ANNUAL VARIATION OF THE GLOBAL PRECIPITABLE WATER AND ITS MAINTENANCE- OBSERVATION AND CLIMATE-SIMULATION

The annual variation of the global-mean precipitable water [W] and the associated hydrological cycle were analyzed with the upper-air data g enerated by the Global Data Assimilation System of the National Meteor ological Center for 1981-1991 and the European Centre for Medium Range Weather Forecasts from 1983-1991. It was found that the annual variat ion of [W] coincides with that of the Northern Hemisphere precipitable water [UT](NH). The hemispheric-mean ([])water budget analysis shows that water vapor is transported from the winter to the summer hemisphe re across the equator by the Hadley circulation, and that the annual v ariations in the water vapor sink [P - E] for both hemispheres also fo llow the same seasonal march. The amplitudes of the annual variations in these two hydrological processes are comparable in both hemispheres . Thus, the annual variations of [W](NH) and [W](SH), are the result o f slight imbalances between the cross-equator wafer vapor transport an d the water vapor sink, particularly in the spring and fall. The clima tological hemispheric-mean water budgets reveal that the Southern Hemi sphere is a water vapor source and the Northern Hemisphere is a water vapor sink. The cross-equator water vapor transport constitutes a majo r source acting to maintain [W](NH), and in turn [W]. The hydrological mechanism maintaining the observed [W] annual variation is consistent with that obtained from the hydrological cycle in a 10-year (1979-198 8) climate simulation done at the Goddard Laboratory for Atmospheres a s part of their participation in the Atmospheric Model Intercomparison Project (AMIP).