Accumulated Cyclone Energy Index

Developed by the Tropical Prediction Center (TPC) and regularly employed by the National Oceanic and Atmospheric Administration (NOAA) and the National Hurricane Center (NHC) since the latter part of the 1990s, the Accumulated Cyclone Energy (ACE) index assists meteorologists and forecasters in analyzing, quantifying, and in some cases, predicting, the total seasonal activity of tropical cyclones within each of the Earth’s tropical cyclone generation zones. Determined through the use of mathematical formulas and models, the ACE index—which according to the NHC is a “wind energy index, defined as the sum of the squares of the maximum sustained surface wind speed (knots)”—is calculated every six hours for existing North Atlantic tropical systems of tropical storm intensity and higher. By including additional data such as the number of tropical storms and hurricanes, the number of major hurricanes, and geographical location, 

NOAA can use the ACE to categorize hurricane, typhoon, and cyclone seasons as being above-normal, near-normal, and below-normal. Since implementing the ACE Index, meteorologists at the NHC have (retroactively) applied the technique to North Atlantic tropical cyclones dating from 1950–2005, and used the ACE Index to determine a base period from 1951–2000. These studies indicate that the median annual index for this base period was 87.5.

In terms of definitional clarity, an above-normal season generally possesses an ACE Index value above 103, or 117 percent of the median, and includes either 10 tropical storms, six hurricanes, or two major hurricanes. A near-normal season carries an ACE Index value of between 66 and 103, and is generally representative of 75 percent to 117 percent of the median. Additionally, a near-normal season is characterized by having less than the long-term average number of tropical storms, hurricanes, or major hurricanes. And a below-normal season has an ACE Index value of 65 and below, which generally translates to 75 percent of the median as determined by the base period data.

According to records maintained by NOAA, the top five North Atlantic tropical cyclone seasons as determined by the ACE Index were: 2005, 1950, 1995, 2004, and 1961. In the case of the extraordinarily active 2005 North Atlantic hurricane season, an ACE Index of 248 was derived. In addition to the severity of the storms produced during that season, there were a total of 27 named tropical systems, with seven of them reaching major hurricane status. Although the 1950 season produced less than half the number of named storms observed during the 2005 season (a total of 13 tropical systems), it generated an above-average number of major hurricanes (eight), which gave it an ACE Index value of 243.