1. Climate Change in the Sahel Michela Biasutti biasutti@ldeo.columbia.edu in collaboration with : Alessandra Giannini, Adam Sobel, Isaac Held

2. Also see, e.g. Nicholson 1986 (J. Clim. App. Met.) Observed, Annual Mean, Continental Scale Rainfall Variability

3. http://www.pbs.org/wnet/africa/explore/sahel/sahel_overview.html The Sahel

4. Sahel variability and relation to SST: OBSERVATIONS Giannini et al., 2003, 2005 EOF1: Sahel rainfall Associated SST

5. Giannini et al. Sahel variability and relation to SST: ATMOSPHERIC GCMs

6. Current questions and challenges: Was the Sahel drought (and associated SST variations) of natural or anthropogenic origin? Are the 20th century Sahel drought and SST anomalies consistent with internal climate variability? Are they reproduced in forced runs? In runs forced with natural forcings? In runs forced by anthropogenic forcings? How will Sahel rainfall change in the greenhouse future?

7. NASA/GISS IPCC Simulations GCMs 20th Century Simulation (XX) Global Warming Scenario (A1B) Pre-Industrial Control (PI) XXPI A1B

8. The forced component: Sahel XX-PI Rainfall Change Biasutti and Giannini, 2006

9. XX-PI Rainfall Change

10. XX-PI SST Change

11. Importance of Internal Noise 60 XX Simulations 1950-1985 Trend 1950-1999 Trend 1930-1999 Trend something missing?

12. The role of land surface feedback: Zeng et al. 1999 QTCM  OBS atmos+ocean +land atmos+ocean +land +vegetation and dust?

13. Internal Noise vs. Forced Signal

14. Effect of GHG 4x(yrs50:70)-PI Mean Rainfall Change Robustness of Rainfall Change Surface Temperature 20

15. NASA/GISS SULFATE AEROSOL FORCINGS (1850-1997) Temp RESPONSE Effect of Reflective Aerosols ROTSTAYN AND LOHMANN ‘02 Precip RESPONSE

16. 20 th Century drying of the Sahel is reproduced by almost all IPCC AR4 models  it is (partly) externally forced. (But natural, internal variability is substantial.) The forcing was anthropogenic, with the most robust (*) signal coming from the sulfate aerosol forcing.* The response to GHG increase alone is inconsistent across models, which implies an uncertain outlook for the Sahel. Some Conclusions

17. IPCC Scenarios for the 21st Century IPCC 2001

18. GFDL Precipitation Response in the Sahel

19. Is it SST?:  different SST anomalies?  different sensitivity to same SST anomalies? Is it something else?  direct GHG influence? What are the possible causes of discrepancy?

20. Relationship of Sahel rainfall & SST (pre-industrial, not forced) Biasutti et al., 2007

21. Linear Multi-Regressive Model: from SST (Indo-Pacific & Atlantic Gradient) to Sahel Rainfall interannual (=detrended) XX A1B goodness of model PI (training run)

22. 2000-2100 Trends Sahel Rainfall Indo-Pacific SST Atlantic SST Gradient

23. Linear Multi-Regressive Model trained on (detrended) PI: from SST ( Indo-Pacific & Atlantic Gradient ) to Sahel Rainfall interannual  interannual + trend PI XX A1B goodness of model nb: same results if NTA & STA are used (3 predictors).

24. interannual  interannual + trend XX goodness of model A1B Linear Multi-Regressive Model trained on (unfiltered) XX: from SST ( Indo-Pacific & Atlantic Gradient ) to Sahel Rainfall

25. ~30%(?) of 20 th Century drying of the Sahel was externally forced. The forcing was anthropogenic, with the most robust signal coming from the sulfate aerosol forcing. In the 21 st Century, when GHG are the dominant forcing, the Sahel response is inconsistent across models. Global SST changes can explain the 20 th Century trend, but, in most models, not the 21 st Century one (at least not through the same mechanisms active in the past). A model’s good representation of the past is no indication of a trustworthy prediction of the future. How can we reduce the uncertainty of our climate outlook? Conclusions

26. Strengthened paleo monsoons follow orbitally-forced increases in seasonality & land/sea contrast. GHGs force an enhanced contrast. Should we expect enhanced monsoons? Current Direction: Explore the role of land/sea contrast

27. Model ‘Composite’ on Sahel response to GHG 1%to4xCO 2 and A1B simulation  A1B-XX  A1B-PI  4x( @XX )-PI  4x( @4x )-PI wet models dry models miroc ncar gfdl

28. Dry/Wet Composites: (2075  2100) - (1975  2000)

29. Dry/Wet Composites: (2075  2100) - (1975  2000)

30. Sahara Low = = Tropical Z 850 - Sahara Z 850 interannual  interannual + trend Sahara Low & Sahel Rainfall

31. What controls the Sahara Low? Is it a heat low? Is it dynamically influenced from afar? Is it an effect, instead of a cause, of enhanced Sahel rain?

32. extra slides

33. Heat Low? Baroclinic variability in Sahara Z 850 sfc 850m b 500mb 200mb

34. Other Influences? Barotropic variability in Sahara Z 850 sfc 500mb 200mb

35. Possible Influences on Sahara Low

36. Sahel Giannini et al., 2003

37. 19 Coupled GCM : XX-PI SST Change OBSERVED XX(1975-2000)-PI(1880-1900) NASA/GISS analysis of surface temp – linear trend 1950-2000 Hansen et al. 1999 (J. Geophys. Res.)

38. Natural or Anthropogenic? natural & anthroanthropogenic

39. Observed T sfc anomalies due to the Pinatubo Eruption Robock, Rev. Geophys., 2000 Natural or Anthropogenic? Composite of Volcanic Years (1975-1999 period) in Models with Volcanism

41. 10-year Running Average of Aug-Oct NH Surface T and MDR SST

42. Best Fit Linear Combination of Global Warming and Aerosol Forcing (red) versus Tropical Atlantic SST (blue) Mann and Emanuel (2006) MDR SST Global mean T + aerosol forcing

43. Lu and Delworth, 2005 Role of Indian and Pacific back

44. Changes in Sahel/SST Regression Coefficients Interannual Only Interannual & Trend PI XX A1B

45. Dry/Wet Composites: (2075  2100) - (1975  2000)

46. Dry/Wet Composites: (2075  2100) - (1975  2000)

47. 20th Century (XX) Pre-Industrial (PI) 1% /yr to 4 times pre-industrial CO 2 (4x) stabilization @ 720ppm (A1B) CGCM experiments for IPCC AR4

49. ukmo hadgem1ANTHROINDIRECTORGANIC CBLACK C ukmo hadcm3ANTHROINDIRECT ncar pcm1NAT+ANTHRO ncar ccsm3NAT+ANTHROBLACK C mri cgcm2NAT+ANTHRO mpi echam5ANTHROINDIRECT miroc3.2 medresNAT+ANTHROLAND USEINDIRECTORGANIC CBLACK C miroc3.2 hiresNAT+ANTHROLAND USEINDIRECTORGANIC CBLACK C ipsl cm4ANTHROINDIRECT inm cm3NAT+ANTHRO iap fgoalsANTHRO giss e rNAT+ANTHROLAND USEINDIRECTORGANIC CBLACK C giss e hNAT+ANTHROLAND USEINDIRECTORGANIC CBLACK C giss aomANTHRO gfdl cm2.1NAT+ANTHROLAND USEORGANIC CBLACK C gfdl cm2.0NAT+ANTHROLAND USEORGANIC CBLAC C csiro mk3ANTHRO cnrm cm3ANTHRO cccma cgcm3ANTHRO 20 th Century Forcings in XX runs back