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 hurricane's wind and storm surge was better related to the square of the maximum wind speed (Vmax²) than simply to the maximum wind speed.[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 adjusted 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). NOAA also renamed it the accumulated cyclone energy index.[2] Since the scale was adjusted 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 categorize how active a tropical cyclone season was as well as to identify possible long-term trends in a certain area such as the Lesser Antilles.[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 summing at six-hourly intervals is an approximation to a time integral. The numbers are usually divided by 10,000 to make them more manageable. One unit of ACE equals 10⁻⁴ 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. However unlike the measure defined above, it is also proportional to the mass (corresponding to the size of the storm), and does not involve a time integral. Thus this statistic is not a measure of kinetic energy, nor of "accumulated energy".

Accumulated cyclone energy of North Atlantic hurricanes.[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, 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]

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]

Historical ACE in recorded Pacific hurricane history

100

200

300

400

1971

1980

1990

2000

2010

2020

Observed monthly values for the PDO index, 1900–present.

Historical East Pacific Seasonal Activity, 1981–2015.

Data on ACE is considered reliable starting with the 1971 season.

Classification criteria

  Above-normal

  Near-normal

  Below-normal

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	116	19	10	4	Above normal
2023	155	15	10	8	Season ongoing

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][15]

• Atlantic hurricane
• Cyclone Freddy – Produced the highest accumulated cyclone energy amount worldwide
• Hurricane/Typhoon Ioke - The second-most ACE producing tropical cyclone on record
• Saffir–Simpson scale – Alternative intensity scale

• 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