of seawater varies rather little, by only a few %, but, as we shall see, these variations turn out to be central to the dynamics. Cold water is denser than warm water; salty water is denser than fresh water; pressure increases the density of water. Density depends on temperature T, salinity S, and pressure p in a rather complicated, nonlinear way (deduced by very careful laboratory measurements), which we represent symbolically as:

Salinity is a measure of the amount of salt dissolved in the water, about 85% of which is sodium and chloride (see Table 9.2). Modern salinity measurements are dimensionless; the Practical Salinity Scale defines salinity in terms of a conductivity ratio very nearly equal to gkg-1 or, equivalently, o/oo (parts per thousand). Typically, seawater has a salinity of 34.5 psu (practical salinity units ). Thus one kg of seawater typically has 34.5 g of salt dissolved in it. Seawater is almost incompressible; not quite, since at enormous pressures in the interior ocean compressibility effects are not always negligible.2

2It is interesting to note that if seawater were really incompressible, sea level would be about 50 m higher than it is.

TABLE 9.3. Physical properties of liquid water.

Specific heat Latent heat of fusion Latent heat of evaporation Density of fresh water Viscosity

Kinematic viscosity Thermal diffusivity cw

Lf Le

Pfresh ftwater V _ fiwater

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