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Abstract

The effect of gravitational separation on the Ar/N2 ratio is a useful, observable indicator to detect changes in stratospheric transport due to its strong relationship with age-of-air (AoA). In this paper, we present new evidence for the effect of stratospheric transport on the surface interannual variability of Ar/N2 with comparison of lower top (~60 km) and higher top (~70 km) simulations using the chemical transport model TOMCAT. Both runs show a similar regular variation in N2O and Ar/N2 at the surface with an average period of ~28 months in the tropics and middle latitudes, indicating a clear quasi-biennial oscillation (QBO) signal in the surface rates of change, in line with NOAA ESRL observations. The higher top simulation, using the full vertical extent of ERA5 reanalysis, shows a signature of the deep branch of the Brewer-Dobson circulation and mesospheric circulations in AoA and Ar/N2 which is absent in the lower top one. The higher top simulation also gives a better coupled QBO signal in the surface interannual variability of Ar/N2 and N2O growth rates. The comparisons indicate the higher model top can better simulate stratospheric variability and stratosphere–troposphere coupling, highlighting the importance of future models with well-resolved stratosphere and lower mesosphere. This study also suggests an influence on the interannual variability in surface Ar/N2 from stratospheric transport. The implications on whether this influence is substantial enough to be used for a better estimation of ocean heat uptake using tropospheric Ar/N2 will be discussed.