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

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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.