Darss Sill

At the eastern flank of the relatively shallow and topographically even Darss SiU is the MARNET station Darss Sill (DS, see Chapter 3), where operational ADCP current measurements have been carried out since 2004. This is the region from where the most important proportion of saline and oxygenated water masses from the Kattegat occasionally spreads within the near-bottom layer down the slope into the deep of the Arkona Sea. Table 6.8 shows the annual and monthly means of the year 2005. 

Regarding the annual means at Darss Sill, the statistics provide no hints of an inflow. The mean currents in 2005 are directed toward north-northwesterly directions throughout the water column. Intermittent inflow events within the bottom layer are not obvious from Table 6.8. They are hidden in the annual means, which agree with averaged results of current measurements during 1973-1979 at the same location (Francke, 1983). 

TABLE 6.8 Current Statistics for 2005 at Darss Sill (DS). Annual and Monthly Means at 6 and 18 in depth. There are No Data from 27 April to 8 June 

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The measured barotropic velocity projected onto the channel axis at the Darss Sill MARNET station reveals a rather Gaussian distribution, while the distributions of both the sea level difference between Viken and Skanoer and the transport derived according to Equation 2.14 differ significantly from the Gaussian distribution. However, the minimum of the wave-controlled and the frictional-controlled transports depicts a quite reasonable Gaussian distribution. 

FIGURE 2.4 Probability distributions of the barotropic current component at the Darss Sill toward 68 T (upper left panel) of the sea level difference between the Kattegat (Viken) and the northwestern par t of the Arkona Basin (Skanoer) (upper right panel), the frictional controlled transport proportional to the square root of the sea level difference (lower left panel), and the minimum of the wave and the frictional controlled transport. 

AB, Arkona Basin DS, Darss Sill OB, Oder Bank C, conductivity T, temperature. 

Mockel and Seehase (1986) developed the idea of establishing a measuring platform at the Darss Sill in the 1980s based on a floating aluminum mast, which was connected to a heavy... 

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 mean magnitudes at Drogden Sill in 2005 are about 50% higher than that at Darss Sill in the same year. The strongest monthly speeds occurred in February 2005 toward the northeast. During several months, the mean speeds at the lower level distinctively exceed... 

FIGURE 7.25 Wind and wave conditions during cyclone Gudrun/Erwin Measurements (solid line) Model data (dashed line) (a) Darss Sill Station (54°42 N, 12 42 E) (b) Arkona MARNET Station (54°53 N. 13°52 E). 

More recent investigations on the mean atmospheric circulation patterns have shown that there are typical circulation patterns that trigger MBls and are necessary for their occurrence (Matthaus and Schinke, 1994). One of the basic conditions linked to major events is, on average, a mean continuous increase in wind speed from westerly directions over several weeks as, for instance, very distinctly observed in 1951 (Wyrtki, 1954) and 1993 (Matthaus and Lass, 1995). This increase begins about 2 weeks before the main inflow period and reaches maximum values on the day before the overflow of water with salinities >17 psu across the Darss Sill. The higher the mean wind speed and its duration during the main inflow period, the stronger the inflow. 

Attention must also be paid to the baroclinic summer inflows (cf. Section 10.5). These inflows below the MBI category are caused by long-lasting calm weather conditions over central Europe, which most likely occur in late summer and early autumn. Strong summer inflows can transport substantial amounts of exceptionally warm and saline water across the Darss Sill into the intermediate layers of the central Baltic Sea (Feistel et al., 2003c, 2004a). A typical example for weak baroclinic inflows and their effects between the Darss Sill and the Bornholm Basin is illustrated in Fig. 10.2. 

FIGURE 10.2 Thermohaline stratification between the Darss Sill and the Shipsk Channel in September 2005 caused by weak baroclinic inflows. (See color plate)... 

TABLE 10.1 Estimated Propagation Speed of the Water Penetrated during Some Important Inflow Events between Both Darss Sill - Bornholm Deep (Distance 240 km) and Darss Sill - Gotland Deep (Distance w640 km) Time is Measured between Inflow Event (MBI) at the Darss Sill and Bottom Water Renewal (BWR)... 

For the first time. Wolf (1972) analyzed major Baltic inflows by means of a quantitative definition. To distinguish between regular inflows and MBIs, he suggested empirical criteria for their identification in the entrance area to the Baltic Sea using limiting values of stratification and bottom salinity at the 1/v Gedser Rev (GR) in the Darss Sill area ... 

Different estimations of the strength of the January 1993 event (cf. Section 10.4.3) led to the reassessment of the intensity. Instead of the mean salinity, Fischer and Matthaus (1996) used the amount of salt q (in Gt = IO kg), entering the Baltic Sea over both the Darss Sill (DS) and the Drogden Sill (DR), as the criterion for the intensity of MBls. On this basis, they recalculated the intensity of 90 events that took place between 1897 and 1976. The new method permitted the quantitative estimation of the strength of MBls using the total salt transport into the Baltic Sea. 

In addition to the criteria GR 1 and GR 2 used for identifying MBls at the Darss Sill, Fischer and Matthaus (1996) developed the following criteria to ascertain inflow days at the Drogden Sill belonging to each identified event ... 

FIGURE 10.5 Rise of the mean Baltic sea level (station Landsort) during precursory and main inflow periods (definition cf. MBI 1975-1976) of selected strong and very strong MBIs (Matthaus, 1993, modified and updated). Hatched lines mark the MBI start al the Darss Sill. 

FIGURE 10.8 Frequency distributions of the water volumes (in km ) penetrating before the beginning of the MBI at the Darss Sill (a), during the event (b), and during the complete inflow interval (c) (Matthaus and Franck, 1990). 

Using the same inflow event, Francke and Hupfer (1980) studied ciurent, wind, sea level, and salinity conditions in the Darss Sill area. On the basis of measurements at the permanent buoy station at the Darss Sill, they analyzed currents in the surface and bottom layers in December 1975 and January 1976. Lass and Schwabe (1990) examined the dynamics of the... 

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