青藏高原积雪年代际变率归因与全球气候效应研究

As one of the most active components of the TP land surface-atmosphere coupled system Tibetan Plateau (TP) snow cover has a large decadal variability. Previous research demonstrates that heavy TP snowfall during winter and spring decreases surface sensible heating over the TP, due to more solar energy reflected or consumed to melt excessive snow cover. In summer, this leads to a cooler TP land surface temperature and a reduced thermal contrast between the Eurasian continent and the oceans, and thereby a weaker East Asian summer monsoon circulation, a wetter summer over the Yangtze River valley and a drier summer in the southeastern and northern China. However, the associated ocean processes connecting spring TP snow anomalies and East Asian summer climate have not been identified clearly. Moreover, further research is needed to examine impacts of decadal TP snow changes on global climate changes. This study first examines the forcing of decadal changes of TP snow cover extent and snow depth through an attribution study, and then uses numerical simulations to identify and attribute global impacts on climate caused by realistic (observation-based) decadal variations of TP snow conditions. We propose to force atmospheric general circulation models using observed high-frequency snow conditions over the TP in model runs starting 1979. Specifically, simulated variables of snow water equivalent and snow cover extent will be replaced with the corresponding observed values every day or every week to improve snow feedbacks. Through observational and modeling diagnostics, we will examine contributions and mechanisms of decadal changes in TP snow cover extent and snow depth to explain their effects on global climate. The forced changes of the East Asian Summer Monsoon, ocean-atmosphere interactions in the tropics, and troposphere-stratosphere interactions, will be examined in detail in the simulations. This study is expected to clarify the skill of global climate system predictability related to decadal TP snow changes, provide a scientific basis for improving predictability of seasonal and decadal climate changes, and contribute significantly to the major scientific questions and overall target of this Key Scientific Research Plan.

高原积雪是青藏高原地-气耦合系统变化中相对活跃部分，具有明显的年代际变化特征。过去研究表明冬春季节高原积雪面积或者雪深增加，会减弱高原的热力作用，导致东亚夏季风减弱，造成长江中下游地区降水增多，华南和华北降水减少。但海洋在以上冬春高原积雪显著影响东亚夏季风过程的作用仍有待理清，同时高原积雪年代际变率的全球气候效应需要深入研究。本项目旨在通过归因分析，阐明青藏高原积雪年代际变率成因；进行一系列多模式多集合数值试验，使用高原地区卫星观测的积雪覆盖面积和雪水当量代替模式预报量,以较准确模拟观测高原积雪年代际变率的全球气候效应，揭示高原积雪年代际变率对热带海气系统，平流层和东亚气候变化等的影响及其机理。最终阐明全球气候系统变化与青藏高原积雪年代际变率有关的可预报性，为提高季节和年代际气候预测水平提供理论根据，对解决重大研究计划核心科学问题和总体目标将有直接重要贡献。

积雪是青藏高原地-气耦合系统变化中相对活跃部分，过去大量研究讨论冬春季高原积雪异常通过反照率和融雪水文效应对亚洲夏季风产生显著影响，但较少研究高原积雪的全球气候影响。我们前期观测和模拟研究揭示了秋冬季高原及周围地区积雪异常能够激发冬季类似太平洋-北美(PNA)遥相关型的大气响应，其机理与高原积雪强迫激发的Rossby波能量东传及东亚-北太平洋地区瞬变涡度反馈作用有关。本项目围绕青藏高原地-气耦合系统变化及其全球气候效应重大研究计划的核心科学问题“青藏高原地-气耦合系统变化如何影响亚洲和全球气候系统？”，结合观测分析和多重设计的数值模拟试验，进一步全面深入评估和揭示了四个季节高原积雪变率的全球气候效应及其机制。我们利用CESM模式进行一系列多集合海气耦合和非耦合大气环流数值试验，使用青藏高原地区卫星观测积雪覆盖度和雪水当量代替模式预报量,较准确模拟出观测高原积雪年际和年代际变率对北半球及热带大气环流和海洋的影响。项目成果清晰地揭示了观测高原积雪变率的全球气候效应，包括对北半球大气环流及北极涛动，北大西洋涛动，PNA和北太平洋涛动等模态，北极增暖和海冰减少，热带海气系统和平流层的显著影响，以及其相应的地面温度和降水变化；清晰阐明了北太平洋海气相互作用在秋冬春高原积雪气候影响中重要作用，揭示了冬春季高原积雪-东亚夏季风隔季相关的可能的海洋途径。项目还通过观测和CMIP5/CMIP6模拟资料诊断分析，检查了气候自然变率和人为辐射强迫的相对作用，基本理清了青藏高原东部雪深年代际变率的成因，特别是人为辐射强迫对近年高原增温和雪深减少的显著贡献。项目取得了重要的创新性进展，主要成果支撑重大研究计划的核心科学问题，具有相当重要的科学意义，并阐明了全球气候变化与高原积雪变率有关的可预报性，为提高季节和年代际气候预测水平提供理论根据。