Royal Netherlands Meteorological Institute
Section of Atmospheric Composition
The upward and downward mass fluxes of air across the tropopause, defined as the potential vorticity=3.5 PVU-surface, have been diagnosed for the extratropical Northern Hemisphere, using high spatial and temporal resolution (0.5 degr. latitude and longitude, 31 levels, 3-hourly) ECMWF-(European Centre for Medium-range Weather Forecasts)-circulation data for January 1994. The diagnosed net cross-tropopause transport is 0.35 x 10**17 kg for January 1994 (downward) for the area north of 28 N. This net transport is smaller than that across the 2.5- and 1.5 PVU-surfaces, and larger than the 'downward control'-result. This indicates that the net downward transport across the tropopause is a combination of transport from the upper stratosphere to the troposphere and transport from the lower stratosphere to the troposphere. The latter transport indicates the existence of a circulation in which air is transported from the subtropical upper troposphere to the upper troposphere at middle and higher latitudes via the lower stratosphere without ever reaching the upper stratosphere. The diagnosed 'local' and 'instantaneous' cross-tropopause fluxes (i.e. the fluxes at the smallest resolved scales) are, on average, about 40 times as large as their area- and time-mean value. This confirms the existence of a relatively large bidirectional exchange in the extratropics between the lower stratosphere and the troposphere. The diagnosed local and instantaneous cross-tropopause flux does not notably change if the horizontal and temporal resolution of the circulation data are decreased to 1 degr. and 6h. A further resolution decrease to 3.75 degr. and 12h, however, generally leads to quite different results for the local and instantaneous flux. The diagnosed zonal and monthly mean flux is not very sensitive to the latter resolution decrease, except at subpolar and subtropical latitudes. This small sensitivity in a large range of latitudes indicates that fluxes across the tropopause, which are quite concentrated onthe small scales, are nonetheless irrelevant for the global mass budgets of the troposphere and the stratosphere.
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