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

泛美洲大陆干旱的定义是影响多种水资源和农业区的干旱，其对社会造成的压力可能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，造成了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大吃一惊。它们似乎不容易用海洋强迫来解释，也不能用目前可操作的季节性预报系统来预测。然而，在仪器记录中，泛大陆干旱反复发生，在过去1000年里，也出现在北美气候和降水的树轮重建中。本项目将从季节到数十年的时间尺度来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、过去千年的替代气候记录、由历史海洋表面温度强制的大气模型、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最后一个千年和未来几十年。假设是太平洋-北美-大西洋大气-海洋耦合系统的优先状态导致了泛大陆干旱，包括在季节和更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水分和植被条件对泛大陆干旱发生和演变的作用。将审查新更新的以千年年轮为基础的北美干旱地图集中区域和泛大陆干旱的记录。研究小组将评估目前的气候系统是否也能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)。我们将考察当代气候模型模拟泛大陆干旱的能力，以及采用何种机制。最后，对过去和当前模型预测的分析将被用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.