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.
Project Outcome: This grant supported research on the understanding and possible prediction of the long-term trend and the decadal variability of fall precipitation over the United States. Using observed datasets derived from weather stations throughout the region, and suites of global climate models used in the most recent and upcoming Intergovernmental Panel on Climate Change Assessment reports, analysis was performed with several key goals in mind: first, to determine what mechanisms are important to produce trends and decadal variations in fall precipitation across the United States and how well are they represented in current generation climate models? Second, in what ways and why these mechanisms are particularly dominant in fall and not in other seasons? And third, can we identify physical linkages between decadal variations in fall precipitation and Pacific or Atlantic Sea Surface Temperatures? Water is a vital human resource, whether and how rainfall changes during the 21st Century have important consequences for the people that live there. Therefore it is necessary to evaluate the uncertainty associated with the global climate model predictions and improve our understanding of the mechanisms that drive precipitation variability and change. Global warming driven by rising greenhouse gases (GHGs) is expected to cause wet regions of the tropics and mid to high latitudes to get wetter and subtropical dry regions to get drier and expand poleward. Over southwest North America, climate models project a steady drop in precipitation minus evapotranspiration, P − E, the net flux of water at the land surface leading to, for example, a decline in Colorado River flow. This would cause widespread and important social and ecological consequences. However in the southeast United States, the decline in surface water is less obvious and observations indicated an increase in precipitation in the region over the past several decades. Further studies showed that this increase is mainly caused by the fall season rainfall increase, which distinguishes the southeast from the southwest. It is motivated by this observations that we propose to understand further the cause of the future changes in precipitation in the United States and the seasonal dependence of the rainfall trend. This research includes several key outcomes. First, that the climate models, by averaging across 20 or so models, provide an estimate of the possible impact of the greenhouse gases and other anthropogenic induced changes on precipitation. This multimodel mean precipitation trend is found to have a north-south pattern over North America, indicating the mid and high latitudes wetting trend and the subtropics drying trend. This is generally true throughout the year with the exception of the autumn season, when the rainfall trend is showing an east-west pattern, with the west getting drier and the east getting wetter in the future (see Figure 1). This result indicates that the southwest North America is experiencing a drier condition throughout the year while the southeast US gets a break from the drying due to the autumn rainfall increase. How and why the autumn season differs from the rest of the year is an open question and will be further examined in the future. Second, the Southwest North America is already experiencing the drying trend and the reduction in Colorado River flow will reach 10-20% by the middle of this Century. Texas, in particular, will experience drier soil condition due to the self-sufficient water supply within the states. Next, it is concluded that the subtropical highs over the Atlantic will intensify and extend further west so that the southeast US rainfall may experience stronger fluctuations due to the impact of subtropical high in the region. This may signal an enhanced variability in the region so that the Southest may experience more extreme hydroclimate fluctuations. Finally, we found that the Atlantic multidecadal variability, a multidecadal sea surface temperature fluctuation, will complicate the United States hydroclimate fluctuations on multidecadal time scales. It is thus important to understand the observed drying that is driven by human induced changes or due to natural climate fluctuations within the coupled ocean-atmosphere system, in order for better future projections of the hydroclimate changes in the United States.
