Roles of mean meridional circulation and eddy diffusion in the transport of trace substances in the lower stratosphere

Recent estimates of mean meridional motions and eddy diffusion coefficients in the lower stratosphere and upper troposphere are used to study the roles of mean circulation and eddy mixing in determining the spread of tungsten 185 injected into the lower stratosphere in the Hardtack test series of 1958. The observed distribution of tungsten 185 is reproduced satisfactorily for a period of 18 months after the injection, provided that the diffusion coefficients estimated from heat flux data are reduced by a factor of about 8. The mean meridional circulation is found to play a minor role in the transport process, its chief contribution being to raise the center of maximum concentration by 2 to 3 km in the equatorial region, in agreement with the observations. The mean residence time for a tracer injected into the equatorial stratosphere between 18 and 20 km is found to be about 10 months, with most of the transport into the troposphere occurring at middle latitudes.