Accumulated cyclone energy


Accumulated cyclone energy is a metric used to compare overall activity of tropical cyclones, utilizing the available records of windspeeds at six-hour intervals to synthesize storm duration and strength into a single index value. The ACE index may refer to a single storm or to groups of storms such as those within a particular month, a full season or combined seasons. It is calculated by summing the square of tropical cyclones' maximum sustained winds, as recorded every six hours, but only for windspeeds of at least tropical storm strength ; the resulting figure is divided by 10,000 to place it on a more manageable scale.
The calculation originated as the Hurricane Destruction Potential index, which sums the squares of tropical cyclones' maximum sustained winds while at hurricane strength, at least 64 knots at six-hour recorded intervals across an entire season. The HDP index was later modified to further include tropical storms, that is, all wind speeds of at least 34 knots, to become the accumulated cyclone energy index.
The highest ACE calculated for a single tropical cyclone on record worldwide is 85.26, set by Hurricane Ioke in 2006.

History

The ACE index is an offshoot of Hurricane Destruction Potential, an index created in 1988 by William Gray and his associates at Colorado State University who argued the destructiveness of a hurricane's wind and storm surge is better related to the square of the maximum wind speed
than simply to the maximum wind speed. The HDP index is calculated by squaring the estimated maximum sustained wind speeds for tropical cyclones while at hurricane strength, that is, wind speeds of at least 64 knots. The squared windspeeds from six-hourly recorded intervals are then summed across an entire season. This scale was subsequently modified in 1999 by the United States National Oceanic and Atmospheric Administration to include not only hurricanes but also tropical storms, that is, all cyclones while windspeeds are at least 34 knots. Since the calculation was more broadly adjusted by NOAA, the index has been used in a number of different ways such as to compare individual storms, and by various agencies and researchers including the Australian Bureau of Meteorology and the India Meteorological Department. The purposes of the ACE index include to categorize how active tropical cyclone seasons were as well as to identify possible long-term trends in a certain area such as the Lesser Antilles.

Calculation

Accumulated cyclone energy is calculated by summing the squares of the estimated maximum sustained velocity of tropical cyclones when wind speeds are at least tropical storm strength at recorded six-hour intervals. The sums are usually divided by 10,000 to make them more manageable. One unit of ACE equals and for use as an index the unit is assumed. Thus:
where is estimated sustained wind speed in knots at six-hour intervals.

Atlantic Ocean

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. These four categories are extremely active, above-normal, near-normal, and below-normal, and are worked out using an approximate quartile partitioning of seasons based on the ACE index over the 70 years between 1951 and 2020. The median value of the ACE index from 1951 to 2020 is 96.7 x 104 kt2.
SeasonTSHUMHACE
193320116258.57
200528157245.3
189312105231.15
19261186229.56
199519115227.10
20041596226.88
201717106224.88
195016116211.28
19611285188.9
199814103181.76

Individual storms in the Atlantic

The highest ever ACE estimated for a single storm in the Atlantic is 73.6, for the San Ciriaco hurricane in 1899. A Category 4 hurricane which lasted for four weeks, this single storm had an ACE higher than many whole Atlantic storm seasons. Other Atlantic storms with high ACEs include Hurricane Ivan in 2004, with an ACE of 70.4, Hurricane Irma in 2017, with an ACE of 64.9, the Great Charleston Hurricane in 1893, with an ACE of 63.5, Hurricane Isabel in 2003, with an ACE of 63.3, and the 1932 Cuba hurricane, with an ACE of 59.8.
Since 1950, the highest ACE of a tropical storm was Tropical Storm Philippe in 2023, which attained an ACE of 9.4. The highest ACE of a Category 1 hurricane was Hurricane Nadine in 2012, which attained an ACE of 26.3. The record for lowest ACE of a tropical storm is jointly held by Tropical Storm Chris in 2000 and Tropical Storm Philippe in 2017, both of which were tropical storms for only six hours and had an ACE of just 0.1225. The lowest ACE of any hurricane was 2005's Hurricane Cindy, which was only a hurricane for six hours, and 2007's Hurricane Lorenzo, which was a hurricane for twelve hours; Cindy had an ACE of just 1.5175 and Lorenzo had a lower ACE of only 1.475. The lowest ACE of a major hurricane was Hurricane Gerda in 1969, with an ACE of 5.3.
The following table shows those storms in the Atlantic basin from 1851–2021 that have attained over 50 points of ACE.
StormYearPeak classificationACEDuration
Hurricane Three1899Category 4 hurricane73.628 days
Hurricane Ivan2004Category 5 hurricane70.423 days
Hurricane Irma2017Category 5 hurricane64.913 days
Hurricane Nine1893Category 3 hurricane63.520 days
Hurricane Isabel2003Category 5 hurricane63.314 days
Hurricane Fourteen1932Category 5 hurricane59.815 days
Hurricane Donna1960Category 4 hurricane57.616 days
Hurricane Carrie1957Category 4 hurricane55.821 days
Hurricane Inez1966Category 5 hurricane54.621 days
Hurricane Sam2021Category 4 hurricane53.814 days
Hurricane Luis1995Category 4 hurricane53.715 days
Hurricane Allen1980Category 5 hurricane52.312 days
Hurricane Esther1961Category 5 hurricane52.218 days
Hurricane Matthew2016Category 5 hurricane50.912 days

Historical ACE in recorded Atlantic hurricane history



Records of tropical storms, hurricanes, and major hurricanes before the satellite era are subject to an undercount bias, due to the difficulty of identifying storms.
Classification criteria
SeasonACETSHUMHClassification
185136.24631Below normal
185273.28551Near normal
185376.49842Near normal
185431.00531Below normal
185518.12541Below normal
185648.94642Below normal
185746.84430Below normal
185844.79660Below normal
185955.73871Below normal
186062.06761Below normal
186149.71860Below normal
186246.03630Below normal
186350.35950Below normal
186426.55530Below normal
186549.13730Below normal
186683.65761Near normal
186759.97971Below normal
186834.65430Below normal
186951.021071Below normal
187087.811102Near normal
187188.39862Near normal
187265.38540Below normal
187369.47532Below normal
187447.05740Below normal
187572.48651Below normal
187656.05542Below normal
187773.36831Near normal
1878180.8512102Extremely active
187963.63862Below normal
1880131.081192Above normal
188159.25740Below normal
188259.4675642Below normal
188366.7432Below normal
188472.06441Below normal
188558.3860Below normal
1886166.16512104Extremely active
1887181.2619112Extremely active
188884.945962Near normal
1889104.0425960Near normal
189033.345421Below normal
1891116.1051071Near normal
1892115.8375950Near normal
1893231.147512105Extremely active
1894135.42754Above normal
189568.765620Below normal
1896136.0825762Above normal
189754.54630Below normal
1898113.23751151Near normal
1899151.0251052Above normal
190083.345732Near normal
190198.9751360Near normal
190232.65530Below normal
1903102.071071Near normal
190430.345640Below normal
190528.3775511Below normal
1906162.881163Extremely active
190713.06500Below normal
190895.111061Near normal
190993.341264Near normal
191063.9531Below normal
191134.2875630Below normal
191257.2625741Below normal
191335.595640Below normal
19142.53100Below normal
1915130.095653Above normal
1916144.012515105Above normal
191760.6675422Below normal
191839.8725641Below normal
191955.04521Below normal
192029.81540Below normal
192186.53752Near normal
192254.515531Below normal
192349.31941Below normal
1924100.18751152Near normal
19257.2525410Below normal
1926229.55751186Extremely active
192756.4775841Below normal
192883.475641Near normal
192948.0675531Below normal
193049.7725322Below normal
193147.8351331Below normal
1932169.66251564Extremely active
1933258.5720116Extremely active
193479.06751371Near normal
1935106.2125853Near normal
193699.7751771Near normal
193765.851141Below normal
193877.575942Near normal
193943.6825631Below normal
194067.79960Below normal
194151.765643Below normal
194262.4851141Below normal
194394.011052Near normal
1944104.45251483Near normal
194563.4151152Below normal
194619.6125730Below normal
194788.491052Near normal
194894.97751064Near normal
194996.44751672Near normal
1950211.282516116Extremely active
1951126.3251283Above normal
195269.081152Below normal
195398.50751473Near normal
1954110.881673Near normal
1955158.171394Above normal
195656.67251241Below normal
195778.6625832Near normal
1958109.69251273Near normal
195977.10751472Near normal
196072.9842Below normal
1961188.89751285Extremely active
196250.45740Below normal
1963112.091073Near normal
1964153.04251375Above normal
196586.71751041Near normal
1966145.21751573Above normal
1967121.7051361Near normal
196845.0725950Below normal
196914918123Above normal
197040.181472Below normal
197196.52751361Near normal
197235.605730Below normal
197347.85841Below normal
197468.1251142Below normal
197576.0625963Near normal
197684.17251062Near normal
197725.3175651Below normal
197863.21751252Below normal
197992.9175962Near normal
1980148.93751192Above normal
1981100.32751273Near normal
198231.5025621Below normal
198317.4025431Below normal
198484.2951351Near normal
198587.98251173Near normal
198635.7925640Below normal
198734.36731Below normal
1988102.99251253Near normal
1989135.1251172Above normal
199096.80251481Near normal
199135.5375842Below normal
199276.2225741Near normal
199338.665841Below normal
199432.02730Below normal
1995227.102519115Extremely active
1996166.18251396Extremely active
199740.9275831Below normal
1998181.767514103Extremely active
1999176.52751285Extremely active
2000119.14251583Near normal
2001110.321594Near normal
200267.99251242Below normal
2003176.841673Extremely active
2004226.881596Extremely active
2005245.328157Extremely active
200678.5351052Near normal
200773.8851562Near normal
2008145.71751685Above normal
200952.58932Below normal
2010165.482519125Extremely active
2011126.30251974Above normal
2012132.632519102Above normal
201336.121420Below normal
201466.725862Below normal
201562.6851142Below normal
2016141.25251574Above normal
2017224.877517106Extremely active
2018132.58251582Above normal
2019132.20251863Above normal
2020180.372530147Extremely active
2021145.55752174Above normal
202294.42251482Near normal
2023145.55652073Above normal
2024161.582518115Extremely active
2025132.41354Above normal

Eastern Pacific

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. These four categories are extremely active, above-, near-, and below-normal and are worked out using an approximate tercile partitioning of seasons based on the ACE index and the number of tropical storms, hurricanes, and major hurricanes over the 30 years between 1991 and 2020.
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.
The mean value of the ACE index from 1991 to 2020 is 108.7 × 104 kt2, while the median value is 97.2 × 104 kt2.

Individual storms in the Pacific

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.
The following table shows those storms in the Eastern and Central Pacific basins from 1971 through 2023 that have attained over 30 points of ACE.
StormYearPeak classificationACEDuration
Hurricane Fico1978Category 4 hurricane62.820 days
Hurricane John 1994Category 5 hurricane54.019 days
Hurricane Kevin1991Category 4 hurricane52.117 days
Hurricane Hector 2018Category 4 hurricane50.513 days
Hurricane Dora 2023Category 4 hurricane48.412 days
Hurricane Tina1992Category 4 hurricane47.722 days
Hurricane Trudy1990Category 4 hurricane45.816 days
Hurricane Lane2018Category 5 hurricane44.213 days
Hurricane Dora 1999Category 4 hurricane41.413 days
Hurricane Jimena2015Category 4 hurricane40.015 days
Hurricane Guillermo1997Category 5 hurricane40.016 days
Hurricane Norbert1984Category 4 hurricane39.612 days
Hurricane Norman2018Category 4 hurricane36.612 days
Hurricane Celeste1972Category 4 hurricane36.316 days
Hurricane Sergio2018Category 4 hurricane35.513 days
Hurricane Lester2016Category 4 hurricane35.414 days
Hurricane Olaf2015Category 4 hurricane34.612 days
Hurricane Jimena1991Category 4 hurricane34.512 days
Hurricane Doreen1973Category 4 hurricane34.316 days
Hurricane Ioke 2006Category 5 hurricane34.27 days
Hurricane Marie1990Category 4 hurricane33.114 days
Hurricane Orlene1992Category 4 hurricane32.412 days
Hurricane Greg1993Category 4 hurricane32.313 days
Hurricane Hilary2011Category 4 hurricane31.29 days
Hurricane Kiko2025Category 4 hurricane30.89 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 Eastern/Central Pacific are included.

Historical ACE in recorded Pacific hurricane history



Data on ACE is considered reliable starting with the 1971 season.
Classification criteria
SeasonACETSHUMHClassification
197113918126Above normal
19721361484Above normal
19731141273Above normal
19749018113Near normal
19751121794Above normal
19761211595Above normal
197722.3840Below normal
1978207.719147Extremely active
197957.41064Below normal
1980771473Near normal
198172.81581Below normal
198216123125Above normal
1983206.221128Extremely active
1984193.721137Extremely active
1985193.124138Extremely active
19861071793Near normal
198713220104Above normal
1988127.51573Above normal
19891101794Near normal
1990249.521166Extremely active
1991178.914105Extremely active
1992294.6271610Extremely active
1993201.815119Extremely active
1994185.220105Extremely active
1995100.21073Near normal
199653.5952Below normal
1997167.91997Above normal
19981341396Above normal
199990.2962Near normal
200095.91962Near normal
200190.61582Near normal
2002124.41686Above normal
200356.61670Below normal
200471.11263Below normal
200596.61572Near normal
2006156.919116Above normal
200752.61141Below normal
200883.81772Near normal
2009126.42085Above normal
201051832Below normal
2011120.911106Above normal
201298.517105Near normal
201374.92091Below normal
2014202.422169Extremely active
2015290.6261611Extremely active
2016184.822136Extremely active
2017100.81894Near normal
2018318.1231310Extremely active
201997.61974Near normal
202077.21743Near normal
202194.51982Near normal
2022114.619104Above normal
2023165.917108Above normal
202484.61453Near normal
2025126.820114Above normal

Western Pacific

TSR forecasts
Date		Tropical
storms		Total
Typhoons		Intense
TCs
Average 25.516.09.3301

Historical ACE in recorded Western Pacific typhoon history

There is an undercount bias of tropical storms, typhoons, and super typhoon before the satellite era, due to the difficulty in identifying storms.
Classification criteria
SeasonACETSTYSTYClassification
1950160.218121Below normal
1951283.425161Near normal
195233829206Extremely active
1953362.624175Extremely active
1954305.519155Near normal
1955249.831204Below normal
1956305.626185Near normal
1957440.222188Extremely active
1958445.823219Extremely active
1959397.625188Extremely active
1960326.730192Near normal
1961365.627208Extremely active
196242330236Extremely active
196338625198Extremely active
1964403.138267Extremely active
1965436.3342111Extremely active
1966302.230203Near normal
1967398.134195Extremely active
1968356.827204Extremely active
1969203.719132Below normal
1970287.524127Near normal
1971380.235256Extremely active
197241329222Extremely active
1973148.121123Below normal
1974205.332160Below normal
197517120143Below normal
1976301.325144Near normal
1977164.119113Below normal
1978236.828151Below normal
1979278.423144Near normal
1980237.823152Below normal
198122728162Below normal
1982356.125192Extremely active
1983219.723124Below normal
198427427162Near normal
1985231.229151Below normal
1986334.229193Extremely active
1987356.523176Extremely active
1988227.925131Below normal
198930530215Near normal
1990377.830214Extremely active
1991414.329205Extremely active
1992470.131214Extremely active
1993267.129203Near normal
1994454.634206Extremely active
1995255.126155Below normal
1996416.530166Extremely active
1997570.4292311Extremely active
1998152.91893Below normal
1999109.923111Below normal
2000252.925144Below normal
2001307.329213Near normal
2002390.624168Extremely active
2003337.422175Extremely active
2004480.631206Extremely active
2005309.924183Near normal
2006321.321136Near normal
2007219.522165Below normal
2008178.427122Below normal
2009278.123155Near normal
2010121.41491Below normal
2011189.718104Below normal
2012302.325154Near normal
2013276.327165Near normal
2014277.820128Near normal
2015462.926209Extremely active
2016261.926176Near normal
2017169.426132Below normal
2018361.629137Extremely active
2019276.829175Near normal
2020152.823122Below normal
2021209.623105Below normal
2022163.222123Below normal
2023268.516114Near normal
2024210.126136Below normal
2025187.127141Below normal
20262.2100Current Season

North Indian

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. 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. 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 based on their best track analysis which goes back to 1982.