Southern Ocean ventilation inferred from seasonal cycles of atmospheric N₂O and O₂/N₂ at Cape Grim, Tasmania

Abstract

The seasonal cycle of atmospheric N₂O is derived from a 10-yr observational record at Cape Grim, Tasmania (41°S, 145°E). After correcting for thermal and stratospheric influences, the observed atmospheric seasonal cycle is consistent with the seasonal outgassing of microbially produced N₂O from the Southern Ocean, as predicted by an ocean biogeochemistry model coupled to an atmospheric transport model (ATM). The model—observation comparison suggests a Southern Ocean N₂O source of ~0.9 Tg N yr−1 and is the first study to reproduce observed atmospheric seasonal cycles in N₂O using specified surface sources in forward ATM runs. However, these results are sensitive to the thermal and stratospheric corrections applied to the atmospheric N₂O data. The correlation in subsurface waters between apparent oxygen utilization (AOU) and N₂O production (approximated as the concentration in excess of atmospheric equilibrium ΔN₂O) is exploited to infer the atmospheric seasonal cycle in O₂/N₂ due to ventilation of O₂-depleted subsurface waters. Subtracting this cycle from the observed, thermally corrected seasonal cycle in atmospheric O₂/N₂ allows the residual O₂/N₂ signal from surface net community production to be inferred. Because N₂O is only produced in subsurface ocean waters, where it is correlated to O₂ consumption, atmospheric N₂O observations provide a methodology for distinguishing the surface production and subsurface ventilation signals in atmospheric O₂/N₂, which have previously been inseparable.