An observational study of the tropical-subtropical semiannual oscillation

The upper-air wind data generated by the Global Data Assimilation System of the National Meteorological Center for 1986-1992 are used to depict the three-dimensional structure of the semiannual oscillation of tropical stationary eddies in terms of the eddy streamfunction. An eddy streamfunction budget analysis was also performed to disclose the cause of this semiannual oscillation. The major findings are: 1) The tropical stationary eddies exhibit a seesaw semiannual oscillation between the eastern and Western Hemisphere with a phase reversal vertically at 400-500 mb and meridionally at the equator. 2) It is inferred from the streamfunction budget analysis that the semiannual oscillation of the tropical stationary eddies is caused by the semiannual east-west seesaw of the global divergent circulation between the areas of the Asian-Australian (AA) monsoon (60°E-120°W) and the extra-AA monsoon (120°W-60°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 strong effect of the land-sea contrast on the east-west interseasonal phase change of the stationary eddies in the Northern Hemisphere.