| Large-scale cross-tropopause mass fluxes are
diagnosed globally from 1979 to 1989 for Northern Hemisphere winter conditions
(December, January, and February). Results of different methods of approaches
with regard to the definition of the tropopause and the way to calculate the
mass fluxes are compared and discussed. The general pattern of the mass exchange
from the tropopause into the stratosphere and vice versa agrees fairly well when
using different methods, but the absolute values can differ up to 100%.
An inspection of the temporal development of the mass fluxes
for solstice conditions indicates a complex picture. Whereas a permanent
significant downward flux from the stratosphere into the troposphere is detected
for latitude regions nearly between 25°N and 40°N and
between 30°S and 50°S (initiated by the poleward
branches of the Hadley cells), a non-uniform behaviour is observed at higher
latitude bands. Periods of strong mass exchange from the troposphere into the
stratosphere are disrupted by periods of an opposite mass exchange. A comparison
of the stratoshere-troposphere (ST) exchange with the exchange at higher
altitudes through surfaces, quasi-parallel to the tropopause, excludes a general
connection. Only a few strong upward directed ST mass exchange events have
counterparts at higher altitudes. The composition of the stratosphere may be
influenced directly by the ST exchange only in a thin layer above the tropopause. |