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  • 1
    Publication Date: 2024-01-30
    Description: 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.
    Description: Plain Language Summary: The upper ocean is characterized by a well‐mixed surface layer, below which temperature decreases rapidly with depth, forming the so‐called thermocline region. A corresponding salinity increase with depth is typically anticipated for stable density stratification to occur. Temperature and salinity inversions can, however, emerge in the upper ocean. Such thermohaline inversions have been observed in different regions of the world's oceans, and various mechanisms have been proposed to explain their generation. Here, the central basin of the Baltic Sea is used as a natural laboratory to explore the formation of distinct salinity minima in the thermocline region during summer conditions. Using high‐resolution numerical simulations and measurements from a field campaign, we show that inversions are abundant and can emerge throughout the entire basin. They increase with increasing wind speeds and concentrate mainly in regions with strong lateral salinity differences. We propose that thermocline salinity minima can occur during summer when the wind transports saltier water over less saline surface waters. This is a generic mechanism that can therefore be responsible for the formation of the salinity inversions observed worldwide in areas with distinct thermoclines and shallow mixed layers.
    Description: Key Points: Observations collected in the central Baltic Sea during summer indicate patches of distinct salinity minima in the thermocline region. Realistic high‐resolution simulations are used to explore the origin of the salinity minima and to identify the hotspots of their genesis. Lateral surface salinity gradients interacting with wind‐induced differential advection are shown to generate most of the inversions.
    Description: German Research Foundation
    Description: http://doi.io-warnemuende.de/10.12754/data-2022-0001
    Keywords: ddc:551.46 ; salinity inversions ; thermohaline intrusions ; subduction ; submesoscales ; differential advection ; Baltic Sea
    Language: English
    Type: doc-type:article
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  • 2
    Publication Date: 2021-07-04
    Description: Narrow baroclinic fronts are observed in the surface mixed layer (SML) of the Baltic Sea following an autumn storm. The fronts are subjected to hydrodynamic instabilities that lead to submesoscale and turbulent motions while restratifying the SML. We describe observations from an ocean glider that combines currents, stratification, and turbulence microstructure in a high horizontal resolution (150–300 m) to analyze such fronts. The observations show that SML turbulence is strongly modulated by frontal activity, acting as both source and sink for turbulent kinetic energy. In particular, a direct route to turbulent dissipation within the front is linked to shear instability caused by elevated nongeostrophic shear. The turbulent dissipation of frontal kinetic energy is large enough that it could be a significant influence in the evolution of the front and demonstrates that small‐scale turbulence can act as a significant sink of submesoscale kinetic energy.
    Description: Key Points: An autonomous ocean glider observed turbulence, currents, and stratification in surface mixed layer submesoscale fronts following a storm. Submesoscale fronts provide both a damping and generation of surface mixed layer turbulence. Shear instability within the front could represent a significant energy transfer in frontal evolution.
    Description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659
    Description: Helmholtz Association http://dx.doi.org/10.13039/501100001656
    Keywords: 551 ; ocean turbulence ; submesoscales ; physical oceanography ; ocean mixing
    Type: article
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