Environmental Engineering Reference
In-Depth Information
and the upper layer outflow is about 610 km
3
y
−
1
which gives 303 km
3
y
−
1
for
the vertically integrated net transport out through the Bosporus [52]. The value
of 303 km
3
y
−
1
reflects the balance between river inflow (350 km
3
y
−
1
) plus
303 km
3
y
−
1
) minus evaporation (350 km
3
y
−
1
) (Table 1).
These inputs result in strong vertical stratification with a fresh, lower density
layer at the surface and a salty higher density layer in the deep water. Full
scale (0-2200 m) salinity, potential temperature and density (sigma-theta) are
shown in Figs. 4a, b, c. These CTD data were obtained during a research
cruise on the R/V Knorr in the center of the western gyre in May 1988 [39].
Salinity increases continuously from low values of about S = 18 at the surface to
deepwater values of over S = 22.3. Density (
σ
θ
), which is controlled primarily
by the salinity, increases similarly. Temperature is seasonally variable at the
surface and decreases with depth to a feature with a temperature minimum
called the cold-intermediate layer (CIL) located at about 50 m. This layer
forms in the winter on both the NW shelf and in the center of the eastern and
western gyres. Its extent of replenishment varies from year to year depending
on the climate [44]. Below the CIL the temperature increases continuously all
the way to the bottom. The values of S, T and density are extremely uniform
in the deep water from about 1750 m to the bottom and form a homogeneous
bottom boundary layer (Fig. 4d, e, f) [39]. This layer appears to be formed due
to bottom heating of the Black Sea by the upward flux of geothermal heat flow
(which destabilizes density) superimposed on the downward increasing salinity
(which stabilizes density).
A temperature-salinity diagram can be used to illustrate the relationships
between the distributions of temperature and salinity. The data from Fig. 4a are
shown as a T-S plot in Fig. 5. The high temperature and low salinity data on the
left are near the surface. Temperature decreases to a minimum of about 7.6
◦
C
in the Cold Intermediate Layer (CIL) and then both salinity and temperature
increase continuously into the deep water.
When the T-S data from the Black Sea is plotted with data from the Bosporus
(which reaches T = 15
◦
C and S = 36) it is apparent that, to a first approximation,
the deep water of the Black Sea forms from linear two-end member mixing
of the Bosporus inflow with the CIL. The magnitude of the Bosporus outflow
averages 313 km
3
y
−
1
but is variable in response to changing local wind con-
ditions. This implies that local synoptic meteorological conditions exert strong
controls on the magnitude of transport. Numerical model results have been used
to estimate short term [45] and longer term, decadal time scale [66] variability.
The net transport varies from 200 to 350 km
3
y
−
1
over the decadal time scale.
The CIL has two sources that are highly variable in intensity depending on
climate. The first is the shallow northwest shelf where the water gets very cold
in the winter [70]. The second site is in the eastern and western gyres where
surface water can become sufficiently cold to rejuvenate the CIL during some
precipitation (
∼
