1. Sahel Rainfall change in the future? Papers review

2. References  Giannini et al., 2008: A climate model-based review of drought in the Sahel: Desertification, the re-greening and climate change  Cook, 2008: The mysteries of Sahel droughts  Biasutti et al., 2009: The role of Sahara low in summertime rainfall variability and change in CMIP3 models  Nicholson, 2009: A revised picture of the structure of the ”monsoon” and land ITCZ over West Africa

3. “ Sahel drought “  Phenomenon: drying from 1960 recovery from 1980 (disputable)  Different projections between GCM models under GHGs forcing

4. Current motivation of the study A Open Question: What will the future precipitation be in the climate change? What contributes to the different simulations of the model? → required more understanding of potential mechanisms involved to drive Sahel rainfall (and its variability)

5. Early studies  Human land-use practice (Charney, 1975)  Global SSTs : (1) Atlantic interhemespheric gradient (atmosphere-ocean feedback to shift ITCZ) (2) Tropical Pacific (ENSO warming whole tropics) (3) Tropical Indian (deep convections shift, Chou) → Ocean forcing intensified by land-atmosphere feedbacks

6. Ocean forcing ?  SST and rainfall relationship hold for past/present but not future by statistics study → future change may control by (1) nonlinearity and details of SST pattern (2) other mechanisms

7. Other mechanisms ? (1) Biasutti: (Models) “West African Monsoon“ → land-sea thermal contrast Sahara heat low (3D structure) (2) Nicholson (Obs.& Reanalysis) “Local up-level circulation” → African Westerly Jet

10. Biasutti et al., 2009  but what causes variability of SHL?

11. Nicholson 2009, 2007  Decouple the land ITCZ and rainbelt  Features in the upper-level circulation override the surface factors

13. Nicholson 2009, 2007  Moisture source: locally recycled rather than come from Atlantic

14. Nicholson 2009, 2007  Identified 4 rainfall patterns  Mechanism controlling the location of AEJ?

15. Nicholson 2009, 2007  Bimodal state of WAM: strengthen of AWJ

16. Nicholson 2009, 2007  Development of AWJ  Strong cross-equatorial pressure gradient  Inertial instability

17. Nicholson 2009, 2007  Strong cross-equatorial pressure gradient caused by surface temperature gradient → what causes the variability of AWJ?