Diagnostics of mixing in the upper troposphere/lower stratosphere region reveal interesting seasonal and interannual variability and illustrate how the subtropical tropopause can be identified with a region of minimum mixing, a partial barrier to the transport of air between the troposphere and stratosphere. Both the strength and the location of this barrier show substantial seasonal variability, with a stronger barrier and weaker mixing observed during winter. The interannual variability of the mixing intensity near the tropopause on the 350 K isentropic surface suggests a correlation with the phase of the El Ni¿o-Southern Oscillation, with weaker mixing occurring during strong El Ni¿o years. The diagnostics are based on the analyses and reanalyses from the European Centre for Medium-Range Weather Forecasts over the 21 year period 1979--2000 and include the exponential stretching rates of material contours and Nakamura's <1996> modified Lagrangian mean effective diffusivity/equivalent length. As well as providing information about the spatial and temporal distribution of mixing intensity, the diagnostics also indicate sensitivity to changes in the analyses/reanalyses data sets. In particular, a stronger seasonal cycle and greater interannual variability is found in the more recent (1994--2000) analyses than in the earlier (1979--1993) reanalyses.