2 Topography, Hydrography, Circulation and Modelling of the Baltic Sea
35
an area of 115,516 km 2 , a volume of 6,369 km 3 and a mean depth of 55 m. The next
one in size is the Bornholm Basin. Its area is 38,942 km 2 , its volume 1780 km 3 and
its mean depth 46 m. The Gulf of Finland covers an area of 29,498 km 2 (8 % of the
Baltic Sea) and has a volume of 1,098 km 3 (5 % of the Baltic Sea). Its mean depth is
37 m, clearly less than that of the entire Baltic Sea. The Danish Straits, the Arkona
Basin, and the Gulf of Riga have relatively small volumes.
The shallowness of the sea becomes clearly visible if the depth distribution is
presented as a hypsographic curve (not shown). Only about 12 % of the total area
has a depth of more than 100 m, and only 2.7 % of more than 150 m. Consequently,
a big part of the Baltic Sea belongs to a shallow, coastal-like area. As a rule of thumb
it can be said that about 50 % of the area of the Baltic has a depth of 50 m or less.
2.2 Basic Hydrography
2.2.1 Salinity
In the Baltic Sea the salinity mostly determines the stratification of the water masses.
When the inflowing saltier water masses from the North Sea enter the brackish
Baltic Sea, they sink to the bottom, move further as dense bottom currents and fill
the deep water pools (see for details Chap. 6 for the Gulf of Finland). As a result,
the Baltic Sea water body has a permanent two-layer structure (Fig. 2.3): the upper layer and the bottom layer (or lower layer), separated by a halocline (see also
Fig. 2.4). The depth of the location of the halocline is usually 40–80 m, but in the
shallow south-western basins it is even less. The upper layer is homohaline, while
the bottom layer is continuously stratified. The salinity stratification is regularly
modified by the formation of the seasonal surface layer in summer. This is a warm,
well-mixed layer, isolated from the deeper water due to lower density. The spring
and summer runoff additionally lower its salinity.
The thickness of the permanent halocline is 10–20 m. Its depth is determined
by advection, wind-induced and convective mixing and the sill depths. This depth
changes very little in time. An exception is the area of the Danish Straits, where
the haline stratification is different from that, for example, in the Gotland Sea. Its
wedge-shaped structure moves back and forth in reaction to prevailing wind conditions. In areas where the vertical stratification of salinity is weak, the halocline can
disappear in certain specific conditions. For example in the western Gulf of Finland
the predominant south-westerly winds work against the standard estuarine circulation. Long-lasting and strong winds push a large amount of relatively fresh surface
water into the gulf. The resulting increase in the hydrostatic pressure may lead to
a gradual export of the salt wedge at the bottom layer of the basin (Elken et al.
2003). The reversal may occur if the average speed of south-westerly winds exceeds
4–5.5 m/s.
Another specific region is the easternmost Gulf of Finland, where the fresh water from the River Neva has a strong effect on the stratification (see Chap. 6). In
35
an area of 115,516 km 2 , a volume of 6,369 km 3 and a mean depth of 55 m. The next
one in size is the Bornholm Basin. Its area is 38,942 km 2 , its volume 1780 km 3 and
its mean depth 46 m. The Gulf of Finland covers an area of 29,498 km 2 (8 % of the
Baltic Sea) and has a volume of 1,098 km 3 (5 % of the Baltic Sea). Its mean depth is
37 m, clearly less than that of the entire Baltic Sea. The Danish Straits, the Arkona
Basin, and the Gulf of Riga have relatively small volumes.
The shallowness of the sea becomes clearly visible if the depth distribution is
presented as a hypsographic curve (not shown). Only about 12 % of the total area
has a depth of more than 100 m, and only 2.7 % of more than 150 m. Consequently,
a big part of the Baltic Sea belongs to a shallow, coastal-like area. As a rule of thumb
it can be said that about 50 % of the area of the Baltic has a depth of 50 m or less.
2.2 Basic Hydrography
2.2.1 Salinity
In the Baltic Sea the salinity mostly determines the stratification of the water masses.
When the inflowing saltier water masses from the North Sea enter the brackish
Baltic Sea, they sink to the bottom, move further as dense bottom currents and fill
the deep water pools (see for details Chap. 6 for the Gulf of Finland). As a result,
the Baltic Sea water body has a permanent two-layer structure (Fig. 2.3): the upper layer and the bottom layer (or lower layer), separated by a halocline (see also
Fig. 2.4). The depth of the location of the halocline is usually 40–80 m, but in the
shallow south-western basins it is even less. The upper layer is homohaline, while
the bottom layer is continuously stratified. The salinity stratification is regularly
modified by the formation of the seasonal surface layer in summer. This is a warm,
well-mixed layer, isolated from the deeper water due to lower density. The spring
and summer runoff additionally lower its salinity.
The thickness of the permanent halocline is 10–20 m. Its depth is determined
by advection, wind-induced and convective mixing and the sill depths. This depth
changes very little in time. An exception is the area of the Danish Straits, where
the haline stratification is different from that, for example, in the Gotland Sea. Its
wedge-shaped structure moves back and forth in reaction to prevailing wind conditions. In areas where the vertical stratification of salinity is weak, the halocline can
disappear in certain specific conditions. For example in the western Gulf of Finland
the predominant south-westerly winds work against the standard estuarine circulation. Long-lasting and strong winds push a large amount of relatively fresh surface
water into the gulf. The resulting increase in the hydrostatic pressure may lead to
a gradual export of the salt wedge at the bottom layer of the basin (Elken et al.
2003). The reversal may occur if the average speed of south-westerly winds exceeds
4–5.5 m/s.
Another specific region is the easternmost Gulf of Finland, where the fresh water from the River Neva has a strong effect on the stratification (see Chap. 6). In