328
T. Soomere
to the entire Baltic Sea. The test areas not only play a key role in the functioning of
the sea (as described in Chaps. 2 and 6) but also exhibit different challenges for the
technique.
10.3.2 The Western and Eastern Gateways to the Baltic Sea
The Danish Straits, the Belt Sea and the Arkona Basin form the gateway through
which all the ships travelling between the North Sea and the Baltic Sea have to pass
(see Figs. 9.1 and 9.2 in Chap. 9). This mostly shallow domain with moderately
sloping bottom serves as a transit area for the outflow of excess water through the
Danish Straits. More importantly, it is the gate through which the deeper parts of
the Baltic Sea are sporadically supplied under specific atmospheric conditions with
saltier, oxygen-rich North Sea waters (Matthäus and Lass 1995).
The complicated geometry of the coasts, the presence of narrow straits and the
small internal (baroclinic) Rossby radius R 1 (about 5 km in the Arkona Basin and
the Pomeranian Bay, Osi´ nski et al. 2010, and as small as 1–4 km in the shallow
Belt Sea, Fennel et al. 1991) make numerical modelling of water masses extremely
challenging. In order to properly resolve mesoscale dynamics, it is necessary to use
a horizontal grid size not larger than about 1/2 of the baroclinic Rossby radius (Drijfhout 1989; Lindow 1997). Most medium-resolution Baltic Sea circulation models
have a grid step around 2 nautical miles (nm) and are barely eddy-permitting here.
Furthermore, fine vertical resolution is needed here to resolve the straits’ topography
and sharp stratification (Lu et al. 2012).
The Gulf of Finland (see Chap. 6) is an elongated estuary (length over 400 km,
width 48–135 km) in the north-eastern (NE) Baltic Sea with a mean depth of 37 m
only (Alenius et al. 1998). As it is quite narrow in some places and the water is too
shallow for marine transportation in others, there are several narrow passages where
the concentration of traffic is exceptionally high. Particularly intense passenger traffic crosses the narrow section of the gulf between Tallinn and Helsinki (Parnell et al.
2008; Kurennoy et al. 2011).
The gulf is widely open to the Northern Gotland Basin. As there is no sill at
its entrance, continuous water exchange occurs between the two water bodies and
the dynamics of the Baltic Proper 1 strongly affects currents in the western part of
the gulf. This region evidently plays a key role in the functioning of the northern
Baltic Sea. Under specific wind conditions the typical estuarine circulation may be
reversed, the salt water wedge may be exported from the gulf and the vertical stratification may almost completely vanish (Elken et al. 2003). This process may lead
to enhanced vertical mixing of water masses. The eastern end of the gulf receives
voluminous fresh water inflow from the River Neva. This gives rise to a strong
east–west gradient in salinity and sea-level and leads to a small baroclinic Rossby
1 We shall use the notion Baltic Proper to denote the Eastern, Northern and Western Gotland Basin
(Table 2.1 in Chap. 2), Bornholm Basin and Gda´ nsk Bay.
T. Soomere
to the entire Baltic Sea. The test areas not only play a key role in the functioning of
the sea (as described in Chaps. 2 and 6) but also exhibit different challenges for the
technique.
10.3.2 The Western and Eastern Gateways to the Baltic Sea
The Danish Straits, the Belt Sea and the Arkona Basin form the gateway through
which all the ships travelling between the North Sea and the Baltic Sea have to pass
(see Figs. 9.1 and 9.2 in Chap. 9). This mostly shallow domain with moderately
sloping bottom serves as a transit area for the outflow of excess water through the
Danish Straits. More importantly, it is the gate through which the deeper parts of
the Baltic Sea are sporadically supplied under specific atmospheric conditions with
saltier, oxygen-rich North Sea waters (Matthäus and Lass 1995).
The complicated geometry of the coasts, the presence of narrow straits and the
small internal (baroclinic) Rossby radius R 1 (about 5 km in the Arkona Basin and
the Pomeranian Bay, Osi´ nski et al. 2010, and as small as 1–4 km in the shallow
Belt Sea, Fennel et al. 1991) make numerical modelling of water masses extremely
challenging. In order to properly resolve mesoscale dynamics, it is necessary to use
a horizontal grid size not larger than about 1/2 of the baroclinic Rossby radius (Drijfhout 1989; Lindow 1997). Most medium-resolution Baltic Sea circulation models
have a grid step around 2 nautical miles (nm) and are barely eddy-permitting here.
Furthermore, fine vertical resolution is needed here to resolve the straits’ topography
and sharp stratification (Lu et al. 2012).
The Gulf of Finland (see Chap. 6) is an elongated estuary (length over 400 km,
width 48–135 km) in the north-eastern (NE) Baltic Sea with a mean depth of 37 m
only (Alenius et al. 1998). As it is quite narrow in some places and the water is too
shallow for marine transportation in others, there are several narrow passages where
the concentration of traffic is exceptionally high. Particularly intense passenger traffic crosses the narrow section of the gulf between Tallinn and Helsinki (Parnell et al.
2008; Kurennoy et al. 2011).
The gulf is widely open to the Northern Gotland Basin. As there is no sill at
its entrance, continuous water exchange occurs between the two water bodies and
the dynamics of the Baltic Proper 1 strongly affects currents in the western part of
the gulf. This region evidently plays a key role in the functioning of the northern
Baltic Sea. Under specific wind conditions the typical estuarine circulation may be
reversed, the salt water wedge may be exported from the gulf and the vertical stratification may almost completely vanish (Elken et al. 2003). This process may lead
to enhanced vertical mixing of water masses. The eastern end of the gulf receives
voluminous fresh water inflow from the River Neva. This gives rise to a strong
east–west gradient in salinity and sea-level and leads to a small baroclinic Rossby
1 We shall use the notion Baltic Proper to denote the Eastern, Northern and Western Gotland Basin
(Table 2.1 in Chap. 2), Bornholm Basin and Gda´ nsk Bay.