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K. Myrberg and T. Soomere
6.5 Concluding Remarks
The mesoscale and basin-scale dynamics of the Gulf of Finland appear very complex, not only due to the pronounced variations in forcing functions and energy
budget but also owing to rapidly changing hydrography and complex bathymetry.
As in the entire Baltic Sea, the four fundamental mechanisms to induce currents in
the gulf are: (i) wind stress, (ii) surface pressure gradient, (iii) thermohaline horizontal gradient of density and (iv) tidal forces. Voluminous river runoffs can produce
considerable local changes in the sea level height and drive significant currents. The
resulting currents are steered by the Coriolis acceleration, topography and friction.
Due to the small depth and size of the gulf, bottom friction exerts far stronger impact
on the currents compared to the deep ocean and even many shelf seas.
The general circulation pattern is typical for a stratified system and mimics the
similar pattern in the entire Baltic Sea. Inflowing waters into a basin are placed at
the depth where the ambient water has an equal density: The fresher water goes into
the upper layer and saltier water masses into a certain lower layer. Owing to strong
stratification the fresher waters leave the gulf near the surface whereas the inflow of
more saline water takes place mostly at larger depths.
The motions in the gulf represent many different time scales. It is customary to
associate the motions averaged over longer time intervals (from several months to
years) with a baroclinic, virtually wind-independent circulation. Its presence is intuitively expected due to the positive fresh water budget and resulting large horizontal
gradient of salinity but its identification from measurements and simulations is still
a challenge (Elken et al. 2011). 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 by one
order of magnitude larger than the average ones.
Drift currents produce in coastal areas upwelling and downwelling features that
may be further affected by Kelvin-type waves. The water is laterally mixed by
mesoscale eddies and deep-water circulation. In the time scale from 1 hour to 1
day, there are several periodic dynamical processes. The most important are inertial
oscillations (about 14 hours) partially overlapping with tidal flows (12–13 or 24–26
hours) and seiches (less than 40 hours). The near-bottom layer below the halocline
is governed by advection and mixing and is virtually decoupled from direct wind
forcing.
A major challenge in the modelling of the vertical structure of currents and especially the properties of transport driven by surface currents, performed in the following chapters, is that the circulation may be substantially different in the very surface
layer and in the one below it. The uppermost layer with a thickness of about 2 m is
typically characterized by Ekman-type flow. The resulting average circulation pattern, at least in certain model simulations (Soomere et al. 2011a) is not necessarily
cyclonic. The layer below, at depths starting from about 3 m is apparently much
more strongly involved in the dynamics of the entire water column and expresses
classical features such as a quasi-permanent cyclonic circulation superposed with
mesoscale eddies. Under certain wind conditions the uppermost layer is eventually
‘entrained’ into the motions in the subsurface layer (Gästgifvars et al. 2006).
K. Myrberg and T. Soomere
6.5 Concluding Remarks
The mesoscale and basin-scale dynamics of the Gulf of Finland appear very complex, not only due to the pronounced variations in forcing functions and energy
budget but also owing to rapidly changing hydrography and complex bathymetry.
As in the entire Baltic Sea, the four fundamental mechanisms to induce currents in
the gulf are: (i) wind stress, (ii) surface pressure gradient, (iii) thermohaline horizontal gradient of density and (iv) tidal forces. Voluminous river runoffs can produce
considerable local changes in the sea level height and drive significant currents. The
resulting currents are steered by the Coriolis acceleration, topography and friction.
Due to the small depth and size of the gulf, bottom friction exerts far stronger impact
on the currents compared to the deep ocean and even many shelf seas.
The general circulation pattern is typical for a stratified system and mimics the
similar pattern in the entire Baltic Sea. Inflowing waters into a basin are placed at
the depth where the ambient water has an equal density: The fresher water goes into
the upper layer and saltier water masses into a certain lower layer. Owing to strong
stratification the fresher waters leave the gulf near the surface whereas the inflow of
more saline water takes place mostly at larger depths.
The motions in the gulf represent many different time scales. It is customary to
associate the motions averaged over longer time intervals (from several months to
years) with a baroclinic, virtually wind-independent circulation. Its presence is intuitively expected due to the positive fresh water budget and resulting large horizontal
gradient of salinity but its identification from measurements and simulations is still
a challenge (Elken et al. 2011). 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 by one
order of magnitude larger than the average ones.
Drift currents produce in coastal areas upwelling and downwelling features that
may be further affected by Kelvin-type waves. The water is laterally mixed by
mesoscale eddies and deep-water circulation. In the time scale from 1 hour to 1
day, there are several periodic dynamical processes. The most important are inertial
oscillations (about 14 hours) partially overlapping with tidal flows (12–13 or 24–26
hours) and seiches (less than 40 hours). The near-bottom layer below the halocline
is governed by advection and mixing and is virtually decoupled from direct wind
forcing.
A major challenge in the modelling of the vertical structure of currents and especially the properties of transport driven by surface currents, performed in the following chapters, is that the circulation may be substantially different in the very surface
layer and in the one below it. The uppermost layer with a thickness of about 2 m is
typically characterized by Ekman-type flow. The resulting average circulation pattern, at least in certain model simulations (Soomere et al. 2011a) is not necessarily
cyclonic. The layer below, at depths starting from about 3 m is apparently much
more strongly involved in the dynamics of the entire water column and expresses
classical features such as a quasi-permanent cyclonic circulation superposed with
mesoscale eddies. Under certain wind conditions the uppermost layer is eventually
‘entrained’ into the motions in the subsurface layer (Gästgifvars et al. 2006).