Abstract

The Pacific Decadal Oscillation (PDO) is defined by sea-surface temperatures (SSTs) in the North Pacific and is a primary driver of climate in the Gulf of Alaska (GOA) (Mantua and Hare, 2002). Characterized by decades-long regime shifts of warm or cool SSTs along the GOA, fluctuations in the PDO have the potential to impact marine resources in the North Pacific and adjacent North American climate (Mantua et al., 1997). The short observational record of the PDO (1900-present) only covers 4-5 regime shifts, thus longer records are needed to characterize how sensitive this phenomenon is to climate forcing. While the mechanisms behind PDO shifts are not well understood, modeling experiments suggest that large stratospheric volcanic eruptions have previously shifted the PDO into a negative mode (Wang et al., 2012). Stratospheric tropical eruptions eject sulfate aerosols into the atmosphere, modifying the Earth’s albedo and eventually cooling regional SSTs. Several large volcanic eruptions documented in ice cores are known to cool SSTs in the North Pacific as evidenced by SST-sensitive tree ring records across the GOA (D’Arrigo et al., 2013).

This study uses yellow cedar (Cupressus nootkatensis) tree ring width data from Southeast Alaska to provide additional evidence of volcanic cooling following the estimated 1458, 1600, 1695, and 1808 eruption events. Dude Mountain, located in Ketchikan, Alaska, was chosen as the study site for its high-elevation (863m) and minimized stand disturbance. This well replicated (1350-2023) ring-width record serves as a proxy for past temperatures and shows negative correlations with May, June, July Sitka temperatures during the 1920s through the 1940s, but not with the PDO. Possibly suggesting that this series does not respond to shifts in the PDO, but instead shifts in the Pacific current, which tend to be at their strongest during May and June. Stratospheric volcanic events can induce extended cooling to the North Pacific Current, delivering colder waters, winds, and temperatures into the Gulf of Alaska. These temperature fluctuations represent a key component of the PDO as it manifests in our study region due to the unique geospatial setting of Dude Mountain. We propose that our proxy may better reflect the Pacific Current through North Pacific Ocean heat content, which could serve as a recorder of more consistent ocean temperatures associated with PDO phases.

Advisor

Wiles, Greg

Second Advisor

Moreno, Carlo

Department

Earth Sciences; Environmental Studies

Disciplines

Atmospheric Sciences | Climate | Other Earth Sciences | Other Environmental Sciences | Other Physical Sciences and Mathematics

Keywords

Dendrochronology, Dendroclimatology, Climate, Atmospheric Sciences, Yellow Cedar, Alaska, Southeast Alaska, Volcanoes

Publication Date

2025

Degree Granted

Bachelor of Arts

Document Type

Senior Independent Study Thesis

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© Copyright 2025 Amanda Flory