A seasonal climatology of effective diffusivity in the stratosphere

A 7‐year (1992–1998) seasonal climatology of effective horizontal diffusivity is presented for the stratosphere (350–1900 K). As in previous studies, the diagnosis is based on the equivalent length of a test tracer advected on isentropic surfaces, in this case using the van Leer flux‐limiting scheme driven by the United Kingdom Meteorological Office assimilated winds. Although the magnitude of equivalent length is resolution dependent, its structure is shown to be reasonably robust for quantifying the inhomogeneous mixing in the stratosphere. The van Leer calculation agrees well with a more expensive spectral transform calculation at a comparable resolution, suggesting that the diagnostic is not sensitive to the advection scheme or subgrid representation of the models. A first attempt is also made at estimating the implicit numerical diffusion of the van Leer scheme, which is necessary to convert equivalent length to effective diffusivity. Compared with the spectral calculations with a constant diffusion coefficient, the van Leer results show notably greater diffusivity within the winter polar vortices despite the comparable equivalent lengths, suggesting that the scheme may be overdiffusing in the vicinities of mobile tracer extrema. The climatology of equivalent length is then presented, and the seasonal evolution is discussed in detail for the lower, middle, and upper stratosphere. The major mixing barriers (winter and summer polar vortices and tropical eddy transport barrier) are easily identified, as well as the strong mixing regions (surf zones). Generally, equivalent length tends to be small in regions of strong zonal wind and large in regions of weak wind. Large values of equivalent length are observed in the summer lower stratosphere and in the middle stratosphere during the breakup of the polar vortices. Interhemispheric asymmetry is observed both after the winter polar vortex breakup, where in the middle stratosphere the Southern Hemisphere shows much larger equivalent lengths, and in the summer tropical region, where larger equivalent lengths are observed in the Northern Hemisphere, likely related to the monsoon circulation. A monthly averaged equivalent length climatology is presented, and implications for two‐dimensional stratospheric chemistry‐transport modeling are discussed.