Transformation of eastward spreading saline water at the Słupk Sill of the Baltic Sea: an estimate based on microstructure measurements

Measurements performed by a loosely tethered free-falling microstructure profiler in the vicinity of the Słupsk Sill revealed a high turbulence dissipation spot immediately beyond the sill to the east in the near-bottom layer filled with eastward spreading saltwater. An approach is developed to quantitatively estimate the role of a topographic obstacle like the Słupsk Sill in mixing/transformation of inflow waters using data of microstructure measurements. To do this, first, based on vertical profiles of specific dissipation rate of kinetic energy of turbulence and potential density, the rate of entrainment of low salinity water from the overlying layer to the near-bottom turbulent saltwater layer is calculated. Then, assuming that in the near-bottom saltwater flow, the critical value of the Froude number is reached directly at the Sill, the flow volume rate is estimated. Finally, from the balance between advection and turbulent entrainment, the change in salinity of the eastward spreading saltwater due to intensification of mixing at the Sill can be evaluated. Using data of microstructure measurements available, the mixing hot spot at the Słupsk Sill was found to be responsible for approximately 5 percent of the inflow water salinity decrease en route from the Arkona Basin to the Gotland Deep.

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