Wintertime asymmetry of upper tropospheric water between the Northern and Southern Hemispheres

Abstract

WATER vapour is an important greenhouse gas^1–3 and yet its abundance in the upper troposphere is poorly known. Upper-tropospheric water vapour is particularly important despite its low mixing ratios, because it has large effects on the flux of infrared radiation near the tropopause². In addition, the distribution and supply of water vapour are central to cloud formation; the effects of cloud on the Earth's radiation budget are in turn central to understanding the climate response to increasing atmospheric concentrations of greenhouse gases. From airborne measurements of total water (vapour plus ice crystal)⁴ during the winters of 1987 in the Southern Hemisphere and of 1988–89 in the Northern Hemisphere, we find that the upper troposphere in middle, subpolar and high latitudes is a factor of 2–4 drier during austral winter than during boreal winter. As the lower-latitude air moves towards the pole in austral winter, it is forced to cool to lower temperatures than in the north—more of the water vapour therefore condenses to form ice crystals, which then precipitate, thereby removing moisture from the air mass. Clearly, climate models must be able to reproduce this asymmetry if their predictions are to be credible. We also note that the asymmetry in water vapour implies an asymmetry in the production rate of the hydroxyl radical, and hence in the tropospheric chemistry of each hemisphere, for example in the rate of methane loss⁵.