Baroclinic inflow

An exceptional high number of Vb lows appeared in the year of 2002, leading, for example, to the century flood of Dresden, and in particular to the first observed significant baroclinic inflow of warm, highly saline water from the Kattegat into the central Baltic Sea (Feistel et al., 2003, 2004a Nausch et al., 2003 see also Chapter 10). 

In the summer of 2003, an Omega situation led to the second ever recognized important baroclinic inflow of warm saline water into the central Baltic Sea that substituted entirely the cold water of the Major Baltic Inflow of January 2003 residing there (Feistel et al., 2004b, 2006). 

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)... 

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 ... 

Baroclinic summer inflows seem to be an indication for a new quality in long-term behavior of the Baltic Sea. The possible relation of their appearance to global climatic change (Schar and Jendritzky, 2004) is an open scientific question. 

THE EFFECTS OF MBls AND BAROCLINIC SUMMER INFLOWS IN THE CENTRAL BALTIC DEEPWATER... 

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 10.14 Long-term variation of temperature in three levels of the deep water of the Gotland Deep. Signals of baroclinic summer inflows (BSI) are indicated by small arrows, and the effects of MBls are indicated by heavy arrows. 

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. 

Alternatively, the vertical grid cell size may vary horizontally following the depth variations. This is the so-called ct-coordinate approach, which is expected to give a better representation of dense inflowing water masses over varying bottom topography. However, the accurate calculation of baroclinic pressure gradients creates problems for this type of coordinates. Moreover, the vertical acceleration is also excluded by the hydrostatic approximation. 

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