Modelling the Impact of Surfactants on Anthropogenic CO2 Transfer between the Ocean and Atmosphere

During the industrial revolution, anthropogenic carbon dioxide (CO₂) emissions have rapidly increased, raising worries about their impact on global climate change. The oceans are an important sink of anthropogenic CO₂, taking in about 30% of emissions. Despite the important role of surfactants in reducing gas exchanges between the atmosphere and the ocean, their effect on oceanic intake of CO₂ has received limited attention in ocean models. In this exploratory work, we examine the impact of a parameterization of surfactants on CO₂ fluxes between the ocean and the atmosphere using the NEMO (Nucleus for European Modelling of the Ocean) ocean engine in the ORCA2-LIM-PISCES configuration. Understanding and quantifying the potential effect of surfactants on the ocean's absorption of anthropogenic CO₂ is the main goal of our modelling. Using documented atmospheric CO₂ concentrations from 1750 till present, we carried out three simulations of the intake of anthropogenic CO₂ by the ocean, one in which the presence of surfactants was ignored and two in which different formulations of the impact of surfactants on air-sea gas exchange were used. According to the simulations, the impact of the presence of surfactant on net, basin-scale anthropogenic CO₂ fluxes into the ocean is rather small, on the order of just a few percent. However, in regions where, in the model, the intake of anthropogenic CO₂ is largest, such as the Southern Ocean and the Kuroshio and Gulf Stream region, the reduction in these fluxes can attain between 10% and 40%. On seasonal timescales, the global effect of surfactants is to slightly enhance the amplitude of the annual cycle of fluxes by between 10% and 15%. The presence of surfactants leads to a reduction in the total mass of anthropogenic dissolved inorganic carbon (DIC) in the global ocean by about 6% at the end of the integration. Regionally, the concentrations of anthropogenic DIC in the water column (mass of DIC per unit area) are up to 10% lower in the Southern Ocean and in the northern branch of the Kuroshio when surfactants are present.