Accumulated cyclone energy explained

Accumulated cyclone energy (ACE) 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.[1] 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.[2] 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 (≥ 34 kn; 63 km/h; 39 mph);[3] the resulting figure is divided by 10,000 to place it on a more manageable scale.[1]

The calculation originated as the Hurricane Destruction Potential (HDP) index, which sums the squares of tropical cyclones' maximum sustained winds while at hurricane strength, at least 64 knots (≥ 119 km/h; 74 mph)[3] at six-hour recorded intervals across an entire season.[4] The HDP index was later modified to further include tropical storms, that is, all wind speeds of at least 34 knots (≥ 63 km/h; 39 mph),[3] to become the accumulated cyclone energy index.[5]

The highest ACE calculated for a single tropical cyclone on record worldwide is 87.01, set by Cyclone Freddy in 2023.[6]

History

The ACE index is an offshoot of Hurricane Destruction Potential (HDP), 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 (

2
v
max
)than simply to the maximum wind speed (

vmax

). 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 (≥ 119 km/h; 74 mph).[3] The squared windspeeds from six-hourly recorded intervals are then summed across an entire season.[5] This scale was subsequently modified in 1999 by the United States National Oceanic and Atmospheric Administration (NOAA) to include not only hurricanes but also tropical storms, that is, all cyclones while windspeeds are at least 34 knots (≥ 63 km/h; 39 mph).[3] 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.[7] 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.[8]

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 (≥ 34 kn; 63 km/h; 39 mph)[3] 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:

ACE=10-4\sum

2
v
max
(for

vmax

≥ 34 kn),

where

vmax

is estimated sustained wind speed in knots at six-hour intervals.[5]

Kinetic energy is proportional to the square of velocity. However, unlike the measure defined above, kinetic energy is also proportional to the mass

m

(corresponding to the size of the storm) and represents an integral of force equal to mass times acceleration,

F=m x a

, where acceleration is the antiderivative of velocity, or

vmax

. The integral is a difference at the limits of the square antiderivative, rather than a sum of squares at regular intervals. Thus, the term applied to the index, accumulated cyclone energy, is a misnomer since the index is neither a measure of kinetic energy nor "accumulated energy."

Atlantic Ocean

See main article: Atlantic hurricane season. 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.[8] 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.[8] The median value of the ACE index from 1951 to 2020 is 96.7 x 104 kt2.[8]

Classification criteria
Category ACE Index % of 1951–2020
median
Extremely active > 159.6 > 165%
Above-normal > 126.1 > 130%
Near-normal 73–126.1 75–130%
Below-normal < 73 < 75%
Reference:
Season! scope="col"
TSHUMHACE
20 11 6 258.57
28 15 7 245.3
12 10 5 231.15
11 8 6 229.56
19 11 5 227.10
15 9 6 226.88
17 10 6 224.88
16 11 6 211.28
12 8 5 188.9
14 10 3 181.76

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.[9] 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 (Category 3 or higher), was Hurricane Gerda in 1969, with an ACE of 5.3.[10] The following table shows those storms in the Atlantic basin from 1851–2021 that have attained over 50 points of ACE.[10]

StormYearPeak classificationACEDuration
Hurricane Three1899bgcolor=#73.628 days
Hurricane Ivan2004bgcolor=#70.423 days
Hurricane Irma2017bgcolor=#64.913 days
Hurricane Nine1893bgcolor=#63.520 days
Hurricane Isabel2003bgcolor=#63.314 days
Hurricane Fourteen1932bgcolor=#59.815 days
Hurricane Donna1960bgcolor=#57.616 days
Hurricane Carrie1957bgcolor=#55.821 days
Hurricane Inez1966bgcolor=#54.621 days
Hurricane Sam2021bgcolor=#53.814 days
Hurricane Luis1995bgcolor=#53.715 days
Hurricane Allen1980bgcolor=#52.312 days
Hurricane Esther1961bgcolor=#52.218 days
Hurricane Matthew2016bgcolor=50.912 days

Historical ACE in recorded Atlantic hurricane history

There is an undercount bias of tropical storms, hurricanes, and major hurricanes before the satellite era (prior to the mid–1960s), due to the difficulty in identifying storms.

Classification criteria

Season! scope="col"
ACETSHUMHClassification
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
1961188.91285Extremely active
196235.5675740Below normal
1963117.93251073Near normal
19641531375Above normal
196584.331041Near normal
1966145.21751573Above normal
1967121.7051361Near normal
196845.0725950Below normal
1969165.737518123Extremely active
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
202448.2531Current season

Eastern Pacific

See main article: Pacific hurricane. 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.[11] 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 and the number of tropical storms, hurricanes, and major hurricanes over the 30 years between 1991 and 2020.[11]

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.[11]

The mean value of the ACE index from 1991 to 2020 is 108.7 x 104 kt2, while the median value is 97.2 x 104 kt2.[11]

Category! scope="col"
ACE Index% of 1991–2020
median		Tropical
storms		HurricanesMajor
hurricanes
Above-normal> 115 > 120% 15 or more 8 or more 4 or more
Near normal80–115 80–120% 15 or fewer 8 or fewer 4 or fewer
Below-normal< 80 < 80% colspan=3
Reference:

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.[12]

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.[13]

StormYearPeak classificationACEDuration
Hurricane Fico1978bgcolor=#62.820 days
1994bgcolor=#54.019 days
Hurricane Kevin1991bgcolor=#52.117 days
2018bgcolor=#50.513 days
2023bgcolor=#48.412 days
Hurricane Tina1992bgcolor=#47.722 days
Hurricane Trudy1990bgcolor=#45.816 days
Hurricane Lane2018bgcolor=#44.213 days
1999bgcolor=#41.413 days
Hurricane Jimena2015bgcolor=#40.015 days
Hurricane Guillermo1997bgcolor=#40.016 days
Hurricane Norbert1984bgcolor=#39.612 days
Hurricane Norman2018bgcolor=#36.612 days
Hurricane Celeste1972bgcolor=#36.316 days
Hurricane Sergio2018bgcolor=#35.513 days
Hurricane Lester2016bgcolor=#35.414 days
Hurricane Olaf2015bgcolor=#34.612 days
Hurricane Jimena1991bgcolor=#34.512 days
Hurricane Doreen1973bgcolor=#34.316 days
2006bgcolor=#34.27 days
Hurricane Marie1990bgcolor=#33.114 days
Hurricane Orlene1992bgcolor=#32.412 days
Hurricane Greg1993bgcolor=#32.313 days
Hurricane Hilary2011bgcolor=#31.29 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

Season! scope="col"
ACETSHUMHClassification
197113918126Above normal
19721361484Near normal
19731141273Near normal
19749018113Near normal
19751121794Near normal
19761211595Above normal
197722840Below normal
197820719147Above normal
1979571064Below normal
1980771473Below normal
1981721581Below normal
198216123125Above normal
198320621128Above normal
198419321137Above normal
198519224138Above normal
19861071793Near normal
198713220104Above normal
19881271573Near normal
19891101794Near normal
199024521166Above normal
199117814105Above normal
1992295271610Above normal
199320115119Above normal
199418520105Above normal
19951001073Near normal
199653952Below normal
19971671997Above normal
19981341396Above normal
199990962Near normal
2000951962Near normal
2001901582Near normal
20021251686Near normal
2003561670Below normal
2004711263Below normal
2005961572Near normal
200615519116Above normal
2007521141Below normal
2008831772Near normal
20091272085Above normal
201052832Below normal
201112111106Above normal
20129817105Near normal
2013762091Below normal
201420122169Above normal
2015290261611Above normal
201618422136Above normal
20171001894Near normal
2018318231310Above normal
2019971974Near normal
2020731743Below normal
2021931982Near normal
202211619104Above normal
202316417108Above normal
202415.68610Current season

Western Pacific

See main article: Pacific typhoon season.

Historical ACE in recorded Western Pacific typhoon history

There is an undercount bias of tropical storms, typhoons, and super typhoon before the satellite era (prior to the mid–1950s), due to the difficulty in identifying storms.

Classification criteria

Season! scope="col"
ACETSTYSTYClassification
1950160.218121Below normal
1951283.425161Above normal
195233829206Above normal
1953362.624175Extremely active
1954305.519155Above normal
1955249.831204Near normal
1956305.626185Above normal
1957440.222188Extremely active
1958445.823219Extremely active
1959397.625188Extremely active
1960326.730192Above normal
1961365.627208Extremely active
196242330236Extremely active
196338625198Extremely active
1964403.138267Extremely active
1965436.3342111Extremely active
1966302.230203Above normal
1967398.134195Extremely active
1968356.827204Extremely active
1969203.719132Near normal
1970287.524127Above normal
1971380.235256Extremely active
197241329222Extremely active
1973148.121123Below normal
1974205.332160Near normal
197517120143Below Normal
1976301.325144Above normal
1977164.119113Below normal
1978236.828151Near normal
1979278.423144Near normal
1980237.823152Near normal
198122728162Near normal
1982356.125192Extremely active
1983219.723124Near normal
198427427162Near normal
1985231.225171Near normal
1986334.226193Above normal
1987356.523176Extremely active
1988227.925131Near normal
198930530215Above normal
1990377.830214Extremely active
1991414.329205Extremely active
1992470.131214Extremely active
1993267.129203Near normal
1994454.634206Extremely active
1995255.126155Near normal
1996416.533216Extremely active
1997570.4292311Extremely active
1998152.91893Below normal
1999109.923111Below normal
2000252.925144Near normal
2001307.329213Above normal
2002390.624168Extremely active
2003337.422175Above normal
2004480.631206Extremely active
2005309.924183Above normal
2006321.321136Above normal
2007219.522165Near normal
2008178.427122Below normal
2009278.123155Near normal
2010121.41491Below normal
2011189.718104Below normal
2012302.325154Above normal
2013276.327165Near normal
2014277.820128Near normal
2015462.926209Extremely active
2016261.926176Near normal
2017169.426132Below normal
2018361.631167Extremely active
2019276.830185Near normal
2020152.823122Below normal
2021209.623105Near normal
2022163.222123Below normal
2023268.516114Near normal
202449.26930Ongoing

North Indian

See main article: North Indian Ocean tropical cyclone. 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.[14] 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 17kn based on their best track analysis which goes back to 1982.[15]

Season! scope="col"
DDDCSSCSVSCSESCSSUCSACE
12 11 8 6 6 3 1 93
9 7 6 5 4 3 0 55
11 8 4 2 2 2 1 46.1
10 6 5 4 3 1 0 45.6
10 8 5 3 3 2 1 44.3
References:

Historical ACE in recorded North Indian cyclonic history

See also

External links

Notes and References

  1. Web site: Measuring overall activity: The Accumulated Cyclone Energy (ACE) index . National Weather Service Climate Prediction Center . NOAA . 30 January 2024.
  2. Web site: Climate Change Indicators: Tropical Cyclone Activity . United States Environmental Protection Agency . EPA . 30 January 2024.
  3. Web site: Glossary of NHC Terms. National Oceanic and Atmospheric Administration, National Hurricane Center and Central Pacific Hurricane Center . 31 January 2024.
  4. Forecast of Atlantic Seasonal Hurricane Activity for 1988 . May 26, 1988 . 13–14 . Colorado State University. Gray, William Mason.
  5. Bell, Gerald D . Halpert, Michael S. Schnell, Russell C. Higgins, R. Wayne. Lawrimore, Jay. Kousky, Vernon E. Tinker, Richard. Thiaw, Wasila . Chelliah, Muthuvel. Artusa, Anthony. Climate Assessment for 1999. Bulletin of the American Meteorological Society. June 2000. 81. 6. S19. 10.1175/1520-0477(2000)81[s1:CAF]2.0.CO;2. free.
  6. Web site: Real-Time Southern Hemisphere Statistics by Storm for 2022/2023 . live . https://web.archive.org/web/20230311184841/http://tropical.atmos.colostate.edu/Realtime/index.php?loc=southernhemisphere . 11 March 2023 . 11 March 2023 . Colorado State University.
  7. Weekly Tropical Climate Note July 14, 2020 . Australian Bureau of Meteorology. September 4, 2020. July 14, 2020. https://web.archive.org/web/20200904160004/http://www.bom.gov.au/climate/tropical-note/archive/20200714.archive.shtml. September 4, 2020. live.
  8. Web site: United States Climate Prediction Center. Background information: North Atlantic Hurricane Season. May 22, 2019. August 22, 2020. https://web.archive.org/web/20200724071552/https://www.cpc.ncep.noaa.gov/products/outlooks/Background.html. July 24, 2020. live.
  9. Web site: Real-Time North Atlantic Ocean Statistics compared with climatology . 2023-10-05 . tropical.atmos.colostate.edu.
  10. Web site: Atlantic hurricanes by ACE - 1950-2012. Policlimate. 7 August 2017. https://web.archive.org/web/20170810205229/http://policlimate.com/tropical/atlantic_storms_ace_maxw.dat. 10 August 2017. dead.
  11. Web site: United States Climate Prediction Center. Background information: Eastern Pacific Hurricane Season. May 22, 2019. August 22, 2020. https://web.archive.org/web/20200724071552/https://www.cpc.ncep.noaa.gov/products/outlooks/Background.html. July 24, 2020. live.
  12. Web site: Eastern Pacific Best Track Data - (1949 - present) . National Hurricane Center . 12 August 2018.
  13. Web site: Webb . Eric . Hurricane Hector 2018 - Discussion . Twitter . 12 August 2018.
  14. Mohapatra M. Vijay Kumar, V. Interannual variation of tropical cyclone energy metrics over North Indian Ocean. Climate Dynamics . March 2017 . 48 . 5–6 . 1431–1445 . 10.1007/s00382-016-3150-3. 2017ClDy...48.1431M . 130486452.
  15. Tropical Cyclone Energy Matrix over North Indian Ocean . India Meteorological Department . September 3, 2020 . https://web.archive.org/web/20200718221357/http://www.rsmcnewdelhi.imd.gov.in/images/bulletin/tcenergymatrix.pdf . July 18, 2020. 2020. live.

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