Hurricanes and El Nino



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Sophia Turczynewycz

WCP 401


6 May 2005

Final Paper



Hurricanes and El Nino

“Oceans cover more than two-thirds of our blue planet. The waters move in a global circulation system, driven by subtle density differences and transporting huge amounts of heat”



(Rahmstorf 207).
Two weather phenomena, hurricanes and El Nino, are direct results of the dance between the atmosphere and the ocean. The word hurricane comes from the god of stormy weather, or an evil god that causes destruction and loss of life. The ancient Mayans of South and Central American called this god Hunraken and the Taino people of the Caribbean called the god Huracan. El Nino translates to “the little boy” or “Christ Child” and was named by Peruvian fisherman in the late 1800s because the event happened during Christmas time. Both phenomena have been around and observed for a long time, and research has shown a correlation between El Nino events and the number of hurricanes that occur during a particular season. By looking at the ingredients needed for hurricanes to occur, looking at ingredients needed for El Nino to occur, and seeing how they overlap contributed to confirming this research. Data collected about hurricanes and their occurrence and intensity throughout the past hundred years and comparing it to the occurrence of El Nino events are also explored in hopes to confirm this prior research.

Hurricanes


Hurricanes fall under the category of tropical cyclones. These cyclones are storms that form over warm waters in the Tropics near the equator, spinning around a calm center of low pressure that moves. There are three types of tropical cyclones – tropical depressions, tropical storms and hurricanes. Tropical depressions are cyclones that have a steady wind speed of 39 mph or less. Once those winds reach 39 mph or higher, they are known as tropical storms. If the winds reach 74 mph or higher, they are known as hurricanes in the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline and the South Pacific Ocean east of 160E (http://www.aoml.noaa.gov/hrd/tcfaq/A1.html 2004).

All hurricanes begin life the same way – in the warm, moist atmosphere over tropical ocean waters (low pressure systems). The atmosphere gathers heat energy through contact with ocean waters that are above 80 degrees F to a depth of about 200 feet. The warm, moist air soars upward and as it rises upward, it cools. Then moisture evaporating from the warm waters enters the atmosphere, thus beginning the hurricane. The low-pressure system acts like a chimney where the warm air is drawn on at the bottom, rises to the top in a column and then spreads out. The hurricane forms bands of clouds and winds near the ocean surface that spiral inward, forming thunderstorms, heating the air further and forcing winds to rise higher into the atmosphere and the spinning to increase. The air travels in a spiral and in the Northern Hemisphere, the spiraling winds travel counterclockwise. In the Southern Hemisphere, these winds travel clockwise. (Simon 4)

A hurricane has many parts to it – the eye, the eye wall and spiral bands. NOAA defines the eye as a “roughly circular area of comparatively light winds and fair weather found at the center of a severe tropical cyclone” (http://www.aoml.noaa.gov/hrd/tcfaq/A11.html 2004). Within the eye the winds may be calm and there is little or no precipitation. It is the region of lowest surface temperature and warmest temperatures where most of the air is sinking. Surrounding the eye of the hurricane is the eye wall, which is “the roughly circular ring of deep convection which is the area of highest surface winds in the tropical cyclone” (http://www.aoml.noaa.gov/hrd/tcfaq/A11.html 2004). The eye wall has a net upward flow of air and is the strongest part of the hurricane. And the lastly, outside the eye wall are the spiral bands. Convection in tropical cyclones is organized into long, narrow rain bands, which seem to spiral into the center of the cyclone: these are the spiral bands. Hurricanes are categorized according to how fast the winds are and are measured on the Saffir-Simpson Hurricane Scale:


Scale Number

Winds (mph)

Winds (knots)

Damage

1

74-95

64-82

Slight

2

96-110

83-95

Moderate

3

111-130

96-113

Extensive

4

131-155

114-135

Extreme

5

156 or higher

135 or higher

Catastrophic

(http://www.nhc.noaa.gov/aboutsshs.shtml)

Hurricane season occurs during the times of the year when the seas are at their warmest and the air above them becomes heavy with moisture. The season always begins in the tropics and the only ocean where these storms do not appear is the South Atlantic. In the western Pacific, hurricane season occurs from June through October. Along the Atlantic and Gulf coasts, hurricane season is from June through November and the height of hurricane season for this area is August and September. The east coast averages five hurricanes a year. (Simon 1)

“Hurricanes not only are threatening because of wind, but also because of the lightning, tornados, rain and flooding they bring about. They also create storm surges, which is the high waves that come about right before a hurricane hits land. The storm tide is the combination of a storm surge and high tide, and prove to be very dangerous and violent” (Effect of El Nino/La Nina on Hurricanes 2004). The powerful winds of a hurricane can rip off roofs, take down trees and power lines and turn anything that is loose into a dangerous weapon. The greatest destructive force is the storm surge that claims 9 out of 10 victims in a typical hurricane. The storm surge causes flooding on coastal towns and cities, as well as pushes extra water into bays, rivers and creeks, making them overflow. (Souza 33) Because hurricanes are so destructive, it is important to understand how they form, when they form and how to predict them. By understand that, as well as how it is so interconnected with El Nino, will help meteorologists in the future.

El Nino


El Nino is the most powerful weather phenomenon that occurs every 3-7 years, altering the climate across the globe. This phenomenon can be compared to a big hot tub full of warm water, where the “hot tub” is the Pacific Ocean. This event is responsible for all kinds of weather, including tornadoes, hurricanes and floods. The main features of El Nino are ocean warming, changing winds and change in air pressure (southern oscillation). El Nino is part of the tropical Pacific ocean-atmosphere system and develops from the interaction between the ocean and the atmosphere in the tropical Pacific region. In a normal year, water near the equator in the Pacific Ocean is very warm because the sun is directly overhead. Trade winds that blow from east to west across the ocean push this warm water toward the western Pacific, away from South America. Once this warm water is pushed away, cooler water rises to the surface along the coast of South America. The cool water is rich with plankton, which feeds fish and other sea creatures, providing them with important nutrients.

During an El Nino year, the trade winds that normally blow from east to west weaken, and can blow in opposite directions. This moves the warm water towards South America instead of away from it. Sea surface temperatures near the equator may rise as much as 5 degrees Centigrade. Now that there is warm water, the plankton is not as abundant so fish leave the area – hence the fisheries in Peru losing a lot of business. Besides the fisheries in Peru, EL Nino has an affect on weather patterns all around the globe. During an El Nino event, the warm water in the western Pacific Ocean heats up the air above it. This air then rises, adding moisture to the atmosphere, changing the pattern of the jet stream. The jet stream is a fast moving flow of air in the upper atmosphere and carries weather systems to all parts of the earth. When this jet stream is disrupted or changed, the typical weather of an area is also changed. El Nino contributes to droughts, forest fires, floods, mudslides, tornados and hurricanes.

No two El Nino events are alike, and what determines an El Nino event depends on what is measured and where it is measured. A measuring system created by the Japanese Meteorological Agency (JMA) is often used. This JMA index shows that an EL Nino year is one where the ocean temperature measured along the equator between 150 degrees east and 90 degrees west average more than 0.5 degrees Centigrade above normal for at least six months. A typical El Nino event begins in the northern hemisphere spring or summer and lasts for a year. For the United States, an El Nino event usually means above-normal rainfall conditions for the southeastern part of the U.S. and dry conditions in the western part.

Hurricanes and El Nino


Research shows a correlation between ENSO events and the number of hurricanes during a particular season. Hurricanes are tropical storms with sustained winds over 74 mph. Warm tropical water helps to build and sustains this weather phenomenon. Past studies have suggested that during El Nino years, there is a decrease in hurricanes in the Atlantic and an increase in hurricanes in the Pacific. During El Nino years hurricane activity is suppressed across the North Atlantic and occasionally enhanced across the eastern North Pacific. Strong vertical shear typically inhibits tropical storm intensification, while weak vertical shear aids tropical storm development and because El Nino is a strengthening of westerly winds, it causes increased shear over the Atlantic basin. There have been yearly variations in hurricane activity, especially during the 1990s because of the active El Nino cycle that has been occurring during this period. While El Nino events tend to suppress hurricane activity in the Atlantic, La Nina events increase activity as shown in the 1995-1995 La Nina events where there were 11 hurricanes that occurred (Bulletin of the American Meteorological Society1998). Other proof that hurricane activity is suppressed in the Atlantic during the El Nino years is that during 1900-1983, 54 major hurricanes struck the U.S. coast and only 4 occurred during the 16 El Nino years while 50 occurred during the 68 non-El Nino years. The probability of two or more hurricanes making landfall during El Nino events is 21% while during neutral conditions it is 46%. After a reanalysis of this data, the probability of hurricanes during an El Nino year is 28% and during neutral events 48% and lastly during La Nina events, 66%. During the past 98 years, there has not been more than one major US hurricane that has been associated with El Nino (Bove 2477).

In contrast to the reduction of hurricanes in the Atlantic Ocean the number of hurricanes in other parts of the world increase during an ESO event. “For many years, meteorologists have known that ENSO strongly affects tropical cyclone activity around the world. In some basins, El Nino events increase tropical cyclone activity (e.g. the central North Pacific near Hawaii, he South Pacific, and the northwest Pacific between 160 degrees E and the date line” (Pielke 1999). Because of the economic and human loss that hurricanes can produce it is crucial that the role that ENSO events play in the development and intensity of hurricanes be clearly understood.

Hurricane season in the Atlantic occurs from June through November with the peak during late August into mid-September. Hurricane season in the Northeast pacific occurs from late May or early June thought late October or early November; with a peak in late August and September. A slightly earlier season for tropical activity affects the Pacific with a focus along the Central American coastline and into the Gulf of California. The greater frequency of named storms in this region is the focus of increased ENSO events with a correlation of Pacific tropical activity in association with a strengthened sub-topical jet veering storm activity eastward and toward land. Despite all the research that has shown hurricane activity is suppressed in the Atlantic region during El Nino events, that does not mean that hurricanes will not occur especially because predictions of El Nino or La Nina events are always uncertain.



Data


To test the research that states there is a decrease in hurricane activity in the Atlantic basin and an increase in the Pacific, I compiled data from NOAA’s hurricane website, looking at the number of hurricanes that occurred per year and in which region. I was able to construct charts using data from the past 10 years, as well as from 1954.


Year

# Hurricanes in Atlantic

# Hurricanes in Pacific

El Nino year?

2004

9

6

NO

2003

7

7

YES


2002

4

6

YES


2001

9

8

NO

2000

8

6

La Nina

1999

8

6

La Nina

1998

10

9

La Nina

1997

3

9

YES


1996

9

5

NO

1995

11

7

NO

Data compiled from NOAA’s hurricane center (http://www.nhc.noaa.gov/pastall.shtml)
This first table shows the number of hurricanes that occurred when and where, and if it happened during an El Nino year. One of the biggest El Nino events occurred in 1997, and as the chart shows, it confirms the past research, having a significantly lower number of hurricanes occur in the Atlantic and a higher number of hurricanes occur in the Pacific. It shows the same for the El Nino that occurred in 2002. The data does not confirm that there are more hurricanes in the Pacific Ocean during La Nina events. During the La Nina years listed here there are still more hurricanes occurring in the Atlantic than in the Pacific Ocean. I realized as I collected this data and looked at this chart, that just because there are still more hurricanes occurring in the Atlantic during those years of La Nina events, does not mean hurricane activity did not rise in the Pacific. I did not take into consideration that there are less hurricanes occurring in the Pacific, so 8 hurricanes in the Pacific in 1998 could be an increase in hurricane activity for the Pacific, which could normally be 4 or 5.

This graph shows the total number of hurricanes that have occurred in the United States since 1954. El Nino years are as follows: 1957, 1958, 1965, 1969, 1972, 1973, 1976, 1982, 1983, 1986, 1987, 1991, 1992, 1994 and 1997. If you look at these years on the map, you can see that during these El Nino years, the number of hurricanes is lower then the Non El Nino years.


Data for previous two graphs compiled from http://www.oar.noaa.gov/k12/html/elnino2.html and

http://www.nhc.noaa.gov/pastall.shtml
This last graph shows the number of hurricanes that have occurred since 1954 and illustrate which years are El Nino and which are non El Nino years. Basically the same as the line graph, just in bar form. It would have been useful to separate the graph even more and see which hurricanes occurred where, to see if more occurred in the Pacific or Atlantic, but this still gives a pretty good idea.

Conclusion


Both hurricanes and El Nino are fascinating weather phenomenon that affect the climate around the world. The same ingredients are needed for both events to occur – warm water, winds, and the basic interaction between the two, a dance between the atmosphere and the ocean. El Nino events tend to generate hurricanes in certain areas of the world, specifically the Pacific Ocean. Data collected and analyzed shows that El Nino does result in a lower number of hurricanes occurring in the US, specifically in the Atlantic. This is because hurricanes need warm waters and moist air to occur in order to form. El Nino raises the temperatures of the sea, but in the Pacific Ocean, thus making the hurricane activity decrease in the Atlantic. Strong vertical shear typically inhibits tropical storm intensification, while weak vertical shear aids tropical storm development and because El Nino is a strengthening of westerly winds, it causes increased shear over the Atlantic basin thus decreasing the amount of hurricane activity that will occur. However, unusual events could occur and you could have years where El Nino does not have an effect on the hurricanes at all, or the number of hurricanes could increase instead. Just because there is an El Nino event predicted to occur for a certain year does not automatically make that year have less hurricanes.

References


Arnold, Caroline. El Nino: Stormy Weather for People and Wildlife. Clarion Books, New York:

1998
Atlantic Oceanographic and Meteorological Laboratory and NOAA. 2004



http://www.aoml.noaa.gov/hrd/tcfaq/A11.html Retrieved 5 April 2005
Bove, Mark C., Elsner, James B., Landsea, Chris W., Niu, Xufeng and O’Brien, James. “Effect

of El Nino on U.S. Landfalling Hurricanes, Revisted.” Bulletin of the American



Meteorological Society. November 1998
Bredson, Carmen. El Nino and La Nina: Deadly Weather. Enslow Publishers, Inc. Hew Jersey:

2002
El Nino. June 2004. http://www.oar.noaa.gov/k12/html/elnino2.html Reetrieved 6 April 2005


“Hurricane Experts Assess the 1997 Atlantic and Pacific Hurricane Seasons.” Bulletin of the

American Meteorological Society. January 1998
 Lauber, Patricia. Hurricanes: Earth's Mightiest Storms. Scholastic Press, New York: 1996
National Hurricane Center. http://www.nhc.noaa.gov/pastall.shtml Retrieved 5 April 2005
Pielke, Roger A. and Landsea, Christopher N. “La Nina, El Nino and Atlantic Hurricane

Damages in the United States.” Bulletin of the American Meteorological Society. October

1999
Rahmstorf, Stefan. “Ocean Circulation and Climate During the past 120,000 Years.” Nature.

September 2002


Simon, Seymour. Hurricanes. Harper Collins, China: 2003

 

Souza, D.M. Hurricanes. Carolrhoda Books Inc., Minnesota: 1996



 
 
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