206
K. Myrberg and T. Soomere
(see Sect. 2.3.4 in Chap. 2) and seiches of the gulf and of the combined Gulf of
Finland–Northern Gotland Basin system additionally complicate the current system.
Although many studies have been devoted to the problem of determining the
mean circulation in the Gulf of Finland, the question is still partially open. The early
analyses (Palmén 1930; Hela 1952) were based on observations from the lightships
of Helsinki and Tallinn. They represent the very surface layer and it is necessary
to eliminate the local drift currents in order to extract the residual (more or less
stationary) flow. The drift current was defined as the flow in a stationary wind and
current system (which very seldom exist). The dependence of the resulting residual
flow on the wind (Hela 1952) indicates that the interactions between different forcing and steering factors are very complex and it is difficult to give an unambiguous
interpretation of the results. Another way to roughly estimate the residual flow is to
calculate the resultant of the currents observed during calm weather (Hela 1952).
The sum of the residual flow and an additional (Ekman) current deflected to the
right of the wind was denoted as the characteristic current in Hela (1952). It was
very slow, 0.6–2.0 cm/s, off Helsinki and somewhat faster, 1.7–4.9 cm/s, off Tallinn.
The best match between the total surface current v c and the wind speed W w was
v c = 0.0137W w . Thus, as a rule of thumb, the characteristic surface current has a
speed of about 1.4 % of the prevailing wind speed and is deflected by 19° to the right
of the wind direction. This deflection angle is naturally not valid near coasts. These
historical results are still of importance for understanding the circulation dynamics
of the Gulf of Finland and have a direct application for the forecast and hindcast of
pollution and contaminants in the surface layer during a large part of the year. Their
outcome is equivalent to that of the studies of net current-driven transport over time
scales larger than the synoptic scale (Soomere et al. 2011a).
The question of a residual flow has been further addressed using a linear regression between the easterly and northerly wind components and the easterly component of the current at a depth of 5 m off Kotka in the northern Gulf of Finland based
on 25 days of observations (Sarkkula 1989, 1991; Pitkänen et al. 1993). The proportionality coefficient of 4.8 cm/s (r 2 = 0.69) was interpreted as the speed of the
residual flow driven by the joint effect of horizontal salinity gradient and bottom
relief (Sarkisyan et al. 1975).
The obtained speed was somewhat larger than in the above estimates for the icefree period. Note that Palmén (1930) used 5 years of observations in which seasonal
differences tend to cancel each other while the more recent results mostly reflect a
single wind event. Another source of the difference stems from the short-term basinwide dynamics of the gulf. The predominant south-western winds pile up the water
at the eastern end of the gulf. If the wind stress vanishes, the westward barotropic
return flow towards an equilibrium stage may simply override the baroclinic effects
and circulation caused by the river water runoff. The linear regression tends to overestimate the residual flow because the wind stress is seldom zero for a long enough
time for the equilibrium stage to be reached. Despite several debatable issues, this
was the first time when the background current was actually identified.
Importantly, the measurements were carried out at a depth of 5 m, which is quite
substantial in the light of the existing vertical structure of water masses in the Gulf
K. Myrberg and T. Soomere
(see Sect. 2.3.4 in Chap. 2) and seiches of the gulf and of the combined Gulf of
Finland–Northern Gotland Basin system additionally complicate the current system.
Although many studies have been devoted to the problem of determining the
mean circulation in the Gulf of Finland, the question is still partially open. The early
analyses (Palmén 1930; Hela 1952) were based on observations from the lightships
of Helsinki and Tallinn. They represent the very surface layer and it is necessary
to eliminate the local drift currents in order to extract the residual (more or less
stationary) flow. The drift current was defined as the flow in a stationary wind and
current system (which very seldom exist). The dependence of the resulting residual
flow on the wind (Hela 1952) indicates that the interactions between different forcing and steering factors are very complex and it is difficult to give an unambiguous
interpretation of the results. Another way to roughly estimate the residual flow is to
calculate the resultant of the currents observed during calm weather (Hela 1952).
The sum of the residual flow and an additional (Ekman) current deflected to the
right of the wind was denoted as the characteristic current in Hela (1952). It was
very slow, 0.6–2.0 cm/s, off Helsinki and somewhat faster, 1.7–4.9 cm/s, off Tallinn.
The best match between the total surface current v c and the wind speed W w was
v c = 0.0137W w . Thus, as a rule of thumb, the characteristic surface current has a
speed of about 1.4 % of the prevailing wind speed and is deflected by 19° to the right
of the wind direction. This deflection angle is naturally not valid near coasts. These
historical results are still of importance for understanding the circulation dynamics
of the Gulf of Finland and have a direct application for the forecast and hindcast of
pollution and contaminants in the surface layer during a large part of the year. Their
outcome is equivalent to that of the studies of net current-driven transport over time
scales larger than the synoptic scale (Soomere et al. 2011a).
The question of a residual flow has been further addressed using a linear regression between the easterly and northerly wind components and the easterly component of the current at a depth of 5 m off Kotka in the northern Gulf of Finland based
on 25 days of observations (Sarkkula 1989, 1991; Pitkänen et al. 1993). The proportionality coefficient of 4.8 cm/s (r 2 = 0.69) was interpreted as the speed of the
residual flow driven by the joint effect of horizontal salinity gradient and bottom
relief (Sarkisyan et al. 1975).
The obtained speed was somewhat larger than in the above estimates for the icefree period. Note that Palmén (1930) used 5 years of observations in which seasonal
differences tend to cancel each other while the more recent results mostly reflect a
single wind event. Another source of the difference stems from the short-term basinwide dynamics of the gulf. The predominant south-western winds pile up the water
at the eastern end of the gulf. If the wind stress vanishes, the westward barotropic
return flow towards an equilibrium stage may simply override the baroclinic effects
and circulation caused by the river water runoff. The linear regression tends to overestimate the residual flow because the wind stress is seldom zero for a long enough
time for the equilibrium stage to be reached. Despite several debatable issues, this
was the first time when the background current was actually identified.
Importantly, the measurements were carried out at a depth of 5 m, which is quite
substantial in the light of the existing vertical structure of water masses in the Gulf