Isentropic transport and the seasonal cycle amplitude of CO₂

Carbon‐concentration feedbacks and carbon‐climate feedbacks constitute one of the largest sources of uncertainty in future climate. Since the beginning of the modern atmospheric CO 2 record, seasonal variations in CO 2 have been recognized as a signal of the metabolism of land ecosystems, and quantitative attribution of changes in the seasonal cycle amplitude (SCA) of CO 2 to ecosystem processes is critical for understanding and projecting carbon‐climate feedbacks far into the 21st Century. Here the impact of surface carbon fluxes on the SCA of CO 2 throughout the Northern Hemisphere troposphere is investigated, paying particular attention to isentropic transport across latitudes. The analysis includes both a chemical transport model GOES‐Chem and an idealized tracer in a gray‐radiation aquaplanet. The results of the study can be summarized by two main conclusions: (1) the SCA of CO 2 roughly follows surfaces of constant potential temperature, which can explain the observed increase in SCA with latitude along pressure surfaces and (2) increasing seasonal fluxes in lower latitudes have a larger impact on the SCA of CO 2 throughout most of the troposphere compared to increasing seasonal fluxes in higher latitudes. These results provide strong evidence that recently observed changes in the SCA of CO 2 at high northern latitudes (poleward of 60°N) are likely driven by changes in midlatitude surface fluxes, rather than changes in Arctic fluxes.