Global water vapour distribution in the upper troposphere and lower stratosphere during CRISTA 2 Original Research Article

Water vapour measurements during the second mission of the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) instrument are presented in the altitude regime 8–20 km. Mixing ratios are shown on isentropic surfaces (300–500 K) as global zonal means and as averages in 60° longitude sectors. Transports are indicated to occur preferentially on isentropic surfaces in the northern hemisphere, but not in the tropics and in the south. The hygropause is found shifted away from the equator towards the winter hemisphere. The amount of shift is longitude dependent and can be as great as 20–30°. The water content of the hygropause area shows considerable zonal asymmetries. A circumpolar toroidally shaped layer of enhanced water vapour is observed at high southern latitudes (60°S). This belt extends from somewhat above the tropopause up to the 450 K level. Polar stratospheric clouds (PSC) are observed near and above the area of highest water vapour mixing ratios. Evaporation of PSC particles and rehydration of the atmosphere are thus suggested as the origin of the enhanced water vapour belt. A tongue-like ridge of enhanced water vapour extends at 30°W from the belt northward to the equator and beyond. It appears to moisten the TTL and tropical lower stratosphere. Related time scales are one or a few days. Intrusions of this kind appear to be not unfrequent and may therefore play a role for the water vapour budget of this atmospheric regio