Environmental Engineering Reference
In-Depth Information
the density field [37]. All these events appear to be connected with the weather
conditions associated with the North Atlantic Oscillation (NAO) [44].
The decrease of intensity of CIL formation should lead to an increase in
its core temperature and a decrease in its oxygen content. We calculated the
average concentrations of dissolved oxygen in the CIL (for the layer
σ
θ
=
14.45-14.60 kg/m
3
) (Fig. 17e). After 1999-2000 the integrated oxygen content
decreased significantly. Minimum concentrations were observed in 2001-2002.
In 2003-2004 the oxygen content in this layer increased back to values typical
for the early 1990s.
These results give some hints for how the Black Sea may respond to fu-
ture global climate change. Increases in sea surface temperature may lead to
reductions in ventilation of the CIL and lack of replenishment of its oxygen
content. The oxygen inventory in the CIL acts as a buffer for the consumption of
oxygen due to organic matter respiration and controls the boundary condition
for the downward diffusive flux of oxygen into the suboxic zone. Variations
in the oxygen content of the CIL should lead to changes in the structure of
the suboxic layer [25]. Following the distributions of physical and chemical
parameters in the density field in the Black Sea should be an excellent approach
for monitoring variations due to global climate change.
7. CONCLUSIONS AND FUTURE RESEARCH
The suboxic zone is a feature that exists at the boundary between the aerobic
surface layer and sulfidic deep layer of the Black Sea. It is a region where
both oxygen and sulfide have extremely low concentrations with no perceptible
vertical gradients. In most of the Black Sea its upper and lower boundaries are
at reasonably similar density values. In the southwest region of the Black Sea
this structure is disrupted by ventilation by the Bosporus Plume which injects
dissolved oxygen and results in enhanced sulfide oxidation, probably coupled
with Mn oxidation and reduction. A complex interleaving of water layers exists
in that region and there is no resolved suboxic layer.
The processes controlling the origin and variability of the suboxic zone in-
clude:
1. The source of oxygen by ventilation from the surface. Surface ventilation
occurs down to the depth of the cold intermediate layer (CIL) (
σ
θ
= 14.5). This
ventilation occurs in the winter on both the NW shelf and the center of the
western and eastern gyre. The intensity of ventilation is determined by climate
forcing which may be determined by large scale climate patterns like the North
Atlantic Oscillation (NAO). This ventilation sets the upper boundary conditions
for the downward transport of O
2
.
