Atmospheric Mixing in a Lagrangian Framework

Inaccurate representation of mixing in chemistry transport models strongly
influence the time evolution of all relevant trace gases and, in particular, the
qualitative determination of the stratosphere-troposphere exchange (STE). For
this reason, a physics-based numerical representation of mixing is required but
remains an uncertain piece for the atmospheric transport models. However, the
Lagrangian view of transport offers an alternative to exploit the numerical diffusion
for parametrization of the physical mixing rather than to find ways of
avoiding this effect.
Using the standard version of the Chemical Lagrangian Model of the Stratosphere
(CLaMS) with mixing parametrization triggered by strong flow deformations,
a remarkable Sudden StratosphericWarming (SSW) case is investigated to
reexamine transport, especially mixing, through analyzing the variation of stratospheric
composition and of the tracer-tracer correlations. The case study of SSW
demonstrates the intensified sub-seasonal variability of polar descent and tropical
upwelling, which further motivates the study of the long-term impact of SSWs
on the variability of the water vapor in the tropical lower stratosphere based on
a CLaMS 35-year run. A sub-seasonal SSW-associated dehydration effect in
the tropical lower stratosphere modulated by the two quasi-biennial oscillation
(QBO) phases is found. The cooling and drying at the tropical tropopause, as a
result of enhanced breaking of planetary waves in the subtropics during SSWs,
is more intensive in the easterly QBO phase than in the westerly QBO phase