Tropicalexpansiondrivenbypolewardadvancingsubtropicalfront

HuYang1,Gerrit Lohmann1,JianLu2,EvanJ.Gowan1,Xiaoxu Shi1,Jiping Liu3,Qiang Wang1

1. AlfredWegenerInstitute,HelmholtzCentreforPolarandMarineResearch,Bremerhaven,Germany2. AtmosphericSciencesandGlobalChangeDivision,PacificNorthwestNationalLaboratory,Richland,Washington,USA

3. DepartmentofAtmosphericandEnvironmentalSciencesUniversityatAlbany,StateUniversityofNewYork,Washington,USA

07.May.2020 1

https://www.joboneforhumanity.org/the_mystery_of_the_expanding_tropics

Tropospheric temperature trends for 1979 to 2005 based on satellite-borne MSU observations. Fu. et al. (2006)

o StormtracksYin,(2005)o JetStreamsArcherandCaldeira,(2008)o WesterlywindsChenetal,(2008)o PrecipitationpatternScheff

andFrierson,(2012)

o CouldNorrisetal,(2016)o Large-scaleoceangyres

Yangetal,(2020)

2

Previousmechanisms

• IncreasingGHG

• DecreasingozoneoverAntarctic

• IncreasingaerosoloverNorthernHemisphere

• Naturalvariations(PDO) Staten et al., 2018

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MajorGaps• NoobservationalsupportshowingthatGHG,ozone,aerosolcorrelatewiththevariationsoftropicalwidth (Allen&Kovilakam,2017).

• Thesubtropicalclimatezoneshiftedduringpastglacial-interglacialcycles,withoutevidenceofchangesinozoneoraerosol (Gersonde etal.,2005;Toggweiler &Russell,2008;Bard&Rickaby,2009;Benzetal.,2016)

• GHGalone(withoutcontributionfromSST)playsaminorroleinwideningtropics (Grise&Polvani,2014)

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Yangetal,(2020)

Polewardshiftofthemajoroceangyres

detectedinawarmingclimate

Whetheroceanplayacrucialrolein

expandingtropics?

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Data andMethod

Ø Observations(ERA5,HadISST,NOAA-OISST)

ØCMIP5multi-model(24)simulations(piControl,1pctCO2).

Ø Sensitivitysimulation(FESOM,ECHAM6)

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ModelName InstitutionACCESS1.0 CentreforAustralianWeatherandClimateResearchACCESS1.3 CentreforAustralianWeatherandClimateResearchCanESM2 CanadianCentreforClimateModellingandAnalysisCNRM-CM5 CentreNationaldeRecherchesMeteorologiques/CentreEuropeendeRechercheet

FormationAvanceesenCalculScientifique(CNRM/CERFACS)CNRM-CM5.2 CentreNationaldeRecherchesMeteorologiques/CentreEuropeendeRechercheet

FormationAvanceesenCalculScientifique(CNRM/CERFACS)CSIROMk3.6.0 CommonwealthScientificandIndustrialResearchOrganisation incollaborationwiththe

QueenslandClimateChangeCentreofExcellenceGFDL-CM3 GeophysicalFluidDynamicsLaboratory

GFDL-ESM2G GeophysicalFluidDynamicsLaboratoryGFDL-ESM2M GeophysicalFluidDynamicsLaboratoryGISS-E2-H NASAGoddardInstituteforSpaceStudiesGISS-E2-R NASAGoddardInstituteforSpaceStudies

HadGEM2-ES MetOfficeHadleyCentreINM-CM4 InstituteforNumericalMathematics

IPSL-CM5A-LR Institut PierreSimonLaplaceIPSL-CM5A-MR Institut PierreSimonLaplaceIPSL-CM5B-LR Institut PierreSimonLaplaceMIROC-ESM JapanAgencyforMarine-EarthScienceandTechnology,AtmosphereandOceanResearch

Institute(TheUniversityofTokyo),andNationalInstituteforEnvironmentalStudies

MIROC5 AtmosphereandOceanResearchInstitute(TheUniversityofTokyo),NationalInstituteforEnvironmentalStudies,andJapanAgencyforMarine-EarthScienceandTechnology

MPI-ESM-LR MaxPlanckInstituteforMeteorologyMPI-ESM-MR MaxPlanckInstituteforMeteorologyMPI-ESM-P MaxPlanckInstituteforMeteorologyMRI-CGCM3 MeteorologicalResearchInstituteNorESM1-M NorwegianClimateCentreNorESM1-ME NorwegianClimateCentre

List of CMIP5 models used in our study

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Edgeoftropics: u-windchangesfromeasterlytowesterly(USF)

Individuallyfromfiveoceanbasins

Waugh et al., 2018 8

Yangetal,(2020)Underreview onGRL.

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MeridionalSSTgradientanomaliesassociated withwidertropics.

Definitionofofmeridionalpositionofsubtropicalfronts 10

ClimatologymeridionalSSTgradients

Greyline:meridional positionofsubtropicalfront,Colouredline:edge oftropics.Positivevaluesindicate poleward shift,viceversa.

11Yangetal,(2020) Underreview onGRL

Yangetal,(2020) Underreview onGRL.Eriksen et al., 2014

Subtropicalconvergencezone(SCZ)Regionswithheavyplasticaccumulation

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Fu. et al. (2006)

Yangetal,(2020) Underreview onGRL.13

Aqua-PlanetFESOM

Yangetal,(2020) Underreview onGRL.

SCZ

SCZ

SCZSCZ

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SSTwarmingpatternunderuniform1degreewarmerSATforcing

Yangetal,(2020) Underreview onGRL.

SCZ

SCZ

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Warmersubtropicalocean(polewardshiftofsubtropicalfront)driveswidertropics

Yangetal,(2020) Underreview onGRL. 16

Yangetal,(2020) Underreview onGRL.17

Answerstothepreviousquestions

ü Theaerosol,especiallyforblackcarbon,contributetowarming overthemid-latitudes (Allenetal.,2012),whichhasthepotentialtosustainapolewardshiftofatmosphericsubtropicalfront.

ü TheozoneholeovertheAntarcticcools the Antarctic(Randel&Wu,1999; Thompsonetal.,2011;Orretal.,2012)contributingtothemigrationofthemid-latitudetemperaturegradients.

ü Thetropicsarenarrowerduring glacialperiod(Sonetal.,2018),becausehigh-latitudecoldwaterexpandstowardslowerlatitudes(Annan&Hargreaves,2013),causinganequatorwardcontractionofthetemperaturegradients.

(Allenetal.,2012)18

Conclusions

• Bothobservationsandclimatesimulationsindicatethatthewidthofthetropics followsthemeridionalvariationsofsubtropicalfronts.

• Underglobalwarming,thesubtropicalfrontsadvancetowardspoles duetoenhancedwarmingoverthesubtropicalocean.

• Polewardadvancingofthesubtropicalfrontsshifts theedgesofthetropics,westerlies,jetstreamsandstormtrackstowardspoles.

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