Multi-year Variability of Tropospheric Water Vapor

A better understanding of the spatial distribution and temporal variation of water vapor is essential for investigations of the hydrologic cycle and climate. Previous work by Ross and Elliott (1996, 1998) examined the distribution and trends of tropospheric water vapor over the Northern Hemisphere using radiosonde data. Water vapor increases for the period 1973-95 were found over most of the Northern Hemisphere. However, temporal changes in water vapor are not monotonic and can be expected to be intertwined with circulation changes on many time scales. We examine the relationships among humidity variations on various time scales from interannual to decadal or longer and multi-year atmospheric variations.

Humidity data from Northern Hemisphere radiosonde stations with reasonably complete, temporally homogeneous records form the primary data set for this analysis. To quantify the interannual variations of water vapor, principal component analysis has been used as an objective means of determining patterns of coherent variation. Correlations between the pattern loadings and various circulation indices, such as representing ENSO, the North Atlantic Oscillation and the North Pacific Oscillation, will be shown. Patterns describing the interannual variations of water vapor will also be compared to those of temperature and precipitation. Finally, we will discuss how interannual variability of water vapor influences the interpretation of linear trends in humidity.