东亚-太平洋型季节内演变和维持机理研究

利用850hPa的纬向风异常建立一个逐候东亚-太平洋(East Asian Pacific,EAP)型指数,研究其季节内演变特征,发现东亚-太平洋型经向波列是东亚夏季风季节内变化的主要模态.其演变过程为:扰动首先出现在北太平洋中部,并通过正压不稳定过程从基本气流中获得能量而发展,在高层罗斯贝波能量向南频散,激发热带对流异常和赤道罗斯贝波,并相互锁相,因赤道罗斯贝波受β效应影响而共同向西移动.热带对流和环流异常在菲律宾附近达到最强,此时在东亚沿岸出现经向三极型波列,此后中低纬度异常继续向西北方向移动,使降水异常在长江流域能维持较长时间.东亚-太平洋型在东亚发展和维持有以下原因:首先,菲律宾暖水上空的对流和低层环流之间存在正反馈;其次,由于海陆热力差异导致暖大陆和冷海洋之间存在特殊的纬向温度梯度和北风垂直切变,东亚-太平洋型在经向上有向北倾斜的斜压结构,能通过斜压能量转换从平均有效位能中获得能量,同时,也能从经向温度梯度的平均有效位能中获得能量.

Researeh of the intraseasonal evolution of the East Asia Pacific pattern and the maintenance mechanism

A pentad East Asia Pacific(EAP) pattern index based on 850 hPa tonal wind anomalies is established to analyze the intrascasonal EAP variability.It is shown that the EAP-like meridional wave train is the dominant mode of the East Asian Summer Monsoon(EASM) intrascasonal variability.The evolution process is as follows; disturbances firstly occur in the middle of the North Pacific, barotropic energy conversion from the middle latitude mean flow facilitates the development of the middle latitudc center of the anomaly circulation,and the southward dispersion of the Rossby wave energy excites convection anomaly and equatorial Rossby waves which tend to reinforce each other over the tropical western Pacific region to form the main EAP pattern.The low and mid-latitude anomalies then locked together and both move westward because of the equatorial Rossby-wave beta-effect control on its tropical center.The anomalous convection and circulation become strongest near Philippines when a tripolar wave train appears in the East Asian coast, and then the low and mid-latitude anomalies continue moving northwest-ward and weakened though in the Yangtze valley basin precipitation anomaly is able to maintain.The reasons why the EAP pattern develops and maintains in East Asian are as follows:first of all,there exists the positive feedback between convection over the Philippines sea warm pool and the low-level circulation;secondly, the EAP pattern tilts to the north in the meridional direction,resulting from a significant energy supply from baroclinic energy conversion from mean available potential energy associated with land-ocean tonal temperature contrast and thus the meridional wind shear between high and low levels.It also draws energy from mean available potential energy owing to the meridional temperature gradient.