Simulated impact of double‐diffusive mixing on physical and biogeochemical upper ocean properties

A global ocean circulation model coupled with a simple marine ecosystem model including the biogeochemical cycles and air–sea fluxes of oxygen and carbon dioxide is used to investigate the impact of double‐diffusive mixing on upper ocean physical and biogeochemical properties. By comparing results for two different parameterizations of double‐diffusive mixing, we also examine the sensitivity of our estimates to the particular representation of this process in general circulation models. Differences between the two parameterizations considered turned out to be much smaller than the difference with respect to a model run without double‐diffusive mixing. For both parameterizations, the impact on upper ocean temperatures and salinities is relatively small (±1°C, ±0.25 psu regionally and 0.04°C, 0.01 psu as global rms difference over the top 50 m) and changes in surface heat flux amount to 0.05 W m −2 globally. However primary production and export production in the oligotrophic subtropics are found to increase by up to 80% and 120%, respectively, when double diffusion is switched on in the model. Double‐diffusive nutrient supply generates an additional oceanic carbon uptake of about 0.4 g C m −2 year −1 , amounting to 0.14 Gt C year −1 globally.