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 ECHAM/MESSy Atmospheric Chemistry (EMAC) global chemistry-climate model. 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 300–100 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.