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Abstract:
In energetic, tidally-forced, shelf seas there is a delicate balance between the formation of stratification by solar insolation and the mixing produced by the different turbulence sources. The accurate modelling of the evolution of stratification by these processes is important for the ecosystem functioning, and in particular the formation of the subsurface chlorophyll maximum (SCM). We argue first, that the formation of the SCM is fuelled with the nutrients from deeper waters transported by the turbulence locally generated within the strongly stratified base of the pycnocline. Second, that this turbulence production can arise entirely from tidally-driven currents in the bottom boundary layer and does not require wind forced near-inertial motions. Turbulent mixing is produced through the interaction between a highly sheared pycnocline base that is close to marginal stability and large-scale eddies that are formed within in the bottom boundary layer. These eddies are the size of the bottom boundary layer and periodically supply enough shear to trigger instability, thus providing the mechanism for mixing nutrients into the SCM in this way. These insights are provided by both observations using autonomous ocean gliders equipped with turbulence microstructure sensors, as well as highly-resolved large eddy simulations. The specific conditions examined are typical for extensive regions of the inner North Sea, with data presented from the German Bight, as well as other shallow shelf seas that have strong tidal currents.
