Meiji University

　

Title

Warm-Core Events of the Arctic Winters Appeared

in the Annual March of the Monthly Mean Air

Temperatures北極域の月平均気温変化に現れる冬季の

ウォームコア現象

Author(s) 梅本,亨

Citation 駿台史學, 103: 79-94

URL http://hdl.handle.net/10291/6101

Rights

Issue Date 1998-03-26

Text version publisher

Type Departmental Bulletin Paper

DOI

　　　　　　　　　　　　　　　　　　　　　　　　　　　https://m-repo.lib.meiji.ac.jp/

駿 台 史 学 第103号79-94頁,1998年3月

SUNDAISHIGAKU(SundaiHistoricalReview)

No.103,Mar.1998,pp.79-94.

Warm-CoreEventsoftheArcticWintersAppearedinthe

AnnualMarchoftheMonthlyMeanAirTemperatures

TohruUMEMOTO

DepartmentofGeography,MeijiUniversity

Abstract:Awarm-coreeventisthewarmspellobservedduringthecoreless

typewinteroftheannualmarchofsurfacetemperatureinthehighlatitudes .Wecanexplainsomepossiblecausesforthewarmspell:1)thelargescale

warmadvectionaroundtheactioncenterssuchastheAleutianandthe

Icelandiclows,2)passingofcyclonicdisturbancesthatbreakthestrong

surfacetemperatureinversion,and,3)existenceofcut-offwarmanticyclones

thatderivedfromblockingridges.Thispapershowsthegeographical

distributionofoccurrencefrequencyofthewarm-coreeventsinthenorthern

hemispherebyusingthemonthlymeantemperaturedataofNMCoctagonal

grids.Warm-coreeventsarefoundnotonlyonthesurfacegridsbutalsoonthe700hPaand500hPagrids.Thegeographicaldistributionofeachgrid

systemshowsasamepattern,sothatthelargescalewarmadvectionisthe

mostlmportantcauseforthewarm-coreevents.

Keywords:warmcore,corelesswinter,Arctic,blocking ,airtemperature.

1 Introduction

Theannualmarchofmonthlymeansurfacetemperaturesinthehigh

latitudesoftenshowsthecorelesstypeinwinter.IntheAntarctic ,wecan

observethetypicalcorelesswintersthatassociatewiththestrongsurface

temperatureinversion.Kawaguchi(1979)pointedoutonepossiblecausethatthe

upwardterrestrialradiationbalanceswiththedownwardatmosphericradiation

duringthepolarnight,sothatthemonthlymeansurfacetemperaturesstagnate

aroundabottomvalue.

AlthoughthelargescaletopographyoftheArcticisquitereversetothe

Antarctic,corelesswintersalsooccurinthenorthernhemisphere .Inthis

situation,theverticaltemperatureprofileshowsastrongsurfaceinversion(Lg .,

Streten,1969;Kahl,1990).Therefore,oneofthemostimportantfactors

controllingtheArcticwintertemperaturefieldisthedownwardlongwave

79

T.UMEMOTO

radiation.Thisfactorisstronglyinfluencedbythebehaviorofthepolarvortex

throughthevariationoftemperature,humidity,andcloudiness(Overlandetal.,

1997).

VowinckelandOrvig(1970)statedthatcoldspellsovertheArcticOceanare

notcausedbythecoldadvectionbutbytheradiativecooling,andwarmspells

arecausedbycyclonesthatdistracttheinversion.Theyalsomentionedthatthis

surfaceinversionisordinarilyintensifiedbytheupper-levelwarmadvection.

Thewarmadvection,however,occasionallyraisessurfacetemperatureand

causesawarmspell.ThereforetheyconcludedthattheArcticsurfaceinversion

isacomplexphenomenon.Recently,Serrezeetal.(1992)ana.1yzedstationdata

includingaerologicalsoundingsandtheRussiandrifting-icestations.Theyshowed

thattemperatureinversionsusuallyappearintheEurasiansideoftheArctic

BasinandthethicknessofinversionlayerisaboutlOOOminwinter.

Figurelshowsthethreetypesofsurfacetemperaturechangeinwinter.

Thecold-coretypeisordinaryinthemiddlelatitudes,andthecorele∬typeis

typicalinthepolarregion.Whenawarmspellofrelativelylongtermisobserved

inthepolarwinter,itmakesawarm-coreupontheflatcorelessline(hereafter

referredtoaswarm-coreez/ent).

SomemeteorologicalstationsaroundtheBeringSea(inJanuary)andthe

BarentsSea(inFebruary)showthewarm-coreastheclimaticvaluesofmonthly

meansurfacetemperature.Forexample,ShiraiwaandSawagaki(1992)observed

awarm-coreeventduringthewinterofl990inSpitsbergen.

Rubinshteyn(1962)showedtheoccurrencefrequencymapofthewarm-core

eventsbyusingmonthlymeantemperatureofllOstationsaroundtheArctic

Basin.Hefiguredouttwohighfrequencyareas:theNorwegianSea(fromthe

eastcoastofGreenlandtothewestcoastofNovayaZemlya),andtheBering

Sea.Healsoreviewedthediscussionofthecauseforwarm-coreevents.He

summarizedthreeprincipalmechanisms:

●thewarmadvectionfromthelowerlatitudes,

●thesensibleheattransportfromtheArcticOceanthroughtheseaice,

●thereleaseoflatentheatattheseaiceformation .

Thewarmadvectionistheonlyonepossiblecauseforthewarm-coreamong

them,becausewecanobservewarm-coreeventsoutsidetheArcticBasinand

80

Warm-CoreEventsoftheArcticWinters

Φ」⊃ρ"5」Φ○∈

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〉一エピ⊂OΣ

エ9エ

≧OＬ

Cold-core

Warm-core

Coreless

EarlyWinterMidWinterLateWinter

Season

FigurelSchematicsforthewarm-coretypetemperaturechange.Thecold-coretypeisotdinaryinthemiddlelatitudesandthecorelessistypicalintheArcticregion.

evenintheinlandregionofthecontinents.

Thepurposeofthispaperistoshowthetemporalandspatialcharacteristics

ofthewarm-coreeventsintheArcticregion.Inordertodiscussthestructureof

large-scalewarmadvection,theauthorpaidaspecialattentiontorecognizethe

geographicaldistributionofoccurrencefrequencyintheupPertroposphere,andhe

extendstheareaofanalysisuptothemiddlelatitudesofthenorthern

hemisphere.

2Dataandthewarm-coredefinition

ThedatausedinthisstudyaretheNMC(NationalMeteorologicalCenter,

USA)octagonalgridsofthemonthlymeantemperatureofthenorthern

hemisphere.Theauthorusedthetemperatureofthreelevels(surface,700hPa,

and500hPa)for22winterseasonsfroml965-66tol986-87.Thewinterseasonis

definedasthe5-monthperiodfromNovembertoMarch.Weordinarilydefinethe

81

T.UMEMOTO

northernhemispherewinterseasonasthe3-monthperiodfromDecemberto

February.Inthepresentanalysistheauthorfocusedontheannualmarchof

temperatureintheArcticregion,sothatheaddedNovemberandMarch.

Ofcourse,thetermwarm-coremeansawarm-coreり ρρArcticwinter,then

thetermistobeusedonlyforthehighlatitudes.Awarm-coreeventinthe

Arcticregionimpliestheexistenceofthelarge-scalepositivetemperature

anomalythatmayextendintothemiddlelatitudes.Thismeansthatwarmspells

ofthemiddlelatitudescloselyrelatetotheArcticwarm-core.Theauthornamed

bothArcticwarm-coresandwarmspellsinthemiddlelatitudesashigh

temperatureevents.Ahightemperatureeventforamonthinawinterseasonat

agivengridpointmeetsthefollowingsimplecondition:

tm.1trn+1.

Intheaboveexpression,tisthemonthlymeantemperatureofonegridpoint,

andthesubscriptsm-1,m,andm十ldenotethepreviousmonth,theverymonth

(fromNovembertoMarch),andthefollowingmonthrespectively.

Theaboveproceduiemaybetoodescriptivetomakeaphysicalimage.The

authorgivesanexamplethatshowsthespatialrelationshipsamongthe

geopotentialheight,temperatureanomaly,andhightemperatureevents.Figure2

showsthemonthlymeanheightandtemperatureanomalyof500hPasurfacefor

Januaryl977.Themoststrikingfeatureofthismonthwasthesimultaneously

developedblockingridgesintheAtlanticandthePacific.Thesetworidges

coupledovertheArcticOceanandbecameintoasingleanticyclonicsystem

(Fig.21eft).

Byusingthestationdata,Umemoto(1985)showedthatthissynopticevent

producedanextremelycoldweatheroverthebroadareaofEastAsia,andatthe

sametime,awarmweatheroverthehighlatitudes.TheNMCgridsof500hPa

levelforJanuaryl977alsoindicatethelargepositivetemperatureanomalyover

thehighlatitudes(Fig.2right).Relativelylargeanomalyareas(forexample,above

十6degree)aresituatedinthepositionsofthetwoblockingridges(theNortheast

PacificandtheNorthwestAtlantic)andofthecutoffhighovertheArcticBasin.

Figure3showsthegeographicaldistributionofhightemperatureeventsfor

Januaryl977.Hightemperatureeventsofthe500hPalevelshowseveral

82

Warm-CoreEventsoftheArcticWinters

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ofthe500hPasurfaceforJanuaryl977:(1eft)heightinmeter,andthedottedlinesshow

theridges,(right)contoursdrowninevery2degrees,shadedareashowspositive

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thehighlatitudes(Fig.2right).Bothofthesurfaceandthe500hPachartsshow

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83

T.UMEMOTO

statisticallydefinedhightemperatureevents(thismeanswarm-coreeventsinthe

Arctic)reflectasubstantialatmosphericsystemofthesynopticscale.

3GeographicalDistributionofWarm-coreEvents

HightemperatureeventsaredetectedatallpointsoftheNMCoctagonalgrid

systemfor22winters.Oneoctagonalgridcontains1977points,butsomepoints

outsidethelatitudinalcircleof20Nareomittedbecauseofpresentation.Figures

4,5,and6showthegeographicaldistributionofoccurrencefrequencies.Inthese

figures,theauthordidnotuseanykindsofarea-normalizationorspatial

smoothingbutjustcalculatedtheoccurrenceratio(timesofoccurrenceare

dividedby22).ContoursaredrawnineverylO%andshadedpartsshowthearea

withthevalueabove30%.

a)Surface

Manyhightempetatureeventsoccurredthroughoutthewinter,butthe

frequencyisverylowinNovember.Therearetwohighfrequencyareasinthe

ArcticregionforJanuary(Fig.4c):thefirstonecentersontheBeringStraitand

itcoversabroadareaoftheEastSiberiaandAlaskasector,thesecondis

situatedaroundGreenland.Bothoftheseshowthevaluesof40%ormore.

AsmallpeakcanbeseentothenorthofNovayaZemlya.Ontheother

hand,hightemperatur6eventsofthemiddlelatitudesoccurredintheEast

PacificandtheEastAtlanticOceans.ThepeakoftheEastPacificseemstobe

apartofthehighlatitudeswarm-corearea.However,thepeakoftheEast

Atlanticseemstohavenorelationtothewarm-corearoundGreenland.

InFebruary(Fig.4d),thesituationistotallydifferent.Onlyonemajorareaof

thewarm-coreappearsovertheNorwegianSeaandtheGreenlandSea.Another

smallpeakisintheeastpartofAlaskaanditcoversthecoastalareaoftheGulf

ofAlaska.Besidesthis,wecannotfindanynotableareasofhightemperature

event.Inothermonths,thefrequencyofwarm-coreislow.Oneexceptionisthe

BarentsSeainMarch(Fig.4e).

Wecanseesomehighfrequencyareasofhightemperatureeventsinthelow

latitudestoo(forexample,theEastPacificOceanofNovember,themiddle

84

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Warm-CoreEventsoftheArcticWinters

鱗

●SURFACE(1966-1987)●

Occurrencefrequency(×10%)

ofthehightemperatureevents

definedas

tm.,tm+1

t:monthlymeantemperatureofgrid・points

(subscripts}

m:month(fromNovembertoMarch)

m-1:thepreviousmonth,m+1:thefollowingmonth

(Data:NMCoctagonalgrids)

a)November,b)Decemberc)January,d)Februarye)March

Figure4Surfaceoccurrencefrequencyofthe

hemisphere.ContoursweredrawnineverylO

30%.

hightemperatureeventsinthenorthern

percent,andashadedareashowsabove

85

T.UMEMOTO

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definedas

tm .ftm+t

f:monthlymeantemperatureofgrid・points

(subscripts)

m:month(lromNovembertoMarch)

m・1:thepreviousmonth,m+1:thefollowingmonth

(Data:NMCoctagonalgrids)

a)November,b)Decemberc)January,d)Februarye)March

Figure5700hPaleveloccurrencefrequencyofthehightemperatureeventsinthe

northernhemisphere.ContoursweredrawnineverylOpercent,andashadedareashows

above30%.

86

Warm-CoreEventsoftheArcticWinters

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Occurrencefrequency(×10%)

ofthehightemperatureevents

definedas

fm-1fm+1

「:monthlymeantemperatureofgrid・points

(subscripts)m:month(fromNovembertoMarch)

m-1二thepreviousmonth,m+1:thefollowingmonth

(Data:NMCoctagonalgrids)

a)November,b)Decemberc)January,d)Februarye)March

Figure6500hPaleveloccurrencefrequencyofthehightemperatureeventsinthe

northernhemisphere.ContoursweredrawnineverylOpercent,andashadedareashows

above30%.

87

T.UMEMOTO

AtlanticOceanofFebruary,etc.).Buttheseareashavenodirectrelationstothe

atmosphericcirculationoftheArctic.Moreover,theprocedureforfindinghigh

temperatureeventsusedinthispapermaybemeaninglessfortheTropics.The

author,thereforedidnottakethesepeaksintoconsiderationonthewarm-core

events.

b)Upperlevels

Thedistributionofhightemperatureeventsof700hPaand500hPain

Novembershowsafamiliarpatternthatrecallsustothecifcumpolarflowof

zonalwave-numberthree(Figs.5aand6a).Highfrequencyareasaresituatedin

theEastPacificsectorcenteredinAlaska,tothesouthofGreenland,andalong

theUralMountains.

InDecember,wecanseetwomajorareasofhighfrequencyinthemiddle

latitudesof500hPalevel(Fig.6b):theNorthAtlanticOceanandtheNorth

PacificOcean.TheNorthAtlanticoneismarkedinthe700hPaleveltoo,butthe

NorthPacificoneissomewhatobscureinthatlevel(Fig.5b).Thesetwoareas

arenotnotableinthesurfacechart(Fig.4b).

InJanuary,ahighfrequencyareaofwarm-coreeventsexistsintheArctic

(Figs.5cand6c).Thedistributionpatternissimilartothesurfacechart.Inthe

middlelatitudes,ahighvalueareainthenortheasternPacificextendstothe

ArcticthroughAlaska.Twootherhighfrequencyareasaredistinguishedover

MongoliaandthesouthwestpartofEurope.

InFebruary(Figs.5dand6d),wecanseeabroadareawiththehighervalue

ofwarm-coreeven .tsovertheNorthAmericansideoftheArctic.Itcoversfrom

AlaskatothenorthwesternEuropethroughtheCanadianArctic.The

westernmostpartofthearea(i.e.,aroundAlaska)of700hPachart(Fig.5d)

extendssoUthwardtothemiddlelatitudesofthewestcoastoftheNorth

America.

ThedistributionofhightemperatureeventsofMarchshowsthreepeaks

(Figs.3eand6e).Arelativelysmallareaofthewarm-coreeventsappearsover

theBarentsSea.TheothertwoaresituatedinthemiddlelatitudesoftheNorth

PacificandtheNorthAtlantic.Eachofthepeakstakessomewhateastpositions

incomparisonwithJanuary.

88

Warm-CoreEventsoftheArcticWinters

4Discussions

Thedistributionofwarm-coreeventsintheArcticshowsastrikingfeature

thatthepositionofhighvalueareaseemstosiftanticlockwisebyabout60

degreesfromJanuarytoFebruary.TheAtlanticside(theeasternmostpart)

changesitspositionfromGreenlandtotheNorwegianSea,andthePacificside

(thewesternmostpart)changesitfromtheBeringSeatoAlaska.Inotherwords,

bothsideschangetheirpositionseastward.

PerlwitzandGraf(1995)showedasimilargeographicalpattern.They

performedthecanonicalcorrelationanalysisandtheempiricalorthogonalfunction

analysisforthethreewintermonths(December,January,andFebruary)inorder

toinvestigate"barotropic"relationbetweenthetroposphericandthestratospheric

variationingeopotentialheightandtemperaturefields.Theirfirstandsecond

modeof850hPatemperature(Figs9candlOcoftheirpaper)areconsideredto

correspondtotheJanuaryandtheFebruarypatternsofthepresentpaper.This

impliesthatthevarianceoflowertropospherictemperature.fieldisstatistically

connectedwiththeuppertroposphericpersistentpositivetemperatureanomalyof

JanuaryandFebruary.

Thesefeaturessuggestthatsomelargescaleatmosphericsystemswiththe

durationofatleastonemonthmaycontrolthetemperaturefields.Wecaneasily

indicatetwoofsuchsystems,theoneistheAleutianLowandtheotheristhe

IcelandicLow.Thesesystemshavethecharacteristicsofstandingwavesinthe

northernhemisphericwesterlies,sothattheytransportsensibleheatpoleward

continuouslyforalongperiod.Ifthesesystemschangethosepositionsslightly,

thentheyproduceadrasticchangeinthetemperaturefield(Umemoto,1985).

Forexample,Mosesetat.(1987)showedthereversalpositionalrelationship

betweentheIcelandicLowandtheAzoresHighfromtheviewpointoftheNorth

AtlanticOscillation.

Overlandetal.(1996)showedthreepathwaysofpolewardheatfluxacross

thelatitudinalcircleof70N:theGreenlandSea,theBeringStrait,andthe

northernCanada.Theformertwoarethemainpathwaysandshowrelatlvely

largeinterannualvariability.Theyalsoindicatedthatabout50%ofthetotalheat

89

T.UMEMOTO

fluxisbroughtbythetransienteddies,andabout25%ofitisduetothe

standingeddies.Thetransientandstandingeddiesoftheiranalysisareconsidered

tobeincludingblockingactivitiesandactioncentersrespectively.

Ontheotherhand,theNorthPacificblockingmadethelarge-scalepositive

(negative)surfacetemperatureanomalyoverthehigh(middle)latitudesinwinter

(DoleandGordon,1983;Umemoto,1986).Blockingphenomenafrequentlyoccur

evenintheArcticBasininwinter(e.g.,KnoxandHay,1985;Mullen,1994).

Colucci(1985)showedthatblockinganticycloneshastwokindsofthewarming

effect:

●theadvectivewarmingofwinds,

●theadiabaticwarmingofthesinkingair.

Therefore,thewarmadvectionbythepersistentblockingridgeoranticyclonecan

produceawarm-coreevent.Blockingphenomenatakeplaceespeciallyinthe

NorthAtlantic,theNorthPacificandthewesternSiberia.Butthecut-offwarm

highsfromblockingridgesdonotalwaysstayintheoccurrencearea.

Thetermblocleingisoriginallyderivedfromitseffecttoblockthemovingof

synopticorsub-synopticscaledisturbances.Inmanycases,ablockingflow

patterndoesnotreallyblockthecyclonepassage,butfixesitstracks.Thiseffect

exertsanimportantinfluenceuponthede『tructionofthestrongsurface

temperaturelnverSlon.

ConsequentlytheblockingphenomenaintheArctichavethreekindsof

contributiontoproducewarm-coreevents:1)thewarmairmassinsidethe

blockinghighanditssubsidence,2)thestronghorizontalwarmadvectionalong

persistentblockingridges,and3)thefixingofthecyclonetrackthatbreaksthe

surfacetemperatureinversion.

InFebruary,aqueerphenomenoncanbefoundinthenorthwesternpartof

Canadanearthemeridianofl20W.Thisareadoesnotshowahighvaluein

surfacechart(Fig.4d),butitshowsabout40%intheupperlevels(Figs5dand

6d).AsimilarphenomenoncanbeseenoverMongoliainJanuary.Arelatively

largeareaofhighfrequency(above30%ormore)isanalyzedatupperlevels.

Bothareasaresituatedintheinlandregionofthecontinents,andatthesame

time,inthecoreareaofwintercontinentalhighs.Theauthor,thereforeconsiders

thatthisphenomenonmaybecausedbythegradualformationofthicklayerof

90

Warm-CoreEventsoftheArcticWinters

temperaturelnverslon.

Ontheotherhand,wecanseesomeareaswithhighfrequencyvaluesover

theoceans.Theseareascanbedistinguishedclearlyintheupperlevels.One

markedhighvalueareaappearsovertheAtlanticOceanoffthecoastofEurope

inDecember,January,andMarch,butitappearsovertheSubtropicsin

February.Iftheseareashaveanysubstantialrelationsoneanother,itis

noteworthythatthesepeakschangetheirpositiongraduallyfromthenorthtothe

south.Thesepositionsare:

・November:thesouthofGreenland,

●December:thewestofthenorthwesternEurope,

●January:thesouthwestoftheWestEurope,

・February:thewestofAfrica,

・March:thesouthwestoftheWestEuropeagain.

OvertheEastPacificOcean,thesituationisalmostthesameasabove:

●November:thewestofAlaska,

.December:thesouthoftheAleutianIslands,

●January:thewestoftheUSA,

●February:aroundCaliforniaPeninsula,

●March:thewestoftheUSAagain.

Thissituationmaybeastatisticalimageofthemagnitudeofmonthlymean

temperaturevariance.Theauthorthinksthatthisgradualdisplacementofthe

hightemperatureeventsreflectstheseasonalsiftoftheactioncentersandthe

magnitudeofcoldoutbreaks.Thisviewmaybesupportedbythefactthatthe

positionofthefrequencypeakreturnsfromthesouthtothenorthinMarch,the

endofwinter.

5Concludingrernarks

Thewarm-coreeventsoftheArcticwinterappearedintheannualmarchof

monthlymeanairtemperaturesweredetectedbyusingtheNMCoctagonalgrid

dataofthenorthernhemispherefroml966tol987.Thegeographicaldistribution

ofthewarm-coreoccurrencefrequencywasmappedforthesurface,700hPa,and

500hPalevels.Theresultsoftheanalysisissummarizedasfollows;

91

T.UMEMOTO

・Mostcasesofhightemperatureevents(relativelywarmerspellsrepresentedby

monthlymeantemperatures)appearinthemiddleandhighlatitudesofthe

northernhemisphere.Theoccurrencefrequencyismuchhigherinthehigh

latitudesthanthemiddlelatitudes.

・IntheArctic,warm-coreeventsoccurintensivelyinJanuaryandFebruary,but

thedistributionofoccurrencefrequencyisdifferentineachmonth.InJanuary,

thehighfrequencyareaisfromtheBeringSeatoGreenlandthroughthe

CanadianArctic.InFebruary,itisfromAlaskatotheNorwegianSeathrough

Iceland.

・Theabovegeographicalfeaturesareincommonamongthreelevelsofthe

troposphere(surface,700hPa,and500hPa).

Theauthorproposestwocausesofwarmspellsmakingwarm-coreevents:

1)warmanticyclonescutofffromblockingridgesthatdriftintotheArctic

Basin,and2)thelarge-s6alewarmadvectioninthenortheasternsideofthe

IcelandicandAleutianlows.

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Colucci,S.J.,1985:Explosivecyclogenesisandlarge-scalecirculationchanges:

Implicationsforatmosphericblocking。 ノottrnaloftheAtmosphericS`iences,42,

2701-2717.

Dole,R.M.andN.D.Gordon,1983:Persistentanomaliesoftheextratropicalnorthern

hemispherewintertimecirculation:Geographicaldistributionandregionalpersistence

characteristics.ルfonthlyIpVeather1～et/iew,ll1,1567-1586.

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93

T.UMEMOTO

【要旨】

北極域の月平均気温変化に現れる冬季のウォームコア現象

梅 本 亨

極地 の 冬 は長 い が,気 温 が 低 下 し続 け るわ け で は な い。 中緯 度 で は 真 冬 の一 ヶ月 を 谷 底 とす

るV字 型 の 気温 変化 グ ラ フ とな るが,極 地 で は早 い時 期 に気 温 が底 を 打 ち,鍋 底 状 のU字 型 と

な る。 これ を コ ア レス ウ ィ ン タ ー とい うが,そ の 中 に暖 か い 月 が 出現 す る こ とが あ り,ウ ォー

ム コア現 象 と呼 ん で い る。北 極 圏 で は 探検 時 代か ら気 象 学 者 に注 目 され た 現象 だが ,こ れ まで

に提 出 され た成 因 に は 多 くの難 点 が あ る。 また,南 極 大 陸 に も ウ ォー ム コ アが 出現 す る が,大

陸 氷 床 上 の斜 面 滑降 風 に よ る逆 転 層 の破 壊 が 主 因で あ る点 で 北 極 域 とは異 な る。 本 論 で は,北

極 域 の ウ ォー ム コア を,中 緯 度 の大 規 模 な寒 波 と対 を 為 す 現 象 と して捉 え る。 そ の発 生 原 因 は

大 気 の作 用 中心 とブ ロ ッキ ング リッジ が も た らす 総 観 規 模 の 暖 気 移 流 で あ る と の仮 説 を立 て,

ウ ォー ム コア の 出現 頻 度 の地 理 的 分 布 に よ って これ を 検 証 す る。

使 用 した 気 温 デ ー タ は,北 極 圏 の対 流 圏 全 体 で 長 期 に わ た り信 頼 の お け るNMC(National

MeteorologicalCenter,USA)の 北 半 球 八 角 形 グ リ ッ ドデ ー タ で あ る。 冬 季 に お い て前 後 の 月

よ り高 温 と な った 格 子 点 を1965-66年 か ら1986-87年 の22冬 に つ いて 求 め,そ の 出 現 頻 度 を 地

上,700hPa面,500hPa面 につ い て月 別 に 示 した。 そ の結 果,ウ ォー ム コ アは 北 極 域 の1月 と

2月 に よ く発 生 す るが,1月 に は 北 ア メ リカ大 陸寄 りに,ま た2月 に は ヨ ー ロ ッパ 寄 りに,い

ず れ もブ ロ ッキ ン グが 多 発 す る領 域 を 中 心 に 分 布 す る傾 向 が 明 らか で あ る。 この 特 徴 は,各 高

度 に 共通 す るの で,必 然 的 に 総 観 ス ケ ール で 背 が高 く順 圧 的 な構 造 を もつ 暖気 移 流場 の存 在 を

示 唆 す る 。 よ って,ウ ォ ー ム コア の 主 因 は ブ ロ ッキ ング と作 用 中心 の相 互作 用 に よ る暖 気 移 流

効 果 で あ る と結 論 した。

な お,本 論 は 人為 的 な 温 室効 果 ガ ス の放 出 に よる いわ ゆ る地 球 温 暖化 問題 と は視 点 を異 に す

る こ とを 明記 して お く。

キ ー ワー ド:ウ ォー ム コア,コ ア レス ・ウ ィ ンタ ー,北 極 圏,ブ ロ ッキ ン グ,気 温

94