Improving Representation of the Nitrous Oxide Cycle in Ocean Biogeochemical Models

The processes governing the marine nitrous oxide cycle, oceanic distribution, and flux to the atmosphere display distinct heterogeneity. The primary pathway for N₂O production in the oxygenated open ocean is believed to be nitrification during the oxidation of ammonium to nitrate. However, mechanisms of marine N₂O production and consumption display significant sensitivity to local oxygen concentration. Oxygen minimum zones such as the Arabian Sea and Eastern Equatorial Pacific are characterized by large gradients in sub-surface N₂O, and high rates of N₂O turnover that significantly exceed those observed in the open ocean. A range of processes is believed to govern N₂O formation in these regions, including enhanced nitrification, and a coupling of nitrification and denitrification pathways. N₂O is also depleted via denitrification in anoxic zones. This spatial heterogeneity presents challenges to the development of effective model parameterizations for ocean N₂O; i.e., parameterizations that also display reliable predictive capability under conditions of changing ocean circulation, productivity, and oxygen distribution. In this analysis we use the ocean biogeochemistry model NEMO-PlankTOM to evaluate a range of recent empirical parameterizations for marine N₂O formation. We contrast these parameterizations with a recently developed process-based model of oceanic N₂O. Simulations are evaluated using a global database of oceanic N₂O measurements. Evaluation metrics include surface concentrations, depth profiles, and regional averages. We also discuss the challenges of developing a successful representation of the marine N₂O cycle, given specific limitations of the present generation of global ocean biogeochemistry models.