| Summary: | The ocean plays a central role in the climate system by absorbing excess anthropogenic heat and carbon dioxide. Moreover, the ocean circulation distributes heat from the tropics towards the poles. Due to the large ocean stratification, vertical exchanges between the ocean interior and the surface are limited. Subduction links the ocean surface and its interior and occurs in winter at mid- or high-latitudes, where the mixed layers (MLs) are deep. In subtropical regions, temperature and salinity decrease below the ML. Temperature has thus a stabilising effect, while salinity has a destabilising effect, a stratification regime called alpha ocean. Opposite, in polar regions, temperature and salinity increase below the ML, and salinity is the stabilising factor, a regime called beta ocean. In between these two regimes lies the polar transition zone (PTZ), where both temperature and salinity are stabilising. Despite the importance of the alpha-beta distinction, the underlying mechanisms controlling these regimes remain unclear. This thesis investigates the factors influencing the upper ocean stratification and the deep MLs adjacent to the PTZs. From observational profiles, we produce novel climatologies of the upper ocean properties. These climatologies confirm that MLs are deep on the poleward flanks of the alpha oceans. Deep MLs are also present in the beta ocean along the coast of Antarctica. In winter, the transition between the different regimes is abrupt. In summer, both temperature and salinity stratify almost the entire ocean. Based on idealised numerical simulations and observations, we find that the buoyancy fluxes largely determine the position of the PTZ. By stabilising the water column poleward of the PTZ, buoyancy fluxes inhibit convection, permitting beta-ocean formation. The exact position of the PTZ and the adjacent deep MLs are determined by the competition between the winter buoyancy loss and the strength of the existing stratification. Importantly, the impact of heat flux on buoyancy is scaled by ...
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