Moisture Effects on Midlatitude Static Stability in a Hierarchy of Models

The atmospheric static stability is fundamental to any theory of the general circulation: it determines the buoyancy frequency of dry perturbations in the vertical, the speed of gravity waves, and growth rates and length scales in linear baroclinic instability problems. We examine the determination of the static stability of the midlatitudes in a hierarchy of atmospheric models, focusing in particular on the effect of moisture and latent heat release on the static stability.

The simplest model we consider is an idealized moist GCM, which consists of the primitive equations on the sphere, a zonally symmetric aquaplanet lower boundary, and idealized parameterizations of moisture and other physics. We compare a wide range of fixed SST simulations using this model with simulations using a comprehensive atmospheric GCM run over the same boundary conditions. Dry baroclinic adjustment theories for the midlatitude static stability are invalid over the entire parameter range considered in the simplified GCM, and for much of the parameter range considered in the comprehensive GCM. A moist theory similar to that proposed by Juckes, on the other hand, works remarkably well in predicting the midlatitude stability over the entire parameter range for both models.

We then examine changes in the static stability in simulations of global warming in the WCRP CMIP3 multi-model dataset. The moist theory of Juckes predicts an increase of dry static stability with increasing temperatures and moisture content. An increase in stability is indeed seen with global warming, in all hemispheres and seasons, but especially in the summer and in the Southern Hemisphere. The moist scaling theories work well in the Southern Hemisphere but fail in the Northern Hemisphere, where enhanced surface warming over land reduces the increase in stability.