S. Davis, E. Ray, K. Rosenlof
Variability and trends in effective diffusivity in reanalyses
Effective diffusivity is one means of quantifying mixing
in the upper troposphere and stratosphere. Here, we
calculate the normalized effective diffusivity from
reanalysis potential vorticity fields, and remove
variability associated with known cycles (e.g., annual
cycle, QBO, solar cycle) to compute trends from
multiple reanalyses. We compare the variability and
trends, highlighting regions of agreement and
disagreement among the reanalyses. Although not in
complete agreement, several reanalyses contain
positive trends in effective diffusivity in the
southern hemisphere stratosphere (> ~400 K) along
the boundary between the upwelling and downwelling
branches of the Brewer-Dobson circulation. These
increases in effective diffusivity, if real, represent
an increase in mixing between the so-called "tropical
pipe" and midlatitudes.
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