人们常常有这样的感受：冬天刚刚过去，夏天就已经来到，而春天总是匆匆一下就过去了。这并不是来自我们感官的错觉，而正是大气环流季节性突变的反映。最早的对大气环流季节突变的系统研究是由中科院大气所的叶笃正、陶诗言和李麦村三位先生做出的。由于太阳辐射的季节变化是缓变的，这种突变现象很难用线性的理论来解释。难能可贵的是，叶－陶－李三位先生当时就对其机理做出了正确的猜想，并提出了用于验证其猜想的实验（模式）研究。或许更出人意料的，他们的猜想要在整整半个世纪以后，才有来自大洋对岸的年轻理论家和他的助手证实，而后者并不知道叶先生等半个世纪以前的建议。

   近日，笔者和美国加州理工学院的Tapio Schneider 教授为《大气科学进展》（Advances in Atmospheric Sciences, AAS）纪念叶笃正先生百年诞辰专刊合作撰写的综述《大气环流季节突变－演化中的视角》（Evolving perspectives on abrupt seasonal changes of the general circulation）（http://159.226.119.58/aas/EN/10.1007/s00376-017-7068-4）讲述了这一大气环流季节突变的相似理论观点在跨越了半个世纪后的对话。而这历史的回声，正如文章摘要的第一句所说的那样，充分说明了叶笃正先生对这一大气环流重要现象的理论认识领先于他所处的时代整整几十年。

　　1957年7月，为庆祝罗斯贝（现代气象学和海洋学的开拓者）即将到来的六十寿辰，他的同事和朋友们邀请了当时国际上最负盛名的气象学家和海洋学家，准备出版专集以回顾罗斯贝的科学贡献；但一个月后罗斯贝的骤然离世让这本专集变成了永久的罗斯贝纪念专集，这就是于1959年出版的《运动中的大气和海洋》（The Atmosphere and The Sea in Motion）一书。

　　在这本书里，来自中国科学院的叶笃正、陶诗言和李麦村首次向英语世界的同行们介绍了他们关于大气环流季节突变的最新成果。这篇文章作为叶笃正先生的经典著作之一在中国广为人所知。由于其理论上的挑战性，大气环流的季节突变在以后数十年里一直吸引着一流的理论家，比如中科院大气物理研究所的曾庆存先生和吴国雄先生都曾就此发表过论文。尽管如此，我们很难说对相关的机理已经了解得比较清楚。这是因为有很多的因素都可能对大气环流的季节突变产生影响，而单纯的观测或非理想化的数值实验很难对这些因素的作用做出明确的分离。

　　几乎半个世纪以后的2008年，Nature Geoscience杂志发表了来自加州理工学院的博士后Simona Bordoni及其合作导师Tapio Schneider的《Monsoons as eddy-mediated regime transitions of the tropical overturning circulation》一文。该文首次提出了在理想化的没有下垫面非均匀性（比如海陆差异或者大地形的存在）条件下，只要改变下垫面的热惯性就足于产生季风及其季节突变现象。在此基础上，他们指出季节突变现象本身并不依赖于上述下垫面的非均一性，而主要地是由于环流内部的反馈，尤其是来自中纬度的大尺度涡旋和平均的纬向环流及热带的经圈翻转环流之间的相互作用和反馈导致的。这一研究的意义在于通过采用理想化的纬向均匀的模式大气和海洋设置，有效地隔离出了对造成季风和大气环流季节突变的本质性因素和非本质的调节性因素，从而为对这些现象的进一步认识开辟了道路。

　　有意思的是，Bordoni和Schneider并没有意识到叶笃正先生等文章的存在，所以也就没有可能认识到他们所采用的模式实验方法几乎完全和叶笃正、陶诗言和李麦村（1959）文中为机理研究所提议的实验一样，这其中包括均一的下垫面条件。而且叶笃正先生等在对可能机理的猜测中，指出在太阳辐射强迫的季节缓变性条件下大气环流季节突变的广泛存在，说明很可能是由于某种环流内部的“不稳定机制”导致了季节突变的发生。显然这种环流型的“不稳定性”和Bordoni和Schneider所说的环流的（正）反馈也是相符合的，虽然叶笃正先生等的工作由于时代条件限制还只是一个猜想，而五十年后的数值实验已经可以为理论提供定量的分析基础。

　　在Bordoni和Schneider（2008）发表5年以后的一次小型学术会议上，本文的两位作者在和Michael McIntyre教授一起午餐讨论时相遇相识。笔者遂向Tapio Schneider介绍了叶笃正先生等1959的工作，并把文章扫描给了他。当时叶笃正先生已经住院多时并于四个月后离世，所以他们不再可能就此进行面对面交流。尽管如此，相信读者通过本文能够感受到1959年和2008年两篇文章在思想、方法和主要结论上的强烈共鸣。

　　这无意间产生的跨越了半个世纪的对话和历史的回声，何尝不是世间的“最美和声”？而其余音，像这篇纪念综述的作者所希望的那样，指向迄今阙如的大气环流的统一理论。谁又能担保叶笃正先生等（1959）和Bordoni和Schneider（2008）的观点不会在以后的日子里，产生更大的历史的回声呢？

It is not unusual that two similar scientific ideas on a particular phenomenon may echo each other across a long time span, unknown to the authors. Such an unintentional duet over history illustrates the amazing beauty of science by clearly showing that progress of science is not necessarily a linear process, but it is often accumulative.  

A recent paper  "Evolving perspectives on the abrupt seasonal changes of the general cicrculation", which has just appeared in issue No. 10 of Advances in Atmospheric Sciences, tells of such a story in the field of atmospheric science.  

Carl-Gustaf Rossby, the founder of modern dynamical meteorology, wrote in a short note published in 1951 that "there is [...] every reason to expect, during the next few years, an extremely vigorous development of Chinese meteorology and, as a result, many significant realistic contributions from that part of world." Exemplifying Rossby's expectation, Tu-cheng Yeh (Duzheng Ye) -- one of Rossby's protégés -- together with his colleagues Shih-yen Dao and Mei-tsun Li from Chinese Academy of Sciences published "The abrupt change of circulation over the Northern Hemisphere during June and October" in the Rossby Memorial Volume eight years later.

Yeh-Dao-Li (1959) viewed the abrupt seasonal change of the general circulation, of which monsoons are a part, as a broad phenomenon, and further conjectured that it was caused by a certain type of "instability" in the atmosphere. They went on to propose a model experiment to test whether such an instability can occur without the inhomogeneities of the underlying boundary conditions (e.g., land-sea contrasts), which have traditionally been taken to be essential for monsoons and abrupt seasonal changes.The advent of numerical general circulation models (GCMs) put the experiments proposed by Yeh-Dao-Li (1959) within reach. Indeed, numerical simulations successfully reproduce abrupt seasonal transitions of the general circulation, including monsoon transitions. However, by including lower-boundary inhomogeneities, these modeling studies were still not the numerical version of the experiment proposed by Yeh-Dao-Li (1959).

Half a century passed after the publication of Yeh-Dao-Li (1959) before GCM experiments corresponding to the laboratory experiment proposed by Yeh-Dao-Li (1959) were conducted (Schneider and Bordoni, 2008; Bordoni and Schneider, 2008), without the authors of these recent studies being aware of Yeh-Dao-Li's proposal decades earlier. They did indeed find abrupt seasonal transitions of the circulation, without inhomogeneities at the lower boundary.

The paper in Advances in Atmospheric Sciences summarizes the circulation feedbacks that lead to the abrupt seasonal transitions found in the model experiments without lower-boundary inhomogeneities. It also emphasizes the work that still remains to be done to establish a comprehensive theoretical framework of seasonal transitions and to answer open questions such as:

(1) How does the strength and structure of a Hadley cell depend on external factors such as the solar declination angle?

(2) What controls when and how rapidly the circulation undergoes seasonal transitions?

(3) How and to what extent do surface inhomogeneities such as land-sea contrasts modulate the internal rearrangement of the atmospheric circulation during the seasonal transitions?

(4) Can the seasonal transitions be understood as an instability of the circulation as forcing parameters move through a critical region, as originally conjectured by Yeh-Dao-Li (1959)?  
我们期待着有更多理论工作者对上述问题产生兴趣，开展更加深入的研究。

文章链接：http://159.226.119.58/aas/EN/10.1007/s00376-017-7068-4

 [感谢AAS编辑部林征和Tapio 对上面中英文稿的修改］