158
E.V. Stanev and X. Lu
The circulation pattern of the Baltic Sea contains several sub-basin gyres. One
important sub-system is the Gulf of Finland (see Chap. 6 where the absence of sill
makes the hydrodynamic connection between the Northern Gotland Basin and the
Gulf of Finland strong (Andrejev et al. 2004). The dynamics in this area sometimes
reveals counter-estuarine transport forced by winds (Elken et al. 2003, 2006), which
adds to the mixing of water masses in this area. Although as a whole the Baltic Sea
can be considered as a two-layer stratified system, the coupling between surface
and bottom circulation is at times relatively strong in the Gulf of Finland. More
details about the Gulf of Finland and the modelling of its circulation are available
in Chap. 6.
Another interesting sub-basin is the Sea of Bothnia. Its connection with the central Baltic Sea via the Northern Gotland Basin is restricted by the Archipelago Sea,
which leads to the formation of contrasting water masses. There seems to be a similarity between this basin and the Adriatic Sea in the sense that both of them are
separated from the deep part of the sea by sills (with different depths) and that they
have the potential to provide an important source for cold water masses penetrating
into the deeper layers of the main body of the basin. Myrberg and Andrejev (2006)
presented an extensive review of the circulation in the Sea of Bothnia.
5.3.2.2 Black Sea
The circulation in the Black Sea is usually structured in two connected gyre systems
(eastern and western gyres) encompassing the basin. The structure in Fig. 5.18 compares well with the scheme of the Black Sea surface circulation based on a synthesis
of dynamic heights derived from hydrographic observations by Oguz et al. (1993).
The baroclinic Rossby radius R 1 in the Black Sea is about 20 km. Between the Rim
Current and the coast a number of coastal eddies (most of them anticyclonic ones)
exist. The names of these eddies are associated with geographic names, for example, capes and towns. The major sub-basin gyres in the Black Sea are known as the
Batumi and Sevastopol eddies.
Surface velocities in the Black Sea are typically 0.3–0.5 m/s, reaching sometimes 1 m/s in the area of the Rim Current (Staneva et al. 2001). The corresponding
difference between the sea level in the coastal and open sea is about 0.2 m, with
the largest values located along the continental slope. Currents in the deeper layers
reach several cm/s and are essentially barotropic.
The horizontal transport almost doubles in winter, which is the season of more
intense circulation. The transition between summer and winter circulation is controlled by baroclinic eddies and is well revealed by the changing balance between
vorticity in the open ocean and the coastal sea (Stanev and Staneva 2000). The
transition between different circulation states (one-gyre circulation in winter and a
system of multiple sub-basin-scale eddies in summer) is enhanced by the large seasonal stratification cycle above a relatively shallow and strong pycnocline (Staneva
et al. 2001).
E.V. Stanev and X. Lu
The circulation pattern of the Baltic Sea contains several sub-basin gyres. One
important sub-system is the Gulf of Finland (see Chap. 6 where the absence of sill
makes the hydrodynamic connection between the Northern Gotland Basin and the
Gulf of Finland strong (Andrejev et al. 2004). The dynamics in this area sometimes
reveals counter-estuarine transport forced by winds (Elken et al. 2003, 2006), which
adds to the mixing of water masses in this area. Although as a whole the Baltic Sea
can be considered as a two-layer stratified system, the coupling between surface
and bottom circulation is at times relatively strong in the Gulf of Finland. More
details about the Gulf of Finland and the modelling of its circulation are available
in Chap. 6.
Another interesting sub-basin is the Sea of Bothnia. Its connection with the central Baltic Sea via the Northern Gotland Basin is restricted by the Archipelago Sea,
which leads to the formation of contrasting water masses. There seems to be a similarity between this basin and the Adriatic Sea in the sense that both of them are
separated from the deep part of the sea by sills (with different depths) and that they
have the potential to provide an important source for cold water masses penetrating
into the deeper layers of the main body of the basin. Myrberg and Andrejev (2006)
presented an extensive review of the circulation in the Sea of Bothnia.
5.3.2.2 Black Sea
The circulation in the Black Sea is usually structured in two connected gyre systems
(eastern and western gyres) encompassing the basin. The structure in Fig. 5.18 compares well with the scheme of the Black Sea surface circulation based on a synthesis
of dynamic heights derived from hydrographic observations by Oguz et al. (1993).
The baroclinic Rossby radius R 1 in the Black Sea is about 20 km. Between the Rim
Current and the coast a number of coastal eddies (most of them anticyclonic ones)
exist. The names of these eddies are associated with geographic names, for example, capes and towns. The major sub-basin gyres in the Black Sea are known as the
Batumi and Sevastopol eddies.
Surface velocities in the Black Sea are typically 0.3–0.5 m/s, reaching sometimes 1 m/s in the area of the Rim Current (Staneva et al. 2001). The corresponding
difference between the sea level in the coastal and open sea is about 0.2 m, with
the largest values located along the continental slope. Currents in the deeper layers
reach several cm/s and are essentially barotropic.
The horizontal transport almost doubles in winter, which is the season of more
intense circulation. The transition between summer and winter circulation is controlled by baroclinic eddies and is well revealed by the changing balance between
vorticity in the open ocean and the coastal sea (Stanev and Staneva 2000). The
transition between different circulation states (one-gyre circulation in winter and a
system of multiple sub-basin-scale eddies in summer) is enhanced by the large seasonal stratification cycle above a relatively shallow and strong pycnocline (Staneva
et al. 2001).