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 σθ = 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 σθ = 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 σθ = 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 σθ = 26.8 kg m-3 with attenuated amplitudes at latitudes of 40°N-45°N in the OML on σθ = 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.
|Affiliation||Japan Meteorological Agency|
|Are you a SFB 754 / Future Ocean member?||No|