4 Studying the Baltic Sea Circulation with Eulerian Tracers
117
artificial mixing during salt water inflows (Meier et al. 2004), too strong vertical
stratification in the Gulf of Finland (Meier 2007), and numerical noise affecting the
sea surface temperature occasionally (Löptien and Meier 2011). Other Baltic Sea
circulation models have similar shortcomings.
For further details of the RCO model the reader is referred to Meier (2001, 2007),
Meier et al. (1999, 2003). Table 4.1 summarizes the most important characteristics
of the RCO model.
4.3 Eulerian Tracer Methods
We define an Eulerian tracer as a field that obeys a classical advection-diffusion
equation driven with currents of the ocean circulation model. The tracer is said to be
passive if it has no impact on ocean dynamics. Sources, sinks and initial and boundary conditions are specific for the substance under consideration. For instance, in
biogeochemical models dissolved nutrients are treated as passive Eulerian tracers
with riverine nutrient loads as the most important external sources and the internal
transformation to organic material and export, burial in the sediments and denitrification as the most important sinks (e.g., Fennel and Neumann 2004). However,
Eulerian tracers could also be just concentrations of markers of water masses or of
their characteristics, defined by simplified sources, sinks, and initial and boundary
conditions. To analyse ocean dynamics with tracer methods either completely artificial or observed tracers, like colours or radioactive substances, were used in the
literature.
An example is the concept of age distribution and transit time (Bolin and Rodhe
1973) which has been applied, for instance, to Himmerfjärden Estuary (Engqvist
1996), the Gulf of Finland (Andrejev et al. 2004b), the Gulf of Bothnia (Myrberg
and Andrejev 2006), and the entire Baltic Sea (Meier 2005, 2007). In these studies
a passive tracer was added to the model variables to characterize the average age
of sea water in the reservoir with prescribed values at the lateral open boundaries
or at the sea surface. For instance, Meier (2005) found for the period 1903–1998
mean age of the bottom water of one year at Bornholm Deep, five years at Gotland
Deep, and seven years at Landsort Deep. For the whole Baltic Sea a maximum age
of about 11 years appeared in the bottom water at Landsort Deep. These numbers
are important information to characterize the climatological renewal process of the
Baltic Sea deep water which is not possible to derive just from observations (see
Chap. 2).
In the study by Meier (2005) the age of sea water is the time elapsed since a water
particle left the sea surface. If the age of the inflowing water at the source regions
(i.e., the lateral open boundaries towards the Baltic Proper and the river mouths)
is set to zero, Andrejev et al. (2004b) and Myrberg and Andrejev (2006) found
maximum water age of around two years in the Gulf of Finland and of around 7.4
years in the Gulf of Bothnia, respectively. These studies have in common that the age
of sea water is estimated from the equation of the age of pure water (Deleersnijder
et al. 2001). Hence, it is not possible to distinguish between different water masses.
117
artificial mixing during salt water inflows (Meier et al. 2004), too strong vertical
stratification in the Gulf of Finland (Meier 2007), and numerical noise affecting the
sea surface temperature occasionally (Löptien and Meier 2011). Other Baltic Sea
circulation models have similar shortcomings.
For further details of the RCO model the reader is referred to Meier (2001, 2007),
Meier et al. (1999, 2003). Table 4.1 summarizes the most important characteristics
of the RCO model.
4.3 Eulerian Tracer Methods
We define an Eulerian tracer as a field that obeys a classical advection-diffusion
equation driven with currents of the ocean circulation model. The tracer is said to be
passive if it has no impact on ocean dynamics. Sources, sinks and initial and boundary conditions are specific for the substance under consideration. For instance, in
biogeochemical models dissolved nutrients are treated as passive Eulerian tracers
with riverine nutrient loads as the most important external sources and the internal
transformation to organic material and export, burial in the sediments and denitrification as the most important sinks (e.g., Fennel and Neumann 2004). However,
Eulerian tracers could also be just concentrations of markers of water masses or of
their characteristics, defined by simplified sources, sinks, and initial and boundary
conditions. To analyse ocean dynamics with tracer methods either completely artificial or observed tracers, like colours or radioactive substances, were used in the
literature.
An example is the concept of age distribution and transit time (Bolin and Rodhe
1973) which has been applied, for instance, to Himmerfjärden Estuary (Engqvist
1996), the Gulf of Finland (Andrejev et al. 2004b), the Gulf of Bothnia (Myrberg
and Andrejev 2006), and the entire Baltic Sea (Meier 2005, 2007). In these studies
a passive tracer was added to the model variables to characterize the average age
of sea water in the reservoir with prescribed values at the lateral open boundaries
or at the sea surface. For instance, Meier (2005) found for the period 1903–1998
mean age of the bottom water of one year at Bornholm Deep, five years at Gotland
Deep, and seven years at Landsort Deep. For the whole Baltic Sea a maximum age
of about 11 years appeared in the bottom water at Landsort Deep. These numbers
are important information to characterize the climatological renewal process of the
Baltic Sea deep water which is not possible to derive just from observations (see
Chap. 2).
In the study by Meier (2005) the age of sea water is the time elapsed since a water
particle left the sea surface. If the age of the inflowing water at the source regions
(i.e., the lateral open boundaries towards the Baltic Proper and the river mouths)
is set to zero, Andrejev et al. (2004b) and Myrberg and Andrejev (2006) found
maximum water age of around two years in the Gulf of Finland and of around 7.4
years in the Gulf of Bothnia, respectively. These studies have in common that the age
of sea water is estimated from the equation of the age of pure water (Deleersnijder
et al. 2001). Hence, it is not possible to distinguish between different water masses.