Diagnostic Study on Seasonality and Interannual Variability of Wind Field

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