SEASONAL-VARIATION OF MASS-TRANSPORT ACROSS THE TROPOPAUSE

The annual cycle of the net mass transport across the extratropical tr opopause is examined. Contributions from both the global-scale meridio nal circulation and the mass variation of the lowermost stratosphere a re included. For the northern hemisphere the mass of the lowermost str atosphere has a distinct annual cycle, whereas for the southern hemisp here, the corresponding variation is weak. The net mass transport acro ss the tropopause in the northern hemisphere has a maximum in late spr ing and a distinct minimum in autumn. This variation and its magnitude compare well with older estimates based on representative Sr-90 mixin g ratios. For the southern hemisphere the seasonal cycle of the net ma ss transport is weaker and follows roughly the annual variation of the net mass flux across a nearby isentropic surface.