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

Abstract

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 ((Figure presented.) m2 s−1– (Figure presented.) m2 s−1) can lead to a ∼70% change in annual SO air-sea CO2 fluxes in the first year of perturbations, and around a ∼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.