Accumulated cyclone energy

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Accumulated cyclone energy (ACE) is a metric used by various agencies to express the energy used by a tropical cyclone during its lifetime. The calculation takes a tropical cyclone's maximum sustained winds every six hours and multiplies it by itself to generate the values. These values are then added together which become a total for a storm and can either be divided by 10,000 to make them more manageable or added to other totals in order to work out a total for a particular group of storms. The calculation was originally created by William Gray and his associates at Colorado State University as the Hurricane Destruction Potential index, which took each hurricane's maximum sustained winds above 65 kn (120 km/h; 75 mph) and multiplied it by itself every six hours. This index was subsequently tweaked by the United States National Oceanic and Atmospheric Administration in 2000 to include all tropical cyclones, with winds above 35 kn (65 km/h; 40 mph) and renamed accumulated cyclone energy. The index has since been used by various other agencies to calculate a storm's accumulated cyclone energy, including the Australian Bureau of Meteorology as well as the India Meteorological Department.

The highest ACE calculated for a single storm is 82, for Hurricane/Typhoon Ioke in 2006.^[1]

History[]

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]

Calculation[]

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).

Atlantic basin[]

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, 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.^[6] The median value of the ACE index from 1951 to 2020 is 96.7 x 10⁴ kt².^[6]

Classification criteria

Category	ACE Index	% of 1951-2020 median
Extremely active	Above 159.6	Above 165%
Above-normal	Above 126.1	Above 130%
Near-normal	73-126.1	75% - 130%
Below-normal	Below 73	Below 75%
Reference:[6]

Top 10 Atlantic hurricane seasons

Season	TS	HU	MH	ACE
1933	20	11	6	258.57
2005	28	15	7	250.13
1893	12	10	5	231.15
1926	11	8	6	229.56
1995	19	11	5	227.10
2004	15	9	6	226.88
2017	17	10	6	224.88
1950	16	11	6	211.28
1961	12	8	5	205.39
1998	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, likely because it was a Category 4 hurricane which lasted for 4 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.^[8]

Since 1950, the highest ACE of a tropical storm was Tropical Storm Laura in 1971, which attained an ACE of 8.6. The highest ACE of a Category 1 hurricane was Hurricane Nadine in 2012, which attained an ACE of 26.3. The lowest ACE of a tropical storm were tropical storms Chris (2000) and Philippe (2017), both of which were tropical storms for only six hours and had an ACE of just 0.1. 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.^[9] The following table shows those storms in the Atlantic basin from 1950–2021 that have attained over 50 points of ACE.^[9]

Storm	Year	Peak classification	ACE	Duration
Hurricane Ivan	2004	Category 5 hurricane	70.4	23 days
Hurricane Irma	2017	Category 5 hurricane	64.9	13 days
Hurricane Isabel	2003	Category 5 hurricane	63.3	14 days
Hurricane Donna	1960	Category 4 hurricane	57.6	16 days
Hurricane Carrie	1957	Category 4 hurricane	55.8	21 days
Hurricane Inez	1966	Category 4 hurricane	54.6	21 days
Hurricane Sam	2021	Category 4 hurricane	53.8	14 days
Hurricane Luis	1995	Category 4 hurricane	53.5	16 days
Hurricane Allen	1980	Category 5 hurricane	52.3	12 days
Hurricane Esther	1961	Category 5 hurricane	52.2	18 days
Hurricane Matthew	2016	Category 5 hurricane	50.9	12 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

Accumulated Cyclone Energy - Atlantic

Season	ACE	TS	HU	MH	Classification
1851	36.24	6	3	1	Below normal
1852	73.28	5	5	1	Near normal
1853	76.49	8	4	2	Near normal
1854	31.00	5	3	1	Below normal
1855	18.12	5	4	1	Below normal
1856	48.94	6	4	2	Below normal
1857	46.84	4	3	0	Below normal
1858	44.79	6	6	0	Below normal
1859	55.73	8	7	1	Below normal
1860	62.06	7	6	1	Below normal
1861	49.71	8	6	0	Below normal
1862	46.03	6	3	0	Below normal
1863	50.35	9	5	0	Below normal
1864	26.55	5	3	0	Below normal
1865	49.13	7	3	0	Below normal
1866	83.65	7	6	1	Near normal
1867	59.97	9	7	1	Below normal
1868	34.65	4	3	0	Below normal
1869	51.02	10	7	1	Below normal
1870	87.8	11	10	2	Near normal
1871	88.39	8	6	2	Near normal
1872	65.38	5	4	0	Below normal
1873	69.47	5	3	2	Below normal
1874	47.05	7	4	0	Below normal
1875	72.48	6	5	1	Below normal
1876	56.05	5	4	2	Below normal
1877	73.36	8	3	1	Near normal
1878	180.85	12	10	2	Extremely active
1879	63.63	8	6	2	Below normal
1880	131.08	11	9	2	Above normal
1881	59.25	7	4	0	Below normal
1882	59.4675	6	4	2	Below normal
1883	66.7	4	3	2	Below normal
1884	72.06	4	4	1	Below normal
1885	58.3	8	6	0	Below normal
1886	166.165	12	10	4	Extremely active
1887	181.26	19	11	2	Extremely active
1888	84.945	9	6	2	Near normal
1889	104.0425	9	6	0	Near normal
1890	33.345	4	2	1	Below normal
1891	116.105	10	7	1	Near normal
1892	115.8375	9	5	0	Near normal
1893	231.1475	12	10	5	Extremely active
1894	135.42	7	5	4	Above normal
1895	68.765	6	2	0	Below normal
1896	136.0825	7	6	2	Above normal
1897	54.54	6	3	0	Below normal
1898	113.2375	11	5	1	Near normal
1899	151.025	10	5	2	Above normal
1900	83.345	7	3	2	Near normal
1901	98.975	13	6	0	Near normal
1902	32.65	5	3	0	Below normal
1903	102.07	10	7	1	Near normal
1904	30.345	6	4	0	Below normal
1905	28.3775	5	1	1	Below normal
1906	162.88	11	6	3	Extremely active
1907	13.06	5	0	0	Below normal
1908	95.11	10	6	1	Near normal
1909	93.34	12	6	4	Near normal
1910	63.9	5	3	1	Below normal
1911	34.2875	6	3	0	Below normal
1912	57.2625	7	4	1	Below normal
1913	35.595	6	4	0	Below normal
1914	2.53	1	0	0	Below normal
1915	130.095	6	5	3	Above normal
1916	144.0125	15	10	5	Above normal
1917	60.6675	4	2	2	Below normal
1918	39.8725	6	4	1	Below normal
1919	55.04	5	2	1	Below normal
1920	29.81	5	4	0	Below normal
1921	86.53	7	5	2	Near normal
1922	54.515	5	3	1	Below normal
1923	49.31	9	4	1	Below normal
1924	100.1875	11	5	2	Near normal
1925	7.2525	4	1	0	Below normal
1926	229.5575	11	8	6	Extremely active
1927	56.4775	8	4	1	Below normal
1928	83.475	6	4	1	Near normal
1929	48.0675	5	3	1	Below normal
1930	49.7725	3	2	2	Below normal
1931	47.835	13	3	1	Below normal
1932	169.6625	15	6	4	Extremely active
1933	258.57	20	11	6	Extremely active
1934	79.0675	13	7	1	Near normal
1935	106.2125	8	5	3	Near normal
1936	99.775	17	7	1	Near normal
1937	65.85	11	4	1	Below normal
1938	77.575	9	4	2	Near normal
1939	43.6825	6	3	1	Below normal
1940	67.79	9	6	0	Below normal
1941	51.765	6	4	3	Below normal
1942	62.485	11	4	1	Below normal
1943	94.01	10	5	2	Near normal
1944	104.4525	14	8	3	Near normal
1945	63.415	11	5	2	Below normal
1946	19.6125	7	3	0	Below normal
1947	88.49	10	5	2	Near normal
1948	94.9775	10	6	4	Near normal
1949	96.4475	16	7	2	Near normal
1950	211.2825	16	11	6	Extremely active
1951	126.325	12	8	3	Above normal
1952	69.08	11	5	2	Below normal
1953	98.5075	14	7	3	Near normal
1954	110.88	16	7	3	Near normal
1955	158.17	13	9	4	Above normal
1956	56.6725	12	4	1	Below normal
1957	78.6625	8	3	2	Near normal
1958	109.6925	12	7	3	Near normal
1959	77.1075	14	7	2	Near normal
1960	72.9	8	4	2	Below normal
1961	205.395	12	8	5	Extremely active
1962	35.5675	7	4	0	Below normal
1963	117.9325	10	7	3	Near normal
1964	169.7675	13	7	5	Extremely active
1965	84.33	10	4	1	Near normal
1966	145.2175	11	7	3	Above normal
1967	121.705	8	6	1	Near normal
1968	45.0725	8	5	0	Below normal
1969	165.7375	18	12	5	Extremely active
1970	40.18	11	5	2	Below normal
1971	96.5275	13	6	1	Near normal
1972	35.605	7	3	0	Below normal
1973	47.85	8	4	1	Below normal
1974	68.125	11	4	2	Below normal
1975	76.0625	9	6	3	Near normal
1976	84.1725	10	6	2	Near normal
1977	25.3175	6	5	1	Below normal
1978	63.2175	12	5	2	Below normal
1979	92.9175	9	6	2	Near normal
1980	148.9375	11	9	2	Above normal
1981	100.3275	12	7	3	Near normal
1982	31.5025	6	2	1	Below normal
1983	17.4025	4	3	1	Below normal
1984	84.295	13	5	1	Near normal
1985	87.9825	11	7	3	Near normal
1986	35.7925	6	4	0	Below normal
1987	34.36	7	3	1	Below normal
1988	102.9925	12	5	3	Near normal
1989	135.125	11	7	2	Above normal
1990	96.8025	14	8	1	Near normal
1991	35.5375	8	4	2	Below normal
1992	76.2225	7	4	1	Near normal
1993	38.665	8	4	1	Below normal
1994	32.02	7	3	0	Below normal
1995	227.1025	19	11	5	Extremely active
1996	166.1825	13	9	6	Extremely active
1997	40.9275	8	3	1	Below normal
1998	181.7675	14	10	3	Extremely active
1999	176.5275	12	8	5	Extremely active
2000	119.1425	15	8	3	Near normal
2001	110.32	15	9	4	Near normal
2002	67.9925	12	4	2	Below normal
2003	176.84	16	7	3	Extremely active
2004	226.88	15	9	6	Extremely active
2005	250.1275	28	15	7	Extremely active
2006	78.535	10	5	2	Near normal
2007	73.885	15	6	2	Near normal
2008	145.7175	16	8	5	Above normal
2009	52.58	9	3	2	Below normal
2010	165.4825	19	12	5	Extremely active
2011	126.3025	19	7	4	Above normal
2012	132.6325	19	10	2	Above normal
2013	36.12	14	2	0	Below normal
2014	66.725	8	6	2	Below normal
2015	62.685	11	4	2	Below normal
2016	141.2525	15	7	4	Above normal
2017	224.8775	17	10	6	Extremely active
2018	132.5825	15	8	2	Above normal
2019	132.2025	18	6	3	Above normal
2020	180.3725	30	14	7	Extremely active
2021	142.365	21	7	4	Above normal
2022	TBD	TBD	TBD	TBD	Upcoming season

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.^[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 and the number of tropical storms, hurricanes, and major hurricanes over the 30 years between 1991 and 2020.^[10]

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.^[10]

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.^[10]

Classification criteria

Category	ACE Index	% of 1991-2020 median	Tropical storms	Hurricanes	Major hurricanes
Above-normal	Above 115	Above 120%	17 or more	9 or more	5 or more
Near-normal	80-115	80% - 120%	16 or fewer	8 or fewer	4 or fewer
Below-normal	Below 80	Below 80%	Not defined
Reference:[10]

Top 10 Pacific hurricane seasons

Season	TS	HU	MH	ACE
2018	23	13	10	318
1992	27	16	10	295
2015	26	16	11	287
1990	21	16	6	245
1978	19	14	7	207
1983	21	12	8	206
1993	15	11	9	201
2014	22	16	9	199
1984	21	13	7	193
1985	24	13	8	192

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.^[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 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

Accumulated Cyclone Energy - Pacific

Season	ACE	TS	HU	MH	Classification
1971	139	18	12	6	Above normal
1972	136	14	8	4	Near normal
1973	114	12	7	3	Near normal
1974	90	18	11	3	Near normal
1975	112	17	9	4	Near normal
1976	121	15	9	5	Above normal
1977	22	8	4	0	Below normal
1978	207	19	14	7	Above normal
1979	57	10	6	4	Below normal
1980	77	14	7	3	Below normal
1981	72	15	8	1	Below normal
1982	161	23	12	5	Above normal
1983	206	21	12	8	Above normal
1984	193	21	13	7	Above normal
1985	192	24	13	8	Above normal
1986	107	17	9	3	Near normal
1987	132	20	10	4	Above normal
1988	127	15	7	3	Near normal
1989	110	17	9	4	Near normal
1990	245	21	16	6	Above normal
1991	178	14	10	5	Above normal
1992	295	27	16	10	Above normal
1993	201	15	11	9	Above normal
1994	185	20	10	5	Above normal
1995	100	10	7	3	Near normal
1996	53	9	5	2	Below normal
1997	167	19	9	7	Above normal
1998	134	13	9	6	Above normal
1999	90	9	6	2	Near normal
2000	95	19	6	2	Near normal
2001	90	15	8	2	Near normal
2002	125	16	8	6	Near normal
2003	56	16	7	0	Below normal
2004	71	12	6	3	Below normal
2005	96	15	7	2	Near normal
2006	155	19	11	6	Above normal
2007	52	11	4	1	Below normal
2008	83	17	7	2	Near normal
2009	127	20	8	5	Above normal
2010	52	8	3	2	Below normal
2011	121	11	10	6	Above normal
2012	98	17	10	5	Near normal
2013	76	20	9	1	Below normal
2014	199	22	16	9	Above normal
2015	287	26	16	11	Above normal
2016	183	22	13	6	Above normal
2017	100	18	9	4	Near normal
2018	318	23	13	10	Above normal
2019	97	19	7	4	Near normal
2020	73	17	4	3	Below normal
2021	93	19	8	2	Near normal
2022	TBD	TBD	TBD	TBD	Upcoming 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.^[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][13]

Top 10 North Indian Ocean cyclone seasons

Season	D	DD	CS	SCS	VSCS	ESCS	SUCS	ACE
2019	12	11	8	6	6	3	1	77.95
1989	10	6	3	2	1	1	1	45.54
2013	10	6	5	4	3	1	0	38.87
2018	14	9	7	5	3	1	0	35.61
2015	12	9	4	2	2	2	0	33.92
1999	10	8	5	3	3	2	1	33.60
2007	11	8	4	2	2	2	1	32.74
1982	19	11	8	5	4	3	0	30.04
1992	13	11	7	2	1	1	0	27.23
2010	8	6	5	4	2	1	0	26.90
References: [4][13]

See also[]

• Atlantic hurricane

References[]

External links[]

• 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