Tropical Cyclone Activity

2023-03-22

Table of contents

Cyclone
Climate Change Indicators: Tropical Cyclone Activity
Key Points
Background
About the indicator
About the Data
- Notes on the indicator
- Data Sources

Cyclone

In meteorology, a cyclone is a large mass of air that rotates around an intense center of low atmospheric pressure, counterclockwise in the northern and the southern hemispheres when viewed from above.

Cyclones are characterized by inward-spiraling winds that revolve around an area of low pressure. The largest low-pressure systems are the largest-scale polar vortices and extratropical cyclones. Hot-core storms, such as tropical and subtropical cyclones, also fall within the synoptic scale.

Climate Change Indicators: Tropical Cyclone Activity

This indicator examines the frequency, intensity, and duration of hurricanes and other tropical storms in the Atlantic Ocean, the Caribbean, and the Gulf of Mexico.

Figure 1. Number of Hurricanes in the North Atlantic, 1878–2020

This chart shows the number of hurricanes that formed in the North Atlantic Ocean each year from 1878 to 2020, along with the number that made landfall in the United States.

The orange curve shows how the total in the green turn can be adjusted to try to account for the lack of aircraft and satellite observations in the early years. All three curves were smoothed using a five-year average plotted in the middle year. The most recent standard was devised in 2018.

Year	Total hurricanes (adjusted)	Total hurricanes (unadjusted)	Hurricanes reaching the United States
1880	9.63874	6.6	2.6
1881	8.24872	5.2	2.4
1882	7.82772	4.8	1.8
1883	7.23736	4.2	1.2
1884	8.38944	5.4	2.2
1885	9.7223	6.8	2.6
1886	10.20804	7.4	3
1887	10.43916	7.8	3.2
1888	9.49582	7	3
1889	8.84482	6.4	2
1890	7.589	5.2	1.2
1891	8.3433	6	1.6
1892	8.21306	5.8	1.8
1893	8.23152	5.8	2
1894	7.97938	5.6	2.2
1895	7.5356	5.2	2.4
1896	6.46862	4.2	2
1897	6.43264	4.2	2.2
1898	6.62844	4.4	2.2
1899	6.66464	4.4	2
1900	6.67938	4.4	1.8
1901	7.14486	4.8	1.6
1902	6.87034	4.6	1.4
1903	6.33056	4.2	1.2
1904	6.19772	4.2	1.6
1905	5.47598	3.6	1.6
1906	5.13668	3.4	1.4
1907	5.4591	3.8	2
1908	5.81044	4.2	2.4
1909	5.15556	3.6	2
1910	5.8964	4.4	2.4
1911	5.36032	4	2.8
1912	4.1679	2.8	1.8
1913	4.55718	3.2	2.2
1914	6.0257	4.6	2.6
1915	5.84202	4.2	2.4
1916	6.1842	4.2	2.2
1917	6.62338	4.6	2.4
1918	6.4119	4.4	1.8
1919	5.26592	3.4	1.4
1920	5.20796	3.6	1.2
1921	4.87144	3.6	1
1922	5.30032	4.2	1.4
1923	4.62592	3.6	1.2
1924	5.17576	4.2	1.6
1925	5.32514	4.4	1.6
1926	5.3125	4.4	1.8
1927	4.89324	4	1.6
1928	5.06812	4.2	1.6
1929	4.08822	3.2	0.8
1930	4.54568	3.6	1.2
1931	5.97882	5	1.8
1932	6.8203	5.8	2
1933	7.42614	6.4	2.4
1934	8.199	7.2	3
1935	7.72284	6.8	2.6
1936	6.26074	5.4	2
1937	5.5374	4.6	1.6
1938	5.90118	4.8	1.6
1939	5.49464	4.2	1.4
1940	5.91232	4.2	1.8
1941	6.40042	4.4	1.6
1942	7.47134	5.4	2
1943	7.41478	5.2	2.2
1944	7.2462	5	2
1945	7.2019	5.2	2.2
1946	7.25388	5.4	2.6
1947	6.86364	5.2	2.6
1948	7.75086	6.4	2.6
1949	8.49848	7.4	2.4
1950	8.302	7.4	2
1951	8.34138	7.6	2
1952	8.30516	7.6	2
1953	7.86162	7.2	1.8
1954	7.03774	6.4	2
1955	6.60828	6	2
1956	6.58736	6	1.6
1957	6.56382	6	1.6
1958	5.53962	5	1.6
1959	6.32076	5.8	1.6
1960	6.32586	5.8	1.4
1961	6.32194	5.8	1.4
1962	6.10684	5.6	1.6
1963	6.09758	5.6	1.4
1964	5.79868	5.4	1.6
1965	6.28954	6	1.8
1966	5.58426	5.4	1.8
1967	6.693	6.6	1.4
1968	6.8	6.8	1.4
1969	6.6	6.6	1.6
1970	6	6	1.6
1971	6	6	1.4
1972	4.4	4.4	1.2
1973	4.6	4.6	1.2
1974	4.6	4.6	0.8
1975	5	5	0.8
1976	5.2	5.2	0.8
1977	5.4	5.4	1.2
1978	6	6	1.2
1979	6.2	6.2	1
1980	5.6	5.6	0.8
1981	5.2	5.2	1
1982	5.2	5.2	0.6
1983	4.8	4.8	1.6
1984	4.2	4.2	2
1985	4.4	4.4	2.2
1986	4.8	4.8	2.2
1987	5.2	5.2	2.6
1988	5.4	5.4	1.4
1989	5.4	5.4	1.2
1990	5.6	5.6	1.2
1991	5.4	5.4	1.2
1992	4.6	4.6	0.6
1993	5.2	5.2	1
1994	6.2	6.2	1.2
1995	6	6	1.2
1996	7.2	7.2	1.6
1997	8.2	8.2	2.2
1998	7.6	7.6	1.8
1999	7.6	7.6	1.4
2000	7.8	7.8	1.4
2001	7.2	7.2	1.2
2002	7.4	7.4	1.8
2003	8.8	8.8	3
2004	8	8	3
2005	8.4	8.4	3
2006	8.6	8.6	3.2
2007	7.4	7.4	2
2008	6.8	6.8	0.8
2009	7.2	7.2	1
2010	8	8	1.2
2011	6.8	6.8	0.6
2012	7.4	7.4	0.8
2013	5.8	5.8	0.8
2014	5.8	5.8	1
2015	5.8	5.8	1.2
2016	7	7	1.6
2017	7	7	1.8
2018	8.8	8.8	3

Figure 1. Number of Hurricanes in the North Atlantic, 1878–2020
Data source: NOAA, 2021; Vecchi and Knutson, 2011
Web update: April 2021
Units: Number of hurricanes

Figure 2. North Atlantic Tropical Cyclone Activity According to the Accumulated Cyclone Energy Index, 1950–2020

This figure shows annual totals of the Cyclone Accumulated Energy Index corresponding to cyclone strength, duration, and frequency from 1950 to 2020.

The National Oceanic and Atmospheric Administration defined “near normal,” “above normal,” and “below normal” ranges based on the distribution values of the ACE Index for 30 years from 1981 to 2010.

Year	Adjusted ACE Index (as % of 1981-2010 median)
1950	228.1081
1951	136.2162
1952	74.59459
1953	107.027
1954	120
1955	170.8108
1956	61.62162
1957	85.40541
1958	118.9189
1959	83.24324
1960	78.91892
1961	221.6216
1962	38.91892
1963	127.5676
1964	183.7838
1965	90.81081
1966	156.7568
1967	131.8919
1968	48.64865
1969	179.4595
1970	43.24324
1971	104.8649
1972	38.91892
1973	51.89189
1974	73.51351
1975	82.16216
1976	90.81081
1977	27.02703
1978	68.10811
1979	100.5405
1980	161.0811
1981	108.1081
1982	34.59459
1983	18.37838
1984	90.81081
1985	95.13514
1986	38.91892
1987	36.75676
1988	111.3514
1989	145.9459
1990	104.8649
1991	38.91892
1992	82.16216
1993	42.16216
1994	34.59459
1995	246.4865
1996	179.4595
1997	44.32432
1998	196.7568
1999	191.3514
2000	128.6486
2001	118.9189
2002	72.43243
2003	190.2703
2004	245.4054
2005	270.2703
2006	85.40541
2007	80
2008	157.8378
2009	57.2973
2010	178.3784
2011	136.2162
2012	139.4595
2013	38.91892
2014	72.43243
2015	68.10811
2016	152.4324
2017	241.0811
2018	144.8649
2019	143.7838
2020	194.5946

Figure 2. North Atlantic Tropical Cyclone Activity According to the Accumulated Cyclone Energy Index, 1950–2020
Data source: NOAA, 2021
Web update: April 2021
Units: Accumulated Cyclone Energy index

Figure 3. North Atlantic Tropical Cyclone Activity According to the Power Dissipation Index, 1949–2019

This number represents the annual values of the Power Dissipation Index, which corresponds to the cyclone’s strength, duration, and frequency. Tropical sea surface temperature trends in the North Atlantic are shown for reference.

Note that sea surface temperature is measured in different units, but the values have been plotted along with the PDI to see how they compare. The lines have been smoothed using a five-year weighted average planned in the middle year. The most recent standard was scheduled in 2017.

Year	Smoothed SST (F)	Smoothed Power Dissipation Index (10^11 m^3/s^2)
1949
1950
1951	82.1066	2.6784
1952	82.21388	2.249
1953	82.15412	2.1383
1954	82.01354	2.3011
1955	81.9455	2.2156
1956	82.02902	1.9505
1957	82.09598	1.783
1958	82.02362	1.778
1959	81.87494	2.0556
1960	81.8078	2.4492
1961	81.8978	2.5897
1962	81.9482	2.5354
1963	81.85316	2.4939
1964	81.75254	2.6379
1965	81.7466	2.7074
1966	81.77846	2.419
1967	81.81104	2.1363
1968	81.87872	1.9656
1969	81.91652	1.9072
1970	81.78728	1.9082
1971	81.55508	1.6771
1972	81.4136	1.424
1973	81.37796	1.3788
1974	81.41198	1.5631
1975	81.51332	1.718
1976	81.61736	1.5895
1977	81.68396	1.5082
1978	81.78962	1.9018
1979	81.97178	2.6182
1980	82.03964	2.8708
1981	81.88304	2.2747
1982	81.6503	1.4132
1983	81.50918	1.1123
1984	81.4901	1.3397
1985	81.60332	1.4556
1986	81.84002	1.4057
1987	82.05314	1.7075
1988	82.17536	2.3569
1989	82.16564	2.6223
1990	82.02452	2.2292
1991	81.82904	1.6827
1992	81.67334	1.3428
1993	81.734	1.5155
1994	81.95414	2.4098
1995	82.17032	3.3501
1996	82.37804	3.654
1997	82.553	3.5959
1998	82.6268	3.7471
1999	82.55858	3.8891
2000	82.41872	3.3709
2001	82.41242	2.7995
2002	82.60628	3.231
2003	82.86494	4.5551
2004	83.07068	5.5545
2005	83.06402	5.1439
2006	82.87646	3.8943
2007	82.71716	2.9178
2008	82.74092	2.6936
2009	82.93082	2.911
2010	83.06348	3.0732
2011	83.07806	2.9344
2012	83.00318	2.342
2013	82.9085	1.7643
2014	82.85504	1.7555
2015	82.84784	2.6053
2016	82.8437	3.8005
2017	82.7609	4.53
2018
2019

Figure 3. North Atlantic Tropical Cyclone Activity According to the Power Dissipation Index, 1949–2019
Data source: Emanuel, 2021
Web update: April 2021
Units: °F; Power Dissipation Index

Key Points

Since 1878, about six to seven hurricanes have formed in the North Atlantic each year. About two a year come ashore in the United States.

The total number of storms and the number making landfall in the United States do not indicate a clear overall trend since 1878 (see Figure 1).
According to the ACE Annual Total Index, cyclone intensity has increased noticeably over the past 20 years, and eight of the ten most active years since 1950 have occurred since the mid-1990s (see Figure 2).

Relatively high levels of cyclone activity were also observed during the 1950s and 1960s.
The PDI (see Figure 3) shows fluctuating cyclone intensity for most of the mid to late 20th century, followed by a noticeable increase since 1995.

These trends are compared with associated sea surface temperature variations in the tropical North Atlantic (see Figure 3).
Despite the apparent increase in tropical cyclone activity in recent years, as shown in Figures 2 and 3, changes in observational methods over time make it difficult to determine whether low storm activity has increased over time.

Background

Hurricanes, tropical storms, and other intense rotating storms fall into a general category called cyclones. There are two main types of cyclones: tropical and extratropical.

Tropical cyclones gain energy from warm tropical oceans. Extratropical cyclones gain power from the jet stream and temperature differences between cold, dry air masses from higher latitudes and warm, moist air masses from lower margins.

This indicator focuses on tropical cyclones in the Atlantic, Caribbean, and Gulf of Mexico. Tropical cyclones are most common during the “hurricane season,” which runs from June to November.

The effects of tropical cyclones are numerous and well-known. At sea, storms disrupt and threaten ships. When hurricanes make landfall, heavy rains and strong winds can cause severe property damage, loss of life, soil erosion, and flooding.

The associated storm surge—a large volume of ocean water pushed ashore by a cyclone’s strong winds—can cause severe flooding, erosion, and destruction.

Climate change is expected to affect tropical cyclones by increasing sea surface temperatures, a key factor influencing cyclone formation and behavior.

The US Global Change Research Program and the Intergovernmental Panel on Climate Change project that tropical cyclones will become more intense with higher wind speeds and heavier rainfall during the 21st century.

About the indicator

Records of tropical cyclones in the Atlantic Ocean have been collected since the 1800s. The most reliable long-term papers focus on hurricanes, the strongest category of tropical cyclones in the Atlantic, with winds of at least 74 miles per hour.

This indicator uses historical data from the National Oceanic and Atmospheric Administration to track the number of hurricanes per year in the North Atlantic and the number that have hit the United States since 1878.

Some storms over the ocean may have disappeared before airplanes and satellites were launched observations. Hence, scientists used other evidence, such as shipping records, to estimate the number of hurricanes that may have formed in earlier years.

This indicator also looks at the Cyclone Accumulated Energy Index and the Energy Dissipation Index, two ways of monitoring tropical cyclones’ frequency, strength, and duration based on wind speed measurements.

Each cyclone has an ACE Index value based on the maximum wind speed measured at six-hour intervals for the entire hurricane is classified as at least a tropical storm.

Therefore, the ACE Index value of a storm corresponds to both strength and duration. The National Oceanic and Atmospheric Administration calculates the total ACE index value for the entire hurricane season by summing the values for all named storms, including subtropical storms, tropical storms, and hurricanes. The resulting annual sum corresponds to the cyclone’s strength, duration, and frequency.

For this indicator, the index has been converted to a scale where 100 equals the mean value for the base period from 1981 to 2010. The thresholds in Figure 2 define whether the ACE index for a given year is close to normal or significantly above average.

Like the ACE Index, the PDI is based on wind speed measurements but uses a different calculation method that puts more emphasis on storm intensity.

This indicator shows the annual PDI value, the sum of the PDI values for all named storms during the year.

About the Data

Notes on the indicator

Over time, data collection methods have changed as technology has improved. For example, wind speed collection methods have evolved substantially over the past 60 years, while aircraft surveys began in 1944 and satellite tracking around 1966.

Figure 1 shows how older hurricane counts have been adjusted to account for the lack of aircraft and satellite observations. Changes in data collection technologies could substantially affect the overall patterns in Figures 2 and 3.

The effects of these changes on data consistency over the lifetime of the indicator would benefit from further research.

While Figures 2 and 3 cover several aspects of tropical cyclones, other essential factors are not shown here, including the size of each storm, the amount of rain, and storm surge height.

The reason for the recent divergence between cyclone activity and sea surface temperature in Figure 3 has not been conclusively identified. Still, it may be related to other factors that influence storm formation, such as the difference in wind speeds at different levels in the atmosphere.

Data Sources

Hurricane counts were compiled using the methods described in Knutson et al. Data for Figures 1 and 2 are from the National Oceanic and Atmospheric Administration’s Hurricane Research Division.

Kerry Emanuel of the Massachusetts Institute of Technology calculated the values for Figure 3. The ACE index and the PDI are based on wind speed measurements compiled by the National Oceanic and Atmospheric Administration.