Ocean ventilation as a driver of interannual variability in atmospheric potential oxygen

We present observations of interannual variability on 2–5 yr timescales in atmospheric potential oxygen (APO≈O₂+CO₂) from the Scripps Institution of Oceanography global flask sampling network. Interannual variations in the tracer APO are expected to arise from air–sea fluxes alone, because APO is insensitive to exchanges with the terrestrial biosphere. These interannual variations are shown to be regionally coherent and robust to analytical artefacts. We focus on explaining a feature dominant in records from the Northern Hemisphere stations, marked by increasing APO in the late 1990s, followed by an abrupt drawdown in 2000–2001. The timing of the drawdown matches a renewal of deep convection in the North Atlantic, followed the next year by a severe winter in the western North Pacific that may have allowed ventilation of denser isopycnals than usual. We find a weak correlation between changes in the interhemispheric APO difference and El Ni˜no indices, and the observations show no strong features of the 1997–98 El Ni˜no. Comparisons with estimates of variations in ocean productivity and ocean heat content demonstrate that these processes are secondary influences at these timescales. We conclude that the evidence points to variability in ocean ventilation as the main driver of interannual variability in APO.