Semiannual variations of several atmospheric hydrological processes are identified from two sets of data: (1) the monthly mean precipitation data generated by the Global Precipitation Climate Project (GPCP) and (2) the monthly mean upper air wind and moisture data generated by the Climate Data Assimilation System of National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis Project for the period 1986--1993. Three major findings of this study follow. First is that the existence of semiannual variations in precipitation and water vapor flux are revealed from (1) time series of area-mean precipitation and divergence of water vapor flux over the Asian-Australian (AA) monsoon (60 ¿E--120 ¿W) and extra-AA monsoon (120 ¿W--60 ¿E) hemispheres and (2) the power spectra of that time series. Second is that the semiannual precipitation variation exhibits an east-west seesaw oscillation. In January and July, positive precipitation anomalies concentrate in the AA monsoon hemisphere, while major negative precipitation anomalies exist in the extra-AA monsoon hemisphere. Opposite spatial patterns occur in April and October. Third is that to maintain the semiannual variation of precipitation, water vapor diverges out of the extra-AA (AA) monsoon hemisphere and converges toward the AA monsoon (extra-AA monsoon) hemisphere in January and July (April and October). ¿ 1998 American Geophysical Union