亚洲中高纬环流春夏季节转换异常机制及其对中国北方夏季气候的影响

The spring to summer seasonal transition of the atmospheric circulation is a classical research topic. In the past, the research with regard to the seasonal transition has been focused on the tropical and subtropical circulation, whereas little has been done for the seasonal transition of the mid- and high latitude circulation. In fact, the latter is associated closely with the weather and climate of China in the following summer. In this project, we regard the anomalous seasonal transition of the mid- and high latitude circulation as a core issue, and depict its diversity and anomalous features. Then, we objectively define the main types of the seasonal transition; On the interannual and intraseasonal time scales, We explore influences of the spring snow ammount and snow melting process over the Eurasian continent, and the tropical Pacific and North Atlantic sea surface temperatures on the anomalous seasonal transition. From the perspectives of the low-frequency Rossby wave dispersion, the feedback forcing of the high-frequency transient eddies, and wave-breaking process, we investigate in depth the roles of the internal atmospheric dynamics for the anomalous seasonal transition; Moreover, with a particular emphasis, we explore the synergistic effect of these external forcing processes and the internal dynamical processes and give its physical picture. Finally, we investigate the influence of the anomalous seasonal transition on the following summer climate in China, and it is also supposed to give a new thinking or method with regard to the summer climate prediction in this region.

大气环流的春夏季节转换问题是一个经典的研究课题。过去的研究更侧重于热带和副热带环流季节转换，而对亚洲中高纬环流春夏季节转换的研究则很少。实际上，亚洲中高纬环流的春夏季节转换与随后的中国夏季天气气候有十分密切的联系。本项研究拟以亚洲中高纬环流的春夏季节转换异常为核心问题，刻画季节转换的多样性和异常特征，并对其主要类型予以客观合理的界定；在年际和季节内尺度上，揭示春季的欧亚大陆积雪及其消融过程、热带太平洋海温和北大西洋海温等外强迫异常对季节转换异常的影响；从低频Rossby波能量频散过程、高频瞬变波反馈强迫以及波破碎过程等角度，深入研究大气内部动力学过程对季节转换异常的作用机制；并通过数值模拟等手段，重点揭示这些外强迫与大气内部动力学过程协同作用的物理图像；最后，研究亚洲中高纬环流春夏季节转换异常对我国夏季气候的影响，并为夏季气候预测提供新的思路和方法。

亚洲中高纬环流春夏季节转换过程是东亚大气环流变化中的一个经典研究问题。亚洲中高纬环流春夏季节转换异常现象是其随后夏季气候形势的一个重要前奏。本项目以亚洲中高纬春夏季节转换异常为核心问题，刻画了其关键特征和演变规律，揭示了下垫面外强迫与大气内部过程对它的影响机理，可望为我国夏季气候预测提供新的依据和方法。. 课题组圆满完成了项目研究任务，并获得了如下新的研究成果：（1）提出了一个界定亚洲中高纬春夏季节转换的新方法，并利用该方法确定了北亚和东亚中纬度地区的入夏时间。同时揭示了亚洲中高纬环流春夏季节转换的5个类型及其关键环流特征。（2）从亚洲大地形的热力和机械强迫作用以及谐波竞争的视角，揭示了东北亚低压和东北亚脊形成的动力学机理，由此解释了亚洲中高纬环流春夏季节转换的根本原因。（3）揭示了积雪融化和土壤湿度等下垫面热力状况的季节性变化对亚洲中高纬春夏季节转换过程的驱动机理，并证实该季节转换过程的早晚归因于北大西洋海温异常强迫及其对应的准定常低频波列。（4）系统总结了亚洲中高纬春夏季节转换过程的早晚及多样性特征与中国初夏和梅雨期气候之间的十分密切的联系。. 本项目研究成果将对东北亚夏季气候研究具有参考价值，并对中国北方和江淮地区夏季气候预测提供重要依据。