Multidecadal trends of oxygen and their controlling factors in the western North Pacific

The rate of change of dissolved oxygen (O-2) concentrations was analyzed over 1987-2011 for the high-frequency repeat section along 165 degrees E in the western North Pacific. Significant trends toward decreasing O-2 were detected in the northern subtropical to subtropical-subarctic transition zones over a broad range of isopycnal horizons. On 25.3 sigma between 25 degrees N and 30 degrees N in North Pacific Subtropical Mode Water, the rate of O-2 decrease reached -0.45 0.16 mu mol kg(-1) yr(-1). It is largely attributed to a deepening of isopycnal horizons and to a reduction in oxygen solubility associated with ocean warming. In North Pacific Intermediate Water, the rate of O-2 decrease was elevated (-0.44 0.14 mu mol kg(-1) yr(-1) on 26.8 sigma) and was associated with net increases in apparent oxygen utilization in the source waters. On 27.3 sigma in the subtropical Oxygen Minimum Layer (OML) between 32.5 degrees N and 35 degrees N, the rate of O-2 decrease was significant (-0.22 0.05 mu mol kg(-1) yr(-1)). It was likely due to the increases in westward transport of low-oxygen water. These various drivers controlling changes in O-2 along the 165 degrees E section are the same as those acting along 137 degrees E (analyzed previously) and also account for the differences in the rate of O-2 decrease between these sections. Additionally, in the tropical OML near 26.8 sigma between 5 degrees N and 10 degrees N, significant trends toward increasing O-2 were detected in both sections (+0.36 0.04 mu mol kg(-1) yr(-1) in the 165 degrees E section). These results demonstrate that warming and circulation changes are causing multidecadal changes in dissolved O-2 over wide expanses of the western North Pacific.