2 Topography, Hydrography, Circulation and Modelling of the Baltic Sea
45
determined by the wind forcing, but, however, inertial oscillations (see below) and
long shallow-water waves are at times excited by the internal dynamics of the sea.
The wind-driven transport of surface waters builds up pressure gradients, which
provide forcing to deeper waters. The resulting motion is called wind-generated
secondary circulation.
2.3.3 Dynamics of Surface Currents
There are four mechanisms to induce currents in the Baltic Sea: wind stress at the
sea surface, surface pressure gradient, thermohaline horizontal gradient of density
and tidal forces. Furthermore, the currents are steered by the Coriolis acceleration,
topography and friction, all of which play a relatively large role in the Baltic Sea owing to its location at high latitudes, its shallowness and to its complicated bathymetry
and geometry. Voluminous river runoffs can produce relatively strong local changes
in the sea level height and consequently contribute substantially to the formation of
currents. Due to the small size of the Baltic Sea basins, friction caused by the bottom
and the shores damp the currents remarkably. The general circulation is typical for
a stratified system. Inflowing waters into a basin interleave at the depth where the
ambient water has an equal density. While the fresher water from rivers goes into
the upper layer, the more salty water masses from the North Sea, optionally partly
mixed with the existing waters in the Baltic Sea, go into a certain intermediate or
lower layer.
In the longest time scale—from several months to years—a baroclinic basic circulation appears, independent of short-term variations in the wind properties. This
pattern is mainly driven by the positive fresh water budget and the resulting large
horizontal gradient of salinity. The fresh waters leave the Baltic Sea in the nearsurface layers, whereas the inflow of saline water masses takes place in the lower
layer.
In short time scales (1–10 days) the currents are caused by the wind stress. Due
to the large variability of the winds, the resulting long-term wind-driven mean circulation is weak, and transient currents are larger than the average ones by one order
of magnitude. Drift currents produce in coastal areas upwelling and downwelling
features that are affected by Kelvin-type waves. The water is laterally mixed by
mesoscale eddies and deep-water circulation (see, e.g., Fennel and Sturm 1992; Lass
and Talpsepp 1993; Raudsepp 1998; Stigebrandt et al. 2002; Elken and Matthäus
2008). In the time scale from 1 hour to 1–2 days, there are several periodic dynamical processes. The most important are seiches (less than 40 hours) and inertial
oscillations (13.2–14.5 hours, discussed in the next section). Another phenomenon
of specific interest in the Baltic Sea conditions is the Ekman drift in the upper layer
(Sect. 2.3.5) that is not only responsible for frequent up- and downwellings (Myrberg and Andrejev 2003; Lehmann and Myrberg 2008; Lehmann et al. 2012) but
also for large surface transport.
In conclusion, the observed features of the long-term mean surface circulation
in the Baltic Sea are created by a non-linear combination of the wind-independent
45
determined by the wind forcing, but, however, inertial oscillations (see below) and
long shallow-water waves are at times excited by the internal dynamics of the sea.
The wind-driven transport of surface waters builds up pressure gradients, which
provide forcing to deeper waters. The resulting motion is called wind-generated
secondary circulation.
2.3.3 Dynamics of Surface Currents
There are four mechanisms to induce currents in the Baltic Sea: wind stress at the
sea surface, surface pressure gradient, thermohaline horizontal gradient of density
and tidal forces. Furthermore, the currents are steered by the Coriolis acceleration,
topography and friction, all of which play a relatively large role in the Baltic Sea owing to its location at high latitudes, its shallowness and to its complicated bathymetry
and geometry. Voluminous river runoffs can produce relatively strong local changes
in the sea level height and consequently contribute substantially to the formation of
currents. Due to the small size of the Baltic Sea basins, friction caused by the bottom
and the shores damp the currents remarkably. The general circulation is typical for
a stratified system. Inflowing waters into a basin interleave at the depth where the
ambient water has an equal density. While the fresher water from rivers goes into
the upper layer, the more salty water masses from the North Sea, optionally partly
mixed with the existing waters in the Baltic Sea, go into a certain intermediate or
lower layer.
In the longest time scale—from several months to years—a baroclinic basic circulation appears, independent of short-term variations in the wind properties. This
pattern is mainly driven by the positive fresh water budget and the resulting large
horizontal gradient of salinity. The fresh waters leave the Baltic Sea in the nearsurface layers, whereas the inflow of saline water masses takes place in the lower
layer.
In short time scales (1–10 days) the currents are caused by the wind stress. Due
to the large variability of the winds, the resulting long-term wind-driven mean circulation is weak, and transient currents are larger than the average ones by one order
of magnitude. Drift currents produce in coastal areas upwelling and downwelling
features that are affected by Kelvin-type waves. The water is laterally mixed by
mesoscale eddies and deep-water circulation (see, e.g., Fennel and Sturm 1992; Lass
and Talpsepp 1993; Raudsepp 1998; Stigebrandt et al. 2002; Elken and Matthäus
2008). In the time scale from 1 hour to 1–2 days, there are several periodic dynamical processes. The most important are seiches (less than 40 hours) and inertial
oscillations (13.2–14.5 hours, discussed in the next section). Another phenomenon
of specific interest in the Baltic Sea conditions is the Ekman drift in the upper layer
(Sect. 2.3.5) that is not only responsible for frequent up- and downwellings (Myrberg and Andrejev 2003; Lehmann and Myrberg 2008; Lehmann et al. 2012) but
also for large surface transport.
In conclusion, the observed features of the long-term mean surface circulation
in the Baltic Sea are created by a non-linear combination of the wind-independent