Oceanic & General Atmospheric Circulation - Video & Lesson Transcript | santemontreal.info
The surface of the tropical Pacific Ocean is nice and warm, but the atmosphere just doesn't seem interested. Will these two crazy kids get in. Decade-long cooling cycle: Middle atmosphere in sync with ocean. Relationship between decadal variations in temperatures in the Pacific and. The oceans and atmosphere interact in many different ways. There can be a net exchange of heat, salt, water and momentum between them.
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.
Oceanic & General Atmospheric Circulation
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.
Dynamics of ocean atmosphere exchange
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.The connections between ocean and atmosphere
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.
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.
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.
Global Energy Transfer, Atmosphere, Climate
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. Massive landslides can also generate tsunamis. New Zealand is an excellent place to study biogenic sulfate from the ocean because there is a much smaller industrial pollution background than there is in the northern hemisphere.
First, we study the biological factors governing DMS production and, second, we study the atmospheric processes that connect DMS with clouds. Both require a combination of observational measurements and modelling work Ocean measurements are being made from the RV Tangaroa in the highly productive marine areas around New Zealand and show that high levels of DMS are associated with large plankton blooms.
We have also measured changes in DMS in the remote Southern Ocean which were stimulated by addition of iron as a micronutrient to the ocean during an international project run by NIWA. Atmospheric measurements are carried out at the Baring Head clean air station near Wellington to determine variations in sulfate aerosol and relate these to atmospheric chemistry. We have developed a computer model of the large number of chemical reactions involved and used this to assess the role of different oxidants.
To quantify the potential climatic impact of DMS, it is important to be able to distinguish between DMS conversion to sulfate adding to existing particles and conversion that forms new particles. We are one of very few groups able to make this distinction by using sulfur isotopes heavy and light versions of the sulfur atom.
This technique relies on the fact that formation of new particles or accumulation on existing particles affects the ratio of heavy to light sulfur atoms differently. Ocean atmosphere carbon exchange Carbon dioxide CO2 is a soluble gas which dissolves in the oceans and is taken up by marine plants phytoplankton.
A natural cycle results in which CO2 is absorbed from the atmosphere in some generally cooler and more biologically active parts of the ocean and released back to the atmosphere in other generally warmer and less biologically active parts. This natural cycle has been modified through the addition of CO2 to the atmosphere by human activities.
Increasing CO2 concentrations in the atmosphere tend to increase the amount dissolved in the surface ocean. Currently about 29 billion thousand million tonnes of CO2 are being added to the atmosphere each year due to fossil fuel burning and deforestation and the oceans are removing about 7 billion tonnes. A similar amount is removed due to increases in plant biomass and soil carbon.
Predicting how the net ocean uptake will change in the future is critical to understanding how atmospheric CO2 concentrations will change in future and so to estimating long term climate change.
Our measurements have shown that large areas of the oceans around New Zealand remove CO2 from the atmosphere. Work is continuing to determine the patterns of this CO2 uptake, its magnitude, and the factors controlling spatial and temporal variability. Large-scale spatial variability is being investigated through open ocean voyages to different water bodies such as subantarctic waters to the east and south of NZ, subtropical waters to the north, and the subtropical frontal area over the Chatham Rise.
These studies are complemented by a regular series of measurements on voyages to the east of the South Island at approximately two-monthly intervals. This time series provides an understanding of the seasonal cycle and inter-annual variability of CO2 in subantarctic waters, and has shown a seasonal cycle in uptake which appears to be strongly influenced by the growth of phytoplankton. Development of techniques capable of high precision measurements is an integral part of the programme.