Seasonal variation of mass transport across the tropopause

The annual cycle of the net mass transport across the extratropical tropopause is examined. Contributions from both the global‐scale meridional circulation and the mass variation of the lowermost stratosphere are included. For the northern hemisphere the mass of the lowermost stratosphere has a distinct annual cycle, whereas for the southern hemisphere, the corresponding variation is weak. The net mass transport across the tropopause in the northern hemisphere has a maximum in late spring and a distinct minimum in autumn. This variation and its magnitude compare well with older estimates based on representative 90 Sr mixing ratios. For the southern hemisphere the seasonal cycle of the net mass transport is weaker and follows roughly the annual variation of the net mass flux across a nearby isentropic surface.