4 Studying the Baltic Sea Circulation with Eulerian Tracers
113
4.2.2.2 Bottom Topography
The Baltic Sea is a semi-enclosed sea consisting of a number of sub-basins that are
connected by narrow and shallow channels (Chap. 2). Hence, there is a large impact
of the bottom topography on currents. In the RCO model the topography by Seifert
et al. (2001) is used which is based upon a gridded data set of digitized charts of the
bathymetry with a horizontal resolution of 1 nautical mile.
4.2.2.3 Initial Conditions and Spin-Up
For the calculation of initial temperature and salinity fields observations are assimilated into available model results from previous experiments. Initial currents and
sea level height are assumed to be zero.
A spin-up integration has to be performed to guarantee that current and density
fields fit dynamically together. Two time scales are important. The first is the socalled advective time scale which describes the time needed to develop density gradients and to adjust currents. This time scale amounts to approximately a couple of
weeks to months. However, such a spin-up would be insufficient because not only
advection affects the horizontal and vertical density distributions. Even diffusion
(and here especially diapycnal mixing) with a much longer time scale contributes to
changes of density gradients. Consequently, a proper spin-up should at least cover
the overturning time scale of the Baltic Sea which amounts to 30 years, approximately (Meier 2002). The latter is the second important time scale which should be
taken into account.
4.2.2.4 Lateral Boundary Conditions
The open boundary conditions are chosen to be located in the northern Kattegat
because available computational resources allow only a limited model domain. In
addition, the water exchange between North Sea and Baltic Sea should be calculated
explicitly. The boundary conditions require water temperature and salinity profiles
in case of inflow. Sensitivity experiments showed that water temperature and salinity in the Baltic Sea domain limited by the Danish Straits depend only on deep water
salinity in the Kattegat (Meier et al. 2003). Boundary conditions for water temperature in the whole water column and salinity of the surface layer impact the Kattegat
hydrography but not the Baltic Sea interior. The reason is that even during major
inflow events the inflowing water volume is smaller than the volume of the Kattegat
surface layer in the model. Hence, usually no water from outside the model domain
is advected through the Danish Straits into the Baltic Sea. As measurements suggest that the variability of the deep water salinity is negligible, sufficient boundary
conditions are built upon climatological mean profiles for water temperature and
salinity in the northern Kattegat.
113
4.2.2.2 Bottom Topography
The Baltic Sea is a semi-enclosed sea consisting of a number of sub-basins that are
connected by narrow and shallow channels (Chap. 2). Hence, there is a large impact
of the bottom topography on currents. In the RCO model the topography by Seifert
et al. (2001) is used which is based upon a gridded data set of digitized charts of the
bathymetry with a horizontal resolution of 1 nautical mile.
4.2.2.3 Initial Conditions and Spin-Up
For the calculation of initial temperature and salinity fields observations are assimilated into available model results from previous experiments. Initial currents and
sea level height are assumed to be zero.
A spin-up integration has to be performed to guarantee that current and density
fields fit dynamically together. Two time scales are important. The first is the socalled advective time scale which describes the time needed to develop density gradients and to adjust currents. This time scale amounts to approximately a couple of
weeks to months. However, such a spin-up would be insufficient because not only
advection affects the horizontal and vertical density distributions. Even diffusion
(and here especially diapycnal mixing) with a much longer time scale contributes to
changes of density gradients. Consequently, a proper spin-up should at least cover
the overturning time scale of the Baltic Sea which amounts to 30 years, approximately (Meier 2002). The latter is the second important time scale which should be
taken into account.
4.2.2.4 Lateral Boundary Conditions
The open boundary conditions are chosen to be located in the northern Kattegat
because available computational resources allow only a limited model domain. In
addition, the water exchange between North Sea and Baltic Sea should be calculated
explicitly. The boundary conditions require water temperature and salinity profiles
in case of inflow. Sensitivity experiments showed that water temperature and salinity in the Baltic Sea domain limited by the Danish Straits depend only on deep water
salinity in the Kattegat (Meier et al. 2003). Boundary conditions for water temperature in the whole water column and salinity of the surface layer impact the Kattegat
hydrography but not the Baltic Sea interior. The reason is that even during major
inflow events the inflowing water volume is smaller than the volume of the Kattegat
surface layer in the model. Hence, usually no water from outside the model domain
is advected through the Danish Straits into the Baltic Sea. As measurements suggest that the variability of the deep water salinity is negligible, sufficient boundary
conditions are built upon climatological mean profiles for water temperature and
salinity in the northern Kattegat.