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

美国西部(美国)在过去的二十年里，墨西哥北部和北部经历了严重和长期的干旱，导致水资源减少，野火活动加剧，并引发了人们对水资源政策是否针对未来各种可能性而设计的担忧。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，最低限度的变暖，以及反复的洪水，但古证据表明，该地区仍然容易发生毁灭性的十年规模的干旱。目前尚不清楚这些截然不同的趋势是否在动态上相关，以及它们是否得到了人为强迫的帮助。推动这项研究的观测到的低频水气候事件在观测记录中是不正常的，但古气候重建表明，水气候的极端、长期波动通常是自然发生的。该项目的目标是将一系列空间上连续的干旱度量延长至少1200年。该项目将使用过去1000年来年度水文气候变率的新网格重建，以更好地了解美国和墨西哥北部低频、年代际尺度水文气候变率的驱动因素，使用三种研究方法：(1)确定近几十年来人为变暖改变观测到的土壤水分的千年标准的程度；(2)重建美国和墨西哥北部近千年来干旱年际到年代际变化的主要模式，特别关注一种对加州水资源至关重要的模式和一种反映东西偶极子的模式，该模式可以提供对观察到的数十年趋势的洞察；以及(3)重建和诊断美国各地干旱变化的东西偶极子，并确定这种模式中自然数十年变化的预期大小如何映射到预测的气候趋势。更广泛的影响包括对早期职业科学家的支持，以及开发跨越过去千年对水资源管理者有价值的美国水文气候变化的区域和大陆尺度重建。

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.