Mechanisms of low-frequency oxygen variability in the North Pacific

This study investigates the mechanisms of interannual and decadal variability of dissolved oxygen (O2) in the North Pacific using historical observations and a hindcast simulation using the Community Earth System Model (CESM). The model captures the observed variability of upper ocean (200 m) O2 where there is relatively high sampling density. The dominant mode of O2 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 identified by which the PDO controls the upper-ocean O2 variability. Vertical movement of isopycnals (“heave”) drives O2 variations in the deep tropics; isopycnal surfaces are depressed in the eastern tropics under the positive (El Niño-like) phase of PDO, leading to O2 increases in the upper water column. Isopycnal heave is the leading control on O2 variability in the tropics, but the heave alone cannot fully explain the amplitude of tropical O2 variability, likely indicating reinforcing changes from the biological O2 consumption. In contrast to the tropics, changes in the subduction are the primary control on extra-tropical O2 variability. These hypotheses are tested by contrasting O2 anomalies with the heave-induced component of variability calculated from potential density anomalies. Mid-latitude O2 variability indeed reflects ocean ventilation downstream of the subduction region where O2 anomalies are correlated with the depth of winter mixed layer. These mechanisms, synchronized with the PDO, yield a basin-scale pattern of O2 variability that are comparable in magnitude to the projected rates of ocean deoxygenation in this century under "unchecked" emission scenario.