Michael Cooley

    4/12/07

     MET4002 – Climatology

        Hurricane Research Paper

 

Hurricanes are an important weather phenomenon that we must live with as part of science and society. "It all starts far out in the ocean with simple, summer-type thunderstorms and if the ingredients are mixed in just the right proportions and at just the right time, mighty chef nature produces a storm of enormous energy: a hurricane." (White, 2007) The more in depth research we can acquire into these deadly and costly storms the better prepared we can be to live with them. "Storms come, for the most part, in two flavors: tropical cyclones and extratropical cyclones. (Norcross, 2006, pg. 55)" If we know a vast amount of information regarding tropical and extratropical cyclones today, then why are we still struggling from storm impacts such as Hurricane Katrina’s impact on the Mississippi Gulf Coast in August of 2005? Maybe this question can be answered through further research on tropical cyclone activity and understanding how tropical cyclone activity varies from year to year. Seasonal hurricane activity varies from year-to-year because of many factors. These factors are ultimately responsible for the number of storms each hurricane season has and each year these factors change. These changing factors can lead to an above average, average, or below average hurricane season.

"Fluctuations in tropical cyclone activity are of obvious importance to society, especially as populations of afflicted areas increase. (Emmanuel, 2005)" The major factors influencing how hurricane numbers can vary from year to year are a combination of various things. These factors are sea surface temperatures and the effect of global warming, thermohaline circulation in the Atlantic Ocean, the state of El Niño, the state of the North Atlantic Oscillation (NAO), state of the El Niño-Southern Oscillation (ENSO), wind shear and the location of frontal boundaries, pressure systems, and steering currents such as the Bermuda high. Without one or a combination of these factors the likelihood of an above average hurricane season occurring becomes less and less probable. According to CBS Hurricane Analyst Bryan Norcross, "Water temperatures in the Atlantic Basin (north of the equator) go up and down in twenty- to forty-year cycles. El Niños and La Niñas occur in three- to seven-year cycles, although there is considerable variability." (Norcross, 2006, pg. 18) "Tropical cyclone intensity change has long been known to be the result of a combination of internal dynamics, atmospheric environmental influences, and the impact of the ocean below." (Aberson et al., 2006) Sea surface temperatures and the state of El Niño can have a significant impact on the Atlantic Hurricane Season. "Gray (1984) showed that overall Atlantic hurricane activity is reduced during El Niño and increased during La Niña." (Tartaglione et al., 2003) "Research shows that climate factors influence hurricane frequency differentially. The effect of the El Niño–Southern Oscillation (ENSO) on the frequency of hurricanes forming over the deep Tropics is significant, but its effect on the frequency of hurricanes over the subtropics is small. The North Atlantic Oscillation (NAO) plays a statistically significant role in modulating coastal hurricane activity." (Elsner and Jagger, 2006)

As sea surface temperatures continue to rise due to global warming, hurricane intensity has the potential to become greater and greater. ENSO plays a vital role in determining how busy an Atlantic Hurricane Season might be. In fact, "The relationship between ENSO phases and hurricane landfalls in the Caribbean is similar to that for hurricane activity in the entire Atlantic basin." (Tartaglione et al., 2003) Scientists have collected data over the past 50 years showing that as the world is warming up so are the ocean’s waters. Adding more warmth to the world’s oceans will only be adding more "fuel to the fire" once hurricane season starts. "The Atlantic is already 11 years into a natural decades-long warm phase that intensifies hurricanes, so more warming is the last thing we need." (Svoboda, 2006) NOAA scientists have found that "the top 1,000 feet of the world’s oceans warmed by about a half a degree Fahrenheit in the last half of the twentieth century. In January 2001, the Intergovernmental Panel on Climate Change (IPCC), a large group of scientists from around the world who conducted an extensive study of climate change, concluded in a report that there is no evidence that the peak wind speeds or amounts of precipitation in tropical cyclones had increased during the last half of the twentieth century." (Sheets and Williams, 2001 pg. 266-267) This goes to show that the warm-up of ocean waters cannot be directly linked to the overall strength of tropical cyclones in the last half of the twentieth century. As time progresses, global warming continues to be a large debatable factor on how it impacts the fluctuation of tropical cyclone activity every year. Many researchers have studied global warming and its affect on seasonal hurricane activity. According to researcher Christopher Landsea, "The Atlantic hurricane-intensity record by itself is not long enough to infer any connection between hurricanes and either global warming or multi-decadal cycles, but the high correlation between hurricane activity and tropical SST is remarkable. (Landsea, 2005)"

Not only do sea surface temperatures combined with global warming and the state of El Niño affect the fluctuation of tropical cyclone activity, but so does the thermohaline circulation that is present within the ocean as well as steering currents. Thermohaline circulation that is found within the Atlantic is believed to be the primary source behind the changes in ocean salinity levels. These changes in ocean salinity in the Atlantic over multiple decades are known as the Atlantic Multi-Decadal Oscillation. A large fluctuation in the number of major or intense hurricanes and tropical cyclones occurred during the 2004 and 2005 seasons. "This large increase in Atlantic major hurricanes is primarily a result of the multi-decadal increase in the Atlantic Ocean thermohaline circulation (THC) that is not directly related to global temperature increase. (Klotzbach and Gray, 2005)" "A large portion of U.S. storm and hurricane landfall variability is related to where the storms form and whether steering currents are favorable or unfavorable for bringing them to the United States. (Lyons, 2003)" Steering currents such as the Bermuda high help steer storms which can make landfall of a storm more and more eminent or less and less likely with time. During past Atlantic Hurricane Seasons with high occurrences of landfalls, "Atlantic steering currents were more efficient at bringing storms and hurricanes to the U.S. coast. (Lyons, 2003)"

All of these mentioned factors that influence why tropical cyclone activity varies from year to year are also studied thoroughly by scientists and forecasters. The two most dominant factors in forecasting how many storms will develop in a hurricane season are the water temperatures of the Atlantic basin and whether an El Niño or a La Niña is present. Scientists and forecasters study global warming, sea surface temperatures, El Niño and La Niña conditions, ENSO conditions and the Atlantic Multi-Decadal Oscillation as well as the Atlantic Thermohaline Circulation (Atlantic THC) and the location of frontal boundaries and steering currents in relation to tropical cyclones. In terms of storm intensity, researcher Elizabeth Svoboda states, "Experts look at surface temperatures in the Atlantic and Pacific oceans and wind patterns in the atmosphere's lower layers--the troposphere and stratosphere--to gauge a season's intensity." By studying all of these factors we can reach a better understanding of why tropical cyclone activity fluctuates from year to year and how to better plan and prepare for such frequent and sometimes intense storms.

Other factors responsible for the fluctuation of land falling hurricanes in particular are the presence of dry and stable air, wind shear, pressure systems, troughs, and frontal boundaries. During the 2005 hurricane season, wind shear was not much of a factor in determining the amount of tropical systems that formed. However, in 2006 there was a considerable amount of wind shear present in the Atlantic basin that caused many storms to dissipate before they could fully develop. As winds blew dust plumes from the west African coast, wind shearing accompanied it. "The dust plumes contain very dry air, and also had high wind shear (winds that blow storms apart)." (NASA, Jan 18^th 2007) A year-round high-pressure system constantly exists in the Atlantic Ocean. This high-pressure system, known as the Bermuda high, is responsible for hampering any development of storms around the high pressure and steering storms either closer to or farther from the US mainland. If this high-pressure system is situated closer to the eastern coast of the US the likelihood of a tropical system entering the Caribbean Sea and Gulf of Mexico becomes more and more likely. Likewise, if this Bermuda high is more towards the center of the Atlantic Ocean, it will decrease the likelihood of a tropical system entering the Caribbean or Gulf of Mexico. Each year the Bermuda high plays a vital role in determining the path that a tropical system will take and where specific storms may form in the Atlantic. In 2005, the Bermuda high was situated more to the West-Southwest in the Atlantic and steered systems west into the warmer Caribbean and Gulf waters. Last year in 2006, the Bermuda high was smaller and shifted into the eastern Atlantic, "So it turned storms like Florence, Gordon, Helene and Isaac north into cooler waters of the central Atlantic, and directed them eastward into colder waters in the North Atlantic Ocean." (NASA, Jan 18^th 2007)

Low-pressure systems can also affect the fluctuations of storms and the paths they take during the course of a hurricane season. Low-pressure systems act just the opposite as high-pressure systems do. While a high-pressure system blocks and impedes any tropical development, a low-pressure system can inhibit and promote tropical development. A great example of this was in August of 2004 when Hurricane Charley made landfall in Port Charlotte, Florida. On August 13^th 2004, Hurricane Charley roared ashore stronger and farther south than what forecasters had predicted. This rapid intensification and eastward movement of the hurricane was believed to have been associated with a low-pressure trough. It was later confirmed by meteorologists such as Meteorologist Ernie Jillson with the National Weather Service in Ruskin, "A persistent trough of low pressure in the middle and upper levels of the atmosphere over the eastern United States blunted Charley's northward migration." (Zaloudek, 2004) I believe that Hurricane Charley’s landfall in 2004 appropriately and clearly exemplifies that a low-pressure system or trough can affect a tropical cyclone by influencing its intensity and movement.

One complex situation that may arise when trying to understand the fluctuation in tropical cyclone activity on a year-to-year basis is whether the frequency of storms or the intensity of storms is the biggest threat to the environment. According to Kerry Emmanuel, "Although the frequency of tropical cyclones is an important scientific issue, it is not by itself an optimal measure of tropical cyclone threat." (Emmanuel, 2005) What Emmanuel is saying is that we must look at not only tropical cyclone frequency throughout the hurricane season, but also each tropical cyclone’s power and destructiveness in measuring the greatest threat from these storms. Fluctuation of tropical cyclone activity and cyclone destructiveness has been linked together since the mid-1970s in a worldwide debate on global warming. Kerry Emmanuel has done research into why since the mid-1970’s storms have increased in frequency and intensity. In his study, Emmanuel developed an index of the potential destructiveness of hurricanes and based this index on the total dissipation of power of a hurricane during its lifetime. Emmanuel’s research suggests that, "Future warming may lead to an upward trend in tropical cyclone destructive potential, and—taking into account an increasing coastal population—a substantial increase in hurricane-related losses in the twenty-first century." (Emmanuel, 2005) Hurricane Katrina during the 2005 hurricane season may have only been the start to this destructive trend.

When comparing the 2005 season with the 2006 season, "the 2006 Atlantic basin hurricane season was much less active than the 2005 season. This is the first year that there have been no land falling hurricanes along the U.S. coastline since 2001." (Klotzbach and Gray, 2006) There are multiple reasons that the 2006 season was not nearly as active as the 2005 season. These reasons were the presence of weak El Niño conditions, changes with the steering flow of winds surrounding the Bermuda high, and lower sea surface temperatures. NASA researchers have suggested that the lower sea surface temperatures may have been caused by dust storms coming from Africa by blocking out the sun’s light and as a result, cooling the ocean waters. "These cooler waters may have impeded some storminess since hurricanes rely on warm waters to form." (NASA, Mar 3^rd 2007) Sea surface temperatures "were found to be as much as 1 degree Celsius (1.8 degrees Fahrenheit) cooler than in 2005." (NASA, Mar 3^rd 2007) Other researchers however, have speculated, "That atmospheric dust may have had relatively little influence on the 2006 hurricane season compared to the effects of underlying El Niño conditions. (NASA, Mar 3^rd 2007)

In addition to the 2005 season having more storms than the 2006 season, the 2005 season also had more intense storms than the 2006 season. In 2005, there were a total of 7 intense hurricanes (Dennis, Emily, Katrina, Maria, Rita, Wilma and Beta) while 2006 saw only 2 intense hurricanes (Gordon and Helene) that didn’t make landfall anywhere. The rapid intensification of storms during the 2005 season was caused by a vast amount of warm ocean water for the storms to thrive on and low vertical wind shear of the storms. The contrast between these two seasons can be associated with the positioning of the Bermuda high steering current and the weak El Niño during the 2005 season versus the much stronger El Niño during the 2006 season. The strong El Niño combined with a high amount of vertical wind shear helped blow any storms apart that may have formed during the 2006 hurricane season.

Many people are wondering what is in store for the 2007 hurricane season. For the 2007 season, I am predicting a total of eighteen tropical storms, ten hurricanes, and five intense hurricanes. I am basing my prediction on the rapid and continuing dissipation of El Niño, the possibility of a reduced amount of vertical wind shear from a weaker El Niño, the possibility of a weak to moderate La Niña, and the above average-warm sea surface temperatures of the Gulf of Mexico and Atlantic waters. I am also basing my prediction on the fact that right now ENSO conditions are currently neutral showing the continued dissipation of El Niño. I am also anticipating a westward shift of the Bermuda high steering tropical systems and hurricanes more towards the Caribbean and Gulf of Mexico. I believe that the wind shear and dry, stable air associated with the dust plumes from Africa will have a minimal impact on the hurricane season. I feel that the most important factor in determining how busy the 2007 Atlantic Hurricane Season lies in the state of El Niño and La Niña. If we experience weak to moderate La Niña conditions combined with upper-level winds that are favorable for tropical development and little wind shear, then we all could be in for a very active season. As all of these previously mentioned factors become more and more influential on this year’s 2007 Atlantic Hurricane Season, the probability of meeting or even exceeding my prediction becomes more and more likely. As a society we all must realize that "tropical cyclones affect U.S. tourism, transportation, the coastal ecosystem. They can devastate the coast and bring massive flooding inland." (Lyons, 2003)

The 2004 and 2005 hurricane seasons were record setting seasons while last year’s hurricane season was very minimal in terms of the number of storms. What lies ahead for the 2007 season has yet to be determined, but one can observe a great amount of fluctuation in the number of tropical cyclones over the past few seasons. It is believed that as the Atlantic basin enters an active period, hurricane seasons to follow are generally active and this period can last from 10-20 years. Early warning signs and factors are pointing to another above average season and much more active than what we saw last season. Hopefully hurricane season’s past such as the 2004, 2005, and 2006 seasons have made people aware that each season varies in terms of the frequency and intensity of storms. We as a society cannot let our guard down as the 2007 hurricane season is vastly approaching us.

As more and more coastal locations become built-up and urbanized, the more these areas could become greatly impacted by a tropical cyclone. In order to protect such areas we, as a society, must develop an even greater understanding of tropical cyclone activity and how it can fluctuate on a yearly basis. We must also analyze the certain factors that cause fluctuations in tropical activity such as the state of ENSO and whether or not an El Niño or La Niña is present. In doing so we can create better forecast models and have the ability to warn those in a storm’s path better. We must also develop a better understanding of a storm’s intensity and develop an even greater understanding into what controls hurricane intensity. By doing all of this we can help minimize the impacts of storms as much as possible and help protect the lives of many people. Understanding the factors that cause fluctuations in tropical cyclone activity from season to season is just the start. In the days and even months following Hurricane Katrina in 2005, there was a struggle between the Federal Government, Federal Emergency Management Association (FEMA), and the Army Corps of Engineers on who should take the downfall for not being prepared for such a disaster. Instead of various government agencies finger-pointing at one another all of the time, agencies such as FEMA and the Environmental Protection Agency (EPA) must continue to work with one another in order to help keep people and property safe and minimize impacts from tropical weather. By working together as a society, we can help keep one another informed as to the potential dangers of the upcoming season. We all must also adhere to hurricane season predictions and plan accordingly. Planning for the worst and hoping for the best during a hurricane season is much better than planning very little or not planning at all. If the factors that are favorable for tropical development such as the presence of La Niña conditions, a westward positioned Bermuda high, warm sea surface temperatures, and very little wind shear, then the 2007 hurricane could be another one of the most active on record. Only time and Mother Nature will tell what the 2007 hurricane season will bring and if it will be anything like the 2005, 2006 hurricane season, or a season all of its own.

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http://www.nasa.gov/mission_pages/hurricanes/archives/2007/hurricane_dust.html

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