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

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12 CHAPTER 3. PHYSICAL PROPERTIES OF SEA WATER
tel-00545911, version 1 - 13 Dec 2010
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Figure 3.2: θ-S-diagram, potential density 0 dbar lines and σ-values are shown in blue. It
is clearly seen that the mixture of two watermasses (dots) of equal densty, which lies on the
dashed line, is always denser than the initial water masses.
due to the atmospheric pressure p atmos and the product of density ρ and gravitational acceleration
of the overlaying fluid. Please note that also for oceanographers the upward direction
is the positive direction, although they mostly speak of depth, this often leads to confusion.
When using the hydrostatic pressure we neglect the usually small variations of pressure due to
the fluid motion (acceleration of fluid). In the equations of motion (see 5.1 – 5.3), it is not the
pressure, but its gradient that matters, which means, that only changes in pressure but not the
absolute values are of importance to the dynamics. This allows oceanographers to furthermore
neglect the atmospheric pressure and define that at the ocean surface p = 0. Other units of
pressure are bars (1 bar = 10 5 Pa), or decibar (1 dbar = 10 4 Pa) which is roughly the increase
of pressure when the ocean depth increases by 1m.
Attention: pressure is a scalar quantity, that is, has no direction!
3.5 Density and σ
Density is measured in kg/m 3 and typical values for sea water range from 1020 − 1050kg/m 3
the density of sea water is usually given in sigma-values σ T (T,S) = (ρ(T,S,O) − 1000kg/m 3 )
/(1kg/m 3 ), that is a water of ρ(10 o , 35, 0) = 1031.0kg/m 3 has σ(10 o , 35) = 31.0 (no units!).
The σ T (sigma-sub-T) value refers to the density a water mass at temperature T and Salinity
S has at the ocean surface.