1. Past and Future Changes in Southern Hemisphere Tropospheric Circulation and the Impact of Stratospheric Chemistry-Climate Coupling Collaborators: Steven Pawson, Richard Stolarski, Paul Newman, Eric Nielsen, Judith Perlwitz CIRES-Climate Diagnostics Center University of Colorado Martin Hoerling Bill Neff

2. Outline Introduction –Southern Hemisphere Annular Mode (SAM) Answers of)Fourth Assessment Report (AR4) on past and future changes in Southern Hemisphere Circulation Will the answers be different when analyzing experiments with Chemistry Climate models? Summary and Implications

3. Southern Hemisphere Annular mode Thompson and Wallace, 2000

4. Reconstruction of SH summer SAM index (Jones and Widman, 2004)

5. Change in Southern Hemisphere Polar Cap Temperature and Geopotential Heights 1969-1998 Based on Radiosonde Data (Thompson and Solomon, 2002)

6. Change in 500 hPa geopotential height (1979-2000), surface temperature (1969- 2000) and 925 hPa wind (1979-2000) Thompson and Solomon, 2002

7. Response to stratospheric ozone loss in the Southern Hemisphere (Gillett and Thompson, 2003) Strengthening of westerly flow from the stratosphere to the troposphere Cooling in the troposphere occurs one to two month after the maximum cooling in the stratosphere Model response agrees well with observations. Z 500hPa [m] DJF Z [m] T [K]

8. Fourth Assessment Report (AR4) of Intergovernmental Panel on Climate Change Released 2007 Describes our improved understanding in observed changes, attribution, and estimates of future change Based on AR4 model simulations

9. Past Changes of SAM index IPCC report: On average, a larger positive trend is projected during the late 20th century by models that include stratospheric ozone changes (red) than those that do not (blue), Miller et al, 2006

10. Attribution of recent changes? IPCC Report: The cause of the positive SAM trend in the second half of the 20th century is mainly attributed to stratospheric ozone depletion, evidenced by the fact that the signal is largest in the lower stratosphere in austral spring through summer (Thompson and Solomon, 2002; Arblaster and Meehl, 2006).

11. Attribution of recent SAM trend However, increases in greenhouse gases are also important factors (Shindell and Schmidt, 2004; Arblaster and Meehl, 2006) for the year-round positive SAM trend induced by meridional temperature gradient changes (Brandefelt and Källén, 2004).

12. Future Change of SAM index IPCC report: during the 21st century, when ozone changes are smaller, the SAM trends of models with and without ozone are similar. Miller et al. 2006

13. Future Change of SAM? IPCC Report: During the 21st century, although the ozone amount is expected to stabilise or recover, the polar vortex intensification is likely to continue due to the increases in greenhouse gases (Arblaster and Meehl, 2006).

14. How realistic are simulated SST changes?

15. How realistic are future projections of stratospheric polar ozone changes used in AR4 models? Ozone Assessment Report 2006: Black Observations: Over the Antarctic, most models predict that ozone amounts will increase to 1980 values close to the time when Antarctic equivalent effective stratospheric chlorine (EESD) decreases to 1980 values. A new empirical model, based on observations, indicates a return of Antarctic EESD values between 1960-1975

16. Stratosphere is not well resolved in these models Stratospheric dynamical processes are not well represented AR4 Models do not simulate feedback mechanism of chemistry climate coupling

17. GEOS4-CCM Model GEOS4-GCM –Model Top: 0.01 hPa (~80km) –55 levels –Horizontal resolution: 2 o x2.5 o Stratospheric Chemistry Module Role of SST changes and changes in atmospheric composition in recent changes of tropospheric SAM Role of greenhouse gas increase and ozone recovery in future SAM changes

18. Correlation coefficients between SAM index at 10 hPa and at all pressure levels between 1000 and 1 hPa for time lags between -60 and 60 days.

19. Experiments: Two Historic Simulations with Observed SST (1950-2004) Two simulations using SST from CCM2, HadGem1 (1971- 2049) Two Future Simulations using CCM3, HadGem1 SST (2000- 2099) Historic and Future simulations where Clorine is kept at 1960 values

20. Past Climate Change 1960-2004 Monthly Changes in Polar Cap Temperature (90S-65S) Monthly changes in Zonal Wind 70S-50S Maps of 500 hPa and surface pressure changes

21. Monthly Linear Trend in Polar Cap Temperature and Zonal Wind 70S-50S (1960-2004) Polar Cap Temperature Zonal Wind 70S-50S

23. Change in tropospheric circulation 500 hPa Geopotential Height Surface Pressure

25. 1950-1999 500 hPa geopotential heights [gpm] DJF GFDL-AM2.0GFDL-AM2.1NCAR-CAM3 OF AM2.0 Response to observed SST changes In tropospheric models AMIP Tropical Indian Ocean warming

26. Simulations of Recent Past Consistent with many other studies: Observed increase of SAM index is caused by stratospheric polar ozone depletion In GEOS4-CCM observed SST changes from 1960-2004 cause negative SAM trend, due to increased upward flux of wave activity into the stratosphere during winter Impact of SST changes is model dependent

27. Future Climate Change 2000-2099

28. Monthly Linear Trend in Polar Cap Temperature and Zonal Wind 70S-50S (2000-2099) Polar Cap Temperature Zonal Wind 70S-50S

29. Change in tropospheric circulation 500 hPa Geopotential Height Surface Pressure

31. Seasonal development of SAM trend 2000- 2049 2000- 2099 500 hPa 50 hPa

32. Changes in SAM index during 21st century CCM simulations without changes in stratospheric polar ozone: –Consistent with AR4 models and previous studies, CCM shows increase of tropospheric SAM index CCM simulation with ozone recovery: –Ozone recovery dominates future development of tropospheric SAM during summer – leading to a negative trend –Changes are most pronounced during first half of 21 century.

33. Two runs with same SST are compared. 2000-2049 In lower stratosphere, positive trend at South Pole is larger in C21D than in CCSM3. 2050-2099 In C21D, positive trend continues. Small negative trend in CCSM3.

34. Implications One-model-result –Multi-model approach necessary similar to IPCC –will be realized in SPARC CCMval project (Validation of Chemistry Climate Models) SST changes were prescribed using simulations that give a different answer (Implications of ozone recovery on ocean circulation and possible atmosphere/ocean interactions are not simulated correctly) – Simulations with coupled CCM/Ocean model are necessary Ozone recovery scenario for 5th IPCC report should be defined