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

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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.