Impact of Near-Inertial Waves on Vertical Mixing and Air-Sea CO₂ Fluxes in the Southern Ocean

We report the significant impact of near-inertial waves (NIWs) on vertical mixing and air-sea carbon dioxide (CO₂) fluxes in the Southern Ocean using a biogeochemical model coupled to an eddy-rich ocean circulation model. The effects of high-frequency processes are quantified by comparing the fully coupled solution (ONLINE) to two offline simulations based on 5-day-averaged output of the ONLINE simulation: one that uses vertical mixing archived from the ONLINE model (CTRL) and another in which vertical mixing is recomputed from the 5-day average hydrodynamic fields (5dAVG). In this latter simulation, processes with temporal variabilities of a few days including NIWs are excluded in the biogeochemical simulation. Suppression of these processes reduces vertical shear and vertical mixing in the upper ocean, leading to decreased supply of carbon-rich water from below, less CO₂ outgassing in austral winter, and more uptake in summer. The net change amounts up to one third of the seasonal variability in Southern Ocean CO₂ flux. Our results clearly demonstrate the importance of resolving high-frequency processes such as NIWs to better estimate the carbon cycle in numerical model simulations.