P2C2: Continental Scale Droughts in North America: Their Frequency, Character and Causes Over the Past Millennium and Near Term Future

泛北美大陆干旱，定义为影响多个水资源和农业地区的干旱，可能对社会造成严重压力，超过更局部但同样严重的干旱。例如，2012年的干旱袭击了平原和中西部，造成数百亿美元的农作物损失。然而，像2012年泛大陆干旱这样的事件让公众和气候研究人员都感到惊讶。它们似乎不容易用海洋强迫来解释，也不能用目前的季节预报系统来预测。然而，泛大陆干旱在仪器记录上反复发生，也出现在过去一千年来北美气候和降水的树木年轮重建中。本项目将在季节到数十年的时间尺度上研究北美泛大陆干旱的原因和动力机制。这项工作将使用仪器记录、过去一千年的代用气候记录、受历史海面温度影响的大气模型、耦合的大气环流模型和理想化的模型实验。感兴趣的时间段将是上个千年和未来几十年。该假说认为，太平洋-北美-大西洋大气-海洋耦合系统的优选状态导致泛大陆干旱，包括在季节性和较长时间尺度上不可预测的内部大气模式的潜在贡献。该项目还将确定土壤湿度和植被条件对泛大陆干旱的发生和演变的作用。将审查最近更新的千年树木年轮《北美干旱地图集》中的区域和泛大陆干旱记录。研究小组将评估当前的气候系统是否也可以解释过去千年的记录，或者是否需要根本不同的运作模式（例如太平洋和大西洋的气候变化）。将研究当前一代气候模式模拟泛大陆干旱的能力，以及采用何种机制。最后，将利用对过去和当前模型预测的分析来确定未来几十年泛大陆干旱可能性的变化。

Pan-continental North American droughts, defined as droughts that impact multiple water resource and agricultural areas, can exert severe stress on society in excess of more localized, but equally severe, droughts. For example, the drought of 2012 struck both the Plains and the Midwest and caused tens of billions of dollars in crop losses. However, events like the 2012 pan-continental drought take both the public and climate researchers by surprise. They do not appear to be easily explained in terms of ocean forcing, and cannot be predicted by current operational seasonal forecasting systems. However, pan-continental droughts have occurred repeatedly over the instrumental record and also appear in tree-ring reconstructions of climate and precipitation in North America over the last thousand years.This project will examine the causes and dynamical mechanisms of pan-continental droughts in North America on seasonal to multidecadal timescales. The work will use instrumental records, proxy climate records of the past millennium, atmosphere models forced by historical sea surface temperatures, coupled general circulation models and idealized model experiments. The time period of interest will be the last millennium and the next few decades. The hypothesis is that preferred states of the coupled Pacific-North America-Atlantic atmosphere-ocean system cause pan-continental droughts, including potential contributions from internal atmospheric modes that are unpredictable on seasonal and longer timescales. The project will also determine the role of soil moisture and vegetation conditions on the occurrence and evolution of pan-continental droughts. The record of regional and pan-continental drought in the newly updated millennium-long tree-ring based North American Drought Atlas will be examined. The research team will assess whether the current climate system can also explain the record of the past millennium or whether fundamentally different modes of operation (e.g. climate shifts in the Pacific and Atlantic Oceans) are required. The ability of current generation climate models to simulate pan-continental droughts, and with what mechanisms, will be examined. Finally, analyses of the past and current model projections will be used to determine changes in the likelihood of pan-continental drought in the coming decades.