Stratospheric Transit Time Distributions Derived From Satellite Water Vapor Measurements

Stratospheric transit time distributions (age-of-air spectra) are estimated using time series of satellite water vapor (H₂O) measurements from the Microwave Limb Sounder over 2004 to 2021 assuming stationary transport. Latitude-altitude dependent spectra are derived from correlations of interannual H₂O anomalies with respect to the tropical tropopause source region, fitted with an inverse Gaussian distribution function. The reconstructions accurately capture interannual H₂O variability in the “tropical pipe” and near-global lower stratosphere, regions of relatively fast transport (∼1–2 years) in the Brewer-Dobson circulation. The calculations provide novel observational estimates of the corresponding “short transit-time” part of the age spectrum in these regions, including the mode. However, the H₂O results do not constrain the longer transit-time “tail” of the age spectra, and the mean age of air and spectral widths are systematically underestimated compared to other data. We compare observational results with parallel calculations applied to the WACCM chemistry-climate model and the CLaMS chemistry-transport model, and additionally evaluate the method in CLaMS by comparing with spectra from idealized pulse tracers. Because the age spectra accurately capture H₂O interannual variations originating from the tropical tropopause, they can be used to identify “other” sources of variability in the lower stratosphere, and we use these calculations to quantify H₂O anomalies in the Southern Hemisphere linked to the Australian New Years fires in early 2020 and the Hunga volcanic eruption in 2022.