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Effect of temperature and salinity on seawater density

Evolution of the maximum density and the freezing point with salinity

With increasing salinity a cross-over is observed between the lines of maximum density and freezing point at 24.695 salinity and -1.33°C.Consequences are:1/ For low Sal. waters (lakes; Baltic Sea; ..), decreasing temperatures will bringsurface water to reach first its state of maximum density; this water will sink to the bottom. Further cooling then lightens the topmost water which does notsink. Eventually ice forms at the surface.

2/ For high Sal waters (S > 25), decreasing temperatures induce convection which continues without the state of maximum density being reached. Thetemperature decreases till the whole water column is at the freezingtemperature. However, freezing of the whole water column can occur only in shallower water.

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Seawater density as a function of temperature and salinity

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Characteristic seasonal evolution of the temperature gradient atweather station « P » in the N.E. Pacific (50°N – 145°W)

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Temperature distribution in the ocean

Density of seawater increases with decreasing temperature, so we expectcolder water below the surface (not necessarily so in polar waters!). However surface to deep temperature transition is not gradual.

Wind stress on the sea surface induces a surface mixed layer about 50 to 200m deep. Below the mixed layer the temperature usually drops sharplyover a short depth interval, i.e. the region of the seasonal thermocline. Below the seasonal thermocline temperature decreases again in a more gradual way.

In low and middle latitudes there is a year round thermocline between 200 and 2000m, called the permanent thermocline

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SalinityCl- Na+ K+ Mg2+ Ca2+ SO4

2- together represent 99.8% of the mass ofsolutes dissolved in seawater. Na+ and Cl- alone represent 86% !

Theoretically, salinity = mass of dissolved salt ions present in 1 Kg ofseawater; 99% of all seawater has a salinity ranging between 33 and 37.

Measuring salinityOriginally salinity was calculated from the Cl- content (i.e. chlorinity), keepingin mind that the major ions stand in constant proportions to each other. Chlorinity represents the mass in g of halides that can be precipitated from1000 g of seawater by Ag+ using a standard AgNO3 solution

3 Ag+ + Cl- + Br+ + I- → AgCl + AgBr + AgI

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Chlorinity = [ Cl atomic weight x moles of Ag consumed ]1000 g seawater

Salinity relates to Chlorinity as follows: Sal = 1.80655 x Chlorinity

From 1979 onward this mode of assessing salinity was replaced by thePractical Salinity, which is based on the relationships between conductivity, chlorinity, salinity and density of seawater, and is obtained from a conductivitymeasurement. Salinity is subsequently deduced using the Practical SalinityScale.

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The Practical Salinity Scale is based on a standard seawater having at 15°C and atmospheric pressure, an electrical conductivity equal to that of a standard KCl solution containing exactly 32.4357 g of KCl per Kg solution.

The standard seawater comes from the North Atlantic and is known as ‘Copenhagen’ water. Its chlorinity equals 19.374 x 10-3 and it has received a practical salinity value equal by definition to 35 ‰. Seawater at a practicalsalinity of 35 has a conductivity of 4.29 Siemens m-1 at 15°C or 2.904 S m-1 at0°C.

The practical salinity of a seawater sample is defined in terms of the K15 or R15

factor, the ratio of the sample’s electrical conductivity at 15°C, 1 atm pressure to the conductivity of the Copenhagen water under the same conditions oftemperature and pressure. We use K15 if a KCl solution is used as the referenceand R15 if the reference is Copenhagen water.

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Electric conductivity (mS / cm) as a function of temperature and salinity atatmospheric pressure

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Salinity is obtained from the following relationship:

S = a0 + a1 K151/2 + a2K15 + a3K15

3/2 + a4K152 + a5K15

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∑Ai = 35.0000; for S = 35.0000 it follows that K15 = 1values for a coefficients are reported in the literature (The PracticalSalinity Scale and the International Equation of State of Seawater, UNESCO technical paper N°36, 1981)

In practice there are two ways to measure conductivity:1/ with laboratory salinometer: Rt = C (S, T, 0) / C (35, T, 0)2/ with in-situ conductivity meter (CTD): R = C (S, T, P) / C (35, 15, 0)with R = RP . RT . rT where RP = C (S, T, P) / C (S, T, 0) to account for P effect, and rT = C (35, T, 0) / C (35, 15, 0) to account for T effect for thereference water.Once RP and rT are known, RT and thus S can be calculated from in-situ results:RT = R / (RP . rT)

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Yearly averaged Temperature, Salinity and Density of AtlanticOcean surface water.

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Distribution of the difference between evaporation (V) andprecipitation (N), in cm / yr. Arrows indicate directions of ofwater vapour transport

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1st row: mixing between 2 homogenous water masses; 2nd row: mixing between 3 homogenous water masses; Left: vertical distribution of T and Sal at inititial condition A and at two stages of advanced mixing B and C. Right: The 3 conditions A, B and C represented in a T-S diagram; numbers indicate depths.

Relationship between T and S at 5 stations in the western basin of the Atlantic Ocean

For B & C: T = m1T1 + m2T2m1 + m2

S = m1S1 + m2S2m1 + m2

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Relationship between T and S at two close-by stations ‘Atlantis 1227 and 1229 in the Gulfstream east of Cape Hatteras; Vertical profile of T (a), S (b) and T-S (c). Numbers indicate depths.

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T-S diagram for the Southern Ocean water column profiles

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Seasonal evolution of density, temperature and salinity for surface seawater inthe Atlantic Ocean (east of Newfoundland: 40° - 45° N; 40° - 45°W)

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C represents equalproportions of A and B, which have samedensity. The mixture C is more dense than theend-members andsinks. As a rsult a discontinuity, or front ismaintained betweenwaters A and B

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Left: T and S profiles for Mediterranean outflow water (30°60’N – 08°38’W). Right: T-S diagram indicating occurrence of salt fingering. This is because heat transfer is faster than molecular diffusion of salt, creating instabilities in the density structure.

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Variation in seawater density as a function of temperature with salinity at 35‰. This relationship, in which the density of sea water increases less rapidly as the temperature drops towards freezing point, has profound consequences in partly determining ocean stratification at high latitudes and sequestration of atmospheric CO2 to the deep sea.

François, 2004

Seawater density: potentialControl on climate change

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Warming of climate → warming of the S.O. water column → decreases the surface to deep density gradient → increases the possibility for deep convection and release of CO2 stored in the deep ocean → positive feed-back on climate

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Sigman et al., 2004

Freezing temp.of seawater

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Cooling of climate → cooling of the S.O. water column → increases the surface to deep density gradient → decreases the possibility for deep convection and keeps the CO2 stored in the deep ocean there →positive feed-back on climate

Freezing temp.of seawater

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