Surface CO2 variability from the stratosphere, or not

Fluctuations in atmospheric CO2 can be measured with great precision and are used to identify human-driven sources as well as natural cycles of ocean and land carbon.  One source of variability is the stratosphere, where the influx of aged CO2-depleted air can produce fluctuations at the surface.  This process has been speculated a potential source of interannual variability (IAV) in CO2 that might obscure the quantification of other sources of IAV.  Given the recent success in demonstrating that the stratospheric influx of N2O- and chlorofluorocarbon-depleted air is a dominant source of their surface IAV in the southern hemisphere, we here apply the same model and measurement analysis to CO2.  Using chemistry-transport modeling or scaling of the observed N2O variability, we find that the stratosphere-driven surface variability in CO2 is at most 10% of the observed IAV and is not an important source.  Diagnosing the amplitude of the CO2 annual cycle and its increase from 1985 to 2021 through the annual variance gives rates similar to traditional methods in the northern hemisphere (BRW, MLO), but can identify the emergence of small trends (0.08 ppm per decade) in the southern hemisphere (SMO, CGO).

Methods
The observational data used for this work are taken from the public NOAA data archives as described in the manuscript.  These are not archived here, find them at the referenced sites below.

Dlugokencky, E.J., Crotwell, A.M., Mund, J.W., Crotwell, M.J., and Thoning, K.W.: Atmospheric Nitrous Oxide Dry Air Mole Fractions from the NOAA GML Carbon Cycle Cooperative Global Air Sampling Network, 1997-2020, Version: 2021-07-30, doi: 10.15138/53g1-x417, 2021b.

Dlugokencky, E.J., Mund, J.W., Crotwell, A.M., Crotwell, M.J., and Thoning, K.W.: Atmospheric Carbon Dioxide Dry Air Mole Fractions from the NOAA GML Carbon Cycle Cooperative Global Air Sampling Network, 1968-2020, Version: 2021-07-30, doi: 10.15138/wkgj-f215, 2021a.

The modeled data are from 28-year simulations of the UCI CTM using ECMWF IFS Cy38r1 forecasts and simulating N2O-like and CO2-like tracers as described in the text.