气候系统内部变率对火山强迫的响应和调制机理研究

Internal variability has emerged as an important player in causing climate change, with the recent occurrence of global warming hiatus and extreme hydrological events. Prediction of future climate change depends on the accurate role evaluation of the external forcing and internal variability of the Earth's climate system. Historical volcanic eruptions provide a valuable tool to study the role of external forcing vs. internal variability， due to their strong radiative forcing and intermittent explosive nature. Previous studies，however, commonly homogenize the forcing by adapting the Superposed Epoch Analysis and fail to differentiate the responses and feedback of the internal variabilities. This synthesize treatment may have contributed to the discrepancies between and among the model results and paleo-reconstructions regarding the climatic consequence of historical volcanism. .The goals of this proposed research project is to test the hypothesis that the internal variability such as ENSO will influence the climatic consequence of volcanic perturbation, which may have caused the model-proxy discrepancies, and identify the underlying mechanism and control factors. We will evaluate the characteristics and diversity of the volcanic radiative forcing for the past millennium; conduct sensitivity simulations with CESM to analyze the relationship between the volcanic forcing and climate response. Then we will isolate the volcanic-induced and internal variability-induced climate signals of the paleoreconstructions and the CESM model results in East Asian Monsoon region; quantify the responses and feedbacks of internal variability to external volcanic forcing, and identify the key process and mechanism. One of the key questions is how to isolate the influence of the concurring El Nino event from the hydroclimate response in Asian Monsoon region, and identify whether the El Nino-like signals is a system response to volcanic perturbation or is merely part of the natural variability. .The project will provide, to our best knowledge, the first assessment of the diversity in historical volcanic forcing and the climatic response associated with the diversity. The methodology and results are valuable extension of the homogenizing method, and may provide additional explanation for the difference in volcanic response between climate model and proxy reconstructions. Volcanic perturbation and ENSO are key representatives of the external forcing and internal variability effecting annual to decadal climate, results from this project therefore provide a new angle to evaluate the interaction and relative role of the two. They will also lead the way to an enhanced understanding of the environmental consequence of anthropogenic aerosols, including those from the geoengineering sources, and future volcanic eruptions.

全球变暖“停滞”等气候变化突显系统内部变率的气候调制作用及其认知局限。历史火山的强辐射效应和间歇性爆发特性为研究外部驱动和内部变率的相对作用提供工具。已有研究均一化处理火山辐射强迫特征，未区分内部变率的响应与调制作用，集成处理方法导致模式与重建结果差异。本项目的目标是量化内部变率对火山强迫的响应和调制作用，表征作用机理和影响因素。拟梳理过去千年火山活动的辐射强迫特征，开展敏感性试验确定火山强迫特征与气候响应之间的关系；针对东亚季风区，分离气候重建指数和CESM模式结果中的火山强迫响应和内部变率信号，评估后者的调制作用，识别关键反馈过程。拟解决的关键问题包括区分厄尔尼诺的影响，并判断其内部调制、抑或外强迫响应源。项目从历史角度评估厄尔尼诺对火山强迫的响应和调制，提供审视变暖“停滞”驱动因素和机理的新思路；多样性评估拓展均一化研究结果，同时为评估人为源气溶胶、未来火山活动的气候风险提供参考。

气候变化往往表现为内部变率及其对外部强迫响应的综合结果。历史火山事件的瞬时冲击特征和强辐射效应为研究内部变率对外部驱动因素的响应和反馈提供了有力的工具。本项目通过调研，提出大型火山喷发事件的气候效应取决于直接的辐射强迫，加上气候内部变率对最初辐射扰动的放大或抑制反馈作用。在此基础上，构建了分离气候重建指数和模式结果中针对东亚季风区的火山强迫响应和内部变率信号的方法；综合观测、史料、代用数据等重建结果，对比分析了典型火山事件的气候效应，明确火山强迫特征与气候响应之间的关系，系统性评估了过去2000年火山事件的气候和社会环境效应，探讨了以火山喷发为代表的突发性气候压力在社会变迁历史中可能扮演的重要角色，及其作用路径；通过开展CESM敏感性试验，结合地球系统多模式集成模拟结果，较为全面地评估了ENSO代表的内部变率的对火山强迫响应的潜在调制作用。研究结果表明，火山爆发引起的厄尔尼诺现象以及与其他气候系统反应的相互作用如暖池海-气相互作用等，加剧了东亚夏季季风降水，ENSO的响应和调制作用常常掩盖或抵消火山爆发造成的季风水文减弱。未来全球变暖情景下，随着大气层水汽含量的增加，等量的火山强迫将会激发更强的赤道太平洋西风异常和厄尔尼诺响应。陆地尤其是热带非洲大陆在其中起关键作用，主要机制是削弱的非洲季风激发了Kelvin波西风异常，并通过Bjerknes反馈作用引发了厄尔尼诺响应。作为外部强迫与内部变异性交互作用的一个典型例子，本研究揭示的火山爆发与厄尔尼诺/南方涛动变化之间的交织关系证明了气候系统的复杂性，并为解释气候模式与重建结果之间的差异提供新的视角。以本项目为第一、二资助，发表第一和通讯作者SCI论文5篇，其他论文多篇；培养硕士研究生2人。另外，本项目综合重建和CESM模拟结果指出，气候系统对热带火山强迫以热力学响应为主，而对高纬火山强迫以动力学响应为主，后者在北半球中高纬的单位辐射强迫响应强度是前者的两倍。在此基础上提出题为“Laki火山强迫和内部变率对18世纪末极端气候事件的影响、机制及其对气候工程的启示研究”的面上项目申请并获资助。