| A new data base of tropical precipitable water, covering the tropics (30°N to 30°S) with daily grids from 1979 to 1995, has been prepared using satellite soundings from the Television InfraRed Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) instrument on National Oceanographic and Atmospheric Administration (NOAA) satellites. The algorithm is based on statistical regression, with separate equations for each satellite and major circumstance that alters the relationship between precipitable water and satellite radiances. The data has been extensively validated for accuracy using radiosonde data at all levels from large-scale averages and trends to individual retrievals. The retrieval quality is compared with almost all other instruments used for water vapor remote sensing and with over 100 reported retrieval methods. Evaluation of the radiosonde record shows little or no spurious drying due to instrument changes.; Grid averaged precipitable water shows 3% average drying from 1979--87 to 1989--95, with nearly steplike drying in late 1988 and early 1989. Using only radiosonde data back to 1973, the tropics moistened quickly in the late 1970s. The late 1970s moistening occurs in a broad worldwide equatorial band, consistent with a well-documented decadal-scale climate shift in 1976--77, apparently starting with tropical Pacific warming. The late 1980s drying shows narrow moistening areas in the Inter-Tropical Convergence Zone, the South Pacific Convergence Zone, and the Americas, more than offset by large drying areas in subtropical highs and over deserts, consistent with a hypothesized climate shift in 1988--89, which contracted the Northern Hemisphere polar vortex and allowed the tropical overturning circulation to intensify. While the current data base is too short to prove the water vapor role in greenhouse warming, the 1988--89 shift is consistent with the negative feedback theory of the response of water vapor to greenhouse warming, where warming intensifies atmospheric circulations, enhancing convection which is balanced by larger drying areas, which increases the earth's heat loss to space and partly offsets the warming. |
