How Does the Stratification and Circulation in the Mdilatitude Respond to an Increase in Atmospheric Temperature

As the atmosphere warms, its water vapor content increases substantially, in agreement with the Clausius-Clapeyron relationship. Such increase in humidity affects the dynamics of convective motions and midlatitudes storm systems which both play a key role in determining the temperature and wind structure of the atmosphere. In the Tropics, there is a strong consensus that an increase in water content would lead to a stronger stratification and a weakening of the meridional circulation (Held and Sobel, 2006). In contrast, there is much less agreement for the changes in the extratropical atmosphere and stormtracks. For instance, while Frierson (2008) shows that the extratropical-stratification in an idealized model followed the moist baroclinic adjustment introduced by Juckes (2000), Schneider and O’Gorman (2008) obtained the opposite conclusion based on a very similar set of numerical simulations. In this presentation, I will show how this discrepancy can be reconciled and analyze the impacts of a temperature increase on the extra-tropical circulation and stratification.

The atmospheric response to different solar forcing and greenhouse gas concentration is simulated using an idealized general circulation model similar to that of Frierson (2008) and and Schneider and O’Gorman (2008). Changes are assessed for the tropopause and for the meridional circulation on dry and moist isentropes. It is found that, in warm climates, the tropopause is decoupled from the dry isentropic circulation in that the later becomes increasingly confined to the lower troposphere. In contrast, there is a strong connection between the poleward branch of the circulation on moist isentropes and the tropopause. From a physical point of view, this implies that, in warm climates, the extratropical tropopause is determined by the value of the equivalent potential temperature of the subtropical warm moist air that feeds the midlatitude stormtracks.

Frierson, DMW, 2008: Midlatitude Static Stability in Simple and Comprehensive General Circulation Models, J. Atmos. Sci, 65, 1049-1062.

Held, I.M., and B.J. Sobel, 2006: Robust Responses of the Hydrological Cycle to Global Warming. J. Climate. 19, 4686-5699.

Juckes, M. N., 2000: The static stability of the midlatitude troposphere: The relevance of moisture. J. Atmos. Sci., 57, 3050–3057.

 Schneider, T. and P. O’Gorman, 2008: Moist Convection and the Thermal Stratification of the Extratropical Troposphere. J. Atmos. Sci., 65, 3571-3583.