Abstract
Abstract. We investigated to what extent the isentropic, non-geostrophic formulation of zonally averaged circulation derived for stratospheric conditions is applicable to climatological transport in the extratropical troposphere and lower stratosphere. The study is based on 10 years of daily data of ECMWF analysis and on the ECHAM3 climate model of the German Climate Computing Centre. The main result is a scalar isentropic mixing coefficient, Kyy, and a mean meridional transport circulation consistently derived from the same data base. For both data sources, isentropic mean meridional circulation is derived from horizontal mass flow rate for 4 representative months. Alternatively, a mean meridional circulation is calculated from total diabatic heating rates of the ECHAM3 model. It is shown that only the latter is in good agreement with the ECMWF mean meridional circulation. Isentropic analysis also comprises the seasonal cycle of the climatological meridional gradient and flux of Ertel's potential vorticity (PV). Application of Tung's flux-gradient relation yields that for all seasons Kyy is positive in height-latitude regions where statistical significance is reached. Large Kyy values, marking regions of more efficient mixing, have been found in the subtropical vertical band of weak westerly wind and in mid-latitudes in regions of upward-propagating baroclinic wave activity in the middle and upper troposphere. Based on the ECMWF data and results of baroclinic-wave behaviour, strong indications are presented that positive zonally averaged PV flux polewards of the jet core in the NH is strengthened by stationary waves and nonlinear effects. Reduced eddy transport is apparent in winter and spring slightly below the subtropical tropopause jet. The seasonal cycle of Kyy from ECHAM3 data is to a great extent in agreement with the result based on ECMWF analysis. In the model, reduced interannual variability enlarges the height-latitude range where sign of Kyy is significant.Key words. Meteorology and atmospheric dynamics · Climatology · General circulation · Middle atmosphere is significant.
Highlights
The global climatological distribution of an atmospheric tracer is determined more by dynamical transport the longer its chemical lifetime
The main drawback is the possible appearance of large relative errors in regions with small climatological potential vorticity (PV) gradient where parameterization on the basis of zonal mean PV distribution is questionable
Experiments with a systematic downward shift of this upper boundary position yielded no noticeable eect on Wv only below 390 K, and so we have to restrict our considerations to this region
Summary
The global climatological distribution of an atmospheric tracer is determined more by dynamical transport the longer its chemical lifetime. The zonally averaged distribution of long-living, quasi-conservative tracers is inuenced by transport of stationary and transient waves. In a climatological time-scale, zonal mean tracer distribution can be determined in an eective way with the help of zonally averaged transport (2D) models. The contribution to zonal mean transport which is not resolved in such models comes from longitudinal correlations of large-scale eddies. It has to be parameterized by zonally averaged quantities. Two-dimensional models become an indispensable part of the hierarchy of transport models if a large number of chemical species, coupled with a complex system of chemical reactions, has to be considered, e.g
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