166
E.V. Stanev and X. Lu
(Stanev and Staneva 2001), demonstrated that both the largest heating (reached in
June) and the largest cooling (reached in early winter) amounted to about 200 W/m 2 .
The amplitude of the seasonal thermal buoyancy flux exceeded that of the haline
buoyancy flux by an order of magnitude (Fig. 5.23). However, the net thermal buoyancy flux was about four times smaller than the net haline flux. As buoyancy fluxes
shape the density field (Stanev et al. 2003), we conclude that the annual mean stratification in the Black Sea is dominated by the dilution of surface waters by rivers,
while the seasonal variability is created mostly by air–sea heat exchange.
The specific combination of different forcing mechanisms tends to create a stable salinity stratification, which changes very little with time; however, there is an
extremely high variability in the upper ocean thermal structure. The formation and
character of the CIL is one of the main consequences of the present-day balances of
buoyancy (Fig. 5.23).
The stagnant conditions in the Black Sea are well illustrated by the fact that
temperature signals in the numerical simulations and observations cannot be traced
much deeper than 200 to 400 m. The low rate of vertical mixing (see Sect. 5.3.1.2)
is a unique feature of the Black Sea (and other estuarine seas) and is explained by
the fact that the intensity of vertical mixing in strongly stratified fluids tends to reach
the level of molecular mixing (Stanev 1990; Gregg and Özsoy 1999).
The ideas of Walin (1982) were applied to the Mediterranean Sea by Tziperman
and Speer (1994). Using the same concepts and analysing the evolution of temperature in density coordinates, Stanev et al. (2003) demonstrated in a numerical
study of Black Sea water mass formation that ventilation occured only during certain parts of winter, unlike in the ocean, where seasonal variability was manifested
by the north-south excursions of outcropping isopycnal surfaces during the whole
year. The above considerations become clearer (Stanev et al. 2003) if we remember that in the Black Sea, the ratio of the net heat flux through the ocean surface
to its seasonal amplitude is a small number (∼10 −2 ). Therefore, the ventilation of
the intermediate layer is maintained by the seasonal signal (not by the permanent
structures of the isopycnal surfaces), which pumps cold water periodically into the
CIL (Fig. 5.24a). The ventilation in the Black Sea is confined to a very thin surface layer and is manifested by the outcropping isopycnal surfaces in winter, which
completely submerge in summer as they are overlain by light surface water. Note
also the haline dominated density (Fig. 5.24b) and the up-and-down excursions of
isopycnal surfaces (Fig. 5.24c).
Numerical simulations with the 5-minute-resolution Black Sea MOM helped to
estimate the rates of water mass formation. From the results of numerical simulations one could expect that the area south of the Kerch Strait where the largest
heat fluxes occur in winter would have the largest contribution to the intermediate
water mass formation. It is, however, the area west of the Crimea Peninsula where
most of the cold surface water intrudes the pycnocline (Fig. 5.25). The explanation of this ‘discrepancy’ is that the atmospheric cooling around Kerch is almost
completely compensated by the advection of warm water by the Rim Current, thus
the dynamics reduces the water mass formation rates. Just the opposite situation
is observed in the western Black Sea where the Rim Current transports cold water
E.V. Stanev and X. Lu
(Stanev and Staneva 2001), demonstrated that both the largest heating (reached in
June) and the largest cooling (reached in early winter) amounted to about 200 W/m 2 .
The amplitude of the seasonal thermal buoyancy flux exceeded that of the haline
buoyancy flux by an order of magnitude (Fig. 5.23). However, the net thermal buoyancy flux was about four times smaller than the net haline flux. As buoyancy fluxes
shape the density field (Stanev et al. 2003), we conclude that the annual mean stratification in the Black Sea is dominated by the dilution of surface waters by rivers,
while the seasonal variability is created mostly by air–sea heat exchange.
The specific combination of different forcing mechanisms tends to create a stable salinity stratification, which changes very little with time; however, there is an
extremely high variability in the upper ocean thermal structure. The formation and
character of the CIL is one of the main consequences of the present-day balances of
buoyancy (Fig. 5.23).
The stagnant conditions in the Black Sea are well illustrated by the fact that
temperature signals in the numerical simulations and observations cannot be traced
much deeper than 200 to 400 m. The low rate of vertical mixing (see Sect. 5.3.1.2)
is a unique feature of the Black Sea (and other estuarine seas) and is explained by
the fact that the intensity of vertical mixing in strongly stratified fluids tends to reach
the level of molecular mixing (Stanev 1990; Gregg and Özsoy 1999).
The ideas of Walin (1982) were applied to the Mediterranean Sea by Tziperman
and Speer (1994). Using the same concepts and analysing the evolution of temperature in density coordinates, Stanev et al. (2003) demonstrated in a numerical
study of Black Sea water mass formation that ventilation occured only during certain parts of winter, unlike in the ocean, where seasonal variability was manifested
by the north-south excursions of outcropping isopycnal surfaces during the whole
year. The above considerations become clearer (Stanev et al. 2003) if we remember that in the Black Sea, the ratio of the net heat flux through the ocean surface
to its seasonal amplitude is a small number (∼10 −2 ). Therefore, the ventilation of
the intermediate layer is maintained by the seasonal signal (not by the permanent
structures of the isopycnal surfaces), which pumps cold water periodically into the
CIL (Fig. 5.24a). The ventilation in the Black Sea is confined to a very thin surface layer and is manifested by the outcropping isopycnal surfaces in winter, which
completely submerge in summer as they are overlain by light surface water. Note
also the haline dominated density (Fig. 5.24b) and the up-and-down excursions of
isopycnal surfaces (Fig. 5.24c).
Numerical simulations with the 5-minute-resolution Black Sea MOM helped to
estimate the rates of water mass formation. From the results of numerical simulations one could expect that the area south of the Kerch Strait where the largest
heat fluxes occur in winter would have the largest contribution to the intermediate
water mass formation. It is, however, the area west of the Crimea Peninsula where
most of the cold surface water intrudes the pycnocline (Fig. 5.25). The explanation of this ‘discrepancy’ is that the atmospheric cooling around Kerch is almost
completely compensated by the advection of warm water by the Rim Current, thus
the dynamics reduces the water mass formation rates. Just the opposite situation
is observed in the western Black Sea where the Rim Current transports cold water