On the processes controlling the seasonal cycles of the air–sea fluxes of O2 and N2O: A modelling study

Abstract

The seasonal dynamics of the air–sea gas flux of oxygen (O2) are controlled by multiple processes occurring simultaneously. Previous studies showed how to separate the thermal component from the total O2 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 O2 but also the cycles of nitrous oxide (N2O), argon (Ar) and nitrogen (N2). For this study, we apply a technique to distinguish the fluxes of O2 driven separately by thermal forcing, NEZP, and address the role of ocean ventilation by carrying separate O2 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° in each hemisphere. This also implies that neglecting the role of ventilation in the subtropical areas would lead to overestimation of the component of O2 flux due to NEZP by 20–30%. Model results also show that the ventilation components of air–sea O2 and N2O fluxes are strongly anti-correlated in a ratio that reflects the subsurface tracer/tracer relationships (~0.1 mmol N2O/mol O2) as derived from observations. The results support the use of simple scaling relationships linking together the thermally driven fluxes of Ar, N2 and O2. Furthermore, our study also shows that for latitudes polewards of 30° 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 ~2–3 weeks.

Keywords

• air–sea fluxes
• ocean biogeochemical cycles
• ocean modeling
• gas flux theory
• general circulation models