Accumulated cyclone energy

The accumulated cyclone energy index was originally created by William Gray and his associates at Colorado State University, as the Hurricane Destruction Potential index (HDP).[2] They argued that the destruction of a hurricanes wind and storm surge was better related, to the square of the storms maximum winds (Vmax²) than to the storms maximum winds.[3] The index was calculated by squaring the estimated maximum sustained wind speed by themselves, for all tropical cyclones with windspeeds of above 65 kn (120 km/h; 75 mph) every six hours over the entire season.[2][3] This scale was subsequently tweaked by the United States National Oceanic and Atmospheric Administration (NOAA) to include all tropical cyclones, with winds above 35 kn (65 km/h; 40 mph) who also renamed it the accumulated cyclone energy index.[2] Since the scale was tweaked by NOAA, the storm totals have been used in a number of different ways, by various agencies and researchers, including the Australian Bureau of Meteorology and the India Meteorological Department.[4][5] These purposes include to categorise how active a tropical cyclone season was as well as to identify possible long-term trends in a certain area such as the Lesser Antiles.[6]

The accumulated cyclone energy of a season is calculated by summing the squares of the estimated maximum sustained velocity of every tropical cyclone that has wind speeds of 35 kn (65 km/h; 40 mph) or higher, at six-hour intervals. The numbers are usually divided by 10,000 to make them more manageable. One unit of ACE equals 10⁴ kn², and for use as an index the unit is assumed. Thus:

${\displaystyle {\text{ACE}}=10^{-4}\sum v_{\max }^{2}}$

where v_max is estimated sustained wind speed in knots.

Kinetic energy is proportional to the square of velocity, and by adding together the energy per some interval of time, the accumulated energy is found. As the duration of a storm increases, more values are summed and the ACE also increases such that longer-duration storms may accumulate a larger ACE than more-powerful storms of lesser duration. Although ACE is a value roughly proportional to the definite integral over time of the kinetic energy of the system, it is not a direct calculation of energy (the mass of the moved air and therefore the size of the storm would show up in a real energy calculation).

Accumulated cyclone energy of North Atlantic hurricanes 1850-2019[7]

Within the Atlantic Ocean, the United States National Oceanic and Atmospheric Administration and others, use the ACE index of a season to classify the season into one of four categories.[6] These four categories are extremely active season, above-, near- and below-normal average, and are worked out using an approximate tercile partitioning of seasons based on the ACE index, number of tropical storms, hurricanes and major hurricanes over the 30 years between 1981 and 2010.[6] The mean value of the ACE Index from 1981 to 2010 is 105.6 x 10⁴ kt², while the median value is 92.4 x 10⁴ kt².[6]

For a season to be defined as above normal, the ACE index criterion and two or more of the other criteria given in the table below must be satisfied. For a season to be defined as below normal, either the ACE index criterion must be satisfied, or all three of the other criteria must be satisfied.[6]

An extremely active season is simply defined as one with an ACE index above 152.5 x 10⁴ kt².[6] Confusingly, this means that a season could simultaneously be extremely active and near-normal (or even below normal). This is currently the case only for the 1906 season.

Within the Eastern Pacific Ocean, the United States National Oceanic and Atmospheric Administration and others, use the ACE index of a season to classify the season into one of three categories.[10] These three categories are above-, near- and below-normal and are worked out using an approximate tercile partitioning of seasons based on the ACE index, number of tropical storms, hurricanes and major hurricanes over the 30 years between 1981 and 2010.[10]

The highest ever ACE estimated for a single storm in the Eastern or Central Pacific, while located east of the International Date Line is 62.8, for Hurricane Fico of 1978. Other Eastern Pacific storms with high ACEs include Hurricane John in 1994, with an ACE of 54.0, Hurricane Kevin in 1991, with an ACE of 52.1, and Hurricane Hector of 2018, with an ACE of 50.5.[11]

The following table shows those storms in the Eastern and Central Pacific basins from 1971–2018 that have attained over 30 points of ACE.[12]

Storm	Year	Peak classification	ACE	Duration
Hurricane Fico	1978	Category 4 hurricane	62.8	20 days
Hurricane John	1994	Category 5 hurricane	54.0	19 days
Hurricane Kevin	1991	Category 4 hurricane	52.1	17 days
Hurricane Hector	2018	Category 4 hurricane	50.5	13 days
Hurricane Tina	1992	Category 4 hurricane	47.7	22 days
Hurricane Trudy	1990	Category 4 hurricane	45.8	16 days
Hurricane Lane	2018	Category 5 hurricane	44.2	13 days
Hurricane Dora	1999	Category 4 hurricane	41.4	13 days
Hurricane Jimena	2015	Category 4 hurricane	40.0	15 days
Hurricane Guillermo	1997	Category 5 hurricane	40.0	16 days
Hurricane Norbert	1984	Category 4 hurricane	39.6	12 days
Hurricane Norman	2018	Category 4 hurricane	36.6	12 days
Hurricane Celeste	1972	Category 4 hurricane	36.3	16 days
Hurricane Sergio	2018	Category 4 hurricane	35.5	13 days
Hurricane Lester	2016	Category 4 hurricane	35.4	14 days
Hurricane Olaf	2015	Category 4 hurricane	34.6	12 days
Hurricane Jimena	1991	Category 4 hurricane	34.5	12 days
Hurricane Doreen	1973	Category 4 hurricane	34.3	16 days
Hurricane Ioke	2006	Category 5 hurricane	34.2	7 days
Hurricane Marie	1990	Category 4 hurricane	33.1	14 days
Hurricane Orlene	1992	Category 4 hurricane	32.4	12 days
Hurricane Greg	1993	Category 4 hurricane	32.3	13 days
Hurricane Hilary	2011	Category 4 hurricane	31.2	9 days

– Indicates that the storm formed in the Eastern/Central Pacific, but crossed 180°W at least once, therefore only the ACE and number of days spent in the EPAC/CPAC are included.

There are various agencies over the North Indian Ocean that monitor and forecast tropical cyclones, including the United States Joint Typhoon Warning Center, as well as the Bangladesh, Pakistan and India Meteorological Department.[4] As a result, the track and intensity of tropical cyclones differ from each other, and as a result, the accumulated cyclone energy also varies over the region.[4] However, the India Meteorological Department has been designated as the official Regional Specialised Meteorological Centre by the WMO for the region and has worked out the ACE for all cyclonic systems above 17 knots (31 km/h; 20 mph) based on their best track analysis which goes back to 1982.[4][14]

• Atlantic hurricane

• The International Best Track Archive for Climate Stewardship (IBTrACS)
• Colorado State University's Real Time Tropical Cyclone Statistics
• Ryan Maue's Global Tropical Cyclone Activity