2 Topography, Hydrography, Circulation and Modelling of the Baltic Sea
53
2.3.7 Surface Circulation of the Baltic Sea and the Gulf of Finland
First studies of the Baltic Sea circulation were based on observations collected onboard lightships already since the beginning of the 1900s (Witting 1912; Palmén
1930). The measurements were based on quite simple instrumentation (surface
drifters, ‘flow crosses’, deployed and followed from lightships) but they provided
the basic knowledge, and hence already 100 years ago an overall picture of the surface circulation was available for the northern Baltic Sea. These early measurements
showed that the mean circulation in the Baltic Sea main basins (Gotland Sea, Gulf
of Finland, Gulf of Riga, Sea of Bothnia and Bay of Bothnia) is cyclonic (counterclockwise, Fig. 2.9). This feature, although heuristically obvious, has important reflections in the transport processes in the Baltic Sea. The mean circulation transports
salt and heat and therefore the water is warmer and saltier in the eastern sides of the
basins than in the western parts. In the narrow, west-east oriented Gulf of Finland
such a difference induced by mean circulation is observed between the northern and
southern side (Fig. 2.9).
The Baltic Sea does not host permanent, stable current structures such as the Gulf
Stream or the Kuroshio in the Atlantic and Pacific Oceans. Even though the longterm mean currents are weak, average speeds are some 5 cm/s and the persistency of
the circulation system is in some areas relatively strong (Andrejev et al. 2004); see
also Chap. 9 for the discussion of semi-persistent transport patterns). During storms
the wind drift currents can reach 50 cm/s, in straits up to 100 cm/s; on average the
speed of the surface current is 2–3 % of the wind speed and the direction is 20–30 ◦
to the right from the wind direction. This is a realization of the Ekman spiral on the
surface.
Palmén (1930) defined the persistency R of the direction of the mean circulation
as the ratio
R =
||U|
U
.
(2.18)
The vector ||U| is the mean current velocity, while U is the mean current speed.
If the direction is constant the persistency is 100 %. When the persistency reaches
zero also the mean flow and net transport of water are, on average, equal to zero. In
an asymmetric, bimodal coastal current, the persistency equals the difference of the
modes; for instance, if the flow is 75 % to west and 25 % to east with the same speed,
the persistency is 50 %. In practice the persistency of the surface circulation is 20–
40 %, and can be even higher in specific locations such as north of the longitudinal
axis of the Gulf of Finland (Andrejev et al. 2004).
The described general current pattern also becomes visible from the directional
distribution of currents. Figure 2.10 shows the distributions for current velocities
(‘current rose’) in the Sea of Bothnia. Near the coast the currents are parallel to
the shoreline and thus have generally a bimodal structure. In the open sea the steering effect of the coast is negligible, the current system is more isotropic and thus
the persistency of currents is lower. This is partially because the offshore current
field contains many mesoscale vortices that manifest themselves through short-term
fluctuating currents.
53
2.3.7 Surface Circulation of the Baltic Sea and the Gulf of Finland
First studies of the Baltic Sea circulation were based on observations collected onboard lightships already since the beginning of the 1900s (Witting 1912; Palmén
1930). The measurements were based on quite simple instrumentation (surface
drifters, ‘flow crosses’, deployed and followed from lightships) but they provided
the basic knowledge, and hence already 100 years ago an overall picture of the surface circulation was available for the northern Baltic Sea. These early measurements
showed that the mean circulation in the Baltic Sea main basins (Gotland Sea, Gulf
of Finland, Gulf of Riga, Sea of Bothnia and Bay of Bothnia) is cyclonic (counterclockwise, Fig. 2.9). This feature, although heuristically obvious, has important reflections in the transport processes in the Baltic Sea. The mean circulation transports
salt and heat and therefore the water is warmer and saltier in the eastern sides of the
basins than in the western parts. In the narrow, west-east oriented Gulf of Finland
such a difference induced by mean circulation is observed between the northern and
southern side (Fig. 2.9).
The Baltic Sea does not host permanent, stable current structures such as the Gulf
Stream or the Kuroshio in the Atlantic and Pacific Oceans. Even though the longterm mean currents are weak, average speeds are some 5 cm/s and the persistency of
the circulation system is in some areas relatively strong (Andrejev et al. 2004); see
also Chap. 9 for the discussion of semi-persistent transport patterns). During storms
the wind drift currents can reach 50 cm/s, in straits up to 100 cm/s; on average the
speed of the surface current is 2–3 % of the wind speed and the direction is 20–30 ◦
to the right from the wind direction. This is a realization of the Ekman spiral on the
surface.
Palmén (1930) defined the persistency R of the direction of the mean circulation
as the ratio
R =
||U|
U
.
(2.18)
The vector ||U| is the mean current velocity, while U is the mean current speed.
If the direction is constant the persistency is 100 %. When the persistency reaches
zero also the mean flow and net transport of water are, on average, equal to zero. In
an asymmetric, bimodal coastal current, the persistency equals the difference of the
modes; for instance, if the flow is 75 % to west and 25 % to east with the same speed,
the persistency is 50 %. In practice the persistency of the surface circulation is 20–
40 %, and can be even higher in specific locations such as north of the longitudinal
axis of the Gulf of Finland (Andrejev et al. 2004).
The described general current pattern also becomes visible from the directional
distribution of currents. Figure 2.10 shows the distributions for current velocities
(‘current rose’) in the Sea of Bothnia. Near the coast the currents are parallel to
the shoreline and thus have generally a bimodal structure. In the open sea the steering effect of the coast is negligible, the current system is more isotropic and thus
the persistency of currents is lower. This is partially because the offshore current
field contains many mesoscale vortices that manifest themselves through short-term
fluctuating currents.