Spatial Variability in Year‐to‐Year, Near‐Bottom Hypoxia Over the United States Pacific Northwest Continental Shelf

Abstract The continental shelf off the northwest coast of North America experiences seasonal upwelling driven by summertime southward winds. Nutrient‐rich, low‐oxygen water is upwelled onto the shelf from a depth of about 150 m seaward of the continental shelfbreak. Though nutrients fuel a productive food web, respiration of organic matter can lead to hypoxic conditions (dissolved oxygen < 61 μmol/kg ∼ 1.4 ml/l) near the seafloor and sometimes reaching to over half of the lower water column. We compare data collected inshore of the 200‐m isobath during the summers of 2021–2024 to determine the magnitude and spatial distributions of near‐bottom hypoxic conditions on the shelf and whether these patterns are similar year‐to‐year. We found widespread hypoxia across the shelf during the summers of 2021 (52% of the shelf), 2022 (28%), 2023 (29%) and 2024 (39%). The largest (smallest) area of near‐bottom hypoxia was 17,079 (9,196) square kilometers in 2021 (2022), when upwelling winds were particularly strong (weak). The spatial distribution of near‐bottom dissolved oxygen exhibits similar patterns across years reinforcing the importance of oceanographic processes in setting those patterns. We investigate relationships between hypoxia and possible forcing factors including dissolved oxygen levels of upwelled source waters—declining over time—and cumulative upwelling‐favorable wind over the upwelling season. There is some evidence for stronger cumulative upwelling‐favorable seasonal winds to drive more hypoxia on the shelf. This implies that should upwelling‐favorable winds continue to increase due to climate change, more hypoxia will occur over the US Pacific Northwest continental shelf.