Empirical age spectra for the midlatitude lower stratosphere from in situ observations of CO2: Quantitative evidence for a subtropical "barrier" to horizontal transport

Empirical age spectra for the northern midlatitude lower stratosphere were derived using a genetic algorithm constrained by in situ observations of CO2 and N2O. An age spectrum is the probability distribution function describing the ensemble of transit times between the tropical tropopause and a point in the stratosphere. Age spectra with two distinct maxima fit observed CO2 mixing ratios significantly better than age spectra consisting of a single peak. The mean transit time (mean age) of the younger peak increased with altitude from the tropopause to ∼1 year at 19.5 km, while the mean age of the older peak was 5.4±0.4 years throughout this altitude region. Near the tropopause, ∼10% of the air was represented by the older peak, increasing to ∼50% at 19.5 km. We argue that the younger peak represents rapid quasihorizontal transport of air from the tropics, while the older peak represents downwelling of air from higher altitudes in the mean meridional circulation. Mean ages for the younger and older peaks provide estimates of the residence times for air in the lower and middle stratosphere, respectively. The clear separation of the peaks implies a significant altitude region where there is little meridional exchange between midlatitudes and the tropics.