The Sensitivity of Southern Ocean Air-Sea Carbon Fluxes to Background Turbulent Diapycnal Mixing Variability

The Southern Ocean (SO) connects major ocean basins and hosts large air-sea carbon fluxes due to the resurfacing of deep nutrient and carbon-rich waters. While wind-induced turbulent mixing in the SO mixed layer is significant for air-sea fluxes, the importance of the orders-of-magnitude weaker background mixing below is less well understood. The direct impact of altering background mixing on tracers, as opposed to the response due to a longer-term change in large-scale ocean circulation, is also poorly studied. Topographically induced upward propagating lee waves, wind-induced downward propagating waves generated at the base of the mixed layer, shoaling of southward propagating internal tides, and turbulence under sea ice are among the processes known to induce upper ocean background turbulence but typically are not represented in models. Here, we show that abruptly altering the background mixing in the SO over a range of values typically used in climate models (O10-4 m2 s-1- O10-5 m2 s-1) can lead to a similar to 70% change in annual SO air-sea CO2 fluxes in the first year of perturbations, and around a similar to 40% change in annual SO air-sea CO2 fluxes over the 6-year duration of the experiment, with even greater changes on a seasonal timescale. This is primarily through altering the temperature and the dissolved inorganic carbon and alkalinity distribution in the surface water. Given the high spatiotemporal variability of processes that induce small-scale background mixing, this work demonstrates the importance of their representation in climate models for accurate simulation of global biogeochemical cycles. The Southern Ocean (SO) connects the world's major oceans and plays a crucial role in the exchange of carbon between the atmosphere and the ocean. Vertical mixing in the ocean is responsible for moving both natural dissolved carbon from deeper parts of the ocean to the surface where it can interact with the atmosphere, and anthropogenic carbon from the surface waters into the deep ocean. While we understand the impact of wind-induced mixing in the upper ocean layers on carbon exchange, we know less about the significance of mixing in the ocean interior. By using a model of the SO, we show that adjusting the strength of mixing in the ocean interior causes a significant alteration in the annual exchange of carbon between the ocean and the atmosphere. This study highlights the importance of accurately representing the strength of ocean interior mixing in climate models to improve our understanding of carbon exchange between the atmosphere and the ocean. Total air-sea carbon fluxes in the Southern Ocean are altered by up to 66% annually by background mixing variationsResolving or skillfully parameterizing the small-scale turbulent mixing in the Southern Ocean is essential to model air-sea carbon fluxesStrong vertical gradients in tracer concentrations in the Southern Ocean increase the sensitivity to vertical mixing rates

Automated summary

The Southern Ocean plays a crucial role in the exchange of carbon between the atmosphere and the ocean, and understanding the mixing processes in this region is important for accurate climate modeling. This study shows that altering the background mixing in the Southern Ocean can significantly impact the annual air-sea CO2 fluxes. The representation of small-scale turbulent mixing in climate models is essential for simulating global biogeochemical cycles accurately.