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

对温室气体增加的气候影响的模拟结果非常一致，即地中海盆地将在世纪大幅度干涸。 预测的未来干旱与世纪中期以来降水量的减少是一致的，这已经给该地区造成了困难。 一个主要的例子是2007-2010年的叙利亚干旱，这是第二年开始的内战和难民危机的一个促成因素。 多方面的证据表明，未来的干旱，以及其严重的社会破坏的潜力，激励协调一致的努力，以了解地中海降雨量变化的动态。 在这项工作中的一个首要问题是如何辐射强迫作用在行星尺度上，在这种情况下，从温室气体的增加，产生这样一个强降水响应在这个特定的region.Work执行这里考虑的见解，从地中海降水响应另一种行星尺度强迫：太阳光的变化，由地球接收到的轨道周期，产生冰河时代。 该项目利用了来自东地中海（EM）的沉积物岩心和洞穴沉积物，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（15 kya至今）的潮湿和干燥时期。在这些时期的干旱的变化被认为是由于季节性的变化，与潮湿的条件下，夏季日照是在其高峰期和干燥期时，秋季日照高峰。主要研究人员（PI）认为，秋季日照较强，导致北大西洋上空的纬度表面温度对比在冬季更强，这将导致北移的大西洋急流和冬季天气系统的路径，沿沿着它移动。北移的天气系统导致大量的年降雨量减少，因为地中海盆地在冬季收到的大部分降雨。 由于轨道周期引起的日照变化当然与温室效应不同，但PI指出，温室气体的增加导致了类似的纬度温度差异，因为西非大陆比邻近的大西洋更热，北大西洋北方存在一个“暖洞”，进一步增强了南北温度的差异。日观测记录、古气候代用数据和过去、现在和预测未来气候的模式模拟。 需要解决的一个问题是，在代用记录中，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，古气候记录中的干旱变化是不稳定的，这表明海洋和冰盖的动态作用。 这些问题的研究使用专门的模拟与社区大气模型，社区地球系统模型（CESM）的大气组成部分进行。这项工作是社会以及科学的兴趣，在EM干旱化的深远影响，如上所述。 PI通过该地区的组织，包括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，加强其工作在现实世界的影响。 PI还参加在其家乡机构的外联活动，包括拉蒙开放日和访问当地高中。 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NSF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.