Vapor-Buoyancy Feedback in an Idealized GCM

Abstract

Humid air is lighter than dry air at the same temperature and pressure because the molecular weight of water vapor is less than that of dry air. This effect is known as vapor buoyancy (VB). In this work we use experiments in an idealized general circulation model (GCM) to show that VB warms the tropical free troposphere and leads to a significant increase in outgoing longwave radiation (OLR). This radiative effect increases with climate warming, causing a negative climate feedback there. We call this the VB feedback. Although this VB feedback was first corroborated in simplified models, it was heretofore unclear whether the VB feedback is significantly impaired by planetary rotation, clouds, or a countervailing water vapor feedback. However, our GCM simulations show that the VB feedback is robust and maintains an appreciable magnitude when considering these factors.