Observations and analyses of water vapor and clouds in the Tropics over the past decade show that the boundary between regions of high and low free-tropospheric relative humidity is sharp, and that upper-level cirrus and high free-tropospheric relative humidity tend to coincide. Most current studies of atmospheric climate feedbacks have focused on such quantities as clear sky humidity, average humidity, or differences between regions of high and low humidity, but the data suggest that another possible feedback might consist of changes in the relative areas of high and low humidity and cloudiness. Motivated by the observed relation between cloudiness (above the trade wind boundary layer) and high humidity, cloud data for the eastern part of the western Pacific from the Japanese Geostationary Meteorological Satellite-5 (which provides high spatial and temporal resolution) have been analyzed, and it has been found that the area of cirrus cloud coverage normalized by a measure of the area of cumulus coverage decreases about 22% per degree Celsius increase in the surface temperature of the cloudy region. A number of possible interpretations of this result are examined and a plausible one is found to be that cirrus detrainment from cumulus convection diminishes with increasing temperature. The implications of such an effect for climate are examined using a simple two-dimensional radiative–convective model. The calculations show that such a change in the Tropics could lead to a negative feedback in the global climate, with a feedback factor of about −1.1, which if correct, would more than cancel all the positive feedbacks in the more sensitive current climate models. Even if regions of high humidity were not coupled to cloudiness, the feedback factor due to the clouds alone would still amount to about −0.45, which would cancel model water vapor feedback in almost all models. This new mechanism would, in effect, constitute an adaptive infrared iris that opens and closes in order to control the Outgoing Longwave Radiation in response to changes in surface temperature in a manner similar to the way in which an eye's iris opens and closes in response to changing light levels. Not surprisingly, for upper-level clouds, their infrared effect dominates their shortwave effect. Preliminary attempts to replicate observations with GCMs suggest that models lack such a negative cloud/moist areal feedback.
*Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts.
+Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland.