Southern Ocean mixed layer depth change in winter influenced by wind variability

The Southern Ocean is characterized by its deep mixed layer that reaches a few hundreds of meters in winter and allows the vertical exchange of properties such as heat and carbon dioxide (CO₂). Wind plays a key role in shaping mixed layer depth (MLD) by providing shear stress at the surface of the ocean. In the last decade, the wind showed the inter-annual to decadal variability and it has been suggested as a trigger for the fluctuation of CO₂ uptake in the Southern Ocean. In this study, we use 5 Argo gridded data products and show that the variability in the wintertime MLD is also significantly linked to the wind changes. Overall these products are in line with the wintertime MLD deepening in the Pacific sector, which can be easily more than 10 m/year in some areas during 2004~2011. However, during 2012~2018 the wintertime MLD shoaled in the Pacific sector in which more than 10% of the grid points show shoaling with the rate of 10 m/year or greater. Even with inevitably large uncertainties in the MLD stemmed from multiple layers of processes in the calculation, these trends are statistically significant. The MLD changes are positively correlated with the wind speed: increasing during 2004~2011 winter but decreasing during 2012~2018 at the same region. Considering that the wintertime MLD controls the CO₂ outgassing, there can be an indirect influence of wind on CO₂ flux through MLD changes.