RAPID: Mechanisms of 20th Century Mediteranean Drying: The Role of External Forcing Versus Natural Variability

这是政府间气候变化专门委员会第五次评估报告（IPCC AR5）中鼓励对气候模式模拟进行诊断分析的致同事信（NSF 11-006）资助的16个快速响应（Rapid）项目之一。在这些项目中进行的研究预计将导致更详细的模式相互比较，更好地理解稳健模式行为，更好地理解和量化未来气候模拟中的不确定性。该项目考虑了为AR5准备的20世纪气候模拟集合所描绘的20世纪下半叶地中海地区的干旱情况。推动该项目的核心问题是，这种干燥是由于自然变化还是人为温室变暖造成的。pi指出，地中海的持续干燥是21世纪温室气体引起的变暖的气候模型预测集合的一个强有力的结果，因此有理由相信该地区容易受到全球变暖的影响。另一方面，观测到的地中海的大部分干燥可以与20世纪后期北大西洋涛动（NAO）处于高相的长时间有关，因为高相与大西洋风暴路径向北移动有关，该风暴路径为地中海提供了大部分冬季降水。全球变暖对NAO的影响程度尚不清楚，但已经提出了几种影响机制。本项目的工作主要采用信噪最大化经验正交函数（EOFs）的统计分析形式，以确定自然变率和温室气体引起的全球变暖在20世纪气候模拟中产生地中海干燥的相对重要性。利用总体平均水分收支分解的附加分析用于区分全球变暖引起的干燥的可能机制。该项目的更广泛影响在于它对IPCC第五次评估报告的支持，该报告旨在向全球决策者提供有关气候变化及其后果的信息。特别是，这项工作将为有关地中海干旱的决策者提供指导，因为近几十年来的干旱可能会或可能不会逆转，这取决于它是自然波动的一部分还是长期全球变暖的结果。本项目探索的自然变率和强迫变化机制也可能对包括美国半干旱亚热带地区在内的世界其他地区发生的气候变化产生重大影响。

This is one of 16 Rapid Response (RAPID) projects funded as the result of a Dear Colleague Letter (NSF 11-006) encouraging diagnostic analyses of climate model simulations prepared for the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5). Research conducted in these projects is expected to lead to more detailed model intercomparisons, better understanding of robust model behaviors, and better understanding and quantification of uncertainty in future climate simulations.The project considers the drying of the Mediterranean region over the second half of the 20th century as portrayed in the ensemble of 20th century climate simulations prepared for the AR5. The central question motivating the project is whether this drying occurred due to natural variability or anthropogenic greenhouse warming. The PIs note that continued drying of the Mediterranean is a robust result across the ensemble of climate model projections of greenhouse gas-induced warming over the 21st century, so there is reason to believe that the region is susceptible to drying as a consequence of global warming. On the other hand, much of the observed drying of the Mediterranean can be linked to the prolonged period in the late 20th century during which the North Atlantic Oscillation (NAO) was in its high phase, as the high phase is associated with a northward shift of the Atlantic stormtrack that provides the Mediterranean with much of its wintertime precipitation. The extent to which the NAO is influenced by global warming is not known, and several mechanisms of influence have been proposed. Work performed in this project uses statistical analysis, primarily in the form of signal-to-noise maximizing empirical orthogonal functions (EOFs), to determine the relative importance of natural variability and greenhouse gas-induced global warming in producing Mediterranean drying in 20th century climate simulations. Additional analysis using a decomposition of the ensemble-mean moisture budget is used to discriminate between possible mechanisms for global warming-induced drying.The broader impact of the project lies in its support of the IPCC AR5, which is intended to provide information on climate change and its consequences to decision makers worldwide. In particular, the work will provide guidance for decision makers concerned with Mediterranean drying, as the drying of recent decades may or may not reverse depending on whether it is part of a natural fluctuation or a consequence of secular global warming. The mechanisms of natural variability and forced change explored in this project may also be consequential for climate change occuring in other parts of the world including the semiarid subtropical regions of the United States.