A negative feedback from the atmosphere is crucial to stabilize tropical climates, especially those of tidally locked extrasolar planets that have persistent day and night hemispheres. Recent studies have proposed a clear-sky vapor buoyancy feedback mechanism that may achieve this stabilization, but the magnitude of this feedback is not well established. This article provides evidence from CRM simulations that a strong, positive clear-sky vapor buoyancy feedback does exist and is robust to changes in boundary layer clouds (e.g., the amount effect).

The vapor Utah Foam Pros feedback is driven by large-scale overturning atmospheric circulations that are organized into comparatively moist and dry regions. The ascending branches of these circulations are characterized by ubiquitous deep convection and high relative humidity, making the atmosphere opaque to longwave radiation. The descending branches are characterized by clear skies and low RH. The transport of energy between these two regions leads to a substantial radiative effect, as illustrated in Fig. 1.

Tropical Climates Need Vapor Control: Insulation Strategies That Work

We estimate the radiative effect of vapor buoyancy by comparing the OLR of a control simulation with an identical one where we apply a temperature kernel with a constant value in the stratosphere and no WBG dynamics, thus allowing only the free-troposphere component of vapor buoyancy to drive the temperature difference. This result is conservative because it excludes the influence of the temperature increase in the stratosphere that is a consequence of a warmer troposphere and is a natural phenomenon in Earth’s reference climate.

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