Simulation of the SF6 tracer with the middle atmosphere MAECHAM4 model:: Aspects of the large-scale transport

The large-scale transport in a general circulation model is evaluated by analyzing a multiyear simulation that included a passive tracer, with emissions linearly increasing with time, aimed at representing the sources of sulphur hexafluoride, SF6, a compound of anthropogenic origin. A motivation to analyze the large-scale passive tracer transport is the expected use of such general circulation models in simulations including active feedbacks with chemical models. It has been found that the time evolution and detrended distribution of the simulated SF6 concentration are comparable to those estimated by available observations of SF6, both displaying high values in the upper troposphere and low values in the stratosphere. The simulation with a general circulation model of a tracer with surface sources linearly increasing in time and no sinks allows derivation of transport timescales. The age of air from the SF6 simulation is consistent with the general features of age of air estimates from observations, as, for instance, a rapid increase in age in the lower stratosphere and older age in the polar regions of the middle and upper stratosphere. The meridional and vertical gradients that characterize the age distribution in the stratosphere are the manifestation of barriers in transport. The average behavior of the transport barriers is diagnosed by means of the potential vorticity gradient. The simulated barriers in potential vorticity gradient in the lower-middle stratosphere are comparable to that deduced from European Centre for Medium-Range Weather Forecast reanalysis.