The Variability of Water Vapor in the Stratosphere

Monthly measurements of stratospheric water vapor concentration, initiated as an IQSY program, have continued from a site near Washington, D.C., and now constitute a six-year time series of homogeneous data which may be examined for evidence of stratospheric water vapor variability. A regression analysis was performed for six pressure levels (100–mb) to examine the variability of water vapor concentration in terms of two components, a linear trend and an annual cycle. A highly significant trend of increasing mixing ratio from 2 to 3 ppm is found for all pressure levels over the six-year interval. A significant annual cycle is indicated for the higher pressure levels. Study of the literature indicates that similar annual cycles and trends have been observed in the height of the tropical tropopause and in temperatures at or near the tropopause. It is suggested that variability in the drying potential of the tropical tropopause region leads to corresponding variability in stratospheric mixing ratio as observed at mid-latitudes.