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