The technique is applied first to an idealized GCM in which the dynamics are dry and forced using the Held-Suarez thermal relaxation, but the model carries a passive water-like tracer which is emitted at the surface and lost due to large-scale condensation with zero latent heat release and no condensate retained. The technique provides an accurate reconstruction of the simulated water vapor field, and application in this context allows us to assess some of the (relatively small) errors associated with resolution and averaging. The technique is then applied to the NCEP/NCAR Reanalysis for periods during the winters of 1997 and 1998.
In both the idealized model and the Reanalysis, the dry air in the subtropical troposphere is produced primarily by isentropic exchange with the extratropics. While it is still true, as assumed in some recent studies, that most of the water vapor in the subtropical dry zones comes from the tropics, the implication of our results is that, under climate change, humidity changes in the subtropical dry zones could be driven to a large extent by extratropical processes, to the extent that these could change the fraction of subtropical air which experiences extratropical isentropic drying.