The Influence of Surface Buoyancy Flux and Ekman transport on Upper Ocean Pycnocline Stratification in the Southern Ocean.

In this study, we examine the factors that influence upper ocean pycnocline (UOP) stratification in the Southern Ocean. The UOP is a layer located just below the mixed layer and its stratification controls the rates of exchange of heat, carbon, and nutrients between the ocean interior and the atmosphere. We classify regions of the UOP based on the relative roles of temperature and salinity in stabilizing the layer, resulting in alpha (temperature-stabilized), beta (salinity-stabilized), or transition (temperature and salinity-stabilized) zones. Our analysis uses observation profiles from the EN4.2 database and calculates annual mean buoyancy fluxes by combining existing heat and freshwater flux products and accounting for the effect of Ekman transport. Our results show that the polar transition zone has a complex structure, with interlocking beta pools and local intrusions into alpha zones. Deep mixed layers are found in the southernmost flank of the alpha region, with the exception of the southeast Pacific sector where they are located in the polar transition zone. Regions of negative buoyancy flux show mixed layer deepening along the water path, but deep mixed layers only form when the buoyancy flux is negative throughout the path. Ekman transport contributes also to the formation of deeper mixed layers throughout the Southern Ocean by bringing cold water northward. Overall, our findings reveal that boundaries between alpha, transition and beta regions are generally consistent with more traditional frontal definitions and provide a comprehensive view of upper ocean pycnocline stratification in the Southern Ocean.