The seasonal variations of the wind fields mainly at 850mb and the outgoing long wave radiation (OLR) over Asian and Australian monsoon regions are examined using 5-day mean data.
Some abrupt changes of the circulation systems and the OLR distribution are recognized in transitional seasons almost simultaneously over a wide region. They occur in early March, late March, mid-April, mid-May, mid-June, late July, early September, late September, late October, mid-November and the end of the year. According to these distinct periods, eleven natural seasons are established. A close relationship is shown between these natural seasons and the advance and retreat of each monsoon system. The seasonal composite maps of low level (850mb) wind and the OLR fields are constructed and the features of each season are described.
The main factors influencing these abrupt seasonal changes are discussed by utilizing difference maps of seasonal mean 500mb height and 300mb temperature between two consecutive seasons. They are the deepening and shallowing, and the displacement of the quasi-stationary long wave trough at the east coast of the Eurasian Continent, the warming and cooling over the Tibetan Plateau and the wave-like interaction between tropical convection and midlatitude westerlies, part of which may be related to the behavior of the long-wave trough mentioned above.
Lastly, the regional divisions are performed based on the mean wind of two extreme seasons at both 850mb and 1000mb. Three main regions with seven sub-regions are distinguished.