Changes in Antarctic temperature wind and precipitation inresponse to the Antarctic Oscillation

Michiel R van den BROEKE1 Nicole P M van LIPZIG2

1Institute for Marine and Atmospheric Research Utrecht PO Box 80005 Utrecht University Princetonplein 53508 TA Utrecht The Netherlands

E-mail broekephysuunl2Royal Netherlands Meteorological Institute Postbus 201 3730 AE De Bilt The Netherlands

ABSTRACT Output of a 14 year integration with a high-resolution (55 km 55 km) regional atmos-pheric climate model is used to study the response of Antarctic near-surface climate to the AntarcticOscillation (AAO) the periodical strengthening and weakening of the circumpolar vortex in theSouthern Hemisphere In spite of the relatively short record wind temperature and precipitation showwidespread and significant AAO-related signals When the vortex is strong (high AAO index)northwesterly flow anomalies cause warming over the Antarctic Peninsula (AP) and adjacent regions inWest Antarctica and the Weddell Sea In contrast cooling occurs in East Antarctica the eastern Ross IceShelf and parts of Marie Byrd Land Most of the annual temperature signal stems from the monthsMarchndashAugust The spatial distribution of the precipitation response to changes in the AAO does notmirror temperature changes but is in first order determined by the direction of flow anomalies withrespect to the Antarctic topography When the vortex is strong (high AAO index) the western APbecomes wetter while the Ross Ice Shelf parts of West Antarctica and the Lambert Glacier basin EastAntarctica become drier

1 INTRODUCTIONTemperature trends in Antarctica have not been uniform inrecent decades A strong warming in the Antarctic Peninsula(258C in the last 45 years) (Vaughan and others 2001 Kingand others 2003) has led to the rapid disintegration of thenorthern ice shelves (Vaughan and Doake 1996) likelycaused by increased meltwater ponding and associated ice-shelf weakening (Scambos and others 2000) On the otherhand a rapid cooling between 1986 and 2000 has beensignalled in the Dry Valleys Victoria Land East Antarctica(Doran and others 2002) This asymmetry in Antarctictemperature trends has been ascribed to Southern Hemi-sphere circulation variability On month-to-month time-scales the dominant mode of variability of the SouthernHemisphere extratropical circulation is the Antarctic Oscil-lation (AAO) (Thompson and Wallace 2000 Hall andVisbeck 2002) The AAO represents the periodical strength-ening and weakening of the circumpolar vortex the belt oftropospheric westerlies surrounding the Antarctic continentIt is also named Southern Annular Mode because it has anapproximate polar-symmetric shape The AAO is a truestanding mode ie it does not resemble a propagating wavelike the Antarctic Circumpolar Wave (White and Peterson1996) the latter explains a significant part of Antarcticclimate variability on multi-annual time-scales (White2004)

Unfortunately meteorological observations in Antarcticasuffer from various problems the station density is low andstation locations are heavily biased towards the coast wheregradients in meteorological variables and topography are attheir greatest and local influences potentially large TheAntarctic observational record is relatively short (lt50 years)

and the large natural climate variability reduces the signifi-cance of trends To structure the Antarctic climate record itis clear that we must resort to modelling and remote-sensingtechniques

Many researchers have used US National Centers forEnvironmental PredictionNational Center for AtmosphericResearch (NCEPNCAR) re-analysis (1948ndash2002) (NRA) datato study Southern Hemisphere climate variability (Kalnayand others 1996) Unfortunately no pre-1968 Antarcticobservations were used in the analysis and before 1970NRA suffers from unphysical mass loss at high southernlatitudes (Hines and others 2000) Comparison with radio-sonde data shows that NRA data have limited accuracy inthe Antarctic region (Marshall 2002a) Given the scarcity ofobservations before the satellite era any future re-analysis ofthe climate of Antarctica before the early 1970s will alsoremain uncertain

Recent studies used remotely sensed Antarctic tempera-tures to solve the problem of poor data coverage over thecontinent Comiso (2000) compiled a dataset of Antarcticsurface temperatures from satellite thermal infrared obser-vations These data were used by King and Comiso (2003) tostudy the spatial coherence of Antarctic Pensinsula (AP)warming and by Kwok and Comiso (2002) to analyze theresponse of Antarctic temperatures to the AAO and to the ElNinondashSouthern Oscillation Schneider and Steig (2002) usedAntarctic ice-sheet microwave brightness temperatures tostudy Antarctic climate variability Problems specific toremotely sensed temperature data are the temporal andspatial biases associated with cloud cover in the thermalinfrared (Shuman and Comiso 2002) and snowpack pene-tration and non-stationarity of the microwave emissivity forbrightness temperatures

In this paper we use regression analysis on output of aregional atmospheric climate model (RACMOANT1 VanLipzig 1999) to investigate in more detail the influence of

Annals of Glaciology 39 2004

Present address British Antarctic Survey Natural Environment ResearchCouncil Madingley Road Cambridge CB3 0ET UK

119

the AAO on the near-surface temperature wind andprecipitation in Antarctica

2 MODEL AND DATARACMOANT1 is based on the ECHAM4 model of the MaxPlanck Institut fur Meteorologie The RACMOANT1 domainof 122130 gridpoints covers the Antarctic continent andpart of the surrounding oceans (Fig 1) The horizontalresolution of approximately 55 km 55 km enables areasonably accurate representation of the coastal ice slopesand the ice shelves fringing the coast In the vertical 20hybrid levels are used An additional layer at 6ndash7 m abovethe surface was included to better capture the strongtemperature and wind-speed gradients near the surface Atthe lateral boundaries RACMOANT1 is forced by EuropeanCentre for Medium-Range Weather Forecasts (ECMWF) re-analysis (1980ndash93 ERA 15) Daily variations in sea-ice coverand sea surface temperatures are prescribed from obser-vations The modelling technique is described in more detailin Van Lipzig and Van den Broeke (2002) The performanceof RACMOANT1 is a great improvement on earlier modelsfor instance the decoupling of the lowest atmospheric layerunder conditions of strong stability that occurs in ERA 15 isabsent in RACMOANT1 which uses a more sophisticatedturbulence scheme In a detailed comparison with stationdata Van Lipzig (1999) and Van Lipzig and others (1999)showed that annual mean temperature wind speed anddirectional constancy are simulated with root-mean-squareerrors of 15 K 2 m sndash1 and 012

Because the RACMOANT1 model domain is too small tocalculate hemispheric circulation indices from we use the

1980ndash93 monthly mean AAO index based on the firstprincipal component of the NRA 850 hPa extratropicalheight field (20ndash908 S Thompson and Wallace 2000)Because this AAO index shows an upward trend that isassociated with unphysical atmospheric mass loss at highsouthern latitudes (Hines and others 2000) we use thedetrended 1980ndash93 AAO time series (Fig 2) As can be seenthe AAO is very variable on monthly time-scales and doesnot exhibit any coherent interannual behaviour High valuesof the AAO index indicate a strong circumpolar vortex andlow-amplitude Rossby waves in the polar front while lowvalues of the AAO index indicate a weak circumpolar vortexand high-amplitude Rossby waves

3 REPRESENTATION OF PRESENT-DAY CLIMATEFigure 3andashd show modelled annual mean (1980ndash93) surfacepressure ps (over the ocean only) 10 m wind speed andvector V10m surface potential temperature s and totalprecipitation P Here we present s instead of absolutetemperature Ts because the latter is dominated by theelevation effect However all subsequent regressions areperformed using Ts to avoid the pressure dependency of sTo enable subsequent sections to be better understood abrief discussion of the main climate features is given below

The model faithfully reproduces the structure of thecircumpolar trough (CPT) in surface pressure with its threeclimatological pressure minima (Fig 3a) North of the CPTwe find strong westerlies prevailing near the surface whilesouth of the CPT easterlies are found in the Antarctic coastalzone (Fig 3b) Over the coastal ice sheet we see a well-developed katabatic wind system with annual mean 10 m

Fig 1 Model domain and topography Stippled areas ice shelves light shaded average July sea-ice extent dark shaded average Januarysea-ice extent Surface elevation (m asl) is contoured every 500 m RIS Ross Ice Shelf FRIS FilchnerndashRonne Ice Shelf

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO120

wind speeds of 10ndash15 m sndash1 In the absence of a significantslope to force katabatic winds the flat ice-sheet domes andice shelves are characterized by relatively low wind speedstypically 5 m sndash1 Unrealistically low values of 10 m windspeed are modelled in regions where the model surfaceroughness for momentum has an important subgrid-topo-graphical component (Transantarctic Mountains parts ofDronning Maud Land and the AP) These values should notbe compared to locally observed values

High values of surface potential temperature s (Fig 3c)are found over the coastal ice sheet these are lsquowarmsignaturesrsquo of katabatic winds that mix relatively warm airtowards the surface (Bromwich 1989 Van den Broeke andothers 1999) Low values of s are found over the flat ice-sheet interior ice shelves and sea ice These areas arecharacterized by a surface temperature inversion (Connol-ley 1996) Note the close relation between the large-scalecirculation with these lsquopoolsrsquo of cold air and northwardsea-ice extent the greatest northward extent of sea ice isfound to the west of the climatological low-pressure areaswhere northward advection of cold air is promoted In theRoss and Weddell Seas these circulation systems arereferred to as the Ross and Weddell gyres The formationof Antarctic Bottom Water in these regions plays a majorrole in driving the global ocean circulation Variations in thestrength of these lsquogyresrsquo are important for the sensitivity ofAntarctic temperature to changes in the large-scale circula-tion

The largest annual precipitation amounts (Fig 3d) inAntarctica are found in the western AP coastal WestAntarctica and coastal Wilkes Land (about 1000ndash1500 mm we) These regions are characterized by stronginteraction of the ice-sheet topography with the large-scaleflow resulting in forced convection The interior of EastAntarctica receives lt100 mm we andash1 of precipitation Nomaps of observed precipitation in Antarctica are availablebut the modelled precipitation minus sublimation (P ndash E notshown) is in qualitative agreement with recent compilationsof Antarctic mass balance (Vaughan and others 1999) Allprecipitation in Antarctica falls as snow except for occa-sional rainfall events reported in the northernmost AP

4 AAO REGRESSION RESULTSWe regressed monthly mean anomalies of surfacepressure ps surface temperature Ts precipitation P and10 m wind speed and wind vector components (U10m V10m)

onto the monthly mean detrended AAO index 1980ndash93(N frac14 168) The fields presented in this section are thusrepresentative of a strengthening of the circumpolar vortex(from low to high AAO index) The slopes of the linearregressions were multiplied by the standard deviation of theAAO index (081) to obtain anomaly fields that have units ofthe dependent variable and are related to 1 standarddeviation of the AAO index (Fig 4andashd) Dashed contoursdelineate areas where correlation confidence reaches 99The time series of AAO index in Figure 2 do not displaysignificant autocorrelation so no correction was applied forthis

Surface pressure and 10 m windThe surface pressure anomaly pattern (Fig 4a) showsstrongly negative values over continental Antarctica andpositive values at lower latitudes a common feature of theAAO Except for a small band around zero change the 99confidence level is reached everywhere and is not shown Inspite of the lsquoannularrsquo character of the AAO the pattern inFigure 4a is far from zonally symmetric because of theclimatological low-pressure centres around Antarctica(Fig 3a) In Figure 4a a pronounced surface pressuredecrease is found in the Amundsen Sea which coincideswith the lsquopole of variabilityrsquo in the Southern Hemisphere andis associated with a changed location of the AmundsenndashBellingshausen Sea low (Connolley 1997) A weaker localminimum is found over Wilkes Land East Antarctica Themaximum gradient in the surface pressure anomaly fieldoccurs just north of the Antarctic coastline and represents anincrease of the surface geostrophic zonal wind of about2 m sndash1 At 500 hPa zonal winds have increased by abouttwice this amount (not shown) indicative of an enhancedhorizontal meridional temperature gradient under condi-tions of positive AAO polarity

Figure 4b presents 10 m wind anomalies expressed as avector composed of the anomalies in the individualhorizontal components Background colours indicatechanges in absolute wind speed Following the annularcharacter of surface pressure anomalies (Fig 4a) a 1 m sndash1

increase of the westerlies north of Antarctica is found ieabout half of the change in the surface geostrophic windspeed Some important deviations from zonal symmetryoccur over the continent flow anomalies are directedtowards West Antarctica where they cross the WestAntarctic ice divide and descend towards the Ross Ice

Fig 2 Time series of detrended monthly mean AAO index 1980ndash93

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 121

Shelf In the Weddell Sea northerly flow anomalies followthe east coast of the AP continuing over the FilchnerndashRonneIce Shelf and on to the East Antarctic plateau Thisrepresents a weakening of the persistent southerly flowalong the eastern border of the AP which Schwerdtfeger(1975) identified as barrier winds Anomalous southerlygeostrophic flow occurs over the Ross Ice Shelf and inwestern Dronning Maud Land

The sign of the wind-speed anomaly (background coloursin Fig 4b) depends on whether the vector wind anomaly hasa component in the direction of the local climatological

wind direction or opposed to it (cf vectors in Figs 2band 4b) For example over the ocean and sea ice thepressure anomaly field enhances the westerlies north of theCPT by up to 1 m sndash1 but weakens the coastal easterlies alongthe East Antarctic coast by a similar amount Over the icesheet the climatological wind direction is determinedmainly by the katabatic component which acts in thedownslope direction This generates cross-slope winds witha downslope component near the surface due to friction(Fig 2b) (Van den Broeke and others 2002)

The dependency of the near-surface katabatic wind field

Fig 3 (a) 1980ndash93 modelled annual mean sea-level pressure in hPa (b) 10 m vector wind and wind speed (background colour) in m sndash1(c) surface potential temperature in K and (d) annual precipitation in mm

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO122

on slope direction explains the dipole patterns of 10 m wind-speed change that are seen in interior Dronning Maud Landand in West Antarctica in Figure 4b In Wilkes Land thelarge-scale pressure-gradient force (PGF) and the katabaticPGF generally act in the same direction ie downslope Inthis region both PGFs become equally important in thenear-surface momentum budget (Parish and Cassano 2001Van den Broeke and Van Lipzig 2003) Because theperturbation large-scale PGF is directed opposite to theaverage large-scale PGF a marked decrease in near-surfacewind speed is found in this area (Fig 4b)

Surface temperature

A stronger circumpolar vortex (high AAO index) inducessignificant cooling over most of East Antarctica and signifi-cant warming over the entire AP (Fig 4c) This pattern is verysimilar to that presented by Kwok and Comiso (2002) whoused surface temperatures derived from thermal infraredimagery (1982ndash99) For wintertime conditions Van denBroeke and Van Lipzig (2002) showed that East Antarcticcooling during strong vortex conditions has two com-ponents a general tropospheric cooling of about 2 K over

Fig 4 (a) AAO regression slope of surface pressure in hPa (b) 10 m vector wind and wind speed (background colour) in m sndash1 (c) surfacepotential temperature in K and (d) annual precipitation in Values correspond to a 1 standard deviation anomaly in the AAO Dashedcontours enclose areas where the 99 confidence level is reached

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 123

the whole of East Antarctica probably caused by suppressedmeridional air exchange and additional cooling near thesurface in areas where a weakening of near-surface windshas intensified the surface temperature inversion

From Figure 4c it appears that areas with a surfacetemperature inversion (roughly coinciding with low poten-tial temperatures in Fig 2c) are prone to potentially strongersurface warming when the circumpolar vortex is strong (highAAO index) through destruction of the temperature inver-sion An example is the AP ice shelves where warming islarger than in the immediate surroundings This process maybe dominated by changes in horizontal heat advection inregions where flow anomalies are directed perpendicular tolarge horizontal temperature gradients such as are found

near the continental margin and the sea-ice edge Forinstance the strong warming over the western AP and theWeddell Sea is likely to be at least partly caused by a greaternortherly wind component in combination with decreasedsea-ice cover in the Bellingshausen Sea (Marshall and King1998 Marshall 2002b) A more quantitative assessment ofthe contributions of various processes to regional tempera-ture change based on calculation of the individual terms inthe heat budget is a topic of future study

PrecipitationWhen the circumpolar vortex is strong (high AAO index)three areas over the continent show changes in precipitationthat reach the 99 confidence level (Fig 4d) An increase of

Fig 5 Seasonal subsets of AAO regression slope of surface pressure in hPa (solid contours) and surface temperature in K (colours) for DJF (a)MAM (b) JJA (c) and SON (d) Values correspond to a 1 standard deviation anomaly in the AAO Dashed contours enclose areas where the99 confidence level is reached

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO124

up to 30 is found over the western AP while western MarieByrd Land and the adjacent Ross Ice Shelf show decreases ofa similar magnitude Interestingly in the precipitationshadow at the eastern side of the AP zero precipitationchange is found over the northerly ice shelves an increase intheir mass balance may have slowed down their disintegra-tion A relative decrease of about 15 is found over theLambert Glacier basin These patterns clearly reflect theinteraction between circulation anomalies (Fig 4a and b)and Antarctic topography Of interest to ice-core researchersis that western Dronning Maud Land experiences aprecipitation increase of up to 10 (mostly reaching the90ndash95 confidence level) and at the same time significantcooling (Fig 4c) Elsewhere in Antarctica increases inprecipitation are in general associated with warming

Regression on seasonal subsetsIt is well known that the Antarctic temperature inversion isstrongest in winter and greatly reduced in summer Otherimportant factors determining Antarctic surface temperaturesuch as sea-ice extent and strength of the katabatic windsalso show a pronounced annual cycle From this we wouldexpect a seasonal dependence of the temperature responseto the AAO To investigate this we regressed seasonalsubsets of 3 months (DJF MAM JJA and SON) on the AAOindex Figure 5 shows the combined results for surfacetemperature (colours dashed contours where 99 con-fidence is reached) and surface pressure (solid contours) Inspite of the reduced number of points used in the regressionall seasons show extensive areas with significant surfacecooling over East Antarctica in response to a strongercircumpolar vortex (high AAO index) For instance inWilkes Land just east of the Lambert Glacier basin coolingis significant in all four seasons Warming in the AP is mostpronounced in autumn and winter when the circulationanomalies have their greatest northerly component so thatmost of the annual temperature signal in Figure 4c derivesfrom the months MarchndashAugust (Fig 5b and c) This is inagreement with observations (Marshall 2002a)

Impact of wind and temperature changes on thesurface sensible heat fluxTo investigate what causes the surface cooling in EastAntarctica under conditions of strong circumpolar vortex(high AAO index) we selected from Figure 4c the location inWilkes Land where the correlation between the AAO indexand temperature attains a minimum For this locationFigure 6 shows the correlation between the AAO index andanomalies in the surface turbulent flux of sensible heat(SHF) SHF is an important component of the wintertimeAntarctic surface energy balance and provides the surfacewith heat that it loses through net emission of longwaveradiation Because turbulence and hence the SHF in thestably stratified wintertime Antarctic atmosphere is gener-ated mainly by wind shear a decrease in 10 m wind leads toa decrease in SHF and hence a lower surface temperatureThis link of the AAO with East Antarctic surface tempera-tures through the katabatic wind field and the sensible heatflux is clearly apparent from Figure 6

6 SUMMARY AND FUTURE WORKWe used output of a 14 year integration with a high-reso-lution regional atmospheric climate model (RACMOANT1)

to study the response of the (near-) surface Antarctic climateto the AAO which is a measure of the strength of thecircumpolar vortex or westerlies In spite of the relativelyshort period significant AAO-related anomalies were foundin surface pressure 10 m wind surface temperature andprecipitation When the vortex is strong (high AAO index)surface pressure changes show an asymmetric pattern withthe largest pressure falls in coastal Marie Byrd Land Thiscauses northerly flow anomalies and significant warmingover the AP and adjacent regions in West Antarctica and theWeddell Sea In response to decreased meridional airexchange and weaker near-surface winds significant cool-ing occurs throughout East Antarctica The strongest signalsare found in Wilkes Land but strong cooling is also foundover the Ross Ice Shelf and the adjacent part of WestAntarctica Repeating the regression on subsets of 3 monthsreveals that most of the annual temperature signal derivesfrom the months MarchndashAugust Precipitation anomaliesreflect the interaction of circulation anomalies and topog-raphy and do not generally mirror temperature changesSignificant wet anomalies are found on the western AP anddry anomalies occur over the Ross Ice Shelf the adjacentWest Antarctic ice sheet and in the Lambert Glacier basinEast Antarctica

The regional modelling approach has proved very usefulfor clarifying physical processes underlying climate changein Antarctica To improve statistical significance in studies ofthis kind a 45 year run with a regional atmospheric climatemodel (forced by ERA40) is ongoing

REFERENCESBromwich D H 1989 Satellite analyses of Antarctic katabatic

wind behavior Bull Am Meteorol Soc 70(7) 738ndash749Comiso J C 2000 Variability and trends in Antarctic surface

temperatures from in situ and satellite infrared measurementsJ Climate 13(10) 1674ndash1696

Connolley W M 1996 The Antarctic temperature inversion Int JClimatol 16(12) 1333ndash1342

Fig 6 Correlation of monthly-mean AAO index (1980ndash93) withanomalies of surface temperature and turbulent flux of sensible heatfor a centre of action in Wilkes Land (see white cross in Fig 4c)

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 125

the AAO on the near-surface temperature wind andprecipitation in Antarctica

2 MODEL AND DATARACMOANT1 is based on the ECHAM4 model of the MaxPlanck Institut fur Meteorologie The RACMOANT1 domainof 122130 gridpoints covers the Antarctic continent andpart of the surrounding oceans (Fig 1) The horizontalresolution of approximately 55 km 55 km enables areasonably accurate representation of the coastal ice slopesand the ice shelves fringing the coast In the vertical 20hybrid levels are used An additional layer at 6ndash7 m abovethe surface was included to better capture the strongtemperature and wind-speed gradients near the surface Atthe lateral boundaries RACMOANT1 is forced by EuropeanCentre for Medium-Range Weather Forecasts (ECMWF) re-analysis (1980ndash93 ERA 15) Daily variations in sea-ice coverand sea surface temperatures are prescribed from obser-vations The modelling technique is described in more detailin Van Lipzig and Van den Broeke (2002) The performanceof RACMOANT1 is a great improvement on earlier modelsfor instance the decoupling of the lowest atmospheric layerunder conditions of strong stability that occurs in ERA 15 isabsent in RACMOANT1 which uses a more sophisticatedturbulence scheme In a detailed comparison with stationdata Van Lipzig (1999) and Van Lipzig and others (1999)showed that annual mean temperature wind speed anddirectional constancy are simulated with root-mean-squareerrors of 15 K 2 m sndash1 and 012

Because the RACMOANT1 model domain is too small tocalculate hemispheric circulation indices from we use the

1980ndash93 monthly mean AAO index based on the firstprincipal component of the NRA 850 hPa extratropicalheight field (20ndash908 S Thompson and Wallace 2000)Because this AAO index shows an upward trend that isassociated with unphysical atmospheric mass loss at highsouthern latitudes (Hines and others 2000) we use thedetrended 1980ndash93 AAO time series (Fig 2) As can be seenthe AAO is very variable on monthly time-scales and doesnot exhibit any coherent interannual behaviour High valuesof the AAO index indicate a strong circumpolar vortex andlow-amplitude Rossby waves in the polar front while lowvalues of the AAO index indicate a weak circumpolar vortexand high-amplitude Rossby waves

3 REPRESENTATION OF PRESENT-DAY CLIMATEFigure 3andashd show modelled annual mean (1980ndash93) surfacepressure ps (over the ocean only) 10 m wind speed andvector V10m surface potential temperature s and totalprecipitation P Here we present s instead of absolutetemperature Ts because the latter is dominated by theelevation effect However all subsequent regressions areperformed using Ts to avoid the pressure dependency of sTo enable subsequent sections to be better understood abrief discussion of the main climate features is given below

The model faithfully reproduces the structure of thecircumpolar trough (CPT) in surface pressure with its threeclimatological pressure minima (Fig 3a) North of the CPTwe find strong westerlies prevailing near the surface whilesouth of the CPT easterlies are found in the Antarctic coastalzone (Fig 3b) Over the coastal ice sheet we see a well-developed katabatic wind system with annual mean 10 m

Fig 1 Model domain and topography Stippled areas ice shelves light shaded average July sea-ice extent dark shaded average Januarysea-ice extent Surface elevation (m asl) is contoured every 500 m RIS Ross Ice Shelf FRIS FilchnerndashRonne Ice Shelf

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO120

wind speeds of 10ndash15 m sndash1 In the absence of a significantslope to force katabatic winds the flat ice-sheet domes andice shelves are characterized by relatively low wind speedstypically 5 m sndash1 Unrealistically low values of 10 m windspeed are modelled in regions where the model surfaceroughness for momentum has an important subgrid-topo-graphical component (Transantarctic Mountains parts ofDronning Maud Land and the AP) These values should notbe compared to locally observed values

High values of surface potential temperature s (Fig 3c)are found over the coastal ice sheet these are lsquowarmsignaturesrsquo of katabatic winds that mix relatively warm airtowards the surface (Bromwich 1989 Van den Broeke andothers 1999) Low values of s are found over the flat ice-sheet interior ice shelves and sea ice These areas arecharacterized by a surface temperature inversion (Connol-ley 1996) Note the close relation between the large-scalecirculation with these lsquopoolsrsquo of cold air and northwardsea-ice extent the greatest northward extent of sea ice isfound to the west of the climatological low-pressure areaswhere northward advection of cold air is promoted In theRoss and Weddell Seas these circulation systems arereferred to as the Ross and Weddell gyres The formationof Antarctic Bottom Water in these regions plays a majorrole in driving the global ocean circulation Variations in thestrength of these lsquogyresrsquo are important for the sensitivity ofAntarctic temperature to changes in the large-scale circula-tion

The largest annual precipitation amounts (Fig 3d) inAntarctica are found in the western AP coastal WestAntarctica and coastal Wilkes Land (about 1000ndash1500 mm we) These regions are characterized by stronginteraction of the ice-sheet topography with the large-scaleflow resulting in forced convection The interior of EastAntarctica receives lt100 mm we andash1 of precipitation Nomaps of observed precipitation in Antarctica are availablebut the modelled precipitation minus sublimation (P ndash E notshown) is in qualitative agreement with recent compilationsof Antarctic mass balance (Vaughan and others 1999) Allprecipitation in Antarctica falls as snow except for occa-sional rainfall events reported in the northernmost AP

4 AAO REGRESSION RESULTSWe regressed monthly mean anomalies of surfacepressure ps surface temperature Ts precipitation P and10 m wind speed and wind vector components (U10m V10m)

onto the monthly mean detrended AAO index 1980ndash93(N frac14 168) The fields presented in this section are thusrepresentative of a strengthening of the circumpolar vortex(from low to high AAO index) The slopes of the linearregressions were multiplied by the standard deviation of theAAO index (081) to obtain anomaly fields that have units ofthe dependent variable and are related to 1 standarddeviation of the AAO index (Fig 4andashd) Dashed contoursdelineate areas where correlation confidence reaches 99The time series of AAO index in Figure 2 do not displaysignificant autocorrelation so no correction was applied forthis

Surface pressure and 10 m windThe surface pressure anomaly pattern (Fig 4a) showsstrongly negative values over continental Antarctica andpositive values at lower latitudes a common feature of theAAO Except for a small band around zero change the 99confidence level is reached everywhere and is not shown Inspite of the lsquoannularrsquo character of the AAO the pattern inFigure 4a is far from zonally symmetric because of theclimatological low-pressure centres around Antarctica(Fig 3a) In Figure 4a a pronounced surface pressuredecrease is found in the Amundsen Sea which coincideswith the lsquopole of variabilityrsquo in the Southern Hemisphere andis associated with a changed location of the AmundsenndashBellingshausen Sea low (Connolley 1997) A weaker localminimum is found over Wilkes Land East Antarctica Themaximum gradient in the surface pressure anomaly fieldoccurs just north of the Antarctic coastline and represents anincrease of the surface geostrophic zonal wind of about2 m sndash1 At 500 hPa zonal winds have increased by abouttwice this amount (not shown) indicative of an enhancedhorizontal meridional temperature gradient under condi-tions of positive AAO polarity

Figure 4b presents 10 m wind anomalies expressed as avector composed of the anomalies in the individualhorizontal components Background colours indicatechanges in absolute wind speed Following the annularcharacter of surface pressure anomalies (Fig 4a) a 1 m sndash1

increase of the westerlies north of Antarctica is found ieabout half of the change in the surface geostrophic windspeed Some important deviations from zonal symmetryoccur over the continent flow anomalies are directedtowards West Antarctica where they cross the WestAntarctic ice divide and descend towards the Ross Ice

Fig 2 Time series of detrended monthly mean AAO index 1980ndash93

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 121

Shelf In the Weddell Sea northerly flow anomalies followthe east coast of the AP continuing over the FilchnerndashRonneIce Shelf and on to the East Antarctic plateau Thisrepresents a weakening of the persistent southerly flowalong the eastern border of the AP which Schwerdtfeger(1975) identified as barrier winds Anomalous southerlygeostrophic flow occurs over the Ross Ice Shelf and inwestern Dronning Maud Land

The sign of the wind-speed anomaly (background coloursin Fig 4b) depends on whether the vector wind anomaly hasa component in the direction of the local climatological

wind direction or opposed to it (cf vectors in Figs 2band 4b) For example over the ocean and sea ice thepressure anomaly field enhances the westerlies north of theCPT by up to 1 m sndash1 but weakens the coastal easterlies alongthe East Antarctic coast by a similar amount Over the icesheet the climatological wind direction is determinedmainly by the katabatic component which acts in thedownslope direction This generates cross-slope winds witha downslope component near the surface due to friction(Fig 2b) (Van den Broeke and others 2002)

The dependency of the near-surface katabatic wind field

Fig 3 (a) 1980ndash93 modelled annual mean sea-level pressure in hPa (b) 10 m vector wind and wind speed (background colour) in m sndash1(c) surface potential temperature in K and (d) annual precipitation in mm

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO122

on slope direction explains the dipole patterns of 10 m wind-speed change that are seen in interior Dronning Maud Landand in West Antarctica in Figure 4b In Wilkes Land thelarge-scale pressure-gradient force (PGF) and the katabaticPGF generally act in the same direction ie downslope Inthis region both PGFs become equally important in thenear-surface momentum budget (Parish and Cassano 2001Van den Broeke and Van Lipzig 2003) Because theperturbation large-scale PGF is directed opposite to theaverage large-scale PGF a marked decrease in near-surfacewind speed is found in this area (Fig 4b)

Surface temperature

A stronger circumpolar vortex (high AAO index) inducessignificant cooling over most of East Antarctica and signifi-cant warming over the entire AP (Fig 4c) This pattern is verysimilar to that presented by Kwok and Comiso (2002) whoused surface temperatures derived from thermal infraredimagery (1982ndash99) For wintertime conditions Van denBroeke and Van Lipzig (2002) showed that East Antarcticcooling during strong vortex conditions has two com-ponents a general tropospheric cooling of about 2 K over

Fig 4 (a) AAO regression slope of surface pressure in hPa (b) 10 m vector wind and wind speed (background colour) in m sndash1 (c) surfacepotential temperature in K and (d) annual precipitation in Values correspond to a 1 standard deviation anomaly in the AAO Dashedcontours enclose areas where the 99 confidence level is reached

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 123

the whole of East Antarctica probably caused by suppressedmeridional air exchange and additional cooling near thesurface in areas where a weakening of near-surface windshas intensified the surface temperature inversion

From Figure 4c it appears that areas with a surfacetemperature inversion (roughly coinciding with low poten-tial temperatures in Fig 2c) are prone to potentially strongersurface warming when the circumpolar vortex is strong (highAAO index) through destruction of the temperature inver-sion An example is the AP ice shelves where warming islarger than in the immediate surroundings This process maybe dominated by changes in horizontal heat advection inregions where flow anomalies are directed perpendicular tolarge horizontal temperature gradients such as are found

near the continental margin and the sea-ice edge Forinstance the strong warming over the western AP and theWeddell Sea is likely to be at least partly caused by a greaternortherly wind component in combination with decreasedsea-ice cover in the Bellingshausen Sea (Marshall and King1998 Marshall 2002b) A more quantitative assessment ofthe contributions of various processes to regional tempera-ture change based on calculation of the individual terms inthe heat budget is a topic of future study

PrecipitationWhen the circumpolar vortex is strong (high AAO index)three areas over the continent show changes in precipitationthat reach the 99 confidence level (Fig 4d) An increase of

Fig 5 Seasonal subsets of AAO regression slope of surface pressure in hPa (solid contours) and surface temperature in K (colours) for DJF (a)MAM (b) JJA (c) and SON (d) Values correspond to a 1 standard deviation anomaly in the AAO Dashed contours enclose areas where the99 confidence level is reached

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO124

up to 30 is found over the western AP while western MarieByrd Land and the adjacent Ross Ice Shelf show decreases ofa similar magnitude Interestingly in the precipitationshadow at the eastern side of the AP zero precipitationchange is found over the northerly ice shelves an increase intheir mass balance may have slowed down their disintegra-tion A relative decrease of about 15 is found over theLambert Glacier basin These patterns clearly reflect theinteraction between circulation anomalies (Fig 4a and b)and Antarctic topography Of interest to ice-core researchersis that western Dronning Maud Land experiences aprecipitation increase of up to 10 (mostly reaching the90ndash95 confidence level) and at the same time significantcooling (Fig 4c) Elsewhere in Antarctica increases inprecipitation are in general associated with warming

Regression on seasonal subsetsIt is well known that the Antarctic temperature inversion isstrongest in winter and greatly reduced in summer Otherimportant factors determining Antarctic surface temperaturesuch as sea-ice extent and strength of the katabatic windsalso show a pronounced annual cycle From this we wouldexpect a seasonal dependence of the temperature responseto the AAO To investigate this we regressed seasonalsubsets of 3 months (DJF MAM JJA and SON) on the AAOindex Figure 5 shows the combined results for surfacetemperature (colours dashed contours where 99 con-fidence is reached) and surface pressure (solid contours) Inspite of the reduced number of points used in the regressionall seasons show extensive areas with significant surfacecooling over East Antarctica in response to a strongercircumpolar vortex (high AAO index) For instance inWilkes Land just east of the Lambert Glacier basin coolingis significant in all four seasons Warming in the AP is mostpronounced in autumn and winter when the circulationanomalies have their greatest northerly component so thatmost of the annual temperature signal in Figure 4c derivesfrom the months MarchndashAugust (Fig 5b and c) This is inagreement with observations (Marshall 2002a)

Impact of wind and temperature changes on thesurface sensible heat fluxTo investigate what causes the surface cooling in EastAntarctica under conditions of strong circumpolar vortex(high AAO index) we selected from Figure 4c the location inWilkes Land where the correlation between the AAO indexand temperature attains a minimum For this locationFigure 6 shows the correlation between the AAO index andanomalies in the surface turbulent flux of sensible heat(SHF) SHF is an important component of the wintertimeAntarctic surface energy balance and provides the surfacewith heat that it loses through net emission of longwaveradiation Because turbulence and hence the SHF in thestably stratified wintertime Antarctic atmosphere is gener-ated mainly by wind shear a decrease in 10 m wind leads toa decrease in SHF and hence a lower surface temperatureThis link of the AAO with East Antarctic surface tempera-tures through the katabatic wind field and the sensible heatflux is clearly apparent from Figure 6

6 SUMMARY AND FUTURE WORKWe used output of a 14 year integration with a high-reso-lution regional atmospheric climate model (RACMOANT1)

to study the response of the (near-) surface Antarctic climateto the AAO which is a measure of the strength of thecircumpolar vortex or westerlies In spite of the relativelyshort period significant AAO-related anomalies were foundin surface pressure 10 m wind surface temperature andprecipitation When the vortex is strong (high AAO index)surface pressure changes show an asymmetric pattern withthe largest pressure falls in coastal Marie Byrd Land Thiscauses northerly flow anomalies and significant warmingover the AP and adjacent regions in West Antarctica and theWeddell Sea In response to decreased meridional airexchange and weaker near-surface winds significant cool-ing occurs throughout East Antarctica The strongest signalsare found in Wilkes Land but strong cooling is also foundover the Ross Ice Shelf and the adjacent part of WestAntarctica Repeating the regression on subsets of 3 monthsreveals that most of the annual temperature signal derivesfrom the months MarchndashAugust Precipitation anomaliesreflect the interaction of circulation anomalies and topog-raphy and do not generally mirror temperature changesSignificant wet anomalies are found on the western AP anddry anomalies occur over the Ross Ice Shelf the adjacentWest Antarctic ice sheet and in the Lambert Glacier basinEast Antarctica

The regional modelling approach has proved very usefulfor clarifying physical processes underlying climate changein Antarctica To improve statistical significance in studies ofthis kind a 45 year run with a regional atmospheric climatemodel (forced by ERA40) is ongoing

REFERENCESBromwich D H 1989 Satellite analyses of Antarctic katabatic

wind behavior Bull Am Meteorol Soc 70(7) 738ndash749Comiso J C 2000 Variability and trends in Antarctic surface

temperatures from in situ and satellite infrared measurementsJ Climate 13(10) 1674ndash1696

Connolley W M 1996 The Antarctic temperature inversion Int JClimatol 16(12) 1333ndash1342

Fig 6 Correlation of monthly-mean AAO index (1980ndash93) withanomalies of surface temperature and turbulent flux of sensible heatfor a centre of action in Wilkes Land (see white cross in Fig 4c)

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 125

wind speeds of 10ndash15 m sndash1 In the absence of a significantslope to force katabatic winds the flat ice-sheet domes andice shelves are characterized by relatively low wind speedstypically 5 m sndash1 Unrealistically low values of 10 m windspeed are modelled in regions where the model surfaceroughness for momentum has an important subgrid-topo-graphical component (Transantarctic Mountains parts ofDronning Maud Land and the AP) These values should notbe compared to locally observed values

High values of surface potential temperature s (Fig 3c)are found over the coastal ice sheet these are lsquowarmsignaturesrsquo of katabatic winds that mix relatively warm airtowards the surface (Bromwich 1989 Van den Broeke andothers 1999) Low values of s are found over the flat ice-sheet interior ice shelves and sea ice These areas arecharacterized by a surface temperature inversion (Connol-ley 1996) Note the close relation between the large-scalecirculation with these lsquopoolsrsquo of cold air and northwardsea-ice extent the greatest northward extent of sea ice isfound to the west of the climatological low-pressure areaswhere northward advection of cold air is promoted In theRoss and Weddell Seas these circulation systems arereferred to as the Ross and Weddell gyres The formationof Antarctic Bottom Water in these regions plays a majorrole in driving the global ocean circulation Variations in thestrength of these lsquogyresrsquo are important for the sensitivity ofAntarctic temperature to changes in the large-scale circula-tion

The largest annual precipitation amounts (Fig 3d) inAntarctica are found in the western AP coastal WestAntarctica and coastal Wilkes Land (about 1000ndash1500 mm we) These regions are characterized by stronginteraction of the ice-sheet topography with the large-scaleflow resulting in forced convection The interior of EastAntarctica receives lt100 mm we andash1 of precipitation Nomaps of observed precipitation in Antarctica are availablebut the modelled precipitation minus sublimation (P ndash E notshown) is in qualitative agreement with recent compilationsof Antarctic mass balance (Vaughan and others 1999) Allprecipitation in Antarctica falls as snow except for occa-sional rainfall events reported in the northernmost AP

4 AAO REGRESSION RESULTSWe regressed monthly mean anomalies of surfacepressure ps surface temperature Ts precipitation P and10 m wind speed and wind vector components (U10m V10m)

onto the monthly mean detrended AAO index 1980ndash93(N frac14 168) The fields presented in this section are thusrepresentative of a strengthening of the circumpolar vortex(from low to high AAO index) The slopes of the linearregressions were multiplied by the standard deviation of theAAO index (081) to obtain anomaly fields that have units ofthe dependent variable and are related to 1 standarddeviation of the AAO index (Fig 4andashd) Dashed contoursdelineate areas where correlation confidence reaches 99The time series of AAO index in Figure 2 do not displaysignificant autocorrelation so no correction was applied forthis

Surface pressure and 10 m windThe surface pressure anomaly pattern (Fig 4a) showsstrongly negative values over continental Antarctica andpositive values at lower latitudes a common feature of theAAO Except for a small band around zero change the 99confidence level is reached everywhere and is not shown Inspite of the lsquoannularrsquo character of the AAO the pattern inFigure 4a is far from zonally symmetric because of theclimatological low-pressure centres around Antarctica(Fig 3a) In Figure 4a a pronounced surface pressuredecrease is found in the Amundsen Sea which coincideswith the lsquopole of variabilityrsquo in the Southern Hemisphere andis associated with a changed location of the AmundsenndashBellingshausen Sea low (Connolley 1997) A weaker localminimum is found over Wilkes Land East Antarctica Themaximum gradient in the surface pressure anomaly fieldoccurs just north of the Antarctic coastline and represents anincrease of the surface geostrophic zonal wind of about2 m sndash1 At 500 hPa zonal winds have increased by abouttwice this amount (not shown) indicative of an enhancedhorizontal meridional temperature gradient under condi-tions of positive AAO polarity

Figure 4b presents 10 m wind anomalies expressed as avector composed of the anomalies in the individualhorizontal components Background colours indicatechanges in absolute wind speed Following the annularcharacter of surface pressure anomalies (Fig 4a) a 1 m sndash1

increase of the westerlies north of Antarctica is found ieabout half of the change in the surface geostrophic windspeed Some important deviations from zonal symmetryoccur over the continent flow anomalies are directedtowards West Antarctica where they cross the WestAntarctic ice divide and descend towards the Ross Ice

Fig 2 Time series of detrended monthly mean AAO index 1980ndash93

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 121

Shelf In the Weddell Sea northerly flow anomalies followthe east coast of the AP continuing over the FilchnerndashRonneIce Shelf and on to the East Antarctic plateau Thisrepresents a weakening of the persistent southerly flowalong the eastern border of the AP which Schwerdtfeger(1975) identified as barrier winds Anomalous southerlygeostrophic flow occurs over the Ross Ice Shelf and inwestern Dronning Maud Land

The sign of the wind-speed anomaly (background coloursin Fig 4b) depends on whether the vector wind anomaly hasa component in the direction of the local climatological

wind direction or opposed to it (cf vectors in Figs 2band 4b) For example over the ocean and sea ice thepressure anomaly field enhances the westerlies north of theCPT by up to 1 m sndash1 but weakens the coastal easterlies alongthe East Antarctic coast by a similar amount Over the icesheet the climatological wind direction is determinedmainly by the katabatic component which acts in thedownslope direction This generates cross-slope winds witha downslope component near the surface due to friction(Fig 2b) (Van den Broeke and others 2002)

The dependency of the near-surface katabatic wind field

Fig 3 (a) 1980ndash93 modelled annual mean sea-level pressure in hPa (b) 10 m vector wind and wind speed (background colour) in m sndash1(c) surface potential temperature in K and (d) annual precipitation in mm

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO122

on slope direction explains the dipole patterns of 10 m wind-speed change that are seen in interior Dronning Maud Landand in West Antarctica in Figure 4b In Wilkes Land thelarge-scale pressure-gradient force (PGF) and the katabaticPGF generally act in the same direction ie downslope Inthis region both PGFs become equally important in thenear-surface momentum budget (Parish and Cassano 2001Van den Broeke and Van Lipzig 2003) Because theperturbation large-scale PGF is directed opposite to theaverage large-scale PGF a marked decrease in near-surfacewind speed is found in this area (Fig 4b)

Surface temperature

A stronger circumpolar vortex (high AAO index) inducessignificant cooling over most of East Antarctica and signifi-cant warming over the entire AP (Fig 4c) This pattern is verysimilar to that presented by Kwok and Comiso (2002) whoused surface temperatures derived from thermal infraredimagery (1982ndash99) For wintertime conditions Van denBroeke and Van Lipzig (2002) showed that East Antarcticcooling during strong vortex conditions has two com-ponents a general tropospheric cooling of about 2 K over

Fig 4 (a) AAO regression slope of surface pressure in hPa (b) 10 m vector wind and wind speed (background colour) in m sndash1 (c) surfacepotential temperature in K and (d) annual precipitation in Values correspond to a 1 standard deviation anomaly in the AAO Dashedcontours enclose areas where the 99 confidence level is reached

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 123

the whole of East Antarctica probably caused by suppressedmeridional air exchange and additional cooling near thesurface in areas where a weakening of near-surface windshas intensified the surface temperature inversion

From Figure 4c it appears that areas with a surfacetemperature inversion (roughly coinciding with low poten-tial temperatures in Fig 2c) are prone to potentially strongersurface warming when the circumpolar vortex is strong (highAAO index) through destruction of the temperature inver-sion An example is the AP ice shelves where warming islarger than in the immediate surroundings This process maybe dominated by changes in horizontal heat advection inregions where flow anomalies are directed perpendicular tolarge horizontal temperature gradients such as are found

near the continental margin and the sea-ice edge Forinstance the strong warming over the western AP and theWeddell Sea is likely to be at least partly caused by a greaternortherly wind component in combination with decreasedsea-ice cover in the Bellingshausen Sea (Marshall and King1998 Marshall 2002b) A more quantitative assessment ofthe contributions of various processes to regional tempera-ture change based on calculation of the individual terms inthe heat budget is a topic of future study

PrecipitationWhen the circumpolar vortex is strong (high AAO index)three areas over the continent show changes in precipitationthat reach the 99 confidence level (Fig 4d) An increase of

Fig 5 Seasonal subsets of AAO regression slope of surface pressure in hPa (solid contours) and surface temperature in K (colours) for DJF (a)MAM (b) JJA (c) and SON (d) Values correspond to a 1 standard deviation anomaly in the AAO Dashed contours enclose areas where the99 confidence level is reached

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO124

up to 30 is found over the western AP while western MarieByrd Land and the adjacent Ross Ice Shelf show decreases ofa similar magnitude Interestingly in the precipitationshadow at the eastern side of the AP zero precipitationchange is found over the northerly ice shelves an increase intheir mass balance may have slowed down their disintegra-tion A relative decrease of about 15 is found over theLambert Glacier basin These patterns clearly reflect theinteraction between circulation anomalies (Fig 4a and b)and Antarctic topography Of interest to ice-core researchersis that western Dronning Maud Land experiences aprecipitation increase of up to 10 (mostly reaching the90ndash95 confidence level) and at the same time significantcooling (Fig 4c) Elsewhere in Antarctica increases inprecipitation are in general associated with warming

Regression on seasonal subsetsIt is well known that the Antarctic temperature inversion isstrongest in winter and greatly reduced in summer Otherimportant factors determining Antarctic surface temperaturesuch as sea-ice extent and strength of the katabatic windsalso show a pronounced annual cycle From this we wouldexpect a seasonal dependence of the temperature responseto the AAO To investigate this we regressed seasonalsubsets of 3 months (DJF MAM JJA and SON) on the AAOindex Figure 5 shows the combined results for surfacetemperature (colours dashed contours where 99 con-fidence is reached) and surface pressure (solid contours) Inspite of the reduced number of points used in the regressionall seasons show extensive areas with significant surfacecooling over East Antarctica in response to a strongercircumpolar vortex (high AAO index) For instance inWilkes Land just east of the Lambert Glacier basin coolingis significant in all four seasons Warming in the AP is mostpronounced in autumn and winter when the circulationanomalies have their greatest northerly component so thatmost of the annual temperature signal in Figure 4c derivesfrom the months MarchndashAugust (Fig 5b and c) This is inagreement with observations (Marshall 2002a)

Impact of wind and temperature changes on thesurface sensible heat fluxTo investigate what causes the surface cooling in EastAntarctica under conditions of strong circumpolar vortex(high AAO index) we selected from Figure 4c the location inWilkes Land where the correlation between the AAO indexand temperature attains a minimum For this locationFigure 6 shows the correlation between the AAO index andanomalies in the surface turbulent flux of sensible heat(SHF) SHF is an important component of the wintertimeAntarctic surface energy balance and provides the surfacewith heat that it loses through net emission of longwaveradiation Because turbulence and hence the SHF in thestably stratified wintertime Antarctic atmosphere is gener-ated mainly by wind shear a decrease in 10 m wind leads toa decrease in SHF and hence a lower surface temperatureThis link of the AAO with East Antarctic surface tempera-tures through the katabatic wind field and the sensible heatflux is clearly apparent from Figure 6

6 SUMMARY AND FUTURE WORKWe used output of a 14 year integration with a high-reso-lution regional atmospheric climate model (RACMOANT1)

to study the response of the (near-) surface Antarctic climateto the AAO which is a measure of the strength of thecircumpolar vortex or westerlies In spite of the relativelyshort period significant AAO-related anomalies were foundin surface pressure 10 m wind surface temperature andprecipitation When the vortex is strong (high AAO index)surface pressure changes show an asymmetric pattern withthe largest pressure falls in coastal Marie Byrd Land Thiscauses northerly flow anomalies and significant warmingover the AP and adjacent regions in West Antarctica and theWeddell Sea In response to decreased meridional airexchange and weaker near-surface winds significant cool-ing occurs throughout East Antarctica The strongest signalsare found in Wilkes Land but strong cooling is also foundover the Ross Ice Shelf and the adjacent part of WestAntarctica Repeating the regression on subsets of 3 monthsreveals that most of the annual temperature signal derivesfrom the months MarchndashAugust Precipitation anomaliesreflect the interaction of circulation anomalies and topog-raphy and do not generally mirror temperature changesSignificant wet anomalies are found on the western AP anddry anomalies occur over the Ross Ice Shelf the adjacentWest Antarctic ice sheet and in the Lambert Glacier basinEast Antarctica

The regional modelling approach has proved very usefulfor clarifying physical processes underlying climate changein Antarctica To improve statistical significance in studies ofthis kind a 45 year run with a regional atmospheric climatemodel (forced by ERA40) is ongoing

REFERENCESBromwich D H 1989 Satellite analyses of Antarctic katabatic

wind behavior Bull Am Meteorol Soc 70(7) 738ndash749Comiso J C 2000 Variability and trends in Antarctic surface

temperatures from in situ and satellite infrared measurementsJ Climate 13(10) 1674ndash1696

Connolley W M 1996 The Antarctic temperature inversion Int JClimatol 16(12) 1333ndash1342

Fig 6 Correlation of monthly-mean AAO index (1980ndash93) withanomalies of surface temperature and turbulent flux of sensible heatfor a centre of action in Wilkes Land (see white cross in Fig 4c)

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 125

Shelf In the Weddell Sea northerly flow anomalies followthe east coast of the AP continuing over the FilchnerndashRonneIce Shelf and on to the East Antarctic plateau Thisrepresents a weakening of the persistent southerly flowalong the eastern border of the AP which Schwerdtfeger(1975) identified as barrier winds Anomalous southerlygeostrophic flow occurs over the Ross Ice Shelf and inwestern Dronning Maud Land

The sign of the wind-speed anomaly (background coloursin Fig 4b) depends on whether the vector wind anomaly hasa component in the direction of the local climatological

wind direction or opposed to it (cf vectors in Figs 2band 4b) For example over the ocean and sea ice thepressure anomaly field enhances the westerlies north of theCPT by up to 1 m sndash1 but weakens the coastal easterlies alongthe East Antarctic coast by a similar amount Over the icesheet the climatological wind direction is determinedmainly by the katabatic component which acts in thedownslope direction This generates cross-slope winds witha downslope component near the surface due to friction(Fig 2b) (Van den Broeke and others 2002)

The dependency of the near-surface katabatic wind field

Fig 3 (a) 1980ndash93 modelled annual mean sea-level pressure in hPa (b) 10 m vector wind and wind speed (background colour) in m sndash1(c) surface potential temperature in K and (d) annual precipitation in mm

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO122

on slope direction explains the dipole patterns of 10 m wind-speed change that are seen in interior Dronning Maud Landand in West Antarctica in Figure 4b In Wilkes Land thelarge-scale pressure-gradient force (PGF) and the katabaticPGF generally act in the same direction ie downslope Inthis region both PGFs become equally important in thenear-surface momentum budget (Parish and Cassano 2001Van den Broeke and Van Lipzig 2003) Because theperturbation large-scale PGF is directed opposite to theaverage large-scale PGF a marked decrease in near-surfacewind speed is found in this area (Fig 4b)

Surface temperature

A stronger circumpolar vortex (high AAO index) inducessignificant cooling over most of East Antarctica and signifi-cant warming over the entire AP (Fig 4c) This pattern is verysimilar to that presented by Kwok and Comiso (2002) whoused surface temperatures derived from thermal infraredimagery (1982ndash99) For wintertime conditions Van denBroeke and Van Lipzig (2002) showed that East Antarcticcooling during strong vortex conditions has two com-ponents a general tropospheric cooling of about 2 K over

Fig 4 (a) AAO regression slope of surface pressure in hPa (b) 10 m vector wind and wind speed (background colour) in m sndash1 (c) surfacepotential temperature in K and (d) annual precipitation in Values correspond to a 1 standard deviation anomaly in the AAO Dashedcontours enclose areas where the 99 confidence level is reached

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 123

the whole of East Antarctica probably caused by suppressedmeridional air exchange and additional cooling near thesurface in areas where a weakening of near-surface windshas intensified the surface temperature inversion

From Figure 4c it appears that areas with a surfacetemperature inversion (roughly coinciding with low poten-tial temperatures in Fig 2c) are prone to potentially strongersurface warming when the circumpolar vortex is strong (highAAO index) through destruction of the temperature inver-sion An example is the AP ice shelves where warming islarger than in the immediate surroundings This process maybe dominated by changes in horizontal heat advection inregions where flow anomalies are directed perpendicular tolarge horizontal temperature gradients such as are found

near the continental margin and the sea-ice edge Forinstance the strong warming over the western AP and theWeddell Sea is likely to be at least partly caused by a greaternortherly wind component in combination with decreasedsea-ice cover in the Bellingshausen Sea (Marshall and King1998 Marshall 2002b) A more quantitative assessment ofthe contributions of various processes to regional tempera-ture change based on calculation of the individual terms inthe heat budget is a topic of future study

PrecipitationWhen the circumpolar vortex is strong (high AAO index)three areas over the continent show changes in precipitationthat reach the 99 confidence level (Fig 4d) An increase of

Fig 5 Seasonal subsets of AAO regression slope of surface pressure in hPa (solid contours) and surface temperature in K (colours) for DJF (a)MAM (b) JJA (c) and SON (d) Values correspond to a 1 standard deviation anomaly in the AAO Dashed contours enclose areas where the99 confidence level is reached

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO124

up to 30 is found over the western AP while western MarieByrd Land and the adjacent Ross Ice Shelf show decreases ofa similar magnitude Interestingly in the precipitationshadow at the eastern side of the AP zero precipitationchange is found over the northerly ice shelves an increase intheir mass balance may have slowed down their disintegra-tion A relative decrease of about 15 is found over theLambert Glacier basin These patterns clearly reflect theinteraction between circulation anomalies (Fig 4a and b)and Antarctic topography Of interest to ice-core researchersis that western Dronning Maud Land experiences aprecipitation increase of up to 10 (mostly reaching the90ndash95 confidence level) and at the same time significantcooling (Fig 4c) Elsewhere in Antarctica increases inprecipitation are in general associated with warming

Regression on seasonal subsetsIt is well known that the Antarctic temperature inversion isstrongest in winter and greatly reduced in summer Otherimportant factors determining Antarctic surface temperaturesuch as sea-ice extent and strength of the katabatic windsalso show a pronounced annual cycle From this we wouldexpect a seasonal dependence of the temperature responseto the AAO To investigate this we regressed seasonalsubsets of 3 months (DJF MAM JJA and SON) on the AAOindex Figure 5 shows the combined results for surfacetemperature (colours dashed contours where 99 con-fidence is reached) and surface pressure (solid contours) Inspite of the reduced number of points used in the regressionall seasons show extensive areas with significant surfacecooling over East Antarctica in response to a strongercircumpolar vortex (high AAO index) For instance inWilkes Land just east of the Lambert Glacier basin coolingis significant in all four seasons Warming in the AP is mostpronounced in autumn and winter when the circulationanomalies have their greatest northerly component so thatmost of the annual temperature signal in Figure 4c derivesfrom the months MarchndashAugust (Fig 5b and c) This is inagreement with observations (Marshall 2002a)

Impact of wind and temperature changes on thesurface sensible heat fluxTo investigate what causes the surface cooling in EastAntarctica under conditions of strong circumpolar vortex(high AAO index) we selected from Figure 4c the location inWilkes Land where the correlation between the AAO indexand temperature attains a minimum For this locationFigure 6 shows the correlation between the AAO index andanomalies in the surface turbulent flux of sensible heat(SHF) SHF is an important component of the wintertimeAntarctic surface energy balance and provides the surfacewith heat that it loses through net emission of longwaveradiation Because turbulence and hence the SHF in thestably stratified wintertime Antarctic atmosphere is gener-ated mainly by wind shear a decrease in 10 m wind leads toa decrease in SHF and hence a lower surface temperatureThis link of the AAO with East Antarctic surface tempera-tures through the katabatic wind field and the sensible heatflux is clearly apparent from Figure 6

6 SUMMARY AND FUTURE WORKWe used output of a 14 year integration with a high-reso-lution regional atmospheric climate model (RACMOANT1)

to study the response of the (near-) surface Antarctic climateto the AAO which is a measure of the strength of thecircumpolar vortex or westerlies In spite of the relativelyshort period significant AAO-related anomalies were foundin surface pressure 10 m wind surface temperature andprecipitation When the vortex is strong (high AAO index)surface pressure changes show an asymmetric pattern withthe largest pressure falls in coastal Marie Byrd Land Thiscauses northerly flow anomalies and significant warmingover the AP and adjacent regions in West Antarctica and theWeddell Sea In response to decreased meridional airexchange and weaker near-surface winds significant cool-ing occurs throughout East Antarctica The strongest signalsare found in Wilkes Land but strong cooling is also foundover the Ross Ice Shelf and the adjacent part of WestAntarctica Repeating the regression on subsets of 3 monthsreveals that most of the annual temperature signal derivesfrom the months MarchndashAugust Precipitation anomaliesreflect the interaction of circulation anomalies and topog-raphy and do not generally mirror temperature changesSignificant wet anomalies are found on the western AP anddry anomalies occur over the Ross Ice Shelf the adjacentWest Antarctic ice sheet and in the Lambert Glacier basinEast Antarctica

The regional modelling approach has proved very usefulfor clarifying physical processes underlying climate changein Antarctica To improve statistical significance in studies ofthis kind a 45 year run with a regional atmospheric climatemodel (forced by ERA40) is ongoing

REFERENCESBromwich D H 1989 Satellite analyses of Antarctic katabatic

wind behavior Bull Am Meteorol Soc 70(7) 738ndash749Comiso J C 2000 Variability and trends in Antarctic surface

temperatures from in situ and satellite infrared measurementsJ Climate 13(10) 1674ndash1696

Connolley W M 1996 The Antarctic temperature inversion Int JClimatol 16(12) 1333ndash1342

Fig 6 Correlation of monthly-mean AAO index (1980ndash93) withanomalies of surface temperature and turbulent flux of sensible heatfor a centre of action in Wilkes Land (see white cross in Fig 4c)

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 125

on slope direction explains the dipole patterns of 10 m wind-speed change that are seen in interior Dronning Maud Landand in West Antarctica in Figure 4b In Wilkes Land thelarge-scale pressure-gradient force (PGF) and the katabaticPGF generally act in the same direction ie downslope Inthis region both PGFs become equally important in thenear-surface momentum budget (Parish and Cassano 2001Van den Broeke and Van Lipzig 2003) Because theperturbation large-scale PGF is directed opposite to theaverage large-scale PGF a marked decrease in near-surfacewind speed is found in this area (Fig 4b)

Surface temperature

A stronger circumpolar vortex (high AAO index) inducessignificant cooling over most of East Antarctica and signifi-cant warming over the entire AP (Fig 4c) This pattern is verysimilar to that presented by Kwok and Comiso (2002) whoused surface temperatures derived from thermal infraredimagery (1982ndash99) For wintertime conditions Van denBroeke and Van Lipzig (2002) showed that East Antarcticcooling during strong vortex conditions has two com-ponents a general tropospheric cooling of about 2 K over

Fig 4 (a) AAO regression slope of surface pressure in hPa (b) 10 m vector wind and wind speed (background colour) in m sndash1 (c) surfacepotential temperature in K and (d) annual precipitation in Values correspond to a 1 standard deviation anomaly in the AAO Dashedcontours enclose areas where the 99 confidence level is reached

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 123

the whole of East Antarctica probably caused by suppressedmeridional air exchange and additional cooling near thesurface in areas where a weakening of near-surface windshas intensified the surface temperature inversion

From Figure 4c it appears that areas with a surfacetemperature inversion (roughly coinciding with low poten-tial temperatures in Fig 2c) are prone to potentially strongersurface warming when the circumpolar vortex is strong (highAAO index) through destruction of the temperature inver-sion An example is the AP ice shelves where warming islarger than in the immediate surroundings This process maybe dominated by changes in horizontal heat advection inregions where flow anomalies are directed perpendicular tolarge horizontal temperature gradients such as are found

near the continental margin and the sea-ice edge Forinstance the strong warming over the western AP and theWeddell Sea is likely to be at least partly caused by a greaternortherly wind component in combination with decreasedsea-ice cover in the Bellingshausen Sea (Marshall and King1998 Marshall 2002b) A more quantitative assessment ofthe contributions of various processes to regional tempera-ture change based on calculation of the individual terms inthe heat budget is a topic of future study

PrecipitationWhen the circumpolar vortex is strong (high AAO index)three areas over the continent show changes in precipitationthat reach the 99 confidence level (Fig 4d) An increase of

Fig 5 Seasonal subsets of AAO regression slope of surface pressure in hPa (solid contours) and surface temperature in K (colours) for DJF (a)MAM (b) JJA (c) and SON (d) Values correspond to a 1 standard deviation anomaly in the AAO Dashed contours enclose areas where the99 confidence level is reached

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO124

up to 30 is found over the western AP while western MarieByrd Land and the adjacent Ross Ice Shelf show decreases ofa similar magnitude Interestingly in the precipitationshadow at the eastern side of the AP zero precipitationchange is found over the northerly ice shelves an increase intheir mass balance may have slowed down their disintegra-tion A relative decrease of about 15 is found over theLambert Glacier basin These patterns clearly reflect theinteraction between circulation anomalies (Fig 4a and b)and Antarctic topography Of interest to ice-core researchersis that western Dronning Maud Land experiences aprecipitation increase of up to 10 (mostly reaching the90ndash95 confidence level) and at the same time significantcooling (Fig 4c) Elsewhere in Antarctica increases inprecipitation are in general associated with warming

Regression on seasonal subsetsIt is well known that the Antarctic temperature inversion isstrongest in winter and greatly reduced in summer Otherimportant factors determining Antarctic surface temperaturesuch as sea-ice extent and strength of the katabatic windsalso show a pronounced annual cycle From this we wouldexpect a seasonal dependence of the temperature responseto the AAO To investigate this we regressed seasonalsubsets of 3 months (DJF MAM JJA and SON) on the AAOindex Figure 5 shows the combined results for surfacetemperature (colours dashed contours where 99 con-fidence is reached) and surface pressure (solid contours) Inspite of the reduced number of points used in the regressionall seasons show extensive areas with significant surfacecooling over East Antarctica in response to a strongercircumpolar vortex (high AAO index) For instance inWilkes Land just east of the Lambert Glacier basin coolingis significant in all four seasons Warming in the AP is mostpronounced in autumn and winter when the circulationanomalies have their greatest northerly component so thatmost of the annual temperature signal in Figure 4c derivesfrom the months MarchndashAugust (Fig 5b and c) This is inagreement with observations (Marshall 2002a)

Impact of wind and temperature changes on thesurface sensible heat fluxTo investigate what causes the surface cooling in EastAntarctica under conditions of strong circumpolar vortex(high AAO index) we selected from Figure 4c the location inWilkes Land where the correlation between the AAO indexand temperature attains a minimum For this locationFigure 6 shows the correlation between the AAO index andanomalies in the surface turbulent flux of sensible heat(SHF) SHF is an important component of the wintertimeAntarctic surface energy balance and provides the surfacewith heat that it loses through net emission of longwaveradiation Because turbulence and hence the SHF in thestably stratified wintertime Antarctic atmosphere is gener-ated mainly by wind shear a decrease in 10 m wind leads toa decrease in SHF and hence a lower surface temperatureThis link of the AAO with East Antarctic surface tempera-tures through the katabatic wind field and the sensible heatflux is clearly apparent from Figure 6

6 SUMMARY AND FUTURE WORKWe used output of a 14 year integration with a high-reso-lution regional atmospheric climate model (RACMOANT1)

to study the response of the (near-) surface Antarctic climateto the AAO which is a measure of the strength of thecircumpolar vortex or westerlies In spite of the relativelyshort period significant AAO-related anomalies were foundin surface pressure 10 m wind surface temperature andprecipitation When the vortex is strong (high AAO index)surface pressure changes show an asymmetric pattern withthe largest pressure falls in coastal Marie Byrd Land Thiscauses northerly flow anomalies and significant warmingover the AP and adjacent regions in West Antarctica and theWeddell Sea In response to decreased meridional airexchange and weaker near-surface winds significant cool-ing occurs throughout East Antarctica The strongest signalsare found in Wilkes Land but strong cooling is also foundover the Ross Ice Shelf and the adjacent part of WestAntarctica Repeating the regression on subsets of 3 monthsreveals that most of the annual temperature signal derivesfrom the months MarchndashAugust Precipitation anomaliesreflect the interaction of circulation anomalies and topog-raphy and do not generally mirror temperature changesSignificant wet anomalies are found on the western AP anddry anomalies occur over the Ross Ice Shelf the adjacentWest Antarctic ice sheet and in the Lambert Glacier basinEast Antarctica

The regional modelling approach has proved very usefulfor clarifying physical processes underlying climate changein Antarctica To improve statistical significance in studies ofthis kind a 45 year run with a regional atmospheric climatemodel (forced by ERA40) is ongoing

REFERENCESBromwich D H 1989 Satellite analyses of Antarctic katabatic

wind behavior Bull Am Meteorol Soc 70(7) 738ndash749Comiso J C 2000 Variability and trends in Antarctic surface

temperatures from in situ and satellite infrared measurementsJ Climate 13(10) 1674ndash1696

Connolley W M 1996 The Antarctic temperature inversion Int JClimatol 16(12) 1333ndash1342

Fig 6 Correlation of monthly-mean AAO index (1980ndash93) withanomalies of surface temperature and turbulent flux of sensible heatfor a centre of action in Wilkes Land (see white cross in Fig 4c)

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 125

the whole of East Antarctica probably caused by suppressedmeridional air exchange and additional cooling near thesurface in areas where a weakening of near-surface windshas intensified the surface temperature inversion

From Figure 4c it appears that areas with a surfacetemperature inversion (roughly coinciding with low poten-tial temperatures in Fig 2c) are prone to potentially strongersurface warming when the circumpolar vortex is strong (highAAO index) through destruction of the temperature inver-sion An example is the AP ice shelves where warming islarger than in the immediate surroundings This process maybe dominated by changes in horizontal heat advection inregions where flow anomalies are directed perpendicular tolarge horizontal temperature gradients such as are found

near the continental margin and the sea-ice edge Forinstance the strong warming over the western AP and theWeddell Sea is likely to be at least partly caused by a greaternortherly wind component in combination with decreasedsea-ice cover in the Bellingshausen Sea (Marshall and King1998 Marshall 2002b) A more quantitative assessment ofthe contributions of various processes to regional tempera-ture change based on calculation of the individual terms inthe heat budget is a topic of future study

PrecipitationWhen the circumpolar vortex is strong (high AAO index)three areas over the continent show changes in precipitationthat reach the 99 confidence level (Fig 4d) An increase of

Fig 5 Seasonal subsets of AAO regression slope of surface pressure in hPa (solid contours) and surface temperature in K (colours) for DJF (a)MAM (b) JJA (c) and SON (d) Values correspond to a 1 standard deviation anomaly in the AAO Dashed contours enclose areas where the99 confidence level is reached

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO124

up to 30 is found over the western AP while western MarieByrd Land and the adjacent Ross Ice Shelf show decreases ofa similar magnitude Interestingly in the precipitationshadow at the eastern side of the AP zero precipitationchange is found over the northerly ice shelves an increase intheir mass balance may have slowed down their disintegra-tion A relative decrease of about 15 is found over theLambert Glacier basin These patterns clearly reflect theinteraction between circulation anomalies (Fig 4a and b)and Antarctic topography Of interest to ice-core researchersis that western Dronning Maud Land experiences aprecipitation increase of up to 10 (mostly reaching the90ndash95 confidence level) and at the same time significantcooling (Fig 4c) Elsewhere in Antarctica increases inprecipitation are in general associated with warming

Regression on seasonal subsetsIt is well known that the Antarctic temperature inversion isstrongest in winter and greatly reduced in summer Otherimportant factors determining Antarctic surface temperaturesuch as sea-ice extent and strength of the katabatic windsalso show a pronounced annual cycle From this we wouldexpect a seasonal dependence of the temperature responseto the AAO To investigate this we regressed seasonalsubsets of 3 months (DJF MAM JJA and SON) on the AAOindex Figure 5 shows the combined results for surfacetemperature (colours dashed contours where 99 con-fidence is reached) and surface pressure (solid contours) Inspite of the reduced number of points used in the regressionall seasons show extensive areas with significant surfacecooling over East Antarctica in response to a strongercircumpolar vortex (high AAO index) For instance inWilkes Land just east of the Lambert Glacier basin coolingis significant in all four seasons Warming in the AP is mostpronounced in autumn and winter when the circulationanomalies have their greatest northerly component so thatmost of the annual temperature signal in Figure 4c derivesfrom the months MarchndashAugust (Fig 5b and c) This is inagreement with observations (Marshall 2002a)

Impact of wind and temperature changes on thesurface sensible heat fluxTo investigate what causes the surface cooling in EastAntarctica under conditions of strong circumpolar vortex(high AAO index) we selected from Figure 4c the location inWilkes Land where the correlation between the AAO indexand temperature attains a minimum For this locationFigure 6 shows the correlation between the AAO index andanomalies in the surface turbulent flux of sensible heat(SHF) SHF is an important component of the wintertimeAntarctic surface energy balance and provides the surfacewith heat that it loses through net emission of longwaveradiation Because turbulence and hence the SHF in thestably stratified wintertime Antarctic atmosphere is gener-ated mainly by wind shear a decrease in 10 m wind leads toa decrease in SHF and hence a lower surface temperatureThis link of the AAO with East Antarctic surface tempera-tures through the katabatic wind field and the sensible heatflux is clearly apparent from Figure 6

6 SUMMARY AND FUTURE WORKWe used output of a 14 year integration with a high-reso-lution regional atmospheric climate model (RACMOANT1)

to study the response of the (near-) surface Antarctic climateto the AAO which is a measure of the strength of thecircumpolar vortex or westerlies In spite of the relativelyshort period significant AAO-related anomalies were foundin surface pressure 10 m wind surface temperature andprecipitation When the vortex is strong (high AAO index)surface pressure changes show an asymmetric pattern withthe largest pressure falls in coastal Marie Byrd Land Thiscauses northerly flow anomalies and significant warmingover the AP and adjacent regions in West Antarctica and theWeddell Sea In response to decreased meridional airexchange and weaker near-surface winds significant cool-ing occurs throughout East Antarctica The strongest signalsare found in Wilkes Land but strong cooling is also foundover the Ross Ice Shelf and the adjacent part of WestAntarctica Repeating the regression on subsets of 3 monthsreveals that most of the annual temperature signal derivesfrom the months MarchndashAugust Precipitation anomaliesreflect the interaction of circulation anomalies and topog-raphy and do not generally mirror temperature changesSignificant wet anomalies are found on the western AP anddry anomalies occur over the Ross Ice Shelf the adjacentWest Antarctic ice sheet and in the Lambert Glacier basinEast Antarctica

The regional modelling approach has proved very usefulfor clarifying physical processes underlying climate changein Antarctica To improve statistical significance in studies ofthis kind a 45 year run with a regional atmospheric climatemodel (forced by ERA40) is ongoing

REFERENCESBromwich D H 1989 Satellite analyses of Antarctic katabatic

wind behavior Bull Am Meteorol Soc 70(7) 738ndash749Comiso J C 2000 Variability and trends in Antarctic surface

temperatures from in situ and satellite infrared measurementsJ Climate 13(10) 1674ndash1696

Connolley W M 1996 The Antarctic temperature inversion Int JClimatol 16(12) 1333ndash1342

Fig 6 Correlation of monthly-mean AAO index (1980ndash93) withanomalies of surface temperature and turbulent flux of sensible heatfor a centre of action in Wilkes Land (see white cross in Fig 4c)

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 125

up to 30 is found over the western AP while western MarieByrd Land and the adjacent Ross Ice Shelf show decreases ofa similar magnitude Interestingly in the precipitationshadow at the eastern side of the AP zero precipitationchange is found over the northerly ice shelves an increase intheir mass balance may have slowed down their disintegra-tion A relative decrease of about 15 is found over theLambert Glacier basin These patterns clearly reflect theinteraction between circulation anomalies (Fig 4a and b)and Antarctic topography Of interest to ice-core researchersis that western Dronning Maud Land experiences aprecipitation increase of up to 10 (mostly reaching the90ndash95 confidence level) and at the same time significantcooling (Fig 4c) Elsewhere in Antarctica increases inprecipitation are in general associated with warming

Regression on seasonal subsetsIt is well known that the Antarctic temperature inversion isstrongest in winter and greatly reduced in summer Otherimportant factors determining Antarctic surface temperaturesuch as sea-ice extent and strength of the katabatic windsalso show a pronounced annual cycle From this we wouldexpect a seasonal dependence of the temperature responseto the AAO To investigate this we regressed seasonalsubsets of 3 months (DJF MAM JJA and SON) on the AAOindex Figure 5 shows the combined results for surfacetemperature (colours dashed contours where 99 con-fidence is reached) and surface pressure (solid contours) Inspite of the reduced number of points used in the regressionall seasons show extensive areas with significant surfacecooling over East Antarctica in response to a strongercircumpolar vortex (high AAO index) For instance inWilkes Land just east of the Lambert Glacier basin coolingis significant in all four seasons Warming in the AP is mostpronounced in autumn and winter when the circulationanomalies have their greatest northerly component so thatmost of the annual temperature signal in Figure 4c derivesfrom the months MarchndashAugust (Fig 5b and c) This is inagreement with observations (Marshall 2002a)

Impact of wind and temperature changes on thesurface sensible heat fluxTo investigate what causes the surface cooling in EastAntarctica under conditions of strong circumpolar vortex(high AAO index) we selected from Figure 4c the location inWilkes Land where the correlation between the AAO indexand temperature attains a minimum For this locationFigure 6 shows the correlation between the AAO index andanomalies in the surface turbulent flux of sensible heat(SHF) SHF is an important component of the wintertimeAntarctic surface energy balance and provides the surfacewith heat that it loses through net emission of longwaveradiation Because turbulence and hence the SHF in thestably stratified wintertime Antarctic atmosphere is gener-ated mainly by wind shear a decrease in 10 m wind leads toa decrease in SHF and hence a lower surface temperatureThis link of the AAO with East Antarctic surface tempera-tures through the katabatic wind field and the sensible heatflux is clearly apparent from Figure 6

6 SUMMARY AND FUTURE WORKWe used output of a 14 year integration with a high-reso-lution regional atmospheric climate model (RACMOANT1)

to study the response of the (near-) surface Antarctic climateto the AAO which is a measure of the strength of thecircumpolar vortex or westerlies In spite of the relativelyshort period significant AAO-related anomalies were foundin surface pressure 10 m wind surface temperature andprecipitation When the vortex is strong (high AAO index)surface pressure changes show an asymmetric pattern withthe largest pressure falls in coastal Marie Byrd Land Thiscauses northerly flow anomalies and significant warmingover the AP and adjacent regions in West Antarctica and theWeddell Sea In response to decreased meridional airexchange and weaker near-surface winds significant cool-ing occurs throughout East Antarctica The strongest signalsare found in Wilkes Land but strong cooling is also foundover the Ross Ice Shelf and the adjacent part of WestAntarctica Repeating the regression on subsets of 3 monthsreveals that most of the annual temperature signal derivesfrom the months MarchndashAugust Precipitation anomaliesreflect the interaction of circulation anomalies and topog-raphy and do not generally mirror temperature changesSignificant wet anomalies are found on the western AP anddry anomalies occur over the Ross Ice Shelf the adjacentWest Antarctic ice sheet and in the Lambert Glacier basinEast Antarctica

The regional modelling approach has proved very usefulfor clarifying physical processes underlying climate changein Antarctica To improve statistical significance in studies ofthis kind a 45 year run with a regional atmospheric climatemodel (forced by ERA40) is ongoing

REFERENCESBromwich D H 1989 Satellite analyses of Antarctic katabatic

wind behavior Bull Am Meteorol Soc 70(7) 738ndash749Comiso J C 2000 Variability and trends in Antarctic surface

temperatures from in situ and satellite infrared measurementsJ Climate 13(10) 1674ndash1696

Connolley W M 1996 The Antarctic temperature inversion Int JClimatol 16(12) 1333ndash1342

Fig 6 Correlation of monthly-mean AAO index (1980ndash93) withanomalies of surface temperature and turbulent flux of sensible heatfor a centre of action in Wilkes Land (see white cross in Fig 4c)

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO 125

Connolley W M 1997 Variability in annual mean circulation insouthern high latitudes Climate Dyn 13(10) 745ndash756

Doran P T and 12 others 2002 Antarctic climate cooling andterrestrial ecosystem response Nature 415(6871) 517ndash520

Hall A and M Visbeck 2002 Synchronous variability in theSouthern Hemisphere atmosphere sea ice and ocean resultingfrom the annular mode J Climate 15(2) 3043ndash3057

Hines K M D H Bromwich and G J Marshall 2000 Artificialsurface pressure trends in the NCEPndashNCAR reanalysis over theSouthern Ocean and Antarctica J Climate 13(22) 3940ndash3952

Kalnay E and 21 others 1996 The NCEPNCAR 40 year reanalysisproject Bull Am Meteorol Soc 77(3) 437ndash471

King J C and J C Comiso 2003 The spatial coherence ofinterannual temperature variations in the Antarctic PeninsulaGeophys Res Lett 30(2) 1040 (1010292002GL015580)

King J C J Turner G J Marshall W M Connolley andT A Lachlan-Cope 2003 Antarctic Peninsula climate vari-ability and its causes as revealed by analysis of instrumentalrecords In Domack E W A Burnett A Leventer P ConleyM Kirby and R Bindschadler eds Antarctic Peninsula climatevariability a historical and paleoenvironmental perspectiveWashington DC American Geophysical Union 17ndash30 (Ant-arctic Research Series 79)

Kwok R and J C Comiso 2002 Spatial patterns of variability inAntarctic surface temperature connections to the SouthernHemisphere annual mode and the Southern OscillationGeophys Res Lett 29(14) (1010292002GL015415)

Marshall G J 2002a Analysis of recent circulation and thermaladvection change in the northern Antarctic Peninsula Int JClimatol 22(12) 1557ndash1567

Marshall G J 2002b Trends in Antarctic geopotential height andtemperature a comparison between NCEPndashNCAR and radio-sonde data J Climate 15(6) 659ndash674

Marshall G J and J C King 1998 Southern Hemisphere circula-tion anomalies associated with extreme Antarctic Peninsulawinter temperatures Geophys Res Lett 25(13) 2437ndash2440

Parish T R and J J Cassano 2001 Forcing of the wintertimeAntarctic boundary layer winds from the NCEPndashNCAR globalreanalysis J Appl Meteorol 40(4) 810ndash821

Scambos T A C Hulbe M Fahnestock and J Bohlander 2000The link between climate warming and break-up of ice shelvesin the Antarctic Peninsula J Glaciol 46(154) 516ndash530

Schneider D P and E J Steig 2002 Spatial and temporalvariability of Antarctic ice sheet microwave brightnesstemperatures Geophys Res Lett 29(20) 1964 (1010292002GL015490)

Schwerdtfeger W 1975 The effect of the Antarctic Peninsula onthe temperature regime of the Weddell Sea Mon Weather Rev103(1) 45ndash51

Shuman C A and J C Comiso 2002 In situ and satellitesurface temperature records in Antarctica Ann Glaciol 34113ndash120

Thompson D W J and S Solomon 2002 Interpretation of recentSouthern Hemisphere climate change Science 296(5569)895ndash899

Thompson D W J and J M Wallace 2000 Annular modes in theextratropical circulation Part I Month-to-month variabilityJ Climate 13(5) 1000ndash1016

Van den Broeke M R and N P M van Lipzig 2002 Impact ofvortex variability on the wintertime low-level climate of EastAntarctica results of a regional climate model Tellus 54A(5)485ndash496

Van den Broeke M R and N P M van Lipzig 2003 Factorscontrolling the near surface wind field in Antarctica MonWeather Rev 131(4) 733ndash743

Van den Broeke M R and 6 others 1999 Climate variables alonga traverse line in Dronning Maud Land East AntarcticaJ Glaciol 45(150) 295ndash302

Van den Broeke M R N P M van Lipzig and E van Meijgaard2002 Momentum budget of the East-Antarctic atmosphericboundary layer results of a regional climate model J AtmosSci 59(21) 3117ndash3129

Van Lipzig N P M 1999 The surface mass balance of theAntarctic ice sheet a study with a regional atmospheric model(PhD thesis Utrecht University)

Van Lipzig N P M and M R van den Broeke 2002 A model studyon the relation between atmospheric boundary-layer dynamicsand poleward atmospheric moisture transport in AntarcticaTellus 54A(5) 497ndash511

Van Lipzig N P M E van Meijgaard and J Oerlemans 1999Evaluation of a regional atmospheric model using measurementsof surface heat exchange processes from a site in AntarcticaMon Weather Rev 127(9) 1994ndash2001

Vaughan D G and C S M Doake 1996 Recent atmosphericwarming and retreat of ice shelves on the Antarctic PeninsulaNature 379(6563) 328ndash331

Vaughan D G J L Bamber M B Giovinetto J Russell andA P R Cooper 1999 Reassessment of net surface mass balancein Antarctica J Climate 12(4) 933ndash946

Vaughan D G G J Marshall W M Connolley J C King andR Mulvaney 2001 Climate change devil is in the detailScience 293(5536) 1777ndash1779

White W B 2004 Comments on lsquoSynchronous variability in theSouthern Hemisphere atmosphere sea ice and ocean resultingfrom the annular modersquo J Climate 17(11) 2249ndash2254

White W B and R G Peterson 1996 An Antarctic circumpolarwave in surface pressure wind temperature and sea-ice extentNature 380(6576) 699ndash702

Van den Broeke and Van Lipzig Temperature wind and precipitation changes in response to AAO126