GLOBAL-SCALE INTRASEASONAL AND ANNUAL VARIATION OF DIVERGENT WATER-VAPOR FLUX

The global-scale intraseasonal and annual variations of divergent water-vapor transport and water vapor itself were examined by using outgoing longwave radiation (OLR) and data for 1979-1986 produced by the Global Data Assimilation System of the National Meteorological Center. An effort was also made to contrast results of this study with previous analyses of OLR and upper-level divergent circulation. As for intraseasonal oscillation, positive (negative) precipitable-water (W) anomalies and negative (positive) OLR couple with the convergent (divergent) center of the potential function of water vapor transport (chi-Q) anomalies and the divergent (convergent) center of upper-level divergent-circulation anomalies. It is inferred that the eastward-propagating divergent circulation of intraseasonal oscillation converges water vapor to maintain cumulus convection, which releases latent heat, possibly to support this low-frequency oscillation. Fluctuations of W and cumulus convection associated with this oscillation are large over the equatorial Indian Ocean and the equatorial western Pacific, but small over the tropical Americas and equatorial Africa. Moreover, during northern summer, W anomaly bands migrate regularly northward, following the low-level transient 30-50 day monsoon troughs and ridges over the northern Indian Ocean. To the south of the equator, a regular southward propagation of W anomaly bands is identified in both northern summer and winter. In contrast; over the northwestern Pacific, a signature depicting the north-south intraseasonal oscillation of the north Pacific Convergence Zone can be inferred by W anomalies. The annual cycle components of W and cumulus convection inferred from OLR anomalies exhibit three pairs of maximum-minimum centers over tropical continents. These centers correspond to those of chi-Q and upper-level divergent circulation anomalies. It is shown that landmass cooling in the winter hemisphere and landmass warming in the summer hemisphere establish a pair of upper-level convergent-divergent centers over each tropical continent. Water vapor is converged (diverged) by circulation, in order to maintain maximum (minimum) centers of W and cumulusconvection anomalies over each tropical continent.