Southern Ocean ventilation inferred from seasonal cycles of atmospheric N 2 O and O 2 /N 2 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-deple...