Climate Change in the Sahel

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

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

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

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

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

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  1950-1985 Trend  1950-1999 Trend 60 XX Simulations  1930-1999 Trend something missing?

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

13. Internal Noise vs. Forced Signal

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

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

16. Some Conclusions • 20th 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.

17. IPCC Scenarios for the 21st Century IPCC 2001

18. Precipitation Response in the Sahel GFDL

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

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

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

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

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

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

25. Conclusions • ~30%(?) of 20th 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 21st Century, when GHG are the dominant forcing, the Sahel response is inconsistent across models. • Global SST changes can explain the 20th Century trend, but, in most models, not the 21st 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?

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

27. Model ‘Composite’ on Sahel response to GHG 1%to4xCO2 and A1B simulation ncar wet models miroc dry models gfdl  A1B-XX  A1B-PI  4x(@XX)-PI  4x(@4x)-PI

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

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

30. Sahara Low & Sahel Rainfall Sahara Low = = Tropical Z850 - Sahara Z850  interannual  interannual + trend

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 Z850 sfc 850mb 500mb 200mb

34. Other Influences? Barotropic variability in Sahara Z850 sfc 500mb 200mb

35. Possible Influences on Sahara Low

36. Sahel Giannini et al., 2003

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

38. Natural or Anthropogenic? natural & anthro anthropogenic

39. Natural or Anthropogenic? Observed Tsfc anomalies due to the Pinatubo Eruption Robock, Rev. Geophys., 2000 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) MDR SST Global mean T + aerosol forcing Mann and Emanuel (2006)

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

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

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

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

47. CGCM experiments for IPCC AR4 1% /yr to 4 times pre-industrial CO2 (4x) stabilization @ 720ppm (A1B) 20th Century (XX) Pre-Industrial (PI)

49. 20th Century Forcings in XX runs back