Ocean / Atmosphere and Coastal Systems
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 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.
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.
Circulation in the Atmosphere
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.
Wind Systems
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
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.
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
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.
Ocean Waves
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
period (L/P)
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 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.
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.
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.
During storms waves reach higher levels onto the shoreline and can thus remove structures and sediment from areas not normally reached by the incoming waves.
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.
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.
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.