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

Giannini et al., 2003, 2005

EOF1: Sahel rainfall

Associated SST

The case for SST: 2. AGCMs forced with the historical SST reproduce

the African summer rainfall modes

Giannini et al., 2003, 2005

obs

NSIPPamip ensemble

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?

NASA/GISS

all XX runs

all XX runs

20th Century (XX) Simulations

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)

XX(1975-2000)-PI(1880-1900)

Reynolds (NASA/GISS)

(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.

What are the mechanisms of SST influence on Sahel rainfall?What do we expect from GHGs?

rich get richerpoor get poorer?

margins get dryer?

thermal low deeper?(wrong sign!)

India causes subsidence?(wrong sign!)

XX-PI

…is it really GHG?

Sahel Rainfall Change due to GHG16 Coupled GCM, 4x and A1B simulation

A1B-XX A1B-PI

4x(@XX)-PI 4x(@4x)-PI

The response to GHG4x(yrs50:70)-PI

Mean Rainfall Change

Robustness of Rainfall Change

Surface Temperature

Widespread warming (GHG) of the remote tropics (Indian, Pacific) warms the tropical troposphere and ups the ante for convection… the gradient in Atlantic warming (SO24 ) causes dry advection [(qV’)] and prevents the Sahel from meeting the upped ante.

Proposed mechanism for XX-PI Sahel rainfall changes

NASA/GISS analysis of surface temp – linear trend 1950-2000Hansen et al. 1999 (J. Geophys. Res.)

regression of NASA/NSIPP1 Sahel PC and sfc tempGiannini et al. 2003, 2005

Widespread tropical warming related to drought

Lu and Delworth, in press (Geophys. Res. Lett.)

Role of Indian and Pacific

Widespread warming (GHG) of the remote Tropics (Indian, Pacific) warms the tropical troposphere and ups the ante for convection… the gradient in Atlantic warming (SO24 ) causes dry advection [(qV’)] and prevents the Sahel from meeting the upped ante.

Proposed mechanism for XX-PI Sahel rainfall changes

19 Coupled GCM :XX-PI Z1000 Change

OBSERVED Kaplan SLP ChangeXX(1966-1990) -PI(1866-1890)

OBSERVED Hadley SLP ChangeXX(1975-1998)-PI(1875-1900)

19 Coupled GCM :

XX-PI Z1000 Change

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 (SO24 ) causes dry advection [(qV’)] and prevents the Sahel from meeting the upped ante.

Proposed mechanism for XX-PI Sahel rainfall changes

NASA/GISS

Natural or Anthropogenic?

Natural or Anthropogenic?

Bragazza et al., Climate Dynamics 2004

NH/SHTemperature Gradient:1895-1994 Trend

natural forcings

Natural or Anthropogenic?

NASA/GISS

SULFATE AEROSOL FORCINGS (1850-1997)

Temp RESPONSE to SULFATE AEROSOL

Forced Signal vs. Internal Noise

• The observed African rainfall variability is forced byglobal SST anomalies (likely reddened and amplifiedby 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.

Courtesy of Isaac Held, Jian Lu, Tom Delworth et al, GFDL

XXI century outlook for Sahel precipitation

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?

The remaining questions:

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)

Observed SST anomalies due to the Pinatubo Eruption

Robock, Rev. Geophys., 2000

Natural or Anthropogenic?

Composite of Volcanic Years (1975-1999 period) in Models with Volcanism

19 Coupled GCM forced with XX Century Forcings (CO2, sulfate aerosols, maybe more…):

Sahel Rainfall Trend

1 Coupled GCM forced with XX Century Forcings (CO2, aerosols, solar variability and volcanic activity):

1950 199019101870stan

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Sah

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I. M. Held, T. L. Delworth, J. Lu, K. Findell, and T. R. Knutson, submitted to Nature

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Simulated 1950-2000 regression between African rainfall and SST

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

______________________________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