208
K. Myrberg and T. Soomere
6.4.3 Complexity of Observed Motion Patterns
Detailed investigations of the spatial and temporal variability of the currents during
the last decades have considerably improved our understanding of the circulation in
the Gulf of Finland (Alenius et al. 1998). Although there is a widely spread opinion
that tidal flows and especially seiches (Jönsson et al. 2008) are relatively weak in
the entire gulf, remarkable seiche-driven currents with periods of 31, 24, 19.5, 16
and 11 hours have been identified in the gulf from in situ measurements near the
island of Naissaar (Lilover et al. 2011). The longest seiche periods correspond, albeit not perfectly, to the well-known Baltic Sea–Gulf of Finland oscillations, with a
theoretically estimated period in the range of 27.5–31 hours (Neumann 1941; Wübber and Krauss 1979). The correlation between winds and corresponding currents
varied between 0.69 and 0.9 as in the earlier estimates (Hela 1952; Sarkkula 1991).
The current was rotated 35° to the right from the wind (Lilover et al. 2011), that is,
slightly more than in the classical estimates (Alenius et al. 1998; Elken et al. 2011).
The average speed of tidal flows was close to 10 cm/s on certain occasions (Lilover
2012).
Measurements using moored current meters (Talpsepp 1986; Talpsepp et al.
1994) revealed that current speeds were generally higher near the Estonian coast
than on the Finnish side. Strong currents (about 20–30 cm/s) in the Estonian
nearshore can sometimes even be called coastal jets. The long-term mean current
speed off Helsinki was only about 10 cm/s. The current variability was reported to
be conspicuous for its 18–20 day periods (Talpsepp et al. 1994). Unidirectional currents prevailed over several days and then changed direction (Laakkonen et al. 1981;
Talpsepp et al. 1994).
Mean surface current speeds were low in summer (3–5 cm/s) and in autumn
(4–6 cm/s) off the Hanko peninsula (in the Tvärminne area) (Laakkonen et al. 1981;
Haapala et al. 1990) and near Kotka (Alenius 1986). The mean current direction was
westwards at the northern coast, supporting the cyclonic circulation idea. However,
strong westerly winds turned the currents to the east near the Finnish coast even
for several weeks at a time. Somewhat counter-intuitively, the largest instantaneous
flow speeds were recorded in summer, apparently because the energy of wind-driven
motions is concentrated in the relatively thin upper mixed layer above the thermocline. Eddy-like short-lived structures that affect the circulation in the near-bottom
layer were observed in a number of studies (Mälkki and Talpsepp 1988).
Measurements carried out by the Finnish Institute of Marine Research in summer
and autumn 1994–1995 (unpublished data) at five stations across the entrance region
of the Gulf of Finland showed that the flow direction varied across the gulf as expected for a cyclonic circulation. The typical surface-layer velocity was 10–20 cm/s.
Simultaneous Acoustic Doppler Current Profiler (ADCP) data (Stipa 2004) revealed
more clearly the regions of oppositely flowing currents separated by about 10 km.
Because the flow remained quite steady for several days, the ADCP data across the
gulf can be considered as representative of that situation.
On the Finnish side the mean velocity was 5–9 cm/s westwards during the summer and the persistency of the current was high (near to or above 50 %). In the
K. Myrberg and T. Soomere
6.4.3 Complexity of Observed Motion Patterns
Detailed investigations of the spatial and temporal variability of the currents during
the last decades have considerably improved our understanding of the circulation in
the Gulf of Finland (Alenius et al. 1998). Although there is a widely spread opinion
that tidal flows and especially seiches (Jönsson et al. 2008) are relatively weak in
the entire gulf, remarkable seiche-driven currents with periods of 31, 24, 19.5, 16
and 11 hours have been identified in the gulf from in situ measurements near the
island of Naissaar (Lilover et al. 2011). The longest seiche periods correspond, albeit not perfectly, to the well-known Baltic Sea–Gulf of Finland oscillations, with a
theoretically estimated period in the range of 27.5–31 hours (Neumann 1941; Wübber and Krauss 1979). The correlation between winds and corresponding currents
varied between 0.69 and 0.9 as in the earlier estimates (Hela 1952; Sarkkula 1991).
The current was rotated 35° to the right from the wind (Lilover et al. 2011), that is,
slightly more than in the classical estimates (Alenius et al. 1998; Elken et al. 2011).
The average speed of tidal flows was close to 10 cm/s on certain occasions (Lilover
2012).
Measurements using moored current meters (Talpsepp 1986; Talpsepp et al.
1994) revealed that current speeds were generally higher near the Estonian coast
than on the Finnish side. Strong currents (about 20–30 cm/s) in the Estonian
nearshore can sometimes even be called coastal jets. The long-term mean current
speed off Helsinki was only about 10 cm/s. The current variability was reported to
be conspicuous for its 18–20 day periods (Talpsepp et al. 1994). Unidirectional currents prevailed over several days and then changed direction (Laakkonen et al. 1981;
Talpsepp et al. 1994).
Mean surface current speeds were low in summer (3–5 cm/s) and in autumn
(4–6 cm/s) off the Hanko peninsula (in the Tvärminne area) (Laakkonen et al. 1981;
Haapala et al. 1990) and near Kotka (Alenius 1986). The mean current direction was
westwards at the northern coast, supporting the cyclonic circulation idea. However,
strong westerly winds turned the currents to the east near the Finnish coast even
for several weeks at a time. Somewhat counter-intuitively, the largest instantaneous
flow speeds were recorded in summer, apparently because the energy of wind-driven
motions is concentrated in the relatively thin upper mixed layer above the thermocline. Eddy-like short-lived structures that affect the circulation in the near-bottom
layer were observed in a number of studies (Mälkki and Talpsepp 1988).
Measurements carried out by the Finnish Institute of Marine Research in summer
and autumn 1994–1995 (unpublished data) at five stations across the entrance region
of the Gulf of Finland showed that the flow direction varied across the gulf as expected for a cyclonic circulation. The typical surface-layer velocity was 10–20 cm/s.
Simultaneous Acoustic Doppler Current Profiler (ADCP) data (Stipa 2004) revealed
more clearly the regions of oppositely flowing currents separated by about 10 km.
Because the flow remained quite steady for several days, the ADCP data across the
gulf can be considered as representative of that situation.
On the Finnish side the mean velocity was 5–9 cm/s westwards during the summer and the persistency of the current was high (near to or above 50 %). In the