The accumulated cyclone energy is an index used by meteorologists to examine the intensity and longevity of tropical cyclones in a given season — and it makes it easier to compare individual storms and seasons.
Hurricane Mitch 3D Satellite on Oct. 26, 1998.
The Atlantic hurricane season officially began on June 1, and with the season underway, the potential for devastating storms could occur at any time. While the number of tropical storms and hurricanes that form each year is important to track, experts say that the Accumulated Cyclone Energy (ACE) index is a more reliable way to quantify the true strength of tropical cyclones.
ACE is an index used to measure the intensity and overall activity of tropical cyclones in a given season. It was first introduced by Dr. William Gray and his team at Colorado State University in the 1990s to quantify the destructive potential of a hurricane season.
AccuWeather Senior Meteorologist Dan Kottlowski says this metric takes into account the strength and duration of each named storm and can be used to compare individual storms and seasons with one another.
“Tropical meteorologists use the value to compare storms and seasons in order to help understand what has happened and as a means to help determine how potentially active an up-and-coming season could be,” said Kottlowski. “In certain parts of the world, and within some tropical basins, the ACE value can help determine the statistical threat for land impacts.”
How is ACE calculated?
The ACE metric factors in the maximum sustained wind of each tropical cyclone throughout its lifetime.
A calculation is done every six hours for each named storm that forms and remains active, and the resulting values are then added up over the entire season.
What is the ACE index for an average season?
The average ACE units vary depending on the basin. For the North Atlantic, the 30-year average ACE value is 123. For the East Pacific basin, the average is 132 units. And for the West Pacific, the average is 298 units.
“However, in the Atlantic basin, we usually look at the value from 1951-2020 for the long-term average which is about 96 units,” said Kottlowski. “The reason why we don’t go back before 1951 is because the data for individual storms is unreliable due to the lack of satellite and reconnaissance data.”
For the Atlantic basin, a below-average season has an ACE index lower than 73 units. And an extremely active season would have an index above 159.6 units.
Comparing hurricane seasons using the ACE metric
A season with a higher ACE value can indicate a more intense season, potentially with more damaging storms, than a season with a lower ACE index, even if the seasons have the same number of named storms. Since the measurement takes into account both the intensity and duration of each storm, the ACE value adds more weight to storms that are stronger and longer lasting.
The 2013 Atlantic hurricane season is a prime example of how the number of storms doesn’t necessarily equate to a higher ACE value. The season featured 14 named storms, which was slightly above average at the time. In 2013, the historical average for named storms in the Atlantic Ocean was considered 11.2.
2013 Atlantic Hurricane tropical tracks
However, only two of the 14 named storms reached hurricane status, and it was the first season since 1994 that no cyclones reached major hurricane strength. Additionally, all but three storms that season lasted five days or less.
So, despite the slightly above-average number of storms during the season, many cyclones were short-lived and weak. The season ended with an ACE of 36.1 units.
In comparison, the 1998 hurricane season also had 14 named storms. Unlike 2013, the 1998 hurricane season went down in the record books as one of the deadliest and costliest seasons at the time.
1998 Atlantic Hurricane tropical tracks
During the 1998 season, 10 of the 14 named storms reached hurricane strength and three reached major hurricane — Category 3 or higher — status. The two most significant storms of the season were hurricanes Georges and Mitch.
Georges, which reached Category 4 strength, devastated Caribbean islands, including Saint Kitts and Nevis, Puerto Rico and the Dominican Republic. In total, Mitch was responsible for more than 600 fatalities.
Hurricane Mitch was an even deadlier storm. Mitch underwent rapid intensification and strengthened into a powerful Category 5 hurricane before making landfall on Oct. 29 in Honduras. Mitch tracked across Central America and reformed in the Bay of Campeche before making another landfall in Florida as a tropical storm.
Hurricane Mitch, a Category 5 hurricane with a central pressure of 906 mb and sustained winds of 195 mph, captured with GOES-8 on Oct. 26, 1998. (NOAA/NESDIS Operational Significant Event Imagery)
Honduras and Nicaragua were especially hit hard by Mitch. Floods and mudslides washed away entire villages and killed thousands of people. According to a figure from the National Weather Service, more than 11,000 fatalities were reported from Mitch. Mitch still stands as the second-deadliest hurricane in the Western Hemisphere since the Great Hurricane of 1780, which killed more than 20,000 people in the eastern Caribbean.
The 1998 hurricane season ended with an ACE value of 181.8 units, which is well above average for the Atlantic Ocean basin. Although the number of named storms was the same as in the 2013 season, the strength can clearly be seen in the substantially different ACE values.
What is AccuWeather forecasting for the 2023 Atlantic hurricane season?
As the 2023 Atlantic hurricane season gets underway, which AccuWeather long-range forecasters expect to be near average in terms of the number of named storms, it's essential to keep an eye on the ACE index in order to gain a clearer picture of the scope of the season.
AccuWeather is predicting an ACE of 75-105 units, which is near the long-term average value, according to Kottlowski.
To learn more about what is expected for the season at hand, head over to AccuWeather's 2023 Atlantic hurricane seasonal outlook, which was published in late March.
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