Insolation Gradients and Eastern Mediterranean Aridity: Impacts on Winter Storms and Implications for Climate Projections

温室气体增加对气候影响的模拟结果非常一致地表明，地中海盆地将在21世纪大幅干涸。对未来干旱的预测与自20世纪中期以来的降水减少一致，这已经给该地区造成了困难。一个典型的例子是2007-2010年叙利亚的干旱，这是次年开始的内战和难民危机的一个促成因素。多种证据表明未来会出现干旱，以及可能造成严重的社会混乱，这促使人们齐心协力，了解地中海降雨变化的动态。在这项研究中，一个重要的问题是，在地球尺度上的辐射强迫，在这种情况下是温室气体的增加，是如何在这个特定地区产生如此强烈的降水响应的。这里进行的工作考虑了从地中海降水对另一种行星尺度强迫的响应中获得的见解：在产生冰河期的轨道周期中，地球接收的阳光的变化。该项目利用了东地中海（EM）的沉积物岩心和洞穴标本，这些岩心和洞穴标本记录了13.5万至11万年前（kya）的末次间冰期（LIG，也称为Eemian）和全新世（15kya至今）的湿润和干燥时期。在这些时期，干旱的变化被认为是由于日照的季节性变化，在夏季日照达到高峰时条件较湿润，而在秋季日照达到高峰时条件较干燥。首席研究员（pi）认为，更强的秋季日照导致北大西洋冬季更强的纬向地表温度对比，这导致大西洋急流和沿其移动的冬季天气系统的路径向北移动。天气系统的北移导致年降雨量大幅减少，因为地中海盆地的降雨量主要集中在冬季。轨道周期引起的日照变化当然与温室变暖截然不同，但pi指出，温室气体的增加导致了类似的纬向温度差异，因为西非大陆比邻近的大西洋升温更多，北大西洋北部有一个“变暖洞”，进一步增强了南北温度差异。这项工作包括对当前观测记录、古气候代理数据以及过去、现在和预测未来气候的模式模拟的综合分析。需要解决的一个问题是，温度对比的影响与当前气候变化有关，而北非季风的入侵与当前气候变化无关。另一个问题是，与轨道强迫相比，古气候记录中干旱变化的突然性，这表明海洋和冰盖的动力学作用。这些问题是通过社区大气模式（社区地球系统模式（CESM）的大气部分）进行的专门模拟来研究的。如上所述，鉴于新兴市场干旱的深远影响，这项工作具有社会和科学意义。私人投资机构通过位于约旦安曼的哥伦比亚大学全球中心、中东生态和平非政府组织、魏茨曼研究所和希伯来大学等区域组织加强其工作的现实影响。这些学生还会在自己所在的学校参加外展活动，包括拉蒙特开放日（Lamont Open House）和访问当地高中。该项目还为博士后提供支持和培训，从而为该研究领域的未来劳动力提供支持。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Simulations of the climatic effect of greenhouse gas increases are in remarkable agreement that the Mediterranean Basin will dry out substantially over the 21st century. The projected future drying is consistent with precipitation declines since the mid-20th century, which have already caused hardship in the region. A prime example is the 2007-2010 Syrian drought, which was a contributing factor in the civil war and refugee crisis that began the following year. The multiple lines of evidence pointing to future drying, together with its potential for severe societal disruptions, motivate a concerted effort to understand the dynamics of Mediterranean rainfall change. An overarching question in this effort is how radiative forcing acting on the planetary scale, in this case from greenhouse gas increases, produces such a strong precipitation response over this particular region.Work performed here considers the insights to be gained from the Mediterranean precipitation response to another kind of planetary-scale forcing: the change in sunlight received by the earth over the orbital cycles that produce the ice ages. The project takes advantage of sediment cores and speleothems from the Eastern Mediterranean (EM) that record wet and dry periods during the Last Interglacial (LIG, also called the Eemian), from 135 to 110 thousand years ago (kya), and the Holocene, from 15kya to the present. The changes in aridity during these periods are thought to be due to changes in the seasonality of insolation, with wetter conditions when summer insolation is at its peak and dry periods when fall insolation peaks. The Principal Investigators (PIs) argue that stronger fall insolation leads to a stronger latitudinal surface temperature contrast over the North Atlantic during winter, which leads to a northward shift of the Atlantic jet stream and the paths of winter weather systems that move along it. The northward shift in weather systems causes substantial annual rainfall reductions as the Mediterranean Basin receives most of its rainfall in winter. The insolation changes due to orbital cycles are of course quite distinct from greenhouse warming, but the PIs note that greenhouse gas increases cause a similar latitudinal temperature contrast since the West African landmass heats up more than the adjacent Atlantic Ocean, and the northern North Atlantic features a "warming hole" which further enhances the north-south temperature contrast.The work involves a combination of analysis of the present-day observational record, paleoclimate proxy data, and model simulations of past, present, and projected future climate. One issue to be addressed is the conflation in the proxy record between the effects of the temperature contrast, which are relevant to current climate change, and incursions of the North African monsoon, which are not. A further issue is the abruptness of the aridity changes in the paleoclimate record compared to the orbital forcing, which suggests a role for the dynamics of oceans and ice sheets. These issues are examined using specialized simulations performed with the Community Atmosphere Model, the atmospheric component of the Community Earth System Model (CESM).The work is of societal as well as scientific interest given the profound impacts of aridification in the EM as noted above. The PIs enhance the real-world impact of their work through organizations in the region including the Columbia University Global Center in Amman, Jordan, the Eco-Peace Middle East non-governmental organization, the Weizmann Institute, and Hebrew University. The PIs also participate in outreach activities at their home institution, including the Lamont Open House and visits to local high schools. The project also provides support and training to a postdoc, thereby providing for the future workforce in this research area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.