Variability and trends in effective diffusivity in the stratosphere, and their implications for stratospheric circulation changes

Effective diffusivity is one means to quantify isentropic mixing in an equivalent-latitude based coordinate system, and has been previously applied to studies of both the upper troposphere and stratosphere. Here, we present variability and trends in effective diffusivity calculated using potential vorticity from multiple meteorological reanalyses. We quantify and discuss variability related to seasonal cycles, the quasi-biennial oscillation (QBO), solar cycle, ENSO, and post-2000 circulation changes. We note positive trends in effective diffusivity, implying increased mixing, that are robust across multiple different meteorological reanalyses in the southern hemisphere overworld (> ~450 K). Effective diffusivity is also analyzed relative to the tropical pipe edge defined by the residual circulation to identify changes in mixing between the tropical pipe and midlatitudes. As illustrated in a recent paper by Ray et al. (JGR, 2010), changes in such tropical in-mixing may have important implications for resolving the apparent discrepancy between the observationally-based increase in midlatitude age-of-air and the indication from models and observations of an intensified BD circulation in the lower stratosphere.