1. The Atmospheric Circulation Response to Climate Change-like Thermal Forcings in a Simple GCM Amy H. Butler 1, David W.J. Thompson 2, & Ross Heikes 2 1 Climate Prediction Center/NOAA, 2 Colorado State University Climate Prediction and Diagnostics Workshop Monterey, CA October 26, 2009

2. Observed Changes in the Atmospheric Circulation Poleward shift of the storm tracks/mid-latitude jets Robust shift in SH storm track Weaker shift in NH storm track Associated with significant changes in surface climate [e.g., Hurrell, 1995; Thompson et al., 2000; Thompson and Solomon, 2002; Ostermeier and Wallace, 2003; Liu et al., 2007] Widening of the Hadley Cell by 2-5° latitude over last 25 yrs [e.g., Hudson et al., 2006; Fu et al., 2006; Seidel and Randel, 2007; Seidel et al., 2008]

3. Forced with Past Changes: in CO 2 : Little shift in storm tracks; expansion of Hadley cell but weaker than observed in Ozone: Robust poleward shift in SH storm track; expansion of Hadley cell but weaker than observed Forced with Future Changes: in CO 2 : Poleward shift in both NH and SH storm tracks; expansion of Hadley cell in Ozone: recovery leads to equatorward shift in SH storm track [e.g., Shindell et al., 1999; Fyfe et al., 1999; Kushner et al., 2001; Gillett and Thompson, 2003; Shindell and Schmidt, 2004; Brandefelt and Kallen, 2005; Yin, 2005; Miller et al., 2006; Arblaster and Meehl, 2006; Lorenz and DeWeaver, 2007; Son et al., 2008] Simulated Changes in the Atmospheric Circulation

4. Changing Temperatures Warmer troposphere, colder stratosphere Stronger warming in tropical troposphere Stronger warming in Arctic Strong seasonal polar cooling (but ozone recovery expected ~2065) IPCC AR4

5. Simple Dry Dynamical GCM CSU dynamical core Held-Suarez parameterizations No topography Equinoctal conditions

6. Steady-State Experiments The time-mean average of the 5 years following 1 year of spin-up

7. Tropical Tropospheric Heating Forcing: K*m/s m 2 /s 2 K/day Temp/Eddy Heat Flux ResponseWind/Eddy Mom Flux Response K*m/s Temp/Eddy Heat Flux Response m 2 /s 2 Wind/Eddy Mom Flux Response

8. Tropical Tropospheric Heating Tropical warming alone produces: poleward shifts in jets and expanded/weakened Hadley cell. In our model, also produces weakened Brewer-Dobson Circulation (BDC).

9. ForcingTemp ResponseWind Response

10. ForcingTemp ResponseWind Response

11. ForcingTemp ResponseWind Response

12. Polar Stratospheric Cooling Forcing: K/day K*m/s Temp/Eddy Heat Flux ResponseWind/Eddy Mom Flux Response m 2 /s 2

16. Polar Surface Warming Forcing: m 2 /s 2 K/day Temp/Eddy Heat Flux Response K*m/s Wind/Eddy Mom Flux Response m 2 /s 2

17. Combination Forcing: m 2 /s 2 K/day Temp/Eddy Heat Flux Response K*m/s Wind/Eddy Mom Flux Response m 2 /s 2

18. Conclusions from Steady-State Runs Heating the tropical troposphere leads to a poleward shift of the storm tracks, an expansion of the Hadley cell, and a weakened Brewer- Dobson circulation Cooling in the lower polar stratosphere shifts the tropospheric jet polewards, but the response is sensitive to the level of the forcing Warming at the Arctic surface may play a role in the predicted weaker poleward shift of the NH storm track

19. Transient Experiments 12 ensemble members, each 150 days long (6 hour output), with initial conditions taken from control run 50 days apart. Thermal forcing turned on: day 10.

20. Transient Ensemble Simulation of Tropical Tropospheric Heating Temp: Equator Wind: 60N Temp: Pole Wind: 40N

21. Mechanisms for Tropospheric Circulation Changes A.Changes in meridional temperature gradient  At upper levels: changes in eddy phase speeds [Chen and Held 2007; Chen et al. 2007]  At lower levels: changes in low-level baroclinicity and eddy generation [Yin 2005] B.Changes in vertical temperature gradient  Changes in static stability [Frierson 2008; Lu et al. 2008]  Changes in tropopause height [Lorenz and DeWeaver 2007] Current research involves analyzing the transient runs in the context of these mechanisms

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