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Abstract

In this study, we examine the factors that influence the stratification of the upper ocean pycnocline (UOP), focusing on the Southern Ocean. The UOP is a critical layer just below the mixed layer whose stratification controls the exchange of properties between the ocean and the atmosphere. We classify the UOP regions according to the relative contributions of temperature and salinity in stabilizing the layer, resulting in three categories: alpha (temperature-stabilized), beta (salinity-stabilized), and transition (temperature- and salinity-stabilized) zones. Our analysis uses observational profiles from the EN4.2 database and calculates annual mean buoyancy fluxes by combining existing heat and freshwater flux products. Ekman transport is taken into account as an additional term in the buoyancy flux. In the Southern Ocean, the deep mixed layers are located on the southernmost flank of the alpha region, with the exception of the southeastern Pacific sector where they are located in the polar transition zone. Regions with negative buoyancy flux exhibit mixed layer deepening along the water path, but deep mixed layers only form when the buoyancy flux is negative along the entire path. Ekman transport contributes to the formation of deeper mixed layers throughout the Southern Ocean by bringing cold water northward. Overall, our results reveal that the boundaries between alpha, transition, and beta regions are generally consistent with more traditional definitions of fronts and provide an overview of upper ocean pycnocline stratification in the Southern Ocean.