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

• Published in: Tellus B: Chemical and Physical Meteorology
• Main Authors: Hamme, Roberta C., Keeling, Ralph F.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Tellus B: Chemical and Physical Meteorology 2008
• Subjects: Pacific; Scripps; North Atlantic
• Online Access: http://hdl.handle.net/1828/12593; https://doi.org/10.1111/j.1600-0889.2008.00376.x

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. This work was supported by the National Science Foundation (NSF) under ATM-872037, ATM-9309765, ATM-9612518, ATM-0000923, ATM-0330096, ATM-0651834, by the Environmental Protection Agency (EPA) under IAG#DW49935603-01-2, and by the National Oceanic and Atmospheric Administration (NOAA) under NA77RJ0453A and OAR-CPO-2007-2000636. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of NSF, EPA or NOAA. R. Hamme was supported by a Gary Comer Abrupt Climate Change Fellowship. Faculty Reviewed