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Variability in the Location. of the Antarctic Polar Front (90 ° JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. C13, PAGES 27,825-27,833, DECEMBER 15, !997 Variability in the location.of the Antarctic Polar Front (90ø- 20øW) from satellite sea surface temperature data J. Keith Moore, Mark R. Abbott, and James G. Richman Collegeof Oceanicand AtmosphericSciences, Oregon State University, Corvallis Abstract. The path of the AntarcticPolar Front (PF) is mappedusing satellite sea surfacetemperature data from the NOAA/NASA Pathfinderprogram. The mean path and variabilityof the PF are stronglyinfluenced by bathymetry.Meandering intensity is weaker where the bathymetryis steeplysloped and increasesin areaswhere the bottom is relativelyflat. There is an inverserelationship between meandering intensity and both the width of the front and the changein temperatureacross it. There is a persistent,large separationbetween the surfaceand subsurfaceexpressions of the PF at Ewing Bank on the Falkland Plateau. 1. Introduction of meanderingjets and fronts hasbeen done previouslyfor the PF [Legeckis,1977] and other strongfrontal systems[Hansen TheAntarctic Polar Front (PF) is a strongjet within t•e and Maul, 1970; Olsonet al., 1983; Cornilion, 1986]. Antarctic CircumpolarCurrent (ACG), which flowseastward Satellite altimeter data has shown that there is little zonal continuouslyaround Antarctica [Nowlin and Klinck, 1986].The coherencein the variabilityof the ACC [Fu and Chelton,1984; PF, also known as the Antarctic Convergence,is the location Sandwelland Zhang, 1989; Chelton et al., 1990; Gille, 1994; where Antarctic surfacewaters movingto the north sink rap- Gille and Kelly,1996]. These studies emphasize the importance idly belowSubantarctic waters [Deacon, 1933, 1937].Thus the of local and regionalinstabilities [Chelton et al., 1990;Gille and PF is a regionof elevatedcurrent speeds and stronghorizontal Kelly,1996]. Gille [1994]used Geosat altimeter data and •i gradientsin density,temperature, salinity, and other oceano- meanderingjet model to map the mean locationof the PF and graphicproperties. The PF marksan importantboundary in the SAF. The large changesin sea surfaceheight detectedby terms of air-sea fluxes and the heat and salt budgets of the the two-jet model would likely be associatedwith the subsur- Southern Ocean. The path of the PF exhibits considerable face expressionof the PF [Gille, 1994].Thus comparison of our variabilityin the form of mesoscalemeandering, eddies, and resultswith thosebased on altimeters[Gille, 1994] aswell as in ring formation [Mackintosh,1946; Joyce et al., 1978]. The re- situ subsurfacedata [Orsiet al., 1995] can provide insightsinto gion lyingnorth of the PF and southof the SubantarcticFront relationshipsbetween the surfaceand subsurfaceexpressions (SAF) is termed the Polar Frontal Zone (PFZ). of the PF. The PF has both surfaceand subsurfaceexpressions, whose Barotropicflow in the oceansover smoothlyvarying topog- locationsdo not necessarilycoincide. Strong gradientsin sea raphytends to conserveangular momentum by followinglines surface temperature mark the surface expression[Deacon, of constantpotential vorticity (f + •)/H, where f is the 1933, 1937;Mackintosh, 1946]. Subsurfacedefinitions for the planetaryvorticity, • is relativevorticity, and H is oceandepth. PF mark the location where Antarctic surface waters descend In open oceanareas the planetaryvorticity is muchlarger than rapidly,such as the point where the minimumpotential tem- the relative vorticity, and mean potential vorticity can be ap- peraturelayer sinksbelow 200 m depth [Deacon,1933, 1937; proximatedas f/H. We will use the term planetarypotential Orsi et al., 1995]. Southof Africa, the subsurfaceand surface vorticity for thisf/H approximation.We compare the path of expressionsof the PF are typicallyseparated by distancesless the Polar Front with the planetarypotential vorticity field. than -50 km, althoughseparations up to 300 km have been In the Southern Ocean the large changesin ocean depth noted [Lutjeharmsand Valentine,1984; Lutjeharms, 1985]. associatedwith the mid-ocean ridges and Drake Passagedo Sparrowet al. [1996] concludedthat there is a wide separation not permit the ACC to follow lines of constantplanetary po- between the surfaceand subsurfacePF expressionsin the vi- tential vorticitycircumglobally [Koblinsky, 1990]. The ACC is cinity of the KerguelenPlateau. Deacon [1933] and Guretskii forced at Drake Passageand the north Scotia Ridge across [1987] noted that in someregions of the southwestAtlantic, jsolinesof planetarypotential vorticity, causing inputs of rela- Subantarctic surface waters extend southward over the cold tive vorticity to the water column through the shrinking of Antarctic surface layer, placing the PF surface expression vortex lines. This relative vorticity is likely dissipatedthrough south of the subsurfaceexpression. These southwardexten- nonlinearprocesses such as eddy action or Rossbywaves. sionsof Subantarcticwaters occurredonly in a relatively thin surfacelayer (-100 m deep [Deacon,1933]). 2. Materials and Methods We have used the strong sea surfacetemperature (SST) The positionof the AntarcticPolar Front wasmapped using gradientat the PF to map its surfaceexpression in the region satellite SST data from 1987-1988. The satellite data were the 90ø-20øWover the 2 year period 1987-1988. Similar mapping daily equal-angle"best SST" files (9 km spatialresolution) of Copyright1997 by the American GeophysicalUnion. the NOAA/NASA Pathfinder program. The Pathfinder pro- Paper number 97JC01705. gram data set hashigh-resolution global SST coveragederived 0148-0227/97/97JC- 01705509.00 from the advancedvery highresolution radiometer (AVHRR) 27,825 27,826 MOORE ET AL.: VARIABILITY OF THE ANTARCTIC POLAR FRONT aboard the NOAA Polar Orbiters [Brownet al., 1993]. The spatialdisplacement of individualpaths at right anglesfrom Pathfinder data have better coveragein high latitude, persis- the mean path was calculated. tently cloudy regions than previously available satellite SST We tried several methods to quantify the SST gradient [Smithet al., 1996]. acrossthe Polar Front; all of which gavesimilar results. There- Cloud cover is particularly persistent over the Southern fore we presentonly one method here. To quantify the tem- Ocean [Legeckis,1977; Bishopand Rossow,1991], which lim- perature gradient acrossthe front, we began at the poleward ited our temporal resolution.In order to minimize gapsin the edge of the PF and moved up the temperaturegradient until data due to cloud cover, daily files were compositeaveraged SST did not increaseover a three-pixel(---20-30 km) distance into weekly files usingall availabledata (includingascending or until a missingpixel was reached. The same method was and descendingsatellite passes). The resultingweekly images used to measure the width of the front. Only complete still had considerablegaps due to cloud cover, but at least transects were used to calculate the mean AT and distance portionsof the PF were unobscuredin eachimage. across the PF. The strong SST gradient at the PF was used to map its A high-resolutionpredicted seafloor topography,derived location. Images showingonly the strongSST gradientswere from ship and Geosat altimeter data [Smith and Sandwell, derived from the weekly SST maps.We defined a stronggra- 1994],was used to createmaps of planetarypotential vorticity dient as a temperaturechange ->1.35øC across a distanceof for comparisonwith the PF paths.The predictedtopography ---45km (pixelwidth varies with latitude).For eachpixel in the had a grid spacingof 3 arc min of longitudeby 1.5 arc min of weekly image a nine- by five-pixelbox centeredon that pixel latitude. The predictedtopography was also usedto calculate was examinedfor strongtemperature gradients in four direc- the slopeof the oceanfloor acrossthe PF (45 km to eitherside) tions(N-S, E-W, SW-NE, and SE-NW). The nine-by five-pixel and the mean depth along a swath45 km north and south of dimensionswere chosenso the distanceacross the box (N-S the mean PF path. and E-W) was ---45 km. The distanceacross the box in the SW-NE and SE-NW directions at 55øS would be ---65 km. If a strongtemperature gradient (AT _>1.35øC) was de- 3. Results and Discussion tected in any of the four directions,the centerpixel (with its All paths of the Antarctic Polar Front mapped during the SST value) wasretained in the gradientmap, otherwiseit was years1987-1988 are shownin Figure 1. Variability in the po- set to zero (missing).Thus the gradientmaps were subsetsof sition of the PF has a relative minimum in three regions:at the weekly temperaturemaps. Drake Passage(---62ø-57øW), between ---78 ø and 76øW, and By experiment,we determinedthat the nine- by five-pixel along the easternend of the Falkland Plateau (43ø-40øW). box, with a AT _>1.35øC, was best at retainingthe mesoscale Variability increaseswest of ---82øW,east of ---33øW,and in the featuresthat we wishedto map with an acceptableamount of northern Scotia Sea between ---50 ø and 40øW. noise(pixels retained which are not associatedwith the major The region of high variability in the northern Scotia Sea fronts).The temperatureresolution of the Pathfinderdata is exhibited considerablemeandering and ring formation, such 0.15øC;thus choicesfor A T were restrictedto multiplesof this that a clear path for the PF couldnot be distinguishedat times value. If the AT value is set too low, noise overwhelms the evenunder cloud-free conditions. Gordon et al. [1977]reported front signal.If the AT value is too high, large portionsof the that the PF was highly
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