CO₂ variability and seasonal cycle in the UTLS: Insights from EMAC model and AirCore observational data

Abstract. The complex distribution of CO₂ in the upper troposphere and lower stratosphere (UTLS) results from the interplay of different processes and mechanisms. However, in such difficult-to-access regions of the atmosphere our understanding of the CO₂ variability remains limited. Using vertical trace gas profiles derived from measurements with the balloon-based AirCore technique for validation, we investigate the UTLS and stratospheric CO₂ distribution simulated with the global chemistry-climate model EMAC. By simulating an artificial, deseasonalised CO₂ tracer, we disentangle the CO₂ seasonal signal from long-term trend and transport contribution. This approach allows us to study the CO₂ seasonal cycle in a unique way in remote areas and on a global scale. Our results show that the tropospheric CO₂ seasonal cycle propagates upwards into the lowermost stratosphere and is most modulated in the extra-tropics between 300 hPa and 100 hPa, characterised by a 50 % amplitude dampening and a 4-month phase shift in the northern hemisphere mid-latitudes. During this propagation the seasonal cycle shape is also tilted, which is 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. As the CO₂ seasonal signal carries information about transport processes on different timescales, the newly introduced tracer is a very useful diagnostic tool and would also be a suitable metric for model intercomparisons.