AN OBSERVATIONAL STUDY OF THE TROPICAL-SUBTROPICAL SEMIANNUAL OSCILLATION

The upper-air wind data generated by the Global Data Assimilation Syst em of the National Meteorological Center for 1986-1992 are used to dep ict the three-dimensional structure of the semiannual oscillation of t ropical stationary eddies in terms of the eddy streamfunction. An eddy streamfunction budget analysis was also performed to disclose the cau se of this semiannual oscillation. The major findings are: 1) The trop ical stationary eddies exhibit a seesaw semiannual oscillation between the eastern and Western Hemisphere with a phase reversal vertically a t 400-500 mb and meridionally at the equator. 2) It is inferred from t he streamfunction budget analysis that the semiannual oscillation of t he tropical stationary eddies is caused by the semiannual east-west se esaw of the global divergent circulation between the areas of the Asia n-Australian (AA) monsoon (60 degrees E-120 degrees W) and the extra-A A monsoon (120 degrees W-60 degrees E). This mechanism is particularly clear in the Southern Hemisphere Tropics but less well established in the Northern Hemisphere Tropics, which may be attributed to the stron g effect of the land-sea contrast on the east-west interseasonal phase change of the stationary eddies in the Northern Hemisphere.