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

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10 CHAPTER 3. PHYSICAL PROPERTIES OF SEA WATER
named after William Dittmar who, in 1884, analysed the waters collected by the scientific
expedition of the British corvette, HMS Challenger (1872–1876). The major constituents of sea
salt are shown in table 3.1. Small regional variations of the composition of sea salt are however
present in the ocean and will probably to be included in the determination of a futur equation
of state with a higher degree of accuracy.
tel-00545911, version 1 - 13 Dec 2010
Salt percentage
Chloride 54
Sodium 31
Sulfate 8
Magnesium 4
Calcium 1
Potassium 1
others 1
Table 3.1: Major constituents of sea salt
3.2 Temperature and Potential Temperature
Temperature is measured in degrees Celsius ( o C) and temperature differences in Kelvin (K),
oceanographers are however slow in adapting to the SI unit Kelvin to measure temperature
differences.
The temperature of the world ocean typically ranges from −2 o C (−1.87 o C freezing point for
S = 35 at surface) (freezing temperature of sea water) to 32 o C. About 75% of the world ocean
volume has a temperature below 4 o C. Before the opening of the Drake Passage 30 million years
ago due to continental drift, the mean temperature of the world ocean was much higher. The
temperature difference in the equatorial ocean between surface and bottom waters was about
7K compared to the present value of 26K. The temperature in the Mediterranean Sea is above
12 o C even at the bottom and in the Red Sea it is above 20 o C.
If one takes a mass of water at the surface and descends it adiabatically (without exchanging
heat with the environment) its in situ (latin for: in position; the temperature you actually
measure if you put a thermometer in the position) temperature will increase due to the increase
of pressure. Indeed if you take a horizontal tube that is 5km long and filled with water of salinity
S = 35psu and temperature T = 0 o C and put the tube to the vertical then the temperature in
the tube will monotonically increase with depth reaching T = 0.40 o C at the bottom. To get rid
of this temperature increase in measurements oceanographers often use potential temperature
θ (measured in o C) that is the temperature of a the water mass when it is lifted adiabatically
to the sea surface. It is always preferable to use potential temperature, rather than in situ
temperature, as it is a conservative tracer (see section 3.7). Differences between temperature
and potential temperature are small in the ocean < 1.5K, but can be important in the deep
ocean where temperature differences are small.