Date of Award:

12-2025

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Plants, Soils, and Climate

Committee Chair(s)

Yoshimitsu Chikamoto

Committee

Yoshimitsu Chikamoto

Committee

Wei Zhang

Committee

Johathan Meyer

Committee

Michael J. McPhaden

Committee

Malte F. Stuecker

Abstract

This dissertation investigates the dynamics, predictability, and impacts of multi-year La Niña events. Multi-year La Niña events are referred to as extended periods of unusually cooler tropical Pacific that persist for two or more consecutive winters. These prolonged events can increase the risk of natural hazards such as droughts, floods, and heatwaves in the western US and around the world. Despite their worldwide significance, questions remain about the mechanisms driving their initiation, their predictability, and regional hydroclimate impacts.

The first chapter of the dissertation introduces what scientists currently believe causes them, and how well different types of climate models can predict them. It also explains how these events can impact drought conditions in the western United States. The second chapter focuses on the recent 2020–2023 La Niña as a case study. Our study found the uniqueness of this event compared to the past and examined how the Indian and Atlantic Oceans might have helped trigger this unusual event. In the third chapter, we tested whether climate models were able to predict the initiation of the 2020–2023 La Ni˜na. The results suggested that including ocean information from other basins (Indian, Atlantic, and North Pacific) improved the model’s ability to simulate how the event began. Finally, the dissertation examines how these multi-year La Niñas affect winter conditions in the western United States. We found that dry conditions tended to shift north in the second year of these events, and that changes in the stratosphere conditions (approximately 10 miles above Earth’s surface) might help explain this shift. Together, this work provides new insight into what causes multi-year La Niñas, how we may better predict them, and what they mean for people living in regions vulnerable to water availability

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Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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