Passive microwave observations of water vapor profiles during ENSO events.

摘要

The handling of the water vapor feedback in general circulation models (GCM's) is controversial. Lindzen (1990, 1997) has proposed a mechanism by which water vapor would cause a negative feedback, rather than the positive feedback which is predicted by most GCM's. His hypothesis posits that warming leads to a more intense convective circulation, which leads to a drying of the upper troposphere. In this study, a dataset is presented that is a relevant test for climate models based on observations of the response of water vapor to a regional warming.; Retrievals of water vapor profiles were performed using a physical relaxation algorithm on data from the Special Sensor Microwave-Temperature-2 (SSM/T-2) for November 1995 (during La Niña) and November 1997 (during El Niño). These retrievals were carried out over the region of the tropical and subtropical Pacific and eastern Indian ocean. Water vapor profiles reveal moistening (drying) in the middle and upper troposphere above areas of warming (cooling) near the Equator, and drying (moistening) in the subtropics to the north and south. These observations are consistent with Lindzen's hypothesis.; Adjustments were made to the water vapor field to remove the expected water vapor profile based on both local sea surface temperature and rainfall. After each of these adjustments, the water vapor field still shows the aforementioned changes. Therefore, local effects are unable to completely explain the changes in water vapor; the large-scale circulation must be considered.; Finally, clear-sky outgoing longwave radiation (OLR) was computed from the Maryland Terrestrial Radiation Package (MDTERP) using observed mean profiles of relative humidity and temperature in a 2.5° grid for each month, ignoring the effects of clouds. Increases in OLR of as much as 10 W m−2 is seen in regions of drying, and reduced OLR is seen in regions of moistening. Patterns of computed OLR agree closely with cloud-cleared observations of OLR. Both observed and calculated OLR plots show that water vapor changes have a significant impact on the OLR field.