Understanding Sahel Rainfall Variability Progress and Challenges Michela
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 • 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)
The case for SST: 1. statistical association with African summer rainfall EOF 1: Sahel rainfall Associated SST Giannini et al. , 2003, 2005
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
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. Nicholson et al, 1998; Zeng et al. 1999 QTCM
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?
20 th Century (XX) Simulations all XX runs NASA/GISS GCMs
19 Coupled GCMs : Sahel XX-PI Rainfall Change
19 Coupled GCM : XX-PI Rainfall Change
19 Coupled GCM : XX-PI SST Change
19 Coupled GCM : XX-PI SST Change OBSERVED XX(1976 -2000)-PI(1876 -1900) Kaplan (LDEO) Reynolds (NASA/GISS) XX(1975 -2000)-PI(1880 -1900)
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. (q. V’): • 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.
XX-PI rich get richer poor get poorer? margins get dryer? thermal low deeper? (wrong sign!) India causes subsidence? (wrong sign!) …is it really GHG?
Sahel Rainfall Change due to GHG 16 Coupled GCM, 4 x and A 1 B simulation A 1 B-XX A 1 B-PI 4 x(@XX)-PI 4 x(@4 x)-PI
The response to GHG 4 x(yrs 50: 70)-PI Mean Rainfall Change Robustness of Rainfall Change Surface Temperature
Proposed mechanism for XX-PI Sahel rainfall changes Widespread warming (GHG) of the remote tropics (Indian, Pacific) warms the tropical troposphere and ups the ante for convection… the gradient in Atlantic warming (SO 2 4 ) causes dry advection [ (q. V’)] and prevents the Sahel from meeting the upped ante.
Widespread tropical warming related to drought NASA/GISS analysis of surface temp – linear trend 1950 -2000 Hansen et al. 1999 (J. Geophys. Res. ) regression of NASA/NSIPP 1 Sahel PC and sfc temp Giannini et al. 2003, 2005
Role of Indian and Pacific Lu and Delworth, in press (Geophys. Res. Lett. )
Proposed mechanism for XX-PI Sahel rainfall changes Widespread warming (GHG) of the remote Tropics (Indian, Pacific) warms the tropical troposphere and ups the ante for convection… the gradient in Atlantic warming (SO 2 4 ) causes dry advection [ (q. V’)] and prevents the Sahel from meeting the upped ante.
19 Coupled GCM : XX-PI Z 1000 Change
OBSERVED Hadley SLP Change XX(1975 -1998)-PI(1875 -1900) 19 Coupled GCM : XX-PI Z 1000 Change OBSERVED Kaplan SLP Change XX(1966 -1990) -PI(1866 -1890)
Proposed mechanism for XX-PI Sahel rainfall changes Widespread warming (GHG) of the remote tropics (Indian, Pacific) warms the tropical troposphere and ups the moisture ante for convection… the gradient in Atlantic warming (SO 2 4 ) causes dry advection [ (q. V’)] and prevents the Sahel from meeting the upped ante.
Natural or Anthropogenic? NASA/GISS
Natural or Anthropogenic?
Natural or Anthropogenic? natural forcings NH/SH Temperature Gradient: 1895 -1994 Trend Bragazza et al. , Climate Dynamics 2004
SULFATE AEROSOL FORCINGS (1850 -1997) Temp RESPONSE to SULFATE AEROSOL NASA/GISS
Forced Signal vs. Internal Noise
• 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 AR 4 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.
XXI century outlook for Sahel precipitation Courtesy of Isaac Held, Jian Lu, Tom Delworth et al, GFDL
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?
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)
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
19 Coupled GCM forced with XX Century Forcings (CO 2, sulfate aerosols, maybe more…): Sahel Rainfall Trend
standardized Sahel prcp anomalies observed & GFDL CM 2. 0 1 Coupled GCM forced with XX Century Forcings (CO 2, aerosols, solar variability and volcanic activity): 1870 1910 1950 1990 I. M. Held, T. L. Delworth, J. Lu, K. Findell, and T. R. Knutson, submitted to Nature
Simulated 1950 -2000 regression between African rainfall and SST
_______________ sun volc. aeros. _______________ N N S(D) Lau et al. , 2005 N N S(D) N? S(D) Y Y OC, BC, S(D) N N Y Y OC, BC, S(D, I) N N S(D? ) Y Y S(D? , I) Y Y OC, BC, S(D, I) N N S(D, I 1) Y Y S(D) Y Y BC, S(D) Y Y S(D) N N S(D, I) N N ? S(D) back
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