2 Topography, Hydrography, Circulation and Modelling of the Baltic Sea
39
waters from rivers to stay in the surface layer. This in turn strengthens the density
difference across the thermocline. Thus both the warming and fresh water content of
the surface layer enhance the stability of this layer. In turn, the bottom layer salinity
increases in summer.
A specific feature of the Baltic Sea and many other ice-covered seas is a dicothermal layer at the bottom of the upper layer. The formation of such a layer
takes place in the following way. During the spring the upper layer starts to warm
from the surface while the temperature in the lower part is close to the temperature
of the density maximum or to the freezing point in the north. On the other hand the
bottom layer water is throughout the year at temperatures between 3 ◦ C and 6 ◦ C
but has a much higher density than the upper layer water due to higher salinity.
The dicothermal layer remains throughout the summer and disappears only in the
autumn convection. Consequently, the temperature may have a four-layer structure
during the summer season: surface layer, thermocline, dicothermal layer and lower
(bottom) layer.
Usually in late August or early September, depending on the latitude, the energy
budget of the sea surface becomes negative, the surface waters cool, become heavier and sink due to convection. Simultaneously the sea surface releases heat to the
atmosphere through turbulent fluxes. This all leads to a deepening of the surface
mixed layer and to a weakening of the thermocline, i.e., a weakening of the vertical temperature gradient 3 (Fig. 2.4). This process continues during autumn. At the
same time the lower parts of the upper layer are still warming due to slow thermal
diffusion. For example, at a depth of 30 m the temperature maximum is reached
only in October while on the surface the highest values are usually measured during
late July–early August. During late autumn or early winter the upper layer becomes
isothermal as a consequence of thermohaline convection and wind-induced mixing.
However, it is important to recognize that the mixing does not destroy the halocline,
which remains throughout the year in the central basins.
After the surface temperature has dropped below the temperature of maximum
density, transient weak winter thermoclines may form close to the surface but in
general mixing continues due to forced, mechanical convection, if the necessary
cooling is allowed by the surface heat flux. It is worth noting that as the temperature of maximum density decreases with increasing salinity, the strength of this
maximum decreases at the same time. In other words the difference between the
maximum density and the density at the freezing point decreases with increasing
salinity. The inverse winter thermocline is therefore more stable in fresh waters than
in brackish waters.
In fact the permanent winter thermocline, where temperature increases approximately from the freezing point to the temperature of maximum density, is located
3 The temperature difference between the surface layer and the lower part of the upper layer (above
the halocline) gradually weakens because not all the heat energy is mixed downwards due to convection. A part of the heat energy is released to the atmosphere by turbulent heat fluxes and thus
the surface water cools down and its temperature difference in comparison to the waters below the
thermocline is reduced.
39
waters from rivers to stay in the surface layer. This in turn strengthens the density
difference across the thermocline. Thus both the warming and fresh water content of
the surface layer enhance the stability of this layer. In turn, the bottom layer salinity
increases in summer.
A specific feature of the Baltic Sea and many other ice-covered seas is a dicothermal layer at the bottom of the upper layer. The formation of such a layer
takes place in the following way. During the spring the upper layer starts to warm
from the surface while the temperature in the lower part is close to the temperature
of the density maximum or to the freezing point in the north. On the other hand the
bottom layer water is throughout the year at temperatures between 3 ◦ C and 6 ◦ C
but has a much higher density than the upper layer water due to higher salinity.
The dicothermal layer remains throughout the summer and disappears only in the
autumn convection. Consequently, the temperature may have a four-layer structure
during the summer season: surface layer, thermocline, dicothermal layer and lower
(bottom) layer.
Usually in late August or early September, depending on the latitude, the energy
budget of the sea surface becomes negative, the surface waters cool, become heavier and sink due to convection. Simultaneously the sea surface releases heat to the
atmosphere through turbulent fluxes. This all leads to a deepening of the surface
mixed layer and to a weakening of the thermocline, i.e., a weakening of the vertical temperature gradient 3 (Fig. 2.4). This process continues during autumn. At the
same time the lower parts of the upper layer are still warming due to slow thermal
diffusion. For example, at a depth of 30 m the temperature maximum is reached
only in October while on the surface the highest values are usually measured during
late July–early August. During late autumn or early winter the upper layer becomes
isothermal as a consequence of thermohaline convection and wind-induced mixing.
However, it is important to recognize that the mixing does not destroy the halocline,
which remains throughout the year in the central basins.
After the surface temperature has dropped below the temperature of maximum
density, transient weak winter thermoclines may form close to the surface but in
general mixing continues due to forced, mechanical convection, if the necessary
cooling is allowed by the surface heat flux. It is worth noting that as the temperature of maximum density decreases with increasing salinity, the strength of this
maximum decreases at the same time. In other words the difference between the
maximum density and the density at the freezing point decreases with increasing
salinity. The inverse winter thermocline is therefore more stable in fresh waters than
in brackish waters.
In fact the permanent winter thermocline, where temperature increases approximately from the freezing point to the temperature of maximum density, is located
3 The temperature difference between the surface layer and the lower part of the upper layer (above
the halocline) gradually weakens because not all the heat energy is mixed downwards due to convection. A part of the heat energy is released to the atmosphere by turbulent heat fluxes and thus
the surface water cools down and its temperature difference in comparison to the waters below the
thermocline is reduced.