Diagnostic study on seasonality and interannual variability of wind field

Based on NCEP/ NCAR reanalysis data during 1980-1994, seasonality and inter annual variability of the horizontal wind field are studied. It is shown th at: (1) In the lower troposphere, there exist regions with maximum of seaso nality in the tropics, the subtropics and high latitudes, which is called t he tropical subtropical and temperate-frigid monsoon region respectively. I n the upper troposphere, the subtropical monsoon combines with the tropical monsoon as a nonseparably planetary monsoon system. In the stratosphere, t here is a belt with very large seasonality in each hemisphere caused by the inversely seasonal circulation and by the establishment and collapse of th e night jet. (2) Seasonal variation of the large-scale monsoon may generall y be attributed to that of the zonal wind, however, seasonal variation of t he meridional wind is of great importance in East Asian monsoon region. (3) In monsoon region, interannual variability of the atmospheric general circ ulation is closely related to seasonal variation of monsoon, while in the t ropical Pacific, it may considerably be influenced by the external factors such as sea surface temperature (SST) anomalies associated with El Nino or La Nina event. Moreover, interannual variability undergoes a pronounced ann ual cycle.