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Understanding Sahel Rainfall Variability: Progress and Challenges

Understanding Sahel Rainfall Variability: Progress and Challenges. Michela Biasutti biasutti@ldeo.columbia.edu in collaboration with Alessandra Giannini (IRI) and with input from Isaac Held (GFDL) and Adam Sobel (CU). Sahel. The Sahel drought: Historical hypotheses.

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Understanding Sahel Rainfall Variability: Progress and Challenges

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  1. Understanding Sahel Rainfall Variability: Progress and Challenges Michela Biasutti biasutti@ldeo.columbia.edu in collaboration with Alessandra Giannini (IRI) and with input from Isaac Held (GFDL) and Adam Sobel (CU)

  2. Sahel

  3. The Sahel drought: Historical hypotheses • Local forcing: (anthropogenic) deforestation and desertification cause enhanced albedo and subsidence. • Remote forcing: Sea Surface Temperature (SST) anomalies change the tropical circulation and rainfall. From JISAO’s website (T. Mitchell)

  4. The case for SST: 1. statistical association with African summer rainfall EOF1: Sahel rainfall Associated SST Giannini et al., 2003, 2005

  5. The case for SST: 2. AGCMs forced with the historical SST reproduce the African summer rainfall modes obs NSIPP amip ensemble Giannini et al., 2003, 2005

  6. The role of land surface processes: from forcing to feedback • Observations: - vegetation follows rain - albedo changes unclear • Model simulations: interactive surface processes and vegetation contribute to the strength and life span of rainfall anomalies. QTCM Nicholson et al, 1998; Zeng et al. 1999

  7. Current questions and challenges: • Is the long-term Sahel rainfall change (and associated SST variations) of natural or anthropogenic origin? Are the Sahel drought and SST anomalies reproduced in unforced runs? In runs forced with natural forcings? In runs forced by anthropogenic forcings? • How will Sahel rainfall change in the greenhouse future?

  8. 20th Century (XX) Simulations all XX runs all XX runs NASA/GISS GCMs

  9. 19 Coupled GCMs : Sahel XX-PI Rainfall Change

  10. 19 Coupled GCM : XX-PI Rainfall Change

  11. 19 Coupled GCM : XX-PI SST Change

  12. 19 Coupled GCM : XX-PI SST Change OBSERVED XX(1975-2000)-PI(1880-1900) XX(1976-2000)-PI(1876-1900) Kaplan (LDEO) Reynolds (NASA/GISS)

  13. What are the mechanisms of SST influence on Sahel rainfall? What do we expect from GHGs? • (q’V): • Uniform warming  rich get richer, poor get poorer. • (qV’): • Warmer over land  deeper thermal low  stronger monsoons. • Change in MOC  SST-induced change in surface wind. • ∂ /∂p : • Warmer underneath deep convective regions  warmer Troposphere  “upped ante”  margins get dryer. • Warmer Indian Ocean  more rain there  more subsidence over Northern Africa.

  14. XX-PI rich get richer poor get poorer? thermal low deeper? (wrong sign!) India causes subsidence? (wrong sign!) margins get dryer? …is it really GHG?

  15. Sahel Rainfall Change due to GHG 16 Coupled GCM, 4x and A1B simulation  A1B-XX  A1B-PI  4x(@XX)-PI  4x(@4x)-PI

  16. The response to GHG 4x(yrs50:70)-PI Surface Temperature Mean Rainfall Change Robustness of Rainfall Change

  17. Proposed mechanism for XX-PI Sahel rainfall changes Widespread warming (GHG) of the remote tropics(Indian, Pacific) warms the tropical troposphere andups the ante for convection… the gradient in Atlantic warming (SO24 ) causes dry advection [(qV’)] and prevents the Sahel from meeting the upped ante.

  18. regression of NASA/NSIPP1 Sahel PC and sfc temp Giannini et al. 2003, 2005 NASA/GISS analysis of surface temp – linear trend 1950-2000 Hansen et al. 1999 (J. Geophys. Res.) Widespread tropical warming related to drought

  19. Lu and Delworth, in press (Geophys. Res. Lett.) Role of Indian and Pacific

  20. Proposed mechanism for XX-PI Sahel rainfall changes Widespread warming (GHG) of the remote Tropics (Indian, Pacific)warms the tropical troposphere andups the ante for convection… the gradient in Atlantic warming (SO24 ) causes dry advection [(qV’)] and prevents the Sahel from meeting the upped ante.

  21. 19 Coupled GCM : XX-PI Z1000 Change

  22. OBSERVED Hadley SLP Change XX(1975-1998)-PI(1875-1900) 19 Coupled GCM : XX-PI Z1000 Change OBSERVED Kaplan SLP Change XX(1966-1990) -PI(1866-1890)

  23. Proposed mechanism for XX-PI Sahel rainfall changes Widespread warming (GHG) of the remote tropics (Indian, Pacific)warms the tropical troposphere andups the moisture ante for convection… the gradient in Atlantic warming (SO24) causes dry advection [(qV’)] and prevents the Sahel from meeting the upped ante.

  24. Natural or Anthropogenic? NASA/GISS

  25. Natural or Anthropogenic?

  26. Natural or Anthropogenic? natural forcings NH/SH Temperature Gradient: 1895-1994 Trend Bragazza et al., Climate Dynamics 2004

  27. SULFATE AEROSOL FORCINGS (1850-1997) Temp RESPONSE to SULFATE AEROSOL NASA/GISS

  28. Forced Signal vs. Internal Noise

  29. CONCLUSIONS • The observed African rainfall variability is forced by • global SST anomalies (likely reddened and amplified • by land processes). • XX-PI drying of the Sahel is reproduced by all IPCC AR4 models  it is (partly) anthropogenic!(But the 1950-1995 trend isn’t: partly due to the MOC?) • The robust signal is coming from the sulfate aerosol forcing: a gradient in the Atlantic SST affects the moisture flow into the Sahel. • The response to GHG increase alone is model-dependent.

  30. XXI century outlook for Sahel precipitation Courtesy of Isaac Held, Jian Lu, Tom Delworth et al, GFDL

  31. The remaining questions: Will the rich get richer? Yes, unless the indirect effects of aerosol win (Liepert et al., 2004). Does land behave like ocean? Will RH change? How does land equilibrate to warmer tropospheric temperatures? Other wild cards: do we trust the “El Nino”-like response of these models? Are missing feedbacks important?

  32. Understanding Sahel Rainfall Variability: Progress and Challenges Michela Biasutti biasutti@ldeo.columbia.edu in collaboration with Alessandra Giannini (IRI) and with input from Isaac Held (GFDL) and Adam Sobel (CU)

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

  34. 19 Coupled GCM forced with XX Century Forcings (CO2, sulfate aerosols, maybe more…): Sahel Rainfall Trend

  35. standardized Sahel prcp anomalies observed&GFDL CM2.0 1870 1910 1950 1990 1 Coupled GCM forced with XX Century Forcings (CO2, aerosols, solar variability and volcanic activity): I. M. Held, T. L. Delworth, J. Lu, K. Findell, and T. R. Knutson, submitted to Nature

  36. Simulated 1950-2000 regression between African rainfall and SST

  37. ______________________________ sun volc. aeros. ______________________________ N N S(D) N N S(D) N? N? S(D) Y Y OC,BC,S(D) Y Y OC,BC,S(D) N N S(D) Y Y OC,BC,S(D,I) Y Y OC,BC,S(D,I) N N S(D?) N N S(D?) Y Y S(D?,I) Y Y OC,BC,S(D,I) Y Y OC,BC,S(D,I) N N S(D,I1) Y Y S(D) Y Y BC,S(D) Y Y S(D) N N S(D,I) N N ? Lau et al., 2005 back

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