Mechanisms of Low‐Frequency Oxygen Variability in the North Pacific

This study investigates the mechanisms of interannual and decadal variability of dissolved oxygen (O 2 ) in the North Pacific using historical observations and a hindcast simulation using the Community Earth System Model. The simulated variability of upper ocean (200 m) O 2 is moderately correlated with observations where sampling density is relatively high. The dominant mode of O 2 variability explains 24.8% of the variance and is significantly correlated with the Pacific Decadal Oscillation (PDO) index ( r  = 0.68). Two primary mechanisms are hypothesized by which the PDO controls upper ocean O 2 variability. Vertical movement of isopycnals (“heave”) drives O 2 variations in the deep tropics; isopycnal surfaces are depressed in the eastern tropics under the positive (El Niño‐like) phase of PDO, leading to O 2 increases in the upper water column. In contrast to the tropics, changes in subduction are the primary control on extratropical O 2 variability. These hypotheses are tested by contrasting O 2 anomalies with the heave‐induced component of variability calculated from potential density anomalies. Isopycnal heave is the leading control on O 2 variability in the tropics, but heave alone cannot fully explain the amplitude of tropical O 2 variability, likely indicating reinforcing changes from the biological O 2 consumption. Midlatitude O 2 variability indeed reflects ocean ventilation downstream of the subduction region where O 2 anomalies are correlated with the depth of winter mixed layer. These mechanisms, synchronized with the PDO, yield a basin‐scale pattern of O 2 variability that are comparable in magnitude to the projected rates of ocean deoxygenation in this century under “unchecked” emission scenario.