ocean/atmosphere
Ocean / Atmosphere and Coastal Systems
adapted to html based on notes of
Stephen A. Nelson, Tulane University
The
Ocean-Atmosphere System
Weather
and Climate
The Atmosphere
Circulation
in the Atmosphere
Wind Systems
Effect
of Air Circulation on Climate
Oceans
and Coastal Zones
Tides
Ocean Waves
Coastal
Erosion and Sediment Transport
Transport
of Sediment by Waves and Currents
Types of
Coasts
Coastal
Evolution
Coastal
Hazards
The Ocean-Atmosphere System
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 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.
The Atmosphere
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 troposphere.
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 winds.
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 north.
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
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.
Coastal Zones
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
tides.
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.
Oceanic Currents
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).
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.
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 at headlands.
Coastal Erosion 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.
Storms
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.
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
for construction
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 wave refraction
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 dunes.
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.
Coastal
Evolution
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 forces.
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).
Coastal Hazards
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 coastal cities.
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 beach.
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.