situation is unlikely in the ocean

situation is unlikely in the ocean because density variations due to temperature
and salinity variations are likely to lead to changes in the isobaric slopes with
depth. The classical assumption is that the circulation is mainly driven from the
surface (by the wind) and the density distribution adjusts to bring the current to
zero at mid-depth. However, it is possible that there may be a slope current in
the deep water where T and S variations are small, plus an additional vertical
variation in the upper 1000 m or so. If we add slopes like those of Fig. 8.9(d) to
those of Fig. 8.9(a) we would have such a situation, and there would not be a
level of no motion at any depth. Observations, though they tend to be indirect,
suggest that deep-water slope currents are at least considerably smaller than
near-surface currents. While the deep currents may have lower speeds they may
transport large amounts of water if they extend over a large depth range (as
indicated in the previous section). Surface currents based on geostrophic
calculations are similar to those of Pilot Charts (obtained independently from
ship navigation data), which is probably the best evidence that the average
speeds of the deep waters are at least small compared with near-surface average
speeds.
* These neutrally buoyant floats are sealed aluminum tubes which are ballasted to sink to and
then float at a predetermined level where they then travel with the water; they have a sound source
so that they can be tracked from a ship or shore station. They are named after their inventor, John
Swallow (1955, see also Baker, 1981)

 
GAMBAR 99

 
In Fig. 8.9, (a), (b) and (c) are examples of "baroclinic" situations while (d) is a
"barotropic" one. These terms will be defined in the next section.
8.7 Relations between isobaric and isopycnal surfaces and
currents
An isobaric surface in a fluid is one on which the hydrostatic pressure is
constant, while an isopycnal surface (sometimes called isosteric) is one on which
the density of the fluid is constant. When the density of a fluid is a function of
pressure only (i.e. p = p (p)), as in fresh water of uniform potential temperature,
the isobaric and isopycnal surfaces are parallel to each other—this is
called a barotropic field of mass. If the density is a function of other parameters
as well and actually varies horizontally with them, the isobaric and isopycnal
surfaces may be inclined to each other—the baroclinic field. This situation
could occur in a freshwater lake where the density was a function of
temperature as well as pressure (p = p (i, p) ) or in the sea where density is a
88 INTRODUCTORY DYNAMICAL OCEANOGRAPHY
function of salinity, temperature and pressure (p = p(s, i, p)). With a barotropic
of mass the water may be stationary but with a baroclinic field, having
horizontal density gradients, such a situation is not possible. In the ocean, the
barotropic case is most common in deep water while the baroclinic case is most
common in the upper 1000 m where most of the faster currents occur.
In the barotropic case the isopycnals will be parallel to the isobars which are
all parallel. The velocity VTC will be zero and the slopes of the isopycnals will be
small and undetectable; for V = 0.1 m s ~1 the slopes are of the order of 10 " 6 at
mid-latitudes, i.e. 0.1 m height change in 100 km. This situation is illustrated in
Fig. 8.10(a) for the stationary case and Fig. 8.10(b) for a uniform flow. Note that