Environmental Engineering Reference
In-Depth Information
Fig. 1.24
Thermocline
(a), halocline (b) and
pycnocline (c) in ocean
compartment
zone where density increases rapidly with depth.
Below the thermocline, temperatures are rela-
tively stable, with only small decreases in tem-
perature towards the ocean bottom. Likewise,
below the halocline, salinities are constant down
to the ocean
produces a vertical overturn. If the water column
has the same density from top to bottom, it is
isopycnal. Such neutral stability means there is
no tendency for water in the column to either
sink or rise. A water column that is neutrally
stable can be easily mixed in the vertical direc-
tion by such forces as wind, wave action or
currents.
Any process that increases the density of
surface water will cause vertical movement of
water or vertical mixing. Vertical mixing is an
important process for the exchange of water from
top to bottom throughout
oor. As a result, water that is
deeper than 1,000 m has relatively constant
density (Fig.
1.24
).
fl
Vertical Mixing
When the density of water increases with depth,
the water column from the surface down is said
to be stable. If the top water in a water column is
denser than the water column below it, then the
water column is unstable. Unstable water column
do not persist, because the denser water at the top
sinks and the less dense water below rises to the
surface. This changeover
s oceans.
Since bottom water usually contains abundant
nutrients from the settling of organic matter from
above and the process of decomposition, vertical
mixing provides a means of exchanging this
nutrient-rich bottom water with oxygen-rich
surface water. As density is controlled by
the world
'
in a water column
