The CO2 seasonal signal as a transport diagnostic in the UTLS

The atmospheric distribution and variability of CO₂ result from the interplay of different processes and mechanisms. Although these trace gas patterns contain valuable information on mixing and transport at different timescales, the information is difficult to extract from observed or simulated mole fractions, particularly in the upper troposphere and lower stratosphere (UTLS), due to the combination of long-term increase and the seasonal cycle of CO₂.

Using a compilation of vertical trace gas profiles derived from measurements with the balloon-based AirCore technique together with ECHAM/MESSy Atmospheric Chemistry (EMAC) model data, we investigate how the seasonality of CO₂ in the troposphere propagates into the lowermost stratosphere. Simulating an artificial, deseasonalised CO₂ tracer enables us to separate and study the seasonal cycle in a unique way in remote areas and on a global scale. Our results show that the tropospheric CO₂ seasonal cycle is strongly modulated in the extratropical UTLS region, characterised by a substantial change in amplitude, a phase shift of several months and a tilt in the shape of the seasonal cycle, which can be associated with the transport barrier related to the strength of the subtropical jet. In the stratosphere, we identified both a vertical and a horizontal “tape recorder” of the CO₂ seasonal cycle. Originating in the tropical tropopause region this imprint is linked to the upwelling and the shallow branch of the Brewer-Dobson circulation.

To validate these model-based findings we developed a strategy to isolate the seasonal signal in observational data as well. This requires CO₂-independent Age of Air (AoA) information to disentangle seasonality from the combined effect of transport and long-term trend. To achieve this, we choose an approach using a normalised methane vs. mean age correlation based on independent observational data. We present average vertical profiles of the isolated CO₂ seasonal signal for latitude bands with sufficient AirCore measurement coverage. Statistical analyses are then used to assess the robustness and representativeness of these results and to determine whether AirCore observations can be used to constrain the CO₂ seasonality in the UTLS.