Annual variations of water vapor in the stratosphere and upper troposphere observed by the Stratospheric Aerosol and Gas Experiment II

Abstract

This paper presents a description of the annual variations of water vapor in the stratosphere and the upper troposphere derived from observations of the Stratospheric Aerosol and Gas Experiment II (SAGE II). The altitude‐time cross sections exhibit annually repeatable patterns in both hemispheres. The appearance of a yearly minimum in water vapor in both hemispheres at approximately the same time supports the idea of a common source(s) for stratospheric dry air. Annual patterns observed at northern mid‐latitudes, like the appearance of a hygropause in winter and the weakening and upward shifting of the hygropause from January to May, agree with in situ balloon observations previously obtained over Boulder and Washington, D.C. An increase in water vapor with altitude in the tropics is consistent with methane oxidation in the upper stratosphere to lower mesosphere as a source for water vapor. A poleward gradient is also shown as expected based on a Lagrangian mean circulation. A linear regression analysis using SAGE II data from January 1986 to December 1988 shows that little annual variation occurs in the middle and upper stratosphere with the region of large annual variability near the tropopause. The semi‐annual variability is relatively marked at altitudes of 24 and 40 km in the tropics.