On the processes controlling the seasonal cycles of the air-sea fluxes of O-2 and N2O: A modelling study

The seasonal dynamics of the air-sea gas flux of oxygen (O-2) are controlled by multiple processes occurring simultaneously. Previous studies showed how to separate the thermal component from the total O-2 flux to quantify the residual oxygen flux due to biological processes. However, this biological signal includes the effect of both net euphotic zone production (NEZP) and subsurface water ventilation. To help understand and separate these two components, we use a large-scale ocean general circulation model (OGCM), globally configured, and coupled to a biogeochemical model. The combined model implements not only the oceanic cycle of O-2 but also the cycles of nitrous oxide (N2O), argon (Ar) and nitrogen (N-2). For this study, we apply a technique to distinguish the fluxes of O-2 driven separately by thermal forcing, NEZP, and address the role of ocean ventilation by carrying separate O-2 components in the model driven by solubility, NEZP and ventilation. Model results show that the ventilation component can be neglected in summer compared to the production and thermal components polewards but not equatorward of 30 degrees in each hemisphere. This also implies that neglecting the role of ventilation in the subtropical areas would lead to overestimation of the component of O-2 flux due to NEZP by 20-30%. Model results also show that the ventilation components of air-sea O-2 and N2O fluxes are strongly anti-correlated in a ratio that reflects the subsurface tracer/tracer relationships (similar to 0.1 mmol N2O/mol O-2) as derived from observations. The results support the use of simple scaling relationships linking together the thermally driven fluxes of Ar, N-2 and O-2. Furthermore, our study also shows that for latitudes polewards of 30 degrees of both hemispheres, the Garcia and Keeling (2001) climatology, when compared to our model results, has a phasing error with the fluxes being too early by similar to 2-3 weeks.