Submesoscale Processes and Mixing in a Semi-Enclosed Basin: The Case of the Baltic Sea

Submesoscale processes have drawn the attention mainly due to their ability to extract energy from the large mesoscale flow and transfer it down-scale, thus contributing to the cascade of energy, but also, due to their important role on mixing and vertical transport of momentum and tracers. This research focuses on the quantification of mixing, caused by submesoscale fronts and filaments, and on the different types of instabilities that occur through loss of geostophic balance. A realistic high-resolution numerical simulation, has been applied for the Baltic Sea, based on the General Estuarine and Transport Model. The Baltic Sea, which is used here as a natural laboratory, constitutes an ideal place for studying those processes since it is a non-tidal, brackish system with intense horizontal gradients in the surface. High-resolution shear microstructure transects measured in the Eastern Gotland Basin captured a variety of fronts and filaments, that were used to confirm the numerical simulations. The model has also been validated against satellite data, along with long-term mooring and research vessel data. The results show that during the winter period a strong and persistent lateral thermohaline gradient is found in the basin created by a combination of upwelling favorable winds, at the western part and a warm coastal current in the East. This frontal structure favors the formation of cold elongated filaments. The filaments are characterized by O(1) Rossby and Richardson dynamics, indicating the prevalence of a submesoscale regime, which is accompanied by narrow regions of cyclonic vorticity, surface convergence, and strong downwelling. Localized areas where symmetric instability might occur have been identified. Although the submesoscale features cover part of the domain, the results show that they have a substantial contribution to the upper ocean average mixing, suggesting that submesoscales can be considered as another important mechanism for winter time mixing.