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, 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 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 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 , 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 South We'st'•,n{arcti•'a Pole

Fig. 1. (continued) warm characteristics. Studies of this phenomenon,using buoyant. Figure 4 [afterBromndch, 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 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. Transantarctic Mountains. Also, the horizontal pressure BromwJcheta]. [19924 and Carrascoand BromwJch[1993] gradient strengthens[Carrosco and Bromwich, 1993] which foundthat the broadscalesupport, linked with the presenceof acceleratesthe flow down to Siple Coast and supportsthe warmsignatures coming from southernMarie ByrdLand and katabatic wind propagationacross the Ross Ice Shelf. The 13,048 BROMWICHETAL.: HEMISPHERIC FLOW AND KATABATIC SURGES

15 ø 30 ø 45 ø 45 ø 30 ø

60", 65 ø

65 ø 75'

70"

105'

120 ø Coast

65 ø

120' 165ø 180o

150"

(a) 135 ø 70 165o 150ø

15 ø 15" 30" 45 ø 45" 30 ø

60' 65 ø

65 ø 75 ø

70 ø

105 ø

105 ø

65 ø

165 ø 120" 180 ø [-] <6m/s D 6 m/s < V < 12m/s 150 ø :• 12m/s 18m/s r• MOUNTAINOUSTERRAIN (b) ,•. 70 ø 150 ø 165 ø

Fig. 3. Forty-eight-hoursimulation of wintersurface winds over the Antarcticcontinent. (a) Drainagepattern of near- surfacekatabatic winds and (b) katabatic wind speeds (ms 'l) [afterParish andBrom•ch, 1991]. BROMWICHET AL.: HEMISPHERICFLOW AND KATABATIC SURGES 13,049

Katabafic Wind Speed glaciers,plays a significantrole in maintenanceand/or enlarge- Profile -200 ment of the polynya. 200, • KatabaticTemperature Profile•---....,,/• ß The following sectiondiscusses the role of the katabatic QuiescentTemperature Profile----...• , 150 150- airflowsupon the polynyaadjacent to the northernedge of the RossIce Shelf. In section3 the regionalsynoptic environment ß 100- 100 describedby Bromn4ch ½t al [1992a] and Carcascoand .c_ Bromn4ch[1993] for dark signaturesacross the iceshelf (called 50 ._m 50. signaturedays hereafter, Table 1) is extendedto a hemispheric domain. Changesin the longwave pattern associated with this 0. Warm TIR Katabatic Signature synopticscale variation are described. 45 40 45 Speedin ms-1 Temperature in øC 2. POLYNYA FORMATION NORTH OF THE Ross ICE SHELF Fig. 4. Schematicwinter illustration for the RossIce Shdf near Byrd Glacier showingthe inferredvertical temperature profiles and how a The persistentpolynya just off the northwesternedge of the warmsatellite signature can reveal the presence of a negativelybuoyant Ross Ice Shelf is a prominentfeature on passivemicrowave katabatieairstream [from Bromwic& 19894. imagesof Antarcticsea ice, both on a time-averagedand on an individualbasis [ZwMly etM., 1983]. In mostcases the polynya above was confirmed by numerical simulations(D.H. is locatedjust to the eastof RossIsland and fluctuatesin size Bromn4ch½t M., Numericalsimulation of winter katabafic accordingto the synopticscale forcing [ZwMly ½tM., 1985]. windscrossing the SipleCoast area of WestAntarctica and The seaice in the RossSea sector disintegrates both southward propagatingacross the Ross Ice Shelf, submitted to Monthly from the and northward from the continent, WeatherR½•½w, 1993) which indicatethat althoughthe primarily to the westof 180ø longitude. The polynya along synopticforcing in generalplays only a minorrole in sittingup the northwesternedge of the RossIce Shelfforms the nucleus the convergenceof the windsupslope from SipleCoast, it from which this summer northward retreat commences. significantlyaccelerates thewinds over and beyond the coastal Brorav•ch [1992] observedthat a northward katabatic wind slopes.Thus the pressure gradient force assists the propaga- surgealong the TransantarcticMountains and acrossthe tion of the intensified katabatic winds toward the northwest, westernRoss Sea in earlyNovember 1986 apparently initiated sometimescovering a horizontaldistance of about1000 km. the seaice decayfor that australseason. Oceanic heat appar- Polynyas,or areasof combinedopen water and thin ice entlyalso contributes to the springdisintegration of the seaice surroundedby seaand/or land ice,can be observedin the [Jacobsand Cotalso, 1989]. Historically, the large summer Antarcticduring all seasons [Jacobs and Comiso, 1989; Zwally open-waterarea in the westernRoss Sea allowedexplorers eta/., 1985;Bromn4ch and Kurtz, 1984]. Becausethese Scott and Shackletonto travel far to the southby ship during featuresare much warmer than theadjacent ice, they appear as theirquest to be firstto reachthe geographicsouth pole. dark toneson infraredsatellite images. The coastalpolynya The explanation for this feature must addressits nearly playsimportant roles in the seaice zone that surroundsfixed geographic location, its persistence,and its waxingand Antarctica. These areas of open water interact with the waningin responseto synopticforcing. Surfacewinds are the atmosphere,ocean, and sea ice. Theintense sensible and latent likelycause as they have been found to forcenearly all Antarc- heat lossesfrom the oceanduring winter are suppliedby the tic coastalpolynyas [e.g., Knapp, 1972;Brom•4ch and Kurtz, freezingof seawater[Zwally cta/., 1985]. The very large 1984;Zwally ctal., 1985; Caval•b•qand Martin, 1985]. Auto- amountsof ice that can form contributeto the growth of the matic weather station observations from the Ross Ice Shelf seaice farther offshore, while the rejectedbrine plays a decisive reveal a persistentoffshore-directed airflow that appearsto role in the formation of denseshelf water [Jacobsat M., 1985; crossthe ice shelfedge in the regionof the persistentpolynya Zwally ½tal, 1985;Cavali½•q and MarEn, 1985;Budd, 1991]. [Stearnsand Wendlet, 1988, seetheir Figure 8; Jacobsand Finally,these features serve as nuclei for the springdisintegra- Cotalso, 1989, see their Figure 6; Bromveich,1991, see his tion ofseaice by absorbingmuch more solar radiation than the Figure9]. The primarysource of thisairstream appears to be surroundingsurfaces [Jacobs and Coraiso,1989]; the excess the persistentkatabatic winds blowing from Byrd, Mulock, heat storedin the oceanicsurface layer during spring and andSkelton glaciers [Bromv•ch, 1989a; Parish and Bromn4ch, summerretards sea ice formation during fall. 1987];these winds are proposedas the primary forcingfor the One area of recurrentpolynya formation is locatedat the spatiallyrestricted and persistentpolynya. In additionto the northernedge of the RossIce Shelf,just to the eastof Ross Island. Observationsreveal that polynya fluctuationsin the TABLE 1. Days in Winter 1988for Whicha Katabatie-Wind RossSea are principallydriven by winds[Zwally ½tal, 1985; SignatureWas ObservedExtending Northwestward Across Kurtz and Bromn4ch,1985; Bromveich and Kurtz, 1984]. The theRoss Ice Shelf, Called Sisnature Days synopticsurface environment associated with dark signatures Month SignatureDays acrossthe RossIce Shelfis similarto that describedby Zwally eta/. [1985]for dramaticexpansions of thispolynya. Therefore April 21, 22, 23, 24, 25, 28 May 4, 19, 21, 23, 25 thelocation of thepolynya near the ice shelf front suggeststhat June 3, 5, 15, 30 the katabaticairflow coming from the bottomof the iceshelf, July 11, 12, 22, 24, 30 alongwith thosecoming from Byrd, Mulock, and Skelton August 10, 24 13,050 BROMWICHETAL.: HEMISPHERIC FLOW AND KATABATIC SURGES

Examination of the satelliteimages for signaturedays indicatesthat the enlargementof the polynyais more frequent on the westernside of the ice shelf(Figure 5), but prominent polynyascan form or enlargein otherplaces along the ice shelf front. An exampleof thisis the polynya observed at 0705UTC July 11and 0654 UTC July12, 1988(Figures 5 and 6aand 6b). The satelliteimage at 0705UTC July11 (Figure 6a) showsthat the polynya had alreadybegun to form offshorefrom the centralpart of the iceshelf. The nextimage at 0654UTC July 12(Figure 6 b)reveals a significantenlargement of thepolynya, Fig.5. Schematicillustration ofpolynya formation atthe front of the nowcoveting an area of about 7.3x103 km: (almost double its RossIce Shelf. Solidcurve indicates average northern limit of the sizein 24 hours). The only stationlocated along the northern polynyafor signature days. Dashed curve indicates northern limit for nonsignaturedays. Dashed-dottedcurves outline a polynyathat edgeof theice shelf and adjacent to thearea where the polynya formed on July 11-12, 1988. occurredis AWS 00 (Figure 1b). This station recorded southeasterlywinds between July 10 and 12with a meanspeed associationof favorablesynoptic conditions for major polynya of 8.6ms '• whichis 2.5 ms '• higher(95% confidence level) than expansions offered by Zwally ½ta/. [1985], Kurtz and the monthly averagefor July. Also, over the sameperiod, Bromn4ch [1985] in the same publication noted that the AWS 08, 15, 24, and 25 recordedsignificantly (in a statistical persistentkatabatic winds from Byrd, Mulock, and Skelton sense)stronger wind speedsthan their respectivemonthly glaciersalso appearedto play a role. Subsequently,Carrasco average. The increaseof wind speedat AWS 11 was not and Bromwich [1993] and Bromwich et al [1992a]found that statisticallysignificant. Only AWS 07 did not recordstronger katabatic winds propagatedalong the TransantarcticMoun- windsthan its monthly average.The satelliteimages revealed tains from West Antarctica to the northwesternedge of the a dark signatureextending from Siple Coast area with the Ross Ice Shelf in conjunction with synoptic scale support. contributionfrom the glaciersthat dissectthe Transantarctic These were associatedwith expansionsof the persistent Mountains(not shownin Figure6 becauseof theenhancement polynya in this area, and the accompanyingsynoptic condi- used to deafly resolvethe polynya). This feature covered tionsconcurred with thosedescribed by Zwally et al. [1985]to almost the entire Ross Ice Shelf exceptthe northwestside be associatedwith major openingsof the samepolynya. In where AWS 07 is deployed. addition,these West Antarctic katabatic airflows are accompa- The strongersurface pressure gradients over the RossIce nied by enhancedkatabatic drainagedown the glaciersfrom Shelf on signaturedays with isobarsalmost parallel to the East Antarcticaonto the RossIce Shelf,apparently including TransantarcticMountains provide the pressuregradient force Byrd, Skelton,and Mulock glaciers(hereafter the combined that allows the katabatic airflow descendingfrom West katabatic airstream is referred as a katabatic surge), thus Antarcticato propagatehorizontally toward the northwest,at supportingKurtz and Bromwich• [1985] observationmen- timesreaching the southernRoss Sea [Brom•4ch eta/., 1992•; tioned above. Carrascoand Brom•4ch, 1993]. The strongerwind speeds The averageareal extent on signaturedays was determined recordedat AWS 08 and AWS 25 for July 10-12 suggestthat from the infrared satelliteimages. The sameevaluation was thepolynya offshore from theice shelf is linkedwith katabatic done for a random selection(22 days) of nonsignaturedays. windsextending from the SipleCoast area, with a substantial Resultsare presentedin Figure 5. The edgeof the openwater contributionfrom the katabatic airflow originatingin East and consolidatedsea ice can be objectivelylocated on digital Antarctica. In fact, AWS 08 and 25 showeda significant satelliteimages. On average,the brightness temperature within increaseof the daily averagedwind speedfrom July 11 to 12 the openwater (dark areas)was about -4* C (physicaltempera- (from7.6 ms '• to 12.2ms '• atAWS 08 and from 6.5 ms 'l to9.7 ture, T, equals-I*C assumingan emissivityof 0.95 for water) ms'l at AWS25), which coincides with the enlargement of the and for consolidatedsea ice was -11*C or colder(T • -6'C polynyaobserved on satelliteimages. This suggestsa rapid assumingan ice emissivityof 0.95). This impliesa brightness response(less than 24 hours)of the seaice to the increaseof temperaturecontrast of about 7ø which enablesthe edge of wind speed. Unfortunately,the lack of consecutivesatellite the polynyato be readilylocated. Althoughpartial (or total) imagesprecludes a detailed study of theevolution and time lag cloudcover may occur(see Figure 6), the edgeof the polynya betweenthe onsetof katabaticwinds and the responseof the can be observedthrough thin nonobscuringcloud. The area polynya.For this,case studies with high temporal resolution coveredby polynyawas calculatedfor the regionbounded by satelliteimages and/or model simulations are needed. the RossIce Shelfand the digitallylocated brightness tempera- ture-gradientzone betweenthe open water and the sea ice. 3. HEMISPHERIC ANALYSIS Resultsshow that the areal averageextent for signaturedays wasabout 3.8x10 • km 2 along the entire edge of the shelf. For 3.1. BroadscaleSpanEl Changes Associated With Kataba• nonsignaturedays it wasabout 2.3x10 • km 2. A largerareal SurgesAcross the Ross Ic½ Shelf on Si•natur•Days variation occursalong the westernhalf of the shelf. Here the polynyareached a size of about2.5x10 • km 2 (width ~ 16km) To providea broadscalecontext for thesynoptic circulation for signaturedays compared with the average of 1.3xl0 • km2 changesfound by Brom•4cheta/. [1992a]to beassociated with (width ~ 8 km) for nonsignaturedays. In general,this repre- katabaticsurges across the RossIce Shelf,the hemispheric sentsan enlargementof the polynya of about 50ø/3under the digitalanalyses prepared by theAustralian Bureau of Meteo- presenceof katabatic surges. rologywere examined. These have a horizontalresolution of ROSS ICE SHELF

ROSS ICE SHELF

ß Fig. 6. (a) Infraredsatellite images at 0705UTC July11, 1988,and(b) at 0654UTC July12, 1988showing polynya formation. 13,052 BROMWmHETAL.: HEMISPHERIC FLOW AND KATABATIC SURGES

90øW

90øE

CONTOUR FRON 96e.ae TO le2e • CONTOUR INTERVAL OF 5.aeee PT[3.3I- lele 1 CONTOUR ½RON •?e.ae TO le2e • CONTOUR INTERVAL OF 5.aeae

90øW 9a-9c,which include the significantanomalies (in a statistical sense)given by thet test(95% confidence level) applied to the differencesbetween the averages.The typicalwinter pattern for the southernhemisphere [ van Loon, 1965;TMjaard ct M., 1969;$chwcrdtœcgcr, 1970] can be observedin Figures7a, 8a, and 9a; and Figures6b, 7 b, and 8b providethe mean pattern for signaturedays. The changesfor sea level pressureare revealedby comparisonof Figures7a and 7 b. Note that the quasi-stationarysynoptic cyclone located over the Ross Sea is intensifiedand displacedcloser to RusskayaStation. This is the sameresult that wasfound by Bromwichct M. [1992a]. In general,a strengtheningof the circumpolar trough takes place, withpronounced intensification in the western hemisphere and slightweakening in theeastern hemisphere. A newfeature that appearsis a high-pressurearea () centered over the SouthwestPacific Ocean, dose to New Zealand. The t test (Figure7c) reveals that the displacement and intensification of thequasi-stationary cyclone and the anticyclone are statistical- ly significantat the95% confidence level. Also,it showsthat I thepressure significantly decreases (in a statisticalsense) to the 90øE south of Africa and over the eastern side of South America. A significantincrease of the sealevel pressure occurs over the Fig.7. Southernhemispheric surface analysis for (a) the five-month southern Indian Ocean between Africa and Australia. average,(b) signature days, and (c) t test(95% confidence level) applied Changesin the 500-hPageopotential height field between to theanomalies. (a) and(b) Thecontour interval is 5 hPa. (c)The the five winter months and signaturedays can be seenby ratio of thet valueto the95% confidence value is plotted;the sign of theratio is the same as the sign of the anomaly. Statistically significant comparingFigures 8a and 8b. The changesare shownmore positiveareas are stippled, and negative areas are crossed. The analysis dearly in Figure 8c which presentsareas of statistically issuppressed to the north of 15øSwhere it isstrongly intlueneed by significantdecrease and increase of geopotentialheights. It can spuriousdata near the edge of thedomain. Parallds of latitudeare be notedthat the decreaseof geopotenfialheight that takes every30 ø startingfrom the equator. placeto thenortheast of theRoss Ice Shelf is accompaniedby an increaseover the SouthwestPacific Ocean, just to the north 500km[ Gujq•½r,1986] and are constructed twice a dayat 0000 of New Zealand. Other significantpositive (increase)changes and 1200 UTC. Averagehemispheric fields of sea level occur over the central South Pacific Ocean, over the south pressure,500-hPa geopotential height, and the 1000-to 500- central part of the Indian Ocean, and the central South hPa geopotentialthickness were obtained for the fivewinter AtlanticOcean. Other significant decreases take place to the monthsof April to August1988 and for signaturedays (Table southof Africaand overthe eastern part of theIndian Ocean. 1). Resultsare respectively shown in Figures7a-7c, 8a-8c, and Thesechanges tend to displace toward the east the troughs and BROMWICHET AL.: HEMISPHERIC FLOW AND KATABATICSURGES 13,053

90*W b

., !

, 90*E , 90*E

CONTOURFRON 486e 8 TO 5888 8 CONTOURINTERVAL OF 60 888 PT(3 3)= 5867 4

900W

Bromvffch ct M. [1992a]. The increase of the 500-hPa geopotentialheight just to the northeast of New Zealand coincideswith the high sea level pressureobserved on the surfaceanalysis. Other important features that appearfor signaturedays are the enhanceddiffluence of the 500-hPa isohypsesover the southeastern tip of Australiaand the trough (2 in Figure 8b) to the west of southernSouth America. The enhancement of the diffluent zone at 500 hPa reveals an enhancementof the climatologicalsplit of thejet streamover the Australia-NewZealand region and is associatedwith a negative(positive) anomaly of the sealevel pressure and 500- hPaheight (see Figures 7cand 8c) over the Tasman Sea (to the southof Tasmania). Althoughthese departures are only statisticallysignificant at 90%,they suggest the typical features associatedwith diffluent type blocking events. Further discussionis givenin section5. Similarlysignificant (in a statisticalsense)changes, over the regionsmentioned above, are revealedin the 1000- to 500-hPa • 90*E thicknessfields (Figures 9a-9c). The quasi-zonal thermal winds

CONTOURFRON -2 8888 TO 2 8888 CONTOURINTERVAL OF 8 58888 PT(3 3)- 8 found for the five-monthaverage contrast with thermal troughsand ridgesthat appearin the averagefield for signa- Fig. 8. Samoas Figure6 but for $00 hPa. (a) and (b) Tho contour intervalis 60 geopotentialmeters (gpm), and the heavy solid lines are turedays (compare Figures 9a and9 b). The changescan more troughsdiscussed in the text. easilybe noted in Figure9c, whichgives the statistically significantnegative and positivedepartures from the five- ridges(wave number 3: troughs1, 3, and 5 in Figures8a and monthaverage. Significant positive departures of the 1000-to 8b) observedin higher latitudes(between 45* and 70'S) for 500-hPageopotential thickness over the Bellingshausen Sea for the five-monthaverage and to developthree new troughs(2, 4, signaturedays confirm the existence of a warmridge projecting and 6 in Figure8 b) increasingthe numberof wavesto six. This southwardacross this area, while the negative departures over impliesmore north/south advection of warm and coldair. The the RossSea indicatethe presenceof a cold troughwhich significantdecreases and increasesof the geopotentialheights extendsequatorward from the RossIce Shelf area (concurs for signaturedays (Figure 8c) overlap the majority of the with Bromv•cheta/. [1992a]).In general,areas of increaseand, significantnegative and positivedepartures of the sea level decreaseof the1000- to 500-hPageopotential thickness, respec- pressure(Figure 7c), implyinga quasi-barotropicatmospheric tively, coincidewith areasof increaseand decreaseof the 500- structure in these areas. Thus the location of the intensified hPageopotential heights. Comparing the significant thickness midtroposphericlow over the Amundsen Sea coincideswith changeswith thosethat take placeat sealevel, it can be noted the locationof the synopticlow at the surface,as was found by thatnegative (positive)departures of the geopotential thickness 13,054 BROM'W•C•ETAL.: HEMgSP•EmC FLOW AND KATABATIC SURGES

90øW a

90øE 90øE

CONTOUR FRON 4860 o CONTOURFRON 406S ß TO 5760 0 CONTOURINTERVAL OF 60 00 e PT(3.3)= 5785 7

90øW whichis relatedto eventsonly aroundthe RossIce Shelf. The ßsignificant changes taking place elsewhere confn'm the interac- tion between the Antarctic continent and the atmospheric circulation at lower latitudes.

3.2. Broadscale Time Vadanbn Associated With Kataba•b SurgeEven ts A crossthe Ross Ice Shelf

To studythe set up of thebroadscale atmospheric features associatedwith signaturedays, it is necessaryto move away fromthe biased perspective of satellite imagery which is limited bycloud obscuration of the surface and by data gaps. Based uponthe study of signaturedays by Bromwichet al. [1992a], the continuouslyavailable air temperaturesfrom AWS influencedby katabaticwinds from WestAntarctica were chosenas the bestvariable to gaugethe likely existenceof katabatic airflow acrossthe RossIce Shelf. A katabatic surge event(KSE) was, as a result,defined to occurwhen the daily averagesurface-air temperatures recorded at thebottom of the RossIce Shelf(by either one or bothof AWS 08 and 11)were continuouslywarmer than the respective monthly average for (• 90øE at least2 days(Table 2). Thisperiod corresponds to the likely CONTOURFRON -I 5e00 TO I 5000 CONTOURINTERVAL OF 0 5000e PTI3 3)- ß Fig. 9. Sameas Figure6 but for 1000-to 500-hPathickness. (a) and TABLE 2. Periodsfor Which the SurfaceAir Temperatures (b) The contourinterval is 60 gpm. at AWS 08 and 11 Were Warmer (Colder)Than the RespectiveMonthly Average According to the Definition of a Katabatic SurgeEvent (KSE) field almost overlap areas of negative(positive) sea level (non-katabaticsurge event) (non-KSE) pressureanomaly. However, the positive thickness departures over the central South Pacific Ocean overlap with a smaller Month KSE,days Non-KSE,days area of significantpositive anomaliesof the 500-hPa April (March31oo- 3o0); (18oo- 3000) (4,2-17oo) geopotentialheights. The increaseof the sealevel pressure May (9•2-1100); (14,2- 16,2); (100-8•2); (12oo- 13,2); overthe central part of theSouth Pacific Ocean is not statisti- (2200-26o0) (17a2-21•2);(2700-loo) callysignificant. Other discrepancies occur with the positive June (200-8o0); (14•2- 20•2); (900-13•2); (21•2- 28o0) anomalies over the central South Atlantic Ocean and the (2900-30x:) July (300-14•2); (3000- 31a2) (la:- 200);(15•2- 2900) BellingshausenSea. This indicates that somechanges are more August (9•2-11a2); (16oo- 22a2); (la:- 8•2);(12•2- 15oo); markedin themidtroposphere than at the surface. (26•:-300o) (2300-25a2) The above descriptionsuggests that the occurrenceof katabaticwinds propagating from WestAntarctica onto the AWS, automaticweather station. Subscriptindicates the time in Ross Ice Shelf is not a localized and isolated phenomenon universalcoordinated time (UTC). Parenthesesenclose events. BROMWICHET AL.: HEMISPHERICFLOW AND KATABATICSURGES 13,055 propagationofkatabatie winds across the RossIce Shelf. This ;•:thirteen KSE (62%),a darkkatabatic signature across the Ross 2-day criterion is designedto eliminate daily temperature IceShelf was observed on satelliteimagery on at leastone day. changeswhich may not be causedby katabafieoutbreaks onto For three KSE the Ross Ice Shelf was overcast or the satellite the ice shelf. Also, in orderto overcomethe discontinuityof imageswere not available,so that the presenceof a dark the anomaliesbetween consecutive months the temperature signaturecould not be ascertained.For the remainingtwo a anomaliesfor the last five and first five days of consecutive dark signaturewas observedto the southof AWS 08 and 11. monthswere determined by the differencebetween each daily Thereforefor the ten observableKSE, eight(80%) are assor- temperatureand the averagefor these10 days. Large varia- atedwith at leastone signature day. Sixsignature days do not tionswere found for the duration of the events. On average, correlatewith our definitionof KSE becausethe temperatures they lastedfor 5.2 • 3.6 (1 s.d.)days, but eventsfrom 2 to 12.5 at AWS 08 and/or 11were not warmerthan average. A similar days duration were observed. Thirteen KSE were present comparisonwas made for nonkatabaticsurge events (non- during the period under consideration. For eight of the KSE), definedas the periodbetween KSE for whichthe daily averageair temperaturesrecorded by AWS 08 and 11 were

90øW continuouslycolder than the respectivemonthly averages. Theseperiods probably correspondto the katabatic winds from WestAntarctica being eonœmed to thesoutheastern part of theshelf. The averageduration of theseperiods was around 6.1 ñ 4.1 (1 s.d.). The total cycle(i.e., from the first warmer day to thenext) that includesboth non-KSE and KSE periods was found to be about 13.3 ñ 6.7 (1 s.d.)days, with the total durationvarying from 6 to 28 days. A new evaluationof the Australiansouthern hemisphere analyseswas conductedto obtain the averagecondition for non-KSE and for KSE. Resultsfor KSE (not shown, see Bromwich et al. [1992b]) are very similar to those already describedfor signaturedays. The main featuresat the surface are: the RossSea synopticscale cyclone which is deeperthan thefive-month average but not asstrong as for signaturedays the appearanceof the high-pressurearea near New Zealand, andthe weakening of the SouthPacific anticyclone (see Figure 7). Resultsfor non-KSEshow that theRoss Sea synoptic scale cycloneis weakerthan for signaturedays, KSE, and the five- month average. The changesfrom non-KSE to KSE (not shown;see Bromwich ½t al [1992b])indicate that the most relevantchanges are the appearanceof a high-pressurearea overnorthern New Zealandand the eastwarddisplacement of

b \

Fig. 10. Differencesbetween katabatic surve event (KSE) and non-KSE for (a) sealevel pressure (1-hPa interval), (b) 500-hPageopotential heights (10-gpm interval), and (c) 1000-to 500-hPathickness (10-gpm interval). 13,056 BROMWICHET AL.: HEMISPHERIC FLOW AND KATABATIC SURGES the troughassociated with the quasi-stationarycyclone over ern SouthPacific Ocean and the negativeanomalies over the the Ross Sea area. The intensificationof this low-pressure southwestern South Atlantic Ocean and southeastern Indian center was also revealed by this analysis,implying more Ocean. Thus the changesfrom non-KSEto KSE shownin frequentand/or intense synoptic scale decaying in the Figure10a can be interpretedas a significantincrease of the AmundsenSea/Marie Byrd Land area. For KSE (non-KSE) sealevel pressure over the areaof lqewZealand and to the the averageof the sealevel pressure reveals positive (negative) southof Australia,over the BellingshausenSea, and overthe departuresfrom the five-month average over New Zealand, the southwesternIndian Ocean. Decreasesof the sealevel pressure southeasternPacific Ocean/BellingshausenSea, and the occur over the northeastern Ross Sea, the southwestern southwesternIndian Ocean.Negative (,positive) departures are AtlanticOcean, and the southeasternIndian Ocean. A wave observedover the northernRoss Sea, SouthwestAriantic number3 patternoccurs around Antarctica near 60 ø S. region, and southeasternIndian Ocean adjacent to Queen The 500-hPaaverage geopotential height fields for non- Mary Coast. The three new featuresrevealed by the KSE KSE and KSE (not shown,but Figures1 l a and 11c maybe averageanalyses are the positiveanomaly over the southeast- usedas a referenceas they showalmost the sameresults) indicatethat althoughthe midtropospheric vortex is displaced 90øW slightlytoward the northeast, it does not deepen as much as for signaturedays (Figure 8 b). However,the differences between non-KSEand KSE with respectto thefive-month average (not shown)reveal that the geopotentialheight changed from significantpositive to significantnegative anomalies over the areanortheast of the RossSea, indicating a strengtheningof thevortex. The areas ofnegative (positive)departures for non- KSE correspondalmost exactly to areasof positive(negative) departuresfor KSE. Thuschanges from non-KSEto KSE (Figure10b) show an increaseof the 500-hPageopotential heightsover the southeastern Pacific Ocean, and over the lqew Zealand/southwestern Pacific Ocean sector. Also, increases ., / occurover the Antarcticplateau. In general,the areasthat showan increase(decrease) of the500-hPa geopotential height almostoverlap areas of increase(decrease) of the sealevel pressure.The samegeneral characteristics are foundfor signaturedays (Figures 7c and 8c). This impliesa quasi- barotropicatmospheric structure. The1000- to 500-hPageopotential thicknesses fornon-KSE and KSE (not shown)reveal a more clearlydefined wave 90øE

CONTOUR F.ROn ,•e6e e TO 5eee e CONTOURINTERVAL OF be eee PT(3.3). 586? 1

90øW 90øW b

180 ø

90 ø E 90 ø E

CONTOUR gR0n ,•eee e TO 5eee e CONTOUR INTERVAL. OF' • egg PTI3.3)= 5e67 5

Fig. 11. Southernhemispheric 500-hPa analysis for (a) peakof non-KSE,(b) transitiontime, and (c) peakofKSE (60-gpm contourinterval). (a) and (c) The heavysolid lines are troughs. BROMWICHET AL.: HEMISPHERICFLOW AND KATABATIC SURGES 13,057 number 3 pattern in high latitudesfor the caseof KSE where The main resultsat sealevel are the eastwarddisplacement three troughsare clearly observed:one projectingnorthward of the troughsand ridges,the intensificationand displacement from the Ross Sea area, another over the southwesternAtlantic of the quasi-stationarylow pressurefrom north of the RossSea Ocean, and the third over the central southern Indian Ocean. toward the vicinity of RusskayaStation, and a weakeningof The significant feature for KSE is the trough extending the South Pacifichigh pressure.The most relevantsequence of northward from the Ross Sea, which is not observedfor non- eventstakes place at 500 hPa. Figures 11a, 11b and 11c show KSEs. In addition, at lower latitudesa fourth trough appeared the initial, the transition, and the f'mal stage of the time to the west of southern South America. The thickness differ- evolutionat 500 hPa correspondingto the peak ofnon-KSE to ences(departures) between KSE (non-KSE) with respectto the the peak of KSE, respectively.The most prominent feature five-month average show significant positive (negative) revealedby the sequenceis a ridge that startsto developover departuresof the geopotentialthickness that coverpart of the Wilkes Land and extends southwestward into East Antarctica. Antarctic plateau, the southeasternPacific Ocean, and the It reachesits maximum intensity at the transition time. A regionto the southof Australia;negative (positive) departures similar but weaker ridge appearsover the other side of the are observed over the southern tip of South continent. On the other hand, the at 500 hPa America/southwestern Atlantic Ocean area, to the south of movestoward the east and strengthenssignificantly from the Africa, and to the north of the Ross Sea, including the area peak of non-KSE to peak of KSE. The trough (1 iv Figures around New Zealand. Changesthat take placefrom non-KSE 11a and 11c) associatedwith the 500-hPa vortex, which extends to KSE are shown in Figure 10c. It revealsthat significant along longitude 160øW for peak days of non-KSE, moves increases(decreases) of the thicknessvalues almost coincide eastwardto be locatedalong longitude 140øW for peak days with the areasof positive(negative)departures described above of KSE (Figure 1l c). Note that the intensificationand dis- for KSE (non-KSE). Another significantarea of geopotential placementof the polar vortex takes place from the transition thickness decrease occurs over the eastern side of the South time to the peak of KSE. Another trough (7 in Figure 1l c) Pacific Ocean to west of the northern Chilean coast. In developed to the west of the northern coast of Chile for the general,the departuresfor KSE (and alsofor non-KSE but of peak of KSE. To the south of this trough a midtropospheric opposite sign) at sea level, 500-hPa and 1000- to 500-hPa ridge extendsover the southeasterncorner of the South Pacific (Figures10a-10c) show a wave number3 pattern at 60øS. Ocean and over Ellsworth Land. The time evolution for 1000- The most intriguing feature that appears in the 1000- to to 500-hPa geopotentialthickness showed that the quasi- 500-hPa thicknessfor non-KSE to KSE is the negativeanoma- zonallypattern (barotropic) of the five-monthaverage is ly (decrease)justto the northeastof the New Zealand sector replacedby morepronounced and a largernumber of troughs (compareFigures 10a, 10b, and 10c). Around the same area and ridges.As for signaturedays, a coldtrough extending the sea level pressureand the 500-hPa height show positive fromthe Ross Ice Shelfarea is observedfor thepeak of KSE anomalies(increase). This indicatesthat the increaseof the sea (Figure 9 b). The time evolutionalso reveals warm and cold air level pressure is larger than the increase of the 500-hPa advectionat highand mid-latitudes, probably associated with geopotentialheight. Apart from this area, other changesthat synopticscale features as suggested by thewave number 6 or 7 take place in the 1000- to 500-hPa geopotential thickness pattern. almost concur with the changes of 500-hPa geopotential heights. In general, the decrease(increase)of the sea level 3.3. MesoscMe Vortex Occurrenc• Over the South PaciI7c pressure over the Amundsen Sea for KSE (non-KSE) is Oceanand Katabat•b SurgeEvents Over the Ross Ice accompaniedby a decrease(increase)in both the 1000- to 500- Shelf hPa and the 500-hPafields. No significantdecrease of sealevel pressureto the westof southernSouth America is observedfor Satellite-basedstudies [e.g., Carleton and Ca•7•nter, 1990; KSE, but the 1000- to 500-hPa and 500-hPa fields show a Fitch and Carleton,1992; Carletonand Fitch, this issue; negative anomaly with respectto the f'ave-monthaverage, Turner and Thomas, 1992] have shown that a favorable confirrningthe developmentof a cold trough over that region. environmentfor mesoscalecyclogenesis (cyclones with diame- tersof lessthan 1000km, alsoreferred to aspolar lows) is a To study the time evolution of the KSE and to deal with their varying durations,the following featureswere identified: (1) peak day of each non-KSE which correspondsto the TABLE 3. Peak Days ofNon-KSE (KSE) DefinedAs the Coldest coldestday within the non-KSE eventsaccording to the air (Warmest)Day Within theNon-KSE (KSE)and TransitionTime temperaturerecorded by eitherone or both of AWS 08 and 11; Defined as Period Between the Non-KSE and KSE for Which (2) transitiontime which correspondsto the time of the cycle the SurfaceAir TemperaturesWere Not Colder or Warmer that divides non-KSE from KSE periods, so that the air Than the RespectiveMonthly Average temperatureswere neitherwarmer nor colder than the respec- Month PeakDay of KSE TransitionTime Peakday of Non-KSE tive monthly average;and (3) peak day of each KSE which correspondsto the warmestday within the KSE. Table 3 lists April 1,25 1712 9 the respectivedays/times according to our definitions. Then, May 912, 15, 25 9o0,1400, 2112 7, 1212, 1912 June 5, 18,29 112, 1400 , 2812 lo0,12, 27 all the peak days of non-KSE, all the transition times, and all July 7, 30 212, 2912 112, 19 the peak days of the KSE were averagedto obtain a time August 10,20, 28 9o0,1512 , 26oo 7, 14,24 evolutionfor the sealevel pressure, 500-hPa, and 1000-to 500- hPa fields in relation to the airflow across the Ross Ice Shelf. Subscriptindicates the time in universalcoordinated time (UTC) 13,058 BROMWICH ET AL.: HEMISPHERIC FLOW AND I•kTABATIC SURGES

cold air massmoving equatorwardfrom the polar regions. Thereforeincreased mesoscale activity is likely Also, mesoscalecyclones have been observedin association to be found in conjunctionwith the occurrenceof katabatic with dissipatingcold-core synoptic vortices over the ocean polar air outbreaks(defined as a mass of cold air moving [Zick, 1983]. In addition, recent studies in the southern equatorward from the Antarctic continent and that near the hemispherehave shown that strong and persistentkatabatic surfaceis associatedwith katabaticwinds). The latter hasbeen windssustained by confluencezones in the continentalinterior the subjectof this paper. To investigatewhether or not the [Parish and Bromwich, 1987, 1991] are associatedwith the subsynopticand synopticscale environments associated with formation of mesoscalecyclones just offshore [Brom•4ch, the northwestwardpropagation of the katabatic winds favor 1989b, 1991; Carcascoand Brom•4ch, this issue]. For com- the formation of mesoscalecyclones over the ocean,the results plete reviewsof polar lows, seeBusinger and Reed[1989] and presentedby Fitch and Carleton[1992]were reevaluated. They TurReT½t al. [this issue]. studiedthe occurrenceof mesoscalevortices over high south- ern latitudesduring five months(March, April, June,August, and October) in 1988, which partly overlap with our study 90øW period. Their studyarea covered the half hemispherecentered on the Ross Sea from 100øE to 80øW and from 50øS to the southpole. To studythe synopticenvironment associated with mesoscaleactivity, they used the southernhemisphere synoptic analysesfrom the Australian Bureau of Meteorology. Their resultsindicate that for activedays (more than five mesoscale vorticesobserved on the sameday) the composite1000-hPa field revealeda negativeheight anomaly centeredat 155øW and 68ø S, with a correspondingnegative anomaly at 500 hPa displaced to the south. The 1000- to 500-hPa thickness anomaly field showednegative departures over the Ross Sea area and positivedepartures over Marie Byrd Land and over the easternside of Australia. Comparingwith the large-scale changessurrounding the Ross/Amundsenseas obtained for signaturedays (or KSE), it canbe notedthat similaranomalies take placeon both activeand signaturedays. Figures12a-12care the averagesea level pressure, 500-hPa geopotentialheights, and the 1000- to 500-hPa geopotential thicknessfields for activedays, respectively. They showFitch and Carleton• [1992] results for the entire southern hemi-

CONTOUR FROM 978 00

90øW

90øE 90 ø E

CONTOURINTERVAL OF 60 •g• PT(3 31: 5867 7 CONTOURFROM 486• • TO 576• o CONTOURINTERVAL OF 60 eee PYt3 31: 5784

Fig. 12. Southernhemispheric analyses for activedays at (a) the surface(5-hPa interval), (b) 500hPa (60-gpminterval), and (c)for the 1000-to 500-hPathickness (60-gpm interval). (b) The heavysolid lines are troughs. BROMWICHET AL.: HEMISPHERIC FLOW AND KATABATIC SURGES 13,059

sphere,including the activedays for July 1988 (A.M. Carleton, that favor such displacement. Results from the satellite- personalcommunication, 1991) but excludingthose for March observationalanalysis of this phenomenonindicate that the and October 1988. The changesfor activedays obtainedby northwestward propagation of the katabatic winds from Fitch and Carleton [1992] are generally similar to those southernMarie Byrd Land can occurquite frequently. In fact, describedfor signaturedays, which are also valid for KSE. theywere observed 14% of the time duringthe five months;this Althoughcomparison with Figures7 b, 8 b, and 9 b for signature correspondsto 21% of analyzabledays, defined as a day for days reveal somedifferences in the exact location of troughs which the presenceof surface-windsignatures could be and ridges,the general pattern obtained for signaturedays evaluated on satellite imagery [Bromnqch ½t al., 1992a]. (and for KSE) appliesto activedays. It shouldbe mentioned Consideringthe increaseof the temperatureat AWS 08 and 11 that eight of twelve active days fall within our definition of as an indicator of the northwestward projection of the KSE, the remainingfour within non-KSE. On the other hand, katabatic airflow (definitionof katabatic surgeevent is equal five of seveninactive days are within non-KSE and the other to KSE), the abovepercentage may be higher. two within a KSE. The correlation betweensignature and The immediateeffect of the northwestwardpropagation of active days is poor. Only three of the twelve active days the katabatic airflow seemsto be the enlargement and/or overlap(+ 1 day) with signaturedays. But for six of the twelve maintenanceof the polynya just offshorefrom the Ross Ice active days, examinationof satelliteimages revealed that the Shelf. The broadscale similarities over the South Pacific Ocean RossIce Shelfwas cloudyor the satelliteinformation was not for activedays [Fitch and Carlcron,1992] and for signatures available,preventing the observationof dark signatures.So, days (or KSE) suggest an increase of the mesoscale for six of the observable active days, three overlap with cyclogenesisdue to the northwardadvection of cold boundary signaturedays. On the other hand, six of the seveninactive layer air from the Antarcticcontinent. daysoverlap with nonsignaturedays, with the seventhcoincid- The hemispheric analysis for signature days and for ing with a signatureday. Although this is a fair correlation, katabatic surge events indicates that the northwestward which can be explained by the fact that both studies had propagation of the katabatic surgeis not an isolated and differentpurposes and the deœmitionsof active and signature localizedphenomenon, but otherregional changes within and dayswere totally independentof eachother, the bettercorrela- outsidethe polar environmentaccompanied the katabaticcold tion with KSE suggestsa relation between mesoscale air outbreak. Theselarge-scale variations are not causedby cyclogenesisand katabatic polar air outbreaks. The 500-hPa the katabaticwind propagatingfrom WestAntarctica, but they time evolution(Figures 11) suggestscold air advectionoff East are probably a responseto a perturbationoccurring in East Antarctica which starts before KSE onset and subsequently Antarctica. On average,the main changescan be summarized extendsover the Ross Ice Shelf. This is linked with the upper as follows: level ridge over Wilkes Land and with the northeastward 1. The quasi-stationaryRoss Sea cycloneis deeperand dis- movement of the 500-hPa vortex. Also, the 1000- to 500-hPa placedcloser to the Marie Byrd Land coastcompared to its differencesbetween non-KSE and the five-month average climatologicalintensity and location, respectively;this also showednegative anomalies over and to the north of Wilkes happensto the midtroposphericvortex at 500 hPa. Land and over the SoutheastPacific Ocean, implying that cold 2. A high-pressurearea developsjust to the northeastof New air outbreaksfrom theseregions occur during non-KSE. Fitch Zealand. The sealevel pressure and the 500-hPa height (1000- and Carleton [1992] found large mesoscalecyclone activity to 500-hPa thickness)reveal positive (negative) departures adjacent to the Wilkes Land Coast in 1988 with respectto associatedwith this area. A low-pressurearea over Tasman other regionsin the Pacific sectorof the Antarctic; therefore Seais accompaniedby a high-pressurearea to the south. These mesoscalecyclogenesis may occurwithin non-KSE (katabatic featuresare associatedwith blocking eventsand appear close airflow outbreaksfrom Adelie Land?)affecting the correlation to the primary region for blocking in the southernhemisphere between active and signaturesdays (or KSE). A spatial [Stretch, 1980; Coughlan, 1983; Trcnbcrth and Mo, 1985]. variation of mesoscalecyclogenesis occurrence may be associ- Other increasesof the sea level pressuretake place over the ated with the cold air advectionfirst beingrestricted primarily southeasternPacific Ocean around the BellingshausenSea and to East Antarcticafor non-KSE and then expandingto the east over the southwesternIndian Ocean adjacentto the Antarctic over the Ross Ice Shelf for KSE. This may be associatedwith coast. the eastward movement of the quasi-stationary synoptic 3. The differencesbetween the averageKSE and non-KSE cyclone. Although only one winter seasonhas been studied, with respectto the five-monthaverage reveal a wave number3 the synoptic environmentresults for active days and KSE departurepattern but out of phasewith respectto the trough suggestsa relation betweenmesoscale cyclogenesis occurrence and ridge distributionshown by the five-monthaverage. This and katabatic winds [ Car/eton, 1992]. increasesthe wave number from 3 to 6, as was evident in the analysesfor signaturedays (e.g., Figure 8b) and in the time- SUMMARY averaged evolution sequence(e.g., Figure 11). The same increasewas observedfor the 1000- to 500-hPa geopotential An observationalstudy of katabatic winds propagating thicknessindicating an enhancementof the warm and cold air from West Antarcticahas beencarried out by examinationof advection for both non-KSE and KSE periods. The wave infrared satelliteimages for the 1988winter season(April to number 6 pattern suggeststhat these advectionsmay be August)[Brom•q'ch et al, 1992a; Carrascoand Bromv•ch, associatedwith synopticscale features. 1993]. This studyrevealed that katabaticsignatures extending 4. The 500-hPageopotential heights are higherfor KSE than acrossthe iceshelf are accompaniedby synopticscale changes for non-KSE over the Antarctic plateau. The time evolution 13,060 BROMWlCHET AL.: HEMISPHERIC FLOW AND KATABATIC SURGES for 500-hPageopotential height fields reveals that a ridgestarts midtroposphericvortex and with the developmentof the ridge to developover Wilkes Land from the peak of non-KSE, over East Antarctica. These changesmay also trigger the reachinga maximumintensity at transitiontime. A much advectionsthat take placein mid-latitudes. weakerridge extendspoleward from aroundthe Ronne Ice An immediate effect of the intensified-katabatic wind is the Shelf area. enlargementof the polynyalocated offshore from the north- 5. The negativeand positivedepartures of the 1000-to 500- westernedge of the RossIce Shelf. The averageduration (5.2 hPageopotential thickness, suggest cold (warm) air advection ñ 3.6 (1 s.d.)) of KSE is sufficientfor supportingand/or to the north of Wilkes Land and over the Southeast Pacific increasingthe sea ice productionin the polynya. Thus Oceanfor non-KSE (KSE) and to the north of the RossIce katabaticairflows appear to play an importantrole in main- Shelffor KSE (non-KSE,Figure 10c). Theintensification and tainingan openwater area offshore from the RossIce Shelf, northeastwarddisplacement of the midtropospheric vortex and xvhichforms a nucleusfor the summerdisintegration of the sea thedevelopment of the500-hPa ridge over East Antarctica may icein the RossSea. It hasbeen recognized that southerlywind explainsome of thecold and warm air advectionsthat take componentsplay an importantrole in formationand/or place.The development ofthe ridge may advect warm air from maintenanceof theRoss Sea polynya through the winter. This the southeasternIndian Ocean onto the plateau (associated airflowhas been associated with synopticcyclones located near with thewestern part of the ridge)at the timethat coldair is RusskayaStation, which set up southerlyand/or southeasterly advectedoffshore from the Victoria/WilkesLand area (associ- winds acrossthe northwesternedge of the Ross Ice Shelf atedwith theeastern part of theridge). The displacement and [ZveallyctM.,1985]. The recurringpolynya in TerraNova Bay intensificationof the 500-hPavortex may increasethe area of was extensivelystudied by Bromwichand Kurtx [1984]and polewardwarm air advectionfrom the southeasternPacific KurtxandBromwich[ 1985].They concluded that thebehavior Ocean,that takes place to theeast side of thevortex, while cold of the Terra Nova Bay polynyais primarilycontrolled by the air advectionmay extend over the Ross Ice Shelf area associat- katabaticwinds coming from the high plateau. Our study edwith thesoutherly winds on thewestern side of thevortex. suggeststhat thekatabatic surges across the Ross Ice Shelfare 6. A midtropospherictrough is resolved for signature days and the primary mechanismfor enlargementof the Ross Sea KSE at 500 hPa to the west of the southernwest coast of South polynya,an areawhere the brine rejected during winter sea ice America. This is accompaniedby a weakeningof the South formationplays a centralrole in the salt budgetof the water Pacifichigh-pressure center (although not significantly)on its masseson the Ross Sea continentalshelf [Jacobset al., 1985; southeastern side. Zwally ½tM., 1985]. How thechanges from KSE to non-KSEthat takeplace at The signature-dayaverage at 500 hPa (Figure 8 b) showsa middle and low latitudesaffect the southernpolar environment greaterdfffluence of the isohypsesover the southeasterntip of is underinvestigation. But it is knownthat the anomalies for Australiawith respectto the five-monthaverage. The sameis non-KSE are oppositeto thosefor KSE. notedbut lessmarkedly for KSE (not shown). This enhanced diffluencezone may be themanifestation of the splitof thejet at 500 hPa which coincideswith the surface low- to high- 5. DISCUSSION pressurefeature (blocking)that developsnear Australia for signaturedays and KSE. From theabove summary the northwestward propagation The main feature suggestedby the time evolution is an of the katabatic winds from West Antarctica acrossthe Ross upperlevel ridge over Wilkes Land in East Antarctica,which Ice Shelfis a regionalphenomenon that is accompaniedby precedesthe katabatic airflow eventsfrom West Antarctica changesthat involve the entire southern hemisphere. Decrease and all the large-scalechanges associated with them. The of the sealevel pressure over the AmundsenSea, associated forcingof thisridge is underinvestigation. The northerlyflow with synopticstorms that move into this area, seemsto perpendicularto the Antarcticice mass on the westside of this facilitatethe northwestwarddisplacement of the katabatic ridge may force a topographicRossby wave. James[1988], airflow by orientingthe isobarsmore parallel to the using a simple barotropic model and ray-tracing theory, TransantarcticMountains and providing a supportingpressure studiedthe propagationof the Rossbywave forced by the gradient.The coldtrough that generally extends northward Antarctic continent into middle and low latitudes. In his from the plateauand the RossIce Shelfarea confirms the simulationthe zonally asymmetricpart of the vorticity field assumptionthat the dark (equal to warm)signatures observed resolved a wave number 1 disturbance propagating on satelliteimages and the relativelywarm air temperatures equatorwardfrom Antarctica. His resultsimply that thiswave recordedby AWSare in factdue to coldair advection moving is associatedwith the splitof the troposphericjet in the vicinity awayfrom the West Antarctic plateau accompanied by a cold of Australia and New Zealand, as simulatedby the model at air outbreakfrom East Antarcticathrough the glaciersthat 300 hPa. This split jet almost coincideswith the region of dissect the Transantarctic Mountains. For non-KSE the prevalenthemispheric blocking, and with the climatological negativeanomaly of 1000-to 500-hPathickness with respect to 300-hPawinter mean zonal wind in the southernhemisphere. the five-monthaverage reveals a colderair massover the From our Fourier analysis (not shown) no evidencefor plateau.This moves northward across the Ross Ice Shelf onto propagationof Rossbywave number 1 was found. This and the southwesternPacific Ocean, as suggestedby the area of the localized amplification of the ridge (Figures 10a-10c) negativeanomaly of thegeopotential thickness for KSE. The suggesta stationarymode for the topographicRossby wave advectionpattern in highlatitudes seems to beassociated with and that anothermechanism(s) is associatedwith the enhance- the northeastwarddisplacement and intensificationof the ment of the split of thejet. BROMWICHET AL.: HEMISPHERICFLOW AND KATABATICSURGES 13,061

Fourieranalysis indicated that the amplification of theridge six for KSE (or signaturesdays) as well as for non-KSE. The over Wilkes Land is related to wave number 3 with some KSE (non-KSE) differencesshow wave number three &par- contribution from wave number 2. Tr•abcrth and Mo [1985] tures, but out of phaserespect to the wave number 3 for the studiedthe frequencyof blockingevents in the southern five-monthaverage. The averageduration of the KSE is 5.2 :t hemispherefrom May 1972to November1980. They conclud- 3.6 (1 s.d.),dose to the averageduration of blockingnear New ed that wavenumber 3 frequentlyplays a key rolein blocking Zealand. Other changesalso take place from non-KSE to eventsduring the winterwith the most frequentblocking KSE: for instance,eastward displacement of the troughsand occurringnear New Zealand.The Fourier analysis indicated ridgesand the developmentof a troughto the westof southern thatwave number 3, with a ridgejust to thesouth of Australia, South America. The latter feature can set up appropriate is the dominantfeature during the time evolutionassociated conditionsfor redevelopmentof synopticscale fronts that with KSE. As Tmnbcrthand Mo [1985] concluded,we may approachthe centralpart of Chile, the most typical situation have the casewhere a ridgeassociated with wave number3 associatedwith precipitationover this region (synoptic experi- reinforcesthe prevalent hemispheric blocking over the Austra- enceof the secondauthor). The sealevel analysis for KSE (not lia-NewZealand region and therefore enhances the split of the reproduced,Brornvffch ½t M., 1992;see their Figure 3) showed westerlywinds over this area.However, further analyses are a weakerSouth Pacificsubtropical high than for non-KSE and neededto validate this. As mentionedearlier, Figures 7c, 8c, for the five-month average(note in Figure 10a the negative and 9c showan area of negativeanomalies over Tasman Sea. difference of the sea level pressure around 35øS, 90øW This suggeststhe presenceof weak cutofflow. Trcnbcrth between KSE and non-KSE). Aceituno [1986] noted that [1980]pointed out that this feature occurs in conjunctionwith relativelywet conditionsin centralChile are relatedto negative the blockinghigh, especially in winter,and both featuresare Southern Oscillation events,with a relative weak South Pacific associatedwith largeanomalies of precipitationand tempera- subtropicalhigh beingobserved; thus the negativephase of the Southern Oscillation has some characteristics in common with ture in the Australianand New Zealandregion. Thesefeatures KSE. may be also related to the westernpart of the Southern The cold air advection from the Antarctic continent into the Oscillation(SO) [Smithand Stearns,this issue]. It is inferredfrom theseresults that the sequenceof events South Pacific Ocean observed for KSE is also found for active from non-KSE to KSE (Figure 13) is (1) developmentof a days [Fitch and Carleton, 1992]. Correlationbetween active midtroposphericridge over Wilkes Land, resultingin (2) daysand KSE aswell asthe similaritybetween both broadscale enhancementof the splitjet andreinforcement of the blocking environmentresults is suggestive. Mesoscalecyclogenesis over the Australia-New Zealand region. Then, (3) a greater activity may also occur during periods of non-KSE but in numberand/or more intense synoptic scale cyclones are steered differentregions than for KSE. This suggestsa spatial varia- toward the AmundsenSea/Marie Byrd Land area, (4)where tion of mesoscalecyclogenesis occurrence around the Antarctic they becomenearly stationaryinducing a sea level pressure continentdue to the apparent spatial variation of the north- field over the Ross Ice Shelf with isobars oriented almost ward advectionof cold air suggestedby the 1000- to 500-hPa parallelto theTransantarctic Mountains. This results in (5) an geopotentialthickness anomalies. It is also suggestivethat intensification and northwestward propagation of the Carleton and Fitch [thisissue] found a decreasein the number katabatic winds across the ice shelf. The wave number 3 of mesoscalecyclones observed to the northeast of Wilkes patternthat characterizesthe five-monthaverage increases to Land and over Ross Sea/Ross Ice Shelf sector in winter 1989 compared to winter 1988, while an increaseoccurred in the Amundsen/Bellingshausensea sector. This betweenwinter variation was associatedwith a variation of the large-scale circulation,which showednegative (positive) sea level pressure 18,00 . and 500-hPa geopotentialheight anomalies to the north of the Ross Sea and positive (negative) anomalies over the ' ' ' ' -. 150oW Amundsen/Bellingshausensector in 1988 (1989). For a much shortertime scale,almost the sameanomaly variationswere .." oo•.../ •, , found between KSE and non-KSE. Acknowledgments.This r•soarchwas sponsoredby the Divisionof Polar Programsof the National ScienceFoundation through grant• DPP- 8916921 and DPP-9117448 to D.H. Bromwich.The sat•llit• imagerywas recordedby U.S. Navy pcmonn½lat McMurdo Stationand obtainedfrom RobertWhritncr of the AntarcticRe.arch Centerat ScrippsInstitution of Oceanography(NSF grant DPP-8815818). This is contribution855 of ß ..% ß -.-'..• %•'.. Byrd Polar ResoarchCenter.

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