Salinity inversions in the thermocline due to wind-induced differential advection

Observations from the global ocean have long confirmed the ubiquity of thermohaline inversions in the upper ocean, often accompanied by a clear signal in biogeochemical properties. Their emergence has been linked to different processes such as double diffusion, mesoscale stirring, frontal subduction, and the recently discussed submesoscale features. This study uses the central Baltic Sea as a natural laboratory to explore the formation of salinity inversions in the thermocline region during summer. We use realistic high-resolution simulations complemented by field observations to identify the dominant generation mechanism and potential hotspots of their emergence. We propose that the strongly stratified thermocline can host distinct salinity minima during summer conditions resulting primarily from the interaction between lateral surface salinity gradients and wind-induced differential advection. Since this is a generic mechanism, such salinity inversions can likely constitute a typical feature of the upper ocean in regions with distinct thermoclines and shallow mixed layers.