6 The Gulf of Finland
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stand stationary even under considerable forcing. In harsh winters the entire gulf
can be covered with landfast ice. An ice thickness of 50 cm is usually strong enough
to resist wind forcing, except when the wind is just pushing ice out of the gulf into
the Northern Gotland Basin.
Horizontal turbulence under the ice cover was studied in Smelov et al. (1985)
using radioactive indicators in the northern part of the Neva Bay and in the sea
area to the east of Kotlin. The measurements covered a fairly small horizontal scale
range (up to 100 m) and showed the dependence of the eddy diffusivity coefficient
on the scale of the turbulence and on ice conditions. The influence of ice led to an
amplification of the hydraulic resistance reducing the turbulent diffusivity. It also
induced additional disturbances in the water adjacent to the lower surface of the ice.
6.3.3 Energy Budget
The decoupling of the dynamics of the upper layer from the lower layer has implications also on the energy budget of the water masses. In the Gulf of Finland, like
in the entire Baltic Sea, the heat budget of the upper layer is driven mainly by solar
and atmospheric forcing. The heating and cooling of the largely decoupled lower
layer mostly occurs owing to advection of heat and water masses from the Gotland
Sea.
The heat content of the Gulf of Finland has an annual amplitude of 46 × 10 18 J,
which corresponds to a mean temperature amplitude of 10.2 °C (Jurva 1937). The
main factor is the annual cycle in the solar radiation, which is close to zero in
mid-winter, reaches 200–300 W/m 2 (on daily average) and peak values of about
600 W/m 2 in summer. The monthly mean solar radiation at Jokioinen (60°49 N,
23°30 E) varies from 10 W/m 2 in December–January to 250 W/m 2 in June. The
radiation emitted by the sea surface usually exceeds the terrestrial (backward) radiation from the sky), except for very rare situations. The air–sea surface temperature
difference may have either sign. When the two temperatures are equal, the sea surface loses total radiation of about 50 W/m 2 . The total (terrestrial + solar) radiation
budget turns positive from sometime in March–April and becomes negative for autumn and winter from September–October.
The sensible heat flux is usually within ±50 W/m 2 . The monthly values are typically a few W/m 2 upwards in April, July and August, while a downward flux prevails in May and June. The latent heat flux is mostly negative, with the monthly
mean values about 25 W/m 2 or even more and generally directed upwards from
April to August. Therefore, the turbulent fluxes together usually do not exceed the
terrestrial radiation losses and can predominate only when the temperature or water
vapour pressure difference between air and sea is large and the wind speed high
(e.g., in late autumn). Heat transfer from precipitation takes place as sensible heat
exchange and as phase changes. If the precipitation were solid, the melting would
take heat of 30 W/m 2 and cool down the surface layer.
The net heat flux through the sea surface is on the order of 100 W/m 2 , being
positive in summer and negative in winter (see also Meier and Döscher 2002). This
201
stand stationary even under considerable forcing. In harsh winters the entire gulf
can be covered with landfast ice. An ice thickness of 50 cm is usually strong enough
to resist wind forcing, except when the wind is just pushing ice out of the gulf into
the Northern Gotland Basin.
Horizontal turbulence under the ice cover was studied in Smelov et al. (1985)
using radioactive indicators in the northern part of the Neva Bay and in the sea
area to the east of Kotlin. The measurements covered a fairly small horizontal scale
range (up to 100 m) and showed the dependence of the eddy diffusivity coefficient
on the scale of the turbulence and on ice conditions. The influence of ice led to an
amplification of the hydraulic resistance reducing the turbulent diffusivity. It also
induced additional disturbances in the water adjacent to the lower surface of the ice.
6.3.3 Energy Budget
The decoupling of the dynamics of the upper layer from the lower layer has implications also on the energy budget of the water masses. In the Gulf of Finland, like
in the entire Baltic Sea, the heat budget of the upper layer is driven mainly by solar
and atmospheric forcing. The heating and cooling of the largely decoupled lower
layer mostly occurs owing to advection of heat and water masses from the Gotland
Sea.
The heat content of the Gulf of Finland has an annual amplitude of 46 × 10 18 J,
which corresponds to a mean temperature amplitude of 10.2 °C (Jurva 1937). The
main factor is the annual cycle in the solar radiation, which is close to zero in
mid-winter, reaches 200–300 W/m 2 (on daily average) and peak values of about
600 W/m 2 in summer. The monthly mean solar radiation at Jokioinen (60°49 N,
23°30 E) varies from 10 W/m 2 in December–January to 250 W/m 2 in June. The
radiation emitted by the sea surface usually exceeds the terrestrial (backward) radiation from the sky), except for very rare situations. The air–sea surface temperature
difference may have either sign. When the two temperatures are equal, the sea surface loses total radiation of about 50 W/m 2 . The total (terrestrial + solar) radiation
budget turns positive from sometime in March–April and becomes negative for autumn and winter from September–October.
The sensible heat flux is usually within ±50 W/m 2 . The monthly values are typically a few W/m 2 upwards in April, July and August, while a downward flux prevails in May and June. The latent heat flux is mostly negative, with the monthly
mean values about 25 W/m 2 or even more and generally directed upwards from
April to August. Therefore, the turbulent fluxes together usually do not exceed the
terrestrial radiation losses and can predominate only when the temperature or water
vapour pressure difference between air and sea is large and the wind speed high
(e.g., in late autumn). Heat transfer from precipitation takes place as sensible heat
exchange and as phase changes. If the precipitation were solid, the melting would
take heat of 30 W/m 2 and cool down the surface layer.
The net heat flux through the sea surface is on the order of 100 W/m 2 , being
positive in summer and negative in winter (see also Meier and Döscher 2002). This