影响东亚夏季风的冷空气活动的年际变化及其与海温和欧亚积雪的联系

East Asian summer monsoon is characterized by strong interannual variation, which has profound impact on the drought and flood disasters in China. It has been pointed out that cold air activity in mid-latitude is one of the principal components of East Asian summer monsoon system. However, previous researches on East Asian monsoon mainly focused on the components at mid- and lower latitudes. Some of the characteristics of cold air activities have been revealed, but were mostly from particular cases, not climatologically. What is the interannual variability of cold air activities, how does it connect to sea surface temperature and Eurasian snow cover and its mechanism? This is a problem needs to be well solved urgently. Our project aims at exploring the interannual variation of cold air in East Asian summer monsoon system in recent decades, as well as its relationship with the principle modes of sea surface temperature, and continental snow cover, by using statistical and numerical methods. We firstly plan to analyze the characteristics of cold air activity in summer, focusing on the interannual mode of it. Afterwards, we will distinguish the contribution of each mode in underlying sea surface and Eurasian snow cover to the interannual variation of cold air by using the newly developed technique, Generalized Equilibrium Feedback Analysis (GEFA). Finally, we can make sure what are key factors influencing cold air activity and its possible mechanism by using numerical experiments. The research findings will provide the scientific basis for deepening our understanding on the annual variations in East Asian monsoon.

东亚夏季风的年际变率是我国旱涝产生的重要原因之一，冷空气活动是东亚夏季风系统的主要成员之一。以往对东亚夏季风的研究中主要关注来自中低纬的子系统，对冷空气活动的特征和作用也多注重于对某些特殊年份的个例分析。夏季冷空气活动具有怎样的年际变化特征，它与海表温度和欧亚积雪之间有何联系，其机理是什么？这是个亟待解决的问题。本项目拟通过统计和数值模拟的方法，探索近几十年以来东亚夏季冷空气活动的年际变化特征及其与海温和欧亚积雪主要模态的联系。拟分析夏季冷空气活动的基本特征，主要关注于冷空气活动的年际变化模态，并使用最新的广义平衡反馈方法（GEFA）分离全球海温和欧亚积雪异常模态对冷空气年际变率的各自贡献，确定影响夏季冷空气年际异常的关键因子，并用数值模式研究关键因子的可能影响途径，从而为加深我们对东亚夏季风年际变化规律的理解提供科学依据和参考。

冷空气活动是东亚夏季风与中国东部降水的主要影响因子之一。本项目对影响东亚夏季风的冷空气活动的气候特征、年际变率主要模态及其与海温和欧亚积雪的联系和可能影响过程进行了分析和研究。评估了四套土壤湿度再分析数据的适用性，发现ERA-interim资料与观测结果最接近，后续研究可选用该数据。分析了ENSO演变阶段与东北夏季极端温度事件的联系，结果表明La Nina对东北极端温度事件的影响强于El Nino，La Nina发展年东北极端低温事件显著减少。使用集合典型相关分析方法建立了中国夏季极端气温预测模型，具有一定的预测能力。研究了冷空气和水汽输送协同变化对长江中下游夏季降水的影响，确定了冷空气活动与水汽输送的同等重要作用。分析了北半球夏季冷空气基本态，发现夏季高纬度存在两支较弱的冷空气路径。使用经验正交函数（EOF）方法提取了东亚夏季冷空气活动的主要模态以及影响江淮降水的关键模态的特征，当西太平洋到我国东部地区冷空气偏多时，江淮地区降水易出现偏多分布。进一步研究了冷空气关键模态与海表温度和积雪深度的联系，建立了夏季冷空气活动与前期春季北大西洋三级子型海表温度模、6月亚洲北部积雪异常之间的联系，提出了北大西洋三极子型海温影响东亚夏季冷空气活动的一种可能过程，即春季海温异常模以高纬度积雪为桥梁，影响同期积雪的分布，再通过积雪异常的水份效应影响土壤湿度与后期温度，进而激发经向异常波列，影响后期大气环流与冷空气活动的关键模态。