6 The Gulf of Finland
207
of Finland. The difference between the classical results for the surface flow and
for the currents at a depth of 5 m may be interpreted as an important evidence of
the layered structure of velocity fields in the gulf. Simulations of Andrejev et al.
(2004a) suggest that the uppermost layer with a thickness of 2–3 m may be almost
completely decoupled from the subsurface layer in terms of instantaneous velocities,
persistency and the overall structure of the flow. Although it is very likely that sea
currents are highly variable over the first meters of depth at least for some seasons
(Suursaar 2010), the details of the forecast or hindcast layered structure of currents
still cannot be assumed as true until confirmed by massive in situ experiments. Rapid
development of profiling velocity meters will eventually fill this major gap in our
understanding very soon.
The long-term mean flow towards the west was indirectly determined from the
time difference between the annual minimum of the surface salinity near Utö and the
maximal river runoff (Launiainen 1982). The time lag of 2.5 months implies a mean
flow of a couple of cm/s (Mälkki and Tamsalu 1985). Even if this estimate does
not represent a true assessment of the circulation, it still gives a clue of the scale of
the motion and matches the typical east-west net transport speed in relatively coarse
numerical models (Soomere et al. 2011a).
The separation of the residual circulation from the observed current field is somewhat artificial (and virtually meaningless in contemporary high-resolution simulations of circulation). Its importance mainly becomes evident in long-term estimates
of the drift of substances, but even there the interpretation is not trivial. Leppäranta
and Peltola (1986) compared cases with and without a permanent flow derived from
Palmén (1930) in a random transport model. The probability distribution of the
transport after a one-year period from a starting point at Helsinki had a probability
maximum in the west but the probability density was non-zero in the east. Therefore, eastward net transport does occur in some cases even if the direct impact of
mesoscale features is filtered out. The actual patterns of currents are evidently much
more complex. The wind- and density-driven currents are coupled in a strongly nonlinear manner that becomes evident for example via the formation of eddies.
One of the key questions of the functioning of the Gulf of Finland is how the water exchange occurs between the gulf and the Northern Gotland Basin, with no sill
in between. A proper answer to this question is necessary, for example, to estimate
how long the water remains in the gulf. Early estimates of the exchange volumes and
time scale (Witting 1912; Palmén 1930) were based on rather sparse field measurements, from which short-term variability has been filtered out. They should thus be
treated as ‘educated guesses’ rather than accurate estimates. Straightforward budget
estimates using the classical Knudsen’s formula (Knudsen 1900) resulted in annual
water in- and outflows of 480 and 600 km 3 /yr, respectively (Witting 1912). The corresponding mean current speed ∼1 cm/s is clearly underestimated compared with
more recent observations (Sarkkula 1991; Mikhailov and Tshernyshova 1997). Despite a significant discrepancy, it is remarkable that the excess of water (mainly input
of river water to the Gulf of Finland), around 115 km 3 /yr (Bergström and Carlsson
1994) was recovered correctly.
207
of Finland. The difference between the classical results for the surface flow and
for the currents at a depth of 5 m may be interpreted as an important evidence of
the layered structure of velocity fields in the gulf. Simulations of Andrejev et al.
(2004a) suggest that the uppermost layer with a thickness of 2–3 m may be almost
completely decoupled from the subsurface layer in terms of instantaneous velocities,
persistency and the overall structure of the flow. Although it is very likely that sea
currents are highly variable over the first meters of depth at least for some seasons
(Suursaar 2010), the details of the forecast or hindcast layered structure of currents
still cannot be assumed as true until confirmed by massive in situ experiments. Rapid
development of profiling velocity meters will eventually fill this major gap in our
understanding very soon.
The long-term mean flow towards the west was indirectly determined from the
time difference between the annual minimum of the surface salinity near Utö and the
maximal river runoff (Launiainen 1982). The time lag of 2.5 months implies a mean
flow of a couple of cm/s (Mälkki and Tamsalu 1985). Even if this estimate does
not represent a true assessment of the circulation, it still gives a clue of the scale of
the motion and matches the typical east-west net transport speed in relatively coarse
numerical models (Soomere et al. 2011a).
The separation of the residual circulation from the observed current field is somewhat artificial (and virtually meaningless in contemporary high-resolution simulations of circulation). Its importance mainly becomes evident in long-term estimates
of the drift of substances, but even there the interpretation is not trivial. Leppäranta
and Peltola (1986) compared cases with and without a permanent flow derived from
Palmén (1930) in a random transport model. The probability distribution of the
transport after a one-year period from a starting point at Helsinki had a probability
maximum in the west but the probability density was non-zero in the east. Therefore, eastward net transport does occur in some cases even if the direct impact of
mesoscale features is filtered out. The actual patterns of currents are evidently much
more complex. The wind- and density-driven currents are coupled in a strongly nonlinear manner that becomes evident for example via the formation of eddies.
One of the key questions of the functioning of the Gulf of Finland is how the water exchange occurs between the gulf and the Northern Gotland Basin, with no sill
in between. A proper answer to this question is necessary, for example, to estimate
how long the water remains in the gulf. Early estimates of the exchange volumes and
time scale (Witting 1912; Palmén 1930) were based on rather sparse field measurements, from which short-term variability has been filtered out. They should thus be
treated as ‘educated guesses’ rather than accurate estimates. Straightforward budget
estimates using the classical Knudsen’s formula (Knudsen 1900) resulted in annual
water in- and outflows of 480 and 600 km 3 /yr, respectively (Witting 1912). The corresponding mean current speed ∼1 cm/s is clearly underestimated compared with
more recent observations (Sarkkula 1991; Mikhailov and Tshernyshova 1997). Despite a significant discrepancy, it is remarkable that the excess of water (mainly input
of river water to the Gulf of Finland), around 115 km 3 /yr (Bergström and Carlsson
1994) was recovered correctly.