Climate Change in the Sahel Michela Biasutti biasutti@ldeo.columbia.edu in collaboration with : Alessandra Giannini, Adam Sobel, Isaac Held Observed, Annual Mean, Continental Scale Rainfall Variability Also see, e.g. Nicholson 1986 (J. Clim. App. Met.) The Sahel QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. http://www.pbs.org/wnet/africa/explore/sahel/sahel_overview.html QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. Sahel variability and relation to SST: OBSERVATIONS EOF1: Sahel rainfall Giannini et al., 2003, 2005 Associated SST Sahel variability and relation to SST: ATMOSPHERIC GCMs Giannini et al. 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? IPCC Simulations PI Pre-Industrial Control (PI) NASA/GISS XX 20th Century Simulation (XX) A1B Global Warming Scenario (A1B) GCMs The forced component: Sahel XX-PI Rainfall Change Biasutti and Giannini, 2006 XX-PI Rainfall Change XX-PI SST Change 60 XX Simulations Importance of Internal Noise 1950-1985 Trend 1950-1999 Trend 1930-1999 Trend something missing? The role of land surface feedback: OBS atmos+ocean QTCM atmos+ocean +land atmos+ocean +land +vegetation Zeng et al. 1999 and dust? Internal Noise vs. Forced Signal Effect of GHG 4x(yrs50:70)-PI Mean Rainfall Change Robustness of Rainfall Change 20 Surface Temperature Effect of Reflective Aerosols SULFATE AEROSOL FORCINGS (1850-1997) Temp RESPONSE Precip RESPONSE QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. NASA/GISS ROTSTAYN AND LOHMANN ‘02 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. IPCC Scenarios for the 21st Century IPCC 2001 Precipitation Response in the Sahel GFDL 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? Relationship of Sahel rainfall & SST (pre-industrial, not forced) Biasutti et al., 2007 goodness of model PI (training run) Linear Multi-Regressive Model: from SST (Indo-Pacific & Atlantic Gradient) to Sahel Rainfall XX A1B interannual (=detrended) 2000-2100 Trends Sahel Rainfall Indo-Pacific SST Atlantic SST Gradient goodness of model PI XX Linear Multi-Regressive Model trained on (detrended) PI: from SST (Indo-Pacific & Atlantic Gradient) to Sahel Rainfall nb: same results if NTA & STA are used (3 predictors). A1B interannual interannual + trend goodness of model XX Linear Multi-Regressive Model trained on (unfiltered) XX: from SST (Indo-Pacific & Atlantic Gradient) to Sahel Rainfall interannual A1B interannual + trend 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? 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? 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 Dry/Wet Composites: (20752100) - (19752000) Dry/Wet Composites: (20752100) - (19752000) Sahara Low & Sahel Rainfall Sahara Low = = Tropical Z850 - Sahara Z850 interannual interannual + trend 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? extra slides Heat Low? Baroclinic variability in Sahara Z850 sfc 850m b 500mb 200mb Other Influences? Barotropic variability in Sahara Z850 sfc 500mb 200mb Possible Influences on Sahara Low Sahel Giannini et al., 2003 19 Coupled GCM : XX-PI SST Change OBSERVED NASA/GISS analysis of surface temp – linear trend 1950-2000 Hansen et al. 1999 (J. Geophys. Res.) XX(1975-2000)-PI(1880-1900) Natural or Anthropogenic? natural & anthro anthropogenic 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 10-year Running Average of Aug-Oct NH Surface T and MDR SST 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) Role of Indian and Pacific Lu and Delworth, 2005 back Changes in Sahel/SST Regression Coefficients Interannual Only PI Interannual & Trend XX A1B Dry/Wet Composites: (20752100) - (19752000) Dry/Wet Composites: (20752100) - (19752000) CGCM experiments for IPCC AR4 1% /yr to 4 times pre-industrial CO2 (4x) stabilization @ 720ppm (A1B) QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. 20th Century (XX) Pre-Industrial (PI) QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. 20th Century Forcings in XX runs ukmo hadgem1 ANTHRO INDIRECT ORGANIC C BLACK C ukmo hadcm3 ANTHRO INDIRECT ncar pcm1 NAT+ANTHRO ncar ccsm3 NAT+ANTHRO mri cgcm2 NAT+ANTHRO mpi echam5 ANTHRO miroc3.2 medres NAT+ANTHRO LAND USE INDIRECT ORGANIC C BLACK C miroc3.2 hires NAT+ANTHRO LAND USE INDIRECT ORGANIC C BLACK C ipsl cm4 ANTHRO inm cm3 NAT+ANTHRO iap fgoals ANTHRO giss e r NAT+ANTHRO LAND USE INDIRECT ORGANIC C BLACK C giss e h NAT+ANTHRO LAND USE INDIRECT ORGANIC C BLACK C giss aom ANTHRO gfdl cm2.1 NAT+ANTHRO LAND USE ORGANIC C BLACK C gfdl cm2.0 NAT+ANTHRO LAND USE ORGANIC C BLAC C csiro mk3 ANTHRO cnrm cm3 ANTHRO cccma cgcm3 ANTHRO BLACK C INDIRECT INDIRECT back