A seasonal climatology of effective diffusivity in the stratosphere

Dr. Allen et N. Nakamura, A seasonal climatology of effective diffusivity in the stratosphere, J GEO RES-A, 106(D8), 2001, pp. 7917-7935
Citations number
Categorie Soggetti
Earth Sciences
Year of publication
7917 - 7935
SICI code
A 7-year (1992-1998) seasonal climatology of effective horizontal diffusivi ty 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 o n isentropic surfaces, in this case using the van Leer flux-limiting scheme driven by the United Kingdom Meteorological Office assimilated winds. Alth ough the magnitude of equivalent length is resolution dependent, its struct ure is shown to be reasonably robust for quantifying the inhomogeneous mixi ng in the stratosphere. The van Leer calculation agrees well with a more ex pensive spectral transform calculation at a comparable resolution, suggesti ng that the diagnostic is not sensitive to the advection scheme or subgrid representation of the models. A first attempt is also made at estimating th e implicit numerical diffusion of the van Leer scheme, which is necessary t o convert equivalent length to effective diffusivity. Compared with the spe ctral calculations with a constant diffusion coefficient, the van Leer resu lts show notably greater diffusivity within the winter polar vortices despi te the comparable equivalent lengths, suggesting that the scheme may be ove rdiffusing in the vicinities of mobile tracer extrema. The climatology of e quivalent 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 barr ier) are easily identified, as well as the strong mixing regions (surf zone s). Generally, equivalent length tends to be small in regions of strong zon al wind and large in regions of weak wind. Large values of equivalent lengt h are observed in the summer lower stratosphere and in the middle stratosph ere during the breakup of the polar vortices. Interhemispheric asymmetry is observed both after the winter polar vortex breakup, where in the middle s tratosphere the Southern Hemisphere shows much larger equivalent lengths, a nd in the summer tropical region, where larger equivalent lengths are obser ved in the Northern Hemisphere, likely related to the monsoon circulation. A monthly averaged equivalent length climatology is presented, and implicat ions for two-dimensional stratospheric chemistry-transport modeling are dis cussed.