Fingerprints of climate change in North Pacific oxygen

Oxygen gas dissolved in seawater has been studied as a tracer of physical and biological processes in the ocean for nearly half a century. Analysis of historical oxygen data has revealed widespread changes in subsurface oxygen concentrations over the past few decades, providing important constraints on the impact of late 20th century climate change on the circulation and biological productivity of the ocean. We report results from a hind cast ocean circulation/biogeochemical model that reproduces the spatial patterns of observed subsurface O-2 variability in the North Pacific, where inferred O-2 trends are strongest. We find that decadal North Pacific O-2 variations in the lower ventilated thermocline primarily reflect changes in the basin's large-scale circulation. A southward expansion of the model subtropical gyre explains the observed subtropical O-2 increase from the 1980s to the 1990s, while the simultaneous O-2 decreases seen throughout the midlatitude Pacific are driven largely by reduced communication between the atmosphere and the ocean interior. Similar O-2 decreases are pervasive among mid- and high-latitude water masses, but further research is needed to determine whether these changes reflect a global response to 20th century climate change.