Tuesday, 4 June 2002: 11:30 AM
Subtropical dryness change mediating water vapor negative area feedback in the tropics (Formerly Poster P2.16)
Satellite observations show a strikingly sharp boundary between areas of high moisture around the deep convective tropical cloud systems and adjacent cloud free areas (Held and Soden, 2000). Conceptually the tropical area can be subdivided into three areas: i) a moist cloudy area due to tropical deep convection, ii) a clear sky moist area fed by the evaporation of hydrometeors from clouds and iii) a clear dry area that represents the area of descending air. The outgoing longwave radiation and the greenhouse effect over those areas are quite different and it has been argued that the relative area changes in response to changes in tropical sea surface temperature constitute an important element in the water vapor feedback (Lindzen et al. 2001). Here we present a relevant analysis of cloud and upper tropospheric humidity (UTH) feedbacks from Meteosat-5 observations over the Indian Ocean. It is found that the clear moist area is proportional to the moist high cloud area and the ratio is about 2 during the observational period of January and February 1999. It is also observed that the dry area shrinks and becomes drier in response to the expanded cloud area in the tropics or vice versa. A radiative transfer model is used to examine the impact of the relative area and dryness/wetness changes on the water vapor greenhouse effect. Partly confirming previous work (Lindzen et al., 2001) this analysis shows that the water vapor area feedback operates in tropical regions. However, it is the subtropical high to mediate climate fluctuations in the tropics through the redistribution of water vapor; i.e., one stabilizing element in the climate system. In summary this study reinforces the potential climatic role of upper tropospheric water vapor changes controlled by tropical deep convection and subtropical subsidence.