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

这是由一封亲爱的同事来信(NSF 11-006)资助的16个快速反应(RAPID)项目之一，该信鼓励对为政府间气候变化专门委员会第五次评估报告(IPCC AR5)准备的气候模型模拟进行诊断分析。在这些项目中进行的研究有望导致更详细的模型相互比较，更好地理解稳健的模型行为，以及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20世纪下半叶地中海地区的干旱，如为AR5准备的20世纪气候模拟组合所描绘的那样。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效应造成的。PI指出，地中海的持续干旱是21世纪温室气体导致气候变暖的气候模型预测的一个强劲结果，因此有理由相信，由于全球变暖，该地区容易受到干旱的影响。另一方面，观测到的地中海干燥现象很大程度上与20世纪末北大西洋涛动(NAO)处于高峰期的较长时期有关，因为高峰期与大西洋风暴带北移有关，为地中海提供了冬季降水的大部分。全球变暖对NAO的影响程度尚不清楚，已经提出了几种影响机制。该项目使用统计分析，主要以信噪比最大化经验正交函数(EOFs)的形式，在20世纪气候模拟中确定自然变异性和温室气体引起的全球变暖在地中海干燥产生中的相对重要性。该项目的更广泛影响在于其对IPCC AR5的支持，该报告旨在向世界各地的决策者提供有关气候变化及其后果的信息。特别是，这项工作将为与地中海干燥有关的决策者提供指导，因为近几十年的干燥可能会逆转，也可能不会逆转，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。该项目中探索的自然变化和强迫变化的机制也可能对世界其他地区发生的气候变化产生影响，包括美国的半干旱亚热带地区。

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.