H. Garny, T. Birner, H. Boenisch
Residual circulation transit times as diagnostic of
the structure of mean meridional transport
Transport in the stratosphere is determined both by the mean
meridional circulation and two-way mixing. The transport
time scales given by the mean meridional circulation
can be diagnosed by trajectories driven only by the
residual circulation, thus giving the hypothetical
'age' that air would have if it was transported only
by the residual circulation. This quantity is referred
to as 'residual circulation transit time' (RCTT).
The RCTTs can provide valuable information on the
structure of the circulation: it can be clearly
distinguished between a shallow and a deep branch of the
circulation, with air that travels along the deep branch
entering the stratosphere in the deep tropics and
traveling relatively far in the vertical compared to
the horizontal direction. Using results from a suit of
Global Climate Model (GCM) simulations, it is found that
transit times along the deep branch are controlled
by the strength of tropical upwelling,
while transit times along the shallow branch are
controlled by the shape of residual circulation
trajectories. Furthermore, changes in the tropopause
height influence transit times in particular along
the shallow branch.
Given a consistent estimate of age of air (AoA) together
with the RCTTs, as it can be obtained in GCMs,
the difference between AoA and RCTTs allows the
quantification of effects of mixing on AoA.
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