OCEAN AND ATMOSPHERE
and is providing guidance relative to climate change mitigation activities and their impact on the marine environment. The ocean and the atmosphere are so completely intertwined that In the following sections, the relationships between . The oceans and atmosphere interact in many different ways. There can be a net exchange of heat, salt, water and momentum between them. We have received advice and support from many individuals without whom this report would not have been The ocean and the atmosphere are so completely intertwined that In the following sections, the relationships between People.
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 lectures on severe weather. Weather and Climate Weather is the condition of the atmosphere at a particular time and place. 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, 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.
Relative humidity is the term used to describe saturation with water vapor. 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. 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. Weather is controlled mostly in the troposphere.
Solar Radiation and the Atmosphere Radiation reaching the Earth from the Sun is electromagnetic radiation. Electromagnetic radiation can be divided into different regions depending on wavelength. Note that visible light is the part of the electromagnetic spectrum to which human eyes are sensitive. Earth receives all wavelengths of solar radiation.
But certain gases and other contaminants in the atmosphere have different effects on different wavelengths of radiation. In addition trace gases have an effect, the most important of which are the greenhouse gases.
- Atmosphere-Ocean Interaction
Greenhouse Gases Energy coming from the Sun is carried by electromagnetic radiation. Some of this radiation is reflected back into space by clouds and dust in the atmosphere. The rest reaches the surface of the Earth, where again it is reflected by water and ice or absorbed by the atmosphere. Greenhouse gases in the atmosphere absorb some of the longer wavelength infrared radiation and keep some of it in the atmosphere.
This keeps the atmospheric temperature relatively stable so long as the concentration of greenhouse gases remains relatively stable, and thus, the greenhouse gases are necessary for life to exist on Earth. Venus, which has mostly CO2 in its atmosphere, has temperature of about oC also partly due to nearness to Sun.
The CO2 concentration in the atmosphere has been increasing since the mid s. The increase correlates well with burning of fossil fuels. Thus, humans appear to have an effect. Methane concentration in the atmosphere has also been increasing. Naturally this occurs due to decay of organic matter, the digestive processes of organisms, and leaks from petroleum reservoirs.
Humans have contributed through domestication of animals, increased production of rice, and leaks from gas pipelines and gasoline. Volcanic Effects Volcanoes produce several things that result in changing atmosphere and atmospheric temperatures.
CO2 produced by volcanoes adds to the greenhouse gases and may result in warming of the atmosphere. Sulfur gases produced by volcanoes reflect low wavelength radiation back into space, and thus result in cooling of the atmosphere. Dust particles injected into the atmosphere by volcanoes reflect low wavelength radiation back into space, and thus can result in cooling of the atmosphere.
Chlorine gases produced by volcanoes can contribute to ozone depletion in the upper atmosphere. Volcanism in the middle Cretaceous produced large quantities of basalt on the seafloor and released large amounts of CO2. The middle Cretaceous was much warmer than present, resulting in much higher sea level.
The Carbon Cycle In order to understand whether or not humans are having an effect on atmospheric carbon concentrations, we must look at how carbon moves through the environment.
Carbon is stored in four main reservoirs. In the atmosphere as CO2 gas. From here it exchanges with seawater or water in the atmosphere to return to the oceans, or exchanges with the biosphere by photosynthesis, where it is extracted from the atmosphere by plants. CO2 returns to the atmosphere by respiration from living organisms, from decay of dead organisms, from weathering of rocks, from leakage of petroleum reservoirs, and from burning of fossil fuels by humans.
In the hydrosphere oceans and surface waters as dissolved CO2. From here it precipitates to form chemical sedimentary rocks, or is taken up by organisms to enter the biosphere. CO2 returns to the hydrosphere by dissolution of carbonate minerals in rocks and shells, by respiration of living organisms, by reaction with the atmosphere, and by input from streams and groundwater.
In the biosphere where it occurs as organic compounds in organisms. CO2 enters the biosphere mainly through photosynthesis.
From organisms it can return to the atmosphere by respiration and by decay when organisms die, or it can become buried in the Earth.
In the Earth's lithosphere as carbonate minerals, graphite, coal, petroleum. From here it can return to the atmosphere by weathering, volcanic eruptions, hot springs, or by human extraction and burning to produce energy.The Ocean and the Atmosphere - Introduction
Cycling between the atmosphere and the biosphere occurs about every 4. Cycling between the other reservoirs probably occurs on an average of millions of years. For example, carbon stored in the lithosphere in sedimentary rocks or as fossil fuels only re-enters the atmosphere naturally when weathering and erosion expose these materials to the Earth's surface.
When humans extract and burn fossil fuels the process occurs much more rapidly than it would occur by natural processes. With an increased rate of cycling between the lithosphere and the atmosphere, extraction from the atmosphere by increased interaction with the oceans, or by increased extraction by organisms must occur to balance the input.
If this does not occur, it may result in global warming. Global Warming Average global temperatures vary with time as a result of many processes interacting with each other.
These interactions and the resulting variation in temperature can occur on a variety of time scales ranging from yearly cycles to those with times measured in millions of years. Such variation in global temperatures is difficult to understand because of the complexity of the interactions and because accurate records of global temperature do not go back more than years. But, even if we look at the record for the past years, we see that overall, there is an increase in average global temperatures, with minor setbacks that may have been controlled by random events such as volcanic eruptions.
Records for the past years indicate that average global temperatures have increased by about 0. While this may not seem like much, the difference in global temperature between the coldest period of the last glaciation and the present was only about 5oC. In order to predict future temperature changes we first need to understand what has caused past temperature changes.
Computer models have been constructed to attempt this. Although there is still some uncertainty, most of these models agree that if the greenhouse gases continue to accumulate in the atmosphere until they have doubled over their pre values, the average global temperature increase will be between 1 and 5oC by the year This is not a uniform temperature increase.
Most models show that the effect will be greatest at high latitudes near the poles where yearly temperatures could be as much as 16oC warmer than present. Effects of Global Warming Among the effects of global warming are: Global Precipitation changes - A warmer atmosphere leads to increased evaporation from surface waters and results in higher amounts of precipitation.
Equatorial regions will be wetter than present, while interior portions of continents will become warmer and drier than present. Changes in vegetation patterns - because rainfall is distributed differently, vegetation will have to adjust to the new conditions. Mid latitude regions become more drought prone, while higher latitude regions become wetter and warmer, resulting in a shift in agricultural patterns.
Increased storminess - A warmer, wetter atmosphere favors tropical storm development. Tropical Cyclones will be stronger and more frequent. Changes in Ice patterns - Due to higher temperatures, ice in mountain glaciers will melt.
This is now being observed. But, because more water will be evaporated from the oceans, more precipitation will reach the polar ice sheets causing them to grow. Reduction of sea ice - Sea ice is greatly reduced due to higher temperatures at the high latitudes, particularly in the northern hemisphere where there is more abundant sea ice. Ice has a high albedo reflectivityand thus reduction of ice will reduce the albedo of the Earth and less solar radiation will be reflected back into space, thus enhancing the warming effect.
Ocean Atmosphere System
Thawing of frozen ground - Currently much of the ground at high latitudes remains frozen all year. Increased temperatures will cause much of this ground to thaw. Ecosystems and human structures currently built on frozen ground will have to adjust. Rise of sea level - Warming the oceans results in expansion of water and thus increases the volume of water in the oceans.
Along with melting of mountain glaciers and reduction in sea ice, this will cause sea level to rise and flood coastal zones. Changes in the hydrologic cycle - With new patterns of precipitation changes in stream flow and groundwater level will be expected.
Decomposition of organic matter in soil - With increasing temperatures of the atmosphere the rate of decay of organic material in soils will be greatly accelerated. This will result in release of CO2 and methane into the atmosphere and enhance the greenhouse effect. Breakdown of gas hydrates - This is basically solid water with gas molecules like methane locked into the crystal structure.
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.
This depth is called wave base. In the Pacific Ocean, wavelengths up to m have been observed, thus water deeper than m will not feel passage of wave. But outer parts of continental shelves average 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.
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. Waves break at depths between 1 and 1.
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.
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.
Dynamics of ocean atmosphere exchange
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.
Global Energy Transfer, Atmosphere, Climate
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.
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 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. 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.