Mechanisms Regulating Water Vapor And Clouds in the Tropical and Extratropical Lower Stratosphere

Abstract

Climate models predict that stratospheric water vapor will increase over the 21st century. Here we conduct experiments using a trajectory model driven by meteorology from a chemistry-climate model to study the contribution of tropical tropopause layer (TTL) temperature and convective ice to the long-term trend of the stratospheric water vapor. We find that the moistening due to TTL warming is not large enough to explain the model’s trend. Adding convective ice evaporation to the trajectory model improves the simulation of water vapor entering the stratosphere, especially during boreal summer. Convective ice mainly evaporates over the Asian monsoon region and the tropical western Pacific region, and the convective ice evaporates faster over boreal summer than boreal winter. Net contribution from convective ice to stratospheric water vapor increases faster on higher level, and the net contribution above 400 K potential temperature contributes most to the stratospheric water vapor at the end of the 21st century. Our trajectory model simulation suggests that over the 21st century, 45% of the increase of the stratospheric water vapor content is due to direct increase in TTL temperatures, 13% of the increase is due to an increase in convective ice content, and 42% is due to increased evaporation allowed by a warming TTL.