Stratospheric water vapor

Abstract

We present a tutorial review of our understanding of stratospheric H2O and the processes controlling it. We attempt to synthesize a consistent global picture that requires rejection of a minimum of the conflicting observational data. As such, this synthesis is determined somewhat by the personal opinions and beliefs of the author. We note the paradoxes posed by currently available observational data and suggest ways they might be resolved. Resolution of the stratospheric H2O budget appears to require (1) either that the bulk of our current data provides unrepresentative and misleading latitudinal H2O gradients immediately downstream from the presumed tropical tropopause source of H2O or that there are unidentified polar stratospheric sinks for H2O; (2) that it also provides an unrepresentative vertical gradient in H2O immediately above the tropical tropopause source or that there is a stratospheric sink for H2O in this region; or (3) that there exist within the troposphere a mechanism different from or in addition to the tropical tropopause ‘cold trap’ for drying air to the mixing ratios found in the lower stratosphere. Satisfaction of conditions such as these are required to reconcile a rather large mass of puzzling observational data. Satisfaction of the third option would also permit tropospheric to stratospheric exchange through the mid‐latitude tropopause gaps, a path suggested by minima in O3 profiles.