First Advisor

Paul Loikith

Term of Graduation

Spring 2024

Date of Publication

5-22-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.) in Earth, Environment, & Society

Department

Earth, Environment, & Society

Language

English

Physical Description

1 online resource (xii, 205 pages)

Abstract

This dissertation characterizes the range of atmospheric circulation patterns and select associated meteorology impacting the northwest United States, the onset timing of rain and wind relevant to wildfire across the western United States, and how these may respond to global warming. The ability of a suite of models contributing to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) to simulate observed large-scale atmospheric circulation patterns over the Pacific Northwest of North America is evaluated. Climate model projections of atmospheric circulation patterns, their frequency, and associated temperature and precipitation anomalies under a high-end global warming scenario are assessed over the Pacific Northwest of North America for the final three decades of the 21st century. Future projections of changes in the timing of rain and wind in a suite of downscaled climate models are assessed.

Atmospheric circulation patterns, defined herein as geopotential height anomalies at 500 hPa (Z500), are an information-rich variable commonly used for diagnostic purposes in climate science and meteorology. The range of Z500 patterns over a region can be used to characterize the weather on a climate time scale. A first step in understanding potential changes to the range of patterns due to global warming is to use reference data, in the case of this study reanalysis data, to characterize the variability of possible patterns that impact a region and their basic meteorological effects. Once this is done, the latest suite of state-of-the-art global climate models can be evaluated for their ability to accurately simulate the range of historical circulation patterns. Similarly, the physical link between the circulation patterns in models and temperature and precipitation anomalies is evaluated, as well as pattern frequency occurrence. Evaluation finds that models are able to simulate the patterns, the frequency, and the temperature and precipitation anomalies with reasonable accuracy, although some biases occur.

Uncertainty concerning future changes to atmospheric circulation patterns, and their impacts, is one of the current challenges at the forefront of climate science. Analysis of future projections of climate models reveals that under a high-end global warming scenario, the range of future circulation patterns over the Pacific Northwest generally resembles the past. However, the strength of Z500 anomalies is found to weaken, although significant results are generally confined to summer. High amplitude patterns are found to decrease in frequency, and patterns in all seasons except summer show increased precipitation.

The circulation regime shift from summer to winter in the western United States brings the onset of Pacific storm systems. As a consequence, a distinct fall/winter rainy season and windy season appears over the region, and generally marks the end of active wildfires. However, the increase in amplified circulation patterns also means strong winds, which can exacerbate wildfires, become increasingly likely as the season progresses. Using high-resolution simulations, this rainy and windy season onset is characterized over the western United States, and future projections are analyzed. Results find that by the end of the 21st century, the beginning of the rainy season may be drier in some regions, and the wind may shift later.

Rights

© 2024 Graham Patrick Taylor

In Copyright. URI: http://rightsstatements.org/vocab/InC/1.0/ This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).

Persistent Identifier

https://archives.pdx.edu/ds/psu/42208

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