Dissolved Oxygen Variability on the Canadian Pacific Shelf: Trends, Drivers, and Projections in the Context of Emerging Hypoxia in Queen Charlotte Sound

Hypoxia is an increasing concern along the Northeast Pacific continental margin, driven by dissolved oxygen ($O_2$) declines related to a warming climate. Although the North Pacific $O_2$ inventory has declined for decades, hypoxia was rarely observed on the central coast of British Columbia, Canada, before 2020. Recent observations from Queen Charlotte Sound (QCS)—the largest shelf sea in the Canadian Pacific—indicate that hypoxia is now an emerging issue in this region. This study synthesizes measurements from numerous platforms to describe regional $O_2$ distributions during 2022 and 2023. These observations reveal persistent shelf-wide hypoxia during summer months, including periods of statistically defined extreme hypoxia. Comparisons with the hydrographic record from 2003 to 2021 show that $O_2$ concentrations were lower and hypoxia more common in 2022 and 2023 than in previous years. This recent hypoxia is caused by long-term deoxygenation trends of approximately 5–10 $\mu$mol ${\text{kg}}^{-1}$ per decade at isopycnals representing QCS deep waters, combined with seasonal and interannual variability of similar or larger magnitude. Drivers of regional $O_2$ variability are identified over a broad range of scales. On sub-decadal time scales, upwelling strength, upwelling timing, and utilization drive $O_2$ variability. On decadal time scales, variability is modulated by the offshore supply of $O_2$, which appears to propagate from the northwestern Pacific via North Pacific Gyre circulation with an 8-year lag. Data-based projections indicate that bottom water hypoxia will occur with increasing frequency in the coming decades, while larger portions of the regional deep layer could experience consistent summertime hypoxia by 2050.