Modeling the Tropical Atmosphere-Ocean System: Determining the Causes of Near Future Subtropical Drying

在21世纪气候的模式预估中，亚热带的干燥是一个普遍存在的、强有力的结果。这种干旱影响了几个重要的农业经济和人口增长地区，最明显的是墨西哥北部、美国西南部、南欧、北非和中东。根据模型，这种干燥会在未来立即开始。预计的振幅是如此之大，以至于美国西南部将在未来几年到几十年内陷入永久性的沙尘暴或20世纪50年代的干旱水平。虽然未来的干旱引发了类似中世纪特大干旱的重现的幽灵，这些干旱与持续的拉尼娜现象有关，但未来的干旱似乎是普遍变暖的结果。利用大气环流模式（GCMs）和简化模式对西南和亚热带地区干旱的成因进行研究。热力学变化——即湿度上升导致平均环流放大现有的水汽输送模式，使干燥地区更干燥，使潮湿地区更潮湿——和大气环流的变化都涉及降水和蒸发的转变模式。研究人员将a)确定耦合GCM内的热力和动力机制对副热带干燥的相对贡献，以及大气环流（MMC、瞬态涡旋和静止波）的变化对不同地区干燥的动力贡献；b)使用耦合GCM诊断和简化模式层次来确定大气环流变化的原因。研究热带和温带海温变化和对流层温度的影响，并研究高纬度低空变暖和平流层变冷；c)确定涡旋、平均环流和水汽输送之间的相互作用，这是降水和蒸发带变化和亚热带干燥的基础，研究冬季和夏季；d)将自然干旱的热带强迫动态与人为干旱动态进行对比，作为确定当前和未来干旱原因的一种手段。这项研究的知识价值在于了解热带和全球大气环流和水文循环如何对外部强迫作出反应的基本原理，这将使人们更好地了解一般环流，特别是涡旋、平均气流和潮湿过程之间的相互作用。这项研究的更广泛的影响涉及到北美干旱地区水资源的未来。这项工作将提高对人为引起的气候变化和亚热带干燥的模式预估的理解，从而有助于确定预测的可信度。ppi将向气候研究界和与水有关的决策者传达结果，并通过出版物、公开演讲、媒体和互联网向更广泛的公众宣传这些问题。

Drying of the subtropics is a ubiquitous and robust result within model projections of 21st century climate. This drying impacts several areas with important agricultural economies and rising populations, most notably northern Mexico, the southwestern United States, southern Europe, North Africa and the Middle East. According to the models, this drying sets in immediately in the future. The projected amplitude is such that the U.S. Southwest will go over to a permanent Dust Bowl or 1950s drought level of aridity in the next few years to decades. While the future drought raises the specter of a return of conditions akin to Medieval mega droughts those have been linked to persistent La Nina-like states but future droughts appear to be a result of general warming.Research will be conducted to determine the causes of the drying of the Southwest and the subtropics using coupled General Circulation Models (GCMs) and simplified models. Both thermodynamic changes - whereby rising humidity leads to amplification of existing patterns of water vapor transport by the mean circulation, making dry areas drier and wet areas wetter - and changes in atmospheric circulation that shift patterns of precipitation and evaporation are involved. The investigators will a) determine within coupled GCMs the relative contributions of thermodynamical and dynamical mechanisms to subtropical drying and what changes in atmospheric circulation (MMC, transient eddies and stationary waves) are responsible for the dynamical contribution to drying indifferent areas,b) use diagnoses of coupled GCM and a hierarchy of simplified models to determine the causes of the changes in atmospheric circulation, examine the impacts of changes in tropical and extratropical SSTs and tropospheric temperatures, and study low level warming at higher latitudes and stratospheric cooling,c) determine the interactions between eddies, the mean circulation and water vapor transports that underlie shifts in precipitation and evaporation zones and subtropical drying, examining both winter and summer seasons, and d) contrast the dynamics of tropical forcing of naturally-occurring droughts with the dynamics of anthropogenic drought as a means to identify the causes of current and future drought.Intellectual merit of the research comes from understanding the fundamentals of how the tropical and global atmosphere circulation and hydrological cycle respond to external forcing which will lead to an improved understanding of the general circulation, in particular, the interactions between eddies, the mean flow and moist processes.Broader implications of the research concern the future of water resources in the arid lands of North America. The work will improve understanding of model projections of human-induced climate change and subtropical drying and, hence, aid assigning of confidence to the predictions. The PIs will communicate results to the climate research community and also to those making water-related decisions and work to educate a wider public on these matters through publications, open addresses, the media and the internet.