Decline and Bidecadal Oscillations of Dissolved Oxygen in the Oyashio Region and Their Propagation to the Western North Pacific

Abstract

During the period 1954–2014, concentrations of dissolved oxygen (O2) in the Oyashio region of the western subarctic North Pacific has been oscillating over bidecadal timescales and significantly decreasing over the isopycnal layer spanning σθ = 26.6–27.5 kg/m3. The cycles of oscillation (16.4–19.6 years) are almost consistent and synchronized within 1 year over this density layer and are probably controlled by the nodal tidal cycle of 18.6 years and/or atmospheric forcing. The mean rate of the long‐term decrease is the highest (−0.70 ± 0.06 μmol · kg−1 · year−1) in the temperature minimum layer. The O2 decline there is predominantly attributed to a reduction of ventilation in winter due to warming and freshening. On the other hand, the O2 decline in deeper layers down to the oxygen minimum layer is attributable to a reduction in ventilation in the Sea of Okhotsk associated with a reduction in sea ice formation and propagation of its impact through diapycnal mixing adjacent to the Bussol' Strait. These trends of bidecadal oscillations and decline of O2 were also found in the downstream to the east in the 165°E section at latitudes 30°N–42.5°N on σθ = 26.8 kg/m3 and with attenuated amplitudes at latitudes of 40°N–45°N in the oxygen minimum layer on σθ = 27.4 kg/m3. 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 Pacific Ocean.

The influence of decadal oscillations on the oxygen and nutrient trends in the Pacific Ocean

“…CC BY 4.0 License. σ θ =26.7 kg m -3 the oxygen decline is attributed to the reduction of Dense Shelf Water formation in the Sea of Okhotsk associated with the reduction of sea ice production and freshening (Sasano et al, 2018).…”

“…1) seems to play a role in the oxygen trend as long-term trends indicate an oxygen decrease throughout the cold and warm PDO phases in all but one area. Note that while the most likely basin-wide drivers of oxygen and nutrient variability were investigated here, other contributors might exist depending on the region, as shown in detail for the Oyashio region by Sasano et al (2018).…”

“…Since the overturning in the North Pacific originates from the Okhotsk Sea through dense shelf water the observed freshening to depth of ~500 m during the past four decades could possibly weaken the shallow overturning of the North Pacific (Ohshima et al, 2014). Thus interior ocean circulation is likely slowing because less dense water is forming in the Sea of Okhotsk (Sasano et al, 2015(Sasano et al, , 2018 and hence decreasing the supply of oxygen.…”

“…The oxygen and nitrate of the Oyashio region show significant correlations with the PDO (Table 2) with the PDO lagging by 4 to 5 years, indicating a delay between the surface signal and the changes in the subsurface layer. In a previous study where oxygen concentrations were investigated from 1954 to 2014, a decrease of oxygen was attributed to a reduction of ventilation in winter due to warming and freshening and reduction of dense water formation in the Sea of Okhotsk (Sasano et al, 2018).…”

“…The Oyashio region data are from hydrodata, and updated data collections used in Whitney et al (2013) and Sasano et al (2018). The Aloha station data (at 22°45'N, 158°W were downloaded from http://hahana.soest.hawaii.edu/hot/hot-dogs/bextraction.html status 15 January 2019, time period Table 1.…”