The northward shift of Meiyu rain belt and its possible association with rainfall intensity changes and the Pacific-Japan pattern

(1)

The

northward

shift

of

Meiyu

rain

belt

and

its

possible

association

with

rainfall

intensity

changes

and

the

Paciﬁc-Japan

pattern

Qingjiu

Gao

a,∗

_,

_Yuting

_Sun

b

_,

_Qinglong

_You

a

a_Climate_Dynamics_Research_Center_(CDRC)_of_the_College_of_Atmospheric_Sciences/Key_Laboratory_of_{Meteorological}_Disaster_of_the

MinistryofEducation(ELME)/JointInternationalResearchLaboratoryofClimateandEnvironmentChange(ILCEC)/Collaborative InnovationCenteronForecastandEvaluationofMeteorologicalDisasters(CIC-FEMD),NanjingUniversityofInformationScience& Technology,219NingliuRoad,Nanjing,210044,China

b_Hubei_Province_Public_{Meteorological}_Service_Center,_Wuhan,_430074,_China

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received8July2016

Receivedinrevisedform24August2016 Accepted28August2016

Availableonline29August2016 Keywords:

Northwardshiftofmeiyurainbelt Veryheavyrainfall

Paciﬁc-Japanpattern

a

b

s

t

r

a

c

t

ThemeridionallocationchangeofMeiyurainbeltanditsrelationshipwiththerainfall

intensityandcirculationbackgroundchangesfortheperiod1958–2009areexamined

usingdailyrainfalldatasetsfrom756stationsinChina,the6-hERA-Interimreanalyses,

CRUmonthlytemperatureanddailyoutgoinglong-waveradiation(OLR)datafromthe

USNationalOceanicandAtmosphericAdministration(NOAA).Theresultsindicatethatthe

Meiyurainbeltexperiencedanorthwardshiftinthelate1990sinresponsetoglobal

warm-ing.Moreover,theintensityofinterannualandday-to-dayvariabilityofrainfallwithin

Meiyuperiodhasbeenincreasinginthewarmingclimate.Theampliﬁcationofthe

vari-abilitywithinMeiyuperiodoverthenorthernYangtze-HuaiRiverValley(YHRV)ismuch

largerthanthatofthesouthernYHRV.Thelargedifferenceinthetrendsofvariancewithin

theMeiyuperiodbetweenthesetworegionsinducesaspatialvaryingfordifferent

rain-fallcategoriesintermsofintensity.Moresigniﬁcantpositivetrendsinheavyandextreme

heavyrainfalloccurovernorthernYHRVcomparedwithsouthernYHRV,whichisa

cru-cialindicatorofchangesintherainband,despitetheobservationofanincreaseinheavy

andveryheavyraineventsandadecreaseinweakeventsthroughouttheentireYHRV.

Acompositeoftheatmosphericcirculationindicatesthatintensenorthwardhorizontal

transportandtheconvergenceofwatervaporﬂuxesaretheimmediatecausesoftherain

bandshift.Besides,throughforcinganorthwardextendedconvectionoverthetropics,the

Paciﬁc-Japan(P-J)patterninducesanorthwardexpansionofwesternPaciﬁcSubtropical

High,leadingtointensiﬁedconvergenceandenhancedrainfalloverNorthernYHRV.

BY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Extensiveobservationshaveyieldedevidenceofchangesinprecipitationwithglobalwarming(Giorgi,2002;Guand

Adler,2013;Wentzetal.,2007).Ontheglobalscale,precipitationtendstoincreasewithwarming,whereasthesituation

∗ Correspondingauthorat:#615BuildingofMeteorology,No.219,NingliuRoad,NanjingUniversityofInformationScience&Technology,Nanjing, Jiangsu210044,China.

E-mailaddresses:[email protected],[email protected](Q.Gao),[email protected](Y.Sun).

http://dx.doi.org/10.1016/j.dynatmoce.2016.08.005

(2)

ismorecomplicatedandexhibitsinconsistentchangesonaregionalscale(IPCC,2007;Leeetal.,2013;Trenberthetal., 2003;ZhangandZhou,2011;Zhuetal.,2014).DuetotheinﬂuenceoftheAsiansummermonsoon,theYangtze-HuaiRiver

Valley(YHRV)ofChinaisvulnerabletoﬂoods.Nearly40%oftheﬂoodsinthisregionoccurduringtheMeiyuseason(Li,

1996).ResearchonchangesinthespatialdistributionoftheMeiyuovertheYHRVrepresentsanovelscientiﬁcchallenge

withsigniﬁcantsocietalandeconomicimplications.

Previousstudies(GongandHo,2002;Wang,2001;Xu,2007;Yuetal.,2004)havesuggestedthattheMeiyurainband

underwentasigniﬁcantinterdecadalshiftinthelate1970s,withincreasedrainfallandtheoccurrenceof‘southwetnorth

drought’pattern.However,astheclimaterapidlywarms,therainbeltpatternistendingtobecomemorecomplexand

diverse(Chenetal.,2007;HuandDing,2009;Huetal.,2010;Lietal.,2016).Speciﬁcally,high-intensityrain(Dingetal.,

2007;Xu,2007)andatypicalMeiyurainfallareoccurringmorefrequently(Liangetal.,2009).Inthe1990s,heavyrainand

ﬂoodeventsfrequentlyoccurredintheareassouthoftheYangtzeRiver.However,since2000,Meiyurainfallandﬂoodshave

becomeincreasinglyconcentratedintheHuaiRiverBasin;ﬂoodsareoccurringnearlytwiceasoftenastheclimatological

average(Chenetal.,2007).Huangetal.(2010)examinedthelineartrendsofdifferentdurationsofcontinuousrainfallduring

theMeiyuperiodandfoundthatlong-durationrainfallhasbecomesigniﬁcantlylesssince2000,whereas2-dayand

3-to-4-daycontinuousrainfalleventshavebecomemore.Previousstudiesindicatethattheintensityofextremeprecipitationis

showinganincreasingtendency(KusunokiandMizuta,2008;WangandZhai,2008).Thesecomplicatedchangesmakeit

difﬁculttopredictrainfallduringtheYHRVﬂoodseason.

UnderstandingthemechanismofthechangesinMeiyupropertyisaprerequisiteforimprovingtheseasonalprediction

overMeiyuregion.Recently,thepossiblecausesoftheMeiyurainbeltshifthavebeeninvestigatedfromtheperspective

ofatmosphericcirculationinEastAsia(Sietal.,2010).ResearchersspeculatethatthesubtropicalwesterlyjetoverEast

AsiahasshiftednorthwardandthattheEastAsiantropicalHadleycellhasexpandedpolewardbecauseoftheexpansion

ofthesubtropics;thus,theMeiyurainbelthasalsoshiftednorthward.ThemigrationoftheMeiyurainbeltmaybealso

associatedwiththeprincipalweathersystems(HuandDing,2009).However,theinﬂuencefactorsoftheMeiyurainfallare

complicatedandtheunderlyingmechanismremainsunclear,andthepreviousstudies,despitetheirscientiﬁcsigniﬁcance,

havenotconsideredhowchangesinrainfallcompositionaffectthedistributionoftheMeiyurainbelt.Thus,furtherresearch

isnecessarytogainaninsightfulunderstandingofthetrendsoftheMeiyuphenomenaandtoprovideascientiﬁcbasisfor

thedevelopmentofclimatechangepolicies.

Inthisstudy,weﬁrstexaminethechangesoftheMeiyurainbelt(Section3)andestablishrelationshipsbetweenthese

changesandrainfallcomposition.ThevariousintensitiesofrainfallarepresentedinSection4.InSection5,wediscussthe

possiblecausesoftheMeiyurainbeltchanges.Finally,asummaryanddiscussionarepresentedinSection6.

2. Datasetsandmethods

2.1. Datasets

Dailyraingaugeobservationsrecordedat756stations inChinafrom1958to2009wereprovidedbytheNational

MeteorologicalInformationCenteroftheChinaMeteorologicalAdministration(CMA).

Foreachstation,therainfalldataforvariousperiodsthroughouteachyearwereaggregatedbasedontheMeiyuperiods

(17Juneto11July)deﬁnedbySun(2014).Thetropicalcyclone(TC)rainfallwithintheYHRVduringeachMeiyuseasonwas

extractedusingtheobjectivesynopticanalysistechnique(OSAT)proposedbyRenetal.(2007)andLiuetal.(2013)andwas

subsequentlysubtractedfromthedatatoeliminatetheinﬂuenceoftheTCrainfall.

Amonthlytemperaturetimeseries(TS)wasextractedfromtheCRUTS3.21datasetprovidedbytheClimateResearch

Unit(CRU)attheUniversityofEastAnglia.PreviousversionsoftheCRUTSdatasethavebeenwidelyappliedinscientiﬁc

researchonglobalwarming(Follandetal.,2001).Inthisstudy,weusedthev3.21temperaturedatafrom1958to2012ina

climatebackgroundanalysisofthechangesintheMeiyurainbeltandrainfallcomposition.

Dailyatmosphericcirculationdatawerederivedfromthe6-hourlyERA-Interimreanalysisdatasets(Deeetal.,2011)

providedbytheEuropeanCentreforMedium-RangeWeatherForecasts(ECMWF),whichspana31-yearperiodfrom1979

to2009.Dailygriddedinterpolatedoutgoinglong-waveradiation(OLR)datawereobtainedfromtheUSNationalOceanic

andAtmosphericAdministration(NOAA).Theresolutionsofthesetwodatasetsare2.5◦×2.5◦and1◦×1◦latitude/longitude,

respectively.

2.2. Methods

Atotalof604stationsthroughoutthecountrywerechosenundertheconstraintofatleast90%dailydataavailability

duringtheﬂoodseason[MaytoAugust(MJJA)]from1958to2009tominimizetemporalinconsistencies;ofthesestations,

77stationsthroughouttheYHRV(28◦–34◦N,110◦–112◦E)wereselectedforuseasMeiyurainsmonitoringstationsinthis

study(Fig.1).

Compositionanalysis,arotatedempiricalorthogonalfunction(REOF)andempiricalorthogonalfunction(EOF)analysis,

(3)

Fig.1.Thelocationsofthe77selectedstationsthroughouttheYangtze-HuaiRiverValley(YHRV)inChina.

Fig.2. AnnualmeanglobalandYHRVairtemperatures(a,c;blacksolidlineswithcrossmarks;◦C)andthecorrespondinganomalies(b,d;bars;◦C)and cumulativeanomalies(greenlines;◦C)duringthe1958–2009period.TheresultsofthemovingT-testtechniqueandtheMann-Kendallrankstatisticsare representedbytheredcurvesin(b)and(d),respectively.Thickverticaldashedlinedenotesthedecadalchangepoints.(Forinterpretationofthereferences tocolourinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

Meiyurainfallbeltanditspossiblecauses.Inaddition,thepercentilemethod(Bonsaletal.,2001)wasusedtoidentifyrain

categories.

3. ChangesinthedistributionofMeiyurainfallphenomena

Previousstudies(IPCC,2007;Wuetal.,2011)indicatethattheglobaltemperaturehasexhibitedacontinuousriseover

thelast50years,withamarkedinter-decadalchangeinthelate1970s.AsseenfromFig.2aandc,anincreasingtrendis

observedintheannualmeanairtemperatureovertheYHRV,consistentwiththechangeintheglobaltemperatureduring

the1958–2012period.Anabruptchangeintheglobalmeantemperatureoccurredafterthelate1990s(Fig.2a,b),whereas

anaccumulatedanomalyanalysis(Fig.2c)andtheMann-Kendallstatistic(Fig.2d)revealanabruptchangeintemperature

around1995andasigniﬁcantwarmingafter2000intheYHRV,whichindicatesthattheremarkableinter-decadalchange

inwarmingintheYHRVlaggedbehindthatintheglobaltemperature.Tofacilitateacomprehensiveanalysisofthechange

intheMeiyurainfalldistributionagainstthebroaderbackgroundofclimatewarming,inthefollowingdiscussions,werefer

tothetimeperiodsbefore1979,from1980to1999,andafter1999asthecoolperiod,thetransitionalperiodandthewarm

period,respectively.

ClimatologicalMeiyu-seasonrainfall(averagedoverthe1958–2009period)isconcentratedinthemiddleandlower

reachesoftheYangtzeRiverandtoitssouth.Therainband,withalocalmaximumofapproximately390mmperMeiyu

season,liesnearlyparalleltotheYangtzeRiver(Fig.3a).Duringthecoolandtransitionalperiods(ﬁguresnotshown),the

locationandshapeoftheobservedrainbeltcorrespondwelltotheclimatologicalpattern;however,theamountsofrainfall

arelessandmorethanthemean,respectively.Speciﬁcally,therainfalltothesouthoftheYangtzeRiverwasdistinctlyhigher

thannormalduringthetransitionalperiod.Comparedwiththeearlierperiods,thespatialdistributionoftheMeiyurainfall

(4)

Fig.3.TherainfallpatternduringMeiyuseason(June17-July11)for:(a)theclimatologicalmeanfrom1958to2009,(b)thewarmperiodfrom2000to 2009,and(c)thedifferenceusing2000–2009meanminus1958–1999mean(ValuesareinunitsofmmperMeiyuseason;thethickdashedlinesina,b representthemaximumrainband.).

Fig.4. DistributionsofthelineartrendsinprecipitationduringtheMeiyuperiodfor(a)1958–1999and(b)1958–2009(valuesareinunitsofmm/yr;light anddarkshadedareasarestatisticallysigniﬁcantatthe90%and95%conﬁdencelevels,respectively).

rainfallalsoshowsa10%orgreaterincreasewithrespecttothemean,whereastherainfalltothesouthoftheYangtzeRiver

isdecreasedby20%ataconﬁdencelevelexceeding95%(ﬁguresnotshown).Before2000,aconsistentupwardtrendwas

apparentovertheentireregion,particularlyinareasofthesouthernYangtzeRiverValley(Fig.4a).Withtheextensionof

thestudyperiodtoincludethedatafrom1999to2009(Fig.4a,b),thetrendcoefﬁcientsovertheentireYHRVareobviously

decreased,thoughtheystillshowpositivevalues.Moreover,theareawitharemarkableincreasingtrendshiftsfromthe

southernYHRVtothecentralandnorthernYHRV,indicatingthattheprecipitationhasdecreasedrapidlyinthesouthern

YangtzeRiverValleybuthasincreasedinthenorthernpartoftheregion.WeappliedaREOFanalysistothenormalized

Meiyurainfalldata.TheﬁrstandsecondREOFmodes,whichaccountfor26.4%and25.7%ofthetotalvariance,respectively,

arestatisticallydistinctfromtheremainingeigenvectorsinaccordancewiththeruledeﬁnedbyNorthetal.(1982).The

regionsofhighloadingcorrespondingtotheseﬁrsttwomodesareboundedbytheYangtzeRiver,whichessentiallyoverlaps

withthelatitudelineat31◦N(ﬁguresnotshown).Forsimplicity,inthispaper,weusethelatitudeof31◦Nasaboundaryline

todividetheYHRVintosouthernandnorthernregions.Fig.5aandbdisplaysthestandardizedaveragerainfallseriesinthe

southernandnorthernYangtzeRiverregions,respectively.Thetwoseriesexhibitscoherenttrendsbeforeearly1990sbut

oppositetrendsafterwards.Particularlyafter2000,thesouthernrainfallseriesshowsanotableincreasebutthenorthern

seriesdecreasesrapidly.ThesechangesfurtheremphasizethenorthwardshiftoftheMeiyurainbeltafter2000,whichis

consistentwiththeﬁndingsofrecentstudies(HuandDing,2009;Sietal.,2010).

Againstthebackgroundoftherisingglobalairtemperature,theamountsofMeiyurainfalloccurringinthesouthernand

northernYHRVexhibitsomeinterdecadalvariabilitywithnosigniﬁcantlong-termtrend(Fig.5a,b).However,the11-year

movingstandarddeviationsoftheMeiyurainfallshowasigniﬁcantincreasingtrend(atthe0.01signiﬁcancelevel)during

1958–2009period(Fig.6c),whichindicatesanincreaseintheinterannualvariability.Theincreaseinthestandarddeviation

duringthe1990swassigniﬁcantlystrongerthanthatduringthe2000sinthenorthernYHRVbecauseMeiyurainfallwas

(5)

Fig.5.Thenormalizedrainfallinthe(a)southernand(b)northernYHRVasdividedbythelatitudelineat31◦_N_for_1958–2009_(the_dashed_lines_represent

5-yearrunningmeanvalues).

Fig.6. Temporalvariations(1958–2009)inthevarianceofdailyrainfallduringtheMeiyuperiodoverthe(a)southernand(b)northernYHRV,together withtheirlineartrends(dashedlines),aswellas(c)the11-yearmovingstandarddeviationsoftheMeiyurainfallforthesouthern(solidline)andnorthern (dashedline)YHRV.Thebvaluesrepresenttrendcoefficients,andbold-facedtextindicatesatrendthatisstatisticallysignificantatthe90%(a,b)or99% (c)confidencelevelaccordingtoattest.

contrast,theinterannualvariabilityshowedanabruptincreaseinthelate1990s.Thetemporalvariations(1958–2009)in

thevarianceofdailyrainfallduringtheMeiyuseasonaveragedoverthesetworegionsarealsopresentedinFig.6a,b,andboth

showincreasingtrends,whichareconsistentwiththewarmingtrendsintheglobalandYHRVannualmeantemperatures.

However,theincreaseinthevarianceoverthenorthernpartoftheregionisstrongercomparedwiththatoverthesouthern

part;inparticular,sincethemid1990s,themeanvarianceofthedailyrainfallduringtheMeiyuperiodhasbeenabovethe

previousmaximum.

Atrendinrainfallvarianceisrelatedtoatrendintheavailabilityofatmosphericmoisturecontent(Trenberthetal.,2005),

asgovernedbytheClausius-Clapeyron(C C)equation,whichcorrespondstoachangeofapproximately7%foreach1◦C

changeintemperature(Trenberth,1998;Wentzetal.,2007).Thetrendintheavailabilityofatmosphericmoisturecontent,

inturn,isduetothegradualwarmingoftheairtemperature(Trenberthetal.,2003).Goswamietal.(2006)showedthatthe

long-termincreaseindailyrainfallvariancethathasbeenobservedovercentralIndiaduringthesummermonsoonseason

islikelyduetothewarmingoftheseasurfacetemperatureinthetropicalIndianOceanandtheassociatedincreasewater

vapor.InChina,upwardtrendsintheamountsofprecipitablewaterinallseasonsandintheannualmeanfrom1970to

1990havebeenrevealedbyZhaiandEskridge(1997),andsubsequentanalysesbyTrenberthetal.(2005)andRossand

Elliott(2001)furtherveriﬁedtheseresults.Durreetal.(2009)foundthatthetotalcolumndensityofwatervaporinthe

atmospherehasincreasedbyapproximately0.45mm/decade(1973–2006)overthelandintheNorthernHemisphere.These

observedincreasedwatervaporaffectboththegreenhouseeffect,byprovidingpositivefeedbackforclimatechange,andthe

(6)

Table1

ThethresholdsforvariousrainfallintensitycategoriesaveragedovertheYHRVduringtheMeiyuseason.

Category 1 2 3 4 5 6 7 8 9 10 11

Percentile(%) <10 10–20 20–30 30–40 40–50 50–60 60–70 70–80 80–90 90–95 >95 Threshold(mm/day) <0.3 0.4–0.7 0.8–1.5 1.6–2.9 3.0–5.0 5.1–8.4 8.5–13.5 13.6–21.9 22.0–38.0 38.1–54.5 >54.5

ramiﬁcationsforprecipitation,drivingchangesinprecipitationamountsandrates.However,itisunlikelythatsuchchanges

inmoisturewillbeuniform,althoughtheairtemperatureisincreasingatasimilarratealmosteverywhere(Trenberth

etal.,2003).Ontheonehand,becauseofthenonlineardependenceontemperaturedescribedbytheC Cequation,alarger

absoluteincreaseinmoistureoccursatlowerlatitudesdespitealargerincreaseintemperatureathigherlatitudes.Onthe

otherhand,atmosphericdynamicsmodifytheatmosphericmoisturecontentthroughpreferentialregionalconvergenceand

subsidence(Trenberth,2011).Accordingly,despitesimilartrendsofincrease(ﬁguresnotshown)intheairtemperatures

overthesouthernandnorthernYHRV,thereisaconsiderabledifferenceinthetrendsofdailyrainfallvariabilitybetween

thetworegions.

4. RelationshipbetweenthenorthwardshiftoftheMeiyurainbeltandrainfallinvariousintensitycategories

Intheprevioussection,itwasdemonstratedthatwithrisingtemperature,theconsequentincreaseinwatervaporcauses

thedailyrainfallvarianceduringtheMeiyuperiodtointensify,whichinevitablymanifestsasvariationsinthefrequency

andintensityofrainfall(Trenberthetal.,2003).Accordingtothepercentilemethod(Bonsaletal.,2001),elevencategories

ofrainfall(Table1)duringtheMeiyuperiodwereidentiﬁed.Then,weanalyzedthetrendsforeachrainfallcategoryand

theirinﬂuenceontheshiftintheMeiyurainbeltintermsofthefrequencyofoccurrence(FOC),theamountsofrainfalland

thresholdstodeﬁnetheintensityofrainfall.

TofacilitatetheassessmentofthevariationsintheFOCsofrainfallinvariousintensitycategoriesinthemostrecentdecade

(thewarmperiod)comparedwiththoseinthecoolandtransitionalperiods,itisadvantageoustoseparatelyconstructthe

frequencydistributionsofrainfallofvariousintensitiesatallstationsthroughoutthesouthern(39stations)andnorthern(38

stations)regionsoftheYHRV(Fig.7)duringtheMeiyuseasonforthreeperiodsofequallength:1970–1979,1990–1999and

2000–2009.From1970–1979to1990–1999,thenumbersofraineventsinthethreehighestintensitycategories(9th–11th)

overthesouthernYHRVincreased,whereasthenumberofallraineventsexceptveryheavyrainevents(11thcategory,

rainfallexceedingthe95thpercentile;hereafter,R≥R95p)overthenorthernYHRVdecreased.After2000,increasesinthe

FOCsofeventsofthethreehighestcategories(9th–11th)occurredonlyoverthenorthernYHRV,andtheFOCsofrainevents

inthefourlowestcateoriesandofallraineventsdecreasedoverboththenorthernandsouthernregions.

Infact,theFOCsofraineventsinthetwohighestcategories(10th–11th)havebeenrisingby10%perdecade(witha

conﬁdencelevelexceeding90%)inthenorthernYHRVsince1958,whereasthoseinthesouthernregionwereinitiallyrising

byonly5%perdecade(notasigniﬁcantincrease)andlaterenteredadownturn,decreasingby20%onaverageafter2000

(Fig.8a,b).ThegrowthrateoftheFOCofraineventsinthe9thcategoryinthenorthernregionisalsolargerthanthat

inthesouthernregion(Fig.8c,d).Regardingraineventsinthemoderatecategories,thetrendsofchangeintheFOCsare

barelyperceptible.Fig.8eandfshowsnegativetrendsinthelightcategories(1st–4th)overthetworegionsandsigniﬁcant

declinesinthelightestrainevents(1st,rainfallinthelowest10thpercentile;hereafter,R<R10p).TheFOCsofraineventsin

the2nd–4thcategoriesinthenorthernregionexhibitmoresigniﬁcantdecreasingtrendsthanthoseinthesouthernregion,

whichseemstoindicatethatthevarianceinlightraineventshasplayedanegativeroleinthenorthwardshiftoftheMeiyu

rainbelt;however,intermsofrainfallamout,alargerabsolutedecreaseisseeninthesouthernregion(ﬁguresnotshown).

Theseﬁndingsillustratethatlightandmoderateraineventshavelessimpactontheoveralldistributionofrainfall,whereas

theraineventsintheheaviercategorieshaveanimportanteffectonthespatialdistributionofMeiyurainfall,asindicated

Fig.7.Thenumbers(N)ofraineventsofvariousintensitiesoverthe(a)southernand(b)northernYHRVduringtheMeiyuseasonforthreeperiods: 1970–1979,1990–1999and2000–2009.

(7)

Fig.8.Temporalvariations(1958–2009)inthenumbers(N)ofraineventsinheavyintensitycategories(a,b:R≥R90p,c,d:R80p≤R<R90p)andlight intensitycategories(e,f:R<R10pandR10p≤R<R40p)overthesouthernandnorthernYHRVduringtheMeiyuseason,togetherwiththeirlineartrends (dashedlines).R≥R90pdenotesdailyrainfallexceedingthe90thpercentile.Abold-facedlineindicatesatrendthatisstatisticallysigniﬁcantatthe90% conﬁdencelevelaccordingtoattest.

bythefactthattheirFOCsaresigniﬁcantlycorrelatedwiththeMeiyurainfall(withacorrelationcoefﬁcientofmorethan

0.53).

Wenextexaminechangesintheintensityofheavyrainfallasderivedfromthresholdchanges.Foreachstation,the

dailyrainfallforMaytoAugustofeachyearwereﬁrstrankedinascendingorderandthenusedtocalculatethevalues

correspondingtoeachpercentile.Themagnitudesofthe80th–99thpercentilesrepresenttheintensityofheavyrainfall

inagivenyearbecausemorethan90%oftheraineventsinthosepercentilesoverallstudiedyearsoccurredduringthe

Meiyuseason,andthus,theyearlyvariationsinthesethresholdsrepresentthechangesintheintensityofheavyrainfall.

Largerthresholdsforthe80thto99thpercentilesoverthesouthernYHRVcomparedwiththoseinthenorthernregionare

clearlyevidentinFig.9a,c,andtheseﬁndingsareconsistentwiththeclimaticdistributionoftheMeiyurainfall.Importantly,

however,thesethresholdsalsoshowincreasingtrendsoverbothregions,withalargerincreaseoverthenorthernYHRV.For

instance,theincreaseinthethresholdfromtheheaviestrainfalloverthenorthernYHRV,fromapproximately48mm/day

intheearly1960stoapproximately60mm/day,isequaltoquadrupletheincrementobservedinthesouthernregion(from

55mm/dayto58mm/day).Furthermore,theaverageintensitythresholdsforthe90thto99thpercentiles(Fig.9b,d)show

a1.9mm/dayperdecadeincreaseoverthe52-yearperiodthatissigniﬁcantatthe90%conﬁdencelevelinthenorthern

region,whereasnosigniﬁcantincreaseisapparentinthesouthernYHRV.

TheaboveﬁndingsindicateincreasesintheFOCsandintensitiesofraineventsinheavycategories(9th–11th) and

decreasesinlightcategories(1st–4th)withthewarmingoftheclimatethroughouttheentireYHRV.Theseﬁndingsare

consistentwithmodelprojections(KarlandKnight,1998;Trenberthetal.,2003)andobservationsinotherareas(Goswami

etal.,2006).AcomparisonoftherainfallinthesouthernandnorthernYHRVrevealsthattheFOCsandintensitiesofthe

twoheaviestcategoriesofrainevents(R≥R90p)haveshownasigniﬁcantincreasingtendencyinareasnorthoftheYangtze

River,whereasabarelynoticeableincreasehasoccurredtothesouthoftheriver.Moreover,therainfallamountsofthese

heavyraineventsaresigniﬁcantlycorrelatedwiththetotalMeiyurainfall(withacorrelationcoefﬁcientofmorethan0.9),

andtheircontributiontothetotalMeiyurainfallisashighas33–50.7%.Thus,thelargerincreaseinrainfalleventsinheavy

intensitycategoriesoverthenorthernregion,especiallythetwoheaviestintensitycategories,hashadanimportantimpact

onthenorthwardshiftoftheMeiyurainbelt.

5. ChangesinatmosphericcirculationassociatedwiththenorthwardshiftoftheMeiyurainbelt

BecausethemovementoftheMeiyurainbelthasoccurredpredominantlyoverthelasttwoorthreedecades,andthe

ERA-Interimreanalysisdataexhibitshighhomogeneitywhensatellitedatawascollectedsince1979,thissectionfocuses

(8)

Fig.9. Temporalvariations(1958–2009)in(a,c)the80thto99thpercentilesofrainfalleventsduringthefloodseason(MaytoAugust)and(b,d)themean rainfallamountsforthe90thto99thpercentilesofrainfalleventsinthesouthernandnorthernYHRV.Thecontoursin(a,c)representrainintensityinunits ofmm/day.Thedashedlinesin(b,d)representlineartrends,andabold-facedlineindicatesatrendthatisstatisticallysignificantatthe90%confidence levelaccordingtoattest.

Fig.10. DifferencesinpatternsobservedovertimeduringtheMeiyuperiod(2000–2009minus1979–1999):(a)verticallyintegrated(fromthesurfaceto 300hPa)watervaporﬂuxes(arrows;kg/(ms))andtheirdivergences(contours;10−5_kg/(m2_s),_(b)_sea_level_pressures_(SLPs)_(contours;_hPa)_and_geopotential

heights(shadedregions;gpm)at850hPa,(c)meridionalsectionsofvorticityalong110–140◦_E₍₁₀−6_s−1_),_and_(d)_OLR_(W/m2_)._The_shaded_areas_in_(c,_d)

arestatisticallysigniﬁcantatthe90%conﬁdencelevelaccordingtoattest.

divergenceofwatervaporﬂuxesexhibitsanegativetrendthatiscenteredtothenorthoftheYangtzeRiverandapositive

trendthatiscenteredtothesouthoftheriver(Fig.10a).Theformercanbeinterpretedastheinteractionoftheenhanced

cycloniccirculationinthevicinityoftheBohaiSeawiththeincreasedwatervaporﬂuxfromthenorthernBayofBengal

(BOB),fromtheIndochinaPeninsulatothenorthoftheYangtzeRiver).Bycontrast,theincreasedﬂuxhasbeenattributed

toanenhancementofthesouthwestmonsoonresultingfromanorth-southbarometricgradient,whichisformedbyan

(9)

Fig.11.Pacific-Japan(P-J)teleconnectionpatternsduringtheMeiyuseason(a)from1979to1999and(b)from2000to2009.Solid(dashed)linesindicate regressedpositive(negative)anomaliesof850-hParelativevorticity(units:10−6s−1).Thecontourintervalis0.5.Heavysolidlinesdenoteavalueofzero. Theshadedareasarestatisticallysignificantatthe90%confidencelevelaccordingtoattest.

thatiscenteredonthenorthernYHRVatthemiddlelatitudes(Fig.10b).Thelatterisduetothewestwardexpansionofthe

WPSH,whichdecreasesthenorthwardtransportofwatervaporovertheSCS.

InFig.10b,thelowertroposphericcirculationfeaturesmeridionaldipolesbetweenthelowlatitudes(10◦–30◦N)and

mid-dlelatitudes(30◦–40◦N).Thecorrespondingvorticitychangesshowsimilarwave-likecharacteristics(ﬁguresnotshown)

thatexhibitadistincttiltwithheight(Fig.10d).Thesefeatures characterizethePaciﬁc-Japan(P-J)teleconnection

pat-tern,whichprovidesacrucialconnectionbetweenthetropicsandthemiddlelatitudes.TheprevalenceoftheP-Jpattern

duringtheMeiyuperiodwasidentiﬁedbasedonanempiricalorthogonalfunction(EOF)analysis,followingKosakaand

Nakamura’sstudy(2010).AnEOFanalysiswasappliedtoanomaliesof850-hPavorticityovertheEastAsia-Paciﬁcregion

[0◦–60◦N,100◦–160◦E]duringtheMeiyuperiodsfrom1979to2009.TheﬁrstEOFmode(EOF1)explains12.5%ofthetotal

varianceandisthereforewellseparatedbasedonthecriteriaproposedbyNorthetal.(1982).Then,anomaliesof850-hPa

vorticityfor1979–1999and2000–2009wereregressedontothecorrespondingprincipalcomponent(PC1)timeseries.The

resultsofbothanalysesarecharacterizedbydistinctmeridionalwavetrainsofzonallyelongatedvorticityanomalies,which

arecharacteristicP-Jpatterns(KosakaandNakamura,2006,2010;Kosakaetal.,2013).PC1isdeﬁnedastheP-Jpattern

index.ItexhibitspositivecorrelationswiththeaveragerainfallamountsoverthesouthernandnorthernYHRV,with

corre-lationcoefﬁcientsof0.58and0.34,respectively.Theseresultsindicatealinkbetweenthechangesintherainfalloverthe

YHRVandtheP-Jpattern.Above-normalprecipitationduringtheMeiyuperiodcorrespondstothepositivephaseofthe

P-Jteleconnection,whichexhibitsatripolepatternwithpositive,negativeandpositivevorticityanomaliesclearlylocated

intheregionsouthofthePhilippines,theregionoftheSouthChinaSea(SCS)andPhilippineSea,andtheregion

contain-ingtheYHRVinChinaandsouthernJapan,respectively,whereasbelow-normalprecipitationcorrespondstothenegative

phaseoftheP-Jpattern.AcomparisonbetweenFig.11aandbrevealsthattheP-Jpatternwasdisplacedpolewardbetween

1979–1999and2000–2009;theuppermostboundaryofthephenomenon,whichcorrespondstothenorthernmarginof

thenegativevorticityanomalyextendingfromtheSCStothePhilippineSea,hasbeenpushednorthwardby5◦inlatitude,

whereasthepositiveanomalyareatothenorthofthenegativeanomalyhasshrunktothenorthoftheYangtzeRiver.Itis

thussuggestedthat,asshowninFig.10bandc,thedifferencesinvorticityandpressurecanbedividedintotworegions

atapproximately30◦N.Aconcretecomparisonofvalues(Fig.11a,b)revealsthatthemagnitudeofthenegativevorticity

anomalyhasdecreased,indicatingaweakeningofanticyclonicperturbationandanenhancementincyclonicperturbation.

Correspondingly,theOLRinthevicinityofthePhilippinesisobviouslydecreased(Fig.10d),whichreﬂectsanenhancement

inconvectiveactivity.ThenegativeanomalyOLRmaximumislocatedatapproximately16◦N,whichissomedistancenorth

fromtheclimatologicallocationofthewesternPaciﬁcITCZ(8–12◦N).Apparently,thewesternPaciﬁcITCZistendingto

strengthenandextendnorthward.Conversely,theOLRispositivealong20–30◦N,thenorthernmarginalzoneoftheWPSH,

andconvectionhasdecreased.ThisimpliesthattheWPSHhasalsostrengthenedandshiftednorthward.

Additionally,previousstudies(Lu,2001;LuandDong,2001;WakabayashiandKawamura,2004)havefoundthattheP-J

patterncaneffectawestwardextensionoreastwardretreatoftheNorthPaciﬁchigh.Fig.10dshowsthattheOLRhasslightly

increasedovertheSCS.Suchachange,whichiscontrarytothatinthePhilippineSea,caninduceenhancedanticyclonic

circulation(Fig.10aandb)andananomalouswestwardextensionoftheWPSH,which,inturn,couldleadtotheabove

changesinthewatervaporﬂuxes.

Asdescribedabove,theP-Jpatternhasextendedpolewardsince2000,whichmayhaveinducedanomalouswestwardand

northwardextensionsoftheWPSH,therebystrengtheningconvectiveactivityoverthenorthernYHRVandinthevicinity

oftheBohaiSeawhilesuppressingthatovertheregiontothesouthoftheYangtzeRiver.Theseeffectsmayhaveledto

intenseconvectionandanincreaseinprecipitationovertheregiontothenorthoftheYangtzeRiver,ultimatelyinducing

theobservedchangestotheMeiyurainbelt.

6. Summaryanddiscussion

WehaveillustratedthenorthwardshiftoftheMeiyurainbeltduringthelate1990sbasedondailyrainfalldatasets(after

(10)

oftheregionalMeiyuperiodproposedbySun(Sun,2014).TherelationshipbetweenthechangesintheMeiyurainbelt

overChinaandtheintensityofrainfallaswellasthepossiblecausesofthesechangesduring1958–2009werepreliminarily

examined.TheaverageMeiyurainfallamountsoverthenorthernandsouthernYHRVshownosigniﬁcantlylong-termtrends

withclimatewarming,althoughsomeinterdecadalvariabilityisevident.However,theinterannualvariabilityoftheMeiyu

rainfallhasbeenincreasingduring1958–2009.Andanevidentuptrendinday-to-dayvariabilitywithintheMeiyuperiod

alsocanbeseenwiththefeaturethattheincreasinginthenorthernisstrongerthanthatinthesouthern.Suchanincreasein

variabilityinevitablymanifestsasvariationsinthefrequencyandintensityofrainfall.Overthepast52years,theFOCsand

intensitiesofraineventsinheavyintensitycategories(R≥R80p)havetendedtoincrease,whereaseventsinweakcategories

(R<R40p)haveexhibitedadecreasingtrendinboththesouthernandnorthernYHRVunderclimatewarming.However,

themoresigniﬁcantpositivetrendsinheavyrainfall(R≥R90p)overthenorthernYHRVcomparedwiththesouthernYHRV

haveexertedanimportanteffectonthenorthwardshiftoftheMeiyurainbelt.

ThepossiblecausesoftheMeiyurainbeltmigrationwereexaminedintermsofchangesinatmosphericcirculation.Since

2000,thenorthwardintensiﬁcationofthehorizontaltransportandconvergenceofwatervaporﬂuxeshavecausedrainfall

tobecomeincreasinglyconcentratedinthenorthernYHRV.TheP-Jpattern,awave-likecirculationpatternemanating

fromthetropicstowardtheextratropicalregionsofEastAsiaandtheNorthernPaciﬁc,wasdisplacedpolewardbetween

1979–1999and2000–2009.ThisinducedintenseconvergenceofwatervaporandconvectionoverthenorthernYHRVvia

theenhancementandnorthwardexpansionofconvectionoverthewesternPaciﬁcITCZandareastoitsnorth,forcingthe

WPSHtoexpandnorthwardandwestward.

TherainfalldistributionduringtheMeiyuiscomplex,particularlybecauseofthechangesinrainfallassociatedwith

climatewarming.Thus,furtherstudiesoftheinﬂuenceoftheatmosphericinternalvariabilityonrainfalldistributionare

needed.Whydoesthewatervaportransportchannelchange?Willtherelativecontributionsofwindﬁeldsandwatervapor

tothehorizontalwatervaporﬂuxchangewithclimatewarming?Thesequestionswillbeaddressedinfutureresearch.

Acknowledgments

ThisstudywasjointlysupportedbytheNationalNaturalScienceFoundationofChina(undergrant41475045)andthe

NationalBasicResearchProgramofChina(undergrants2015CB453201andBK20150062).Thisstudywasalsofundedbythe

“PriorityAcademicProgramDevelopmentofJiangsuHigherEducationInstitutions”(PAPD).TheauthorsthanktheNational

MeteorologicalInformationCenteroftheChinaMeteorologicalAdministration(NMIC-CMA)forsupplyingtheobservational

data.

References

Bonsal,B.,Zhang,X.,Vincent,L.,Hogg,W.,2001.CharacteristicsofdailyandextremetemperaturesoverCanada.J.Clim.14(9),1959–1976.

Chen,J.,Cheng,H.,Wang,W.,2007.TheChineseabnormalwarmingisimportantpredictivesignofoccurringtorrentialofrainandheavyﬂoodsinthe YangtzerivervalleyortheHuairiverbasininnextsummer.Prog.Geophys.22(04),1380–1385.

Dee,D.P.,etal.,2011.TheERA-interimreanalysis:conﬁgurationandperformanceofthedataassimilationsystem.Quat.J.R.Meteorol.Soc.137(656), 553–597.

Ding,Y.,etal.,2007.Astudyofthesynoptic-climatologyoftheMeiyusysteminEastAsia.Chin.J.Atmos.Sci.31(06),1082–1101(inChinesewithEnglish abstract).

Durre,I.,Williams,C.N.,Yin,X.,Vose,R.S.,2009.Radiosonde-basedtrendsinprecipitablewaterovertheNorthernHemisphere:anupdate.J.Geophys.Res.: Atmos.(1984–2012),114(D5).

Folland,C.K.,etal.,2001.Globaltemperaturechangeanditsuncertaintiessince1861.Geophys.Res.Lett.28(13),2621–2624.

Giorgi,F.,2002.Variabilityandtrendsofsub-continentalscalesurfaceclimateinthetwentiethcentury.PartI:observations.Clim.Dyn.18(8),675–691.

Gong,D.-Y.,Ho,C.-H.,2002.ShiftinthesummerrainfallovertheYangtzerivervalleyinthelate1970.Geophys.Res.Lett.29(10),1436.

Goswami,B.N.,Venugopal,V.,Sengupta,D.,Madhusoodanan,M.S.,Xavier,P.K.,2006.IncreasingtrendofextremeraineventsoverIndiainawarming environment.Science314(5804),1442–1445.

Gu,G.,Adler,R.F.,2013.Interdecadalvariability/long-termchangesinglobalprecipitationpatternsduringthepastthreedecades:globalwarmingand/or paciﬁcdecadalvariability?Clim.Dyn.,1–14.

Hu,Y.,Ding,Y.,2009.PossiblereasonsfornorthwardshiftofMeiyubeltinYangtze-HuaiheRiverregionduring2000–2005.Meteor.Mon.35(12),37–43 (inChinesewithEnglishabstract).

Hu,Y.,Ding,Y.,Liao,F.,2010.AclassiﬁcationoftheprecipitationpatternsduringtheYangtze-Huaihemeiyuperiodfortherecent52years.ActaMeteor. Sin.68(02),235–247(inChinesewithEnglishabstract).

Huang,D.,Zhu,J.,Kuang,X.,2010.DecadalvariationofdifferentdurationsofcontinuousMeiyuprecipitationandthepossiblecause.Chin.Sci.Bull.56 (4–5),424–431.

IPCC,2007.SummaryforPolicymakersofClimateChange2007:thePhysicalScienceBasis.ContributionofWorkingGroupItotheFourthAssessment ReportoftheIntergovernmentalPanelonClimateChange.CambridgeUniversityPressCambridge4.

Karl,T.R.,Knight,R.W.,1998.Seculartrendsofprecipitationamount,frequency,andintensityintheUnitedStates.Bull.Am.Meteor.Soc.79(2),231–241.

Kosaka,Y.,Nakamura,H.,2006.StructureanddynamicsofthesummertimePaciﬁc–Japanteleconnectionpattern.Q.J.R.Meteorolog.Soc.132(619), 2009–2030.

Kosaka,Y.,Nakamura,H.,2010.Mechanismsofmeridionalteleconnectionobservedbetweenasummermonsoonsystemandasubtropicalanticyclone. PartI:thePaciﬁc–Japanpattern.J.Clim.23(19),5085–5108.

Kosaka,Y.,Xie,S.-P.,Lau,N.-C.,Vecchi,G.A.,2013.OriginofseasonalpredictabilityforsummerclimateovertheNorthwesternPaciﬁc.Proc.Natl.Acad.Sci. 110(19),7574–7579.

Kusunoki,S.,Mizuta,R.,2008.FuturechangesintheBaiurainbandprojectedbya20-kmmeshglobalatmosphericmodel:seasurfacetemperature dependence.Sola4(0),85–88.

Lee,E.-J.,Ha,K.-J.,Jhun,J.-G.,2013.Interdecadalchangesininterannualvariabilityoftheglobalmonsoonprecipitationandinterrelationshipsamongits subcomponents.Clim.Dyn.,1–17.

(11)

Ren,F.,Wang,Y.,Wang,X.,Li,W.,2007.Estimatingtropicalcycloneprecipitationfromstationobservations.Adv.Atmos.Sci.24(4),700–711.

Ross,R.J.,Elliott,W.P.,2001.Radiosonde-basedNorthernHemispheretroposphericwatervaportrends.J.Clim.14(7).

Si,D.,Ding,Y.,Liu,Y.,2010.DecadalnorthwardshiftoftheMeiyubeltandthepossiblecause.Chin.Sci.Bull.54(24),4742–4748.

Sun,Y.,2014.ChangesintheMeiyubeltanditsassociationwithwatervaportransportintermsofglobalwarming.Master’sThesis,NanjingUniversityof InformationScience&Technology.

Trenberth,K.E.,Dai,A.,Rasmussen,R.M.,Parsons,D.B.,2003.Thechangingcharacterofprecipitation.Bull.Am.Meteorol.Soc.84(9),1205–1217.

Trenberth,K.E.,Fasullo,J.,Smith,L.,2005.Trendsandvariabilityincolumn-integratedatmosphericwatervapor.Clim.Dyn.24(7–8),741–758.

Trenberth,K.E.,1998.Atmosphericmoistureresidencetimesandcycling:implicationsforrainfallratesandclimatechange.Clim.Change39(4),667–694.

Trenberth,K.E.,2011.Changesinprecipitationwithclimatechange.Clim.Res.47(1),123–138.

Wakabayashi,S.,Kawamura,R.,2004.ExtractionofmajorteleconnectionpatternspossiblyassociatedwiththeanomaloussummerclimateinJapan.J. Meteorolog.Soc.JapanSer.II82(6),1577–1588.

Wang,X.,Zhai,P.,2008.ChangesinChina’sprecipitationinvariouscategories.J.Trop.Meteor.(inChinese)24(5),459–466(inChinesewithEnglishabstract).

Wang,H.,2001.TheweakeningoftheAsianmonsooncirculationaftertheendof1970’s.Adv.Atmos.Sci.18(3),376–386.

Wentz,F.J.,Ricciardulli,L.,Hilburn,K.,Mears,C.,2007.Howmuchmorerainwillglobalwarmingbring.Science317(5835),233–235.

Wu,Z.,Huang,N.E.,Wallace,J.M.,Smoliak,B.V.,Chen,X.,2011.Onthetime-varyingtrendinglobal-meansurfacetemperature.Clim.Dyn.37(3–4),759–773.

Xu,Q.,2007.RecentstrongdecadalchangeofMeiyuin121years.Adv.WaterSci.18(3),327–335(inChinesewithEnglishabstract).

Yu,R.,Wang,B.,Zhou,T.,2004.TroposphericcoolingandsummermonsoonweakeningtrendoverEastAsia.Geophys.Res.Lett.31(22).

Zhai,P.,Eskridge,R.E.,1997.AtmosphericwatervaporoverChina.J.Clim.10(10),2643–2652.

Zhang,L.,Zhou,T.,2011.Anassessmentofmonsoonprecipitationchangesduring1901–2001.Clim.Dyn.37(1–2),279–296.

Zhu,Z.,Li,T.,He,J.,2014.Out-of-PhaserelationshipbetweenborealspringandsummerdecadalrainfallchangesinsouthernChina*.J.Clim.27(3), 1083–1099.