Water Modification in the Åland Sea and Impacts on Exchange Flow between Central and Northern Baltic Sea

• We present observational oceanographic data together with openly accessible high-resolution bathymetry from a prototypical sill and an adjacent deep channel in the Southern Quark located in the Åland Sea, connecting the Northern Baltic Proper with the Bothnian Sea. Our data were acquired during two one-week cruises in February-March 2019 and 2020 and include high resolution broadband acoustic observations of turbulent mixing, in situ microstructure profiler measurements and current velocities from Acoustic Doppler Current Profilers. A temporally reversing non-tidal stratified flow is observed over the steep bathymetric sill, creating a dynamic and extremely energetic environment. Saltier, warmer, and less oxygenated deep water south of the sill is blocked, the flow is at times hydraulically controlled with hydraulic jumps occurring on both sides of the sill depending on the flow direction, and sub-mesoscale processes in the surface layer leading to high spatial variability at small scales. Mixing and vertical salt flux rates are increased by 3-4 orders of magnitude in the entire water column in the vicinity of the sill compared to reference stations not directly influenced by the overflow. We suggest that underlying mechanisms causing the highly increased mixing across the halocline are a combination of shear and topographic lee waves breaking at the halocline interface. We discuss the resulting deep-water and surface-water modification in the Southern Quark, which in turn impacts exchange processes between the Bothnian Sea and the Northern Baltic Proper. The here observed mixing is hypothesized to be important for the development of oxygen and nutrient conditions in the Bothnian Sea.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Oceanografi, hydrologi och vattenresurser (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Oceanography, Hydrology and Water Resources (hsv//eng)

Nyckelord

Baltic Sea

Turbulent diapycnal mixing

deep water ventilation

circulation in the Baltic Sea

Northern Baltic Sea

Oceanography

oceanografi

Publikations- och innehållstyp

vet (ämneskategori)

ovr (ämneskategori)