Water vapor provides a valuable tracer for studying transport into and within the stratosphere.  We have used the long
record (1991-2000) of UARS HALOE measurements, combined with UARS MLS data in polar regions (not observed
by HALOE), to analyze the global variability of water vapor in the lower stratosphere.  These data reveal intriquing
details on the global transport in this region.  Below are some monthly meridional cross sections: blue is relatively dry
and red is relatively wet air.  The dashed line is the tropopause, and the solid lines are isentropes of 380, 400, 500 and
600 K.   Images for each month can be found here.

The dryest air originates in the tropics in January, as this is the season of coldest temperatures near the tropical
tropopause (and the water vapor is 'freeze-dryed').  This dry air is transported meridionally in the lowest stratosphere,
and covers much of the globe by March.  This seasonally-varying signal is also transported vertically by the mean
upward circulation in the tropics, giving rise to the well-known 'tape-recorder' (note the upward movement between
January and August).  Note that in the lowest stratosphere the patterns of dryest air approximately follow the
380-400 K isentrope layer (which slopes downward with increasing latitude).  This meridional transport is also
clearly seen in a latitude-time section at 390 K.
 

The main results of this work include:

• demonstration of rapid quasi-isentropic transport between the tropics and extratropics in the lowermost stratosphere.  Near-global mixing  occurs with a time-scale of ~1-2 months over isentropes ~ 380-450 K.  Above 500 K the tropics are more isolated from extratropics.
•  The seasonal cycle in lower stratospheric water vapor in extratropics follows that in the tropics.  Very dry air over the North Pole in NH summer  originates in the tropics ~4 months earlier.  The tropical dry air moves into the SH and merges with the residual of Antarctic dehydration in ~February,  so that the dehydrated polar and tropical air masses are no longer distinct.
• The persistence of dry air in the lowest stratosphere throughout summer over both polar regions argues for relatively weak transport through this region.
• The NH summer monsoons exert a strong influence on hemispheric constituent budgets in the lower stratosphere, and are an important mechanism for stratosphere-troposphere exchange.
• Longituninal structures in water vapor (and ozone) identify regions of strong coupling to the troposphere.  An intriguing result is that the solstice minima in tropical water vapor are spatially separated from maximum convection and coldest temperatures, suggesting the importance of transport related to the local Hadley circulation.
• Detailed comparisons with balloon and aircraft measurements show that the HALOE data provide accurate estimates of large-scale variability.

For more information, see our paper:

Seasonal cycle of water vapor in the lower stratosphere observed in Halogen Occultation Experiment data
William J. Randel, Fei Wu, Andrew Gettelman, J.M. Russell III, Joseph.M. Zawodny and Samuel J. Oltmans
J. Geophys. Res., 106, 14313-14326 (July 16, 2001). click here for a pdf file

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