东亚与北美东部降水和环流季节演变差异及其可能机理分析

利用NCEP/ NCAR 再分析资料和CMAP、GPCP卫星反演降水资料,对比分析了东亚与北美东部地区降水和大尺度大气环流季节演变特征的差异。结果显示,东亚和北美东部地区冬季环流形势较为相似,而夏季差异则较大,这正是东亚为季风区,北美为非季风区的表现。此外,基于季风的两大特征量“风”和“雨”,分析了两地降水和低空风场季节变化的显著差异：东亚副热带地区降水季节变率大,呈“夏湿冬干”的季风降水特征,低层盛行风向随季节逆转,冬季盛行偏北风,夏季盛行偏南风,具有显著的副热带季风区特征。北美东部副热带地区全年雨量分配均匀,低层常年盛行偏西风,呈现非季风区特征。进一步的分析发现,作为季风基本推动力的海、陆热力差异在东亚和北美东部地区有着显著的区别：东亚地区的经向和纬向温度梯度随季节反转的特征显著;而北美东部地区虽有纬向温度梯度的季节反转但幅度很小,且经向海、陆热力差异随季节反转不明显。此外,与青藏高原和落基山脉相伴的纬向环流也存在显著差异。鉴于此,提出经向和纬向海、陆热力梯度反转特征的不同以及青藏高原和落基山脉地形的不同作用很可能是造成东亚副热带季风气候而北美东部非季风气候的主要原因,上述结论还有待于数值试验的进一步证实。

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.