Temporal changes in dissolved oxygen of the intermediate water in the subarctic North Pacific

Using an approximately 50‐year data set, the changes in dissolved oxygen (DO) on isopycnals in the intermediate layer of the subarctic North Pacific were analyzed. The temporal variations in DO on a decadal scale in the western subarctic Pacific display a negative correlation with those in the eastern subarctic Pacific. From 1950 to 2000 there is an average increase in the Apparent Oxygen Utilization (AOU) with a rate of 0.3 ± 0.2 (95% confidence interval) μmol kg−1 y−1 in the Western Subarctic Gyre (WSG) and Alaska Gyre (AG). The increase in AOU coincides with an increased temperature in the intermediate layer‐ (0.31 ± 0.28°C century−1 in the WSG and 0.54 ± 0.32°C century−1 in the AG) and decrease in surface‐water salinity in the Bering Sea (−0.32 ± 0.22 century−1). It is hypothesized that the changes are correlated with the North Pacific Index (NPI), which fosters meridional transport of salt to the Bering Sea when it is high. The gradual decrease in NPI thus has caused a freshening and a subsequent decrease in the ventilation resulting in an AOU increase in the intermediate waters of the subarctic North Pacific.