Ocean Deoxygenation Conference | Kiel 2018

This study investigates the mechanisms of interannual and decadal variability of dissolved oxygen (O$_2$) in the North Pacific using historical bottle O$_2$ data and a physical-biogeochemical hindcast simulation. An ocean-ice configuration of the Community Earth System Model (CESM) is used to for the hindcast. The simulated variability of upper ocean (200m) O$_2$ is broadly consistent with observations in the western and eastern Pacific where sampling density is relatively higher. The dominant mode of O$_2$ variability in this depth range explains 24.8% of the variance and is significantly correlated with the Pacific Decadal Oscillation (PDO) index (${r=0.68}$). Two major mechanisms are proposed as null hypotheses by which the PDO controls O$_2$ variability. Vertical movement of isopycnals (``heave'') may drive O$_2$ variability in deep tropics. Isopycnal surfaces are depressed in the eastern tropics under the positive (El Nino-like) phase of PDO, leading to O$_2$ increases in the upper water column. In contrast to the tropics, changes in subduction associated with the PDO are the primary control on extra-tropical O$_2$ variability. These hypotheses are tested by contrasting the anomalies of O$_2$ and heave-induced O$_2$ where the latter is calculated from potential density anomalies. At 200m depth, isopycnal heave is the leading control on O$_2$ variability except for the central subtropics, downstream of the subduction region. Further examination of the amplitude of O$_2$ anomealies reveals that the null hypothesis cannot fully explain the tropical O$_2$ variability, likely indicating the reinforcing changes in the biological O$_2$ consumption. These mechanisms, synchronized with the PDO, develops a basin-scale pattern of O$_2$ variability that are comparable in magnitude to the projected rates of ocean deoxygenation in this century.