Doctoral Dissertation Research: Analysis of Decadal Moisture Anomalies over North America: 1700-1979

最近关于海洋、大气和陆地系统之间复杂相互作用的研究突出表明，需要更全面地了解在季节、年和年代际尺度上影响气候变率的过程。对社会许多不同部门具有特殊意义的脆弱性的一个方面是长期干湿期的趋势，由于降水缺乏或过多，这可能产生严重影响。本博士论文研究项目将分析美国和墨西哥北部旱涝灾害自然模式的时空变化和强迫。分析将集中在区域空间尺度和年代际到世纪的时间尺度上。仪器和树木年轮重建的帕尔默干旱严重指数（PDSI）数据集可用于覆盖美国的6月至7月至8月，测量纬度为2度，经度为3度，仪器数据涵盖1895年至1995年期间，而树木气候数据涵盖1700年至1979年期间。在这项研究中，重建的数据集将扩展到墨西哥北部新的树木年轮年表。本研究的总体目标是更好地了解湿度异常在空间和时间上是如何变化的，以及地球气候系统自然变化中的哪些因素导致了它们的发生。该项目的第一个目标是描述20世纪著名的史诗般的干旱和潮湿的空间和时间特征。这些史诗事件的潜在类似物及其空间位置、进展和严重程度将从长树轮重建的PDSI数据集和文献中的历史证据中得到特征。PDSI地图序列的动画将有助于分析干旱/潮湿法术的进展和消散。对导致或“强迫”干旱/湿期的因素的研究将包括检查太阳日照变化、太平洋海面温度以及太平洋/北美模式、太平洋年代际涛动、南方涛动、北大西洋涛动和北极涛动等指数对干旱/湿期的位置、持续时间和强度的影响。用于这些“强迫”分析的方法将是对强迫时间序列和单个网格点PDSI时间序列之间的成对比较获得的复合海面温度和相关统计数据进行映射。湿度异常和各种强迫因素之间的可能联系也将通过t检验和光谱分析进行评估和绘制。该研究项目将有助于更好地了解美国和墨西哥北部长期干旱和潮湿时期的地理位置、时间、进展、强度和原因。重点将放在长期干旱和潮湿时期，因为它们对环境和社会产生了巨大影响。比如20世纪30年代的大干旱，以及20世纪初和90年代初的湿润期。然而，这些事件在美国气候史上并非没有先例。基于300多年来由树木年轮重建的美国水分异常记录，将识别和表征反复出现的长时间干旱/潮湿期。因果机制，如太阳辐照和海面温度的变化，将在统计上与这些反复出现的湿度异常联系起来，以确定共同的原因。预期的结果是提高对与某些强迫或因果机制相关的旱涝灾害的时间、位置和严重程度的理解。这有望改善旱涝天气的预报，并为继续研究这些深刻的气候异常提供有价值的见解和数据。作为博士论文研究改进奖，该奖项还将提供支持，使有前途的学生建立一个强大的独立研究生涯。

Recent research on the complex interactions among ocean, atmosphere, and terrestrial systems have highlighted the need for more complete understanding of the processes that influence climatic variability at seasonal, annual, and decadal time scales. Once facet of vulnerability that has special significance for many different sectors of society is the tendency for longer-term dry and wet spells, which can have critical impacts because of the absence or overabundance of precipitation. This doctoral dissertation research project will analyze the spatial and temporal variation and forcing of natural modes of drought and wet spells over the U.S. and northern Mexico. Analyses will focus on the regional spatial scale and decadal to century time scales. Instrumental and tree-ring-reconstructed Palmer Drought Severity Index (PDSI) data sets are available as June-July-August means covering the U.S. for cells measuring 2 degrees of latitude by 3 degrees of longitude, with the instrumental data extending over the period from 1895 to1995 period while the dendroclimatological data cover the period from 1700 to 1979. The reconstructed data set will be expanded in this study with new tree-ring chronologies over northern Mexico. The overall goal of this study is to better understand how moisture anomalies vary spatially and temporally and what factors within the natural variation of earth's climate system cause them to occur. The project's first objective is a description of the spatial and temporal characteristics of well-known epic droughts and wet spells during the 20th century. Potential analogs to these epic events and their spatial position, progression, and severity will be characterized from the long tree-ring reconstructed PDSI data set and historical evidence in the literature. Animations of PDSI map sequences will help analyze drought/wet spell progression and dissipation. Studies of factors that lead to or "force" droughts/wet spells will include examination of the effects of solar insolation variation, Pacific sea surface temperatures, and indices like the Pacific/North American Pattern, Pacific Decadal Oscillation, Southern Oscillation, North Atlantic Oscillation, and Arctic Oscillation on the position, duration, and intensity of drought/wet spells. Methods used for these "forcing" analyses will be the mapping of composite sea surface temperatures and correlation statistics obtained from pair-wise comparisons between the forcing time series and individual grid point PDSI time series. The possible link between moisture anomalies and various forcing factorswill also be assessed and plotted using t-tests and spectral analyses.This research project should result in a better understanding of the geographical positioning, timing, progression, intensity, and causes of long-duration drought and wet periods in the U.S. and northern Mexico. The focus will be on long-term drought and wet periods because of the enormous impact they have had on the environment and society. Examples are the 1930s epic drought and the wet periods of the early 1900s and the early 1990s. These events are not unprecedented in the climate history of the U.S., however. Based on a 300-plus-year record of U.S. moisture anomalies reconstructed from tree-rings, recurring long duration drought/wet periods will be identified and characterized. Causal mechanisms such as variations in solar irradiation and sea surface temperature will then be statistically associated with these recurring moisture anomalies to identify common causes. The expected result is an improved understanding of the timing, position, and severity of drought and wet spells associated with certain forcing or causal mechanisms. This is expected to lead to improved forecasts of drought/wet spells and provide valuable insight and data for continued research into these profound climatic anomalies. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.