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

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]

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

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

Simple Dry Dynamical GCM

• CSU dynamical core

• Held-Suarez parameterizations

• No topography

• Equinoctal conditions

Steady-State Experiments

The time-mean average of the 5 years following 1 year of spin-up

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

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).

Forcing Temp Response Wind Response

Forcing Temp Response Wind Response

Forcing Temp Response Wind Response

Polar Stratospheric Cooling

Forcing:

K/day

K*m

/s

Temp/Eddy Heat Flux Response Wind/Eddy Mom Flux Response

m2/s

2

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

Combination

Forcing:

m2/s

2

K/day

Temp/Eddy Heat Flux Response

K*m

/s

Wind/Eddy Mom Flux Response

m2/s

2

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

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.

Transient Ensemble Simulationof Tropical Tropospheric Heating

Tem

p: E

quat

orW

ind: 60NT

emp:

Pol

eW

ind: 40N

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

Thanks!