Collaborative Research: Variations and Trends in Fall Precipitation over the Central United States: Issues of Physical Mechanisms, Circulation Anomalies and Boundary Forcing

气候模型中有一个广泛的共识，即全球变暖将导致美国亚热带大部分地区的气候更加干燥。然而，观测证据表明，在过去的几十年里，美国各地的总降雨量和溪流流量都有所增加，其中最大的增幅通常出现在美国中部的秋季。在目前的气候模型中，对观测到的降水趋势的来源识别可能会因为明显的秋季干燥偏向而变得复杂。大多数耦合的气候模型严重低估了秋季密西西比盆地的降水量，限制了我们熟练地预测未来降水量变化的能力，或者将最近观察到的趋势归因于人为原因。秋季降水的观测趋势与气候模型中的干燥偏差之间的这种明显的不一致促使首席调查员更好地了解和确定产生秋季降水趋势和变化的主要机制。美国冬季和夏季大气环流和地表气候之间的关系一直是许多观测和模拟研究的主题。对秋季降水的关注相对较少，限制了我们对秋季气候的时空变化和可预测性的了解。这项研究的一个关键目标是了解美国中部地区秋季降水的长期趋势和年代际变化。为了实现这一目标，PIs将重点解决三个广泛的问题：(A)哪些机制对于产生美国中部秋季降水的趋势和年代际变化是重要的，以及它们在当代气候模型中的表现如何？(B)在哪些方面以及为什么这些机制在秋季特别占优势，而在其他季节不占主导地位？(C)能否查明秋季降水的年代际变化与太平洋或大西洋表面温度之间的物理联系？PIS将首先扩展他们正在进行的观测数据分析和现有的文献结果，以进一步建立秋季降水变化、环流异常和边界强迫之间的联系。然后，他们将尝试确定可能的物理机制，以解释观察到的相关性和关联联系。这项研究的一个主要成果将是更好地了解和确定秋季降水的空间相关趋势和年代际变化的主要大气过程，以及它们与其他季节的不同之处。这项研究将在季节、年际和年代际时间尺度上处理与秋季降水变异性和趋势的起源和性质有关的问题。现有的研究大多只考虑冬季和夏季的降水变化。在任何未来的气候变化评估中，如政府间气候变化专门委员会(气专委)的第四次评估报告中，都没有单独考虑秋季过渡期。这项研究的结果将为我们提供新的见解，了解为什么秋季降水量主要在美国中部地区出现大幅增加，以及为什么当前的气候模型无法捕捉到这一趋势。塔夫茨大学和哥伦比亚大学之间的这种合作伙伴关系建立在水循环研究、大气动力学和水文学方面相互协同的专业知识的基础上。这一伙伴关系将通过博士生的共同咨询和本科生通过暑期实习的参与而得到进一步加强。PIS将整合这项研究的结果，开发一个关于美国降水量变化的互动多媒体教育模块，作为我们的环境信号处理双级课程(高年级本科生和一年级研究生)为期三周的教学工具。他们将在国家会议和档案期刊上公布他们的成果，并以不同的媒体形式发表他们的成果，以便记者、教师和普通公众随时可以获得。

There is a broad consensus among climate models that a warming world will lead to a drier climate over most of the subtropical United States. Yet, observational evidence suggests that total precipitation and stream flow have increased across the United States over the last several decades with the largest increases generally observed in fall across the central United States. Identification of origins for the observed precipitation trends may be complicated by an apparent fall dry bias in current climate models. Most coupled climate models significantly underestimate precipitation over the Mississippi basin during fall, limiting our ability to skillfully predict future changes in precipitation or attribute the recently observed trends to anthropogenic origins. This apparent inconsistency between observed trends in fall precipitation and the dry bias in climate models motivates the Principal Investigators (PIs) to better understand and identify the dominant mechanisms that produce trends and variations in fall precipitation. The relationship between atmospheric circulations and surface climate over the United States in winter and summer has been the subject of many observational and modeling studies. Relatively little attention has been paid to fall precipitation, limiting our knowledge of the space-time variations and predictability of fall climate. A key goal of this research is to understand the long-term trend and the decadal variability of fall precipitation over the central United States. To achieve this goal, the PIs will focus on three broad questions: (a) What mechanisms are important to produce trends and decadal variations in fall precipitation across the central United States and how well are they represented in current generation climate models? (b) In what ways and why these mechanisms are particularly dominant in fall and not in other seasons? (c) Can the physical linkages between decadal variations in fall precipitation and Pacific or Atlantic Sea Surface Temperature (SST) be identified? The PIs will begin by expanding their ongoing observational data analyses and existing results from the literature to further establish the associational link among fall precipitation variations, circulation anomalies, and boundary forcing. Then, they will attempt to identify possible physical mechanisms that can explain the observed correlation and associational links. A main outcome of this research will be a better understanding and identification of the dominant atmospheric processes responsible for the spatially coherent trends and decadal variations in fall precipitation and how they differ from other seasons. This research would address questions related to origin and nature of fall precipitation variability and trends at the seasonal, inter-annual, and decadal time scales. Most of the existing studies consider precipitation variations in winter and summer seasons only. The fall transition season was not considered separately in any of the future climate change assessments, such as those by the Fourth Assessment Report of the Intergovernmental Panels on Climate Change (IPCC). Results from this research will provide new insight on why a large increase in precipitation is primarily observed in the central United States in fall and why current climate models are unable to capture this trend. This collaborative partnership between the Tufts University and Columbia University builds on mutually synergistic expertise in water cycle research, atmospheric dynamics, and hydrology. This partnership will be further strengthened through co-advising of PhD students and involvement of undergraduate students through summer internships. The PIs will integrate findings from this research to develop an interactive multimedia education module on Precipitation Variations over the United States to be used as a three-week teaching instrument for our dual-level (senior undergraduates and first year graduate students) course on Environmental Signal Processing. They will present their results in national conferences and archival journals and publish their findings in diverse media formats so they will be readily available to journalists, teachers and the general public.