The Rapid Response of Southern Ocean Biological Productivity to Changes in Background Small Scale Turbulence

Background subsurface vertical mixing rates in the Southern Ocean (SO) are known to vary by an order of magnitude temporally and spatially, due to variability in their generating mechanisms, which include winds and shear instabilities at the surface, and the interaction of tides and lee waves with rough bottom topography. There is great uncertainty in the parameterization of this mixing in coarse resolution Earth System Models (ESM), and in the impact that this has on SO biological productivity on sub decadal timescales. Using a data assimilating biogeochemical ocean model we show that SO phytoplankton productivity is highly sensitive to differences in background diapycnal mixing over short timescales. Changes in the background vertical mixing rates alter key biogeochemical and physical conditions. The greatest changes to the distribution of physical and biogeochemical tracers occur in regions with very strong tracer vertical gradients. A combination of reduced nutrient limitation and reduced light limitation causes a strong increase in SO phytoplankton productivity with higher background mixing. This leads to increased summer carbon export but reduced wintertime export over the mixed layer depth, which could alter the strength of the SO biological carbon pump and atmospheric CO2 ${\text{CO}}_{2}$ concentrations on centennial to millennial timescales. This study demonstrates the importance of accurately representing diapycnal mixing in ESM to predict SO biogeochemical dynamics and their broader climatic implications. The vertical mixing of water beneath the surface of the Southern Ocean varies significantly over time and space due to factors such as winds, tides, and the interaction of waves with the ocean floor's topography. This is hard to represent in ocean climate models, which are essential for understanding the ocean's role in global climate systems. In this work, we investigate how this mixing affects the productivity of phytoplankton, tiny plants crucial to the ocean's ecosystem. We demonstrate that even small changes in background mixing rates can profoundly impact phytoplankton productivity in the Southern Ocean over short time periods. Alterations in mixing rates influence key environmental conditions, particularly nutrient and light availability, which are vital for phytoplankton growth. This heightened productivity enhances the ocean's ability to absorb carbon dioxide from the atmosphere, potentially affecting long-term atmospheric CO2 ${\text{CO}}_{2}$ levels. The rate of background vertical mixing is a key control of Southern Ocean phytoplankton productivity over annual timescales Increased mixing enhances the vertical flux of nutrients to the surface, increasing productivity and altering phytoplankton assemblages Mixing rates alter the physical and biogeochemical environment, reducing the carbon exported to below the winter mixed layer depth