Oceanography
An Invitation to Marine Science, 7th
Tom Garrison
Chapter 6
Water and Ocean Structure
Chapter 6 Study Plan
• A Note to the Reader
• The Water Molecule Is Held Together by Chemical
Bonds
• Water Has Unusual Thermal Characteristics
• Surface Water Moderates Global Temperature
• The Ocean Is Stratified by Density
• Refraction Can Bend the Paths of Light and Sound
through Water
• Light Does Not Travel Far through the Ocean
• Sound Travels Much Farther than Light through the
Ocean
Chapter 6 Main Concepts
• Heat is not the same as temperature. Temperature
is an object’s response to an input (or removal) of
heat. Not all substances respond in the same way.
• Water resists rising in temperature as heat is added.
Water gives off heat when it freezes, and absorbs
heat as it thaws. These properties of liquid water
moderate Earth’s surface temperatures.
• The ocean is density stratified. Dense cold and salty
water underlies less dense warm and fresher water.
• Light is quickly extinguished by passage through
water. Sound is not.
• Light and sound can be bent (refracted) by passage
through water masses whose physical
characteristics differ.
The Water Molecule Is Held
Together by Chemical Bonds
• Compounds – substances that contain two
or more different elements in fixed
proportions
• Element – a substance composed of
identical particles that cannot be chemically
broken down into simpler substances
• Atoms – the particles that make up elements
The Water Molecule Is Held
Together by Chemical Bonds
A water molecule is composed of two hydrogen atoms and one oxygen atom.
Water is a polar molecule, having a positive and a negative side.
A molecule is a group of atoms held together by chemical bonds.
Chemical bonds, the energy relationships between atoms that hold them together,
are formed when electrons - tiny negatively charged particles found toward the
outside of an atom - are shared between atoms or moved from one atom to
another.
The Water Molecule Is Held
Together by Chemical Bonds
What holds water molecules
together?
Hydrogen bonds form when the
positive end of one water molecule
bonds to the negative end of another
water molecule.
What are two important properties
of water molecules?
Cohesion – the ability of water
molecules to stick to each other,
creating surface tension.
Adhesion – the tendency of water
molecules to stick to other substances
Not All Substances Have the
Same Heat Capacity
What is the difference between heat
and temperature?
Heat is energy produced by the random
vibrations of atoms or molecules.
Temperature is an object’s response to
input or removal of heat.
Heat Capacity is a measure of the heat
required to raise the temperature of 1g
of a substance by 1C.
Water has a very high heat capacity,
which means it resists changing
temperature when heat is added or
removed.
Water’s Temperature Affects Its
Density
The relationship of
density and
temperature for pure
water.
Note that points C
and D both represent
0°C (32°F) but
different densities
and thus different
states of water. Ice
floats because the
density of ice is lower
than the density of
liquid water.
Water’s Temperature Affects Its
Density
The three common states of matter
– solid, liquid, and gas. A gas is a
substance that can expand to fill
any empty container. Atoms or
molecules of gas are in high-speed
motion and move in random
directions. A liquid is a substance
that flows freely in response to
unbalanced forces but has a free
upper surface in container it does
not fill. Atoms or molecules of a
liquid move freely past one another
as individuals or small groups.
Liquids compress only slightly
under pressure. Gases and liquids
are classed as fluids because both
substances flow easily. A solid is a
substance that resists changes of
shape or volume. A solid can
typically withstand stresses without
yielding permanently. A solid
usually breaks suddenly. On Earth,
water can occur in all three states:
gas, liquid, and solid.
Water Becomes Less Dense
When It Freezes
The lattice structure of an ice
crystal, showing its hexagonal
arrangement at the molecular
level.
The space taken by 24 water
molecules in the solid lattice could
be occupied by 27 water
molecules in liquid state, so water
expands about 9% as the crystal
forms.
Because molecules of liquid water
are packed less efficiently, ice is
less dense than liquid water and
floats.
Water Becomes Less Dense
When It Freezes
For water to evaporate, heat
must be added to water in
the liquid state. After water
reaches 100C, an input of
540 cal/gram is required to
break the hydrogen bonds
and allow evaporation.
The amount of energy
required to break the bonds
is termed the latent heat of
vaporization.
Water has the highest latent
heat of vaporization of any
known substance.
Water Removes Heat from
Surfaces As It Evaporates
The energy input or output associated with water in the three states of
matter. We must add 80 calories of heat energy to change a gram of
ice to liquid water. After the ice is melted, about 1 calorie of heat is
needed to raise each gram of water by 1°C. But 540 calories must be
added to each gram of water to vaporize it – to boil it away. The
process is reversed for condensation and freezing.
Surface Water Moderates Global
Temperature
San Francisco, California, and Norfolk, Virginia, are on the same line of
latitude, yet San Francisco is warmer in the winter and cooler in the
summer than Norfolk. Part of the reason is that wind tends to flow from
west to east at this latitude. Thus, air in San Francisco has moved over the
ocean while air in Norfolk has approached over land. Water doesn’t warm
as much as land in the summer nor cool as much in winter – a
demonstration of thermal inertia.
Ocean-Surface Conditions Depend on
Latitude, Temperature, and Salinity
Average surface temperature and salinity for the world ocean. As you would
expect, temperatures are lowest in the polar regions and highest near the
equator. Heavy rainfall in the equatorial regions “freshens” the ocean near the
equator, whereas hot and dry conditions near the tropic lines (Tropic of
Capricorn and Tropic of Cancer) result in higher surface salinity in those areas.
Ocean-Surface Conditions Depend on
Latitude, Temperature, and Salinity
Sea-surface temperatures during Northern Hemisphere
summer.
Ocean-Surface Conditions Depend on
Latitude, Temperature, and Salinity
Sea-surface average salinities in parts per thousand (‰).
The Ocean Is Stratified by Density
The complex
relationship between
temperature, salinity
and density of
seawater.
Note that two
samples of water can
have the same
density at different
combinations of
temperature and
salinity.
The Ocean Is Stratified into Three Density
Zones by Temperature and Salinity
• The ocean is divided into three density zones
• Surface zone – the upper layer of the ocean,
containing the least dense water. The surface
zone is only about 2% of total ocean volume.
• Pycnocline – a zone in which density
increases with depth, containing about 18%
of all ocean water
• Deep zone – contains about 80% of all ocean
water. There is little change in density
throughout this layer.
The Ocean Is Stratified into Three Density
Zones by Temperature and Salinity
Density stratification in the ocean.
(a) In most of the ocean, a surface zone
(or mixed layer) or relatively warm, lowdensity water overlies a layer called the
pycnocline. Density increases rapidly with
depth in the pycnocline. Below the
pycnocline lies the deep zone of cold,
dense water – about 80% of total ocean
volume.
(b) The rapid density increase in the
pycnocline is mainly due to a decrease in
temperature with depth in this area – the
thermocline.
(c) In some regions, especially in shallow
water near rivers, a pycnocline may
develop in which the density increase
with depth is due to vertical variations in
salinity. In this case, the pycnocline is a
halocline.
The Ocean Is Stratified into Three Density
Zones by Temperature and Salinity
Typical temperature
profiles at polar,
tropical, and middle
(temperate)
latitudes. Note that
polar waters lack a
thermocline.
Refraction Can Bend the Paths of
Light and Sound through Water
• Sound and light both travel in waves:
– Refraction is the bending of waves, which occurs
when waves travel from one medium to another.
– The refractive index is a ratio that expresses how
much light is refracted from one medium to another.
• Sunlight does not travel well in the ocean.
Scattering and absorption weaken light:
– Scattering occurs when light is bounced between air
and water molecules, dust and other objects.
– Absorption occurs when light’s electromagnetic
energy is converted to heat in the molecules of
seawater.
Refraction Can Bend the Paths of
Light and Sound through Water
An analogy for refraction. The
ranks of marchers represent
light or sound waves; the
pavement and sand represent
different media. The marchers
can walk faster if they stay on
the pavement than if they walk
in the sand next to the highway.
(a) If the marchers head off the
pavement at an angle other than
90°, their path will bend (refract)
as they hit the sand because
some will be walking more
slowly than others.
(b) If they march straight off the
pavement, the ranks will slow
down but not bend as they hit
the sand
Water Transmits Blue Light More
Efficiently Than Red
Only a thin film of
seawater is illuminated by
the sun. Except for light
generated by living
organisms, most of the
ocean lies in complete
blackness.
(a) The table shows the
percentage of light
absorbed in the
uppermost meter of the
ocean and the depths at
which only 1% of the light
of each wavelength
remains.
(b) The bars show the
depths of penetration of
1% of the light of each
wavelength (as in the last
column of the table)
Sound Travels Much Farther
Than Light in the Ocean
The relationship
between water
depth and sound
velocity.
Refraction Causes SOFAR
Layers and Shadow Zones
The SOFAR layer,
in which sound
waves travel at
minimum speed.
Sound transmission
is particularly
efficient - that is,
sounds can be
heard for great
distances - because
refraction tends to
keep sound waves
within the layer.
Sonar Systems Use Sound to
Detect Underwater Objects
The principle of active
sonar.
Pulses of high-frequency
sound are radiated from
the sonar array of the
sending vessel.
Some of the energy of this
ping reflects from the
submerged submarine and
returns to the sending
vessel.
The echo is analyzed to
plot the position of the
submarine.
Sonar Systems Use Sound to
Detect Underwater Objects
Side-scan sonar in
action. Sound
pulses leave the
submerged towed
array.
Chapter 6 in Perspective
In this chapter you learned of the polar nature of the water molecule. This polarity
and the hydrogen bonds that form between water molecules result in water’s
unexpected thermal properties. You found that liquid water is remarkably resistant to
temperature change with the addition or removal of heat; and ice, with its large latent
heat of fusion and low density, melts and re-freezes over large areas of the ocean to
absorb or release heat with no change in temperature. These thermostatic effects,
combined with the mass movement of water and water vapor, prevent large swings
in Earth’s surface temperature. And, of course, you also learned that the words heat
and temperature are not interchangeable.
Changes in temperature and salinity greatly influence water density. Ocean water is
usually layered by density, with the densest water on or near the bottom.
The physical characteristics of the world ocean are largely determined by the
physical properties of seawater. These properties include water’s heat capacity,
density, salinity, and its ability to transmit light and sound.
In the next chapter you will learn what happens when solids and gases dissolve in
seawater. Most of the properties of seawater are different from those of pure water
because of the substances dissolved in the seawater.