The oceans and the atmosphere are the two large reservoirs of water in the
Earth's hydrologic cycle. The two systems are complexly linked to
one another and are responsible for Earth's weather and climate. The
oceans help to regulate temperature in the lower part of the
atmosphere. The atmosphere is in large part responsible for the
circulation of ocean water through waves and currents. In this
section we first look at how the atmosphere controls weather and climate,
and we will explore some the introductory material necessary to understand
our upcoming lecture on severe weather. Next we will look at the
oceans, to see what drives ocean currents and causes waves. We will
then briefly discuss coastal erosion.
Weather and Climate
Weather is the condition of the atmosphere at a particular time and
place. It refers to such conditions of the local atmosphere as
temperature, atmospheric pressure, humidity (the amount of water contained
in the atmosphere), precipitation (rain, snow, sleet, & hail), and
wind velocity. Because the amount of heat in the atmosphere varies with
location above the Earth's surface, and because differing amounts of heat
in different parts of the atmosphere control atmospheric circulation, the
atmosphere is in constant motion. Thus, weather is continually
changing in a complex and dynamic manner.
Climate refers to the average weather characteristics of a given
region. Climate, although it does change over longer periods of
geologic time, is more stable over short periods of time like years and
centuries. The fact that the Earth has undergone fluctuation between ice
ages and warmer periods in the recent past (the last ice age ended about
10,000 years ago) is testament to the fact that climate throughout the
world as has been changing through time.
The Earth's weather and climate system represent complex interactions
between the oceans, the land, the sun, and the atmosphere. That these
interactions are complex is evidence by the difficulty meteorologists have
in predicting weather on a daily basis. Understanding climate change is
even more difficult because humans have not been around long enough to
record data on the long term effects of these processes. Still, we do know
that the main energy source for changing weather patterns and climate is
solar energy from the Sun.
Earth's atmosphere consists of a mixture of Nitrogen (N2) and Oxygen
(O2). At the Earth's surface, dry air is composed of about 79%
Nitrogen, 20% Oxygen, and 1% Argon. It can also contain up to 4%
water vapor at saturation, but saturation depends on temperature.
Relative humidity is the term used to describe saturation with water
vapor. When the relative humidity is 100%, the atmosphere is saturated
with respect to water vapor, and precipitation results. Other gases occur
in the atmosphere in small amounts. Among the most important of these
other gases is Carbon Dioxide (CO2).
The atmosphere has a layered structure, as shown here. Each layer is
defined on the basis of properties such as pressure, temperature,
and chemical composition. The layer closest to the surface is called the
troposphere, which extends to an altitude of 10 to 15 km. Temperature
decreases upward in the troposphere to the tropopause (the boundary
between the troposphere and the next layer up, the stratosphere). The
troposphere contains about 90% of the mass of the atmosphere, including
nearly all of the water vapor. Weather is controlled mostly in the
Circulation in the
The troposphere undergoes circulation because of convection. Recall
that convection is a mode of heat transfer. Convection in the
atmosphere is mainly the result of the fact that more of the Sun's heat
energy is received by parts of the Earth near the Equator than at the
poles. Thus air at the equator is heated reducing its the density.
Lower density causes the air to rise. At the top of the troposphere
this air spreads toward the poles. If the Earth were not rotating, this
would result in a convection cell, with warm moist air rising at the
equator, spreading toward the poles along the top of the troposphere,
cooling as it moves poleward, then descending at the poles
The Coreolis Effect -
Since the Earth is in fact rotating, atmospheric circulation
patterns are much more complex. The reason for this is the Coreolis
Effect. The Coreolis Effect causes any body that moves on a rotating
planet to turn to the right (clockwise) in the northern hemisphere and to
the left (counterclockwise) in the southern hemisphere. The effect is
negligible at the equator and increases both north and south toward the
poles. The Coreolis Effect occurs because the Earth rotates out from
under all moving bodies like water, air, and even airplanes. Note
that the Coreolis effect depends on the initial direction of motion and
not on the compass direction. If you look along the initial
direction of motion the mass will be deflected toward the right in the
northern hemisphere and toward the left in the southern hemisphere.
Low Pressure Centers - In zones where air ascends, the air is less dense
than its surroundings and this creates a center of low atmospheric
pressure, or low pressure center. Winds blow from areas of high pressure
to areas of low pressure, and so the surface winds would tend to blow
toward a low pressure center. But, because of the Coreolis Effect,
these winds are deflected. In the northern hemisphere they are
deflected to toward the right, and fail to arrive at the low pressure
center, but instead circulate around it in a counter clockwise fashion as
shown here. In the southern hemisphere the circulation around a low
pressure center would be clockwise. Such winds are called cyclonic
High Pressure Centers - In zones where air descends back to the surface,
the air is more dense than its surroundings and this creates a center of
high atmospheric pressure. Since winds blow from areas of high pressure to
areas of low pressure, winds spiral outward away from the high
pressure. But, because of the Coreolis Effect, such winds, again
will be deflected toward the right in the northern hemisphere and create a
general clockwise rotation around the high pressure center. In the
southern hemisphere the effect is just the opposite, and winds circulate
in a counterclockwise rotation about the high pressure center. Such winds
circulating around a high pressure center are called anticyclonic winds.
The rising moist air at the equator creates a series of low pressure zones
along the equator. Water vapor in the moist air rising at the
equator condenses as it rises and cools causing clouds to form and rain to
fall. After this air has lost its moisture, it spreads to the north
and south, continuing to cool, where it then descends at the mid-latitudes
(about 30o North and South).
Descending air creates zones of high pressure, known as subtropical high
pressure areas. Because of the rotating Earth, these descending
zones of high pressure veer in a clockwise direction in the northern
hemisphere, creating winds that circulate clockwise about the high
pressure areas, and giving rise to winds that blow from the northeast back
towards the equator. These northeast winds are called the trade
winds. In the southern hemisphere the air circulating around a high
pressure center is veered toward the left, causing circulation in a
counterclockwise direction, and giving rise to the southeast trade winds
blowing toward the equator.
Air circulating north and south of the subtropical high pressure zones
generally blows in a westerly direction in both hemispheres, giving rise
to the prevailing westerly winds.
These westerly moving air masses again become heated and start to rise
creating belts of subpolar lows. Meeting of the air mass circulating
down from the poles and up from the subtropical highs creates a polar
front which gives rise to storms where the two air masses meet. In
general, the surface along which a cold air mass meets a warm air mass is
called a front.
The position of the polar fronts continually shifts slightly north and
south, bringing different weather patterns across the land. In the
northern hemisphere, the polar fronts shift southward to bring
winter storms to much of the U.S. In the summer months, the polar
fronts shift northward, and warmer subtropical air circulates farther
Effect of Air
Circulation on Climate
Atmospheric circulation is further complicated by the distribution of land
and water masses on the surface of the Earth and the topography of the
land. If the Earth had no oceans and a flat land surface, the major
climatic zones would all run in belts parallel to the equator. But,
since the oceans are the source of moisture and the elevation of the land
surface helps control where moist air will rise, climatic zones depend not
only on latitude, but also on the distribution and elevation of land
masses. In general, however, most of the world's desert areas
occur along the mid-latitudes where dry air descends along the
mid-latitude high pressure zones
Oceans and Coastal Zones
The oceans play a major role in weather and climate because over 70% of
the Earth's surface is covered by oceans. The atmosphere picks up
most of its moisture and heat from the oceans and thus weather patterns
and climate are controlled by the oceans.
The oceans vary considerably in their depth. The deepest part of the
ocean is called the abyssal plain. As the seafloor starts to rise
toward continental margins it is called the continental rise. The
continental slope is the steep slope rising toward continual
margins. The gently sloping area along the margin of a continent is
called the continental shelf. In addition, deep trenches that occur
along zones where oceanic lithosphere descends back into the mantle are
called oceanic trenches. And, ridges in the deep oceans that
rise above the abyssal plains and where new oceanic lithosphere is created
are called oceanic ridges. These features all effect the circulation
of the oceans and the ecosystems that inhabit the oceans.
A coastal zone is the interface between the land and water. These
zones are important because a majority of the world's population inhabit
such zones. Coastal zones are continually changing because of the dynamic
interaction between the oceans and the land. Waves and winds along the
coast are both eroding rock and depositing sediment on a continuous basis,
and rates of erosion and deposition vary considerably from day to day
along such zones. The energy reaching the coast can become high during
storms, and such high energies make coastal zones areas of high
vulnerability to natural hazards. Thus, an understanding of the
interactions of the oceans and the land is essential in understanding the
hazards associated with coastal zones. Tides, currents, and waves
bring the energy to the coast, and thus we start with these three factors.
Tides are due to the gravitational attraction of Moon and to a lesser
extent, the Sun on the Earth. Because the Moon is closer to the Earth than
the Sun, it has a larger effect and causes the Earth to bulge toward the
moon. At the same time, a bulge occurs on the opposite side of the Earth
due to inertial forces (this is not explained well in the book, but the
explanation is beyond the scope of this course). These bulges remain
stationary while Earth rotates. The tidal bulges result in a rhythmic rise
and fall of ocean surface, which is not noticeable to someone on a boat at
sea, but is magnified along the coasts. Usually there are two high tides
and two low tides each day, and thus a variation in sea level as the tidal
bulge passes through each point on the Earth's surface. Along most coasts
the range is about 2 m, but in narrow inlets tidal currents can be strong
and fast and cause variations in sea level up to 16 m
Because the Sun also exerts a gravitational attraction on the Earth, there
are also monthly tidal cycles that are controlled by the relative position
of the Sun and Moon. The highest high tides occur when the Sun and the
Moon are on the same side of the Earth (new Moon) or on opposite sides of
the Earth (full Moon). The lowest high tides occur when the Sun and the
Moon are not opposed relative to the Earth (quarter Moons). These highest
high tides become important to coastal areas during hurricane season and
you always hear dire predications of what might happen if the storm surge
created by the hurricane arrives at the same time as the highest high
Fluctuations in Water Level
While sea level fluctuates on a daily basis because of the tides, long
term changes in sea level also occur. Such sea level changes can be
the result of local effects such as uplift or subsidence along a coast
line. But, global changes in sea level can also occur. Such
global sea level changes are called eustatic changes. Eustatic sea
level changes are the result of either changing the volume of water in the
oceans or changing the shape of the oceans. For example,
during glacial periods much of the water evaporated from the oceans is
stored on the continents as glacial ice. This causes sea level to
become lower. As the ice melts at the end of a glacial period, the
water flows back into the oceans and sea level rises. Thus, the
volume of ice on the continents is a major factor in controlling eustatic
sea level. Global warming, for example could reduce the amount of
ice stored on the continents, thus cause sea level to rise. Since
water also expands (increases its volume) when it is heated, global
warming could also cause thermal expansion of sea water resulting in a
rise in eustatic sea level.
The surface of the oceans move in response to winds blowing over the
surface. The winds, in effect, drag the surface of oceans creating a
current of water that is usually no more than about 50 meters deep.
Thus, surface ocean currents tend to flow in patterns similar to the winds
as discussed above, and are reinforced by the Coreolis Effect. But,
unlike winds, the ocean currents are diverted when they encounter a
continental land mass. In the middle latitudes ocean currents run
generally eastward, flowing clockwise in the northern hemisphere and
counterclockwise in the southern hemisphere. Such easterly flowing
currents are deflected by the continents and thus flow circulates back
toward the west at higher latitudes. Because of this deflection,
most of the flow of water occurs generally parallel to the coasts along
the margins of continents. Only in the southern oceans, between
South America, Africa, Australia, and Antarctica are these surface
currents unimpeded by continents, so the flow is generally in an easterly
direction around the continent of Antarctica.
Waves are generated by winds that blow over the surface of oceans. In a
wave, water travels in loops. But since the surface is the area affected,
the diameter of the loops decreases with depth. The diameters of loops at
the surface is equal to wave height (h).
Wavelength (L) = distance to complete one cycle
Wave Period (P) = time required to
complete on cycle.
Wave Velocity (V) = wavelength/wave
Wave Base - Motion of waves is only effective at moving water to depth
equal to one half of the Wavelength (L/2). Water deeper than L/2 does not
move. Thus, waves cannot erode the bottom or move sediment in water deeper
than L/2. This depth is called wave base. In the Pacific Ocean,
wavelengths up to 600 m have been observed, thus water deeper than 300m
will not feel passage of wave. But outer parts of continental shelves
average 200 m depth, so considerable erosion can take place out to the
edge of the continental shelf with such long wavelength waves.
When waves approach shore, the water depth decreases and the wave will
start feeling bottom. Because of friction, the wave velocity (= L/P)
decreases, but its period (P) remains the same Thus, the wavelength
(L) will decrease. Furthermore, as the wave "feels the bottom", the
circular loops of water motion change to elliptical shapes, as loops are
deformed by the bottom. As the wavelength (L) shortens, the wave height
(h) increases. Eventually the steep front portion of wave cannot support
the water as the rear part moves over, and the wave breaks. This results
in turbulent water of the surf, where incoming waves meet back flowing
water. Rip currents form where water is channeled back into ocean.
Wave Erosion - Rigorous erosion of sea floor takes place in the surf zone,
i.e. between shoreline and breakers. Waves break at depths between 1 and
1.5 times wave height. Thus for 6 m tall waves, rigorous erosion of sea
floor can take place in up to 9 m of water.
Waves can also erode by abrasion and flinging rock particles against one
another or against rocks along the coastline.
Wave refraction - Waves generally do not approach shoreline parallel to
shore. Instead some parts of waves feel the bottom before other parts,
resulting in wave refraction or bending.
Wave energy can thus be concentrated on headlands, to form cliffs.
Headlands erode faster than bays because the wave energy gets concentrated
and Sediment Transport
Coastlines are zones along which water is continually making changes.
Waves can both erode rock and deposit sediment. Because of the continuous
nature of ocean currents and waves, energy is constantly being expended
along coastlines and they are thus dynamically changing systems, even over
short (human) time scales.
Erosion by Waves
As we discussed previously, the motion of waves is only felt to a depth of
1/2 times the wavelength. Thus, waves can only erode if the water
along a coastline is shallower than 1/2 times the wavelength. But,
when the wave breaks as it approaches the shoreline, vigorous erosion is
possible due to the sudden release of energy as the wave flings itself
onto the shore. In the breaker zone rock particles carried in
suspension by the waves are hurled at other rock particles. As these
particles collide, they are abraded and reduced in size. Smaller
particles are carried more easily by the waves, and thus the depth to the
bottom is increased as these smaller particles are carried away by the
retreating surf. Furthermore, waves can undercut rocky coastlines
resulting in mass wasting processes wherein material slides, falls,
slumps, or flows into the water to be carried away by further wave action.
Transport of Sediment
by Waves and Currents
Sediment that is created by the abrasive action of the waves or sediment
brought to the shoreline by streams is then picked up by the waves and
transported. The finer grained sediment is carried offshore to be
deposited on the continental shelf or in offshore bars, the coarser
grained sediment can be transported by longshore currents and beach drift.
Longshore currents - Most waves arrive at the shoreline at an angle, even
after refraction. Such waves have a velocity oriented in the direction
perpendicular to the wave crests, but this velocity can be resolved into a
component perpendicular to the shore (Vp) and a component
parallel to the shore (VL). The component parallel to the shore can
move sediment and is called the longshore current.
Beach drift - is due to waves approaching at angles to beach, but
retreating perpendicular to the shore line. This results in the swash of
the incoming wave moving the sand up the beach in a direction
perpendicular to the incoming wave crests and the backwash moving the sand
down the beach perpendicular to the shoreline. Thus, with successive
waves, the sand will move along a zigzag path along the beach.
High winds blowing over the surface of the water during storms bring more
energy to the coastline and can cause more rapid rates of erosion.
Erosion rates are higher because:
During storms wave velocities are higher and thus larger particles can be
carried in suspension. This causes sand on beaches to be picked up
and moved offshore, leaving behind coarser grained particles like pebbles
and cobbles, and reducing the width of the beach.
During storms waves reach higher levels onto the shoreline and can thus
remove structures and sediment from areas not normally reached by the
Because wave heights increase during a storm, waves crash higher onto
cliff faces and rocky coasts. Larger particles are flung against the
rock causing rapid rates of erosion.
As the waves crash into rocks, air occupying fractures in the rock becomes
compressed and thus the air pressure in the fractures is increased.
Such pressure increases can cause further fracture of the rock.
Types of Coasts
The character and shape of coasts depends on such factors as tectonic
activity, the ease of erosion of the rocks making up the coast, the input
of sediments from rivers, the effects of eustatic changes in sea level,
and the length of time these processes have been operating.
Rocky Coasts - In general, coastlines that have experienced recent
tectonic uplift as a result of either active tectonic processes (such as
the west coast of the United States) or isostatic adjustment after melting
of glacial ice (such as the northern part of the east coast of the United
States) form rocky coasts with cliffs along the shoreline. Anywhere
wave action has not had time to lower the coastline to sea level, a rocky
coast may occur. Because of the resistance to erosion, a wave cut bench
and wave cut cliff develops. The cliff may retreat by undercutting and
resulting mass-wasting processes. If subsequent uplift of the wave-cut
bench occurs, it may be preserved above sea level as a marine terrace.
Because cliffed shorelines are continually attacked by the erosive and
undercutting action of waves, they are susceptible to frequent
mass-wasting processes which make the tops of these cliffs unstable areas
Along coasts where streams entering the ocean have cut through the rocky
cliffs, wave action is concentrated on the rocky headlands as a result of
Beaches - A beach is the wave washed sediment along a coast. Beaches occur
where sand is deposited along the shoreline. A beach can be divided into a
foreshore zone, which is equivalent to the swash zone, and backshore zone,
which is commonly separated from the foreshore by a distinct ridge, called
a berm. Behind the backshore may be a zone of cliffs, marshes, or sand
Barrier Islands - A barrier island is a long narrow ridge of sand just
offshore running parallel to the coast. Separating the island and coast is
a narrow channel of water called a lagoon. Most barrier islands were built
during after the last glaciation as a result of sea level rise. Barrier
islands are constantly changing. They grow parallel to the coast by beach
drift and longshore drift, and they are eroded by storm surges that often
cut them into smaller islands.
Coral Reefs - Reefs consist of colonies of organisms, like corals,
which secrete calcium carbonate. Since these organisms can only live in
warm waters and need sunlight to survive, reefs only form in shallow
tropical seas. Fringing reefs form along coastlines close to the sea
shore, whereas barrier reefs form offshore, separated from the land by a
lagoon. Both types of reefs form shallow water and thus protect the
coastline from waves. However, reefs are high susceptible to human
activity and the high energy waves of storms.
A rugged coast with many sea cliffs and raised wave cut benches (marine
terraces) is called an emergent coastline and in this case is due to
a recent episode of uplift of the land relative to the sea by tectonic
A submerged coast, and shows submerged valleys, barrier islands, and
gentle shorelines, all due to rise of sea level since last glaciation age
(during glacial ages, seawater is tied up in ice, and sea level is lower;
when the ice melts sea level rises).
Storms - great storms such as hurricanes or other winter storms can cause
erosion of the coastline at much higher rate than normal. During such
storms beaches can erode rapidly and heavy wave action can cause rapid
undercutting and mass-wasting events of cliffs along the coast, as noted
above. Note that the El Niño driven storms on the west coast caused
extensive coastal erosion in 1998.
Tsunamis - a tsunami is a giant sea wave generated by an earthquakes,
volcanic eruptions, or landslides, as we have discussed before. Such waves
can have wave heights up to 30 m, and have great potential to wipe out
Landslides - On coasts with cliffs, the main erosive force of the waves is
concentrated at the base of the cliffs. As the waves undercut the cliffs,
they may become unstable and mass-wasting processes like landslides will
result. Massive landslides can also generate tsunamis.
Adapting to Coastal Erosion
Seacliffs, since they are susceptible to landslides due to undercutting,
and barrier islands and beaches, since they are made of unconsolidated
sand and gravel, are difficult to protect from the action of the waves.
Human construction can attempt to prevent erosion, but cannot always
protect against abnormal conditions. Furthermore, other problems are
sometimes caused by these engineering feats.
Protection of the Shoreline.
Shoreline protection can be divided into two categories: hard
stabilization in which structures are built to reduce the action of the
waves and soft stabilization which mainly refers to adding sediment back
to a beach as it erodes away.
Hard Stabilization - Two types of hard stabilization are often used. One
type interrupts the force of the waves. Seawalls are built parallel to the
coastline to protect structures on the beach. A seawall is usually built
of concrete or piles of large rocks. Waves crash against the seawall
and are prevented from running up the beach. Breakwaters serve a
similar purpose, but are built slightly offshore, again preventing the
force of the waves from reaching the beach and any structures built on the
The other type interrupts the flow of sediment along the beach.
These structures include groins and jetties, built at right angles to the
beach to trap sand and widen the beach. While hard stabilization does
usually work for its intended purpose, it does cause sediment to be
redistributed along the shoreline. Breakwaters, for example cause
wave refraction, and alters the flow of the longshore current.
Sediment is trapped by the breakwater, and the waves become focused on
another part of the beach, not protected by the breakwater, where they can
cause significant erosion. Similarly, because groins and jetties trap
sediment, areas in the downdrift direction are not resupplied with
sediment, and beaches become narrower in the downdrift direction.
Soft stabilization is primarily accomplished by adding sediment to the
coastline, usually by dredging sediment from offshore and pumping it onto
the coastline. Adding sediment is necessary when erosion removes too much
sediment. But, because the erosive forces are still operating, such
addition of sediment will need to be periodically repeated.
Coastal Erosion Controversies
As noted above, hard stabilization usually affects areas in the downdrift
direction of the longshore current. The net result being that some
areas of a coastline are protected while other areas are destroyed.
Nearly all human intervention with coastal processes interrupts natural
processes and thus can have an adverse effect on coastlines.
Barrier Islands show a noticeable difference among the islands that have
been built upon and those that have not. The undeveloped islands have
beaches 100 to 200 meters wide, while the developed islands have beaches
with widths less than 30 m. In order to provide protection from flooding
caused by heavy rainssome places in the world have dammed most of the
streams draining into the ocean . The dams trap sediment that would
normally be carried to the ocean by stream flow. Since this sediment is
not being supplied to the ocean, longshore currents cannot resupply the
beaches with sediment, but do carry the existing sediment in the downdrift
direction, resulting in significant erosion of the beaches.
Large rivers world wide deposit huge volumes of sediment through flooding.
Humans, however, have prevented the rivers from flooding by building
levee systems. As previously deposited sediments become compacted
they tend to subside. But, since no new sediment is being supplied the
subsidence results in a relative rise in sea level. This,
coupled with a current rise in eustatic sea level, is causing coastal
erosion at an incredible rate.
Many have suggested that the best way to adjust to coastal erosion is to
leave the coastlines alone. This, of course will not solve the
problems of southern California or Louisiana, because both areas need the
flood protection measures. Nevertheless, in some coastal areas zoning laws
have been enacted to prevent development along beaches, and in other areas
building codes are enforced to protect the natural environment.