Geoscience Reference
In-Depth Information
Salinity was originally defined as a mass ratio equivalent to the number of grams of
salt in 1 kg of seawater. It is currently defined in terms of a near-equivalent ratio
of the electrical conductivities of seawater and a standard solution of potassium
chloride (KCl). Its value has to be inferred from the electrical conductivity (C) which
varies with both salinity (S) and temperature (T) and to a lesser extent with pressure
(p). Given C, T and p from measurement, S can be determined from accurately
known polynomial functions S
S(C, T, p). It is worth noting here that salinity,
defined by a conductivity ratio, does not have any units. We should specify that we
are using the practical salinity scale (referred to as PSS) when quoting such salinities.
There is a tendency in the literature to use the practical salinity unit (psu), which is not
strictly correct. Density, which varies with salinity, temperature and pressure, is then
recovered from the equation of state r
¼
r(S, T, p) which is another complicated set
of polynomial functions. Since the density of seawater varies by only a few percent
over the whole ocean, the relatively small differences involved which are of great
importance in relation to ocean dynamics are often represented in terms of the
parameter s
STp
which is defined as:
¼
kg m
3
STp
¼ðð
S
;
T
;
p
Þ
1000
Þ½
:
ð
1
:
1
Þ
For many purposes, in comparing the densities of water particles, it is sufficient to
deal in terms of their density at the surface s
ST0
which is usually abbreviated to s
T
.
A good quality CTD will be able to determine temperature with an accuracy of better
than
0.005
C in temperature and
0.01 in salinity. The pressure is also used to
determine the depth of the CTD package from the relation z
∼
∼
the mean
density of the water column above the CTD. There are several internet sources where
you can find the polynomials used to calculate salinity and density, all using the
algorithms in the UNESCO technical paper (Fofonoff and Millard,
1983
). Recently
there have been some important developments towards a thermodynamic equation
of state for seawater (Millero, 2010).
The basic CTD sensor suite is normally attached to a frame along with a set
of water bottles for taking water samples at selected depths (e.g. see
Fig. 1.4
). The
bottles are arranged around the CTD frame in a 'rosette' configuration and can be
triggered by electrical signals sent down the cable from the controlling computer on the
research vessel. This arrangement is vital in physical oceanography to provide samples
for salinity analyses and subsequent calibration of the CTD salinity. The rosette bottles
are also necessary for a lot of interdisciplinary work as water samples are still needed
for laboratory analyses and experiments. As well as vertical profiling from a stationary
vessel, CTD sensor packages can also be carried by a towed undulating vehicle, two
examples of which are shown in
Fig. 1.5
. These vehicles are able to porpoise up and
down in a sawtooth pattern at speeds up to 8 knots (
¼
p
=^
g, with
^
4ms
1
) behind a research
vessel. This mode of operation allows for rapid sampling of the horizontal and
vertical structure of the water column and is particularly useful in regions of high
gradient which occur in oceanographic fronts. Oscillating to a depth of 100 metres,
a horizontal resolution of
∼
500 metres can be achieved.
∼
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