P2C2: Decomposition and Reconstruction of Drought Variability Across the Continental United States and Northern Mexico

在过去的二十年里，美国西部和墨西哥北部经历了强烈而持久的干旱，这减少了水资源，增加了野火活动，并引起了人们对水政策是否为未来可能性设计了足够广泛的关注。与此同时，美国东部的大部分地区经历了百年来的湿润趋势、最小的变暖和反复的洪水，但古证据表明，该地区仍然容易发生十年一次的毁灭性干旱。目前尚不清楚这些截然不同的趋势是否存在动态关联，以及它们是否受到人为强迫的推动。激发本研究的低频水文气候事件在观测记录中是异常的，但古气候重建表明，水文气候的极端、长时间波动经常是自然发生的。该项目的目标是扩展至少1200年的空间连续干旱指标。该项目将利用过去一千年年度水文气候变率的新网格重建，通过三种研究方法更好地理解美国和墨西哥北部的低频年代际水文气候变率的驱动因素：(1)确定近几十年来人为变暖对土壤湿度观测的千年变化的改变程度；(2)重建过去一千年来美国和墨西哥北部地区的年际至多年代际干旱变率的主要模态，特别是对加利福尼亚水资源至关重要的模态和反映东西偶极子的模态，这可能为观测到的几十年趋势提供见解；(3)重建和诊断美国各地干旱变化的东西偶极子，并确定该模式下几十年自然变化的预期幅度如何映射到预测的气候趋势。更广泛的影响包括支持一位早期的职业科学家，以及对过去一千年美国水文气候变化的区域和大陆尺度重建的发展，这对水资源管理者来说是有价值的。

The western United States (U.S.) and northern Mexico have experienced intense and prolonged droughts over the past two decades that have reduced water resources, enhanced wildfire activity, and raised concerns regarding whether water policies are designed for a sufficiently wide range of future possibilities. Concurrently, much of the eastern U.S. experienced a centennial wetting trend, minimal warming, and repeated floods, but paleo evidence indicates that this region is nonetheless prone to devastating decade-scale droughts. It is unclear if these contrasting trends are dynamically related and whether they have been aided by anthropogenic forcing. The observed low-frequency hydroclimate events that motivate this research are abnormal in the observational record, but paleoclimate reconstructions indicate that extreme, prolonged swings in hydroclimate often occur naturally. The aim of the project is to extend a range of spatially continuous drought metrics for at least 1,200 years.This project will use new gridded reconstructions of annual hydroclimatic variability over the past millennium to better understand the drivers of low frequency, decadal scale hydroclimate variability across the US and northern Mexico using three research approaches: (1) determine the degree to which anthropogenic warming has altered the millennial standing of observed soil-moisture in recent decades; (2) reconstruct the leading modes of interannual to multi-decadal drought variability over the past millennium across the U.S. and northern Mexico, with particular focus on a mode critical for California water resources and a mode reflecting an east-west dipole that may lend insights regarding observed multi-decade trends; and (3) reconstruct and diagnose the east-west dipole in drought variability across the U.S. and determine how the expected magnitude of natural multi-decade variations in this mode map onto projected climate trends.The broader impacts involve support for an early career scientist and the development of regional- and continental- scale reconstructions of hydroclimate variability in the United States spanning the past millennium of value to water resource managers.