Physicalanalysisofthemodeldri2intheNorthAtlan5c:theroleoftheatmosphereinthebiasadjustmentEmiliaSanchez-Gomez1,ChristopheCassou1,YohanRuprich-Robert2,ElodieFernandez3andLaurentTerray1

1CERFACS/CNRS,Toulouse,France2NOAA/GFDL,Princeton,USA3Mercator-Ocean,Toulouse,France

Sanchez-Gomezetal.,Clim.Dyn.2015

Themodeldri2

Themodel(theguy)isini5alisedfromanobservedstate,whichiswarmerthanthemodelmeanclimate.Hence,the'modelguy'progressivelyadjusts(hewrapsup)bytofinallyreachtheequilibriumstate.

Anillustra5onofthemodeldri2inaclimatepredic5on.

ObservedClimate

ModelClimate

o  Thedri2isthesequenceofphysicalprocessesbywhichmodeladjusttoitsequilibriumstateora7ractor

o  ModeldriDsareremovedfrompredicIonsforforecastverificaIonandrarelyanalysed…butthedriDanalysiscanprovideusefulinformaIononthephysicalprocessesinvolvedinthedevelopmentofmodelsystema5cbiases.

Mo5va5onandgoals

§  Thegoal:ToinvesIgatesomeofthephysicalprocessesinvolvedinthemodeldriDtounderstandthemechanismsleadingtothemodelsystemaIcerrors

FocusontheNorthAtlanIc

Numericalexperiments

v  Ini5alcondi5ons(IC)-CoupledexperimentinwhichtheoceanisnudgedtowardsNEMOVARoceanreanalysisv Decadalexperiments(DEC)-IniDalisedevery5yearswithin1960-2000(10years,10members)-  FullfieldiniDalisaDonusingIC(Sanchez-Gomezetal.2015)v Historicalexperiments(HIST):-  NoniniDalized,1960-2005,10members-  UsedtoesDmatethemodela7ractor

CoupledModel:CNRM-CM5(Voldoireetal.2013)atmosphere:ARPEGEv5(T127,1.8o)ocean:NEMOv3.2-ORCA1sea-ice:GELATOv5.2

Modelclimatologyversusini5alcondi5ons

HIST–ICdifferenceBlackcontours:HISTclimatology

HIST–ICdifferenceBlackcontour:HISTclimatologyDarkBlue:ICclimatology

Modelclimatologyversusini5alcondi5ons

-Strongergyrecircula5oninIC-GulfStreamlocatednorthwardinIC-GulfStreamandSPGstrongerinIC(~10Sv)

HIST–ICdifferenceBlackcontours:HISTclimatology

HIST–ICdifferenceBlackcontour:HISTclimatologyDarkBlue:ICclimatology

Modelclimatologyversusini5alcondi5ons

-Strongergyrecircula5oninIC-GulfStreamlocatednorthwardinIC-GulfStreamandSPGstrongerinIC(~10Sv)

-StrongerAMOCinIC(~6Sv)-MaximumAMOClocatedatupperlevels(750m)inHIST

HIST–ICdifferenceBlackcontours:HISTclimatology

HIST–ICdifferenceBlackcontour:HISTclimatologyDarkBlue:ICclimatology

Modelclimatologyversusini5alcondi5ons

Duringthemodeldri2,theSPGandtheAMOCinDECwillweakentogetclosetotheHISTvalues…Whatarethephysicalmechanismand>mescalesinvolvedinthisdri@process?

SubpolarGyre:circula5on

WEST EAST

SlackeningoftheSPGcircula5on:•  ThewestSPGinDECrapidlyweakenstoreachHISTa2eraround5

years.•  TheeastSPGweakensandunexpectedlydri2sawayfromHIST

ICDECHIST

SubpolarGyre:circula5on

WEST EASTICDECHIST

SlackeningoftheSPGcircula5on:•  TheeastSPGstrengthensfrom

Yr11-12toreachbarelytheacractora2er30years.

10yearslengthforecasts

30yearslengthforecasts

ç√ ç√

SubpolarGyre:TandSWEST

ICDECHIST

Temperature:-UnIlYr4-5:strongwarming-ADerYr5:StrongcoolingSalinity:-Yr1:systemaIcdecrease-UnIlYr4-Yr5:salinityincrease-ADerYr5:slightsalinitydecrease

Temperature0-700m

Salinity0-700m

SubpolarGyre:TandSWEST

densitydecrease

densityincrease

density

Temperature:-UnIlYr4-5:strongwarming-ADerYr5:StrongcoolingSalinity:-Yr1:systemaIcdecrease-UnIlYr4-Yr5:salinityincrease-ADerYr5:slightsalinitydecrease

WEST

ICDECHIST

Temperature0-700m

Salinity0-700m

SubpolarGyre:TandSEAST

ICDECHIST

EAST

density

densitydecrease

densityincrease

densitystabilisaIon

Temperature:-UnIlYr4-5:strongwarming-ADerYr5:Strongcooling

Salinity:-Yr1:systemaIcdecrease-UnIlYr4-Yr5:salinityincrease-ADerYr5:strongsalinitydecrease

Temperature0-700m

Salinity0-700m

SubpolarGyre:TandSEAST

density

densitydecrease

densityincrease

Temperature:-UnIlYr4-5:strongwarming-ADerYr5:Strongcooling

Salinity:-Yr1:systemaIcdecrease-UnIlYr4-Yr5:salinityincrease-ADerYr5:strongsalinitydecrease

EAST

ICDECHIST

Temperature0-700m

Salinity0-700m

DEC–ICinYr1toYr4 DEC–ICinYr5toYr10

Thedri2oftheAMOC

AMOCslowdown(~6Sv),leadingtoareduc5onofheatandsaltpolewardtransport(coherentwithcoolinganddesalinisa5onofSPGa2erYr5)

Blackcontour:ICclimatology

(hPa)

Dri2intheatmosphericcircula5on

DEC–NCEPinYr1toYr4 DEC–NCEPinYr5toYr10Sealevelpressure

Theatmospherereachesquicklytheabractor.Thecoupledmodelbiasprojectsontoanega5veNAO.

(hPa)

Dri2intheatmosphericcircula5on

DEC–NCEPinYr1toYr4 DEC–NCEPinYr5toYr10Sealevelpressure

-Thestand-aloneatmosphericcomponentbiasprojectsontotheNAO-.-TheintrinsicatmosphericbiasisamplifiedinDEC

Stand-aloneatmosphere-NCEP

Weakeningofthewesterlies

Dri2intheatmosphericcircula5on

TheNAO-!weakeningofwesterliesàini5alwarmingoftheSPG!reduc5onofdeep-waterforma5on!slackeningoftheAMOC!decreaseofthepolewardheatandsalttransport!coolinganddesalinisa5onoftheSPG(Lohmanetal.2009,Barrieretal.2013)

Slope=-0.44hPa/oCCorrelaDon=-0.75AweakSLP/SSTfeedbackisestablised

Stand-aloneatmosphere

Summaryandconclusions

Ø  IntheNorthAtlan5c,thecoupledmodelbiasesprojectsontoaNAO-pacern,whichispresentfromthebeginningoftheforecastsandcanactlikeaforcingfortheocean.

Ø  Theseatmospherecircula5onbiasescanbepar5allyacributedtothebiasesofstand-aloneatmosphericcomponent.

Ø  Thecoupledmodeldri2scanbemostlyinterpretedastheintegra5onby

theoceanoftheseintrinsicatmosphericbiases:theNAO-forcingleadstoaweakeningofwesterlies,aniniIalwarmingintheSPGandareducIonofdeep-waterformaIonyieldingtoaslackeningoftheAMOC,thatinturnswillreducethepolewardheatandsalttransport,yieldingtoacoolinganddesalinisaIonoftheSPGarea.

Sanchez-Gomezetal.,Clim.Dyn.2015

Modeldri2inArc5cSeaIce

DriDaveragedovertheLabradorSeaarea(top)andGINseasarea(bobon).OnlyforDECandfortheSeaiceextent.

Heat flux:

Heatfluxatthesurface

feedbackterm

dTdQ Feedbackcoefficient

=-40W/m2/K

Fresh water flux:

Freshwaterbudgetatthesurface

feedbackterm

Feedbackparameter=-167mm/day

sγ

SeaSurfacerestoring

Seasurfacerestoring

Ini5alisa5onmethod

β=f(depth,space)

Current

NonudgingwithintheEquatorialband(1oN-1oS)and

Nearthecoast(300km)(1/β)=0

Inthethermocline(1/β)=0

DeepOceanβ=360days

Belowthermoclineβ=10days

3DNewtoniandamping

Ini5alisa5onmethod