Reinterpreting the Deoxygenation Trend in the Subarctic NE Pacific with BGC-Argo Data

A long-term deoxygenation trend—a decrease in the mean dissolved oxygen saturation—has been reported in the subarctic NE Pacific from time series data at Ocean Station Papa (50° N, 145° W). The greatest declines (on the order of -10% over 60+ years) have occurred around the 26.5 potential density surface (mean depth = 140 m). However, the deoxygenation trend analysis may be skewed by natural variability in the region. Earlier in the 60+ year time series, measurements occurred year-round. However, since mid-1981, hydrographic measurements have been concentrated in the months of February, June, and August. With the month of June exhibiting the greatest variability in O₂ saturation of all months at Ocean Station Papa this has potentially serious implications for the deoxygenation trend estimates. The interquartile range—describing the spread of values between the 25^th and 75^th percentile of observations—of O₂ saturation for June spans >10%. To better understand the impacts of natural variability on the estimated deoxygenation trend, oxygen measurements from Biogeochemical-Argo floats equipped with optode sensors are synthesized with historical shipboard measurements to increase the temporal resolution of oxygen data coverage in more recent years. An obstacle to this goal is the appreciable (and still not fully characterized) drift that occurs in the optode measurements. We test and develop calibration methods to post-correct for the drift and suggest a workflow for optode calibration. Our primary goal is to determine local and non-local drivers of low oxygen concentrations in these waters and to quantify the potential impacts of the natural variability on the estimated deoxygenation trend.