Mixing processes and exchanges in the tropical and the subtropical UT/LS R. James and B. Legras Laboratoire de Météorologie Dynamique, Ecole Normale Supérieure – CNRS – Université Pierre et Curie, Paris, France
Abstract. Both in situ measurements and satellite observations
indicate evidence of mixing in the upper troposphere (UT) and the
lower-stratosphere (LS). In this study, the measurements performed
during the Pre-AVE and Costa-Rica AVE campaigns are analysed with
diffusive back-trajectories to assess mixing properties in the
tropical and the subtropical UT/LS. A description of
cross-tropopause pathways and mixing time scales is provided.
In the subtropics, Troposphere-Stratosphere mixing processes are
found to differ in the vicinity of the tropopause and at higher
altitudes. Below 350 K, a mixing line observed during Pre-AVE is
shown to result from fast and local cross-tropopause irreversible
exchange, involving two initially distant air masses with distinct
chemical compositions. For measurements located above 350 K, mixing
of the tropospheric air in the subtropical stratosphere occurs over
a period of a month, the origins of the tropospheric source being
localised in the tropical UT and the tropical boundary layer.
In the tropics, quantitative reconstructions of CO and
O3 profiles above 360 K are obtained for one month
back-trajectories calculations, pointing out that long term mixing
is essential to determine the chemical composition in the tropical
ascent. In particular, the existence of two-way meridional
irreversible exchanges between 360 and 450 K is found to export
tropical air in the subtropical stratosphere and to entrain old
stratospheric air in the tropical ascent. Mean age of air
calculated with a Lagrangian model is shown to be in agreement with
the CO2 observations.
Citation: James, R. and Legras, B.: Mixing processes and exchanges in the tropical and the subtropical UT/LS, Atmos. Chem. Phys., 9, 25-38, doi:10.5194/acp-9-25-2009, 2009.