Multiple thermal fronts near the Patagonian shelf break

Barbara C. Franco,1,2 Alberto R. Piola,1,2 Andres L. Rivas,3,4 Ana Baldoni,5

and Juan P. Pisoni2,3

Received 17 September 2007; revised 9 November 2007; accepted 3 December 2007; published 22 January 2008.

[1] Eighteen year (1985–2002) sea surface temperature(SST) data are used to study the intraseasonal variability ofthe Patagonian shelf break front (SBF) in the SW SouthAtlantic Ocean between 39� and 44�S. The cross-shelf breakSST gradients reveal distinct, previously undocumentedthermal fronts located both, offshore and inshore of theSBF. Throughout the year the main SBF, identified as aband of negative SST gradient maxima (relatively strongoffshore temperature decrease), forms a persistent featurelocated closed to the 200 m isobath, while two distinctnegative gradient maxima are located inshore and offshoreof this location. Daily SST images reveal the presence ofthree branches of cold waters whose edges delineate theabove mentioned fronts. The two offshore branches closelyfollow lines of constant potential vorticity ( f/h) and appearto be associated with the Malvinas Current, while a thirdbranch, located further onshore, is not steered by the bottomtopography. South of 40�S the onshore branch forms a quasipermanent 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:10.1029/2007GL032066.

1. Introduction

[2] Downstream of Drake Passage, the northernmostbranch of the Antarctic Circumpolar Current, describes asharp anticyclonic turn and penetrates about 1800 km intothe western Argentine Basin forming the Malvinas Current[e.g., Peterson and Whitworth, 1989]. This is the onlylocation in the Southern Hemisphere where a permanentinjection of cold, nutrient-rich subpolar waters extendsbeyond 40�S [Orsi et al., 1995, Figure 11]. The intrusioncreates unique oceanographic and environmental conditionsin the southwest South Atlantic. Near 38�S the MalvinasCurrent (MC) collides with the southward flowing BrazilCurrent creating one of the most energetic regions of theworld ocean [Gordon, 1981; Chelton et al., 1990]. Theregion is characterized by numerous oceanographic fronts,which are generally associated with high biological produc-

tivity [Longhurst, 1998; Saraceno et al., 2005], enhancedexchanges between the ocean and the atmosphere, andintense vertical circulation. The focus of this work is onthe shelf break front (SBF) which marks the transitionbetween the Patagonia continental shelf and the MalvinasCurrent.[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 theshelf break and the surface some tens of km offshore[Romero et al., 2006]. The SBF represents the transitionbetween diluted subantarctic shelf waters and cold, salty,and relatively nutrient-rich waters of the MC. The SBFinner boundary is located between the 90 and 100 misobaths with a W–E extension of around 80 km at thesurface and 40 km at the bottom [Bogazzi et al., 2005]. Insummer the SBF presents strong thermal gradients(>0.08�C/km) [e.g., Martos and Piccolo, 1988; Saracenoet al., 2005], and weak salinity gradients (0.002 psu/km)[e.g., Romero et al., 2006]. Around 38�–39�S the frontlocation varies seasonally, displacing offshore during sum-mer 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 measure-ments [Saraceno et al., 2005; Romero et al., 2006] showthat the SBF is associated with a band of high chlorophyll-a(chl-a), which is indicative of high phytoplankton concen-tration. The region of high chl-a forms a quasi-continuousband located close to the shelf break during austral springand summer [Podesta, 1988; Longhurst, 1998]. Accordingto Saraceno et al. [2005], local SST gradient and chl-amaxima correspond in time and space and are located at theshelf break, emphasizing the strong topographic control onthe frontal system. In addition, other studies suggest thatsurface chl-a blooms at the shelf break are located inshoreof the thermal front [Romero et al., 2006]. The SBFcoincides spatially with aggregations of zooplankton, scal-lops, fishes and mammals [Brunetti et al., 1998; Thomson etal., 2001; Acha et al., 2004; Bogazzi et al., 2005; Ciocco etal., 2006; Campagna et al., 2007]. High phytoplanktonbiomass associated with the SBF is attributed to nutrientinput by upwelling processes along the shelf break [Carretoet al., 1995; Romero et al., 2006; R. Matano and E. Palma,personal communication, 2007].[5] The thermal expression of the SBF extends approx-

imately between 39� and 44�S and its intensity presentssharp seasonal and somewhat lower interannual variability[Saraceno et al., 2004]. The study also found the SBFclosely follows the 300 m isobath. Despite the strongtopographic control of the front, zonal displacements havebeen reported near its northern boundary. Carreto et al.[1995] report zonal displacements of the SBF near 38�–

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1Departamento Oceanografıa, Servicio de Hidrografıa Naval (SHN),Buenos Aires, Argentina.

2Also at Departamento de Ciencias de la Atmosfera y los Oceanos,FCEyN, Universidad de Buenos Aires, Buenos Aires, Argentina.

3Centro Nacional Patagonico (CENPAT-CONICET), Puerto Madryn,Argentina.

4Also at Facultad de Ciencias Naturales, Universidad Nacional de laPatagonia, Puerto Madryn, Argentina.

5Instituto Nacional de Investigacion y Desarrollo Pesquero (INIDEP),Mar del Plata, Argentina.

Copyright 2008 by the American Geophysical Union.0094-8276/08/2007GL032066$05.00

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39�S. Coastal-trapped waves were suggested as a possiblemechanism leading to the peaks in SST and chl-a variabilityobserved at intraseasonal frequencies along the SBF [Sar-aceno et al., 2005]. Given the ecological importance of theSBF and the evidence of its seasonal zonal displacementsnear 39�S, in this article we discuss the seasonal andintraseasonal frontal variability based on the analysis of18 years of monthly mean SST data.

2. Data and Methods

[6] Satellite-derived SST data were used to locate theSBF climatological monthly mean position and its variabil-ity. As the SBF develops at the transition from warm shelfwaters to colder MC waters, it is associated with negativemaximum cross-shelf break SST gradients near the shelfbreak. Zonal (gx) and meridional (gy) SST gradients werecomputed using a centered difference scheme based on 18years (1985–2002) of monthly mean satellite data from theNOAA/NASA Pathfinder SST [Vazquez et al., 1998].Cross-shelf break gradients (gSST) were then calculatedprojecting those components in a direction 125� from truenorth. The rotation is based on the mean shelf breakdirection between 39� and 44�S in an Equidistant Cylindri-cal projection. Thus, gSST = gx. cos(35�) � gy. sin(35�).[7] We use Pathfinder ‘‘best-SST values’’ daytime data

with 9.28 km resolution. The estimated average accuracy ofthe Pathfinder SST from most daytime matchups is 0.00 ±0.24�C [Kearns et al., 2000]. Because in austral summer theMC advects waters much colder than those present on thecontinental shelf [Rivas and Piola, 2002], the highest SSTgradient intensity is reached in that season. In contrast, inwinter the temperature difference between the continentalshelf and the MC decreases, therefore the SST gradients aresignificantly lower than in summer. Therefore, to analyzethe intraseasonal variability of the SBF, the minimumjgSSTj observed between 39� and 44�S during the climato-logical winter was selected as a threshold value (A. Rivasand J. P. Pisoni, unpublished data, 2007). Although themeasurement accuracy is larger than the frontal thresholdadopted, fronts can be effectively detected based on gradientalgorithms when applied on monthly mean SSTs [Ullmanand Cornillon, 1999; Hickox et al., 2000].

3. Results and Discussion

[8] Climatological (1985–2002) monthly means of sat-ellite-derived gSST were analyzed in the area between 39�and 44�S near the shelf break at 1� latitude intervals (Figure1). In this region the shelf break is located between 110 and165 m depth [Parker et al., 1997]. The analysis shows thatthe SBF is characterized by a negative gSST maximumlocated near the 200 m isobath. The front is persistentthroughout the year. The only exception is at 40�S, wherethe SBF is displaced inshore, close to the 100 m isobath.Similar inshore locations of the SBF near 40�S are apparentin the analysis of Saraceno et al. [2004]. Negative gSSTalong the front reach values <�0.05�C/km mainly fromNovember to May; while gradients slightly lower than�0.02�C/km are persistent along the front during australwinter (July–September). Regardless of the high gradientvariability, depicted by the gSST standard deviations, mainly

during austral autumn (April–June) and spring (October–December), the strongest gradients are located around the200 m isobath (Figure 1). Maximum (negative) gSST arelocated away from the 200 m isobath mainly in the northern(39�S) and southern (42�–44�S) regions, where strongerthermal fronts are apparent at other locations. At 39�S, anegative 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] havereported similar seasonal displacements of the SBF at thislocation. However, our analysis of gSST suggests that thesegradient maxima are not associated with the main thermalband 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 theSBF appears from spring to autumn south of 40�S, while inwinter a distinct thermal front develops offshore from theSBF. During December, January, and February at 44�S thisfeature appears to be displaced further east (Figure 1). Thisoffshore front is associated with well-defined gSST peak at43�S and 44�S during July and August (Figure 1), and isalso apparent as a distinct band of negative gSST separatedfrom the SBF by non-significant (jgSSTj < 0.005�C/km) orweakly 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, (gx2 + gy

2)1/2, used inprevious studies, reveals distinct thermal fronts around theSBF that have not been previously described in the litera-ture. The core of the MC closely follows the 1000 m isobath[Vivier and Provost, 1999a], its equivalent-barotropic struc-ture [Vivier and Provost, 1999b] is likely responsible forlocking the frontal structure to lines of constant potentialvorticity (f/h). For instance, Saraceno et al. [2004] foundthat the SBF approximately follows the line where f/h =2.10�7 m�1.s�1, which lies close to the 300 m isobath.Other studies suggested that the MC bifurcates in thewestern Argentine Basin [Piola and Gordon, 1989]. Theirquasi-synoptic density distribution from winter 1980 sug-gests that at about 43�S the MC upper layer splits in twobranches: an offshore branch describes a sharp cyclonicloop and returns southward, while the westernmost branchcontinues northward along the continental slope [Piola andGordon, 1989, Figure 3]. Our analyses of gSST reveal thesurface expressions of the fronts associated with these twobranches of the MC. During spring and early summer(October–February), between 42� and 44�S, the thermalfront between the MC branches is more evident as low, orpositive gradients (gSST < 0.005�C/km) are observed be-tween (negative) gSST maxima, associated with the SBFand the offshore front (Figure 2). During winter the offshorefront is located closer to the SBF between 41� and 44�S andgSST 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 distinctbranches of relatively cold waters are observed (A, B, andC). The thermal fronts fA and fC shown in Figure 3b

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indicate the negative peaks in gSST formed by the coldbranches A and C. The two offshore temperature minima (Band C), located in the vicinity and east of the 200 m isobath,appear to be associated with the MC and are similar to theMC branches described by Piola and Gordon [1989]. TheSBF, the thermal front associated with branch B, closelyfollows the line where f/h = 5.10�7 m�1.s�1 and the 200 misobath (Figures 1 and 3a). However, the inshore branch (A)

that appears to originate in the outer shelf near 51�S, andextends northward beyond 42�S, and the associated thermalfront fA, do not seem to be effectively steered by aparticular f/h contour. Both present displacements within6.10�7 m�1.s�1 < f/h < 10�6 m�1.s�1. North of 41�S fA islocated onshore from the 100 m isobath (Figure 1), wherethe 10�6 m�1.s�1 f/h contour deflects about 100 km inshore(Figure 3b). Similarly, the SBF is displaced inshore near

Figure 1. Climatological (1985–2002) 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 lowerthan �0.04�C/km are shown in white. Standard deviations are shown below each gSST panel, with sign inverted to easecomparison. The solid black lines indicate the locations of the 100 and 200 m isobaths. Only the 200 m isobath is shown at44�S. The location of the selected area is depicted in Figure 3.

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40�S, close to the 100 m isobath (see Figures 1 and 3). Theapparently wider variability of the inshore branch is mostlikely due to the substantially weaker bottom slopes (andpotential vorticity gradients), and therefore weaker topo-

graphic control over the outer shelf (Figure 3a). Thetransition between the inshore branch and shelf waterscreates the strong negative gSST gradients observed inFigure 3b. South of 40�S the later feature forms a persistent

Figure 2. Climatological (1985–2002) monthly mean of cross-shelf break SST gradients (gSST, �C/km) along thePatagonian shelf break between 39� and 44�S. The purple and white lines are the 100 and 200 m isobaths, respectively.

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front from spring to autumn parallel to the SBF and, duringautumn, it is also apparent further north (Figure 1).

4. Summary and Final Remarks

[11] The intraseasonal variability of the Patagonian shelfbreak front was studied based on 18-year (1985–2002) ofPathfinder SST data. The analysis of cross-shelf break SSTgradients reveals the presence of distinct thermal fronts inthe vicinity of the SBF. The SBF is revealed by (negative)gSST maxima persistent throughout the year, located closeto the 200 m isobath. In addition, other local (negative)maxima are located away from the 200 m isobath near 39�S

and 42�–44�S. In austral autumn between 39� and 44�SgSST lower than �0.02�C/km are observed �40 km on-shore from the mean location of the SBF. South of 40�Sfrom spring to autumn this feature is associated with apersistent front parallel to the SBF. The onshore front isclose to the mean position of surface chl-a blooms at theshelf break during spring and summer [see Romero et al.,2006]. Daily SST images reveal the presence of three mainbranches of relatively cold waters. Two SST minima locatedin the vicinity and offshore from the 200 m isobath appearto be associated with previously described branches of theMC. South of 40�S and onshore from the 200 m isobath we

Figure 3. (a) MODIS/Aqua derived sea surface temperature (SST) image of 4 km resolution for 30 December 2006. Threebranches 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 theGEBCO bathymetry (http://www.ngdc.noaa.gov/mgg/gebco/gebco.html). The region between 6.10�7 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 ofnegative gSST formed along the western edge of cold branches A and C. The f/h (10�7 m�1.s�1) contours are indicated inpurple.

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observed an additional branch of cold water. The laterbranch is also observed north of 40�S during australautumn.[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 SBFmay determine the cross-shelf extension of Patagonianscallop beds (C. Mauna, personal communication, 2007).Given the cross-shelf beds extension (�40 km) is close tothe distance between the mean locations of the inshore front(A in Figure 3) and the main SBF (Figure 1), the branch ofcold waters located onshore of the 200 m isobath might alsoplay a significant role on the ecology of marine species.

[13] Acknowledgments. This work was supported by the Inter-Amer-ican Institute for Global Change Research grant CRN2076, which issupported by the US National Science Foundation (Grant GEO-0452325). Additional support was provided by Agencia Nacional dePromocion Cientıfica y Tecnologica grant PICT04 25533 and GlaciarPesquera, Argentina. SST data were obtained from NASA PhysicalOceanography Distributed Active Center at the Jet Propulsion Laboratory.

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�����������������������A. Baldoni, Instituto Nacional de Investigacion y Desarrollo Pesquero

(INIDEP), Paseo V. Ocampo N� 1, Mar del Plata, B7602HSA, Argentina.B. C. Franco and A. R. Piola, Departamento Oceanografıa, Servicio de

Hidrografıa Naval (SHN), Av. Montes de Oca 2124, Buenos Aires,C1270ABV, Argentina. (ocebcf@furg.br)J. P. Pisoni, Centro Nacional Patagonico (CENPAT-CONICET), Boule-

vard Brown s/n, Puerto Madryn, 9120, Argentina.A. L. Rivas, Centro Nacional Patagonico (CENPAT-CONICET),

Boulevard Brown s/n, Puerto Madryn, 9120, Argentina.

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