Read and analyze the scenario and situation

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  1. Read and analyze the scenario and situation

Situation: Hurricane Katrina began in the Southeastern Bahamas on August 23 as a tropical depression. By August 25 it had become a category 1 hurricane and had hit the southern coast of Florida. On August 28th it had been declared a category 5 hurricane, and it hit land the next day. Hurricane Katrina caused devastation in New Orleans and Mobile, but a lot of the damage was due to the storm surge and levee breaks. Hurricane Katrina not only caused over $81 billion in property damage, but also took 1,500 lives (the third deadliest hurricane on record).

Scenario: Recently there has been an increase in the number of tropical cyclones, with the record number in 2005. The U.S. National Ocean Service has asked us to analyze whether global warming could be causing the increase in hurricane frequency and intensity. We will do this by looking at past data on hurricane activity and also how that activity has changed today. To fully analyze this possible cause we will also have to look more into global warming and what it causes.

  1. List your personal understanding, ideas, or hunches

  • They are formed over warm bodies of water

  • Hurricanes have high winds

  • Hurricanes have an eye

  • When Hurricane Katrina hit land it destroyed a lot of land because of the high winds, but the breaking of the levy was what caused the major destructions

  • There are different categories of hurricanes.

  • Warm waters have an effect on hurricanes.

  1. List what is known

Different Names in Different Places

A tropical cyclone is a low-pressure system over tropical or sub-tropical waters, accompanied by cyclonic surface wind and thunderstorm activity, i.e. organized convection. They require a water temperature of at least 80°F to form. Tropical cyclones that never top a speed of 39 miles per hour are designated as Tropical Depressions, but once that speed is reached, they are called Tropical Storms, and are given a name. Winds must reach 74 miles per hour in order to classify a tropical cyclone as a hurricane, but they can get as high as 160.

Once this speed is reached, the name for the storm differs depending on where it is located. Though called a hurricane in the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E, the same kind of storm is called a typhoon in the Northwest Pacific Ocean west of the dateline, a severe tropical cyclone in the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E, a severe cyclonic storm in the North Indian Ocean, and a tropical cyclone in the Southwest Indian Ocean.

Hurricanes in Particular

With an eye that is usually 30-40 miles wide, the storms can exist for more than two weeks over open water before striking land. The most dangerous part of a hurricane, though, is the Storm Surge: a dome of ocean water that can be 20 feet high at its peak and 50 to 100 miles wide. This surge can devastate coastal communities. Nine out of 10 hurricane fatalities can be attributed to the storm surge, though other dangers that accompany hurricanes include high winds, debris, tornadoes and flooding from heavy rains.
There are positive effects of hurricanes as well. For instance, the precipitation that comes with them can bring much-needed moisture to otherwise dry areas. A prime example of this is Japan, which gets half of its moisture from typhoons. Destruction of property and buildings also encourages new construction projects and redevelopment, thereby increasing property values.
To meet the conditions under which a hurricane can occur, there must be a low-pressure system, warm temperatures over the ocean, a moist environment (precipitation), and tropical wind patterns over the equator.
Interestingly, on average, 100 tropical disturbances develop each year between May and November over the Atlantic Ocean. Half of these begin over Africa near the Cape Verde Islands.  About 25 of these disturbances develop into tropical depressions. Out of these 25, 10 become tropical storms and only 6 of these 10 become hurricanes. Of these 6 hurricanes, 2 are likely to strike the coast of the United States.
The Atlantic hurricane season is between June and November, with very few hurricanes occurring outside of these months. Like water down the drain, hurricanes in the Northern Hemisphere rotate counterclockwise, those in the Southern Hemisphere rotate clockwise.

A Few More Terms to Know

Super-typhoon is a term used by the U.S. Joint Typhoon Warning Center for typhoons. The term designates typhoons that reach maximum sustained 1-minute surface winds of at least 150 mph. This is the equivalent of a strong Saffir-Simpson category 4 or category 5 hurricane in the Atlantic basin or a category 5 severe tropical cyclone in the Australian basin.

Major hurricane, a term utilized by the National Hurricane Center, is for hurricanes that reach maximum sustained 1-minute surface winds of at least 111 mph, which is the equivalent of category 3, 4 and 5 on the Saffir-Simpson scale. Intense Hurricane is sometimes also used in scientific literature, but it is an unofficial term.
What is the structure of a hurricane?

The structure of the Hurricane consists of the eye, the eye wall, and spiral rain bands. The eye is the most commonly used icon when talking about hurricanes because it looks like an eye.

The most central part of the storm is where the eye is located and it has a tendency to be an area of calm. This area is between 20-50km in diameter. It works as the focus of the hurricane, the point about which the rest of the storm rotates and where the lowest surface pressures are found. The eye wall is located outside of the eye of the storm and is the location of the most damaging winds and intense rainfall.

The pressure begins to fall more rapidly while the wind speed simultaneously increases. Within the eye wall, the wind speed reaches its maximum but within the eye, the winds become very light ­ sometimes even calm. The surface pressure continues to drop through the eye wall and into the center of the eye, where the lowest pressure is found. Upon exiting the eye, the wind speed and pressure both increase rapidly. The wind speed again reaches a maximum in the opposite eye wall, and then quickly begins to decrease. The wind and pressure profiles inside a hurricane are roughly symmetrical, so a quick rise in winds and pressure through the eye wall followed by a slower increase in pressure and likewise decrease in wind speed would be expected. Rainfall is all so most extreme in the eye wall, dumping up to 3ft of rain within a couple of days. This is one of the reasons Hurricane Katrina caused such devastation.
Large spirals fan out from the eye wall called spiral rain bands. There are sometimes gaps in between these bands where no rain is found. In fact, if one were to travel between the outer edge of the hurricane to its center, one would normally progress from light rain to dry back to slightly more intense rain again over and over with each period of rainfall being more intense and lasting longer until reaching the eye. Upon exiting the eye and moving towards the edge of the hurricane, one would see the same events as they did going in, but in opposite order.

How do hurricanes develop?

Hurricanes have a full life cycle that can last for a matter of weeks from Birth-Life-Death. They first begin as a cluster of thunderstorms over warm tropical waters. The breakdown into single cell, multicell, and supercell covers the major storm types within the spectrum. One "cell" denotes one updraft/downdraft couplet. Thus, there are several updrafts and downdrafts in close proximity with a multicell storm. Multicell storms can be broken down further into the categories of multicell line and multicell cluster storms. The "intense" updraft storm is almost invariably the supercell, a storm capable of producing the most devastating weather, including violent tornadoes.

Once the disturbance becomes a tropical depression the acceleration to the maturity level of tropical storm can take as little as half a day. Under the right atmospheric conditions the making of a tropical depression have wind speeds in the center about 23-39 mph. Once the storm starts to transform into a tropical storm, the wind speeds jump to about 39-73mph, and the storm starts to take on more of a cyclone appearance. As surface pressures continue to drop, a tropical storm becomes a hurricane when sustained wind speeds reach about 74 mph. A pronounced rotation develops around the central core. This is known as the eye. Wind is also important in creating a hurricane. Wind shear is defined as the amount of change in the wind's direction or speed with increasing altitude. Water temperatures are also important. Most storms are found in tropical areas because the optimal water temperatures need to be about 80°F. The warmer the water, the more energy being poured into the hurricane.
How are hurricanes named and why are they named?

When tropical depressions wind speeds reach 39mph they are given names. Starting in the beginning of the season with A and going to Z if there are more than 26 hurricanes within a season, Greek numbers are then adopted. In 1979 a six-year rotating list of Atlantic storm names was elected to alternate between female and male names. Names are retired if there is substantial damage or other special reasons. Such in the case of Hurricane Katrina… there will never be another Katrina due to insurmountable damage that occurred.

The names assigned for the period between 2004 and 2009 are shown below.

Names for Atlantic Basin Tropical Cyclones





































































































































*Lili was retired after the 2002 season, replacement name to be determined.

How are hurricanes categorized?

In most parts of the world the Saffir-Simpson Hurricane Scale is used to measure the severity of the storms. The scale is from 1-5 with 5 being the worst possible category. The numbers refer to the estimated property damage, flooding, rain fall and wind speed.

Tropical Storm

Winds 39-73 mph (34-63 kt)
Category 1 Hurricane

Winds 74-95 mph (64-82 kt)

No real damage to buildings. Damage to unanchored mobile homes. Some damage to poorly constructed signs. Also, some coastal flooding and minor pier damage.

- Examples: Irene 1999 and Allison 1995
Category 2 Hurricane

Winds 96-110 mph (83-95 kt)

Some damage to building roofs, doors and windows. Considerable damage to mobile homes. Flooding damages piers and small craft in unprotected moorings may break their moorings. Some trees blown down.

- Examples: Bonnie 1998, Georges(FL & LA) 1998 and Gloria 1985
Category 3 Hurricane

Winds 111-130 mph (96-113 kt)

Some structural damage to small residences and utility buildings. Large trees blown down. Mobile homes and poorly built signs destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Terrain may be flooded well inland.

- Examples: Keith 2000, Fran 1996, Opal 1995, Alicia 1983 and Betsy 1965
Category 4 Hurricane

Winds 131-155 mph (114-135 kt)

More extensive curtain wall failures with some complete roof structure failure on small residences. Major erosion of beach areas. Terrain may be flooded well inland.

- Examples: Hugo 1989 and Donna 1960
Category 5 Hurricane

Winds 156 mph and up (135+ kt)

Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Flooding causes major damage to lower floors of all structures near the shoreline. Massive evacuation of residential areas may be required.

- Examples: Andrew(FL) 1992, Camille 1969 and Labor Day 1935.

How has hurricane activity changed in the past? What are some of the major hurricanes that have touched land? Which ones have made the most costly damage, human and economic?
A recent study has been done that has shown the increase in category 4 and 5 hurricanes in the recent years. The following table shows how many category 4 and 5 hurricanes occurred during that time and what percentage of hurricanes in that area were either category 4 or 5.


Number (1975-1989)

Percentage (1975-1989)

Number (1990-2004)

Percentage (1990-2004)

East Pacific Ocean





West Pacific Ocean





North Atlantic





Southwestern Pacific





North Indian





South Indian





Recent hurricanes like Katrina received more notoriety, because of all the drama surrounding FEMA and the nations slow reaction to the disaster. This does not mean that hurricanes are getting worse than they have been in the past. The majority of tropical hurricanes today are hitting places that have always been plagued with hurricanes. Bhola was the most deadly hurricane of all time, killing somewhere between three hundred thousand and one million people. Hurricanes today are more costly because our culture is much more advanced. The most costly hurricane in history was Katrina, causing 81.2 billion dollars in damages.

-Bhola - Bangladesh - 1978 - 300,000 – 1,000,000 dead – storm surge was the cause
-Nina – China – 1975 – 100,000 dead – flooding caused 62 dams to fail
-“The Great Hurricane” – Lesser Antilles – 1780 - 22,000 dead – rainfall and mudslides were the cause
-Andrew – USA - 1992 – 50 dead - $40,700,000,000 in damage
-Katrina – USA – 2005 – 1,836 dead – $81,200,000,000 in damage - H. Andrew

Where do hurricanes tend to be located? Are more severe hurricanes in a certain area?

Hurricanes are also called typhoons or cyclones depending on where they occur. Worldwide about 96 tropical cyclones occur a year and 12% of those occur in the Atlantic Ocean. About 25 typhoons occur each year in the Western North Pacific Ocean. The Indian Ocean also has a high cyclone activity. The Indian Ocean has high activity because thunderstorms develop due to the warm Indian Ocean and the nearby InterTropical Convergence Zone (ITCZ), which is a ring of thunderstorms that surrounds the globe in the tropics. Hurricanes are more severe over water and get weakened as they travel over land. Hurricanes develop over large bodies of warm water; they lose some strength when they travel over land. The severity of hurricanes can be affected by their location due to El Nino and West African Monsoons. Hurricanes that do hit land can cause storm surges. These cause more flooding depending on where mouths of rivers are located and what direction the hurricane is rotating. Storm surges can build up to heights ranging from 3 to 30 feet. The Southwest United States is quite flat and therefore especially vulnerable to flooding and storm surges. Most hurricanes occur on the Intertropical Front (ITF), the Intertropical Convergence Zone (ITCZ) or the monsoon trough. The Intertropical Front or ITF is the meeting point of air brought by the trade winds from the circulation of winds from both the northern and southern hemispheres. The Intertropical Convergence Zone or ITCZ is the area encircling the equator where winds from the northern and southern hemispheres come together. Where the ITCZ merges with a monsoonal circulation, it is called a monsoon trough. Monsoon trough’s are common in Australia and parts of Asia. These three weather system areas are the cause of most of the world’s hurricanes.

Hurricane season in the Atlantic and Northeast Pacific is in the summer and early fall with August and September being the peak months. Typhoons occur in the Northwest Pacific throughout the year, but peak in the summer. The North Indian Ocean cyclone season is May to November, while the South Indian Ocean’s season is December to March.

How do hurricanes affect the environment after they have ended?

They produce strong winds, high waves, torrential rain, floodwaters, and sometimes they spawn tornadoes. Hurricanes affect the costal environment the most. Floodwaters can reach as for as 25 miles inland. The storms can carry much heat and energy inland causing atmospheric circulation. Storm surges, strong winds, flooding, tornadoes and rip tides can cause a massive amount of environmental destruction after the hurricane has calmed down. Heavy rain and flooding has a major affect on the environment after the storm. Standing water can cause disease and transportation routes may have been destroyed. Rescue efforts can be cut off due to breakdowns in communications infrastructure. They can be beneficial to dry areas, because they bring great amounts of rain. They also move heat from the tropics pole ward. Sand is eroded from beaches and coral can be rearranged. Hurricanes can strip trees and scatter forest debris, which makes great fuel for wild fires. A great example of this was hurricane Gilbert created an environment for a forest fire, which burned for three months and destroyed 460 square miles of forest.

What is global warming?
Global warming is the average increase in the earth’s temperature, which causes climate change. Some examples of this would include changes in rainfall patterns, the rising of sea levels, and impacts on plants, animals, and humans. One of the main concerns with global warming is whether or not it is caused by human impact on the environment, or whether it is the earth’s natural heating and cooling trends.

What contributes to global warming?
Although all that causes global warming is still a little controversial, scientists are sure that one of the causes of it has to do with the greenhouses that are released into the air. When greenhouse gases (water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halogenated fluorocarbons (HCFCs) , ozone (O3), perfluorinated carbons (PFCs), and hydrofluorocarbons (HFCs) are released into the earth’s atmosphere, trapping all their energy and making the atmosphere warmer. The main culprit is the burning of fossil fuels, such as coal, oil, natural gases.

These all emit high levels of greenhouse gases within the atmosphere; CO2 being the main gas. Also, believe it or not, cows in large numbers can be harmful on the environment. They release high amounts of methane gas, another greenhouse gas, into the atmosphere. The large amount of gases released from volcanic eruptions, along with deforestation, pollution, and potentially water vapor released from industrial smoke stacks all do their share in releasing greenhouse gases into the atmosphere and contributing to global warming.

How can they predict where a hurricane will start and what its path will be?
Scientists can predict where a hurricane may start by monitoring the ocean and climate using a variety of technology to help collect data. (See “How do they track hurricanes?). By keeping close tabs on the ocean’s temperature, wind and wave height, speed, and direction, scientists can recognize weather patterns characteristic of ocean storms and hurricanes. This includes watching cloud formation over the ocean through satellite imaging and aircraft runs over the ocean to look for a hurricane’s notorious circular cloud rotations. But predicting hurricanes is not always as easy as reading a variety of instruments. Scientists’ predictions can be just that, a prediction, which means that they can be wrong. When a hurricane is formed it can be tracked, and its path can be predicted three to five days in advance. They use a variety of strategic and statistical models to help predict its cone shaped projection. Scientist’s accuracy can be off.

Days Before Landfall

Error in miles











How do they track hurricanes?

Today, long gone are the days when brutal ocean storms would hit with only hours warning to flee; hurricane trackers use a wide variety of modern technology combining years of weather research with computer science. Since 1943, aircrafts make routine flights about the coasts and oceans to get a clear bird’s eye view of the sea and see any bad weather patterns heading towards the coast. Satellite imaging that has been used since the 1960’s. The NOAA's National Environmental has two types of satellites: geostationary operational environmental satellites (GOES) for national, regional, and short-range forecasting and polar-orbiting operational environmental satellites (POES) for global, long-term forecasting and environmental monitoring. Together these two types of satellite imaging complete a “global weather satellite monitoring system”, tracking varying temperatures and providing atmospheric data and cloud images needed to track and understand hurricanes. With this comes the use of the Doppler radar, which reads precipitation levels and shifting patterns, along with a wind data across the atmosphere, providing forecasters with an important profile analysis of a storm. Also, buoys and floats are placed throughout the ocean to measure air and wind temperatures, wave and wind height, speed and direction. Scientist combine all of the data gathered by all of the previous mentioned methods to create computer forecast models, which evaluates of the gathered data to calculate future weather patterns. Hurricane predictions in the end fall into two categories, “seasonal probabilities and tracking the path of a current hurricane,” Seasonal predicted hurricanes are based off of past data and number of hurricanes. Every April scientists begin to predict how many hurricanes will occur and their intensity based off of this data.

How do hurricanes form?

Hurricanes form over warm bodies of water, due to evaporation of the seawater. The evaporated seawater is absorbed into the air making it moist. This moist air rises and it lowers the atmospheric pressure of the air beneath it. Where there is low atmospheric pressure the air between the water and the top of the atmosphere is less dense, and therefore weighs less. Like most things in the world, air moves from areas of high pressure to areas of low pressure this creates wind. The warmer the temperature of the water, the more evaporation, and the more moist the air. This causes a lower atmospheric pressure and thus stronger winds.

Development of a hurricane also depends on the wind shear. Wind shear is the difference in direction and speed between winds at upper and lower elevations. Hurricanes develop best when there is little wind shear because this allows the warm inner core of the storm to stay intact. If there is higher wind shear than this could cause the speed of the wind at different elevations to be different, which could break up the storm. The storm would also be affected if the directions at the elevations were different because it could cause the storm to tip over or for the two to split.

The first step in hurricane formation is tropical disturbance where the rain clouds are building because of the moist air rising and cooling. Warm air can hold more water vapor than cool air so when the warm moist air cools the water changes to droplets of water that form clouds. Some of these clouds may become cumulonimbus clouds that produce heavy rains and thunderstorms. If the tropical disturbance has a lot of heat energy and moisture, than the cumulonimbus clouds will remain for longer this is because the warm moist air from the surface is attracted to low pressure.

Hurricanes continue to grow due to the fact that warm moist air is attracted to low pressure. This causes the winds to increase in speed. A hurricane tends to last for 3 to 14 days and travels at about 10 to 20 mph. Hurricanes in the Northern Hemisphere travel east to west, but as they approach the coast they shift north. In the Southern Hemisphere hurricanes travel westward, then turn southwest, south, and then finally southeast. Although most hurricanes follow these patterns the path of a hurricane is difficult to predict, but all move away from the equator to higher latitudes.

At higher latitudes the air is colder which means there is less moisture in the air. These conditions along with greater wind shears weaken the hurricane because it does not have the warm moist air supplying it. When a hurricane goes over land than it quick loses its supply of energy from the warm moist air from the water.
How are hurricanes affected by the environment?

The environment can affect hurricanes by not only increasing the sea surface temperatures but also other factors can influence hurricane activity. In the past decade the high-pressure ridge in the troposphere across the central and eastern North Atlantic has increased, this also increases the hurricane activity. The African easterly lower atmospheric winds have also increase and these winds increase the development of hurricanes that move westward from the African Coast. The increase of these two things could be another reason why hurricane activity has increased, but the cause for their increase was not found.

Are recent increases in the number and strength of hurricanes evidence of global climate change? Or are they the result of a natural climate cycle?

Hurricane activity has increased as shown by data from the National Oceanic and Atmospheric Administration (NOAA). From 1995-2004 has been an increase in the number of intense hurricanes. The question is whether this is due to global warming or has another possible cause.

Many researchers are looking into whether hurricanes can be affected by global climate change and if this will increase the number and intensity of hurricanes. Global warming has been a focus of many studies because it could increase the sea surface temperatures in tropical waters. Hurricanes are formed in the summer and early fall months in the northern hemisphere when temperatures are warmer. Things that can cause hurricanes are high humidity, light winds, and warm sea surface temperatures (80°F). Therefore an increase in temperatures from global warming is something that does need to be examined.

NASA has presented models on how global warming can change the intensity of hurricanes. They say that if ocean temperatures increase by 2-2.5°C the intensity of hurricanes would increase by 6-10%. Just from 1970, the average ocean temperature has increased by half a degree. Other scientists have reported on the positive correlation on the warmer sea surface temperatures and the increase in the number of intense storms. In 2005 and 2006 there was an increase in the number of category 4 and 5 storms. Also when sea surface temperatures increase the number of cyclones increase.

Warmer sea surface temperatures cause more moisture to evaporate and if this is enough it will trigger thunderstorms, which may become tropical storms and hurricanes. Researchers have found that water vapor levels have increased 15% on average in the last 30 years in the Atlantic hurricane zone. This increase results in more category 4 and 5 hurricanes.

The affects of global warming could not just increase the intensity of hurricanes but they also may create hurricanes in parts that have never before seen hurricanes. For example, In 2004, Brazil was struck with its first hurricane ever recorded. Also in 2003, Hurricane Juan struck the coast of Canada.

Another theory for this change in hurricane activity has been reported that excludes global warming as a cause. It is the Atlantic multi-decadal oscillation (AMO) that says the Atlantic has a natural cycle of 20-40 years where there are periods of increased activity and then decreased activity. The AMO is characterized by changes in the sea surface temperatures. Since the mid 1990s we have been in the warm phase. Some evidence for this theory is that during the 1950s and 1960s there were more intense hurricanes during the warm phase, but for two decades following the hurricane activity decreased. In the mid 1990s hurricane intensity increased once again and it is still at an increased level. The arguments against this theory are that measurements have changed and are more advanced since the 1970s. Also in the 1970s and 1980s there was an increase in aerosols which could have caused a cooling, but the global warming from the 1990s overcame the aerosol cooling.

Researchers with the Woods Hole Oceanographic Institution (WHOI) conducted a study looking at the hurricane activity in the Atlantic for the last 5,000 years by using sedimentation data. They found that there were long stretches of increased activity and then stretches of decreased activity that goes along with the AMO. They also found that the number of intense hurricanes increased when the El Niño was weak and when the West African monsoon was strong. The West African monsoons can affect hurricane activity because they provide the seedlings for hurricane development because the storms lead to easterly atmospheric waves moving across the Atlantic.

The research on this subject is continuing and many different ideas have come forth. It is important to note that while they are using past records for analysis, all predictions are based on models rather than actual experiments which is probably why there are many conflicting views. Bender et al. recently reported in Science that global warming would have too effects on the activity of hurricanes. The number of hurricanes would decrease, but for the hurricanes that are formed they would be more intense. There would be more of the category 4 and 5 hurricanes that cause the most damage. This is based on the fact that increased water vapor will intensify existing storms but will inhibit the formation of new storms. They also state that the largest increase in intensity will be seen in the Western Atlantic, north of 20°N.

Bender, M.A., T.R. Knutson, R. E. Tuleya, J. J. Sirutis, F. A. Vecchi, S. T. Garner, I.S. Held. Jan 22,2010 “Modeled impact of Anthropogenic warming on the Frequency of Intense Atlantic Hurricanes” Science
Kerr, Richard Jan 22, 2010 Science review article “Models Foresee More-Intense Hurricanes in the Greenhouse”
What is the Atlantic Multi-decadal Oscillation?

The Atlantic Multi-decadal Oscillation (AMO) is a 20-40 year cycle that occurs in the North Atlantic Ocean. It causes long-duration change in the sea surface temperature. From the extreme cool and warm phase, the different in sea surface temperature is about 1° F. The area affected by the AMO is in the Atlantic between the equator and Greenland, although some areas in the North Pacific can also be affected. Currently, we have been in the warm phase since the mid 1990s.

AMO not only causes severe Atlantic hurricanes, but it also affects air temperatures, rainfall over much of the Northern Hemisphere, the frequency of droughts in North America, and it also over exaggerates human-induced global warming. A warm AMO will cause there to be more frequent and/or severe droughts in the Midwest and Southwest of North America. The Pacific Northwest and Florida will get more rainfall when there is a positive AMO.

The AMO does not influence the number of weak storms, but it does influence the intensity of the storms. In the warm phase AMO causes there to be more intense hurricanes, but does not alter the average number of hurricanes.

The AMO is caused by changes in the south to north circulation and the overturning of water and heat in the Atlantic Ocean. The AMO will enter a cool phase when the overturning circulation decreases. Scientists have figured out what causes the AMO and because of this they are able to calculate the probability of a change in the AMO, but they are still unable to predict exactly when the AMO will change. Despite this, just the probability of the change has provided useful information for long-term planning in climate sensitive applications. An example of this is water management where scientists can look at what the region could be facing in the long term future if a change in the AMO occurs. If there was a change in the AMO, the Pacific Northwest would just see increased rainfall, but this method would be better used in the Midwest where they would have droughts.

The AMO is different from the North Atlantic Oscillation (NAO) because the NAO is the changes in the atmospheric pressures.

Are hurricanes affected by El Niño?

El Niño causes an increase in ocean surface temperatures near the equator in the East Pacific Ocean. Normally during an El Niño there is an increase in hurricane activity in the eastern Pacific but a decrease in activity in the Atlantic, Gulf of Mexico, and the Caribbean Sea. NASA reports that the probable cause of this difference is how El Niño affects the wind shears. Wind shears is a change of direction and/or speed of wind and increased vertical wind shear helps prevent hurricanes from developing from tropical disturbances. The increased vertical wind shear happens in the Caribbean and the Atlantic, while a reduced wind shear is present in the eastern Pacific.
Current Hurricanes

Currently there are some cyclones present in the South Pacific Basin. Cyclone Oli was generated on February 1, and has reached a category 4 cyclone according to the Saffir-Simpson scale. Cyclone Pat is also in the South Pacific Basin but has only reached a category 2 cyclone. Cyclone Rene has reached a category 3 cyclone. In the South Indian Basin, cyclone Gelane has reached a category 1 cyclone.

ESS Analysis

H>E – The sea surface temperature reaches 80°F and a hurricane can form.

H>A – The warm seawater evaporates producing warm moist air.

H>A>E – The warm moist air cools and lowers the atmospheric pressure providing conditions for hurricanes to form.

E>H>A – The hurricane is fueled by the warm moist air that warm seawater produces.

E>A – Hurricanes can cause tornadoes to form.

E>H – Hurricanes can cause storm surges.

E>H – Water can be brought inland by hurricanes.

E>H>L – The water from a hurricane can cause flooding and mudslides.

E>H>L>B – The flooding can contaminate the water supply and can spread disease.

E>L>B – The Hurricanes can strip trees and scatter the forest debris which makes good conditions for forest fires.

E>L>B>A – The forest fires, if the occur, will not only destroy the plants, but will also put excess CO2 in the atmosphere.

  1. List what is unknown

  • How exactly do the West African monsoons increase hurricane activity?

  • Does the AMO affect El Niño frequency?

  • How are hurricanes able to create such high winds?

  • How does flooding from hurricanes affect the water supply?

  • What are common diseases that occur in plants, animals, and humans after hurricanes hit land?

  • How do hurricanes affect the hydrosphere? Do they have a positive or negative affect on marine life?

  • How does global warming affect sea surface temperature?

  1. List what needs to be done

  • Research more on the Atlantic Multi-decadal oscillation.

  • Research more on West African monsoons.

  • Look into how global warming affects the sea surface temperatures exactly.

  • Look into possible affects of the hurricanes on marine life.

  • If we look at the hurricane activity when the AMO is in a cool phase than the true effect of global warming.

  1. Develop a problem statement

Due to an increase in the number of intense hurricanes, we are to look at whether global warming or a natural cycle could be causing these changes. To discover the truth we will not only have to look at natural cycles, but how hurricanes are formed to analyze how global warming could be increasing the number of intense hurricanes.

  1. Gather information

  • Tropical cyclones require a water temperature of at least 80°F.

  • Tropical depressions have speeds below 39 miles per hour, tropical storms have wind speeds between 39 mph and 74 mph, and hurricanes have wind speeds over 74 mph.

  • Cyclones in the North Atlantic Ocean, Southeast Pacific Ocean, and the Northeast Pacific Ocean are called hurricanes.

  • Typhoons occur in the Northwest Pacific Ocean

  • Cyclones in the Indian Ocean and Southwest Pacific Ocean.

  • Eye of the hurricane can be 30-40 miles wide.

  • Most dangerous part of a hurricane is the storm surge, which is a dome of ocean water that can be 20 feet high at its peak and 50 to 100 miles wide.

  • Hurricanes can also cause high winds, debris, tornadoes, and flooding from heavy rains.

  • Half of the tropical disturbances that develop each year in the Atlantic ocean begin over Africa near the Cape Verde Islands.

  • The Atlantic hurricane season is between June and November.

  • Hurricanes in the Northern Hemisphere rotate counterclockwise, those in the Southern Hemisphere rotate clockwise.

  • The eye is an area of calm where the rest of the storm rotates and has the lowest surface pressure.

  • The eye wall causes the most damage and intense rainfall.

  • Hurricanes develop from clusters of thunderstorms over tropical waters.

  • Goes from tropical disturbance to tropical depression to tropical storm to tropical cyclone.

  • Tropical depressions are given names once their speeds reach 39 mph. At the beginning of the season the names start with A and make their way to Z after all the letters have been used then Green numbers are used.

  • In 1979, they began to use male names also.

  • Hurricanes are categorized using the Saffir-Simpson scale.

  • The number of category 4 and 5 hurricanes has increased.

  • Hurricanes can produce tornadoes.

  • Hurricanes form because the warm moist air rises, which lowers the atmospheric temperature. When the warm moist air rises, it cools which produces clouds and rain.

  • The continued supply of warm moist air from the ocean supplies heat energy to the developing hurricane.

  • All hurricanes move to higher latitudes where there is less warm moist air and greater wind shears, this weakens the hurricane. When it moves overland it quickly loses its supply of heat energy and dies.

  • The African easterly lower atmospheric winds increase the development of hurricanes on the African coast.

  • Global warming could increase the sea surface temperatures.

  • Water vapor levels have increased by 15% on average in the last 30 years in the Atlantic hurricane zone.

  • The increased hurricane activity could be due to the Atlantic multi-decadal oscillation.

  • There has been an increase in the number of intense hurricanes.

  • The AMO has been and currently is in the warm phase since the mid 1990s.

  • El Niño causes an increase in hurricane activity in the Pacific, but a decrease in the Atlantic.

  1. Present findings

The goal of this research project was to determine whether the current increase in hurricane intensity was due to global warming or to a natural cycle. In order to accomplish this goal we had to look at what causes hurricanes to form and how they act once they are formed. This research along with the research on the possible influence of global warming has led us to conclude that although global warming could influence hurricane activity in the future, the current increase is due to a warm phase Atlantic Multi-decadal Oscillation (AMO).

Hurricanes can form when sea surface temperatures exceed 80°F because the warm water causes more warm moist air, which leads into hurricanes. The AMO causes the sea surface temperatures to increase, but only by about 1°F. Since it is a small change we believe that is why it does not cause more hurricanes to form, but increases the intensity of already formed hurricanes. This is because the temperature of the water will be warmer which will create more warm moist air which feeds the growing hurricane.

Since the AMO is currently in a positive phase, we should see an increase in intense hurricanes and that is what is happening. Also research for hurricane activity for the past 5,000 years has shown periods of increased intensity of hurricanes and periods of decreased intensity. This information goes along with the theory that the AMO being in a positive phase is what is causing the intense hurricanes.

Global warming can contribute to hurricane intensity if it warms the sea surface temperature in the tropics. In order to fully exclude any roll that global warming could have on the intensity of hurricanes we need wait and see what happens when the AMO returns to a cool phase. If intense hurricane activity is still at an increased level than it is possible that global warming can be a contributing factor.

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