Dynamic and thermodynamic constraints on the response of tropical circulations to climate changes

The convective upward mass flux integrated over the tropics is constrained by energetic and hydrologic balances, which control how much the precipitation and the specific humidity can change. However, the net vertical mass flux even near the ITCZ is only a fraction of the upward mass flux; that is, even near the ITCZ, upward mass fluxes are partially compensated by downward mass fluxes. Energetic and hydrologic constraints are insufficient for constraining the net vertical mass flux and thus the strength of the Hadley circulation.

Here we discuss the form of the energetic and hydrologic constraints on the tropical upward mass flux and the additional constraints the net mass flux must satisfy. Contrary to widely held beliefs, changes in the near-surface saturation specific humidity alone are generally insufficient to account for changes in tropical upward mass flux. Changes in the saturation specific humidity stratification, and thus in the thermal stratification, also need to be taken into account. The angular momentum balance near the center of the Hadley cells provides an additional constraint on how the strength of the Hadley circulation responds to climate changes.

The theoretical arguments will be illustrated with simulations with an idealized GCM, which shows, for example, that the tropical upward mass flux can be smaller than in the present climate in much warmer and much colder climates.