OEAS 604: Introduction to Physical Oceanography

Properties of SeawaterChapter 1 – Knauss

Chapter 3 – Talley et al.

Outline

• Structure of water molecule and properties of water

• Properties of seawater• Salinity, temperature, density of seawater • Sound• Light

Water• Strange fluid with unexpected properties• Universal solvent• Seawater – about 96.5% is water and 3.5% is

dissolved material that is an ionic solution of different salts

• Water is a polar molecule – 2 hydrogen atoms (H+) and 1 oxygen atom (O=)

• Many possible arrangements of atoms – linear then no net charge – electrically neutral +H -O- H+

Water Structure

• Asymmetric structure• Bind hydrogen atoms at angle

of about 105 degrees• Polar molecule - dipole

moment results in strong forces between water molecules

• Water is a polymer; molecules arrange in chains

• Degree of polymerization is a function of temperature

Water Structure – arrangement of water molecules

Tetrahedral Structure

Ball Pack of Maximum Density Structure

Quartz lattice-like Structure

Water Structure • Arrangement of water molecules

– Tetrahedral structure – Quartz lattice-like structure– Ball pack of greatest density

• Tetrahedral structure is less dense structure and is formed with water freezes – ice floats

• Increase temperature tetrahedral crystals breakdown and water molecules move closer together to form ball pack – more dense and reaches maximum density at 4°C

• Warmer temperatures water undergoes thermal expansion and is less dense - lattice structure

Water Properties

• Specific heat – amount heat needed to raise a given mass by a

given number of degrees• Water has high specific heat

– ocean has capacity to store heat • High surface tension

– supports capillary waves and allows transfer of wind energy to ocean

Seawater

• Temperature, salinity (and density) are important for identifying seawater

• Properties acquired at sea surface• Away from surface these are conservative properties • No internal sources or sinks – except mixing• Allows tracing of particular water masses from origin • Evaporation and precipitation change salinity • Heat transfer processes change temperature

Seawater • Complex ionic solution

– Cl – 55%, Na – 32%, SO4 – 8%, Mg – 4%, K – 1%• Ratios of the weights of abundant elements are independent of

total concentration– constancy of composition

• Original definition – mass in grams of solid material in a kilogram of seawater after water was evaporated (absolute salinity) – determined by titration and units are part per thousand (ppt)

• Salinity measured by electrical conductivity – practical salinity unit• Recent method accounts for other dissolved substances and is

based on ratio of the mass of all dissolved substances in seawater to the mass of the seawater expressed as kg/kg or g/kg - salinity has no units

Salinity Measurement

Bottles for collecting water samples Autosalinometer for running

salinity analyses relative to standard seawater

CTD (conductivity, temperature, pressure) for measuring conductivity in a profile (deployed from ship)

Talley et al. (2011)

Salinity Measurement• Chemical titration – based on chlorine ion

Salinity = 1.80655 x Chlorinity• Electrical Conductivity (S,T) – practical salinity

unit (1978) – based on potassium chloride standard (Natl seawater) and assumes that there are negligible variations in seawatercomposition

• Absolute salinity (2010) – corrects for geographic variations in dissolved materials

Absolute salinity (TEOS-10)

Absolute salinity = reference salinity + correction for other dissolved materials

SA = SR + δSA

δSA corrects for geographic variations in dissolved matter that does not contribute to conductivity variations: silicate, nitrate, alkalinity

To convert practical salinity (Sp) to absolute salinity

SR = 35.16504/35 * Sp= 1.0047*psu

Reference salinity has been corrected for new knowledge (since 1978) about sea water stoichiometry as well as new published atomic weights – based on reference Atlantic seawater.

Accuracy and Precision

Accuracy: reproducibility relative to a chosen standard

Precision: repeatability of an observation by a given instrument or observing system

A very precise measurement could be wildly inaccurate.

Standard Seawater

Salinity accuracy and precision

Accuracy Precision

Old titration salinities (pre-1957)

0.025 psu 0.025 psu

Modern lab samples relative to reference standard

0.002 psu 0.001 psu

Profiling instruments without lab samplesaccuracy and precision to be determined

Talley SIO 210 (2012)

Seawater density ()

Seawater density depends on S, T, and pressure

= (S, T, p) units are mass/volume (kg m-3)

Specific volume

α = 1/ units are volume/mass (m-3 kg)

Pure water has a maximum density (at 4°C, atmospheric pressure) of

(0,4°C,1bar) = 1000 kg m-3 = 1 g cm-3

Seawater density ranges from about 1022 kg m-3 at the sea surface to 1050 kg m-3 at bottom of ocean, mainly due to compression

Density of Seawater

• Density is always ~1000 kg m-3, so common way to express density is as (“sigma”)

(S, T, p) = (S, T, p) - 1000 kg m-3

• Use (S,T,0) – referred to as t – density parcel of water would have for a given T, S and evaluated at surface

• The Equation of State (EOS) is nonlinear – complex dependencies that involve products of T, S, and p with themselves and with each other (i.e. terms like T2, T3, T4, S2, TS, etc.)

Seawater Density (S,T)

Figure 3.1 – Talley et al.

Pressure Effects

• Often omit pressure in (S, T, p) because usually compare water masses over same depth range

• Pressure affects density because water is compressible – change volume and hence density

• Pressure reduces volume and increases density

Pressure Effects

• Pressure changes density • Lower water of 35 and 5°C adiabatically to

4000 m temperature would increase to 5.45°C due to compression

• Raise water of 35 and 5°C adiabatically to surface would cool to 4.56°C due to expansion

• 5°C at 4000 m is in situ temperature • 4.56°C is potential temperature

Temperature• Temperature units: Kelvin and Celsius• Kelvin is absolute temperature, with 0 K at

the point of zero entropy• Celsius 0°C at melting point at standard

atmosphere (and no salt, etc) • TK = TC + 273.16°• Ocean temperature range: freezing point to

about 30°C to 31°C

Talley SIO 210 (2012)

Potential temperature

• Water (including seawater) is compressible

• Compress a volume of water adiabatically (no exchange of heat or salt), then its temperature increases (“adiabatic compression”)

• Potential temperature is the temperature a parcel of water has if moved adiabatically (without heat exchanges or mixing) to the sea surface

• Denoted by

• Potential temperature is always lower than measured temperature except at the sea surface (where they are the same by definition)

Pressure effect on temperature:Mariana Trench (western Pacific Ocean)

Figure 4.9, Talley et al.

Measured temperature (T) has a minimum around 4000 dbar and increases below that.

Potential temperature (Θ) is almost exactly uniform below 5000 m - the water column is “adiabatic”.

Talley SIO 210 (2012)

Temperature and potential temperature

difference in S. Atlantic (25°S)

X

Note temperature and potential temperature minimum at about 1000 m (must be balanced by a salinity feature)

Talley SIO 210 (2012)

Atlantic temperature and potential temperature sections for contrast

Temperature Potential temperature

Talley SIO 210 (2012)

Sound and Light

• Light (electromagnetic) and sound (mechanical) vibration exist in ocean

• Ocean attenuates light more strongly than sound

• Sound is a longitudinal wave – energy moves along line of propagation of wave

• Speed of sound in water depends on compressibility of water and water density

Seawater is compressible/elastic supports compressional waves or pressure waves

Sound speed

β = Adiabatic compressibility of seawater C is small if compressibility is large C is large if compressibility is small

Ocean acoustics: sound speed

Ocean acoustics

• Sound is a compressional wave• Sound speed, cs, is calculated from the

change in density for a given change in pressure1/cs

2 = /p at constant T, S

This quantity is small if a given change in pressure creates only a small change in density (I.e. medium is only weakly compressible)

• Sound speed is faster in water than in air because water is much less compressible than air

Temperature and pressure effects on sound speed

• Warm water is less compressible than cold water (density decreases)– sound speed is higher in warm water

• Sound speed increases with increasing temperature and salinity (non-linear)

• Water at high pressure is less compressible than water at low pressure (density increases) – sound speed is higher at high pressure

• These competing effects create a maximum sound speed at the sea surface (warm) and a maximum sound speed at great pressure, with a minimum sound speed in between

• The sound speed minimum is an acoustic waveguide, called the SOund Fixing and Ranging (SOFAR) channel

FIGURE 3.7Talley et al.

Data from Station Papa in the Pacific Ocean at 39°N, 146°WAugust, 1959

SOFAR Channel

Sound speedcorrections

VerticalProfiles

c = 1448.96 + 4.59T – 0.053T2 + 1.34 (S – 35) + 0.016p (c in m s-1, if T in °C, S in psu, p in dbar)

• c increases ~5m s-1 per °C• c increases ~1m s-1 per psu S• linear increase with pressure (depth)

Equation of state for sound speed (c) depends on T, S, p which is expressed as:

Typical sound speed profiles in open ocean

Talley et al.

FIGURE 3.8Talley et al.

Sound ray diagrams

Shallow source for a sound-speed profile initially increasing with depth in upper mixed layer to a shallow minimum and then decreasing

Sound source near the speed minimum in the sound channel for a typical open ocean sound-speed profile.

Light

• Visible light strongly absorbed by seawater• Some of incident light is reflected

(backscatter) • Major source of heat to the surface ocean

through shortwave radiation

Light

• Visible light strongly absorbed by seawater• Some of incident light is reflected

(backscatter) • Major source of heat to the surface ocean • Attenuation of light follows exponential law

Schematic of optical processes in seawater (from Mobley 1995)Note indicators of seawater heating and photosynthesis

Satellite observation of ocean color based on reflected light FIGURE 3.9Talley et al.

FIGURE 3.10Talley et al.

Attenuation coefficient kl, as a function of wavelength l (mm) for clear ocean water (solid line) and turbid coastal water (dashed line).

Relative energy reaching 1, 10, and 50 m depth for clear ocean water and reaching 1 and 10 m for turbid coastal waters.

Light

• Different wavelengths absorbed differently• Clear water – blue because penetrates

deepest• Turbid water shifts to yellow/green

wavelengths • Green water – phytoplankton • White – glacial melt • Ocean color basis for satellite remote sensing

Next Class

• Global Heat Balance• Chapter 2, 3 Knauss • Chapter 5 Talley et al.