North Atlantic SST and Sahel
rainfall
Workshop on Sahel Climate Change
March 19-21, 2007
Mixed signals from idealized AGM
simulation studies
Hoerling et al. 2006
(ARPEGE model)
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JAS SSTA regression onto JJASO
Sahel index, 1950-2002
Lu and Delworth 2005
(GFDL AM2)
Bader and Latif 2003
(ECHAM4)
North Atlantic (50N-70N)
JAS SSTA (r = 0.35)
Sahel JJASO index
(Mitchell)
Tropical North Atlantic
(5N-35N) JAS SSTA (r =
0.3)
Tropical South Atlantic
(25S-5N) JAS SSTA (r =
0.14)
North Atlantic (50N-70N)
JAS SSTA
Sahel JJASO index
(Mitchell)
Tropical Atlantic ‘Dipole’ JAS
(r = 0.37)
Tropical Indian ocean JAS
(sign reversed; r = -0.24)
Interannual-decadal variations ‘tripole’ SSTA driven by atmosphere
Seager et al, 2000:
MCA between SST and NCEP
winds, JFM 1958-1998
Atlantic Interdecadal Variations
Atlantic Multidecadal
Oscillation (AMO) index
(top)
Regression of AMO on
detrended surface
temperature data (bottom)
(source: Sutton and Hodson)
Atlantic Interdecadal Variations
phase
0
60
120
180
Knight et al. 2005: AMO-like
behavior in the HadCM3 linked to
natural variations in the Atlantic
meridional overturning variations.
Global impacts of AMO
(including Sahel rainfall)
• Knight et al. 2006 - analysis of AMO impacts
in a 1400 year HadCM3 simulation
• Zhang and Delworth 2006 - imposed Atlantic
q-flux forcing on an otherwise fully coupled
model (GFDL CM2.1) to mimic AMO over the
20th century
AMO interpretation challenged (for the
tropical North Atlantic (“MDR region”,
August-October)
Global SST contribution
G(t)
Blue: solid - tropical Atl
ASO SSTA, decadally
smoothed; dashed is the
single parameter fit to the
global SST contribution
Aerosol contribution
S(t)
T t   Gt   St   Rt 
(Mann and Emanuel 2006)
20th century simulations &
aerosols
Late 20th century minus preindustrial JJA anomalies
(Biasutti and Giannini 2006)
Aerosol forcing on Sahel proposed
by Rotstayn and Lohmann 2002
Sahel and abrupt climate
change in the North Atlantic
SAT anomaly
JJA Precip anomaly
Dahl et al. 2005 (GFDL R30 coupled model)
Also:
Southward shift of Tropical Atlantic ITCZ (many studies)
Weaker Indian monsoon: Zhang and Delworth (2005)
Global adjustment to Atlantic thermohaline
slowdown
• CCSM3.0, present day basic state
• 6 member ensemble
• Freshwater pulse applied at Nov of year 0, by
freshening the upper 970m of the North Atlantic and
Arctic Oceans 55N-90N by an average of 2psu
• Surface temp (shaded) and precipitation (0.4mm/d
contour interval) anomalies
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
(CI 0.4mm/d)
JAS anomalies years 4-9
SST and rainfall
(CI 0.4mm/d)
SLP and 1000mb winds
JAS anomalies years 4-9
SLP and 1000mb winds
Sahara JAS air temp and slp
approximately follow Sahel rainfall trends
(noted by Haarsma et al 2005)
NCEP 2m air temp
NCEP slp
MSU TMT
But, changing air temperature
has a strong TOA radiative effect
under clear-sky conditions
Clear-sky OLR increases with
increasing air temperature
Clear-sky radiative
calculations for
different RH
Slope for 25% RH
~2.5 to 3 Wm-2/K
T(0) is the low-level air
temperatures
(from Pierrehumbert 1995)
Sahara JAS OLR anomalies linked to air
temperature anomalies
NCEP 2m air temp
NCEP slp
NCEP TOA net
Observed OLR
Dynamical connection?
Sahel -> Sahara
Gill solution to Sahel-like heating gives
maximum subsidence directly north of it
Sahara -> Sahel
A cooler Sahara results in reduced OLR,
requiring less atmospheric energy import
Thanks
Total JJASO 1898-2002 (correlation)
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Total JJASO 1898-2002 (regression)
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Sahel JJASO correlated with 1950-2002
SSTA (Kaplan)
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Haarsma et al. (2005)
NCEP dry - wet composites
• The following are composites based on
JAS 1950-69 (wet) and JAS 1970-89
(dry).
• Zonal averages from 15W-20E
NCEP vvel x 100
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NCEP u
NCEP phi
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NCEP [vq]bar * 1000
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NCEP T
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NCEP relative humidity
change
Mean relative humidity
Gill divergent solution to ‘Sahellike’ precipitation decrease
x,y length
scale ~ 10
degrees
Indian ocean JAS SSTA 3090E, 30S-30N (sign reversed)
anomalies
Detrended
anomalies
Haarsma et al. (2005)
Regression of NCEP SAT on “Sahara” MSLP timeseries
(derived from an MCA analysis of MSLP and precipitation)
Dipole
TNA
TSA
North Atlantic JAS SSTA (60W20W, 50N-70N, area weighted)
K
Sahel
JJASO
index
Sahel JAS precip under
AMOC slowdown
Sahel JAS precip (15W-25E,14N-19N) for years 1-8 after hosing
Differences for years 2, 4 and after are significant at 5% level
How does the Atlantic SST
influence the Sahel?
“Interhemispheric
gradient” (e.g. Folland et
al. 1986)
Is it similar to the
tropical Atlantic dipole
SST influence on the
marine ITCZ?
Interannual variations - ‘tripole’
SSTA driven by atmosphere
(Seager et al. 2000)
Interdecadal Atlantic SST
variations are linked to Sahel
North Atlantic JAS SSTA
Sahel JJASO index
(Mitchell)
Tropical Atlantic ‘Dipole’ JAS SSTA
Relationship between Sahara
surface pressure and Sahel rainfall
Haarsma et al. (2005)
- surface pressure
gradient variations
control surface
moisture transports
20th century simulations &
aerosols
Sulfate column burden,
present-day minus
preindustrial
Rotstayn and Lohmann 2002
Model annual mean
surface air temperature
difference due to
increased sulfate