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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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