A study of the different characteristics of seasonal variations of the precipitation and large-scale circulation between East Asia and eastern North America and its possible mechanism

Based on the NCEP/ NCAR gridded reanalysis data and the CMAP, GPCP data, a study is conducted with focus on the comparative analysis of seasonal variations of the precipitation and large scale circulation between East Asia and eastern North America. The results suggest that the circulation pattern of the two regions is similar in winter but different in summer. Besides, based on "wind" and "rainfall" which are the two characteristics of monsoon, we find that the seasonal variation of precipitation and low level wind is very different: The seasonal difference of the precipitation over East Asia is significant, presenting the monsoon rainfall characteristic called "wet summer and dry winter". In the lower troposphere, the prevailing wind direction in East Asia in summer is opposite to that in winter. However, the situation in eastern North America is opposite. Furthermore, as the essential driving force of monsoon, the land sea thermal contrast in East Asia is different from that in eastern North America. There is prominent inversion of meridional and zonal temperature gradient along with the seasonal shift in East Asia. On the other hand, the inversion does exist in zonal temperature gradient in eastern North America, but does not in meridional temperature gradient. Besides, that the large scale circulation is different in the two regions is also attributed to the difference between the Tibet Plateau and Rocky Mountains. Therefore, the differences of the seasonal evolution of meridional and zonal temperature gradient and the forcing of Tibet Plateau and Rocky Mountains may be the main causes of the subtropical monsoon climate in East Asia as well as the non-monsoon climate in North America. The numerical experiment will be needed to confirm this conclusion.