Transient Response to Aerosol Radiave Forcing and ...
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Transient Response to Aerosol Radia0ve Forcing and Implica0ons for Hurricanes Ron Miller NASA Goddard Ins0tute for Space Studies Dept of Applied Physics and Applied Math, Columbia University
Transcript of Transient Response to Aerosol Radiave Forcing and ...
TransientResponsetoAerosolRadia0veForcingandImplica0onsforHurricanes
RonMillerNASAGoddardIns0tuteforSpaceStudies
DeptofAppliedPhysicsandAppliedMath,ColumbiaUniversity
An0correla0onofDustAerosolsandAtlan0cHurricaneAc0vity
• Robustan0correla0onofdustandhurricanes(DunionandVelden2004,Evanetal.2006;Wongetal.2008).
• Causal?Mechanism?
– IntrusionofDrySaharanAirLayer(DunionandVelden2004).Dustispassive?
– DustreducesSST(LauandKim2007,Evanetal.2007,Evanetal.2008).Howtoquan0fy? Evanetal.Geophys.Res.Le,.2006
r=0.6
SSTAdjustmenttoDustRadia0veForcing
• SST(To’)istypicallycalculatedaccordingto:
• Restora0onofSSTbysurfacefluxes(red)isneglected,assumingthatSSTadjustsquicklycomparedtosurfaceflux0mescale(afewmonths).
LauandKimEos2007
⇒ Doestheair‐seaheatfluxandcouplingwiththeatmospherechangetheadjustmentofSST?
SimpleModelofCoupledAtmosphereandOcean• AaerSchopfJ.Phys.Ocean.1983:
• Note:• TendencyofTa’dependsuponFTOAinaddi0ontoFSurf,• Theoceanhasmuchlargeriner0athantheatmosphere.
• EquilibriumSolu0on:
⇒ SurfaceairtemperatureTa’iscontrolledbyFTOA.(Cessetal.J.Geophys.Res.1985)
Keyassump0on:convec0vemixingmaintainslapse‐ratedespitecoolingbeneaththeaerosollayer.
AdjustmentByTwoCoupledModes
SlowMode(fewyears):theatmosphereisinquasi‐equilibriumwiththeoceanandbothadjustslowlytotheTOAaerosolforcing.FastMode(aweek):theatmospherequicklywarmsbyradia0veabsorp0onwithintheaerosollayer.Thisdecreasestheair‐seatemperaturedifference,reducingthefluxofheatfromtheoceantotheatmosphere.Thisopposesthecoolingoftheoceanbytheaerosolsurfaceforcing.
Time‐dependentsolu0onconsistsofequilibriumsolu0onplustwotransientmodes(‘eigenmodes’oftheunforcedequa0ons).
Example:AdjustmenttoSteadyForcing
WhyDon’tAerosolsChangeSSTVeryMuch?• SurfaceAerosolForcingisopposedbyheatfluxfromtheoceantotheatmosphere(whichreducesthecoolingbyabouthalfinthisexample).
• Aseasonisshortcomparedtotheadjustment0mescale(mainlydeterminedbythemixedlayerdepth).Despiteanespeciallydustysummer,theoceandoesn’thavemuch0metocool.
• However,dustmightcontributetointerdecadalvaria0onsinhurricanes,becausethis0mescaleislongcomparedtotheadjustment0mescale.
Chiapello, Moulin, Prospero, J. Geophys. Res. 2005
Annual(opentriangles)andmonthly(blacktriangles)surfacedustconcentra0onmeasuredatBarbados
SurfaceTemperatureOvertheEastAtlan0c
MillerandTegenJ.Climate1998
Climatologicalreduc0onofsurfaceairtemperatureislessthan1oCofftheAfricancoast,andissmallerdownwindoverthehurricanemaindevelopmentregion.
Conclusions• Ta’adjuststoaerosolforcingalongwithTo’.Theatmosphere,which
warmsquicklyinresponsetoaerosolradia0vehea0ng,reducesthefluxofheatfromtheoceantotheatmosphere,opposingcoolingoftheoceanbyaerosolsurfaceforcing.
• Duringadustysummer,oceancoolingisabouthalfofthatpredictedbyholdingatmospherictemperaturefixedandnotaccoun0ngforforcingatthetopofatmosphereFTOA.Thecoolingislimitedbytheslow(mul0‐year)response0meoftheoceanmixedlayertemperature.
• Oninterdecadal0mescales,wheretheclimatecanadjustfullytochangesindustradia0veforcing,thecoolingcanbelarger,althoughitshouldbecalculatedusingbothFTOAandFSURF.
• 3DResponse:theatmospherecanmoderatethelocalresponsetodustradia0veforcingbychanginglateraltransportofenergyawayfromthedustlayer.(Needabejermodelwithlateraldependence.)
• Howdoesaerosolforcingchangesurfacemoistureorpoten0alintensity,whichiskeytohurricanedevelopment?
Example:AdjustmenttoEpisodicForcing
Coolingpeaksafewmonthsaaerthemaximumdustforcing.TheannualaveragecoolingisrelatedtotheTOAforcing.
Reflec0veAerosolsandHurricaneDevelopment
Unlikedust,sulfatesdonotwarmbyabsorbingradia0on,socoolingoftheoceanthroughthesurfaceheatfluxisnotreducedasmuch.
SurfaceForcingandSurfaceAirTemperature
MillerandTegenJ.Climate1998
⇒ Dustcausesalargereduc0onofsunlightovertheArabianSea(upto‐55Wm‐2),butsurfaceairtemperatureislargelyunchanged.
⇒ Thereiscoolingonthefringesoftheforcing(whereprecipita0onissmallandairsubsides).
Lapse‐rateChangeByAerosolForcing
• Changeislessthan1oCoverthedepthofthetroposphere(comparedtotheunperturbedvalueof50oC).
⇒ Thecolumnremainswell‐mixeddespitethereduc0oninsunlightatthesurface.
MillerandTegenJ.Climate1998
