Variability of Atmospheric CO₂ Over the Arctic Ocean: Insights From the O-Buoy Chemical Observing Network

As the Arctic climate rapidly warms, there is a critical need for understanding variability and change in the Arctic carbon cycle, but sparse spatial coverage of observations has hindered progress. This work analyzes measurements of atmospheric CO₂ in the Arctic from long-term on-ice measurements (the O-Buoy Network), as well as coastal observatories from 2009 to 2016. The on-ice measurements showed smaller seasonal amplitudes than coastal observatories, in contrast to the general observation of poleward increases of seasonal cycle amplitude. Average on-ice measurements were also lower than their coastal counterparts during winter and spring, contradicting the expectation that CO₂ increases poleward in boreal winter. We compared the observations to CO₂ simulated in an updated version of GEOS-Chem 3-D chemical transport model, which includes new tracers of airmass history and CO₂ sources and sinks. The model reproduced the observed features of the seasonal cycle and showed that terrestrial biosphere fluxes and synoptic transport explain most CO₂ variability (both synoptic and interannual) over the Arctic Ocean surface. The polar airmass partially isolates the Arctic Ocean surface air from terrestrial CO₂ exchange, which explains the reduced seasonal cycle amplitude and winter maxima. All Arctic coastal sites had similar CO₂ interannual variability, particularly in summer, which was largely reproduced by the model. The interannual variability observed over sea ice, however, was distinct from the coastal sites and not reproduced by the model. Air-sea CO₂ exchange in and around sea ice, which was once thought to be negligible, may be an important driver of interannual variability over the Arctic Ocean.