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| Tropical Cyclone Activity |
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 Climate Change Indicators: Tropical
Cyclone Activity
This indicator examines the frequency, intensity, and
duration of hurricanes and other tropical storms in the
Atlantic Ocean, Caribbean, and Gulf of Mexico.
Key Points
Since 1878, about six to seven hurricanes have formed in the
North Atlantic every year. Roughly two per year make
landfall in the United States. The total number of
hurricanes (particularly after being adjusted for
improvements in observation methods) and the number reaching
the United States do not indicate a clear overall trend
since 1878 (see Figure 1).
According to the total annual ACE Index, cyclone intensity
has risen noticeably over the past 20 years, and six of the
10 most active years since 1950 have occurred since the
mid-1990s (see Figure 2). Relatively high levels of cyclone
activity were also seen during the 1950s and 1960s.
The PDI (see Figure 3) shows fluctuating cyclone intensity
for most of the mid- to late 20th century, followed by a
noticeable increase since 1995 (similar to the ACE Index).
These trends are shown with associated variations in sea
surface temperature in the tropical North Atlantic for
comparison (see Figure 3).
Despite the apparent increases in tropical cyclone activity
in recent years, shown in Figures 2 and 3, changes in
observation methods over time make it difficult to know
whether tropical storm activity has actually shown an
increase over time. |
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Background
Hurricanes, tropical storms, and other intense rotating
storms fall into a general category called cyclones. There
are two main types of cyclones: tropical and extratropical
(those that form outside the tropics). Tropical cyclones get
their energy from warm tropical oceans. Extratropical
cyclones get their energy from the jet stream and from
temperature differences between cold, dry air masses from
higher latitudes and warm, moist air masses from lower
latitudes.
This indicator focuses on tropical cyclones in the Atlantic
Ocean, Caribbean, and Gulf of Mexico. Tropical cyclones are
most common during the “hurricane season,” which runs from
June through November. The effects of tropical cyclones are
numerous and well known. At sea, storms disrupt and endanger
shipping traffic. When cyclones encounter land, their
intense rains and high winds can cause severe property
damage, loss of life, soil erosion, and flooding. The
associated storm surge—the large volume of ocean water
pushed toward shore by the cyclone’s strong winds—can cause
severe flooding and destruction.
Climate change is expected to affect tropical cyclones by
increasing sea surface temperatures, a key factor that
influences cyclone formation and behavior. The U.S. Global
Change Research Program and the Intergovernmental Panel on
Climate Change project that, more likely than not, tropical
cyclones will become more intense over the 21st century,
with higher wind speeds and heavier rains. |
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About the Indicator
Records of tropical cyclones in the Atlantic Ocean have been
collected since the 1800s. The most reliable long-term
records focus on hurricanes, which are the strongest
category of tropical cyclones in the Atlantic, with wind
speeds of at least 74 miles per hour. This indicator uses
historical data from the National Oceanic and Atmospheric
Administration to track the number of hurricanes per year in
the North Atlantic (north of the equator) and the number
reaching the United States since 1878. Some hurricanes over
the ocean might have been missed before the start of
aircraft and satellite observation, so scientists have used
other evidence, such as ship traffic records, to estimate
the actual number of hurricanes that might have formed in
earlier years.
This indicator also looks at the Accumulated Cyclone Energy
(ACE) Index and the Power Dissipation Index (PDI), which are
two ways of monitoring the frequency, strength, and duration
of tropical cyclones based on wind speed measurements.
Every cyclone has an ACE Index value, which is a number
based on the maximum wind speed measured at six-hour
intervals over the entire time that the cyclone is
classified as at least a tropical storm (wind speed of at
least 39 miles per hour). Therefore, a storm’s ACE Index
value accounts for both strength and duration. The National
Oceanic and Atmospheric Administration calculates the total
ACE Index value for an entire hurricane season by adding the
values for all named storms, including subtropical storms,
tropical storms, and hurricanes. The resulting annual total
accounts for cyclone strength, duration, and frequency. For
this indicator, the index has been converted to a scale
where 100 equals the median value (the midpoint) over a base
period from 1981 to 2010. The thresholds in Figure 2 define
whether the ACE Index for a given year is close to normal,
significantly above normal, or significantly below.
Like the ACE Index, the PDI is based on measurements of wind
speed, but it uses a different calculation method that
places more emphasis on storm intensity. This indicator
shows the annual PDI value, which represents the sum of PDI
values for all named storms during the year.
Indicator Notes
Over time, data collection methods have changed as
technology has improved. For example, wind speed collection
methods have evolved substantially over the past 60 years,
while aircraft reconnaissance began in 1944 and satellite
tracking around 1966. Figure 1 shows how older hurricane
counts have been adjusted to attempt to account for the lack
of aircraft and satellite observations. Changes in data
gathering technologies could substantially influence the
overall patterns in Figures 2 and 3. The effects of these
changes on data consistency over the life of the indicator
would benefit from additional research.
While Figures 2 and 3 cover several different aspects of
tropical cyclones, there are other important factors not
covered here, including the size of each storm, the amount
of rain, and the height of the storm surge. The reason for
the recent divergence between cyclone activity and sea
surface temperature in Figure 3 has not been identified
conclusively, but it may relate to other factors that
influence the formation of storms, such as the difference in
wind speeds at different levels in the atmosphere (called
vertical wind shear).
Data Sources
Hurricane counts were compiled using methods described in
Knutson et al. (2010). Data for Figures 1 and 2 came from
the National Oceanic and Atmospheric Administration’s
Hurricane Research Division, and are available online at:
www.aoml.noaa.gov/hrd/hurdat/comparison_table.html.
Values for Figure 3 have been calculated by Kerry Emanuel at
the Massachusetts Institute of Technology. Both the ACE
Index and the PDI are based on wind speed measurements
compiled by the National Oceanic and Atmospheric
Administration.
Technical Documentation
Download related technical information PDF |
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 Figure
1. Number of Hurricanes in the North Atlantic,
1878–2015
This graph shows the number of hurricanes that formed in the
North Atlantic Ocean each year from 1878 to 2015, along with
the number that made landfall in the United States. The
orange curve shows how the total count in the green curve
can be adjusted to attempt to account for the lack of
aircraft and satellite observations in early years. All
three curves have been smoothed using a five-year average,
plotted at the middle year. The most recent average
(2011–2015) is plotted at 2013.
Data source: NOAA, 20164; Vecchi and Knutson, 20115 |
 Figure
2. North Atlantic Tropical Cyclone Activity According
to the Accumulated Cyclone Energy Index, 1950–2015
This figure shows total annual Accumulated Cyclone Energy
(ACE) Index values, which account for cyclone strength,
duration, and frequency, from 1950 through 2015. The
National Oceanic and Atmospheric Administration has defined
“near normal,” “above normal,” and “below normal” ranges
based on the distribution of ACE Index values over the 30
years from 1981 to 2010.
Data source: NOAA, 20166 |
 Figure
3. North Atlantic Tropical Cyclone Activity According
to the Power Dissipation Index, 1949–2015
This figure presents annual values of the Power Dissipation
Index (PDI), which accounts for cyclone strength, duration,
and frequency. Tropical North Atlantic sea surface
temperature trends are provided for reference. Note that sea
surface temperature is measured in different units, but the
values have been plotted alongside the PDI to show how they
compare. The lines have been smoothed using a five-year
weighted average, plotted at the middle year. The most
recent average (2011–2015) is plotted at 2013.
Data source: Emanuel, 20167 |
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EPA Page |
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Additional Climate Change Information |
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(Beginner - Listening,
reading)
A video lesson to
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Animals
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The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
Researchers believe Africa may lose as much as 30 percent of its animal and
plant species by the end of this century. |
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