17.6: Salinity, Temperature, Pressure, and Water Density
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Essential to Know
- The density of water increases as the salinity increases.
- The density of seawater (salinity greater than 24.7) increases as temperature decreases at all temperatures above 4°C
- The density of seawater increases with increasing pressure. Density changes about 2% because of the pressure difference between the surface and the deep seafloor. The effect of pressure on density usually can be ignored because most applications require density comparisons between water masses at the same depth.
- Water with a salinity less than 24.7 has an anomalous density maximum. Pure water has its maximum density at about 4°C, but the maximum density of water occurs at lower temperatures as salinity increases.
- Between 4°C and the freezing point, the density of pure water decreases as temperature decreases.
- The relative importance of changes in temperature and salinity in determining seawater density varies with water temperature. Temperature variations are more important in warm ocean waters, whereas salinity variations are more important in cold ocean waters.
Understanding the Concept
Below the ocean surface layers, movements of water masses are caused predominantly by differences in density between water masses (CC1, CC3). For this reason, water density, which depends on temperature, the concentration of dissolved salts, and pressure, is among the most important properties of seawater.
Increasing pressure compresses liquids, squeezing more mass into a smaller volume. Thus, density increases with increasing pressure. However, the molecules of water can be forced together only slightly, even by large pressure increases. Water, unlike air, is therefore almost incompressible, and the effect of pressure on water density is small. The density of seawater is only about 2% greater at the deepest depths of the oceans than it is at the surface. Consequently, the effects of pressure on density are generally ignored because oceanographers are usually interested in density differences between water masses at or near the same depth.
The density of water increases as salinity increases. This is because most ions have a higher density than water molecules do, and dissolved substances reduce the clustering of water molecules.
Decreasing temperature generally causes liquids to contract. Therefore, decreasing temperature generally increases density. However, the behavior of pure water is an exception to this rule. Between its boiling point and 4°C, water behaves normally: density increases as temperature decreases. However, between about 4°C and its freezing point, pure water decreases slightly in density as the temperature decreases. In other words, water density has a maximum at about 4°C (Chap. 5).
As salinity increases, the temperature at which the density maximum occurs decreases. At salinities of 24.7 and higher, the water density maximum is at the freezing point. The salinity of open-ocean waters is generally above 24.7. Therefore, in contrast to pure water and water with salinity less than 24.7, open-ocean seawater increases in density as temperature decreases at all temperatures above its freezing point.
The relationships among seawater density and pressure, temperature, and salinity are complex and are discussed in more detail in Chapter 5 and Figure CC6-1. However, they are illustrated by the following summary:
- Pure water at 4°C and 1 atmosphere (atm) pressure has, by definition, a relative density of exactly 1.
- Density varies almost linearly with salinity. Density increases by about 0.00080 (±0.00004) for each unit of salinity increase, where salinity is measured in practical salinity units, abbreviated PSU. The density of “average” seawater is about 1.028 at salinity 35, 0°C, and 1 atm pressure, which is about 3% more than that of pure water at 4°C.
- Density varies almost linearly with pressure. Density increases by about 0.00045 for every 100 m of depth in the oceans.
- The rate of change of density with temperature is a function of both temperature and salinity. Density increases as temperature decreases, as shown in Table CC6-1.
aThe density of pure water decreases with decreasing temperature below 4°C.
The relative importance of temperature and salinity in determining density varies with water temperature. In cold waters, variations in salinity are more important than variations in temperature. In warm waters, variations in temperature are more important.
The density of seawater in the surface layer is altered continuously by solar heating, which changes temperature, and in turn evaporation, precipitation, and ice formation and melting, which alter salinity. These processes take place at the ocean surface. Vertical motions of water masses in the oceans are caused primarily by the sinking of high-density surface water formed by cooling or evaporation (increased salinity).
The vertical motions resulting from changes in water density are discussed in CC3 and in Chapter 8.