Climate sensitivity and Climate sensitivity and variability with models variability with models

extending into the middle extending into the middle atmosphere atmosphere

F. SassiF. SassiNational Center for Atmospheric National Center for Atmospheric

ResearchResearch

Climate and Global Dynamics Climate and Global Dynamics

The impact of the middle The impact of the middle atmosphere on tropospheric atmosphere on tropospheric climate:climate:The climate sensitivityThe climate sensitivity• Climate sensitivity (CS) is a broadly used tool to

determine the models response to different forcing (increase of GHGs, solar variability, aerosols, etc.)

• CS is defined as the equilibrium change of global surface temperature following a doubling of CO2.

• Gregory et al (2004) have defined a simple method to calculate CS based on linear regression analysis between the change of net heat flux at the TOA and the change of global surface temperature using an atmospheric model coupled to a SOM.

• This method has been used recently by Kiehl et al (2006) to calculate the CS of the CCSM3.

The impact of the middle The impact of the middle atmosphere on tropospheric atmosphere on tropospheric climate:climate:The simulationsThe simulations• Run WACCM and CAM in similar

configurations:– CAM3 + SOM, present day CO2– CAM3 + SOM, 2xCO2– WACCM3 + SOM, present day CO2, full chemistry– WACCM3 + SOM, 2xCO2, full chemistry

• Although by and large the physical parameterization that turn CAM into WACCM are relevant only above the stratopause, WACCM is not exactly identical to CAM:– Efficiency parameter for orographic gravity waves

CAM/ CS=2.2 WACCM/ CS=2.1

Climate Sensitivity

Surface Temperature ANN

Surface Albedo

ANN

• Both models predict an Arctic reduction of albedo ( surfarce warming less sea ice) in the doubled CO2 scenario.

• The Arctic changes are greater in CAM than in WACCM.

• These changes have a seasonal cycle (not shown), being more pronounced in DJF.

• Implications for ocean circulation?

DJF

Sea Level Pressure

• Both models predict a reduction of sea level pressure over the Arctic.

• As before, changes are larger and more ubiquitous in CAM than in WACCM.

• Note that sea level pressure over the north Atlantic is shallower in WACCM than in CAM.

1x CO2 Momentum

Forcing

DJF

Zonal Mean Zonal Wind

• As the strength of the polar vortex decreases, the sea level pressure is expected to decrease: stronger westward drag in the stratosphere stronger mean meridional circulation mass redistribution between polar and low latitudes.

• This similar to the Polvani and Kushner mechanism.

• The sea level pressure change WACCM – CAM poleward of 60N is ~ 230 Pa a total mass redistribution of ~ 8E14 Kg.

• About 50% of that mass change occurs in the stratosphere.

Redistribution of mass in the

vertical column

Approx. location of tropopause at high latitudes

DJF

Zonal Mean Temperature

• Both models produce an increase of temperature in the troposphere and a decrease in the lower stratosphere.

• Upper tropical tropospheric warming is larger in CAM.

• Tropical middle stratospheric cooling is larger in WACCM.

EPD Difference

NH Annular Modes

• Use daily data of geopotential interpolated to standard pressure levels.

• Take only data northward of 20N, zonally averaged.• Calculate the composite annual cycle and the

anomalies against it.• Obtain the leading zonal mean pattern (EOF-1) from

the WACCM 1x simulation. • Project the pattern on the time series ( PC).• Stratospheric influence on the troposphere:

– Calculate lag correlation of all points vrs 10 hPa and near surface

– Calculate composite of stratospheric weak and strong jet events.

Correlation with near surface events

Christiansen 2005

• Correlation with near surface events is amplified in CAM at lag zero: Near 1 hPa, the re-analysis show a correlation less than 0.1, while it is > 0.25 in CAM.

• WACCM is much closer to the re-analysis.

• Both models overestimate the tropospheric correlation.

ERA40

Correlation with stratospheric events

Christiansen 2005

ERA40

• Downward progression of stratospheric anomalies is quite similar between the two models in the stratosphere.

• In the troposphere, there is no downward progression in CAM/WACCM.

Composite of weak stratospheric vortex

events

• Both models show a relatively long persistence of the anomalies in the lower stratosphere. This is consistent with longer newtonian relaxation rate.

• The WACCM simulations show that tropospheric anomalies lead the stratospheric events. This is a realistic feature that is not reproduce in the CAM.

• Downward influence of stratospheric events on the troposphere wanes rapidly in CAM. WACCM suggests longer time scales.

CONCLUSIONS

• Globally averaged measures of climate Globally averaged measures of climate change are identical in both CAM and WACCM change are identical in both CAM and WACCM the presence of a well resolved middle the presence of a well resolved middle atmosphere is irrelevant to metrics like CS.atmosphere is irrelevant to metrics like CS.

• Regional metrics can be different in the two Regional metrics can be different in the two models: surface temperature, sea ice, sea models: surface temperature, sea ice, sea level pressure, zonal mean temperature. By level pressure, zonal mean temperature. By and large, the signatures of change are and large, the signatures of change are amplified in CAM compared to WACCM.amplified in CAM compared to WACCM.

• By and large, stratosphere-troposphere By and large, stratosphere-troposphere coupling is more realistic in WACCM than in coupling is more realistic in WACCM than in CAM.CAM.