The HALOE water vapor observations suggest strong meridional transport out of the tropical lower
stratosphere,such that tropical air is mixed to midlatitudes with a time scale of 1-2 months.  This transport
occurs in the lowest stratosphere over potential temperature levels ~380-450 K (~16-19 km), while above
500 K (~20 km) the tropics are more isolated from extratropics.  We have performed 3-dimensional
trajectory simulations based on observed wind fields to study this transport.  The calculations use an
isentropic coordinate system (theta levels 380, 400, 420,...600 K), with horizontal velocities derived from
UKMO assimilated meteorological analyses, and vertical velocities (diabatic heating rates) derived from
a radiative heating code.  We initialize approximately 250, 000 parcels spaced regularly in the tropics
(20 N-S), and run trajectory calculations for 3 months (from January 1 ).  Below are a few results
showing the distribution of parcels initialized on the 400 K level, after transport for 10 and 60 days.
 

                                Transport after 10 days

                    Transport after 60 days

One clear result is that the parcels are transported out of the tropics and cover the majority of the
globe after 2 months time, similar to the behavior inferred from water vapor observations.  This
global-scale transport occurs mainly in the lowest stratosphere; parcels initialized at and above
500 K (~20 km) are much more confined to the tropics (not shown here).  Analyses of the
space-time spectral characteristics of the calculated transport (work in progress) shows that
transient tropospheric baroclinic waves are the main mechanism at work (the spatial scales can
be inferred from the day 10 'snapshot' above.  We are using many sets of these simulations
to quantify the spatial and temporal variability of 3-dimensional transport both out of and into
the tropical stratosphere.
 

See an animation produced by the NCAR Visualization Laboratory