大气环流的季节突变与季风的建立I·基本理论方法和气候场分析

The Abrupt Seasonal Transitions in the Atmospheric General Circulation and the Onset of Monsoons Part I:Basic Theoretical Method and Its Application to the Analysis of Climatological Mean Observations

将曾庆存等提出的大气环流的季节划分和计算季风的理论方法作了改进,使之更便于研究季风的建立过程.该理论方法将环流突变和季风建立时段,其'变差度'和与其前和其后场的'相似度'等由空间场的泛函随时间的变化(即数学名词上的'流'[flow])一起求出来.研究Ⅰ先分析气候平均场,Ⅱ分析各个别年份的情况及年际变化.研究Ⅰ的结果表明:(1)该方法可以客观定量地定出'突变'时段的关键日期;与季风建立过程联系,即是季风建立的'预兆日期',它比人们用天气-气候学方法(甚至用别的气象要素)定出的可以明显感觉到的或有明显实用价值的'季风来临日期'要早2至4天.(2)在北半球亚澳季风系统区域,夏季风的来临在许多关键地区伴有明显的环流突变,建立和推进者很快,但也有许多区域不表现为当地环流的突变,推进速度也慢.(3)北半球亚澳季风系统低空的热带季风分支,在6月中以前可明确区分为3个子系统,(a)西太平洋暖池和邻近低纬区域,4月中下旬建立;(b)热带东北印度洋(北界与孟加拉湾相邻,但不包括其在内)及索马里东边海洋,4月末至5月初建立;和(c)南海区域,5月上旬从南到5月下旬到北部.南海夏季风向北推进最快,于5月末候即可达北回归线附近,然后与暖池西北区域风场的突变一起,于6月中旬影响到东亚30°N区域;印度洋季风于6月初到达印度半岛东南端,然后逐渐推向印-巴次大陆.7月中以后,热带季风才连成一片,由非洲东岸直至长江下游和菲律宾附近.副热带季风分支于6月中旬可以感到其影响,于7、8月盛行于东亚和西太平洋区域,且结构和演变都比较复杂;6～7月间只表现为在(5～20°N,120～150°E)区域有强的环流突变(与副高增强并北移对应),7月中至8月底,则在上述区域和沿30°N的长江下游和日本以南的洋面上有3个强的环流突变中心(对应于副高又一次增强北移和西伸).这里暂不讨论温寒带季风分支.(4)季风具有鲜明的三度空间斜压结构,尤其是在低空季风'爆发'之前,平流层早已有强的环流突变,季节调整完成,然后突变向下延伸(虽然强度大减),跟着就有当地的低空季风'爆发'(建立).平流层和对流层环流的相互作用及其与季风建立的关系很值得进一步研究.

Zeng＇s previous theoretical methods for the study of seasonal divisions of atmospheric general circulations and the calculations of monsoons are modified in order to apply to the study of monsoon onset process more properly and effectively. These modified methods are able to detect the critical day of abrupt transition in the atmospheric circulation and the ＂presage day＂ of monsoon onset by calculating the ＂normalized finite temporal variation＂. ＂Similarity＂ of two fields etc. from the time series of a functional of the spatial field （the ＂flow＂ in the mathematical terminology）. Part Ⅰ is the application of these methods to the analysis of the climatological mean field, and Part Ⅱ deals with the analysis of the cases of every individual year and the interannual variations. The major results of Part Ⅰ are as follows： （1） Our methods can objectively and quantitatively determine the critical day of abrupt change of atmospheric general circulation in a region, this day can be called as the ＂presage day＂ in the relation with the monsoon onset and is just 2-4 days earlier than the so-called ＂onset day＂ defined by the conventional meteorological method. （2） In the Northern Hemisphere, in the area of Australia-Asian monsoon system, the onset of summer monsoon in many key regions is closely related to the abrupt change of atmospheric circulation in that region, the onset and advance processes are very quick, but in many other regions not related with the abrupt change of circulation and slowly advanced. （3） In the Australia-Asian monsoon area, before the mid-June the tropical monsoon branch can be clearly divided into three sub-branches, （a） the region of Warm Pool and its vicinity （onset in the 4th to 5th pentad of April）, （b） the region of tropical Indian Ocean （onset first in the late-April to early May）, and （c） the region of South China Sea （onset in the second pentad of May in the South and very rapid march to about the Tropic of Cancer in the last pentad of May）. The Indian summer monsoon arrives the south-east edge of India in the beginning of June. After mid-July the tropical monsoon branch is unified as a whole subsystem, occupies the whole Indian Peninsula and extends to the South China Sea, 30°N of eastern China, and the low latitudes in the vicinity of Warm Pool. The subtropical monsoon branch presents its influence on the eastern Asia and West Pacific in the mid-June, and is prevailing during July and August. Its structure and evolutionary process are relatively complicated, there are three centers in the maps of abrupt change of circulation. One is located north-east to Philippine, the other two in the lower reach of the Yangtze River valley and the Pacific Ocean in the south of Japan. The temperate-frigid monsoon branch is not studied in this paper. （4） The summer monsoon system possesses very pronounced 3-D spatial baroclinic structure. Especially, before the onset of monsoon in the lower troposphere, there occur already very strong abrupt change of stratosphere general circulation and its seasonal transition from winter to summer, then the abrupt change extends to the whole column of troposphere （although the strength is weakened）, and the onset of monsoon in the low levels of the troposphere takes place. The interactions between stratospheric and the tropospheric circulations and their relation to the onset of monsoons need further investigation.