144
E.V. Stanev and X. Lu
(2005) that the average age of water masses is most sensitive to perturbations of the
wind speed (decreased wind speed causes significantly increased age of the deep
water).
Gustafsson (2000) quantified the residence times of the bottom water to be about
15 days in the northern and central Kattegat and 34 days in the southern Kattegat.
Analysis of the distribution of age tracers demonstrated that the ventilation of bottom water in the Great Belt significantly influenced oxygen conditions in the southern and western Baltic Sea (Bendtsen et al. 2009). In contrast, the central Kattegat
was primarily ventilated by advection of bottom water from the Skagerrak. An age
tracer representing the ventilation rate of bottom water with either Skagerrak water
or surface water was shown to be inversely correlated to the observed oxygen distribution in the region. Recently, Stedmon et al. (2010) claimed that exchange between
the North and Baltic Sea is so important that water from the German Bight was detectable at salinities down to 12 in the Kattegat and Belt Sea. On average, 23 %
of the coloured dissolved organic matter (CDOM) in bottom waters of the Kattegat, Great Belt, Belt Sea, Arkona Basin (see Fig. 2.2 in Chap. 2) and the Øresund
originated from the German Bight.
The theory of rotating hydraulics appeared useful to understand the dynamics
of topographically constrained currents. To describe rotating hydraulics of outflow
from a wide and shallow basin into a channel, the circulation in the upstream basin
has to be known (Gill 1977; Borenäs and Lundberg 1986). However, the Baltic subbasins are relatively shallow and wide therefore the characteristics of the upstream
basin only are not sufficient to fully describe the dominating processes (Laanearu
and Lundberg 2003). Furthermore, Borenäs et al. (2007) came to the conclusion
that a hydraulic framework, although providing an upper bound of the transport,
is of limited use when dealing with the Stolpe Channel (see Fig. 2.2 in Chap. 2)
overflow because it was more likely that the transport is governed by the combined
effects of friction and wind forcing. Therefore more detailed numerical modelling
is needed to understand the processes in question (Meier et al. 2006).
The first important step in the Baltic Sea oceanography demonstrating the power
of numerical models to provide knowledge on the water exchange through the straits
was done by Sayin and Krauss (1996). One of the important questions they asked
was the following: provided that the cross-sectional areas of the Great Belt and
Øresund were 0.255 and 0.08 km 2 , respectively, do total through-flows obey the
rate 25.5 : 8? With the help of academic and realistic numerical simulations they
proved that this rate was strongly controlled by dynamics. The proportionality between flows and cross-sectional areas was particularly clear during outflow cases
characterized by very strong density gradients. However, Sayin and Krauss (1996)
were not able to establish the dependence of rates of transport on the variability
of wind conditions. Therefore the issue of how wind fluctuations control inflowoutflow regimes presents a challenge for further research.
Another fundamental issue addressed by Sayin and Krauss (1996) was the control of the Arkona Basin circulation on the propagation of inflowing water into the
Gotland Basin. Their results clearly demonstrated that hydrodynamics can strongly
impact the ventilation of the deep sea, an issue which has also been well understood
E.V. Stanev and X. Lu
(2005) that the average age of water masses is most sensitive to perturbations of the
wind speed (decreased wind speed causes significantly increased age of the deep
water).
Gustafsson (2000) quantified the residence times of the bottom water to be about
15 days in the northern and central Kattegat and 34 days in the southern Kattegat.
Analysis of the distribution of age tracers demonstrated that the ventilation of bottom water in the Great Belt significantly influenced oxygen conditions in the southern and western Baltic Sea (Bendtsen et al. 2009). In contrast, the central Kattegat
was primarily ventilated by advection of bottom water from the Skagerrak. An age
tracer representing the ventilation rate of bottom water with either Skagerrak water
or surface water was shown to be inversely correlated to the observed oxygen distribution in the region. Recently, Stedmon et al. (2010) claimed that exchange between
the North and Baltic Sea is so important that water from the German Bight was detectable at salinities down to 12 in the Kattegat and Belt Sea. On average, 23 %
of the coloured dissolved organic matter (CDOM) in bottom waters of the Kattegat, Great Belt, Belt Sea, Arkona Basin (see Fig. 2.2 in Chap. 2) and the Øresund
originated from the German Bight.
The theory of rotating hydraulics appeared useful to understand the dynamics
of topographically constrained currents. To describe rotating hydraulics of outflow
from a wide and shallow basin into a channel, the circulation in the upstream basin
has to be known (Gill 1977; Borenäs and Lundberg 1986). However, the Baltic subbasins are relatively shallow and wide therefore the characteristics of the upstream
basin only are not sufficient to fully describe the dominating processes (Laanearu
and Lundberg 2003). Furthermore, Borenäs et al. (2007) came to the conclusion
that a hydraulic framework, although providing an upper bound of the transport,
is of limited use when dealing with the Stolpe Channel (see Fig. 2.2 in Chap. 2)
overflow because it was more likely that the transport is governed by the combined
effects of friction and wind forcing. Therefore more detailed numerical modelling
is needed to understand the processes in question (Meier et al. 2006).
The first important step in the Baltic Sea oceanography demonstrating the power
of numerical models to provide knowledge on the water exchange through the straits
was done by Sayin and Krauss (1996). One of the important questions they asked
was the following: provided that the cross-sectional areas of the Great Belt and
Øresund were 0.255 and 0.08 km 2 , respectively, do total through-flows obey the
rate 25.5 : 8? With the help of academic and realistic numerical simulations they
proved that this rate was strongly controlled by dynamics. The proportionality between flows and cross-sectional areas was particularly clear during outflow cases
characterized by very strong density gradients. However, Sayin and Krauss (1996)
were not able to establish the dependence of rates of transport on the variability
of wind conditions. Therefore the issue of how wind fluctuations control inflowoutflow regimes presents a challenge for further research.
Another fundamental issue addressed by Sayin and Krauss (1996) was the control of the Arkona Basin circulation on the propagation of inflowing water into the
Gotland Basin. Their results clearly demonstrated that hydrodynamics can strongly
impact the ventilation of the deep sea, an issue which has also been well understood