Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 263

zones of the mid-Atlantic Ridge, with intense turbulent
mixing along the upper interface. Tracer studies at the
interface of other shallow water masses reveal a low value
of the mixing rate, about 10 5 m 2 s 1 . This implies a low
rate of turbulent mixing along density interfaces relative to
lateral spread, a conclusion also established by turbulent
stress calculations. However, it is likely that other mixing
mechanisms exist, for example breaking internal waves
generated during ocean tides, which will lead to much
larger turbulent dissipation.
A feature of deep ocean waters is attributed in part to
the action of thermohaline currents and in part to the
occurrence of deep-sea storms (see discussion in
Section 6.4.5). This is the phenomenon of increased sus-
pended material, revealed by light-scattering techniques
(Fig. 6.32). The source of the suspended sediment in these
bottom nepheloid layers is variable: distant sourcing from

polar regions, local erosional resuspension of ocean-floor
muds by “storms” and enhanced thermohaline currents,
windblown dust, and dilute distal turbidity current flows
probably all have a role. Some nepheloid layers may be up
to 2 km thick, although 100-200 m is a more usual figure.
Sediment in nepheloid layers is usually 2 m in size

although fine silt up to 12 m may be suspended, nor-

mally at concentrations of up to 500 mg l 1 rising to
5000 mg l 1 a few meters off the bottom during deep-sea
“storms.” Nepheloid layers are also known in many areas
from intermediate depths, often at the junction between
different water masses. These are thought to arise through
the erosion of bottom sediments by internal waves
(Section 4.13) and tides, amplified on certain critical bot-
tom slopes. The layers, once formed, intrude laterally into
the adjacent open ocean as layers many tens of meters
thick.

6.5 Shallow ocean

Shallow ( 200 m depth) ocean dynamics (Fig. 6.33) are

more complicated than the open ocean both because of the
effects of the shallow water on wave and tide and proximity
to land. A generalized physical description of the shelf
boundary layer (Fig. 6.34) defines an inner shelf mixed
layer where frictional effects of wave and tide are dominant
in the less than 60 m shallow waters. In the deepening
mid- to outer shelf there is differentiation into surface and
bottom boundary layers separated by a “core” zone. The
shallow water enables waves to directly influence the

bottom and for the longer-period tidal wave to amplify as it
is forced shelfward from the open ocean. Proximity to land
causes interactions of wave and tide with effluent plumes
sourced from river estuaries and delta distributaries
(Fig. 6.35). Coastal geometry also has a strong local influ-
ence upon water dynamics. Shelves have been classified into
tide- and weather-dominated, but most shelves show a
mixture of processes over both time and space. The major-
ity of shelves have a tidal range less than 2 m but this may
be amplified several times around their margins.

Intruding ocean
currents

Tidal
currents

Meteorological
currents

Density
currents

reversing, standing waves or rotary boundary (Kelvin) waves

Cyclic
components

Residual
components

Longshore
and rip currents

Direct wind
shear

Wind
drift

Wind
setup

Landward
bottom
currents

Shelf
riverine
jets and plumes

Shelf
riverine
underflows

Internal
waves

Fig. 6.33 Components of the shelf current velocity field.