200
K. Myrberg and T. Soomere
Fig. 6.12 Extension of ice
cover in the Gulf of Finland
in differently severe ice
winters. Open water is found
to the west of the ice edge
curve (Seinä and Palosuo
1993). Graphics by M. Viška
line freezes. The freezing mostly occurs during January and the ice melts in April,
giving an average length of three months for the extensive ice coverage. The ice
reaches its maximum extent in February or March. The maximum annual thickness
of coastal fast ice varies from 30 to 80 cm near Vyborg and in the Neva Bay.
The ice conditions are not evenly distributed in the different parts of the Gulf
of Finland. The heat inflow due to the coastal current from the Northern Gotland
Basin and the predominance of southern and south-western winds keep the Estonian
coastal area free of ice some time. The north–south asymmetry of the ice conditions
is enhanced by the coastal morphology, which supports a broad, landfast ice zone
along the coast of Finland but leaves almost no fast ice at the Estonian coast. The
width of the landfast ice zone in coastal areas, where the ice is stationary for most
of the ice season, depends on the bottom topography: islands and grounded sea ice
ridges serve as support points to stabilize the ice sheet. The edge of the landfast ice
in the gulf is in the neighbourhood of the 10 m isobath (Leppäranta 1981).
Sea ice has a remarkable influence on virtually all aspects of the physical, biological and chemical oceanography of the Gulf of Finland. Ice formation and melting
influence the stratification. Particularly important is the freshening of the surface
layer in spring. Especially thick ice substantially reduces the transfer of momentum
from the wind to the water body. The circulation is forced only by boundary fluxes
at the ice edge (Leppäranta 2010) and possibly by the ice drift. Under ice cover,
circulation in the subsurface layer (Andrejev et al. 2004a) may thus extend over the
entire upper layer (Soomere and Quak 2007).
Even more dramatic is the non-linear dynamic behaviour of wind- and currentdriven drift ice outside the landfast ice zone, with a highly non-linear mechanical behaviour. In open ice fields the floes drift independently and, like any floating drifters
on the sea surface, with a velocity close to that of the surface current. The theoretical free-drift speed of ice fields is 2 % of the wind speed, and drift direction 30° to
the right of the wind direction (Leppäranta 2010). Wind is a purely external force,
but the dynamics of ice and water are coupled. The interfacial stress depends first
of all on the ice-water velocity difference. Sometimes the drift can be as large as
20–30 km/day. There is no general simple rule for compact drift ice. The mobility
of a compact ice cover has been evaluated from experimental data. The ice with a
thickness <10 cm offers little resistance to wind and can be easily pushed to the
eastern end of the Gulf of Finland. At the other extreme, a compact ice field may
K. Myrberg and T. Soomere
Fig. 6.12 Extension of ice
cover in the Gulf of Finland
in differently severe ice
winters. Open water is found
to the west of the ice edge
curve (Seinä and Palosuo
1993). Graphics by M. Viška
line freezes. The freezing mostly occurs during January and the ice melts in April,
giving an average length of three months for the extensive ice coverage. The ice
reaches its maximum extent in February or March. The maximum annual thickness
of coastal fast ice varies from 30 to 80 cm near Vyborg and in the Neva Bay.
The ice conditions are not evenly distributed in the different parts of the Gulf
of Finland. The heat inflow due to the coastal current from the Northern Gotland
Basin and the predominance of southern and south-western winds keep the Estonian
coastal area free of ice some time. The north–south asymmetry of the ice conditions
is enhanced by the coastal morphology, which supports a broad, landfast ice zone
along the coast of Finland but leaves almost no fast ice at the Estonian coast. The
width of the landfast ice zone in coastal areas, where the ice is stationary for most
of the ice season, depends on the bottom topography: islands and grounded sea ice
ridges serve as support points to stabilize the ice sheet. The edge of the landfast ice
in the gulf is in the neighbourhood of the 10 m isobath (Leppäranta 1981).
Sea ice has a remarkable influence on virtually all aspects of the physical, biological and chemical oceanography of the Gulf of Finland. Ice formation and melting
influence the stratification. Particularly important is the freshening of the surface
layer in spring. Especially thick ice substantially reduces the transfer of momentum
from the wind to the water body. The circulation is forced only by boundary fluxes
at the ice edge (Leppäranta 2010) and possibly by the ice drift. Under ice cover,
circulation in the subsurface layer (Andrejev et al. 2004a) may thus extend over the
entire upper layer (Soomere and Quak 2007).
Even more dramatic is the non-linear dynamic behaviour of wind- and currentdriven drift ice outside the landfast ice zone, with a highly non-linear mechanical behaviour. In open ice fields the floes drift independently and, like any floating drifters
on the sea surface, with a velocity close to that of the surface current. The theoretical free-drift speed of ice fields is 2 % of the wind speed, and drift direction 30° to
the right of the wind direction (Leppäranta 2010). Wind is a purely external force,
but the dynamics of ice and water are coupled. The interfacial stress depends first
of all on the ice-water velocity difference. Sometimes the drift can be as large as
20–30 km/day. There is no general simple rule for compact drift ice. The mobility
of a compact ice cover has been evaluated from experimental data. The ice with a
thickness <10 cm offers little resistance to wind and can be easily pushed to the
eastern end of the Gulf of Finland. At the other extreme, a compact ice field may