Physical Prop
101
Part A | 5.2
5. Physical Properties of Seawater
Wendell S. Brown
This chapter presents the definitions of the principal physical properties of the seawater, including
pressure, temperature, salinity, density, density
anomaly, specific heat, and thermal expansion.
The typical global ocean profiles and distribution maps of surface temperature, salinity, and
density anomaly are presented. Other oceanic
properties, including the stability of oceanic water
columns, use of temperature–salinity diagrams in
identifying water masses, sea water freezing, as
well as oceanic sound transmission and light are
treated.
Sea water can be characterized by its temperature, salinity (the dissolved solids), and pressure.
From these three quantities and the equation of
the state of sea water, the ocean scientist or engineer can calculate other desired quantities, such
as the density, sound velocity, heat capacity, or
electrical conductivity. The oceanographic tables
5.1 Hydrostatic Pressure ............................ 101
5.2 Temperature........................................ 101
5.3 Salinity ............................................... 103
5.4 Density ............................................... 104
5.5 Temperature–Salinity Relationships...... 105
5.6 Specific Heat ....................................... 106
5.7 Freezing of Sea Water and Sea Ice......... 106
5.8 Coefficient of Thermal Expansion.......... 106
5.9 Sound Velocity ..................................... 107
5.10 Acoustic Ambient Noise ........................ 107
5.11 Light Transmission ............................... 108
References................................................... 109
of yesteryear have been replaced by the microcomputer for the calculation of these derived
quantities.
5.1 Hydrostatic Pressure
Assuming a static ocean (zero velocity), the vertical
equation of motion reduces to a static force balance
between the pressure gradient and water parcel weight
according to
@p
@z
D Dg ;
(5.1)
where z is elevation positive upward, is local water
density, and g is the acceleration due to gravity. Upon
integration of (5.1) from the surface (where gauge pressure is zero by definition) to depth z, we obtain the
hydrostatic pressure p h relation
p h .z/ D Dgz ;
(5.2)
in which is the average surface to depth z density.
Hydrostatic pressure dominates the total oceanic pressure field. The hydrostatic relation (5.2) enables us to
convert measured pressure to depth, if one assumes the
density of an average ocean profile. For example, a pressure of 4062 decibars (5891 psi) converts to a depth of
4000 m (Sanders and Fofonoff [5.1], for further details
and limitations).
5.2 Temperature
The temperature T of a water parcel is expressed in degrees Celsius (
ı C) and gives an indication of the energy
or work that has been done on or associated with that
water parcel. Temperature is now being easily and accurately measured by electronic thermometers employing
thermistors or platinum resistance probes as the sens-
101
Part A | 5.2
5. Physical Properties of Seawater
Wendell S. Brown
This chapter presents the definitions of the principal physical properties of the seawater, including
pressure, temperature, salinity, density, density
anomaly, specific heat, and thermal expansion.
The typical global ocean profiles and distribution maps of surface temperature, salinity, and
density anomaly are presented. Other oceanic
properties, including the stability of oceanic water
columns, use of temperature–salinity diagrams in
identifying water masses, sea water freezing, as
well as oceanic sound transmission and light are
treated.
Sea water can be characterized by its temperature, salinity (the dissolved solids), and pressure.
From these three quantities and the equation of
the state of sea water, the ocean scientist or engineer can calculate other desired quantities, such
as the density, sound velocity, heat capacity, or
electrical conductivity. The oceanographic tables
5.1 Hydrostatic Pressure ............................ 101
5.2 Temperature........................................ 101
5.3 Salinity ............................................... 103
5.4 Density ............................................... 104
5.5 Temperature–Salinity Relationships...... 105
5.6 Specific Heat ....................................... 106
5.7 Freezing of Sea Water and Sea Ice......... 106
5.8 Coefficient of Thermal Expansion.......... 106
5.9 Sound Velocity ..................................... 107
5.10 Acoustic Ambient Noise ........................ 107
5.11 Light Transmission ............................... 108
References................................................... 109
of yesteryear have been replaced by the microcomputer for the calculation of these derived
quantities.
5.1 Hydrostatic Pressure
Assuming a static ocean (zero velocity), the vertical
equation of motion reduces to a static force balance
between the pressure gradient and water parcel weight
according to
@p
@z
D Dg ;
(5.1)
where z is elevation positive upward, is local water
density, and g is the acceleration due to gravity. Upon
integration of (5.1) from the surface (where gauge pressure is zero by definition) to depth z, we obtain the
hydrostatic pressure p h relation
p h .z/ D Dgz ;
(5.2)
in which is the average surface to depth z density.
Hydrostatic pressure dominates the total oceanic pressure field. The hydrostatic relation (5.2) enables us to
convert measured pressure to depth, if one assumes the
density of an average ocean profile. For example, a pressure of 4062 decibars (5891 psi) converts to a depth of
4000 m (Sanders and Fofonoff [5.1], for further details
and limitations).
5.2 Temperature
The temperature T of a water parcel is expressed in degrees Celsius (
ı C) and gives an indication of the energy
or work that has been done on or associated with that
water parcel. Temperature is now being easily and accurately measured by electronic thermometers employing
thermistors or platinum resistance probes as the sens-