From alpha to beta ocean: Exploring the role of surface buoyancy fluxes and seawater thermal expansion in setting the upper ocean stratification

Abstract

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 the thermal expansion coefficient (TEC). The TEC is a strong function of the temperature, a property unique to water. This diminishes the buoyancy fluxes over cold waters. We find that the local value of the TEC in the subpolar region is of paramount importance in controlling the winter buoyancy loss and stratification, and thus the position of the PTZ. A larger TEC value would cause the alpha ocean to extend poleward, inhibiting beta-ocean formation. Considering the importance of the beta ocean in sea-ice formation, the Earth’s climate is influenced by the TEC values, which are directly linked to the ocean surface temperature. In summary, this thesis enlightens the central role of the TEC in modulating buoyancy fluxes and thereby controlling the alpha-beta ocean distinction.