Meridional transport in the lower stratosphere
 

The meridional transport of water vapor in the lower stratosphere is most
clearly seen in a latitude-time section on the 390 K isentrope (below).  The
minimum occurs in the tropics during January-March, and mixes into both
hemispheres with a time scale of several months.  Note that the seasonal
variation in midlatitudes of both hemispheres follows that in the tropics,
and also that low tropical water vapor is transported all the way to the Arctic
(and persists there throughout the summer).  In the Antarctic additional
dehydration occurs during July-November in the intensely cold SH polar
vortex: this is seen in the August cross-section above.   This Antarctic
dehydration persists in the lowest stratosphere (below 400 K) at least until
January, after which time it merges with the dry air transported from the
tropics.  The maximum water vapor in NH summer (centered near 30 N in
August) is a result of the Asian and North American  monsoon circulations.