Aerosol, Interhemispheric Gradient, and Climate Sensitivity Ching-Yee Chang Department of Geography University of California Berkeley Lawrence Livermore National Lab Seminar April 27, 2011 Collaborators: John Chiang (UC Berkeley) Michael Wehner (Lawrence Berkeley Lab)
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Aerosol, Interhemispheric Gradient, and Climate Sensitivity Ching-Yee Chang Department of Geography University of California Berkeley Lawrence Livermore.
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Aerosol, Interhemispheric Gradient, and Climate Sensitivity
Ching-Yee ChangDepartment of Geography
University of California Berkeley
Lawrence Livermore National Lab Seminar April 27, 2011
Collaborators: John Chiang (UC Berkeley)
Michael Wehner (Lawrence Berkeley Lab)
Sulfate Aerosols and Climate
Difference in SAT caused by sulfate aerosol indirect effect (Rotstayn & Lohmann 2002)
Direct forcing from anthropogenic sulfate forcing (Kiehl & Briegleb 1993)
IPCC AR4
Large uncertainty in the aerosol forcing
Kiehl 2007
IPCC AR4
Outline
• Sulfate aerosol control of Tropical Atlantic climate over the 20th century (Chang et al. 2011, in press for Journal of Climate)
• Projection of the Interhemispheric Gradient in 21st century
• Interhemispheric Gradient and Transient Climate Response
Sulfate aerosol control of Tropical Atlantic climate over the 20th century
Atlantic interhemispheric SST gradient over the 20th century
Interhemispheric SST index = South – North box
21-yr running mean on annual mean data
Hadley SST, ERSST, Kaplan SST
HADISSTERSST KAPLAN SST
Ensemble Empirical Mode Decomposition (EEMD)
Modes from Ensemble Empirical Mode Decomposition (EEMD) analysis:
Multidecadal (mode 4) and trend (mode 5)
HADISSTERSST KAPLAN SST
The interhemispheric SST gradient and the meridional position of ITCZ
Mode 1 of an MCA of SSTA and 10m winds, and regression of the SSTA mode 1 time series on precipitation (Chiang and Vimont, 2004)
Equatorial meridional winds
ICOADS Winds
Smith et al. 2010: Reconstructed global precip
CRU TS 2.1 land precip.
south-north
June-July-August precipitation
CMIP3 models simulation of Atlantic Interhemispheric Gradient in 20th century climate experiment
Variance explained by this EOF: 49%
• Most of the projection coefficient are positive (Most models have a upward trend in the SSTA
gradient indices)
Projection of EOF1 onto each run
1st EOF of AITG indices from 71 model runs
South Hemisphere warming more than North HemisphereThis trend mitigates after 1980
T-test value=4.41 p-value = 0.00001 (assuming 71 d.o.f. for the 20th
century runs, and 44 for the preindustrial)
Mean of 1900-1982 trend in SSTA gradient significantly different from preindustrial run
Unit: 0.1K/100yr
Regression of SSTA onto model-index EOF1
Hadley SSTA
Stronger warming in the South Atlantic
Model ensemble averaged SSTA
Southward shift of ITCZ
Regression of Precip. Anomaly onto model-index EOF1
Model ensemble averaged Precip. anomalyCRU Precip. anomaly
Attribution the cause of the trend
Trend behavior appears in model ensemble mean
Most likely to be externally forced Single forcing runs
Most models project negative trend in the Pacific interhemispheric gradient – the rate of the warming in the north Pacific speeds up at the end of 21st century
Possibly related to the decrease of the aerosols in the north Pacific in the future, but GHG forcing or other factors may also contribute
Summary II
Interhemispheric Gradient and Climate sensitivity
Trend Statistic from different subsets of models
AIE models simulate the AITG trend closer to observation
AIE models: Models with both Aerosol Direct and Indirect Effect
No-AIE models: Models with only aerosol direct, but no Indirect Effect
X-axis unit: 0.1K/100yr
Kiehl 2007:• Total forcing inversely correlated
to climate sensitivity• Large uncertainty in the aerosol
forcing
Equilibrium Climate sensitivity and Transient Climate Response
Equilibrium Climate Sensitivity
IPCC AR4 Table 8.2
Transient Climate Response
300 ppm
600 ppmCO2 concentration
+ Slab ocean
AGCM
T0
T’
CO2 concentration
AGCM + OGCM300 ppm
600 ppm
T0
T’
Climate sensitivity and Transient Climate Response
IPCC AR4 Table 8.2
1.5 2 2.5 3 3.5 4 4.5 50
0.5
1
1.5
2
2.5
3
Equilibrium Climate Sensitivity (C )Tr
ansi
ent C
limat
e Re
spon
se (C
)
Atlantic SSTA Grad. Trend v.s. Climate Sensitivity
• There seems to be a linear relationship between the gradient trend and the Transient Climate Response (TCR) among most of the models