Ocean Deoxygenation Conference | Kiel 2018

The western North Pacific is one of the oceans where oxygen (O₂) concentration has been declining significantly over the past several decades. Among the studies in the western North Pacific, we have identified trends of declining O₂ along the meridional sections at 137°E (P09) (Takatani et al., 2012) and at 165°E (P13) (Sasano et al., 2015), respectively, where Japan Meteorological Agency (JMA) has been making high-frequency measurements of physical and biogeochemical Essential Ocean Variables including O₂. In this study, we extended the analyses of long-term O₂ changes in the Oyashio region, i.e., the southwestern region of the western boundary current in the western subarctic, by Ono et al (2001) back to the year 1954 over the past 61 years using quality-controlled data acquired by the JMA.
Concentrations of O₂ in the Oyashio region have been declining significantly and oscillating over bidecadal timescales on the isopycnal layers spanning σ_θ = 26.6-27.5 kg m^-3. The mean rate of the long-term O₂ decrease is the highest (-0.70 ± 0.06 µmol kg^-1 yr^-1) on σ_θ = 26.7 kg m^-3 in the temperature minimum layer. The O₂ decline here is predominantly attributed to the reduction of ventilation in winter due to warming and freshening. At σ_θ = 27.4 kg m^-3 in the Oxygen Minimum Layer (OML) at around the depth of 950 m, O₂ concentration is 43 µmol kg^-1 on average and has been declining at a mean rate of -0.14 ± 0.03 µmol kg-1 yr-1 for the past six decades. The trends in the OML are attributable to a reduction in ventilation in the Sea of Okhotsk associated with a reduction in sea ice formation and the propagation of its impact to the Oyashio through diapycnal mixing adjacent to the Bussol' Strait in the Kuril Islands. These trends of O₂ decline is accompanying bidecadal oscillations and have also been found in the downstream to the east in the 165°E section at latitudes 30°N-42.5°N on σ_θ = 26.8 kg m^-3 with attenuated amplitudes at latitudes of 40°N-45°N in the OML on σ_θ = 27.4 kg m^-3. These results indicate that the signal of secular declines of O₂, together with bidecadal oscillations, is being propagated broadly from the Oyashio source region into the interior of the North Pacific Ocean.