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大部分亚热带地区的气候更加干燥。然而，观测证据表明，在过去几十年里，美国各地的总降水量和河流流量都有所增加，通常在美国中部的秋季观测到的增幅最大。由于当前气候模式中存在明显的秋季干燥偏倚，确定观测到的降水趋势的起源可能会变得复杂。大多数耦合气候模式明显低估了秋季密西西比河流域的降水，限制了我们熟练预测未来降水变化或将最近观测到的趋势归因于人为起源的能力。观测到的秋季降水趋势与气候模式中的干旱偏倚之间的这种明显不一致促使首席研究员（pi）更好地理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与地面气候之间的关系一直是许多观测和模式研究的主题。对秋季降水的关注相对较少，限制了我们对秋季气候时空变化和可预测性的认识。本研究的一个关键目标是了解美国中部降水的长期趋势和年代际变化。为了实现这一目标，PIs将关注三个广泛的问题：(a)什么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，它们在当前一代气候模式中的表现如何？(b)这些机制以何种方式以及为什么在秋季而不是在其他季节特别占优势？(c)能否确定秋季降水的年代际变化与太平洋或大西洋海表温度之间的物理联系？ppi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，进一步建立降水变化、环流异常和边界强迫之间的联系。然后，他们将试图确定可能的物理机制，可以解释观察到的相关性和关联联系。本研究的一个主要成果将是更好地理解和确定导致秋季降水空间连贯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差异。这项研究将处理与秋季降水变化的起源和性质以及季节、年际和年代际时间尺度上的趋势有关的问题。现有的研究大多只考虑冬季和夏季的降水变化。在未来的任何气候变化评估中，例如政府间气候变化专门委员会（IPCC）的第四次评估报告，都没有单独考虑秋季过渡季节。这项研究的结果将为以下问题提供新的见解：为什么在秋季主要观察到美国中部降水的大量增加，以及为什么目前的气候模式无法捕捉到这一趋势。塔夫茨大学和哥伦比亚大学之间的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基础上。这种合作关系将通过博士生的共同指导和本科生通过暑期实习的参与进一步加强。pi将整合这项研究的结果，开发一个关于美国降水变化的交互式多媒体教育模块，作为我们的双水平（大四本科生和一年级研究生）环境信号处理课程的三周教学工具。他们将在国家会议和档案期刊上展示他们的成果，并以不同的媒体形式发表他们的发现，以便记者、教师和公众随时可以获得。

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.