Climate Change in the Sahel
Michela Biasutti
biasutti@ldeo.columbia.edu
in collaboration with :
Alessandra Giannini, Adam Sobel, Isaac Held
Observed, Annual Mean, Continental Scale
Rainfall Variability
Also see, e.g. Nicholson 1986 (J. Clim. App. Met.)
The Sahel
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http://www.pbs.org/wnet/africa/explore/sahel/sahel_overview.html
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Sahel variability and relation to SST:
OBSERVATIONS
EOF1: Sahel rainfall
Giannini et al., 2003, 2005
Associated SST
Sahel variability and relation to SST:
ATMOSPHERIC GCMs
Giannini et al.
Current questions and challenges:
• Was the Sahel drought (and associated SST variations) of
natural or anthropogenic origin?
Are the 20th century Sahel drought and SST anomalies
consistent with internal climate variability? Are they
reproduced in forced runs? In runs forced with natural
forcings? In runs forced by anthropogenic forcings?
• How will Sahel rainfall change in the greenhouse future?
IPCC Simulations
PI
Pre-Industrial
Control (PI)
NASA/GISS
XX
20th Century
Simulation (XX)
A1B
Global Warming
Scenario (A1B)
GCMs
The forced component:
Sahel XX-PI Rainfall Change
Biasutti and Giannini, 2006
XX-PI Rainfall Change
XX-PI SST Change
60 XX Simulations
Importance of Internal Noise
 1950-1985 Trend
 1950-1999 Trend
 1930-1999 Trend
something missing?
The role of land surface feedback:
OBS
atmos+ocean
QTCM
atmos+ocean
+land
atmos+ocean
+land
+vegetation
Zeng et al. 1999
and dust?
Internal Noise vs. Forced Signal
Effect of GHG
4x(yrs50:70)-PI
Mean Rainfall Change
Robustness of
Rainfall Change
20
Surface Temperature
Effect of Reflective Aerosols
SULFATE AEROSOL FORCINGS (1850-1997)
Temp RESPONSE
Precip RESPONSE
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NASA/GISS
ROTSTAYN AND LOHMANN ‘02
Some Conclusions
• 20th Century drying of the Sahel is reproduced by almost
all IPCC AR4 models  it is (partly) externally forced. (But
natural, internal variability is substantial.)
• The forcing was anthropogenic, with the most robust(*)
signal coming from the sulfate aerosol forcing.
• The response to GHG increase alone is inconsistent
across models, which implies an uncertain outlook for the
Sahel.
IPCC Scenarios for the 21st Century
IPCC 2001
Precipitation Response in the Sahel
GFDL
What are the possible
causes of discrepancy?
Is it SST?:
 different SST anomalies?
different sensitivity to same SST anomalies?
Is it something else?
 direct GHG influence?
Relationship of Sahel rainfall & SST (pre-industrial, not forced)
Biasutti et al., 2007
goodness of model
PI
(training run)
Linear Multi-Regressive
Model:
from SST
(Indo-Pacific & Atlantic Gradient)
to Sahel Rainfall
XX
A1B
 interannual (=detrended)
2000-2100 Trends
Sahel Rainfall
Indo-Pacific SST
Atlantic SST Gradient
goodness of model
PI
XX
Linear Multi-Regressive
Model
trained on (detrended) PI:
from SST
(Indo-Pacific & Atlantic Gradient)
to Sahel Rainfall
nb: same results if
NTA & STA are used
(3 predictors).
A1B
 interannual
 interannual + trend
goodness of model
XX
Linear Multi-Regressive
Model
trained on (unfiltered) XX:
from SST
(Indo-Pacific & Atlantic Gradient)
to Sahel Rainfall
 interannual
A1B
 interannual + trend
Conclusions
• ~30%(?) of 20th Century drying of the Sahel was externally
forced. The forcing was anthropogenic, with the most
robust signal coming from the sulfate aerosol forcing.
• In the 21st Century, when GHG are the dominant forcing,
the Sahel response is inconsistent across models.
• Global SST changes can explain the 20th Century trend,
but, in most models, not the 21st Century one (at least not
through the same mechanisms active in the past).
• A model’s good representation of the past is no indication
of a trustworthy prediction of the future. How can we
reduce the uncertainty of our climate outlook?
Current Direction:
Explore the role of land/sea contrast
• Strengthened paleo monsoons follow orbitally-forced
increases in seasonality & land/sea contrast.
• GHGs force an enhanced contrast. Should we expect
enhanced monsoons?
Model ‘Composite’ on Sahel response to GHG
1%to4xCO2 and A1B simulation
ncar
wet models
miroc
dry models
gfdl
 A1B-XX
 A1B-PI
 4x(@XX)-PI
 4x(@4x)-PI
Dry/Wet Composites:
(20752100) - (19752000)
Dry/Wet Composites:
(20752100) - (19752000)
Sahara Low & Sahel Rainfall
Sahara Low =
= Tropical Z850 - Sahara Z850
 interannual
 interannual + trend
What controls the Sahara Low?
•Is it a heat low?
•Is it dynamically influenced from afar?
•Is it an effect, instead of a cause, of enhanced Sahel rain?
extra slides
Heat Low?
Baroclinic variability in
Sahara Z850
sfc
850m
b
500mb
200mb
Other Influences?
Barotropic variability in
Sahara Z850
sfc
500mb
200mb
Possible Influences on Sahara Low
Sahel
Giannini et al., 2003
19 Coupled GCM :
XX-PI SST Change
OBSERVED
NASA/GISS analysis of surface temp – linear trend 1950-2000
Hansen et al. 1999 (J. Geophys. Res.)
XX(1975-2000)-PI(1880-1900)
Natural or Anthropogenic?
natural & anthro
anthropogenic
Natural or Anthropogenic?
Observed Tsfc anomalies due to the Pinatubo Eruption
Robock, Rev. Geophys., 2000
Composite of Volcanic Years (1975-1999 period) in Models with Volcanism
10-year Running Average of Aug-Oct NH Surface T and MDR
SST
Best Fit Linear Combination of Global Warming and
Aerosol Forcing (red) versus Tropical Atlantic SST
(blue)
MDR SST
Global mean T
+ aerosol forcing
Mann and Emanuel (2006)
Role of Indian and Pacific
Lu and Delworth, 2005
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Changes in Sahel/SST Regression Coefficients
Interannual Only
PI
Interannual & Trend
XX
A1B
Dry/Wet Composites:
(20752100) - (19752000)
Dry/Wet Composites:
(20752100) - (19752000)
CGCM experiments for IPCC AR4
1% /yr to 4 times pre-industrial CO2
(4x)
stabilization
@ 720ppm
(A1B)
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20th Century
(XX)
Pre-Industrial
(PI)
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20th Century Forcings in XX runs
ukmo hadgem1
ANTHRO
INDIRECT
ORGANIC C
BLACK C
ukmo hadcm3
ANTHRO
INDIRECT
ncar pcm1
NAT+ANTHRO
ncar ccsm3
NAT+ANTHRO
mri cgcm2
NAT+ANTHRO
mpi echam5
ANTHRO
miroc3.2 medres
NAT+ANTHRO
LAND USE
INDIRECT
ORGANIC C
BLACK C
miroc3.2 hires
NAT+ANTHRO
LAND USE
INDIRECT
ORGANIC C
BLACK C
ipsl cm4
ANTHRO
inm cm3
NAT+ANTHRO
iap fgoals
ANTHRO
giss e r
NAT+ANTHRO
LAND USE
INDIRECT
ORGANIC C
BLACK C
giss e h
NAT+ANTHRO
LAND USE
INDIRECT
ORGANIC C
BLACK C
giss aom
ANTHRO
gfdl cm2.1
NAT+ANTHRO
LAND USE
ORGANIC C
BLACK C
gfdl cm2.0
NAT+ANTHRO
LAND USE
ORGANIC C
BLAC C
csiro mk3
ANTHRO
cnrm cm3
ANTHRO
cccma cgcm3
ANTHRO
BLACK C
INDIRECT
INDIRECT
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