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Ventilation of the Baltic Sea deep water: A brief review of present knowledge from observations and models

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Author(s): H. E. Markus Meier | Rainer Feistel | Jan Piechura | Lars Arneborg | Hans Burchard | Volker Fiekas | Nikolay Golenko | Natalia Kuzmina | Volker Mohrholz | Christian Nohr | Vadim T. Paka | Adolf Stips | Victor Zhurbas

Journal: Oceanologia
ISSN 0078-3234

Volume: 48;
Issue: S;
Start page: 133;
Date: 2006;
Original page

Keywords: Baltic Sea | Salt water inflows | Deep water | Ventilation | Entrainment | Turbulent mixing

ABSTRACT
The ventilation of the Baltic Sea deep wateris driven by either gale-forced barotropic or baroclinic salt water inflows.During the past two decades, the frequency of large barotropic inflows(mainly in winter) has decreased and the frequency of medium-intensity baroclinic inflows(observed in summer) has increased. As a result of entrainment of ambient oxygen-rich water,summer inflows are also important for the deep water ventilation.Recent process studies of salt water plumes suggest that the entrainmentrates are generally smaller than those predicted by earlier entrainment models.In addition to the entrance area, the Słupsk Sill andthe Słupsk Furrow are important locations for the transformation of water masses. Passing the Słupsk Furrow, both gravity-driven dense bottom flows and sub-surface cyclonic eddies,which are eroded laterally by thermohaline intrusions,ventilate the deep water of the eastern Gotland Basin.A recent study of the energy transfer from barotropic to baroclinicwave motion using a two-dimensional shallow water model suggests thatabout 30% of the energy needed below the halocline for deep water mixingis explained by the breaking of internal waves.In the deep water decade-long stagnation periods with decreasingoxygen and increasing hydrogen sulphide concentrations might be caused by anomalously largefreshwater inflows and anomalously high mean zonal wind speeds. In differentstudies the typical response time scale of average salinity was estimated tobe between approximately 20 and 30 years.The review summarizes recent research resultsand ends with a list of open questions and recommendations.