Studies of the General Circulation of the Atmosphere with a Simplified Moist GCM

Dargan M. W. FriersonUniversity of Washington, Department of Atmospheric Sciences

PCC Summer Institute, 9-14-11

Collaborators: Yen-Ting Hwang (Uw), Jack Scheff (UW)Understanding Zonal Mean Precipitation Changes in a Warmer ClimateHow Can Precipitation Change?Ill argue that precipitation changes can be usefully separated into two componentsChanges in intensity of featuresWet gets wetter with warmthChanges in position of featuresPoleward shifts of midlatitude storms with warmthTropical precipitation shifts towards the hemisphere with more heatingChanges in Precip in 21st Century SimulationsMulti-model mean precip change With stippling based on a weak significance criteria

21st Century Simulations SummaryMost confidence in high latitude moisteningLots of subtropical drying, occasionally with confidenceTropical moistening (but little confidence)

Why?Changes in Water Vapor in AR4 SimulationsChanges in water vapor content (% increase) versus temperature change (K) for change over 21st century (A1B scenario):

Water vapor increases by 10-25% with 1.5-3 K warming.

Spread among models is mostly due to spread in amount of warming.Held and Soden 2006Why Wet Get WetterMore moisture in the atmosphere q more moisture flux vq wet get wetter

This explains tendency for high latitudes and tropics to moisten

Precipitation Changes with WarmingGlobal average P rises much more slowly thoughSignificantly less than water vapor content increase, around 1-3% per KSo some regions have to dry

Per K increase of P varies by over a factor of 3 in models!

See Angie Pendergrasss poster for why: black carbon prescriptions are the primary culpritHeld and Soden 2006Why Global P Changes SlowlyWeak global average precip change becauseFrom surface budget perspective:

From the atmospheric energy budget perspective:

SolarNet surfacelongwaveSensible heat fluxOcean heat storageEvaporation is constrained by solar radiation input, etc

Net radiativecooling of the atmosphereSensible heatfluxPrecipitation is constrained by the ability of the atmosphere toshed heatWhy Dry Regions Persist/ExpandSo some regions have to dry as wellSubtropics are the main place where this happensPart of this is because more moisture is fluxed away from there(flux gets more negative)

Actual (solid) and thermodynamic prediction(dashed) of P-E change with global warming

Held & Soden 2006Poleward Shift of EddiesEddy kinetic energy changes from Yin 2005 Black contours are current mean, colors are predicted change

Poleward (and upward) shift with global warming

Poleward Shifts of Midlatitude Storm TracksMidlatitude precip moves along with the storm tracks: Moistens high latitudesDries on the equatorward side of the storm track

From Scheff and Frierson (submitted to J. Climate): Storm track shifts are the primary cause of significant dryingMoistening on poleward side

Drying on equatorward sideExtra-tropical summaryWet regions get wetter (high latitudes)Dry regions persist/expand polewardAnd land surfaces tend to get more arid unless precip goes up b/c potential evaporation increases (see Jack Scheffs poster)

What about the tropics? We generally expect moistening but we dont have much confidence thereHadley CirculationThe Hadley Cells transports energy poleward, and moisture into the deep tropics

Dima and Wallace (2003)Energy flux by upper branchesMoisture flux by lower branchesZonally averaged precip and evapHadley cell is key to converging moisture towards the equator

Evap and precip annual means (NCEP Reanalysis 2)14Seasonal Shifts of Tropical PrecipitationTropical rain follows the warmth in the seasonal cycle (TRMM climatology)

Seasonal Hadley Cell/PrecipJanuary (top) and July (bottom) Hadley cells & precip

Energy transportfrom warm to coldPrecip shift southwardEnergy transportfrom warm to cold

Precip shift northwardEnergetics and Tropical Precipitation ShiftsClaim: whenever one hemisphere is heated more, some of that heat is fluxed to the other hemisphereResulting in an ITCZ shift in the other directionEven if the forcing is purely extratropical, the ITCZ will still respond (see Ph.D. thesis of Sarah Kang)Eddies bring some of the response into lower latitudes

Approach: correlate precip shift with cross-equatorial energy transportThen use the atmospheric energy budget to attribute the shift to different heating terms

Change in Precip, AR4 Slab Ocean Models Moistening in tropics and mid/high latitudesDrying in subtropicsPlot by Yen-Ting Hwangmm/year

Change in Precip, AR4 Slab Ocean Models Plot by Yen-Ting Hwang

Huge variance in tropics though!60 cm/yr difference in precip!mm/year

Change in Precip, AR4 Slab Ocean Models Plot by Yen-Ting Hwang

Big differences in SH too: 10 cm/yrmm/year

Precip shift versus cross-eq energy fluxAnticorrelated: Hadley cell governs both

If we can explain the energy flux changes, we can explain the ITCZshifts

Change in Precipitation in Extreme Cases

CCCMA (most S-ward)MPI (most N-ward)mm/yearSeen across most longitudes, and over continents as wellPlot by Yen-Ting Hwang

Surface Albedo + Cloud Feedbacks CCCMA has more net heating in SH: ITCZ shifts south

MPI has more heating in NH: ITCZ shifts northPlot by Yen-Ting HwangFeedbacksSurface albedoCloud Shortwave

Lots of ice meltingLow clouds form1-D Energy Balance Model for Energy FluxesPrescribe latitudinal structure of forcings/feedbacks: Surface albedo changesCloud radiative feedbacksOcean heat uptakeAerosol scattering/absorptionPredict: Energy fluxesTemperature changesClear sky outgoing radiation changesAssumes constant diffusivity!See Frierson and Hwang (in press) for more detailsEBM Prediction for Slab Models

R = 0.91Importance of Extratropical ForcingEBM forced by terms outside of the tropics only (poleward of 20o N/S)

Extratropical forcing explains the range in ITCZ shiftsR = 0.86Relevance to Precipitation Climatology?Why does tropical precipitation peak more in the Northern Hemisphere? Lets study from the energetics perspective

GPCP ClimatologyWhy is the tropical precip mostly in the north?Total energy flux from CERES: More energy inputinto the Southern Hemisphere!!

Ocean heat fluxThe ocean heat flux is northward though

EQMy argumentOcean heat flux (MOC) drains all the cold water out of the AtlanticAtmospheric heat fluxes respond & flux energy back southwardTropical precip goes to the north

The ocean causes the rain to be in the northIf true, this is one of the most important roles of the ocean in the current climate??

See recent work by Neven Fukar (U Hawaii) in a coupled GCMConclusionsIn a warmer climate, wet gets wetter and dry regions persist/expandMidlatitude precip: High latitudes moistenSubtropics dryPoleward shifts are importantTropical precipitation shifts: Uncertain in 21st century primarily due to climate feedbacksOceanic MOC causes tropical precipitation to be in the north?See recent papers by Ting, Jack & I for details