Collaborative Research: Spatiotemporal Variability and Drivers of East-west Hydroclimate Anti-phasing in the Midcontinental United States During the Last 2000 Years

美国中大陆是世界上最大的农业中心之一，居住着7000多万美国人。近几十年来，该地区的水文气候事件加剧，美国中部大陆东部的洪水通常更频繁、持续时间更长，西部的干旱更严重、更持续。这种水文气候变化的东西格局可能是持久和长期的，是自然的大规模海洋-大气环流体制造成的，也可能是人为气候变化的较新表现。考虑到仪器气象记录的跨度通常不超过150年，在数百年至数千年的时间尺度上调查中西部地区水文气候变化的原因需要分析高分辨率的古气候记录，而高分辨率古气候记录的发展是本研究的主要目标。通过对湖泊沉积物的地球化学分析，研究人员将产生一系列跨越过去2000年的湖泊水位和降水变化记录，以确定大陆中部水文气候变异性的时空模式和潜在驱动因素。这项研究将1)产生关于水气候的长期时间尺度观点，为水资源管理战略提供信息；2)阐明水气候对全球气温上升的区域尺度响应。作为该项目的一部分，调查人员将与地方、州和联邦机构合作，包括美国地质调查局，将这项研究的结果纳入洪水模型开发工作和河流侵蚀灾害缓解工作。来自该项目的数据还将为气候-社会相互作用的研究提供信息并使其受益，包括突然和长期的气候变化对美洲原住民社会的影响，这些社会曾经居住在美国中部大陆，但在大约600年前突然放弃了该地区的大部分地区。在方法论上，本研究有三个具体目标。1)研究人员将利用两个水文开阔湖泊沉积物中碳酸盐矿物的氧同位素(D18O)测量，重建过去2000年美国中大陆大气环流和降水季节性的变化。2)研究人员将使用d18O、粒度、碳氮比和对四个水文封闭湖泊沉积物的硅藻分析来记录当地的水气候条件(降水量：蒸发率和湖面变化)。3)研究人员将在从经验古气候记录和模型模拟中获知的大气-海洋变异性的背景下，对开放和关闭的湖泊水文气候记录进行统计评估。利用这些方法，研究人员将量化大气环流和降水季节性对区域大陆性水文气候的影响，并确定海洋-大气过程的潜在作用(S)。具体地说，调查人员将检验三个假设。1)美国中部大陆近2000年来数十年至百年的水气候变化特征是东西向水气候偶极子，其铰链线位于大平原和中西部之间，约96oW。2)水气候偶极子变化是由大气环流驱动的，这种环流类似于太平洋-北美模式，改变了季节降水模式。3)中大陆水气候和大气环流异常主要受太平洋盆地大气-海洋过程的控制。这项研究的一个重要组成部分是对本科生和研究生进行教育和培训，同时积极努力通过招聘和支持科学领域的妇女和代表性不足的少数群体来增加STEM领域的多样性。一名博士、两名硕士和四名本科生将直接得到这个项目的支持，其他几个得到其他来源支持的学生也将提供帮助。因为向公众传播科学是任何有效研究项目的关键组成部分，研究人员和他们的机构将参与外联活动，包括为4至9年级的学生开发一个教育模块，重点是1)中西部气候，包括古气候和现代气候，2)古气候重建技术，3)水资源可变性。该单元将由IUPUI的地球和环境科学中心开发和实施，并公开提供给其他教育工作者使用。研究人员将与印第安纳州立博物馆合作，将项目成果纳入关于中大陆美洲原住民的互动展览中。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The midcontinental United States (US) is one of the world's largest agricultural centers and home to more than 70 million Americans. In recent decades, hydro-climate events in this region have intensified, with generally more frequent and prolonged flooding in the eastern midcontinental US and more severe and sustained droughts in the west. This east-west pattern of hydro-climate variability may be persistent and long-standing, resulting from natural large-scale ocean-atmosphere circulation regimes, or it may be a more recent manifestation of anthropogenic climate change. Given that instrumental meteorological records typically span no more than ~150 years, investigation of the causes of hydroclimate changes in the Midwest on timescales of hundreds to thousands of years requires analysis of high-resolution paleoclimate records, the development of which is the primary objective of this research. Through geochemical analysis of lake sediment, the researchers will produce a series of records of lake-level and precipitation changes that will span the past 2000 years in order to determine the spatiotemporal patterns and underlying drivers of midcontinental hydro-climate variability. This research will 1) produce a long timescale perspective on hydroclimate that will inform water resource management strategies, and 2) elucidate regional-scale responses of hydro-climate to increasing global temperatures. As part of this project, the investigators will be working with local, state, and federal agencies, including the United States Geological Survey, to incorporate the results of this research into flood model development efforts and fluvial erosion hazard mitigation. The data from this project will additionally inform and benefit research into climate-society interactions, including the impact of abrupt and long-term climate changes on Native American societies that once populated the midcontinental US, but abruptly abandoned much of the region ~600 years ago. Methodologically, there are three specific objectives of this research. 1) The researchers will reconstruct changes in atmospheric circulation and precipitation seasonality in the midcontinental US during the last 2000 years using oxygen isotope (d18O) measurements of carbonate minerals in sediment from two hydrologically open lakes. 2) The researchers will produce records of local hydroclimate conditions (precipitation:evaporation ratios and lake-level variability) using d18O, grain size, carbon:nitrogen, and diatom analysis of sediment from four hydrologically closed lakes. 3) The researchers will statistically evaluate the open and closed lake hydro-climate records in the context of atmosphere-ocean variability known from empirical paleoclimate records and model simulations. Using these approaches, the researchers will quantify the respective influences of atmospheric circulation and precipitation seasonality on regional midcontinental hydro-climate and determine the underlying role(s) of ocean-atmosphere processes. Specifically, the investigators will test three hypotheses. 1) Multi-decadal to centennial hydroclimate variability in the midcontinental US during the last 2000 years was characterized by an east-west hydroclimate dipole with its hinge line located between the Great Plains and Midwest at ~96oW. 2) Hydroclimate dipole variability was driven by atmospheric circulation resembling the Pacific North American mode that altered seasonal precipitation patterns. 3) Midcontinental hydroclimate and atmospheric circulation anomalies were predominantly controlled by Pacific basin atmosphere-ocean processes. A critical component of this research is the education and training of undergraduate and graduate students while actively working to increase diversity in STEM fields by recruiting and supporting women and underrepresented minorities in science. One PhD, 2 MSc, and 4 undergraduate students will be directly supported by this project, while several others supported by other sources will also contribute. Because translation of science to the public is a critical component of any effective research project, the researchers and their institutions will engage in outreach activities including the development of an educational module for 4th to 9th grade students focused on 1) Midwest climate, both paleo and modern, 2) paleoclimate reconstruction techniques, and 3) water resource variability. This module will be developed and implemented by IUPUI's Center for Earth and Environmental Sciences and made publicly available for other educators to use. The researchers will work with the Indiana State Museum to incorporate the projects results into an interactive exhibit on midcontinental Native Americans.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.