Declining Oxygen in the Northeast Pacific

Abstract

Climate models predict a decrease in oceanic dissolved oxygen and a thickening of the oxygen minimum zone, associated with global warming. Comprehensive observational analyses of oxygen decline are challenging, given generally sparse historical data. The Newport hydrographic (NH) line off central Oregon is one of the few locations in the northeast Pacific with long oxygen records. Good quality data are available here primarily in two time blocks: 1960–71 and 1998–present. Standard sampling extends from midshelf (bottom depth of 58 m) to 157 km offshore (bottom depth of 2880 m). Shipboard measurements have been supplemented in recent years (2006–present) with data from autonomous underwater gliders. Oxygen declines significantly over this 50-yr period across the entire NH line. In addition to decrease in the vicinity of the oxygen minimum depth (~800 m), oxygen decreases across a range of density surfaces σ[subscript θ] = 26–27 within the thermocline, in the depth range 100–550 m. A core of decreasing oxygen (0.7 ± 0.2 μmol kg⁻¹ yr⁻¹ or 0.016 ± 0.005 ml l⁻¹ yr⁻¹) is also found over the upper slope at 150–200-m depths, within the region of average northward flow associated with the poleward undercurrent. During the summer upwelling season, the largest decline is observed near bottom on the shelf: the dissolved oxygen of upwelled water, already low, is further reduced by shelf processes, leading to near-bottom hypoxia (<60 μmol kg⁻¹) on the Oregon shelf.