CO_2 in the upper troposphere: Influence of stratosphere-troposphere exchange

Abstract

A two-dimensional transport model constrained to measured surface CO_2 concentrations was used to simulate the spatial and temporal variation of CO_2 in the atmosphere for the period from 1975 to 2004. We find that the amplitude, phase and shape of the CO_2 seasonal cycle vary as a function of both altitude and latitude. Cross tropopause exchanges, especially the downward branch of the Brewer-Dobson circulation, which brings stratospheric air to the upper troposphere at middle and high latitudes, change the CO_2 concentration and seasonal cycle in the extra-tropics. The model results match recent aircraft measurements of CO_2 in the upper troposphere (Matsueda et al., 2002) remarkably well. We conclude that upper tropospheric CO_2 volume mixing ratios will provide a valuable tool for validating vertical transport. The implications of the CO_2 variation caused by the stratosphere-troposphere exchange for remote sensing of CO_2 are discussed.