DARGAN M. W. FRIERSON UNIVERSITY OF WASHINGTON, DEPARTMENT OF ATMOSPHERIC SCIENCES

1. Experiments with a Hierarchy of GCMs: ITCZ Response to High Latitude Forcing, and Tropical Variability DARGAN M. W. FRIERSON UNIVERSITY OF WASHINGTON, DEPARTMENT OF ATMOSPHERIC SCIENCES COLLABORATORS: SARAH KANG, ISAAC HELD, MING ZHAO, JIALIN LIN, IN-SIK KANG, DAEHYUN KIM, MYONG-IN LEE, ADAM SOBEL, ERIC MALONEY, GILLES BELLON

2. Modeling Philosophy • Models aren’t reality… • They can only tell us so much about the real atmosphere • A major advantage of using models is ability to play around with parameters • Turning feedbacks on/off • Modifying/simplifying boundary conditions • Changing physical parameterizations

3. Comprehensive and Simplified GCMs • With comprehensive models, not always straight-forward to perform such experiments • Can affect many aspects of model (e.g., convection scheme affects clouds, etc) • Can cause fidelity of simulated climate to decrease • Requires careful experimental design • Simplified GCMs are useful for aiding the above • Here we’ll discuss: • Moist GCM with highly simplified physics (Frierson 2005) • No cloud- or water vapor-radiative feedbacks • Simplified Betts-Miller convection scheme • Aquaplanet full GCM simulations • Realistic geography full GCM simulations

4. Outline • ITCZ response to extratropical forcing • With Sarah Kang & Isaac Held • Convectively coupled Kelvin waves • With Jialin Lin, In-Sik Kang, Daehyun Kim & Myong-In Lee • MJO • With Adam Sobel, Eric Maloney & Gilles Bellon

5. ITCZ Location • Pioneering work by Chiang, Biasutti and Battisti (2004) and Chiang and Bitz (2005): • Showed strong sensitivity of ITCZ to high latitude sea ice and land ice in LGM simulation using CCSM Southward displacement of ITCZ occurs in LGM climate Paleoclimate data is consistent with such a shift Drying Moistening From Chiang and Bitz (2005)

6. Extratropical Influences on ITCZ • Sarah Kang’s thesis work (2009): • Effect of high latitude forcing on ITCZ location/structure/intensity • Simplified moist GCM and aquaplanet full GCM (AM2) runs w/ idealized forcing: Forcing Think glaciers + sea ice in NH, plus warming in SH (to keep global mean temperature the same) SH warming NH cooling From Kang, Held, Fri., & Zhao (2008, J Clim) and Kang, Fri. & Held (in press, JAS)

7. ITCZ Changes • In both models, ITCZ precipitation shifts towards warmed hemisphere Tropical precip in full GCM • Response is sensitive to parameters • which affect cloud feedbacks • Response is significantly larger in • full GCM as compared with simplified • GCM From Kang, Held, Fri., & Zhao (2008, J Clim) and Kang, Fri. & Held (in press, JAS)

8. Mechanism for ITCZ Response • We argue energy flux is of key importance Change in MSE flux in simplified GCM 8 Anomalous energy flux into cooled region Less flux into warmed region

9. Mechanism for ITCZ Response • ITCZ latitude ~ “Energy flux equator” Change in MSE flux in simplified GCM Define “energy flux equator” as zero crossing of energy flux Shifted into SH in perturbed case In tropics, mean circulation does most of the flux => v=0 there => ITCZ is nearby 8 ITCZ location (-) is approximately same as energy flux equator (--) for full GCM

10. Mechanism for Energy Flux Change • Eddies modify fluxes in midlatitudes • Quasi-diffusively: they can be well-approximated with a moist energy balance model • Anomalous Hadley circulation modifies fluxes in tropics See Kang, Held, Fri., & Zhao (2008, J Clim) & Kang, Fri. & Held (in press, JAS) for more

11. Role of Cloud-Radiative Forcing • Differences in cloud-radiative forcing (CRF) affect ITCZ as follows: • CRF = extra forcing at certain latitude bands • Forcing is again propagated away by eddies quasi-diffusively • Changes in energy flux equator then result in changes in ITCZ location • Result in massive differences in ITCZ shift for same forcing! • Similar mechanism seen in energy fluxes in IPCC model simulations of global warming • Current work of my grad student Ting Hwang

12. Role of “Gross Moist Stability” • In idealized model, we can take the energy flux argument one step further • Can predict mass flux response (and hence precip response), with the “gross moist stability” of the tropics: • Changes in parameters of simplified Betts-Miller scheme can change (as shown in Frierson 2007a, JAS) • Larger GMS when convection can easily reach high levels • Smaller GMS when there’s an abrupt trigger for convection

13. Role of “Gross Moist Stability” • For identical forcing and identical energy flux response, the precip response can be significantly different Change in precip See Kang et al 2008; also Frierson 2007a

14. Tropical Variability in Simplified GCM • Convectively coupled Kelvin waves dominate tropical variability in the idealized GCM Unfiltered Hovmoller diagram of precipitation at the equator Does gross moist stability control the speed of these waves (as in simple theories)? From Frierson (2007b, JAS)

15. Convectively coupled Kelvin waves • GMS reduction also leads to slower convectively coupled waves: GMS = 7 K GMS = 4.5 K GMS = 2.5 K Wavespeed can be tuned to essentially any value in this model See Frierson (2007b) for more detail

16. Idealized Moist GCM Kelvin Waves • Kelvin waves are powered by evaporation-wind feedback • Likely not true in reality in Indian Ocean… • Vertical structure is purely first-baroclinic mode • Unrealistic… Composited pressure velocity Longitude See Frierson (2007b) for more detail

17. Equatorial Waves in a Full GCM • Experiments with SNU atmospheric GCM • Run over observed SSTs, realistic geography • Simplified Arakawa-Schubert convection scheme • Varying strength of convective trigger • Wavespeed decreases with stronger moisture trigger • Due to smaller GMS, as in simplified GCM See Lin, Lee, Kim, Kang and Frierson (2008, J Clim) & Fri. et al (in prep) for more

18. Moist Static Energy • Vertical profile of MSE in the North West Pacific ITCZ: • MSE clearly reduced at higher levels (more unstable) • GMS also reduced

19. Vertical structures • In full GCM, the waves show realistic vertical phase tilts (unlike in simplified GCM) Shallow -> deep -> stratiform Gradual moistening of boundary layer/midtroposphere Warm over cold temperature anomalies See Lin et al (2008) and Frierson et al (in prep) for more detail

20. MJO in Realistic GCMs • Work with Sobel, Maloney, & Bellon using GFDL AM2 model w/ realistic geography • First crank up Tokioka “entrainment limiter” to get a better MJO simulation: Obs (NCEP) Modified GFDL model Unmodified GFDL model See SMBF (2008, Nature Geoscience; 2009, J. Adv. Modeling Earth Systems)

21. MJO in GFDL AM2 Model • Ratio of variance in eastward/westward intraseasonal bands: 2.6 for modified GFDL model • Less than the observed value of 3.5, but larger than nearly all models in Zhang et al (2006) comparison • Higher entrainment in convection scheme => more sensitivity to midtropospheric moisture • Next test role of evaporation-wind feedbacks in driving the modeled MJO • Set windspeed dependence in drag law formulation to globally averaged constant value See SMBF (2008, Nature Geoscience; 2009, J. Adv. Modeling Earth Systems)

22. Evap-Wind Feedback in Modeled MJO • MJO greatly weakened when evaporation-wind feedback (EWF) is turned off! With EWF Without EWF See SMBF (2008, Nature Geoscience; 2009, J. Adv. Modeling Earth Systems)

23. Conclusions • ITCZ is affected by high latitude forcing by following processes: • Energy fluxes: “energy flux equator” • Cloud-radiative forcing • Gross moist stability • Convectively coupled waves in simple and full GCM are affected by “gross moist stability” • Full GCM shows second baroclinic mode characteristics • Simulated MJO in full GCM extremely sensitive to evaporation-wind feedback