OEAS 604: Introduction to
Physical Oceanography
Properties of Seawater
Chapter 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
Quartz lattice-like Structure
Ball Pack of Maximum
Density 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 samples
accuracy and precision to be determined
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
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)
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”.
Figure 4.9, Talley et al.
Temperature and
potential
temperature
difference in S.
Atlantic (25°S)
Note temperature and
potential temperature
minimum at about 1000 m
(must be balanced by a
salinity feature)
X
Atlantic temperature and potential
temperature sections for contrast
Temperature
Potential temperature
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
Ocean acoustics: sound speed
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 is a compressional wave
• Sound speed, cs, is calculated from the
change in density for a given change in
pressure
1/cs2 =  /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
Vertical
Profiles
SOFAR Channel
Sound speed
corrections
Data from Station Papa in the Pacific Ocean at 39°N, 146°W
August, 1959
FIGURE 3.7
Talley et al.
Equation of state for sound speed (c) depends on
T, S, p which is expressed as:
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)
Typical sound speed
profiles in open ocean
• c increases ~5m s-1 per °C
• c increases ~1m s-1 per psu S
• linear increase with pressure
(depth)
Talley et al.
Sound ray diagrams
Shallow source for a soundspeed 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.
FIGURE 3.8
Talley et al.
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
dI
= -kI
dz
I(z) = I 0e
-kz
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 3.9
Talley 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.
FIGURE 3.10
Talley et al.
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.