Atlantic
Hurricane/El Niño Project: Teachers Guide
Introduction
and Rationale
This activity provides the student with the opportunity to study
data that will enable them to develop a deeper understanding of
the El Niño phenomena as well as investigate a possible relationship
between El Niño events and frequency/severity of Atlantic Hurricanes.
Rationale for conducting this investigation is based upon the fact
that El Niño events change convective patterns locally over the
Pacific Ocean and globally the general atmospheric circulation.
When the equatorial trade winds weaken, warm waters build up in
the Eastern Pacific. Subsequently, large amounts of water evaporate
at the surface. This sequence of events creates a low pressure system,
or trough in the Eastern Pacific. Researchers are uncertain as to
what triggers the weakening in the trade winds. What is agreed upon
is that these changes impact the the general atmospheric circulation
pattern and can alter climate conditions across the globe.
Results
of Research to Date
According to Gray (Colorado State Unv., 1984) and O'Brien, Richards
and Davis (Florida State Unv., 1995), El Niño is commonly associated
with diminished hurricane activity in the Atlantic. The Florida
State researchers analyzed Hurricanes (only) between 1949-1992 and
found that there was a 21% probability of 2 or more hurricanes striking
the U.S. during an El Niño year. In a "normal" year, there
is a 46% chance of the same event. Notice that this study limited
itself to hurricanes (not tropical storms) that struck the U.S.
mainland only. They also defined an El Niño event according to the
scheme developed by the Japanese Meteorological Agency (reference
JMA Atlas, 1991). According to the JMA, an El Niño is said to have
begun when within the region bounded by +/-4 degrees of the equator
from 150-90 deg. West experiences 6 consecutive months of sea surface
temperatures of at least 0.5 deg. C above normal. They further limited
the criteria by stating that the series of six consecutive months
must begin before September, and must include October, November,
and December. They found that an El Niño typically starts in late
summer and lasts for about a year. According to these definitions,
the winter of 1994-95 was not an El Niño winter, and thus the extreme
hurricane season in 1995 was not a summer following an El Niño winter.
Your students as they work through this investigation will need
to establish their own criteria for when an El Niño begins and ends.
You may wish, depending upon the group, to share with them the JMA's
criteria.
Other
researchers, such as William Gray of Colorado State, using different
criteria, do consider the events of 1995 to be related to the El
Niño. This presents another problem: when El Niño occurs, it clearly
changes the upper atmospheres positions of me an ridges and troughs,
however, it rarely does so the same way twice! Dr. Gray of Colorado
State believes that this past years Hurricane season, while it followed
a El Niño year, was severe in the Atlantic as a result of other
dynamics such as unusually large convective activity over Western
Africa producing an unusual number of tropical waves in the Eastern
Atlantic.
This
lab ideally will allow the student to develop their own criteria
for when an El Niño is occurring. The student will also be able
to look at Atlantic Hurricanes and Tropical Storms from a variety
of perspectives. These include intensity, frequency, and storm track.
I have included storm tracks (acquired from a Purdue Univ. web site)
for the years in question (1981-1996), however, their maps do not
provide hurricane tracks, only tropical storm tracks. At the University
of Hawaii, I accessed more complete maps of storm tracks for all
storms from 1994 and 1995, however I have not yet found maps that
track hurricanes from earlier years. These images are included in
your package, and are titled "improved storm track
1994,
1995."
The Purdue web site does have JPEG files for individual hurricanes
available for download, some samples from the past couple of years
have been included for study here. The web address for accessing
these images appears later in this paper.
Images
have been provided of Pacific Ocean "Sea Surface Temperature
Anomalies" (ssta). These images are in 3 month intervals from
late 1981 to early1996. I have taken the images and used them with
a shareware program called "Graphic Converter." This program
allows the student to view the images in a "slide show"
format. They can control the speed of the slide show and stop at
various images for detailed study. I suggest beginning the activity
by having the students study the "ssta" images, and identify
when El Niño Events have occurred over the past couple of decades.
A key labeled "sstakey.jpg" depicts which colors indicate
above normal sea surface temperatures. The key appears both at the
beginning of the slide show sequence and at the end.
A final
but important note: when you or your students are viewing the sea
surface temperature anomalies, the date appears in coded form in
the upper left corner of the screen. The code is easily analyzed,
the year appears first, followed by the month in numerical format.
For example: "847" means "1984, July." The one
exception to this format used was for October which is labeled as
"99." For example, October 1984 is "8499." This
was done to insure that "Graphic Converter" would create
a slide show in order from oldest to most recent image.
Additional
graphics provided included an image depicting El Niño vs. Normal
sea surface temperature patterns in this folder. It is labeled
"Normal
vs. El Niño Vertical Profile." I suggest that the students
view the series of ssta images to get a feel for the dynamic changes
that take place in the Pacific Ocean. Then study the image "Normal
vs. El Niño Vertical Profile" to learn how to recognize an El
Niño event, as well as to discover what changes take place in the
atmosphere over the Pacific when an El Niño occurs. The students
can then view the slide show again of sea surface temperature anomalies,
and begin to identify when El Niño events have occurred.
Additional
images/diagrams that you may wish to provide the students with include:
"El
Niño 1986-pres. SST tmp/anom": This image presents essentially
a summary of the entire sequence of images prepared for student
analysis. Both sea surface temperatures and anomalies are presented
from 1986-1996.
"El
Niño @ eqtr. temp/wind": This image depicts "normal"
sea surface temperatures at the equator with depth, and compares
this profile to an "El Niño" scenario. Additionally, surface
winds are depicted along the equator across the Pacific under both
"normal" and "El Niño" conditions.
According
to the literature, El Niño events have occurred in 1982-83, 1986-87,
1991-92, 1993, and 1994. By studying the series of "ssta"
images, students should be able to identify within a couple of months
when each event began to develop and later subside. Researchers
are quick to point out that it is unusual for El Niño's to occur
in such rapid succession as has been seen in the first half of this
decade.
You may
wish to share the following with your students, or for your information:
"El Niño was originally recognized by fishermen off the coast
of South America in the late 1800's as the appearance of abnormally
warm water in the Pacific ocean which usually arrived shortly after
the beginning of the year. This localized phenomenon is now known
to be part of a very large and complex system of atmospheric and
oceanic interactions called the El Niño Southern Oscillation or
ENSO for short. The mechanisms of the ENSO are the dynamic and thermodynamic
interactions between the atmosphere, oceans and land surfaces. In
most years it doesn't even occur, and in others, such as 1982, the
effects are felt around the world with devastating impact. An ENSO
event can reverse ocean currents and trade wind patterns, cause
drought and wildfires in one part of the world, and bring torrential
rains to other, usually dry, regions of the earth. In fact, the
1982 El Niño event, may be one of the most powerful climate oscillations
in modern history!
The strength
of this phenomenon varies greatly from year to year. But why does
it appear in certain years and not in others? Can we predict and
prepare for the next episode? These and many other questions are
perplexing scientists today. Currently, there are many scientists
involved in the monitoring, detecting and modeling of the ENSO.
Many are using sophisticated measurement devices and various satellite
instruments to analyze past El Niño events to enable them to model
and predict future events." (This was taken from the "What
is El Niño?" web page.
Atlantic
Hurricane/El Niño Project: Student Activity
Introduction
El Niño is a phenomena that affects many aspects of our planets
major systems. In short, El Niño is a warming of the waters off
of Peru in the Pacific Ocean. This phenomena, first noticed by fishermen
in the late 19th century, has been determined to occur periodically,
usually every three years or so, however lately the frequency seems
to have increased. Why does El Niño occur? What controls when it
occurs? What impacts does El Niño have upon the atmosphere? biosphere?
hydrosphere (other oceans)? In this investigation, you will be provided
with several sets of data and asked to answer one question: Does
El Niño have any impact upon Atlantic Hurricanes?
Lets
ask why is this even a possibility worth considering. It is known
that El Niño can change patterns in the upper atmosphere, particularly
in the Northern Hemisphere. Torrential rains in California and the
Midwest in recent years have been blamed on El Niño. Unusual warmth
in the Northeast in the winter of 1995-96 was also blamed on El
Niño. Based upon these observations, it seems reasonable to ask
whether El Niño is impacting Atlantic Hurricanes.
To answer
whether El Niño is impacting Atlantic Hurricanes, you need to study
the various data sets presented in this activity. The questions
raised in the introductory paragraph, while valuable, can not be
addressed from the data presented here.
Procedure
To begin your study, go to the file called "Pacific
Ocean ssta." You will see a color coded temperature key
first. Note: the data set is showing you "sea surface
temperature anomalies." This means that the colored images
are showing you warmer and cooler than normal temperatures, not
a specific temperature. You may pause to familiarize yourself with
what is above normal and what is below normal. Once familiar, you
can continue to view the data. Note: it covers the years
1981 to 1996. The date is presented in the upper left corner and
is coded as follows: year, month. For example: 837 is July, 1983.
One exception to this pattern is any data from October which is
presented as month "99." This means that October 1986
looks like this: 8699. This was done to insure that the program
would read the images in proper sequence.
Now that
you have viewed the "ssta" data once, you should study
the image called
"Normal
vs El NiñoVertical Profile." This image will reveal to
you precisely where the warming that you are looking for takes place.
After studying this image, report your observations in writing to
your instructor. Note the difference between the two sea surface
temperature patterns. Next, discuss what changes occur in the atmosphere
when El Niño develops. Submit this to your instructor, and upon
approval return to the ssta data and determine when El Niño events
have occurred over the past two decades. (For Example...From my
analysis of the data provided, clearly identified El Niño events
can be seen between July 1982-Jan 1984; Aug. 1986-Jan 1988; Aug
1991-Aug 1992; April 1993-Jan 1994; Oct 1994-May 1995)
Now that
you have identified when El Niño's have occurred, you need to study
the Hurricane data supplied. There are two primary sets of data
for you to study. One set lists all of the storms that occurred
each season, reports the minimum pressure achieved for each storm,
maximum winds, and classifies them according to the
Saffir-Simpson
scale. Study this data in the file called "Hurricane
Info."
Your
Scientific Investigation and Report
Study the data provided, keep an open mind as to what relationship
(if any) may exist. Realize that it might not be a simple relationship
such as El Niño events increase or decrease the number of hurricanes
each season. As you study the data, prepare tables or charts that
will help you to ascertain whether any relationships exist. Be sure
to present these products in the scientific report that you should
prepare to report the results of this investigation.
(Depending
upon the group, you may wish to guide the students through the various
approaches that can be taken in studying the data. For example,
you may choose to direct the students to prepare a table comparing
El Niño years to non-El Niño years vs number of hurricanes. Additional
comparisons can be made between El Niño years and non-El Niño years
by categorizing the storm tracks. You could for example, break down
the tracks storms into those that cross the 75 deg. West longitude
vs. those that do not. Comparing date of occurrence vs type of season
could also yield statistically significant results.
Anticipated
Results (Teacher Information Only)
Years 1981,1982,1984,1985,1988,1989,1990 were clearly not El
Niño years, while 1983,1987,1992,1993 were El Niño years. Arguments
could be made in either direction for 1986,1991,1994 and 1995. During
the El Niño years, the average number of storms was 6.5, and hurricanes
was 3.5. During non-El Niño years, the average number of storms
was 11.1, and hurricanes was 5.8. Opportunities to discuss statistical
significance are presented here, since the data is based upon only
7 seasons for non-El Niño years and 4 seasons for El Niño years.
Similar studies can be conducted for any variations in tracks that
occur or strength or part of the season when the storms occur.
I prefer
to keep investigations such as this as open-ended, and allow the
students to study the data to see what they can make of it.
They are expected to conclude their study with a written
report and an oral defense of their conclusions.
