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Hemispheric Atmospheric Variations and Oceanographic Impacts

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Hemispheric Atmospheric Variations and Oceanographic Impacts JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. D7, PAGES 13,045-13,062, JULY 20, 1993 HemisphericAtmospheric Variations and Oceanographic Impacts Associated With KatabaticSurges Across the RossIce Shelf,Antarctica DAVIDH. BROMWICH,•'2JORGE F. CARRASCO,1'2'3ZHONG LILI, 1'2 AND REN-YOW TZENG' ByrdPolar R e•e,•rchCan t•r, OtubState Uaiv•rdt y, Columbus Numericalsimulations and surface-basedobservations show that katabaticwinds persistently converge towardand blow across the Siple Coast part of West Antarctica onto the Ross Ice Shelf. About 14% of the timeduring winter (April to August 1988), thermal infrared satellite images reveal the horizontal propagation ofthis negatively buoyant katabatic airstream for al•out 1000 km across the ice shelf to its northwestern edge, a trajectorythat nearly parallels the Transantarctic Mountains. This takes place when the pressure field supportssuch airflow, and is caused by synoptic scale cyclones that decay near and/or over Marie Byrd Land. Thenorthwestward propagation of thekatabatic winds is accompanied byother changes in thehemispheric longwave pattern. An upper level ridge develops over Wilkes Land, resulting inan enhancement ofthe split jetin thePacific Ocean. Then, more frequent and/or intensified synoptic scale cyclones are steered toward MarieByrd Land where they become nearly stationary to thenortheast of the climatological location. The resultingisobaric configuration accelerates the katabatic winds crossing Siple Coast and supports their horizontalpropagation across the Ross Ice Shelf. An immediateimpact ofthis katabatic airflow, that crosses fromthe ice shelf to the Ross Sea, is expansion of thepersistent polynya that is present just to theeast of Ross Island.This polynya is a conspicuousfeature on passive microwave images of Antarctic sea ice and plays a centralrole in the saltbudget of watermasses over the RossSea continental shelf. The impactof this katabaticairflow upon mesoscale ,-vcl,,oene•i• over the South Pacific Orean is also di•cu•_qed. 1. INTRODUCTION cloudsare assumedto beblackbody radiators; thus the amount Oneof theprominent features that can be observedwith of radiationemitted by themcan be convertedinto tempera- clearconditions on thermalinfrared satellite images is a dark turevalues. TIR imageryprepared for meteorologicalpurpos- signatureextending from Siple Coast toward the northwestern es shows cold areas as light tones and warm areasas dark tones edgeof the Ross Ice Shelf (Figures 1 and 2). Bromndch½tM. [Rao et al., 1990]. Over the ice shell on a dear day or with [1992a]studied the frequency of this phenomenon during the limited cloud cover, the TIR sensorcaptures the radiation winter(April to August)of 1988.They used all theavailable emittedby the snow surface. Thus the dark signaturesob- NOAA advancedvery high resolution radiometer (AVHRR) servedin this type of satelliteimagery reveal their relatively satelliteimages (277) recorded at McMurdoStation on Ross Island[ Van Wbcrt½t M., 1992].Their results indicate that around14% of thetime, dark signatures were observed with a northwestwardextension and an orientationalmost parallel to the TransantarcticMountains. Numerical simulationsof the nearsurface wind field over the Antarctic continent [Parish and Bromv•ch,1987, 1991] show that katabaticwinds converge OCEAN towardseveral topographic troughs near the coastline of the a0.s. :,oOC•. continent(Figure 3). Oneof these convergence zones is located justupslope from Siple Coast. The location and orientation of ß /r----•J...• • 'X -• • X OCEAN thesesatellite signatures suggest that theyare linkedto the katabaficairflow coming down from southernMarie Byrd Land and from the main glaciers that dissect the l_/ u ! Transantarctic Mountains along the Amundsen Coast ••ilkes [Bromwich½t M., 1992a]. A thermalinfrared (TIR) satelikeimage is a measureof the radiationemitted by the Earth and its atmosphere.At a wavelengthof 10.5- 11.5ism (AVHRR channel 4)and 11.5- 12.5pm (AVHRR channel5) theEarth's surface and thick ocli. // •ByrdPolar Research Center, Ohio State University, Columbus. 2AtmosphericSciences Program, Ohio State University, Columbus. 3Permanentlyat Direccion Meteorologica deChile, Santiago. Fig. 1. (a) Southernhemispheric map, (b) regionalmap of the Ross Sea,Ross Ice Shelf,and surroundingareas. B, Sh,A, and SC,respeo- Copyright1993 by the American Geophysical Union. tively,denote Beardmore, Shaeldeton, Amundsen, and Scott glaciers. Farther north, By, Sk, and Mu locateByrd, Skelton,and Muloek Papernumber 93JD00562. gladers. Dots with attachednumbers indicate autoraatie weather 0148-0227/93/93JD-00562505.00 station(AWS) sites. 13,045 13,046 I•ROMWlCHET AL.: HEMISPHERICFLOW AND K.ATABATIC SURGES 180 o Ross Sea ß ' 5 Bay ß ' ' '1• Ross Island Amundsen McMurdo St. Sea 07 (•(•" Russkaya St. V •ce yfdl . ß ... ß . ,rd St." ' 90ow East Antarctica South We'st'•,n{arcti•'a Pole Fig. 1. (continued) warm characteristics. Studies of this phenomenon,using buoyant. Figure 4 [after Bromndch,1989a] shows the inferred satelliteimagery and automaticweather station (AWS) data temperature and wind profries associatedwith katabatic [Bromndch,1989 a, 1992;Bromadch ½t al., 1992tr, Carrascoand airflowover the Ross Ice Shelf that can explain the coexistence Bromndch,1993], indicatethat when AWS deployedon the of warm katabaticsignatures and negativelybuoyant katabatic Ross Ice Shelf were within a dark signature,the air tempera- winds. This preferentiallyoccurs with clear sky conditions turesmeasured by thesesites were usuallywarmer than the when the boundary layer over the ice shelf is likely to be temperaturerecorded by AWS locatedin surroundingareas stronglystratified. The above argumentswere confumedby outsidethe dark feature. Also, the air temperaturesmeasured two case-studyflights carried out by Parish and Bromndch by the AWS within the dark featurewere usuallywarmer [1989], when dark signaturesover the Nansen Ice Sheet during the eventthan beforeit startedand afterit ended. AWS adjacentto Terra Nova Bay wereobserved on thermalinfrared deployedon the Antarcticcontinent measure air temperature satelliteimages, at the time when strongkatabatic winds and at a nominal height of 3 m above the surface[Stearns and negativelybuoyant air temperatureswere measuredat the Wendlet, 1988]. Therefore dark signaturesobserved on 185-m flight level. Strictly, katabatic winds are defined as a thermalinfrared satellite images reflect the warm characteris- gravity-drivenflow blowing down inclinedterrain. Once the ticsof thesnow surface and the air immediatelyabove it. airflow arrives at the foot of the slope, these two essential Thesewarm features have been interpreted as resulting from components (gravitational force and inclined terrain)for adiabaticwarming [Sndthinbank, 1973; Breckcn•idg½, 1985; formation and maintenance of katabatic winds are lost. D'Aguanno, 1986]that takes placewhen katabaticairflows However, due to the well-defined source of the airflow we descendfrom the plateauthrough the valleysthat dissectthe continueto describeit as katabaticas it propagatesfar across Transantarctic Mountains. The intermittent extension of the flat terrain. warm signaturesfor gmat distancesacross the flat Ross Ice Model simulationsof near-surfacekatabatic winds [Parish Shelfstrongly argues against these features being positively and Bromadch, 1991] over the Antarctic continent show that buoyantairstreams. It appearsthat althoughthe air adjacent the katabatic wind speedsover the gentle slopesnear Siple to the snow surfaceis warmed by turbulentmixing, the Coast are weaker than those linked with the katabatic airflow katabatic air at a few tens of meters above the surface is colder descendingthe steepmargins of EastAntarctica bordering the thanthe surroundingair masses[Bromndch, 1989a; see Figure RossIce Shelf(Figure 3 b). Bromndch[1989a] argued that the 4]. That is, the katabatic airstream is warmer than the sur- horizontalpropagation of katabaticwinds beyond the foot of roundingsat very low levelsbut is colderat higheraltitudes; the terrain slopeis governedby the katabatic masstransport for theboundary layer as a wholethe katabatic jet isnegatively crossingthe break in the slope. Then if dark signaturesare BROMWICHET AL.: HEMISPHERICFLOW AND KATABATIC SURGES 13,047 .:.:.::'::: .i::::.::i:.::i:: .-. .:.:?: ....... i .:.::;:i!()'::.':.:- . :.::::i::::!:i:M:::"..)i:E':.:. .': Fig. 2. Infrared satelliteimage at 0654 UTC June5,1988, showinga dark (equalto warm) katabaticwind signatureextending horizontally acrossthe RossIce Shelf from West Antarctica. only a resultof katabaticwind propagationover the RossIce propagating acrossthe Ross Ice Shelf, is provided by the Shelf,the warm featurescoming from southernMarie Byrd synopticscale cyclones that decaynear RusskayaStation. On Land shouldnot extendas far onto the shelfas thosecoming average,an intensificationand northeastwarddisplacement of fromthe high plateau. However, dark signatures crossing Siple theclimatological quasi-stationary synoptic scale cyclone from Coast can at times extend much farther across the ice shelf than the Ross Sea toward Russkaya Station was found to be thosecoming from glaciersthat dissectthe Transantarctic associatedwith katabatic signaturedays. The temporary Mountains [Bromwich ½t al., 19924. This suggeststhat surfaceposition of synopticscale cyclones creates a surface anothermechanism assists the northwestwardpropagation of pressurefield whereby the isobarsrun almost parallel to the the katabafic airflow descendingfrom West Antarctica.
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