Coupling between moisture and circulation above the clouds: The annular cycle of stratospheric water vapor

Despite the extreme dryness of the stratosphere -- characterized by water vapor concentrations on the order of a few parts per million -- the roughly logarithmic impact of greenhouse gas concentration on radiative transfer implies that small variations of water vapor in the middle atmosphere can significantly impact the circulation, and even the surface; recent work has highlighted a positive feedback of stratospheric water vapor on global temperature in response to greenhouse gas forcing. In this presentation, we investigate the coupling between radiation and circulation that drives the annual cycle in stratospheric water vapor using a new model of an idealized, moist atmosphere.

Stratospheric water vapor is regulated by the tropical cold point, where air enters the middle atmosphere. Somewhat surprisingly, it exhibits an annual cycle (as opposed to the semi-annual signal in solar forcing in the tropics), reaching a minimum/maximum in boreal winter/summer. We first show how differences in land-sea contrast and large scale orography between the hemispheres drive the annual signature of cold point variability (and hence stratospheric water vapor), implicating important roles for both synoptic and planetary scale waves, respectively. We then take advantage of the idealized nature of the model to decouple the radiative impact of water vapor from the circulation, revealing the role of radiative feedback on the cold point structure and the meridional overturning of the stratosphere, or Brewer-Dobson circulation.