Seasonal variation of global surf ace pressure and water vapor

Previous studies have shown that the seasonal variation of global-mean surface pressure (ps) results from variation of global-mean water vapor pressure (pw). The current study, employing the global data generated by Version 1 of the Goddard Earth Observing System (GEOS-1) Data Assimilation System, shows that seasonal variations of regional ps and pw tend to be out of phase (particularly in the subtropics of the two hemispheres) and that the magnitude of the former variation is generally much larger than that of the latter. The seasonal variations of these two quantities are maintained by airmass and water vapor transports by the global divergent circulation, which is driven by the latent heat released by cumulus convection over the water vapor sink, as the “water mass forcing” mechanism predicted. Since pwand ps are used often in depicting the climate system, assessments of climate change in terms of the global-mean and regional variations of these two variables should be interpreted with caution.