NASA / TP--1998 -209003

Deciphering the Long-Term Trend of Atlantic

Basin Intense Hurricanes: More Active Versus

Less Active During the Present Epoch

Robert M. Wilson

Marshall Space Flight Center, Marshall Space Flight Center, Alabama

National Aeronautics and

Space Administration

Marshall Space Flight Center

December 1998

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ii

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TABLE OF CONTENTS

1. INTRODUCTION ..........................................................................................................................

2. RESULTS AND DISCUSSION .....................................................................................................

3. CONCLUSIONS ............................................................................................................................

REFERENCES ....................................................................................................................................

l

2

7

8

iii
LIST OF FIGURES

° Decadal averages of mean temperature anomalies (top), mean temperature

at the Armagh Observatory (middle), and frequency of intense hurricanes (bottom).

For the anomalies, the filled circles refer to the northern hemisphere, while the filled

triangles refer to the globe ......................................................................................................

° Annual mean temperature at Armagh Observatory (top) and the annual frequency

of intense hurricanes (bottom). The thin lines are the yearly values, while the thicker

and thickest lines are the 4- and 10-yr moving averages, respectively. E1 Nifio

occurrences and individual Hale cycles are identified across the top ....................................

4

. Scatter plots of the frequency of intense hurricanes versus mean temperature

at the Armagh Observatory, using 4-yr moving averages (right) and 10-yr moving

averages (left) and employing a lag of 6 yr (temperature leading) ........................................

iv
TECHNICAL PUBLICATION

DECIPHERING THE LONG-TERM TREND OF ATLANTIC BASIN INTENSE HURRICANES:

MORE ACTIVE VERSUS LESS ACTIVE DURING THE PRESENT EPOCH

1. INTRODUCTION

Humans have always demonstrated an affinity to be near the water and the population of the United

States is no exception. In particular, Williams I has shown that the population of counties located along the

Atlantic and Gulf coasts has steadily increased with the passage of time and that, associated with this

burgeoning population growth, an escalating cost of hurricane damage, reaching into the tens of billions of

dollars, has occurred (see also, Gray and Landsea2). In terms of severity, hurricanes are ranked according

to their central pressure, maximum sustained wind speed, and accompanying storm surge using the "SaffirSimpson hurricane damage potential scale" (see Simpson 3) which ranges from 1-5, where, collectively,

those of category 3-5 are the classes of "intense hurricanes." These particular hurricanes have central

pressure <964 mbar, maximum sustained wind speed >50 msec -1, and storm surge >2.5 m.

Previous investigations 4-12 have described the long-term variation of the frequency of hurricanes,

in particular, in relation to E1 Nifio, wet-dry years of West African rainfall, a perceived active-inactive

pattern of hurricane activity, and an inferred long-term steady decline of hurricane activity spanning the

past five decades (see also, Wilson 13 and Landsea et al. 14). In this paper, the long-term trend of the annual

frequency of intense hurricanes is reexamined, specifically, on the basis of its 4- and 10-yr moving averages (moving averages are often used to reduce the effect of random movements on time series datal5),

comparing them against those for the annual mean temperature at the Armagh Observatory (Northern

Ireland), first, to determine the possible effect of climatic change on the frequency of intense hurricanes

and, second, to determine the probable direction of its long-term trend during the current epoch. Recall that

the annual mean temperatures at Armagh are based on the use of maximum and minimum thermometers

and extend back to 1844 (see Butler 16 and Butler and Johnstonl7). Recently, Wilson 18 showed that the

Armagh temperatures can be used as a proxy for climatic change, since they are found to compare quite

favorably with northern hemispheric and global standards (i.e., those of Parker et al. 19 based on global

fields of area-weighted averages of Meteorological Office Historical Sea Surface Temperature). Moreover,

Wilson showed that the current long-term trend of temperature now appears to be upward, being statistically related to the Sun's Hale cycle (also called the "double sunspot cycle"). Because the intensity of

hurricanes may be related to changes in climate, 2°-24 the current warming seems to portend a return to the

more active state of intense hurricane activity, perhaps, like the one that was prevalent prior to the mid

1960's.
2. RESULTS AND DISCUSSION

Figure 1 displays the variation of decadal averages of temperature anomalies (top) from Parker

et al., 19 mean temperatures at Armagh (middle) from Wilson, 18 and the frequency of intense hurricanes in

the Atlantic basin from Wilson. 13 Noticeable is that all three parameters strongly resemble each other; i.e.,

they have higher values in the 1940's, followed by a downward trend, a flattening, and the hint of an

upward trend (with hurricane activity, perhaps, slightly lagging those of the temperature anomalies and

means). Because of the strong resemblance to temperature, one infers that shifts in the activity state for

intense hurricanes (from more active to less active and vice versa) may, in fact, be related to changes in

climate. Hence, a strong statistical association between temperature and the frequency of intense hurricanes is anticipated, especially, one based on moving averages. (Strictly speaking, the comparative data

sets are limited by the length of the reliable record of intense hurricanes of the Atlantic basin as determined

by routine aircraft reconnaissance (see Neumann et al. 25) a timespan of only about 54 yr. Therefore, one

cannot yet rule out the possibility that the inferred association between the annual frequency of intense

hurricanes and temperature may be the result of a mere statistical fluke.)

Figure 2 compares annual mean temperatures at Armagh (top) and the frequency of intense hurricanes (bottom), both plotted as the thin lines. Superposed on the yearly values are the 4- and 10-yr moving

averages, plotted, respectively, as the thicker and thickest lines. Identified across the top are the occurrences of El Nifio (taken from Trenberth 26 and supplemented by Quinn et al. 27, 28 for the years preceding

1950) and the onsets of the Sun's Hale cycles 18-19, 20-21, and 22-23, where each pair of numbers refers

to individual sunspot cycles that comprise them. Recall from Wilson 18 that the Hale cycle represents the

fundamental cycle of solar magnetic activity and that each has a preferred pairing (specifically,

"even-odd," in that order).

From figure 2, one finds that the annual frequency of intense hurricanes has ranged from 0 to 7,

with the annual counts >4 only for the years of 1948, 1950, 1955, 1958, 1961, 1964, 1995, and 1996. The

timeframe before 1965 has previously been identified as being more active than after 1965 (e.g., Gray 5 and

Wilsonl3); the same, however, cannot, as yet, be said for the present epoch (i.e., has a new active era

already begun?). 13, 14

Interestingly, for 41 of the 54 yr displayed (76 percent) the annual frequency of intense hurricanes

has measured 2+1, with only 5 of 54 yr (9 percent) having no occurrences per year and 8 of 54 yr

(15 percent) having an annual frequency >4. Of the five zero occurrence years, only the year of 1962 failed

to be an E1 Nifio year. Previously, on the basis of the seasonal number of hurricane days for the years of

1900-1982, Gray 4 found that in most E1 Nifio years hurricane activity was diminished in comparison to

non-E1 Nifio years. More recently, Wilson, 29 using the detailed listing of El Nifio for the interval of 1950-

1997 by Trenberth, 24 contrasted El Nifio- and non-El Nifio-related hurricane seasons and found the annual

frequency of intense hurricanes to be ~2.8 for non-El Nifio-related seasons and to be only 1.3 for E1 Nifiorelated seasons, where the observed difference is noted to be statistically important at the 0.2 percent

significance level. Predicting the annual frequency for 1998, however, posed a difficult problem at the

beginning of 1998 because of the continuing (though waning) effects of the strong E1 Nifio of 1997-1998

2
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41-50 61-70 81-90

Decadal Time Intervals

Figure 1. Decadal averages of mean

temperature anomalies (top),

mean temperature at the Armagh

Observatory (middle), and

frequency of intense hurricanes

(bottom). For the anomalies, the

filled circles refer to the northern

hemisphere, while the filled

triangles refer to the globe.

Had the event been found to extend beyond June 1998,

the season would have had to be classified as "El Nifiorelated," indicating that probably <2 (thus far, always

<3) intense hurricanes would form in the Atlantic

basin. Now, however, because the event appears to be

officially over (Trenberth, private communication), having ended prior to the start of the current hurricane season, the 1998 season must be recognized as "nonE1 Nifio-related;" consequently, the frequency of intense

hurricanes probably will be >2. Complicating this

issue, however, is the observation that the yearly rate

for the year following one in which a frequency of

1 event per year has been recorded has always been <3

(true for 13 of 13 yr); for 10 of 13 yrthe next year's rate

was <2. Because in 1997 only one intense hurricane was

recorded, it follows that, statistically speaking,

<3 events probably should be expected during the current 1998 season. Thus, it seems quite plausible that an

average hurricane season for 1998 may be the "best

guess" (i.e., 2_+1 intense hurricanes). Through September 1998 two intense hurricanes have formed thus far in

the Atlantic basin--"Bonnie" and "Georges,"

a category 3 and 4 hurricane, respectively (generally

speaking, intense hurricanes occur most frequently

after August 1st during the interval of AugustOctober3°-37). On the other hand, because of the strong

likelihood that the 1999 season will also be "nonE1 Nifio-related" (perhaps, even a "La Nifia-related"

season) 29 one probably should anticipate an increase in

the number of intense hurricanes for next year's season

(i.e., possibly >4).

On the basis of a Poisson distribution for intense

hurricanes (one having a mean of 2.22 events per year--

from the actual data of 120 events per 54 yr), one easily

computes the probability of having no events per year

as 10.9 percent (or, about 1 chance in 9.2), of having >4

events per year as 18.5 percent (or, about 1 chance in

5.4), and of having 2+1 events per year as 70.7 percent

(or, about 1 chance in 1.4). Thus, by always predicting

an "average" number of events for any given year (i.e.,

2-+ 1 events per year), one expects to be correct -71 percent of the time. (The rate of 2 events per year is also

the mode and median values for the observed distribution of intense hurricanes.)
El Nifio

Occurrences

10.5_-- _-I' Hal cle 1 19 al ycl HaleCycle22-23

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=E'_ 8.5

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1940

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1950 1960 1970 1980 1990 2000

Calendar Year

Figure 2. Annual mean temperature at Armagh Observatory (top) and the annual

frequency of intense hurricanes (bottom). The thin lines are the yearly

values, while the thicker and thickest lines are the 4- and 10-yr moving

averages, respectively. E1 Nifio occurrences and individual Hale cycles

are identified across the top.

From figure 2 it is found that the 4- and 10-yr moving averages of the annual frequency of intense

hurricanes appear to be in fairly steady decline from about 1950 through about 1970, then to be relatively

fiat until about 1990, when an upswing seems discernible. Similarly, the 4- and 10-yr moving averages of

the annual mean temperature at Armagh appear to be in decline from the mid 1940's to the mid 1960's, then

flat from the mid 1960's to the mid 1980's, when an upswing also seems discernible. This coordinated

behavior (with temperature leading) suggests the presence of a strong statistical association between them.

4
Figure 3 displays scatter plots of the frequency of intense hurricanes versus temperature using the

4-yr (right) and 10-yr (left) moving averages, both with temperature leading by 6 yr (a lead time that is

associated with the strongest correlations). In each panel the regression line is plotted and the results of the

linear regression analysis are given, as is the probability P of obtaining the observed 2x2 contingency

table, or one more suggestive of a departure from independence, where the table is determined by the

medians, shown as the thin vertical and horizontal lines (i.e., using Fisher's exact test). For the 4- and

10-yr moving averages, both correlations are inferred to be statistically important at <<0.1 percent significance level, suggesting that the leading trend of temperature, indeed, can be used to predict the consequential trend of the frequency of intense hurricanes.

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atArmaghObservatory

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,, ,/I, A, i I i n , _ I _ _ , , I , , , , l , , , , I

8.0 9.0 10.0 11.0

4-yr MovingAveragesot Temperature°C

at ArmaghObservatory

Figure 3. Scatter plots of the frequency of intense hurricanes versus mean temperature

at the Armagh Observatory, using 4-yr moving averages (right) and lO-yr moving

averages (left) and employing a lag of 6 yr (temperature leading).

5
As an example, for the 10-yr moving averages, the leading trend of temperature (see Fig. 2) is

found to exceed its median of 9.235 °C for the years of 1986-1991, implying that the predicted values of

the frequency of intense hurricanes for the years 1992-1997 will be in the upper-right quadrant of figure 3

(i.e., above its median of 1.85 and being indicative of the more active state). Indeed, the observed 10-yr

moving average of frequency for 1992 is found to exceed the median. For 1987-1991, the leading 10-yr

moving averages of temperature, respectively, equal 9.31, 9.29, 9.27, 9.32, and 9.39 °C, all above its

median; hence, one estimates that for the years of 1993-1997, the 10-yr moving average of frequency,

respectively, will be about 2.37, 2.30, 2.23, 2.41, and 2.66 intense hurricanes per year (all above its median

value). Such values are certainly consistent with the notion that a shift to the more active state probably has

already occurred, and that the yearly frequencies of intense hurricanes during the present epoch ( 1998 and

later) will usually be above the median, as well (though modulated, of course, by the appearances of

El Nifio).

As to why a 6-yr lead time should be incorporated in the correlations (with temperature leading),

the answer is not intuitively obvious. Perhaps, it is related to the average length of time between El Nifio

events, being about 3-4 yr in length, 28, 29 and/or to the timing and strength of the quasi-biennial oscillation

(a 2-3 yr oscillation) over the Tropics, both of which have previously been shown to have an influence on

Atlantic tropical cyclone activity. 4, 6, 37, 38 Perhaps, instead, it bespeaks of a subtle relationship between

climate and the Sun, 29, 39 where transitions between states are not driven instantaneously, but require a few

years or so for the change from one state to the other to be manifested. Then again, perhaps, it is simply an

artifact of the analysis, using moving averages based on the rather short interval (54 yr) for which reliable

records exist for determining the annual frequency of intense hurricanes. 1I, 25 While a physical basis for

the 6-yr lead time remains nebulous, the strength of the inferred correlations between temperature and the

annual frequency of intense hurricanes provides encouragement that one can use them (in combination

with the well-established El Nifio-Atlantic hurricane activity relationship 4. 29) in formulating a prediction

for intense hurricane activity during the current epoch (at least, until it fails to have predictive ability).

One of the most important factors affecting a successful prediction of the number of intense hurricanes during any particular season seems to be whether or not the season is deemed to be "El Nifio-related"

or not. 4, 29, 37, 38 Thus, when anomalous periods of E1 Nifio activity suddenly occur, 28, 40-46 as during the

recent interval of 1990-1995, they can play havoc with trying to accurately predict the level of intense

hurricane activity during a specific year, and, in particular, with trying to observe the transition from one

state to the other. Hence, the recent "expected" transition from the less active state of intense hurricane

activity (in vogue after the mid 1960's) to the more active state (that was not really apparent until after the

1995-1996 seasons, having levels reminiscent of the active interval prior to the mid 1960's) essentially

went unnoticed. Obviously, more indepth work modeling the occurrences of these anomalous periods of

El Nifio activity must be accomplished. (Previously, Quinn et al. 28 have noted several extended periods of

time when the amount and/or strength of El Nifio activity and its resulting effects appeared to represent

significant long-term climatic changes. These anomalous periods include 1607-1624, 1701-1728,

1812-1832, 1864-1891, 1925-1932, and now 1990-1995.)

6
3. CONCLUSIONS

In conclusion, on the basis of the most reliable data (1944-1997) of intense hurricanes, this study

has found strong statistical evidence (at <<0.1 percent significance level) linking the annual frequency of

intense hurricanes with climatic change, in particular, as gauged using annual mean temperature at the

Armagh Observatory (Northern Ireland). Hence, by monitoring the trend in temperature (which has been

shown to be strongly related to the Sun's Hale cyclei8), one has at his disposal a tool ['or assisting in the

accurate prediction of the seasonal frequency of intense hurricanes and for observing the transitional phasing from one state to the other (i.e., from more to less active and vice versa). Furthermore, the upswing in

temperature that began in the mid 1980's (associated with the start of the current Hale cycle) strongly

suggests that the present epoch of intense hurricane activity, modulated, of course, by the appearances of

El Nifio, will likely be reminiscent of the more active era that was prevalent befoie the mid 1960's. This

active state, as modulated by El Nifio, should continue until near the end of the first decade of the new

millennium, when a return to the less active era should once again be apparent (due to the onset of a new

Hale cycle in about 2006). Because a Poisson distribution (having a mean of about 2.22 events per season)

yields a good match to the observed distribution of annual frequencies of intense hurricanes, one finds that

the "best guess" prediction for any year is about 2_+1 intense hurricanes per season, being preferentially

lower (more often <2) during E1 Nifio-related seasons and preferentially higher (more often >2) during

non-El Nifio-related seasons, and matching the observed level of activity about 71 percent (or more) of the

time. Because the 1998 season is now recognized as being "non-El Nifio-related" and the 1999 season,

likewise, is expected to be "non-El Nifio-related" (perhaps, even, "La Nifia-related"), one expects >2 intense hurricanes during the seasons, probably 2-3 for 1998 and possibly >4 for the 1999 season.

7
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Vol. 103, pp. 14,169-14,240, 1998.

11
Form Approved REPORT DOCUMENTATION PAGE OMBNo.0704-0188

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1. AGENCY USE ONLY (Leave Blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

December 1998 Technical Publication

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Deciphering the Long-Term Trend of Atlantic Basic Intense

Hurricanes: More Active Versus Less Active During the Present

Epoch

!6. AUTHORS

Robert M. Wilson

7. PERFORMING ORGANIZATION NAMES(S) AND ADDRESS(ES)

George C. Marshall Space Flight Center

Marshall Space Flight Center, AL 35812

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

8. PERFORMING ORGANIZATION

REPORT NUMBER

M-901

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA/TP--1998-209003

11. SUPPLEMENTARY NOTES

Prepared by the Space Sciences Laboratory, Science and Engineering Directorate

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited

Subject Category 47

Standard Distribution

12b, DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

During the interval of 1944-1997, 120 intense hurricanes (i.e., those of category 3 or higher on the Saffir-Simpson

hurricane dama e otential scale) were observed in the Atlantic basin having an annual frequency of 0-7 events per

year, being mo_ aP_ive prior to the mid 1960's than thereafter (hence a possible two-state division: more active versus

less active), and being preferentially lower during El Nifio years as compared to non-El Nifio years. Because decadal

averages of the frequency of intense hurricanes closely resemble those of average temperature anomalies for northern

hemispheric and global standards and of the average temperature at the Armagh Observatory (Northern Ireland), a

proxy for climatic change, it is inferred that the long-term trends of the annualfrequency of intense hurricanes and

temperature may be stanstically related. Indeed, on the basis of 4- and 10-yr moving averages, one finds that there

exists strong linear associations between the annual frequency of intense hurricanes in the Atlantic basin and

temperature (es _ecially, when temperature slightly leads). Because the long-term leading trends of temperature are

now decidedly i ipward, beginning about the mid 1980's, it is inferred that the long-term consequential trends of the

annual frequency of intense hurricanes should now also be upward, having begun near 1990, suggesting that a return

to the more acti ve state probably has already occurred. However, because of the anomalous E1Nifio activity of the

early to mid 1990's, the switch from the less active to the more active state essentially went unnoticed (a marked

increase in the number of intense hurricanes was not observed until the 1995 and 1996 hurricane seasons, following

the end of the anomalous El Nifio activity). Presuming that a return to the more active state has, indeed, occurred, one

expects the number of seasonal intense hurricanes during the present epoch (continuing through about 2012) to

usually be higher than average (i.e., >2), except during El Nifio-related seasons when the number usually will be less

than average.

14. SUBJECT TERMS

global change, climatic change, climate, Atlantic Ocean, intense hurricanes,

trends in, global warming, Armagh Observatory, E1 Nifio, ENSO

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIRCATION 19. SECURITY CLASSIFICATION

OF REPORT OF THIS PAGE OF ABSTRACT

Unclassified Unclassified Unclassified

NSN7_0-0t-280-SS00

15. NUMBER OF PAGES

16

16. PRICE CODE

A03

20. LIMITATION OF ABSTRACT

Unlimited

Standard Form 298 (Rev. 2-89)

PrescribedbyANSISld 239-18

298-102