Empirical age spectra for the lower tropical stratosphere from in situ observations of CO2:: Implications for stratospheric transport

Empirical age spectra for the lower tropical stratosphere (from the tropopause to similar to 19.5 km) have been derived from in situ measurements of CO2, using information provided by the vertical propagation of the tropospheric seasonal cycle and long-term positive trend. Our method provides accurate and unambiguous mean ages for this region which are difficult to obtain by simple analysis of lag times from tracer measurements. We find that the air is 30-40% younger in northern spring than in autumn. For example, at 460 K the mean age (relative to the tropical tropopause) was 0.4 years in March and 0.6 years in September. The phase lag times and attenuation of CO2 seasonal extrema in the stratosphere are shown to depend on seasonal variations in transport rates and on the presence of harmonics in the CO2 boundary condition with frequencies higher than 2 pi/yr. Profiles of stratospheric water vapor, generated from the derived age spectra with a stratospheric boundary condition based on observed tropical tropopause temperatures, are consistent with in situ observations of H2O. Comparison of the predicted water vapor seasonal cycle with satellite observations suggests that satellite-borne instruments underestimate the amplitude near the tropical tropopause. We relate the empirical age spectra to the analytic solution for the 1-D advection-diffusion tracer continuity equation to obtain seasonally resolved estimates of the ascent rate and the vertical diffusion coefficient. The derived age spectra provide a unique observation-based diagnostic for evaluating the simulation of tracer transport in models.