Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 256

6.4 Deep ocean

6.4.1 General: Oceanic boundary layer

3 Atmospheric pressure gradients . Under intense cyclonic
storms (Section 6.2) these drive strong local currents into
the lowered pressure zone at the storm centers where
mean sea level may be up to several decimeters higher than
that which normal tide-producing forces would produce.
Once in motion three major modifying forces arise:
Coriolis force due to planetary vorticity (Section 3.8).
Lateral boundary friction force due to boundary layer
current shear and momentum flux (Sections 4.5 and 6.2).
Internal waves (Section 4.10) break under critical vertical
gradients in imposed shear thereby creating turbulence.

Below a shallow well-mixed surface layer, the OBL has
strong vertical and lateral gradients of temperature, salin-
ity, velocity, and turbulent kinetic energy. As discussed pre-
viously (Section 6.2.3) the OBL is a locus of active
interaction between the moving ABL above and the main
mass of ocean waters beneath. The occurrence of a rela-
tively slow-moving (in comparison with the ABL) large-
scale surface circulation of the OBL, first recognized and
systematically recorded as a practical exercise by mariners
over hundreds of years, has become well known in detail
over the past 20 years since the advent of precise geodetic
observations from satellites. In particular, it has drawn
attention to the existence of oceanic “weather,” that is,
regional variations in surface flow vectors, defining
unsteady and nonuniform flows, in mesoscale eddies . The
OBL may also have a surprisingly deep signature.
The major forces available to drive OBL circulation are:
1 Lateral gradients in hydrostatic pressure . Newtonian fluids
cannot resist lateral pressure gradients, geostrophic or gradi-
ent flow results, the latter influenced by centrifugal effects as
seen in the flow of water round bends (Section 3.7).
2 Interplanetary gravity forces . Chiefly from the Sun and
the Moon, they cause the production of long-period
oceanic waves responsible for tides. These are generally of
very low surface amplitude in the open oceans but are
considerably amplified on shelves and around coastlines.
We consider these further in a subsequent section.

6.4.2 Gradient currents

We have seen that in a well-mixed surface ocean layer,
pressure gradients, d p/ d s , can be brought about purely by
the action of surface forces, chiefly wind shear . In cases
where temperature and density do not vary laterally the
subsurface isobars are parallel to the sea surface; this is the
barotropic condition (Fig. 6.21). However, the existence of
thermohaline gradients in seawater density caused by dif-
ferential means that the barotropic condition may not
apply. Instead, subsurface isopycnal surfaces are not parallel
to the sea surface and the isobars become increasingly
divergent from the overlying sea surface slope with depth.
This is the baroclinic condition (Fig. 6.22). In the open
oceans, away from frictional influences at solid boundaries
(see below), the resulting flow in both baroclinic and

y

Dynamic
topography

NB
b is true slope, i.e.,
xion is normal to gradient

b x

fu

g tan b

b

Sloping isobars

Sloping isopycnals

b indicates flow into

page in N. Hemisphere

A B
Hydrostatic pressures above B > hydrostatic pressures at all equivalent heights above A , by a constant gradient
given by the water surface slope, tan b . The pressure gradient, d p/ d x is r g tan b per unit volume, or g tan b per unit mass.

The pressure gradient and hence magnitude of the gradient current are equal at all depths

In the absence of friction the horizontal pressure force is balanced by the Coriolis force so that
g tan b = fu
or
u = ( g/f ) tan b

Fig.6.21 Barotropic conditions: isobaric and isopycnal surfaces are parallel in well-mixed water bodies (assuming atmospheric pressure is equal
across line of section).