Date of Graduation

5-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Geosciences (PhD)

Degree Level

Graduate

Department

Geosciences

Advisor

David Stahle

Committee Member

Song Feng

Second Committee Member

Ralph Davis

Third Committee Member

Fred Paillet

Keywords

adjusted latewood, dendrometeorology, earlywood, latewood, seasonal climate reconstructions, single day precipitation totals

Abstract

In recent decades, there has been an increase in the development of sub-annual earlywood (EW) and latewood (LW) width tree-ring chronologies that have been used to make long-term inferences about discrete seasonal moisture variability for different regions of North America. This doctoral research developed a new network of EW, LW, and adjusted latewood (LWa) tree-ring chronologies from the western Great Plains. These chronologies were used to reconstruct 300+ years of spring and summer moisture variability over the northern and southern Plains. The reconstructions document new information about the long-term seasonal climate history of the Great Plains, including the unusual nature of persistent spring dryness in the mid- to late-19th century and the unprecedented summer drought conditions of the 1930s. Using daily precipitation data, a regional average of four LWa chronologies from eastern Colorado was shown to be significantly correlated (r = 0.83) with the heaviest rainfall day that occurs during the wettest two weeks of year in late-July. A 238-year reconstruction estimates there has been an increase in the frequency of the most extreme events (>90th percentile), especially during the late-20th century. The heaviest midsummer rainfall days are associated with strong Canadian cold fronts and a negatively tilted upper-level ridge over the western United States that increases atmospheric moisture advection from the Gulf of Mexico and Pacific. Sixty-nine Douglas-fir and ponderosa pine LWa chronologies from the Southwest were compared with gridded daily rainfall data to objectively define the length and timing of the precipitation intervals best correlated with independent LW growth during the warm season. The majority of LWa chronologies are significantly correlated with early-season monsoon precipitation and are related to the start of the local monsoon season. Ponderosa pine is largely correlated with sub-monthly precipitation totals during the early-monsoon season across the Southwest. Douglas-fir tends to be correlated with precipitation summed over longer intervals, and there is substantial geographic variability in terms of the timing of the best warm season precipitation signals. The intensity of the monsoon during the month of July appears to be the primary factor in determining geographically which sites contain a monsoon precipitation signal.

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