Climate Change Indicators in the United States: Tropical Cyclone Activity
www.epa.gov/climatechange/indicators - Updated May 2014
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.
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.1,2
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
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Climate Change Indicators in the United States: Tropical Cyclone Activity
www.epa.gov/climatechange/indicators - Updated May 2014
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.
• 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 associated with variations in sea surface temperature in the tropical Atlantic (see Figure 2).
• Despite the apparent increases in tropical cyclone activity in Figures 2 and 3, changes in
observation methods over time make it difficult to know whether tropical storm activity has
actually shown a long-term increase.3
Key Points
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Climate Change Indicators in the United States: Tropical Cyclone Activity
www.epa.gov/climatechange/indicators - Updated May 2014
Figure 1. Number of Hurricanes in the North Atlantic, 1878-2013
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This graph shows the number of hurricanes that formed in the North Atlantic Ocean each year from 1878
to 2013, along with the number that made landfall in the United States. The blue curve shows how the
total count in the red 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 (2009-2013) is plotted at 2011.
Data source: Knutson, 20144
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d% Climate Change Indicators in the United States: Tropical Cyclone Activity
www.epa.gov/climatechange/indicators - Updated May 2014
Figure 2. North Atlantic Tropica! Cyclone Activity According to the
Accumulated Cyclone Energy Index, 1950-2013
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2020
This figure shows total annual Accumulated Cyclone Energy (ACE) Index values, which account for
cyclone strength, duration, and frequency, from 1950 through 2013. The National Oceanic and
Atmospheric Administration has defined "near normal/' "above normaland "below normal" ranges
based on the distribution of ACE Index values over the 30 years from 1981 to 2010.
Data source: NOAA, 2014
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Climate Change Indicators in the United States: Tropical Cyclone Activity
www.epa.gov/climatechange/indicators - Updated May 2014
Figure 3. North Atlantic Tropical Cyclone Activity According to the
Power Dissipation Index, 1949-2013
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1950 1960 1970 1960 1990 2000 2010 2020
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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 (2009-2013) is plotted at 2011.
Data source: Emanuel, 2014s
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.
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Climate Change Indicators in the United States: Tropical Cyclone Activity
www.epa.gov/climatechange/indicators - Updated May 2014
Data Sources
Hurricane counts are reported on several National Oceanic and Atmospheric Administration websites
and were compiled using methods described in Knutson et al. (2010).7 The ACE Index data (Figure 2)
came from the National Oceanic and Atmospheric Administration's Climate Prediction Center, and are
available online at: www.cpc.noaa.gov/products/outlooks/background information.shtml. Values for
the PDI have been calculated by Kerry Emanuel at the Massachusetts Institute of Technology. Both
indices are based on wind speed measurements compiled by the National Oceanic and Atmospheric
Administration.
1 Melillo, J.M., T.C. Richmond, and G.W. Yohe (eds.). 2014. Climate change impacts in the United States: The third
National Climate Assessment. U.S. Global Change Research Program, http://nca2014.globalchange.gov.
2 IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis.
Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge
University Press, www.ipcc.ch/report/ar5/wgl.
3 IPCC (Intergovernmental Panel on Climate Change). 2012. Managing the risks of extreme events and disasters to
advance climate change adaptation. Cambridge, United Kingdom: Cambridge University Press. http://ipcc-
wg2.gov/SREX.
4 Knutson, T.R. 2014 update to data originally published in: Knutson, T.R., J.L. McBride, J. Chan, K. Emanuel, G.
Holland, C. Landsea, I. Held, J.P. Kossin, A.K. Srivastava, and M. Sugi. 2010. Tropical cyclones and climate change.
Nature Geosci. 3:157-163.
5 NOAA (National Oceanic and Atmospheric Administration). 2014. The Atlantic Hurricane Database Re-analysis
Project, www.aoml.noaa.gov/hrd/hurdat/comparison table.html.
6 Emanuel, K.A. 2014 update to data originally published in: Emanuel, K.A. 2007. Environmental factors affecting
tropical cyclone power dissipation. J. Climate 20(22):5497-5509.
7 Knutson, T.R., J.L. McBride, J. Chan, K. Emanuel, G. Holland, C. Landsea, I. Held, J.P. Kossin, A.K. Srivastava, and
M. Sugi. 2010. Tropical cyclones and climate change. Nature Geosci. 3:157-163.
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