the atmospheric circulation response to climate change-like thermal forcings in a simple gcm amy h....
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The Atmospheric Circulation Response to
Climate Change-like Thermal Forcings in a Simple GCM
Amy H. Butler1, David W.J. Thompson2, & Ross Heikes2
1Climate Prediction Center/NOAA, 2Colorado State University
Climate Prediction and Diagnostics WorkshopMonterey, CA
October 26, 2009
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Observed Changes in the Atmospheric Circulation
•Poleward shift of the storm tracks/mid-latitude jets
•Robust shift in SH storm track
•Weaker shift in NH storm track
•Associated with significant changes in surface climate
[e.g., Hurrell, 1995; Thompson et al., 2000; Thompson and Solomon, 2002; Ostermeier and Wallace, 2003; Liu et al., 2007]
•Widening of the Hadley Cell by 2-5° latitude over last 25 yrs[e.g., Hudson et al., 2006; Fu et al., 2006; Seidel and Randel, 2007; Seidel et al., 2008]
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Forced with Past Changes:
•in CO2: Little shift in storm tracks; expansion of Hadley cell but weaker than observed
•in Ozone: Robust poleward shift in SH storm track; expansion of Hadley cell but weaker than observed
Forced with Future Changes:
•in CO2: Poleward shift in both NH and SH storm tracks; expansion of Hadley cell
•in Ozone: recovery leads to equatorward shift in SH storm track
[e.g., Shindell et al., 1999; Fyfe et al., 1999; Kushner et al., 2001; Gillett and Thompson, 2003; Shindell and Schmidt, 2004; Brandefelt and Kallen, 2005; Yin, 2005; Miller et al., 2006; Arblaster and Meehl, 2006; Lorenz and DeWeaver, 2007; Son et al., 2008]
Simulated Changes in the Atmospheric Circulation
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Changing Temperatures
• Warmer troposphere, colder stratosphere
• Stronger warming in tropical troposphere
• Stronger warming in Arctic
• Strong seasonal polar cooling (but ozone recovery expected ~2065)
IPCC AR4
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Simple Dry Dynamical GCM
• CSU dynamical core
• Held-Suarez parameterizations
• No topography
• Equinoctal conditions
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Steady-State Experiments
The time-mean average of the 5 years following 1 year of spin-up
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Tropical Tropospheric Heating
Forcing:
K*m
/s
m2/s
2
K/day
Temp/Eddy Heat Flux Response Wind/Eddy Mom Flux Response
K*m
/s
Temp/Eddy Heat Flux Response
m2/s
2
Wind/Eddy Mom Flux Response
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Tropical Tropospheric Heating
Tropical warming alone produces: poleward shifts in jets and expanded/weakened Hadley cell. In our model, also produces
weakened Brewer-Dobson Circulation (BDC).
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Forcing Temp Response Wind Response
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Forcing Temp Response Wind Response
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Forcing Temp Response Wind Response
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Polar Stratospheric Cooling
Forcing:
K/day
K*m
/s
Temp/Eddy Heat Flux Response Wind/Eddy Mom Flux Response
m2/s
2
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Polar Surface Warming
Forcing:
m2/s
2
K/day
Temp/Eddy Heat Flux Response
K*m
/s
Wind/Eddy Mom Flux Response
m2/s
2
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Combination
Forcing:
m2/s
2
K/day
Temp/Eddy Heat Flux Response
K*m
/s
Wind/Eddy Mom Flux Response
m2/s
2
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Conclusions from Steady-State Runs• Heating the tropical troposphere leads to a
poleward shift of the storm tracks, an expansion of the Hadley cell, and a weakened Brewer-Dobson circulation
• Cooling in the lower polar stratosphere shifts the tropospheric jet polewards, but the response is sensitive to the level of the forcing
• Warming at the Arctic surface may play a role in the predicted weaker poleward shift of the NH storm track
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Transient Experiments
12 ensemble members, each 150 days long (6 hour output), with initial conditions taken from
control run 50 days apart. Thermal forcing turned on: day 10.
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Transient Ensemble Simulationof Tropical Tropospheric Heating
Tem
p: E
quat
orW
ind: 60NT
emp:
Pol
eW
ind: 40N
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Mechanisms for Tropospheric Circulation Changes
A. Changes in meridional temperature gradient
At upper levels: changes in eddy phase speeds [Chen and Held 2007; Chen et al. 2007]
At lower levels: changes in low-level baroclinicity and eddy generation [Yin 2005]
B. Changes in vertical temperature gradient Changes in static stability [Frierson 2008; Lu et al. 2008]
Changes in tropopause height [Lorenz and DeWeaver 2007]
Current research involves analyzing the transient runs in the context of these mechanisms
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Thanks!