The Static Stability of the Midlatitude Troposphere: The Relevance of Moisture

Abstract

It is generally accepted that the action of baroclinic waves in midlatitude is responsible for raising the static stability of the troposphere above moist neutrality. Previous efforts to quantify this process have concentrated on the stability criteria for normal-mode baroclinic instability. The suggestion presented here is based on the observation that moist convection also plays a significant role in the latter stages of baroclinic disturbances. It is suggested that the minimum stability in midlatitudes is moist neutral, and further that the mean tropopause potential temperature can be estimated as the minimum, that is, moist neutral, plus half the variance. The variance is created by quasi-adiabatic meridional advection of the meridional gradient, so that the final prediction is that the change in saturated equivalent potential temperature between the surface and tropopause should be proportional to the meridional temperature change across the storm tracks. This is found to be in reasonable agreement with the Northern Hemisphere annual cycle in a 16-year climatology. The annual cycle in the Southern Hemisphere, on the other hand, does not fall into the same pattern.