6 The Gulf of Finland
213
accurately the actual hydrodynamic flow field and the corresponding drift in the
Gulf of Finland.
6.4.5 The Intriguing Entrance to the Gulf of Finland
The multitude of processes at the entrance to the Gulf of Finland certainly makes
their modelling extremely difficult. Internal wave activity, a process scarcely reflected in recent studies (Kurkina et al. 2011; Lilover and Stips 2011), is apparently
high in this area. The production of eddies (Pavelson 2005), topographically controlled currents and fronts is frequent; diapycnal mixing is therefore intense in the
entire entrance region. The anisotropic wave regime in the Baltic, with frequent
occurrence of very high waves in the north-eastern part of the Northern Gotland
Basin (Schmager et al. 2008), apparently also contributes to the forming of specific
features of the vertical structure of water at the entrance to the gulf.
The previous dynamic consideration of the water cycle in the Gulf of Finland
(Alenius et al. 1998) mirrored the sporadic one-way salt water inflow through the
Danish Straits (e.g., Lass and Matthäus 1996; Elken and Matthäus 2008) into the
deeper layers of the Baltic Sea (Chap. 2). Instead, a two-way exchange and even
an almost total export of saline bottom water from the gulf, accompanied by drastic
variations in the halocline’s position, or by its almost complete disappearance, may
take place at the entrance of the gulf (Elken et al. 2003).
Whilst the classical estuarine dynamics is governed mostly by the balance between the fresh water flow and the impact of open sea water masses, the strongly
anisotropic wind forcing (Soomere and Keevallik 2003) plays an important role in
the water exchange and in the formation of the vertical structure of the water masses
here. Whereas the standard estuarine circulation is supported by most of the wind
directions, long-lasting, strong south-western winds push a large amount of fresher
surface water back into the gulf. The excess volume of water increases the hydrostatic pressure in the gulf and may lead to a gradual export of the salt wedge in the
bottom layer of the gulf (Elken et al. 2003) if the wind speed exceeds a mean value
of 4–5.5 m/s.
A major consequence of this kind of reversal of the estuarine transport is a weakening of the stratification of water masses at the entrance of the gulf, accompanied by an intensification of vertical mixing (Elken et al. 2006). Its practical consequences for the functioning of the deep-water ecosystem are not fully understood
yet; for example, it may result in unexpectedly effective supply of oxygen to the
deep layers of the Northern Gotland Basin and thus play a great role in the functioning of the entire Baltic Sea.
Perhaps even more importantly, this finding has led to a substantial revision of the
traditional concept of mostly decoupled lower layer dynamics and reveals that both
the surface and near-bottom layers respond rather actively to wind forcing. While
the layers are mostly decoupled on local scales as discussed above, the coupling
here occurs on the scale of the entire basin. The basin-scale barotropic flows are
213
accurately the actual hydrodynamic flow field and the corresponding drift in the
Gulf of Finland.
6.4.5 The Intriguing Entrance to the Gulf of Finland
The multitude of processes at the entrance to the Gulf of Finland certainly makes
their modelling extremely difficult. Internal wave activity, a process scarcely reflected in recent studies (Kurkina et al. 2011; Lilover and Stips 2011), is apparently
high in this area. The production of eddies (Pavelson 2005), topographically controlled currents and fronts is frequent; diapycnal mixing is therefore intense in the
entire entrance region. The anisotropic wave regime in the Baltic, with frequent
occurrence of very high waves in the north-eastern part of the Northern Gotland
Basin (Schmager et al. 2008), apparently also contributes to the forming of specific
features of the vertical structure of water at the entrance to the gulf.
The previous dynamic consideration of the water cycle in the Gulf of Finland
(Alenius et al. 1998) mirrored the sporadic one-way salt water inflow through the
Danish Straits (e.g., Lass and Matthäus 1996; Elken and Matthäus 2008) into the
deeper layers of the Baltic Sea (Chap. 2). Instead, a two-way exchange and even
an almost total export of saline bottom water from the gulf, accompanied by drastic
variations in the halocline’s position, or by its almost complete disappearance, may
take place at the entrance of the gulf (Elken et al. 2003).
Whilst the classical estuarine dynamics is governed mostly by the balance between the fresh water flow and the impact of open sea water masses, the strongly
anisotropic wind forcing (Soomere and Keevallik 2003) plays an important role in
the water exchange and in the formation of the vertical structure of the water masses
here. Whereas the standard estuarine circulation is supported by most of the wind
directions, long-lasting, strong south-western winds push a large amount of fresher
surface water back into the gulf. The excess volume of water increases the hydrostatic pressure in the gulf and may lead to a gradual export of the salt wedge in the
bottom layer of the gulf (Elken et al. 2003) if the wind speed exceeds a mean value
of 4–5.5 m/s.
A major consequence of this kind of reversal of the estuarine transport is a weakening of the stratification of water masses at the entrance of the gulf, accompanied by an intensification of vertical mixing (Elken et al. 2006). Its practical consequences for the functioning of the deep-water ecosystem are not fully understood
yet; for example, it may result in unexpectedly effective supply of oxygen to the
deep layers of the Northern Gotland Basin and thus play a great role in the functioning of the entire Baltic Sea.
Perhaps even more importantly, this finding has led to a substantial revision of the
traditional concept of mostly decoupled lower layer dynamics and reveals that both
the surface and near-bottom layers respond rather actively to wind forcing. While
the layers are mostly decoupled on local scales as discussed above, the coupling
here occurs on the scale of the entire basin. The basin-scale barotropic flows are