Hemispheric asymmetries in water vapor and inferences about transport in the lower stratosphere

Both satellite water vapor measurements and in situ aircraft measurements indicate that the southern hemisphere lower stratosphere is drier than that of the northern hemisphere in an annual average sense. This is the result of a combination of factors. At latitudes poleward of ∼50°S, dehydration in the Antarctic polar vortex lowers water vapor mixing ratios relative to those in the north during late winter and spring. Equatorward of ∼50°S, water vapor in the lower stratosphere is largely controlled by the tropical seasonal cycle in water vapor coupled with the seasonal cycle in extratropical descent. During the tropical moist period (June, July, and August), air ascending in the Indian monsoon region influences the northern hemisphere more than the southern hemisphere, resulting in a moister northern hemisphere lower stratosphere. This tropical influence is confined to levels beneath 60 mbar at low latitudes, and beneath 90 mbar at high latitudes. During the tropical dry period (December, January, and February), dry air spreads initially into both hemispheres. However, the stronger northern hemisphere wintertime descent that exists relative to that of southern hemisphere summer transports the dry air out of the northern hemisphere lower stratosphere more quickly than in the south. This same hemispheric asymmetry in winter descent (greater descent rates during northern hemisphere winter than during southern hemisphere winter) brings down a greater quantity of “older” higher water vapor content air in the north, which also acts to moisten the northern hemisphere lower stratosphere relative to the southern hemisphere. These factors all act together to produce a drier southern hemisphere lower stratosphere as compared to that in the north. The overall picture that comes from this study in regards to transport characteristics is that the stratosphere can be divided into three regions. These are (1) the “overworld” where mass transport is controlled by nonlocal dynamical processes, (2) the “tropically controlled transition region” made up of relatively young air that has passed through (and been dehydrated by) the cold tropical tropopause, and (3) the stratospheric part of the “middleworld” or “lowermost stratosphere”, where troposphere‐stratosphere exchange can occur adiabatically. Satellite water vapor measurements indicate that the base of the “overworld” is near 60 mbar in the tropics, or near the 450 K isentropic surface.