Interactions between the Northern and Southern Hemispheric Atmospheres and the Essence of Monsoon
Zeng Qingcun and Li Jianping(Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029)
Based on the theoretical consideration and the statistical analysis of NCEP/NCAR reanalyses data, it is clear that the causes of seasonal variations of atmospheric general circulation, the cross-equatorial air flows and the interactions between the Northern and Southern Hemispheric atmospheres are primarily due to the seasonal variations of incoming solar radiation. The primary driving forcing of the tropical monsoon, characterized by the cross-equatorial meridional wind component, is the planetary thermal convection, and the semi-permanent planetary waves due to the seasonal variation of the differences between the thermal characters of earth surface (ocean and continent) are the secondary forcing. The calculation results show that the efficiency ratio of the first to second forces, on the average, is 2:1. The two forces are in phase in the eastern hemisphere, resulting in pronounced Asian-Australian monsoon prevails, but out of phase in the western hemisphere, resulting in weak seasonal variations and monsoon over the tropical Atlantic and the tropical eastern Pacific. Besides, the semi-permanent planetary waves in the middle-high latitudinal zones owing to the seasonal variations of thermal contrast of land and ocean and the incoming solar radiation are favourable for the Asian-Australian monsoon to extend to the extratropical regions. At high altitudes, the air flows compensating the low-level monsoons also undergo significant seasonal variations. Thus there is a clear picture of globally three-dimensional general monsoon system.