Stratospheric age spectra derived from observations of water vapor and methane

The stratospheric age spectrum is useful both as a diagnostic for testing models of stratospheric transport and as a tool for calculating the response of the stratosphere to changes in tropospheric composition. Although the age spectrum of an individual air parcel is not an observable quantity, it is possible to estimate a zonal mean age spectrum from long‐term observations of a conserved tracer with sufficient temporal variability. We have constructed a conserved tracer by combining satellite and balloon‐borne observations of water vapor and methane from 1986 to 1998. After discussing the possible roles of tropical sea surface temperatures and the quasi‐biennial oscillation in determining stratospheric humidity, we use the observations to estimate the zonal mean age spectrum in the tropics and northern midlatitudes. The estimated age spectrum peaks at progressively greater lag times as the altitude increases above the tropopause, in agreement with model calculations. The mean age of air calculated from the derived age spectrum in the lower stratosphere is in good agreement with ages calculated from measurements of SF 6 , but the satellite record is still too short to calculate reliable ages from the age spectrum in the middle and upper stratosphere.