Water Vapor–Induced OLR Variations Associated with High Cloud Changes over the Tropics: A Study fromMeteosat-5Observations

Abstract

Subdividing the Indian Ocean domain into three areas: (i) a moist cloudy area due to tropical deep convection, (ii) a moist clear area fed by the evaporation of hydrometeors from adjacent high clouds, and (iii) a dry area represented by descending air over the subtropics, the relationships between upper-tropospheric humidity over these three areas and tropical convections are examined using the European Geostationary Meteorological Satellite (Meteosat-5) observations. It is observed that the clear dry area shrinks and becomes drier in response to expansion of the cloudy area in the Tropics and vice versa. This change in upper-tropospheric humidity over the subtropics appears to mitigate the increase (decrease) in water vapor greenhouse effect caused by the expansion (contraction) of moist convective areas. A simple sensitivity test shows that the strength of the water vapor feedback due to changes in the spatial extent of tropical convection is benign, though slightly negative, if the changes in subtropical dryness are considered.