Natural Disasters

Prof. Stephen A. Nelson

The Ocean-Atmosphere System

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.  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.

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. 

• Dry air is composed of about 79% Nitrogen, 20% oxygen, and 1% Argon. It also contains water, 4% at saturation, but saturation depends on temperature. In addition trace gases have an effect, the most important of which are the greenhouse gases.

   
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.

The most important green house gases are H₂O (water vapor), CO₂ (Carbon Dioxide), CH₄ (methane), and Ozone. H₂O is the most abundant greenhouse gas, but its concentration in the atmosphere varies with temperature. Venus, which has mostly CO₂ in its atmosphere, has temperature of about 500^oC (also partly due to nearness to Sun). The CO₂ concentration in the atmosphere has been increasing since the mid 1800s. The increase correlates well with burning of fossil fuels. Thus, humans appear to have an effect.

Volcanic Effects

Volcanoes produce several things that result in changing atmosphere and atmospheric temperatures.

1. CO₂ produced by volcanoes adds to the greenhouse gases and may result in warming of the atmosphere.
   
   
2. Sulfur gases produced by volcanoes reflect low wavelength radiation back into space, and thus result in cooling of the atmosphere.
   
   
3. Dust particles injected into the atmosphere by volcanoes reflect low wavelength radiation back into space, and thus can result in cooling of the atmosphere.
   
   
4. Chlorine gases produced by volcanoes can contribute to ozone depletion in the upper atmosphere.

The Mt. Pinatubo eruption in 1991 and El Chichķn eruption in 1981 released large quantities of dust and sulfur gases - resulted in short term cooling of 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.

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.

1. 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.
   
   
2. 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.
   
   
3. 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.
    
4. 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.

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 100 years. But, even if we look at the record for the past 100 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 100 years indicate that average global temperatures have increased by about 0.8^oC. 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 5^oC.

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-1860 values, the average global temperature increase will be between 1 and 5^oC by the year 2100.

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 16^oC warmer than present.

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. This is now being observed.  Ice has a high albedo (reflectivity), and 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.
  
  

• 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. Organic compounds  in the frozen ground will be subject to decay, releasing more methane into the atmosphere and enhancing the greenhouse effect. 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.  They occur in oceanic sediments and beneath frozen ground at the high latitudes. Warming of the oceans or warming of the soil at high lattitudes could cause melting of the gas hydrates which would release methane into the atmosphere. Since methane is a greenhouse gas, this would cause further global warming.

Because human history is so short compared to the time scales on which global climate change occurs, we do not completely understand the causes. However, we can suggest a few reasons why climates fluctuate.

• Long term variations in climate (tens of millions of years) on a single continent are likely caused by drifting continents. If a continent drifts toward the equator, the climate will become warmer. If the continent drifts toward the poles, glaciations can occur on that continent.
  
  
• Short-term variations in climate are likely controlled by the amount of solar radiation reaching the Earth. Among these are astronomical factors and atmospheric factors.
  

 

• Astronomical Factors -
  
• Variation in the eccentricity of the Earth's orbit around the sun has periods of about 400,0000 years and 100,000 years.
  
  
• Variation in the tilt of the Earth's axis has a period of about 41,000 years.
  
  
• Variation in the way the Earth wobbles on its axis, called precession, has a period of about 23,000 years.
  
  
• The combined effects of these astronomical variations results in periodicities similar to those observed for glacial - interglacial cycles.
  

 

• Atmospheric Factors- the composition of the Earth's atmosphere can be gleaned from air bubbles trapped in ice in the polar ice sheets. Studying drill core samples of such glacial ice and their contained air bubbles reveals the following:
  
• During past glaciations, the amount of CO₂ and methane, both greenhouse gasses that tend to cause global warming, were lower than during interglacial episodes.
  

• During past glaciations, the amount of dust in the atmosphere was higher than during interglacial periods, thus more heat was likely reflected from the Earth's atmosphere back into space.
  
  
• The problem in unraveling what this means comes from not being able to understand if low greenhouse gas concentration and high dust content in the atmosphere caused the ice ages or if these conditions were caused by the ice ages.
   
• Changes in Oceanic Circulation - small changes in ocean circulation can amplify small changes in temperature variation produced by astronomical factors.
  

 

• Other factors
  
• The energy output from the sun may fluctuate.
  
  
• Large explosive volcanic eruptions can add significant quantities of dust to the atmosphere reflecting solar radiation and resulting in global cooling.

 
Circulation in the Atmosphere

Areas where warm air rises and cools are centers of low atmospheric pressure.  In areas where cold air descends back to the surface, pressure is higher and these are centers of high atmospheric pressure.

• The Coriolis Effect - Again, the diagram above would only apply to a non-rotating Earth.  Since the Earth is in fact rotating, atmospheric circulation patterns are much more complex.  The reason for this is the Coriolis Effect.  The Coriolis 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 Coriolis Effect occurs because the Earth rotates out from under all moving bodies like water, air, and even airplanes.  Note that the Coriolis 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

• 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 Coriolis 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.
  

• 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 Coriolis 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.   

• 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 30^o 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, called the trade winds, that blow from the northeast back towards the equator.  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.
  

• 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.
  
  

Water and Heat

Water has one of the highest heat capacities of all known substances.  This means that it takes a lot of heat to raise the temperature of water by just one degree.  Water thus absorbs a tremendous amount of heat from solar radiation, and furthermore, because solar radiation can penetrate water easily, large amounts of solar energy are stored in the world's oceans. 

Thus, both liquid water and water vapor are important in absorbing heat from solar radiation and transporting and redistributing this heat around the planet.

Air Masses

Due to general atmospheric circulation patterns, air masses containing differing amounts of heat and moisture move into and across North America. 
 

Fronts and Mid-latitude Cyclones
Different air masses with different temperatures and moisture content, in general, do not mix when they run into each other, but instead are separated from each other along boundaries called fronts.

El Niņo

One of the dramatic manifestations of the interaction between the oceans and the atmosphere and its effects on both climate and weather is the Southern Oscillation, one of the consequences of which is El Niņo.  The Southern Oscillation is a back and forth variation in atmospheric pressure between a high pressure system normally located off the west coast of South America and a low pressure system normally located in the western Pacific near Indonesia and Australia.  In the U.S. El Niņo conditions  result in  heavy rains, flooding, landslides, and tornadoes in greater than normal amounts because El Niņo conditions drive abnormal amounts of moist warm air across North America. El Niņo causes flooding in Peru, as well as drought and fires in Indonesia and Australia.

The phenomena is manifested by the arrival of warm water off the coast of Peru around Christmas time, and thus is called El Niņo (Spanish for the boy child) because it arrives at this time. An El Niņo event occurs every 2 to 7 years with various degrees of strength.  Some El Niņo events are more intense than others, and the condition lasts from 18 to 24 months.  The following table lists the years of El Niņo events.

Such nutrient-rich waters help sustain large fish populations.  The upwelling cold water cools the atmosphere above, and prevents rain clouds from forming off the coast of Peru.The warm water pushed to the west by the trade winds, heats as it flows along the equator, so that on arrival in the western Pacific heat is added to the overlying atmosphere causing it to rise, form clouds, and produce extensive rainfall.The moisture depleted upper atmosphere then circulates back to east where it descends off the coast of South America contributing to the dry conditions. During periods of exceptionally strong trade winds the upwelling of cold water off the South America cools the water even further creating a condition called La Niņa (girl child).

El Niņo Conditions - During El Niņo periods there is a weakening of the easterly trade winds and the warm waters of the western Pacific are pushed toward the east.  This causes the thermocline in the eastern Pacific to sink, preventing the upwelling of cold waters from below, depleting the waters in nutrients, and thus leading to starvation of fish populations.  As the warm water shifts eastward so does the development of atmospheric disturbances that lead to upwelling of the atmosphere to form thunderstorms.  

Rising bodies of moist air thus occur closer to the coast of the Americas, leading to increased storminess, not only in South America, but in North America as well.  These low pressure systems that develop in the eastern Pacific can move over the continent and cause severe weather as noted above. In addition, they create upper level winds that tend to shear the tops off of developing tropical storms and hurricanes in the Atlantic Ocean and Gulf of Mexico, leading to a decrease in the number of intense tropical cyclones that develop in these regions.

La Niņa conditions have about the opposite effect of El Niņo conditions.   i.e. better fishing harvests off the west coast of South America, drier conditions in North and South America, more hurricanes in the Atlantic, and wetter conditions in Australia and Indonesia.

Over the past 50 years, the oscillation of warm water back and forth across the tropical Pacific Ocean has created El Niņo conditions 31% of the time, La Niņa conditions 23% of the time, and "normal" conditions 46% of the time.

Examples of questions on this material that could be asked on an exam

1. What is the difference between weather and climate?
   
   
2. What are greenhouse gases, how does their concentration in the atmosphere affect climate, and what are the major greenhouse gases?
   
   
3. What is the evidence for global warming and how is this explicable if one considers the carbon cycle?
   
   
4. Besides warming of the atmosphere, what are some of the other possible effects of global warming?
   
   
5. Why does water in the oceans store solar energy, and how does this solar energy stored in the oceans get into the atmosphere?
   
   
6. What is convection and why does it occur?
   
   
7. What happens at high pressure centers and low pressure centers in the atmosphere?
   
   
8. Explain through words and pictures what happens along a cold front, warm front and occluded front.
   
   
9. What are the effects El Niño and La Niña in terms of their potential to contribute to natural disasters?

References

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