Climatological Ertel's potential-vorticity flux and mean meridional circulation in the extratropical troposphere − lower stratosphere

摘要

We investigated to what extent theisentropic, non-geostrophic formulation of zonally averaged circulation derivedfor stratospheric conditions is applicable to climatological transport in theextratropical troposphere and lower stratosphere. The study is based on 10 yearsof daily data of ECMWF analysis and on the ECHAM3 climate model of the GermanClimate Computing Centre. The main result is a scalar isentropic mixingcoefficient, K_yy, and a mean meridional transport circulationconsistently derived from the same data base. For both data sources, isentropicmean meridional circulation is derived from horizontal mass flow rate for 4representative months. Alternatively, a mean meridional circulation iscalculated from total diabatic heating rates of the ECHAM3 model. It is shownthat only the latter is in good agreement with the ECMWF mean meridionalcirculation. Isentropic analysis also comprises the seasonal cycle of theclimatological meridional gradient and flux of Ertel's potential vorticity (PV).Application of Tung's flux-gradient relation yields that for all seasons K_yyis positive in height-latitude regions where statistical significance isreached. Large K_yy values, marking regions of more efficientmixing, have been found in the subtropical vertical band of weak westerly windand in mid-latitudes in regions of upward-propagating baroclinic wave activityin the middle and upper troposphere. Based on the ECMWF data and results ofbaroclinic-wave behaviour, strong indications are presented that positivezonally averaged PV flux polewards of the jet core in the NH is strengthened bystationary waves and nonlinear effects. Reduced eddy transport is apparent inwinter and spring slightly below the subtropical tropopause jet. The seasonalcycle of K_yy from ECHAM3 data is to a great extent inagreement with the result based on ECMWF analysis. In the model, reducedinterannual variability enlarges the height-latitude range where sign of K_yyis significant.Key words. Meteorology and atmospheric dynamics ·Climatology · General circulation · Middle atmosphere is significant.