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. Plain Language Summary: Stratospheric water vapor (H2O) is mainly controlled by transport across the cold tropical tropopause, which sets the entry value for the global stratosphere. Interannual variations in H2O originate near the tropical tropopause and then propagate throughout the stratosphere with the global Brewer‐Dobson circulation (BDC), where anomalies are lagged in time and smoothed by mixing. We use the observed time series of H2O from the Microwave Limb Sounder during 2004–2021 to calculate transit time distributions (also called age spectra) from the tropical tropopause source region. The results capture H2O variability in the tropics up to 30 km and in the global lower stratosphere, regions of relatively fast transport (∼1–2 years) in the BDC. The calculations provide novel observational estimates of the age spectra in these regions. The calculations are straightforward to apply to global models, and we compare observational results with simulations from the WACCM and CLaMS models. Key Points: Stratospheric transit time distributions (age spectra) are derived from time series of satellite water vapor measurementsWater vapor reconstructions from age spectra capture detailed variability in the tropics up to 30 km and in the global lower stratosphereAge spectrum results are compared between observations and global model simulations [ABSTRACT FROM AUTHOR]