Seasonal variation of water vapor in the lower stratosphere observed in Halogen Occultation Experiment data

The seasonal cycle of water vapor in the lower stratosphere is studied base d on Halogen Occultation Experiment (HALOE) satellite observations spanning 1991-2000. The seasonal cycle highlights fast, quasi-horizontal transport between tropics and midlatitudes in the lowermost stratosphere (near isentr opic levels similar to 380-420 K), in addition to vertical propagation abov e the equator (the tropical "tape recorder"). The rapid isentropic transpor t out of the tropics produces a layer of relatively dry air over most of th e globe throughout the year, and the seasonal cycle in midlatitudes of both hemispheres land over the Arctic pole) follows that in the tropics. Additi onally, the Northern Hemisphere summer monsoon has a dominant influence on hemispheric-scale constituent transport. Longitudinal structures in tropica l water vapor and ozone identify regions of strong coupling to the troposph ere; an intriguing result is that the solstice minima in water vapor and oz one are spatial separated from maximum convection and coldest tropical temp eratures. Detailed comparisons with tropical aircraft measurements and the long record of balloon data from Boulder, Colorado, demonstrate the overall high quality of HALOE water vapor data.