Etudes et évaluation de processus océaniques par des hiérarchies ...

183
Chapter 3
Physical properties of sea water
tel-00545911, version 1 - 13 Dec 2010
Sea water has many physical properties: temperature, salinity, pressure, density, electric conductivity,
thermal conductivity, viscosity, diffusivity of temperature, diffusivity of salt, compressibility,
thermal expansion, thermal capacity, speed of sound, optical refraction index and
many more. If we like to characterize a probe of sea water we do not have to measure all of
these quantities as they are not all independent. Indeed thermodynamics teaches us that sea
water is described by only three independent variables 1 . That means, if we have measured
three of this properties say temperature, salinity and pressure all the other variables can be
calculated (or looked up in a table) and do not have to be measured. The best known relation
between physical properties is the function that allows to calculate the density of sea water
from temperature, salinity and pressure it is called the equation of state .
ρ = ρ(T,S,P). (3.1)
Density, or more precisely density differences, are of primary importance as they act due to
the earths gravitational force on the dynamics of the ocean and is a primary source of motion
in the ocean. We will thus further investigate the four properties appearing in the equation of
state.
3.1 Salinity
Salinity is the easiest to comprehend, its concentration is given in grams of salt dissolved in one
kilogram of sea water and is measured in practical salinity units (psu). If one kilogram of sea
water contains 34.7 grams of salt, the sea water has a salinity of 34.7 psu. Since the 1980s this
is not exactly true as salinity is determined by the conductivity of the water sample: the mass
of dissolved salt in 1kg of sea water is actually around 1.005g times the salinity, depending on
pressure and temperature. Typical values of salinity in the world ocean range from 33 to 37 psu.
In marginal seas they differ from these typical values as these basins are often shallow and have
higher fresh water fluxes per volume. In the Mediterranean Sea (average depth of 1500m) they
vary between 37 and 39 psu, in the Red Sea (average depth of 490m) they typically measure
between 40 and 42 psu, while in the Baltic Sea (average depth of 55m) they range from 10 to
20 psu. Marginal basins play an important role in the global ocean dynamics due to their role
as “factories” of extreme water mass properties (salinity and temperature).
The sea salt is composed of different sorts of salt, although the salinity varies in the world
ocean the ratio of the different salts is rather constant, an observation called Dittmar’s law,
1 We neglect the influence of: dissolved gases, chemical substances other than salt, variations in the composition
of the sea salt and biology.
9