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 CO 2 and N2O. An age spectrum is the probability distribution function describ ing the ensemble of transit times between the tropical tropopause and a poi nt in the stratosphere. Age spectra with two distinct maxima fit observed C O2 mixing ratios significantly better than age spectra consisting of a sing le peak. The mean transit time (mean age) of the younger peak increased wit h altitude from the tropopause to similar to1 year at 19.5 km, while the me an age of the older peak was 5.4 +/- 0.4 years throughout this altitude reg ion. Near the tropopause, similar to 10% of the air was represented by the older peak, increasing to similar to 50% at 19.5 km. We argue that the youn ger 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 pea ks provide estimates of the residence times fur air in the lower and middle stratosphere, respectively. The clear separation of the peaks implies a si gnificant altitude region where there is little meridional exchange between midlatitudes and the tropics.