The longest possible daily gridded satellite-derived data base of total precipitable water (PW) has been prepared by reanalysis of all TIROS Operational Vertical Sounder (TOVS) soundings, currently covering the tropics from 1979 to 1995. Separate retrieval equations are used for each satellite or major circumstance affecting instrument responses, calibrated to and validated by over 2 million radiosondes to eliminate possible artificial trends, biases, and discontinuities caused by satellite transitions or drifts. Individual retrievals have good accuracy in all environments including oceans, land, and in clouds, and there is no significant drift of the data base relative to collocated radiosondes during the entire period.
Worldwide tropical area-average PW shows a drying of 2.9% from 1979 to 1995, with much of the drying occurring in late 1988 and early 1989. Tropical surface temperatures warmed about 0.08 degrees C during the period, implying moistening of about 0.7% using an assumption of unchanged tropospheric dew point depressions. All PW trends and spatial changes appear in a lower resolution analysis of radiosondes alone, and the discrepancy between the expected moistening and observed drying is much too large to be attributed to possible radiosonde instrument or processing discontinuities.
Spatial patterns of moistening and drying are consistent with strengthening of the Hadley and Walker circulations. Moderate moistening occurs in the ITCZ, SPCZ, and much of tropical North and South America. There is strong drying in subtropical subsidence zones, especially from the Sahara westward into the Atlantic, in the cold pool areas west of South America and southern Africa, and over Australia westward into the Indian Ocean.
As hypothesized by Lindzen, a negative water vapor feedback could be caused by greenhouse warming and would be shown by a strengthened tropical convective overturning circulation. Increased evaporation would intensify and deepen convection, enhancing both precipitation and dry convective outflow. The increased outflow would strengthen subsidence in subtropical highs, possibly enlarging their area and reducing the depth of the marine boundary layer. The concentration of long-lived tropospheric radiatively active trace gases increased from 35% in 1979 to 48% in 1995 above preindustrial values (with the increases of all gases expressed in carbon dioxide radiative equivalents from IPCC 1995). The observed drying could be speculatively interpreted, at least in part, as an atmospheric response to greenhouse warming. This data base is planned to be updated and extended worldwide, allowing more confident monitoring of the water vapor signal in regional and worldwide decadal-scale shifts and long-term trends.