H. Garny, T. Birner, and H. Boenisch
The impact of mixing on Age of Air
Transport in the stratosphere is determined both by
the mean meridional circulation and two-way mixing.
Stratospheric age of air (AoA) is a measure
of the integrated effect of all transport processes
that affected an air parcel on its way through the
stratosphere after crossing the tropopause.
Mean AoA is often used to quantify the transport
circulation in the stratosphere, namely the Brewer-Dobson
Circulation (BDC). Global models project an increase
in the mean meridional circulation in a changing
climate, and simultaneously a decrease in AoA.
However, evidence of changes in mean AoA from
observations is weak. To infer from the AoA
measurements on changes in the residual circulation,
that cannot be measured directly, a better
understanding of the relation between AoA and
the residual circulation is necessary. Global models
provide residual circulation data consistent with AoA,
and thus can be used to investigate this relationship.
We use trajectories driven only by the residual
circulation to derive 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 difference between AoA and RCTT is then the
additional aging of air caused by mixing processes. It
is shown that this aging by mixing is positive
throughout the lower stratosphere, only in the lowermost
stratosphere at high latitudes, air is younger than
expected from residual circulation transport only.
The processes of the impact of mixing on AoA are further
investigated using a simple tropical leaky pipe (TLP)
model. The TLP model can explain the general increase
of AoA in the lower stratosphere and above by mixing
with the recirculation of air parcels. Aging by mixing
is dependent both on 1) the mixing strength, that
controls the fraction of air that recirculates,
and 2) the residual circulation strength, that controls
the speed of recirculation. Thus, stronger mixing
increases aging by mixing, as a larger fraction of
air recirculates, while a stronger residual circulation
reduces aging by mixing, as the recirculation speeds up.
However, the mixing strength and the residual circulation
are physically linked as both are driven by breaking waves.
Therefore, an increase in wave breaking leads to
a faster residual circulation but also to stronger mixing.
By ļ¬tting the TLP model to the global model results,
it is shown that in the lower stratosphere the total
effect of stronger wave breaking is to reduce aging
by mixing. Thus, the faster recirculation dominates over
stronger mixing. Therefore, it can be expected that
mixing effects amplify the decrease in AoA caused
by a faster residual circulation.
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