Modes of Variability in Annual Hemispheric Water Vapor and Transport Fields

Abstract

The variability of annually averaged water vapor and water vapor transport fields over the Northern Hemisphere during the fifteen year period 1958?1973 is studied by means of empirical orthogonal function (EOF) analysis. Examined are vertically integrated values of the moisture quantities at a set of 91 stations chosen to cover the hemisphere as uniformly as possible. Analyses of the zonal and meridional transports are performed separately, but in addition a version of the EOF analysis is performed in which the two are treated as components of a single vector. All resulting modes are examined in light of statistical significance criteria established through a form of Monte Carlo testing. The first mode in moisture variability appears to be highly significant, and it is dominated by opposite behavior in the held over Africa and over the western equatorial Pacific. Its time series reflects a sharp change in regime during the period studied. The significant mode of variability in the zonal transport field mirrors that of water vapor content, whereas the first two modes of meridional transport variability point to the Afro-Indian region as being important in explaining changes in the zonal mean Hadley cell during the period.