and Gordon, 1989, Figure 3]. Our analyses of gsst reveal the surface expressions of the fronts associated with these two branches of the MC. During spring and early summer (October February), between 42 and 44 S, the thermal front between the MC branches is more evident as low, or positive gradients (gsst < C/km) are observed between (negative) gsst maxima, associated with the SBF and the offshore front (Figure 2). During winter the offshore front is located closer to the SBF between 41 and 44 S and gsst maxima are observed at 43 and 44 S (Figure 1). [10] Inspection of daily SST images reveals the complex thermal structure and further suggests a multi-branch MC. Figure 3a illustrates a situation in which three distinct branches of relatively cold waters are observed (A, B, and C). The thermal fronts fa and fc shown in Figure 3b 2of6

3 Figure 1. Climatological ( ) monthly mean satellite-derived cross-shelf break SST gradients (gsst, C/km) across the Patagonian shelf break at 39, 40, 41, 42, 43, and 44 S. Contour interval is 0.01 C/km and contours lower than 0.04 C/km are shown in white. Standard deviations are shown below each gsst panel, with sign inverted to ease comparison. The solid black lines indicate the locations of the 100 and 200 m isobaths. Only the 200 m isobath is shown at 44 S. The location of the selected area is depicted in Figure 3. indicate the negative peaks in gsst formed by the cold branches A and C. The two offshore temperature minima (B and C), located in the vicinity and east of the 200 m isobath, appear to be associated with the MC and are similar to the MC branches described by Piola and Gordon [1989]. The SBF, the thermal front associated with branch B, closely follows the line where f/h = m 1.s 1 and the 200 m isobath (Figures 1 and 3a). However, the inshore branch (A) that appears to originate in the outer shelf near 51 S, and extends northward beyond 42 S, and the associated thermal front fa, do not seem to be effectively steered by a particular f/h contour. Both present displacements within m 1.s 1 < f/h <10 6 m 1.s 1. North of 41 S fais located onshore from the 100 m isobath (Figure 1), where the 10 6 m 1.s 1 f/h contour deflects about 100 km inshore (Figure 3b). Similarly, the SBF is displaced inshore near 3of6

4 Figure 2. Climatological ( ) monthly mean of cross-shelf break SST gradients (gsst, C/km) along the Patagonian shelf break between 39 and 44 S. The purple and white lines are the 100 and 200 m isobaths, respectively. 40 S, close to the 100 m isobath (see Figures 1 and 3). The apparently wider variability of the inshore branch is most likely due to the substantially weaker bottom slopes (and potential vorticity gradients), and therefore weaker topographic control over the outer shelf (Figure 3a). The transition between the inshore branch and shelf waters creates the strong negative gsst gradients observed in Figure 3b. South of 40 S the later feature forms a persistent 4of6

5 Figure 3. (a) MODIS/Aqua derived sea surface temperature (SST) image of 4 km resolution for 30 December Three branches of cold waters are labeled A, B and C. Heavy black lines are constant f/h (10 7 m 1.s 1 ) contours based on the GEBCO bathymetry ( The region between m 1.s 1 < f/h <10 6 m 1.s 1 is hatched. (b) SST cross-shelf break gradient ( C/km) between 39 and 44 S. Here fa and fc indicate bands of negative gsst formed along the western edge of cold branches A and C. The f/h (10 7 m 1.s 1 ) contours are indicated in purple. front from spring to autumn parallel to the SBF and, during autumn, it is also apparent further north (Figure 1). 4. Summary and Final Remarks [11] The intraseasonal variability of the Patagonian shelf break front was studied based on 18-year ( ) of Pathfinder SST data. The analysis of cross-shelf break SST gradients reveals the presence of distinct thermal fronts in the vicinity of the SBF. The SBF is revealed by (negative) gsst maxima persistent throughout the year, located close to the 200 m isobath. In addition, other local (negative) maxima are located away from the 200 m isobath near 39 S and S. In austral autumn between 39 and 44 S gsst lower than 0.02 C/km are observed 40 km onshore from the mean location of the SBF. South of 40 S from spring to autumn this feature is associated with a persistent front parallel to the SBF. The onshore front is close to the mean position of surface chl-a blooms at the shelf break during spring and summer [see Romero et al., 2006]. Daily SST images reveal the presence of three main branches of relatively cold waters. Two SST minima located in the vicinity and offshore from the 200 m isobath appear to be associated with previously described branches of the MC. South of 40 S and onshore from the 200 m isobath we 5of6

6 observed an additional branch of cold water. The later branch is also observed north of 40 S during austral autumn. [12] The SBF plays a strong role on the life cycle of a variety of species [e.g., Acha et al., 2004]. For instance, recent studies suggest that zonal displacements of the SBF may determine the cross-shelf extension of Patagonian scallop beds (C. Mauna, personal communication, 2007). Given the cross-shelf beds extension (40 km) is close to the distance between the mean locations of the inshore front (A in Figure 3) and the main SBF (Figure 1), the branch of cold waters located onshore of the 200 m isobath might also play a significant role on the ecology of marine species. [13] Acknowledgments. This work was supported by the Inter-American Institute for Global Change Research grant CRN2076, which is supported by the US National Science Foundation (Grant GEO ). 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B. C. Franco and A. R. Piola, Departamento Oceanografía, Servicio de Hidrografía Naval (SHN), Av. Montes de Oca 2124, Buenos Aires, C1270ABV, Argentina. (ocebcf@furg.br) J. P. Pisoni, Centro Nacional Patagónico (CENPAT-CONICET), Boulevard Brown s/n, Puerto Madryn, 9120, Argentina. A. L. Rivas, Centro Nacional Patagónico (CENPAT-CONICET), Boulevard Brown s/n, Puerto Madryn, 9120, Argentina. 6of6