Chapter 2
Topography, Hydrography, Circulation
and Modelling of the Baltic Sea
Kai Myrberg and Andreas Lehmann
Abstract The hydrography and dynamics of the Baltic Sea, although ruled by the
same principles and forcing factors as any part of the World Ocean, contain several
distinguishing features. Apart from the complicated geometry and bathymetry of
the basin, two major factors contribute to the complexity of the processes here. The
interplay between inflowing saline, dense waters from the North Sea in the bottom
layer with the excess of light, and fresh riverine waters coming into the system in
the upper layer leads to the formation of a permanent two-layer structure of density
separated by a sharp jump layer (halocline). Due to the layered structure, the direct
atmospheric forcing is restricted to the upper layer with a typical thickness of 40–
80 m, while in the bottom layer advection and mixing processes govern the patterns
of the hydrographic fields. On the top of the upper layer, a well-mixed surface layer,
with a typical thickness of 15–20 m, is formed due to summer-time heating, whereas
at the bottom of this layer a rather sharp jump layer of temperature (thermocline)
exists. During autumn the vertical temperature gradient vanishes due to thermal
convection and turbulent mixing. There are four mechanisms which induce currents
in the Baltic Sea: the wind stress at the sea surface, the surface pressure gradient,
the thermohaline horizontal gradient of density and the tidal forces. The currents are
steered furthermore by the Coriolis acceleration, topography and friction, forming
a general (cyclonic) circulation in this stratified system with positive fresh water
budget. Due to the shallowness of the Baltic Sea, bottom friction damps the currents
remarkably. Voluminous river runoffs can produce local changes in the sea level
height and consequently also in currents. Inflowing waters penetrate at depths where
the density of the ambient water matches the inflowing water masses. Due to the
K. Myrberg (B)
Finnish Environment Institute, Marine Research Centre, Mechelininkatu 34 a, P.O. Box 140,
00251, Helsinki, Finland
e-mail: kai.myrberg@ymparisto.fi
K. Myrberg
Department of Geophysics, Klaip˙ eda University, Herkaus Manto Str. 84, Klaip˙ eda, Lithuania
A. Lehmann
Helmholtz Centre for Ocean Research Kiel (GEOMAR), Düsternbooker Weg 20, Kiel, Germany
e-mail: alehmann@geomar.de
T. Soomere, E. Quak (eds.), Preventive Methods for Coastal Protection,
DOI 10.1007/978-3-319-00440-2_2,
© Springer International Publishing Switzerland 2013
31
Topography, Hydrography, Circulation
and Modelling of the Baltic Sea
Kai Myrberg and Andreas Lehmann
Abstract The hydrography and dynamics of the Baltic Sea, although ruled by the
same principles and forcing factors as any part of the World Ocean, contain several
distinguishing features. Apart from the complicated geometry and bathymetry of
the basin, two major factors contribute to the complexity of the processes here. The
interplay between inflowing saline, dense waters from the North Sea in the bottom
layer with the excess of light, and fresh riverine waters coming into the system in
the upper layer leads to the formation of a permanent two-layer structure of density
separated by a sharp jump layer (halocline). Due to the layered structure, the direct
atmospheric forcing is restricted to the upper layer with a typical thickness of 40–
80 m, while in the bottom layer advection and mixing processes govern the patterns
of the hydrographic fields. On the top of the upper layer, a well-mixed surface layer,
with a typical thickness of 15–20 m, is formed due to summer-time heating, whereas
at the bottom of this layer a rather sharp jump layer of temperature (thermocline)
exists. During autumn the vertical temperature gradient vanishes due to thermal
convection and turbulent mixing. There are four mechanisms which induce currents
in the Baltic Sea: the wind stress at the sea surface, the surface pressure gradient,
the thermohaline horizontal gradient of density and the tidal forces. The currents are
steered furthermore by the Coriolis acceleration, topography and friction, forming
a general (cyclonic) circulation in this stratified system with positive fresh water
budget. Due to the shallowness of the Baltic Sea, bottom friction damps the currents
remarkably. Voluminous river runoffs can produce local changes in the sea level
height and consequently also in currents. Inflowing waters penetrate at depths where
the density of the ambient water matches the inflowing water masses. Due to the
K. Myrberg (B)
Finnish Environment Institute, Marine Research Centre, Mechelininkatu 34 a, P.O. Box 140,
00251, Helsinki, Finland
e-mail: kai.myrberg@ymparisto.fi
K. Myrberg
Department of Geophysics, Klaip˙ eda University, Herkaus Manto Str. 84, Klaip˙ eda, Lithuania
A. Lehmann
Helmholtz Centre for Ocean Research Kiel (GEOMAR), Düsternbooker Weg 20, Kiel, Germany
e-mail: alehmann@geomar.de
T. Soomere, E. Quak (eds.), Preventive Methods for Coastal Protection,
DOI 10.1007/978-3-319-00440-2_2,
© Springer International Publishing Switzerland 2013
31