The North Pacific Oxygen Uptake Rates Over the Past Half-Century

The transport of dissolved oxygen (O₂) from the surface ocean into the interior is a critical process sustaining aerobic life in mesopelagic ecosystems, but its rates and sensitivity to climate variations are poorly understood. Using a data-assimilated circulation model, we show that the North Pacific thermocline effectively “breathes in” O₂ primarily by expanding the area through which O₂-rich mixed layer water is detrained into the thermocline. The outcrop area during the critical winter season varies in concert with the Pacific Decadal Oscillation (PDO). When the central North Pacific Ocean is in a cold phase, the winter outcrop window for the Central Mode Water class (CMW; a neutral density range of γ = 25.6 - 26.6) expands southward allowing more O₂ to enter the ocean’s interior. An increase in O₂ supply to the CMW density water is partly compensated by a reduced supply to the shallower densities of Subtropical Mode Water (γ = 24.0 - 25.5). Consistent with strong O₂ uptake rates in the 1980’s, the thermocline becomes better oxygenated. Positive O₂ anomalies appear first near the outcrop and subsequently downstream in the subtropical gyre. In contrast to the O₂ variations within the ventilated thermocline, observed O₂ in Intermediate Water (density range of γ = 26.7 – 27.2) shows a declining trend over the past half-century, a trend not explained by the open ocean water mass formation rate.