WHAT PHYSICAL STRUCTURES MAKE UP EARTH’S OCEANS?
T
he sunlit and wind-driven upper layer of Earth’s oceans rests on relatively colder and
denser waters. At the surface interface between the atmosphere and the ocean, solar
energy is absorbed and stored. Acting on this upper layer, winds drive ocean currents that move heat and water from the warm equatorial regions to the icy poles. With
increasing depth, temperature drops until an abrupt transition separates the churning, windstirred upper layers from the quieter depths below. To understand the physical structure of
our oceans, we must examine the processes that give rise to them along with the unique
nature of water itself.
This theme (Oceans - Scale and Structure) features the properties of water and factors that
influence the physical layering of the ocean. External forces that affect ocean structure are discussed, as well.
Related Themes:
• Thermohaline circulation is also discussed in Oceans - Systems and Interactions.
• Seasonal variations in the upper ocean layers is presented in Oceans - Process and
Change.
• How satellites measure parameters that affect ocean structure is presented in Oceans
- Measurements.
• Water properties such as heat capacity latent heat of vaporization are examined in
Climate - Scale and Structure.
• The “global conveyer belt” model of heat transport is featured in Climate - Systems
and Interactions.
Related Activities:
• Evaporation, Surface Area, Temperature, and Seawater
• Measuring the Density of Water
• Seawater Mixing and Sinking
• Taking the Ocean’s Temperature
• Sound Travels SOFAR in the Ocean
INTRODUCTION
Seventy-one percent of Earth’s surface is covered with
water [Fig. 1]. In fact, the top 1 centimeter of the oceans is
equivalent to the water in all the rivers in the world.
With no external forces applied, the ocean would stratify,
or layer itself, into a simple layered structure based on density (mass divided by volume). Factors that influence
seawater’s density are pressure, temperature, and salt content
(or salinity). The physical structure of oceans is affected by
forces such as wind, heating by the Sun, and the formation Figure 1. Typical view of Earth. In
of sea ice. Yet, despite all of these complexities, a relatively this “bird’s eye” view of our planet
simple model of the ocean’s vertical structure can be drawn. taken from the space shuttle, only a
few traces of land are visible.
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PROPERTIES OF WATER
To understand how ocean
structures form, it is important to
learn about the physical and
chemical properties of water. A
water molecule consists of one
atom of oxygen and two atoms of
hydrogen [Fig. 2]. The chemical
formula for water is H2O. Water
molecules are bound by hydrogen
bonds. Hydrogen bonds form
when molecules have one side
that is electrically positive and one
side that electrically negative.
Separation of charges such as this
occur when the atoms have very
different sizes. For water, the oxygen atom is so much larger than
each hydrogen atom the oxygen’s
larger positive charge pulls the
shared electrons close to itself.
This leaves the oxygen end of the
molecule with a slight negative
charge and a slight positive charge
on the hydrogen end of the molecule. The molecule’s overall
charge, however, is zero.
Figure 2. Hydrogen bonds. Hydrogen bonds form because the
hydrogen end of the water molecule has a slight positive charge
and is attracted to the slightly negative oxygen end of another molecule. These weak bonds are responsible for, among other things,
water freezing and boiling at much higher temperatures than other
molecules of similar size.
PRESSURE
Pressure, force per unit area, increases with depth as the weight of overlying water grows.
For each 10 meters (33 feet) of ocean depth, pressure rises by 1 atmosphere (1 atm). A unit of
atmosphere is defined as the pressure exerted by Earth’s air column from the outer limit of the
atmosphere (about 160 kilometers or 100 miles) to sea level. The deepest part of the ocean is 11
kilometers (6.8 miles) deep. There the pressure is almost 1,000 atmospheres (1,000 kilograms per
square-centimeter or 14,700 pounds per square inch). This is ten times the pressure needed to
compress a block of wood to about half its original volume.
The relationship between pressure and depth is relatively straight-forward because water is
only slightly compressible. This means that, all other factors being equal, density changes only
slightly with depth [Table 1]. Thus the influence of pressure on ocean water density is less complicated than that of either temperature or salinity.
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Table 1. Physical and chemical properties of water.
TEMPERATURE
In an ocean without currents, sea surface temperatures would depend only on latitude with the
bands of warmest waters located along the equator. However, because ocean currents transport
heated water around the globe, the distribution of sea-surface temperatures is somewhat more
complicated [Fig. 3].
The volume of seawater expands with increased temperature and thus its density changes,
too. As the Sun heats the oceans the density of surface waters decreases, causing these water to
float on top of colder, denser waters below. This leads to the general layering of water by temperature.
A graph of the temperature versus depth [Fig. 4] shows a discrete layer where temperatures
drops dramatically. The layer in which this transition occurs is called the thermocline.
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Figure 3. Sea surface temperatures. Temperature bands are generally parallel the equator except where
major ocean currents flow along the margins of continents. Note that the warmest ocean water is 29°C
and the coldest is -2°C.
Figure 4. Change in ocean temperature with
depth. Although temperature generally decreases with depth, there is a layer where temperatures drop abruptly. This thermocline layer
is usually shallower than 1,000 meters.
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SALINITY
Water is notable for its ability to act as a solvent. It is able to dissolve substances, most notably
salts. Water carries salt from land, causing oceans to be salty. This characteristic results from the
hydrogen bonds in water molecules. Salinity is calculated as the amount of salt (in grams) dissolved in 1,000 grams (1 kilogram) of seawater. Seawater usually has a salinity of 3.6%.
Salinity varies throughout the oceans, depending on whether freshwater (salinity = 0) has
been added by precipitation or removed by evaporation. Many regions are affected by processes
that both increase and decrease salinity. For example, near the margins of continents, dry winds
blow off landmasses and increase salinity via high evaporation. In these same areas, salinity is
decreased by the influx of freshwater from nearby rivers.
In general, the higher ocean salinities occur in the centers of ocean basins, where trade winds
evaporate water and rain is rare. Lower salinities occur at high latitudes where the ocean receives fresh water from rain and melting ice [Fig. 5].
Figure 5. Sea surface salinities. Note that highest surface salinities occur in the middle of ocean
basins where the evaporation rate is high and rainfall rate is low. Low salinity areas are often near
sources of fresh water such as major rivers and melting ice.
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Salinity is important because it affects the
seawater’s density and thus influences ocean water layering. Other factors held constant, increasing the salinity of seawater causes its density to
increase. Thus, high salinity seawater should sink
below lower salinity water.
A graph of the salinity versus depth [Fig. 6]
shows a distinct layer where salinity increases
sharply. The layer in which this transition occurs
is called the halocline.
In a few important areas in the North Atlantic,
cold, dry air blows over the ocean, causing evaporation while cooling the surface water. Together
these processes cause seawater to be dense enough
to sink to great depths. This is known as thermohaline circulation (from Latin words for temperature
and salt). Deep water formation in these areas is a
key element of the general circulation model
known as the global conveyer belt.
DENSITY AND LAYERS OF THE OCEAN
Figure 6. Change in ocean salinity with depth.
Although salinity generally increases with depth,
there is a layer where salinity rises markedly. This
halocline layer is usually shallower than 1,000
meters.
Temperature, salinity and, to a lesser degree,
pressure determine the density of ocean water.
Figure 7 shows, in general, a rise in seawater density with depth. Marked variation in density
with depth defines the pycnocline. The position
of the pycnocline directly corresponds to layers
with distinct shifts in temperature (thermocline)
and salinity (halocline).
In a simplified model, the world’s oceans
may be thought of as having three layers [Fig. 8]
where the water conditions such as temperature,
salinity, and density are essentially the same:
• surface (or mixed) layer
• intermediate layer
• deep (or bottom) layer.
Surface or Mixed Layer
These near-surface waters are well-mixed by
winds and waves. This layer, which is about 100
to 500 meters thick, contains only 2% of the to6
Figure 7. Change in ocean density with depth.
Although density generally increases with depth,
there is a layer where density rises sharply. This
pycnocline corresponds to the position of the thermocline and halocline layers.
Figure 8. The three layers of the ocean. The surface and intermediate layers isolate the deep layer from
the atmosphere everywhere expect near high latitudes.
tal ocean volume. Temperature and salinity in this layer change seasonally because of variations
in precipitation, evaporation, cooling, and heating. This zone contains the warmest and least
dense water in the ocean.
Intermediate Layer
This layer is found where water density changes markedly with depth. Its exact depth is
controlled by factors that influence seawater density, namely temperature and salinity. In the
open ocean, where salinity is relatively constant, the depth of the pycnocline corresponds to the
thermocline (i.e., is dependent on temperature). On the other hand, the pycnocline position
coincides with the halocline (i.e., is salinity-dependent) in coastal areas where changes in fresh
water influx dominate temperature variations.
Deep or Bottom Layer
About 80% of the ocean’s volume is found in this layer. It is marked by low temperature, high
salinity, and high density.
CONCLUSION
The physical structure of our oceans results from the characteristics of seawater itself and the
external forces that act on our seas. The general vertical structure of our oceans can be depicted
by a three-layer model based on temperature, salinity, pressure, and the resulting profile of density. These important parameters can be measured directly by shipboard methods. The vertical
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structure can also be measured indirectly because temperature and pressure influence the velocity of sound as it travels through the ocean. In fact, underwater sound is used by marine scientists to measure depth of ocean layers and determine ocean temperature changes. Moreover,
many marine animals use underwater sound to sense their environment, communicate, and find
food.
VOCABULARY
atmosphere
H 2O
pressure
solvent
thermohaline
density
halocline
pycnocline
stratify
global conveyer belt
hydrogen bonds
salinity
thermocline
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