Central Baltic deep water

The definite identification and general classification of all inflow events regardless of their effects on the various deep basins can be done by the salinity conditions in the entrance sill area (Wyrtki, 1954 Wolf, 1972 Matthaus and Franck, 1992). Another occasionally used characterization of the inflow events is based on their effects on the hydrographic and chemical conditions in the central Baltic deep water (cf. e.g., Kalle, 1943 Fonselius, 1969 Nehring, 1990). The latter method has a certain drawback because the effects depend not only on the intensity of the inflows but also on the bottom topography, the density of the bottom water, and the buffering capacity of various deep basins. 

The intensity of MBls can be estimated during the inflow events by the salinity conditions at the Baltic entrance sills (cf. Section 10.4). The effects of MBls in the central Baltic deep water depends on the density of the inflowing water, the mixing with the ambient water on its way through the Baltic Sea (cf. Fig. 10.1), and the density of the deep water in the various basins. The pathways of the inflowing water propagating from the sills through the Arkona Basin into the Bornholm Basin have been studied in detail by Lass et al. (2005). 

The salinity of the Bornholm Basin below the permanent halocline is a measure for the estimation of the impact of weak inflows on the central Baltic deep water. During periods of low inflow activity, salinity and thus density decreases in the deep water of the Bornholm Basin. Depending on the volume of saline water and its density, inflows below the MBI magnitude — but sometimes even MBIs — fill up only that basin, and the saline water does not pass to a greater extent the Slupsk Sill downstream through the Slupsk Channel into the Gotland Basin (cf. Fig. 10.2). 

The Institute of Marine Research and later the Baltic Sea Research Institute in War-nemiinde were laigely involved in investigations of the analysis of specific inflows of highly saline water and the study of the water renewals in the central Baltic, as well as on the stagnation periods following these events in the central Baltic deep water. 

Fonselius (1962) started the detailed description of the effects of MBIs in the central Baltic deep water. He identified MBIs studying long-term variations, mainly of salinity in the Bornholm Basin. Fonselius described the dynamic processes between the Baltic deep basins after MBIs, the importance of the Bornholm Basin as buffer basin, and the overflow of the Eastern Gotland Basin in intermediate depths downstream into the Landsort Deep. 

Air-sea interactions are a main source of the variability observed in seas in time scales ranging from years to centuries. On decadal timescales, variations in atmospheric circulation over the northern Atlantic Ocean and Europe govern fluctuations in the water exchange between the North Sea and the Baltic Sea. The schematic diagram in Fig. 10.10 illustrates the pathway of the influence of the atmospheric circulation on the central Baltic deep water. The variability in atmospheric circulation governs the water exchange of the Baltic Sea with the North Sea, especially the occurrence or absence of not only major inflows but also of baroclinic summer inflows. MBls and summer inflows have essential impacts on the oceanographic conditions in the deepwater. 

FIGURE 10.12 Long-term variation of salinity in the central Baltic deep water. 

Nehring, D., 1987. Temporal variations of phosphate and inorganic nitrogen compounds in central Baltic deep waters. Limnology and Oceanography, 32, 494 99. 

Nehring, D., 1989. Phosphate and nitrate trends and the ratio oxygen consumption to phosphate accumulation in central Baltic deep waters with alternating oxic and anoxic conditions. Beitrage zur Meereskunde, Berlin, 59, 47-58. 

Nehring, D., Matthaus, W., Lass, H.U., Nausch, G., Nagel, K., 1995b. The Baltic Sea 1995 - Beginning of an new stagnation period in its central Baltic deep waters and decreasing nutrient load in its surface waters. Deutsche Hydrographische Zeitschrift, 47, 319-327. 

Hagen, E., Feistel, R., 2001. Spreading of Baltic deep water a case study for the winter 1997-1998. In Matthaus W, Nausch, G. (Eds.), The Hydrographic-Hydrochemical State of the Western and Central Baltic Sea in 1999/2000 and During the 1990s. Meereswissenschaftliche Berichte, Warnemiinde, 45, pp. 99-133. 

In anoxic waters, such as the deep basin of the central Baltic Sea or parts of the shallow Po River delta, N2O is consumed by water-column denitrification. 

N2 in the suboxic deep waters (70—440 m) of the western part of the central Baltic Sea was supersaturated up to 8%, whereas in the surface layer N2 was slightly undersaturated (Ronner and Sorensson, 1985). The accumulation of N2 resulted from denitrification. In the suboxic intermediate water masses (150—1000 m) of the central Arabian Sea, N2 concentrations were found to be significandy enhanced with a maximum in 250 m (Codispoti et al., 2001, 2005). This corresponds to a minimum of of dissolved N2 in the core of the suboxic zone, which drops from 0.6%o... 

FIGURE 2.3 Typical thermohaline stratification of the central Baltic Sea during winter (full line) and summer (partly hatched). (Ai = summer upper layer A2 = summer thermocline A3 = cold intermediate water layer B = cold winter water layer C = permanent discontinuity layer D = deep water layer). 

Besides MB Is, baroclinic summer inflows of exceptionally warm and saline water affect the deeper layers of the central Baltic Sea (cf. Section 10.5). Such inflows do not fulfill the criteria for MB Is (cf. Section 10.3) but can effectively influence the deep water below the halocline in the Bornholm, Gdansk, and Eastern Gotland Basins (Feistel et al., 2003c, 2004a). 

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... 

The Baltic Sea (Fig. 6.30) is a large regional sea, receiving drainage from much of northern and central Europe. The hydrography of the Baltic is complex, consisting of a number of deep basins separated by shallow sills. As a result, the waters of the deep basins can be isolated from exchange with one another—and from the atmosphere—on timescales of years. 

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