On the decay of stratospheric pollutants: Diagnosing the longest-lived eigenmode

[1] On the basis of a one-dimensional (1-D) analysis the decay time of the lowest eigenmode, tau(1), for the stratospheric distribution of a conserved tracer is derived from measured vertical profiles of the mean age of stratospheric air. Two case studies (a and b) give tau(1,a) = 3.8 +/- 0.8 years and tau(1,b) = 5.3 +/- 1.1 years. These semiobservational times are considerably longer than most of the tau(1) derived from 2-D and 3-D models. At the same time they are shorter than the observational eigentime, tau(1,HTO) = 7.7 +/- 2 years, determined from the decay of the tritium (T) content in stratospheric water vapor, following the thermonuclear test explosions in the early 1960s. Part of the differences among the observational eigentimes can be explained by the assumptions that had to be made to extract tau(1),(HTO) from the trend in the T content of stratospheric water vapor ( namely, the cosmogenic background of tritiated water vapor and the trend in stratospheric water vapor). This leads to a revised value tau(1,HTO) = 6.3 +/- 0.9 years for the time period 1975-1983. Allowing for a possible temporal trend in Gamma and hence tau(1), the value for the current tau(1) decreases to 5.3 +/- 1 years.