Interannual variability in effective diffusivity in the upper troposphere/lower stratosphere from reanalysis data

The effective diffusivity based on passive tracer advection is used to evaluate the long-term mixing properties for the period 1980–2012 in the lower stratosphere and upper troposphere (UTLS) using data from the ERA-Interim reanalysis. The regions of strongest interannual variability in effective diffusivity coincide with the regions of strong climatological mixing, such as the winter and spring midlatitude stratosphere, the polar lowermost stratosphere and around the edge of the subtropical jets (especially in summer). The annular modes dominate the variability in the winter polar vortices, and the Quasi-Biennial Oscillation in the tropical stratosphere. El Niño/Southern Oscillation modulates the strength of mixing across the subtropical jets, and has a significant impact on mixing in the summer subtropical lower stratosphere. Long-term trends show a vertical shift in the mixing consistent with the effect of ozone depletion on the zonal wind in the austral summer polar stratosphere. Other significant trends include increased mixing in the austral surf zone and a dipolar pattern in the boreal summer and autumn, with mixing increased on the equatorward part of the subtropical jet and reduced just above. The results are highly consistent with those from the JRA-55 reanalysis when the same horizontal resolution is used. The calculations are also qualitatively consistent with effective diffusivity obtained directly from the potential vorticity field and, in the UTLS, they are broadly consistent with those obtained from the ozone field.

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