Date of Award

Winter 12-2016

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Oceanography

Advisor

Fei Chai

Second Committee Member

Huijie Xue

Third Committee Member

Francisco Chavez

Additional Committee Members

Andrew Thomas

Lawrence Mayer

Abstract

A potential consequence of climate change is a global decrease in dissolved oxygen at depth due to changes in the balance of ventilation, mixing, respiration, and photosynthesis in the oceans. Regionally, the California Current has experienced dissolved oxygen declines since the late 1980s with observations from Oregon and the Southern California Bight. Here, we present observations of declining dissolved oxygen along CalCOFI Line 67 off of Monterey Bay, in the Central California Current region, and investigate likely mechanisms. The hydrographic cruises obtained dissolved oxygen measurements 50-300 km from shore between 1998 and 2013, with quasi-seasonal sampling resolution. Dissolved oxygen decreased along the entire transect over the 16-year period on the σθ 26.6-26.8 isopycnals, corresponding to depths between 250-400 m. At two regions around 130 and 240 km from shore respectively, declines in dissolved oxygen occurred on σθ 25.7-26.5 as well as σθ 26.6-26.8. Variations in oxygen concentration on isopycnals at σθ 25.5, at approximately the bottom of the surface mixed layer, do not show similar decline, but correlate with environmental climate indices including the upwelling index in spring. A box model of the region suggests that multiple mechanisms are at work contributing to the oxygen declines including changes in the oxygen concentration of source waters and local respiration. Our work highlights the need for care in investigating the mechanisms for declining dissolved oxygen in the California Current system due to the complex mechanisms at work.

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