Decadal Trends in Oxygen Concentration in Subsurface Waters of the Northeast Pacific Ocean

Previous studies have shown decreasing oxygen concentration (O-2) in subsurface waters of the continental slope from California to Canada since about 1980. With longer time series we show that from southern California to northern Canada increasing O-2 preceded these decreases from 1950 to about 1980. Because there has been no clear trend since 1950, we cannot yet conclude that anthropogenic climate change is the cause of these decreasing trends after 1980. These findings are based mainly on O-2 on the 26.7 potential density (sigma(theta)) surface in the region north of 30 degrees N and east of 170 degrees W, covering both the continental margin and deep-sea regions. On the continental slope, O-2 increased at most locations by 10 to 20 mu mol kg(-1) to about 1980, followed by declines of similar magnitude in recent years. Changes in O-2 were associated with changes in temperature of the opposite sign south of 37 degrees N, but correlation of temperature and O-2 is irregular in more northerly locations. At all locations, temperature-related solubility change was a minor cause of these O-2 trends. In deep-sea waters, O-2 decreased with time with a more rapid decrease from about 1995 to about 2003. At Ocean Station P (OSP; 50 degrees N, 145 degrees W), which has the longest uninterrupted record of observations, significant linear trends of -0.4 to -0.5 mu mol kg(-1) y(-1) were found on the 26.5, 26.7, and 26.9 sigma(theta) surfaces. In addition, a significant sinusoidal oscillation of period 18.61 years and amplitude of 18 mu mol kg(-1) was found on the 26.9 sigma(theta) surface at OSP and a station 400 km to the east, which fits reasonably well with the lunar nodal cycle. The phase of this oscillation was identical at both locations. Clear evidence of similar variability did not emerge at other open-ocean locations or along the continental slope.