2 Topography, Hydrography, Circulation and Modelling of the Baltic Sea
37
transport of a few cm/s. The stable summer stratification with weak vertical mixing
induces a salinity maximum in the bottom layer. In fall and winter mixing is strong
and the salinity difference between surface and bottom layer is reduced again. This
development is most pronounced near the coast where the halocline might not exist.
The salinity in the Baltic Sea decreases from the south to north and towards east.
This pattern is governed by the inflowing saline waters from the North Sea, the voluminous river water inflow mainly from the north and east and the overall cyclonic
circulation. The surface layer water at the entrance of the Baltic Sea (in the Kattegat
and at Danish Straits; at times also in the Belt Sea and Arkona Basin) is usually
Baltic Sea water (salinity 8–12 ‰), whereas the bottom layer water originates from
the Kattegat with salinities up to 32–33 ‰. Towards east and north the salinity decreases both in surface and bottom layers. In the Gotland Basin the salinity of the
upper layer is 7–8 ‰. Below the halocline the salinity increases approximately linearly with depth to 9–12 ‰ at a depth of 100 m and to 11.5–13 ‰ at 200 m. In
the northern basins surface salinity varies between 0 and 6 ‰ and bottom salinity is
slightly higher, i.e., a weak halocline exists in part of the basins (see Chap. 6).
2.2.2 Temperature
In the Baltic Sea the temperature first follows the two-layer structure determined
by the salinity-driven density changes (Figs. 2.3, 2.4). The temperature of the upper layer experiences a remarkable annual cycle forced by the radiation budget and
air–sea interaction, while the bottom layer below the halocline is decoupled from
atmospheric forcing and largely influenced by the advection from the North Sea.
A warm seasonal mixed layer develops during summer above a seasonal thermocline in the upper layer. In the northern part of the Baltic Sea an inverse thermocline
is formed in wintertime: the temperature is at the freezing point at the surface and
increases with depth to the temperature of maximum density (2–3 ◦ C). A dicothermal layer (see below) forms in summer at the bottom of the upper layer because the
water is heated from the top and cold water remains further down. This cold layer is
also called winter water in the Baltic Sea. The layered structure is clearly evident in
the mean temperature gradient.
The seasonal evolution of temperature in the Baltic Sea is very different in the
upper and lower layers. Due to large seasonal variations of the energy budget at
the sea surface, the surface water temperature reaches its maximum in summer and
minimum in winter, and in winter at least a part of the sea freezes. The seasonal
variability of the temperature in the lower layer is weak. Especially in the southern
Baltic Sea it mostly depends on advection from the North Sea through the Danish
Straits. When the water masses flow northwards they sink down to the bottom due
to their higher salinity compared with the ambient water. This is why the bottom
waters are relatively warm, in the Gotland Basin the bottom temperature is 4–6 ◦ C,
and in the Gulf of Finland, the Gulf of Riga and the Gulf of Bothnia it is 2–4 ◦ C.
In spring, after the melting of the ice, a thin upper layer is heated due to solar radiation. The surface waters quickly reach the temperature of maximum density T m
37
transport of a few cm/s. The stable summer stratification with weak vertical mixing
induces a salinity maximum in the bottom layer. In fall and winter mixing is strong
and the salinity difference between surface and bottom layer is reduced again. This
development is most pronounced near the coast where the halocline might not exist.
The salinity in the Baltic Sea decreases from the south to north and towards east.
This pattern is governed by the inflowing saline waters from the North Sea, the voluminous river water inflow mainly from the north and east and the overall cyclonic
circulation. The surface layer water at the entrance of the Baltic Sea (in the Kattegat
and at Danish Straits; at times also in the Belt Sea and Arkona Basin) is usually
Baltic Sea water (salinity 8–12 ‰), whereas the bottom layer water originates from
the Kattegat with salinities up to 32–33 ‰. Towards east and north the salinity decreases both in surface and bottom layers. In the Gotland Basin the salinity of the
upper layer is 7–8 ‰. Below the halocline the salinity increases approximately linearly with depth to 9–12 ‰ at a depth of 100 m and to 11.5–13 ‰ at 200 m. In
the northern basins surface salinity varies between 0 and 6 ‰ and bottom salinity is
slightly higher, i.e., a weak halocline exists in part of the basins (see Chap. 6).
2.2.2 Temperature
In the Baltic Sea the temperature first follows the two-layer structure determined
by the salinity-driven density changes (Figs. 2.3, 2.4). The temperature of the upper layer experiences a remarkable annual cycle forced by the radiation budget and
air–sea interaction, while the bottom layer below the halocline is decoupled from
atmospheric forcing and largely influenced by the advection from the North Sea.
A warm seasonal mixed layer develops during summer above a seasonal thermocline in the upper layer. In the northern part of the Baltic Sea an inverse thermocline
is formed in wintertime: the temperature is at the freezing point at the surface and
increases with depth to the temperature of maximum density (2–3 ◦ C). A dicothermal layer (see below) forms in summer at the bottom of the upper layer because the
water is heated from the top and cold water remains further down. This cold layer is
also called winter water in the Baltic Sea. The layered structure is clearly evident in
the mean temperature gradient.
The seasonal evolution of temperature in the Baltic Sea is very different in the
upper and lower layers. Due to large seasonal variations of the energy budget at
the sea surface, the surface water temperature reaches its maximum in summer and
minimum in winter, and in winter at least a part of the sea freezes. The seasonal
variability of the temperature in the lower layer is weak. Especially in the southern
Baltic Sea it mostly depends on advection from the North Sea through the Danish
Straits. When the water masses flow northwards they sink down to the bottom due
to their higher salinity compared with the ambient water. This is why the bottom
waters are relatively warm, in the Gotland Basin the bottom temperature is 4–6 ◦ C,
and in the Gulf of Finland, the Gulf of Riga and the Gulf of Bothnia it is 2–4 ◦ C.
In spring, after the melting of the ice, a thin upper layer is heated due to solar radiation. The surface waters quickly reach the temperature of maximum density T m