Tropical Cyclone Activity Page: 2 of 6
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A4 Climate Change Indicators in the United States: Tropical Cyclone Activity
www.epa.gov/climate-indicators - Updated August 2016
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.
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.32
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United States. Environmental Protection Agency. Tropical Cyclone Activity, text, August 2016; (https://digital.library.unt.edu/ark:/67531/metadc949223/m1/2/: accessed April 26, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.