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Changes in climate are likely to drive changes in stratospheric mixing withassociated implications for changes in transport of ozone from tropicalsource regions to higher latitudes, transport of water vapour and source gasdegradation products from the tropical tropopause layer into the mid-latitudelower stratosphere, and changes in the meridional distribution of long-livedtrace gases. To diagnose long-term changes in stratospheric mixing, globalmonthly fields of Lyapunov exponents were calculated on the 450 K, 550 K,and 650 K isentropic surfaces by applying a trajectory model to wind fieldsfrom NCEP/NCAR reanalyses over the period 1979 to 2005. Potential underlyinggeophysical drivers of trends and variability in these mixing fields wereinvestigated by applying a least squares regression model, which includedbasis functions for a mean annual cycle, seasonally dependent linear trends,the quasi-biennial oscillation (QBO), the solar cycle, and the El NiñoSouthern Oscillation (ENSO), to zonal mean time series of the Lyapunovexponents.  Long-term positive trends in mixing are apparent over southern middle to highlatitudes at 450 K through most of the year, while negative trends oversouthern high latitudes are apparent at 650 K from May to August. Wintertimenegative trends in mixing over northern mid-latitudes are apparent at 550 Kand 650 K. Over low latitudes, within 40° of the equator, the QBOexerts a strong influence on mixing at all three analysis levels. This QBOinfluence is strongly modulated by the annual cycle and shows a phase shiftacross the subtropical mixing barrier. Solar cycle and ENSO influences onmixing are generally not significant. The diagnosed long-term changes inmixing should aid the interpretation of trends in stratospheric trace gases

Trends and variability in stratospheric mixing: 1979–2005