Analysis of the Potential Impact of Discrepancies in Stratosphere-troposphere Exchange on Inferred Sources and Sinks of CO2

Abstract:

The upper troposphere and lower stratosphere (UTLS) represents a transition region between the more dynamically active troposphere and more stably stratified stratosphere. The processes that influence the distribution of atmospheric constituents in the UTLS occur on small vertical scales that are a challenge for models to reliably capture. As a consequence, models typically underestimate the mean age of air in the lowermost stratosphere, reflecting excessive vertical transport and/or mixing in the region. Using the GEOS-Chem global chemical transport model, we quantify the potential impact of discrepancies in vertical transport in the UTLS on inferred sources and sinks of atmospheric CO₂. Comparisons of the modeled CO₂ and O₃ in the polar UTLS with data from the HIAPER Pole-to-Pole Observations (HIPPO) campaign show that the model overestimates CO₂ and underestimates O₃ in the region. Using the observed CO₂/O₃ correlations in the UTLS, we correct the modeled CO₂ in the Arctic UTLS (primarily between the 320 K and 360 K isentropic surfaces) and quantify the impact of the CO₂ correction on the flux estimates using the GEOS-Chem data assimilation system together with XCO₂ data from the Greenhouse Gases Observing Satellite (GOSAT). As a result of isentropic transport, the correction is transported down into the subtropical troposphere, where it impacts the regional flux estimates. Our results suggest that discrepancies in mixing in the UTLS could bias the latitudinal distribution of the inferred CO₂ fluxes.