Barotropic inflow

The first deep basin downstream of the entrance sills is the Bornholm Basin (Fig. 10.1). This basin has a maximum depth of more than 90 m and is separated from the next downstream basin by the Slupsk Sill (sill depth 60 m). The buffering properties of the Bornholm Basin play an essential part for the effectiveness of MBls in other central basins. The thermohaline conditions in the Bornholm Basin are also considerably important for the evolution of stagnation in the central Baltic deepwater. In general, there is a frequent inflow of lower amounts of highly saline water that penetrates across the sills into the ArkonaBasin during each baroclinic or weak barotropic inflow event. This water is trapped into the... 

While barotropic inflows can be detected rather rigorously by parameters such as wind speed, sea level, and surface salinity, baroclinic ones are more concealed events. So far, the best indicators seem to be certain features appearing in the time series from the Darss SiU mast ... 

The Bornholm Basin deepwater, which is frequently influenced by each baroclinic or weak barotropic inflow event (cf. Section 10.2), is renewed annually to a certain extent. Consequently, anoxic conditions cannot be observed as often and persistent as in the subsequent basins. The phosphate and nitrate concentrations mirror these frequent changes between oxic and anoxic conditions. The relatively low phosphate values in the 1960s suggest that oxic conditions prevailed during this period. 

FIGURE 20.11 Surface salinity at Gedser Rev, low-pass filtered with an exponential response function with a memory of 10 days. The line at 17 psu indicates a threshold for significant barotropic inflow situations. 

Lamb, H., 1932. Hydrodynamics, 6th edn. Cambridge University Press, New York, Dover. 738 pp. Lass, H. U., 1988. A theoretical study of the barotropic water exchange between the North Sea and the Baltic and the sea level variations of the Baltic. Beitrage zur Meereskunde, Berlin, 58, 19-33. Lass, H. U., Matthaus, W., 1996. On temporal wind variations forcing salt water inflows into the Baltic... 

Fehmarnbelt are blocked by the Darss Sill and recirculated into the Mecklenburg Bight that is, they do not generally contribute to the inflow into the Arkona Sea. Only when high water levels in the Kattegat (cf. Chapters 2 and 3) strengthen the barotropic flow through the narrow straits, the Fehmarnbelt s near-bottom water can spill over the Darss SiU and spread further to the east into the Arkona Sea. 

The ASCII file batrix.asc is the daily Barotropic Transport Index Batrix, compiled from hourly sea level differences between Hornbaek and Gedser (Section 20.3.3), filtered with the linear 10-day-memory response function (20.16) of the volume flow in reaction to the applied barotropic pressure gradient. The Batrix values are measured in cm and are taken at 24 h once a day. Data gaps are padded with the mean difference —4 cm. The Batrix time series shown in Fig. 20.10 is an alternative measure for the inflow activity during the last century. 

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