Factors governing the oceanic nitrous oxide distribution: Simulations with an ocean general circulation model

A global model of the oceanic nitrous oxide distribution is developed to evaluate current understanding of the processes governing nitrous oxide formation and distribution in the open ocean. N₂O is treated as a nonconserved tracer in a global ocean general circulation model subject to biological sources in the oceanic interior and gas exchange at the ocean surface. A simple scalar parameterization linking N₂O production to oxygen consumption (and based on observed correlations between excess N₂O and apparent oxygen utilization) is successful in reproducing the large-scale features of the observed distribution, namely, high surface super-saturations in regions of upwelling and biological productivity, and values close to equilibrium in the oligotrophic subtropical gyres. The majority of the oceanic N₂O source is produced in the upper water column (over 75% above 600 m) and effluxes directly to the atmosphere in the latitude band of formation. The observed structure at depth is not as well reproduced by this model, which displays excessive N₂O production in the deep ocean. An alternative source parameterization, which accounts for processes which result in a depth variation in the relationship between N₂O production and oxygen consumption, yields an improved representation of the deep distribution. The surface distribution and sea-air flux are, however, determined primarily by the upper ocean source and, therefore, are relatively insensitive to changes in the nature of deep oceanic N₂O production.