Natural variability and anthropogenic trends in oceanic oxygen in a coupled carbon cycle–climate model ensemble

Internal and externally forced variability in oceanic oxygen (O 2 ) are investigated on different spatiotemporal scales using a six‐member ensemble from the National Center for Atmospheric Research CSM1.4‐carbon coupled climate model. The oceanic O 2 inventory is projected to decrease significantly in global warming simulations of the 20th and 21st centuries. The anthropogenically forced O 2 decrease is partly compensated by volcanic eruptions, which cause considerable interannual to decadal variability. Volcanic perturbations in oceanic oxygen concentrations gradually penetrate the ocean's top 500 m and persist for several years. While well identified on global scales, the detection and attribution of local O 2 changes to volcanic forcing is difficult because of unforced variability. Internal climate modes can substantially contribute to surface and subsurface O 2 variability. Variability in the North Atlantic and North Pacific are associated with changes in the North Atlantic Oscillation and Pacific Decadal Oscillation indexes. Simulated decadal variability compares well with observed O 2 changes in the North Atlantic, suggesting that the model captures key mechanisms of late 20th century O 2 variability, but the model appears to underestimate variability in the North Pacific. Our results suggest that large interannual to decadal variations and limited data availability make the detection of human‐induced O 2 changes currently challenging.