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Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment
P. J. Webster,1G. J. Holland,2J. A. Curry,1H.-R. Chang1
We examined the number of tropical cyclones and cyclone daysas well as tropical cyclone intensity over the past 35 years,in an environment of increasing sea surface temperature. A largeincrease was seen in the number and proportion of hurricanesreaching categories 4 and 5. The largest increase occurred inthe North Pacific, Indian, and Southwest Pacific Oceans, andthe smallest percentage increase occurred in the North AtlanticOcean. These increases have taken place while the number ofcyclones and cyclone days has decreased in all basins exceptthe North Atlantic during the past decade.
1 School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA. 2 National Center for Atmospheric Research, Boulder, CO, USA.
During the hurricane season of 2004, there were 14 named stormsin the North Atlantic, of which 9 achieved hurricane intensity.Four of these hurricanes struck the southeast United Statesin rapid succession, causing considerable damage and disruption.Analysis of hurricane characteristics in the North Atlantic(1,2) has shown an increase in hurricane frequency and intensitysince 1995. Recently, a causal relationship between increasinghurricane frequency and intensity and increasing sea surfacetemperature (SST) has been posited (3), assuming an accelerationof the hydrological cycle arising from the nonlinear relationbetween saturation vapor pressure and temperature (4). The issueof attribution of increased hurricane frequency to increasingSST has resulted in a vigorous debate in the press and in academiccircles (5).
Numerous studies have addressed the issue of changes in theglobal frequency and intensity of hurricanes in the warmingworld. Our basic conceptual understanding of hurricanes suggeststhat there could be a relationship between hurricane activityand SST. It is well established that SST > 26°C is arequirement for tropical cyclone formation in the current climate(6,7). There is also a hypothesized relationship between SSTand the maximum potential hurricane intensity (8,9). However,strong interannual variability in hurricane statistics (10-14)and the possible influence of interannual variability associatedwith El Niño and the North Atlantic Oscillation (11,12) make it difficult to discern any trend relative to backgroundSST increases with statistical veracity (8). Factors other thanSST have been cited for their role in regulating hurricane characteristics,including vertical shear and mid-tropospheric moisture (15).Global modeling results for doubled CO2 scenarios are contradictory(15-20), with simulations showing a lack of consistency in projectingan increase or decrease in the total number of hurricanes, althoughmost simulations project an increase in hurricane intensity.
Tropical ocean SSTs increased by approximately 0.5°C between1970 and 2004 (21). Figure 1 shows the SST trends for the tropicalcyclone season in each ocean basin. If the Kendall trend analysisis used, trends in each of the ocean basins are significantlydifferent from zero at the 95% confidence level or higher, exceptfor the southwest Pacific Ocean. Here we examine the variationsin hurricane characteristics for each ocean basin in the contextof the basin SST variations. To this end, we conducted a comprehensiveanalysis of global tropical cyclone statistics for the satelliteera (1970–2004). In each tropical ocean basin, we examinedthe numbers of tropical storms and hurricanes, the number ofstorm days, and the hurricane intensity distribution. The tropicalcyclone data are derived from the best track archives of theJoint Typhoon Warning Center and of international warning centers,including special compilations and quality control (22).
Fig. 1. Running 5-year mean of SST during the respective hurricane seasons for the principal ocean basins in which hurricanes occur: the North Atlantic Ocean (NATL: 90° to 20°E, 5° to 25°N, June-October), the Western Pacific Ocean (WPAC: 120° to 180°E, 5° to 20°N, May-December), the East Pacific Ocean (EPAC: 90° to 120°W, 5° to 20°N, June-October), the Southwest Pacific Ocean (SPAC: 155° to 180°E, 5° to 20°S, December-April), the North Indian Ocean (NIO: 55° to 90°E, 5° to 20°N, April-May and September-November), and the South Indian Ocean (SIO: 50° to 115°E, 5° to 20°S, November-April).
[View Larger Version of this Image (28K GIF file)]
Tropical cyclonic systems attaining surface wind speeds between18 and 33 m s–1 are referred to as tropical storms. Althoughstorms of intensity >33 m s–1 have different regionalnames, we will refer to these storms as hurricanes for simplicity.Hurricanes in categories 1 to 5, according to the Saffir-Simpsonscale (23), are defined as storms with wind speeds of 33 to43 m s–1, 43 to 50 m s–1, 50 to 56 m s–1,56 to 67 m s–1, and >67 m s–1, respectively.We define the ocean basins that support tropical cyclone developmentas follows: North Atlantic (90° to 20°W, 5° to 25°N),western North Pacific (120° to 180°E, 5° to 20°N),eastern North Pacific (90° to 120°W, 5° to 20°N),South Indian (50° to 115°E, 5°-20°S), NorthIndian (55° to 90°E, 5°-20°N), and SouthwestPacific (155° to 180°E, 5° to 20°S). Withinthese basins, total tropical storm days are defined as the totalnumber of days of systems that only reached tropical storm intensity.Total hurricane days refer to systems that attained hurricanestatus, including the period when a system was at tropical stormintensity. Total tropical cyclone number or days refers to thesum of the statistics for both tropical storms and hurricanes.
Figure 2 shows the time series for the global number of tropicalcyclones and the number of cyclone days for the period 1970–2004,for hurricanes, tropical storms, and all cyclonic storms. Noneof these time series shows a trend that is statistically differentfrom zero over the period (24). However, there is a substantialdecadal-scale oscillation that is especially evident in thenumber of tropical cyclone days. For example, globally, theannual number of tropical cyclone days reached a peak of 870days around 1995, decreasing by 25% to 600 days by 2003.
Fig. 2. Global time series for 1970–2004 of (A) number of storms and (B) number of storm days for tropical cyclones (hurricanes plus tropical storms; black curves), hurricanes (red curves), and tropical storms (blue curves). Contours indicate the year-by-year variability, and the bold curves show the 5-year running average.
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Figure 3 shows that in each ocean basin time series, the annualfrequency an\
d duration of hurricanes exhibit the same temporalcharacteristics as the global time series (Fig. 2), with overalltrends for the 35-year period that are not statistically differentfrom zero. The exception is the North Atlantic Ocean, whichpossesses an increasing trend in frequency and duration thatis significant at the 99% confidence level. The observationthat increases in North Atlantic hurricane characteristics haveoccurred simultaneously with a statistically significant positivetrend in SST has led to the speculation that the changes inboth fields are the result of global warming (3).
Fig. 3. Regional time series for 1970–2004 for the NATL, WPAC, EPAC, NIO, and Southern Hemisphere (SIO plus SPAC) for (A) total number of hurricanes and (B) total number of hurricane days. Thin lines indicate the year-by-year statistics. Heavy lines show the 5-year running averages.
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It is instructive to analyze the relationship between the covariabilityof SST and hurricane characteristics in two other ocean basins,specifically the eastern and western North Pacific. Decadalvariability is particularly evident in the eastern Pacific,where a maximum in the number of storms and the number of stormdays in the mid-1980s (19 storms and 150 storm days) has beenfollowed by a general decrease up to the present (15 stormsand 100 storm days). This decrease accompanied a rising SSTuntil the 1990–1994 pentad, followed by an SST decreaseuntil the present. In the western North Pacific, where SSTshave risen steadily through the observation period, the numberof storms and the number of storm days reach maxima in the mid-1990sbefore decreasing dramatically over the subsequent 15 years.The greatest change occurs in the number of cyclone days, decreasingby 40% from 1995 to 2003.
In summary, careful analysis of global hurricane data showsthat, against a background of increasing SST, no global trendhas yet emerged in the number of tropical storms and hurricanes.Only one region, the North Atlantic, shows a statistically significantincrease, which commenced in 1995. However, a simple attributionof the increase in numbers of storms to a warming SST environmentis not supported, because of the lack of a comparable correlationin other ocean basins where SST is also increasing. The observationthat increases in North Atlantic hurricane characteristics haveoccurred simultaneously with a statistically significant positivetrend in SST has led to the speculation that the changes inboth fields are the result of global warming (3).
Examination of hurricane intensity (Fig. 4) shows a substantialchange in the intensity distribution of hurricanes globally.The number of category 1 hurricanes has remained approximatelyconstant (Fig. 4A) but has decreased monotonically as a percentageof the total number of hurricanes throughout the 35-year period(Fig. 4B). The trend of the sum of hurricane categories 2 and3 is small also both in number and percentage. In contrast,hurricanes in the strongest categories (4 + 5) have almost doubledin number (50 per pentad in the 1970s to near 90 per pentadduring the past decade) and in proportion (from around 20% toaround 35% during the same period). These changes occur in allof the ocean basins. A summary of the number and percent ofstorms by category is given in Table 1, binned for the years1975–1989 and 1990–2004. This increase in category4 and 5 hurricanes has not been accompanied by an increase inthe actual intensity of the most intense hurricanes: The maximumintensity has remained remarkably static over the past 35 years(solid black curve, Fig. 4A).
Fig. 4. Intensity of hurricanes according to the Saffir-Simpson scale (categories 1 to 5). (A) The total number of category 1 storms (blue curve), the sum of categories 2 and 3 (green), and the sum of categories 4 and 5 (red) in 5-year periods. The bold curve is the maximum hurricane wind speed observed globally (measured in meters per second). The horizontal dashed lines show the 1970–2004 average numbers in each category. (B) Same as (A), except for the percent of the total number of hurricanes in each category class. Dashed lines show average percentages in each category over the 1970–2004 period.
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Table 1. Change in the number and percentage of hurricanes in categories 4 and 5 for the 15-year periods 1975–1989 and 1990–2004 for the different ocean basins.
Period
Basin
1975–1989
1990–2004
Number
Percentage
Number
Percentage
East Pacific Ocean
36
25
49
35
West Pacific Ocean
85
25
116
41
North Atlantic
16
20
25
25
Southwestern Pacific
10
12
22
28
North Indian
1
8
7
25
South Indian
23
18
50
34
Cyclone intensities around the world are estimated by patternrecognition of satellite features based on the Dvorak scheme(25). The exceptions are the North Atlantic, where there hasbeen continuous aircraft reconnaissance; the eastern North Pacific,which has occasional aircraft reconnaissance; and the westernNorth Pacific, which had aircraft reconnaissance up to the mid-1980s.There have been substantial changes in the manner in which theDvorak technique has been applied (26). These changes may leadto a trend toward more intense cyclones, but in terms of centralpressure (27) and not in terms of maximum winds that are usedhere. Furthermore, the consistent trends in the North Atlanticand eastern North Pacific, where the Dvorak scheme has beencalibrated against aircraft penetrations, give credence to thetrends noted here as being independent of the observationaland analysis techniques used. In addition, in the Southern Hemisphereand the North Indian Ocean basins, where only satellite datahave been used to determine intensity throughout the data period,the same trends are apparent as in the Northern Hemisphere regions.
We deliberately limited this study to the satellite era becauseof the known biases before this period (28), which means thata comprehensive analysis of longer-period oscillations and trendshas not been attempted. There is evidence of a minimum of intensecyclones occurring in the 1970s (11), which could indicate thatour observed trend toward more intense cyclones is a reflectionof a long-period oscillation. However, the sustained increaseover a period of 30 years in the proportion of category 4 and5 hurricanes indicates that the related oscillation would haveto be on a period substantially longer than that observed inprevious studies.
We conclude that global data indicate a 30-year trend towardmore frequent and intense hurricanes, corroborated by the resultsof the recent regional assessment (29). This trend is not inconsistentwith recent climate model simulations that a doubling of CO2may increase the frequency of the most intense cyclones (18,30), although attribution of the 30-year trends to global warmingwould require a longer global data record and, especially, adeeper understanding of the role of hurricanes in the generalcirculation of the atmosphere and ocean, even in the presentclimate state.
References and Notes
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22. C. J. Neumann, in Global Guide to Tropical Cyclone Forecasting, G. J. Holland, Ed. (WMO/TD-560, World Meteorological Organization, Geneva, Switzerland, 1993), chap. 1.
31. This research was supported by the Climate Dynamics Division of NSF under award NSF-ATM 0328842 and by the National Center for Atmospheric Research, which is funded by NSF.
Received for publication 22 June 2005. Accepted for publication 18 August 2005.
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