1. The Great 20 th Century Drying of Africa Ninth Annual CCSM Workshop Climate Variability Working Group 9 July 2004, Santa Fe Jim Hurrell, Marty Hoerling, Jon Eischeid Adam Phillips, Taiyi Xu, Gary Bates Climate Analysis Section, NCAR Climate Diagnostics Center, NOAA

2. Motivation Increasing evidence of the controlling effect of global SST variations on regional climate change over the 20 th Century “On the cause of the 1930s Dust Bowl” (Schubert, Suarez, Pegion, Koster, Bacmeister: Science 2004) “The Perfect Ocean for Drought” (Hoerling and Kumar: Science 2003) “Tropical Origins for Recent North Atlantic Climate Change” (Hurrell, Hoerling, Phillips, Bates, Xu: Climate Dynamics: 2004) “Oceanic Forcing of Sahel Rainfall …” (Giannini, Saravanan, Chang: Science: 2003) Others …

3. Atmospheric GCM Integrations Monthly evolving global SST and sea ice (fixed solar insolation, aerosols and trace-gas composition) 80 members: NCAR (CAM2) (15) ARPEGE (3) (8) NASA (NSIPP-1) (23) ECHAM (3) (10) ECHAM (4.5) (24)

4. Coupled Integrations Unforced Control CCSM2: Last 650 of 1000 years

5. Coupled Integrations Unforced Control CCSM2: Last 650 of 1000 years Forced (“Dutch Challenge Experiment”) CCSM1.4: 62 member ensemble 1940-2000: Prescribed GHG, sulfate aerosols, solar radiation, volcanic aerosols 2001-2080: Business as usual GHG

6. NCAR CCM3 First Harmonic Observed Seasonal Cycle of Mean African Rainfall January July

7. Multi-AGCM Observed First Harmonic Seasonal Cycle of Mean African Rainfall January July

8. Seasonal Cycle of 50-yr Seasonal Rainfall Trends (1950-1999) Observed AGCM Simulated (3-month overlapping seasons)

9. Seasonal Cycle of 50-yr Seasonal Rainfall Trends (1950-1999) Observed AGCM Simulated FMA JAS

10. Time Series of Regional African Rainfall (1950-1999) South Africa: FMASahel: JAS Observed AGCM simulated

11. PDF of 50-yr Seasonal Rainfall Trends (1950-1999) Observed AGCM simulated Unforced CCSM control South Africa: FMA Sahel: JAS

12. PDF of 50-yr Seasonal Rainfall Trends (1950-1999) Observed Forced CCSM South Africa: FMASahel: JAS

13. Preliminary Conclusions African drying since 1950 has followed the seasonal migration of monsoon rains The drying has been attributable to the temporal evolution of global SSTs Sub-Saharan drought has been strongly driven by the warming of the South Atlantic relative to the North Atlantic Ocean The progressive warming of the Indian Ocean has played a major role in the austral summer drought over southern Africa Experiments performed to date do not sample all SST influences known to be important for African rainfall variability Coupled climate models do not produce 50-year drying trends as large as observed