The boundary condition for water vapor entering the stratosphere in the tropics (irreversible passage through the 390 K θ surface) is set by the vertical/horizontal temperature structure of the cold trap region over the Western Tropical Pacific through which the quasi-horizontal passage of air provides adequate residence time (coupled potentially with multiple traversals of these cold pools) to desiccate the air to a vapor pressure corresponding to the local, seasonally dependent, temperature minimum.
Understanding the processes governing stratospheric water vapor is broadly recognized as a dominating scientific need. Changes in stratospheric water vapor under future climate regimes would strongly affect both radiative balance and ozone chemistry. In particular, increases in stratospheric water content could dramatically increase ozone losses by:
- increasing the cold aerosol surface area that draws down the NOx/NOy ratio thereby increasing the fraction of reactive halogens and hydrogen species portioned into free radical form (ClO, BrO, IO, OH and HO2)
- promoting formation of the polar stratospheric clouds on which catalytic ozone loss chemistry occurs
- increasing the source of HOx radicals
- radiative cooling of the lower stratosphere
Because of a lack of measurements, however, debate continues about a wide array of potential scenarios by which air entering the stratosphere is dehydrated. As an illustrative example, we note that while the average saturation mixing ratio of the tropical tropopause in January is 3 ppmv, the average water vapor content of air entering the stratosphere is 4 ppmv (Weinstock et al., 2001). Yet, nearly one quarter of the inner tropics in January has a cold point tropopause temperature corresponding to 2 ppmv (or less) co-located with the region of the most intense convection; so what fraction of the air entering the stratosphere (as it moves zonally and quasi-horizontally with convection superimposed on this background motion) has passed through this cold region?