Mechanisms of Low‐Frequency Oxygen Variability in the North Pacific

Abstract

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. The simulated variability of upper ocean (200 m) O2 is moderately correlated with observations where sampling density is relatively high. 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 hypothesized by which the PDO controls 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. In contrast to the tropics, changes in subduction are the primary control on extratropical O2 variability. These hypotheses are tested by contrasting O2 anomalies with the heave‐induced component of variability calculated from potential density anomalies. Isopycnal heave is the leading control on O2 variability in the tropics, but heave alone cannot fully explain the amplitude of tropical O2 variability, likely indicating reinforcing changes from the biological O2 consumption. Midlatitude 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.

Biogeochemical variability in the California Current System

“…The high interannual to decadal O 2 variability observed in the central North Pacific, which reaches its maximum on the isopycnal 26.5, is not included in the climatological boundary conditions, and thus likely accounts for the model bias toward low O 2 variance in the north of the domain. (Ito et al, 2019). Similar correlations are observed in ROMS and the World Ocean Database (Fig.…”

Responses of Horizontally Expanding Oceanic Oxygen Minimum Zones to Climate Change Based on Observations

“…DO min and OMZs changes in response to climate change and involves multiple mechanisms (Deutsch et al., 2011). We recognized that many factors will promote the growth of OMZ areas, including wind, climate modes, and productivity (Ito et al., 2019; Leung et al., 2019), but we focused here on the responses of OMZ areas to increasing sea temperature. In addition to sea temperature, the mechanisms including physical, chemical, and biological processes, responsible for changes in regional OMZ areas require further exploration.…”

Marine Ecosystem Variations Over the North Pacific and Their Linkage to Large-Scale Climate Variability and Change

“…The strong oxygen decrease over the NP may be due to reduced ventilation caused by weaker cooling in the Okhotsk Sea (Nakanowatari et al, 2007). In addition, vertical displacements of isopycnal surfaces and advection on those surfaces play important roles in varying dissolved oxygen concentration in different regions of the NP (Pozo Buil and Di Lorenzo, 2017;Ito et al, 2019). Further studies are necessary to understand the mechanisms of deoxygenation over the NP basin and its influences on marine ecosystems.…”