The sensitivity of the isentropic slope in a primitive-equation dry model

It has long been noted that the extratropical troposphere has a well defined isentropic slope, such that the characteristic isentrope leaving the subtropical boundary layer reaches the tropopause in polar latitudes. This typical isentropic slope is very robust in both hemispheres and throughout the seasonal cycle.

Early theories attributed the robustness of the extratropical isentropic slope to baroclinic adjustment, i.e. to the baroclinic neutralization of the troposphere by the eddies. However, the baroclinic-adjustment constraint has been shown to fail in full-physics GCM experiments, and in idealized moist GCMs when perturbing the moisture content. In contrast, recent experiments by Schneider and others have suggested that the isentropic slope is very strongly constrained in idealized dry GCMs, even when strongly perturbing the heating.

In this study, we investigate the sensitivity of the isentropic slope in an idealized dry GCM similar to Schneider's, and discuss this sensitivity using the global heat and momentum balances. We find that changes in the isentropic slope are associated with changes in the net isentropic mass transport, consistent with heuristic diffusive arguments. As a result, changes in the heating only translate into changes in the isentropic slope when the mass circulation changes, which depends on how the gross stability changes.