NASA/TP--2001-210991

Decadal

Tropical

Robert M. Wilson

Trends of

Cyclones

Atlantic Basin

(1950-1999)

Marshall Space Flight Center, Marshall Space Flight Center, Alabama

May 2001
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NASA/TP--2001-210991

Decadal Trends of Atlantic Basin Tropical

Cyclones (1950-1999)

Robert M. Wilson

Marshall Space Flight Center, Marshall Space Flight Center, Alabama

National Aeronautics and

Space Administration

Marshall Space Flight Center • MSFC, Alabama 35812

May 2001
NASA Center for AeroSpace Information

7121 Standard Drive

Hanover, MD 21076 1320

(301) 621 0390

Available from:

ii

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

(703) 487 4650
TABLE OF CONTENTS

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

2. RESULTS ...................................................................................................................................

3. DISCUSSION AND SUMMARY ..............................................................................................

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

1

2

16

19

iii
LIST OF FIGURES

. (a) The seasonal rate (r) of NS for 1950-1999. Each season is identified

as ENR by filled triangles or as NENR by unfilled triangles (La Nifia-related)

and unfilled circles (interlude-related). (b) The distribution of 10-yr moving

average first difference (fd) values for the number (N) of named storms, N(NS).

The tabular insert gives corresponding 10-yr moving averages (for 1995)

andfd values (for 1994) for selected seasonal rates for the year 2000 season.

See text for additional details ................................................................................................. 2

. Same construction as employed in figure 1 except now it is for tropical storms,

where N(TS) is the number of tropical storms ....................................................................... 4

. Same construction as employed in figure 1 except now it is for hurricanes,

where N(H) is the number of hurricanes ................................................................................ 5

. Same construction as employed in figure 1 except now it is for major (or intense)

hurricanes, where N(MH) is the number of major hurricanes ................................................ 6

. (a) Histograms of the percentages of seasons for the number of named storms

per calendar month (May-December) for the combined data set and for even-odd,

ENR-NENR, and less-more differentiated data sets. (b) Same construction

as panel (a) except now they are histograms of the percentages of the total

number of named storms ........................................................................................................ 13

. (a) Histograms of the percentages of the total number of named storms for ENR

and NENR seasons during even-more and odd-more years. (b) Same construction

as panel (a) except now they are histograms of the percentages of the total number

of hurricanes ........................................................................................................................... 15

V
LIST OF TABLES

° The distribution of the seasonal rate of selected groupings of Atlantic basin

cyclones for 1950-1999 and the associated Poisson distributions for the

combined data set ................................................................................................................. 7

° The distribution of the seasonal rate of selected groupings of Atlantic basin

cyclones for 1950-1999 and the associated Poisson distributions

for ENR seasons ................................................................................................................... 8

° The distribution of the seasonal rate of selected groupings of Atlantic basin

cyclones for 1950-1999 and the associated Poisson distributions

for NENR seasons ................................................................................................................ 9

° The distribution of the seasonal rate of selected groupings of Atlantic basin

cyclones for 1950-1999 and for selected climatic conditions and the associated

Poisson probability distributions .......................................................................................... 10

° The distribution of seasonal rates of selected groupings of Atlantic basin

cyclones for 1950-1999 and the more-NENR climate condition, differentiating

even and odd years ............................................................................................................... 11

° The distribution of the seasonal rate of selected groupings of Atlantic basin

cyclones for 1950-1999 and for the more-ENR climate condition, differentiating

even and odd years ............................................................................................................... 12

vi
LIST OF ACRONYMS AND SYMBOLS

ENR

H

ma

MH

NENR

NS

QBO

S

TS

E1 Nifio related

hurricane

moving average

major hurricane

non-E1 Nifio related

named storm

quasi-biennial oscillation

storm

tropical storm

vii
NOMENCLATURE

fd

i

m

N

T/

P

F

sd

t

first difference

index for summation

mean rate

number

number of individual storms

probability

rate

standard deviation

time

viii
TECHNICALPUBLICATION

DECADAL TRENDS OF ATLANTIC BASIN TROPICAL CYCLONES (1950-1999)

1. INTRODUCTION

Atlantic basin tropical cyclones have been reported since the time of the explorations of Columbus. 1,2 These large-scale cyclonic weather systems form over the warm tropical waters of the North Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea, often posing a hazard to maritime shipping, island

populations, and coastal and inland populations of the continental United States (from Texas to Maine),

Mexico, and Central America. On occasion, they have been associated with tremendous loss of life and

damage reaching into the tens of billions of U.S. dollars. 3 6

Classification of Atlantic basin tropical cyclones depends strictly on the maximum sustained wind

speed near the center of circulation. For example, a tropical depression is one having a maximum sustained

wind speed <17 m sec 1, a tropical storm is one having a maximum wind speed of 17 to <33 m sec 1, and

a hurricane is one having a maximum wind speed of at least 33 m sec 1. Major (or intense) hurricanes have

a maximum wind speed of at least 50 m sec 1 and refer to those of category 3-5 on the Saffir-Simpson

hurricane damage potential scale, 7 9 the most destructive of hurricanes.

In this study, the decadal trends of Atlantic basin tropical cyclones, in particular, those "named"

systems, 2,4 including tropical storms and hurricanes, are investigated for the 50-yr interval of 1950-1999.

Examined are the 10-yr moving averages and associated Poisson probability distributions for various groupings of storms and for differing climatic conditions. As such, this investigation represents an expansion of

Wilson 1° who previously examined the 10-yr moving averages and Poisson probability distributions

associated with major hurricanes (only) in the Atlantic basin. (Decadal variability in the climate system has

received considerable attention in recent years. 11_6)
2. RESULTS

Figure l(a) displays the seasonal rate (r) of "named" storms (NS) occurring in the Atlantic basin

over the past 50 yr (1950-1999). Each season is denoted E1 Nifio-related (ENR), by the filled triangles,

or non-E1 Nifio-related (NENR), by the unfilled triangles and circles, where these latter two groupings

refer specifically to La Nifia- and interlude-related seasons, respectively. 24,27 The thin horizontal line spanning the time series is the median rate, equal to 8, while the thick heavy line is the 10-yr moving average.

(Moving averages are often employed in time series data in order to reduce the effect of random

movements. 28)

(a) 20 j- N(NS)

| m= 8.84

/

I sd= 3.01

i 15r10"n=44_

_ 5

0 I

1950

I I I I I I I I I

1960 1970 1980 1990 2000

Year

Median

(b) 10 N= 44

E_5

Z

70% _ r(2000) lO-yr ma(1995) fd(1994)

l

0 10.00 -0.65

8 10.40 -0.25

II I 19 10.95 0.30

, ,,,,,I I,,,,,, ,,,, ,

-1.00 -0.50 0 0.50 1.00

fd[ N( NS )]lO-yr ma

Figure 1. (a) The seasonal rate (r) of NS for 1950-1999. Each season is identified

as ENR by filled triangles or as NENR by unfilled triangles (La Nifia-related)

and unfilled circles (interlude-related). (b) The distribution of 10-yr moving average

first difference (fd) values for the number (N) of named storms, N(NS). The tabular

insert gives corresponding 10-yr moving averages (for 1995) andfd values (for

1994) for selected seasonal rates for the year 2000 season. See text for additional

details.

2
Inspection of figure 1 shows that the 10-yr moving average of named storms was relatively flat

until about 1978. Since then, however, it has steadily increased in value, so that its last known value (in

1994) equals 10.65, measuring =2.65 units higher than the long-term median rate and being the highest

value yet recorded (over the past 50 yr). While the increase may reflect a real change in the Earth's climate

system, perhaps related to normal climate variability (or, speculatively, to global warming23,29,3°), in contrast, it may be due simply to having a more complete record of tropical cyclones, owing to greater use of

weather satellites. Many of the tropical cyclones, especially, those that do not reach hurricane status, may

occur far from land over the open ocean where one must rely exclusively on qualitative methods based on

satellite imagery to gauge the intensity. Still, because (1) the record of tropical cyclone activity is considered reliable from 1944 when routine aircraft reconnaissance of the Atlantic basin began; 2 (2) the recent

years of 1990, 1995, 1996, and 1998 are the 4 yr having the highest seasonal rates yet observed (14, 19, 13,

and 14, respectively) over the entire 50-yr interval; (3) the trend in the seasonal rates for ENR years (from

1982) appears to be directed upward; and (4) all NENR years (since 1978) have had seasonal rates of 211

(measuring 12 in 1999, an NENR year), one cannot escape the notion that the present epoch is indeed one

of enhanced activity.

Plotted in figure l(b) is the spread offd values of thel0-yr moving average, where the first difference is defined as the following year's 10-yr moving average minus the present year's 10-yr moving

average. One finds that 70 percent of the time, thefd value for the number of NS equals +0.15 units. Hence,

because the last known 10-yr moving average equals 10.65 (for 1994), one expected the next value (for

1995) to be =10.65 + 0.15. Such a range plainly indicated that the number of NS's for the year 2000

hurricane season should be 10 or more (=13 + 3), providing, of course, that the year 2000 season was not a

statistical outlier. Furthermore, it strongly suggested that the year 2000 season should be an NENR season,

which is now known to be true.

Following is a computational example showing exactly how the above-stated seasonal rate was

determined. Recall that, by definition, the 10-yr moving average of the seasonal rate (r) is the average of

two consecutive 10-yr sequences. Mathematically, it is computed as twice the sum of the 9 innermost

values (r's) in a sequence of 11 values (i.e., 2 × Zr i for i = -4 through 4) plus the two end values (r 5 and rs)

and the total quantity divided by 20. To compute the expected seasonal rate (r) for the next season, one

notes that it is equal to 20 times the expected 10-yr moving average minus two times the sum of the

incremented (by 1 yr) seasonal rates (i.e., the new ri for i = -4 to 4) minus the rate for the new r 5.

As applied to the year 2000 season, its rate is computed as (20 × 10.65) - (2 × 93) - 14, where 10.65 is

the expected 10-yr moving average for 1995, 93 is the new sum of seasonal rates between 1991 and 1999,

and 14 is the rate for 1990. Hence, the expected seasonal rate for the year 2000 season was 13. The +3

results from the observed statistical spread found for the first difference in the 10-yr moving averages,

fd[N(NS)] 10-yr ma" (This same technique will be employed in the following discussions of tropical storms,

hurricanes, and major hurricanes.)

Figure 2(a) illustrates the variation of the seasonal rate (r) for tropical storms (TS), constructed

similarly to that of figure 1. Clearly, there has been a general upward progression with time of the seasonal

rate, such that now the 10-yr moving average is about twice what it was in the 1950's. Since 1988, the

observed seasonal rate for tropical storms has exceeded the median rate every year except 1992, and 7 of

the past 9 yr have had a seasonal rate of exactly 4, whether the season was categorized as NENR or ENR.

3
(a) 10 F N(TS)

/ m= 3.12

I sd = 1.72 Ill L n=156 ,-_ _

- v v

Oi _ i i i i i i i i i

Median

1950 1960 1970 1980 1990 2000

Year

(b) 10 N= 44

E_5

Z

Figure 2.

I I I

-1.00 -0.50

_'t 73% _ r(2000) lO-yr ma (1995) fd(1994)

I II 30 4.10 4.25 -0.30 -0.15

8 4.50 0.10

i,II I,,,, , , ,

0 0.50 1.00

fd[N(TS)]lO-yr ma

Same construction as employed in figure 1 except now it is for tropical

storms, where N(TS) is the number of tropical storms.

Figure 2(b) shows the spread offd for tropical storms. One finds that 73 percent of the time, the

10-yr moving average for the following year has been within +0.10 units of the last known value. Hence,

the 10-yr moving average for 1995 was expected to be =4.40 + 0.10, indicating that r for the year 2000

season should be >4 (=6 + 2).

Figure 3(a) similarly exhibits the seasonal rate (r) for hurricanes (H). From 1950 through the mid1960's, the 10-yr moving average was above the long-term median rate (5.5), while from the mid-1960's

until about 1992, it was below the median rate. The 10-yr moving average rose once again above the longterm median rate about 1992, although its last known value (6.25 in 1994) falls short of the previous high

(6.85 in 1951). Perhaps, this is an indication that the 10-yr moving average for hurricanes will continue

to grow (see below). Interestingly, 11 of the last 11 ENR seasons have had a seasonal rate below the

long-term median rate (actual seasonal rates measuring 2-5), while 9 of the last 11 NENR seasons have

had a seasonal rate of >7 (actual seasonal rates measuring 5-11).

Figure 3(b) shows the spread offd for hurricanes. One finds that 68 percent of the time, the 10-yr

moving average for the succeeding year has been within +0.15 units of the last known year's value. Hence,

the 10-yr moving average for 1995 was expected to be =6.25 + 0.15, indicating that r for the year 2000

season should be >4 (=7 + 3).

4
(a) 20

_10

o

a_

N(H)

m = 5.72

sd = 2.30

n=286

0 I I I I I I I I I I

1950 1960 1970 1980 1990 2000

Year

Median

(b) 10 N=44

E_5

Z

I I I

-1.00 -0.50

68% _'_

,,,,,llll,I, ,,

0

fd[N(H)llO-yr ma

r(2OOO) lO-yr ma (1995) fd(1994)

0 5.90 -0.35

5.5 6.18 -0.07

11 6.45 0.20

I I, , , ,

0.50 1.00

Figure 3. Same construction as employed in figure 1 except now it is for hurricanes,

where N(H) is the number of hurricanes.

Figure 4(a) plots the seasonal rate (r) for major (or intense) hurricanes (MH), those of category

3-5 on the Saffir-Simpson hurricane damage-potential scale, the most destructive of hurricanes. The

behavior of the 10-yr moving average is found to mimic that for the combined set of all hurricanes, shown

previously in figure 3. Namely, a peak is observed to have occurred in the early 1950's, followed by a

relatively steady decline 31,32 to values below the median rate beginning about the mid-1960's. The values

continued below the median rate until about 1992 when they once again were found to exceed it (in

response to the seasonal rate recorded in 1995). On the basis of the relative behavior of the 10-yr moving

average to the long-term median rate (equal to 2), Wilson 1° described the behavior of major hurricanes in

terms of "intervals of persistent activity." Specifically, he divided the record into portions of two "more"

active intervals separated by a single "less" active interval that lasted =25 yr. 32 34 As yet, a peak in the 10-

yr moving average for the second, more active interval has not been seen, nor is the largest observed value

(equal to 2.35) commensurate with that of the first, more active interval (equal to 3.30). Hence, it may be

that higher than median rate values will continue for some time into the future before beginning a downward trek, with actual seasonal values being modulated by the random occurrences of E1 Nifio. 18,35 (The

two-tiered classification scheme for hurricanes, specifically, major hurricanes, employed in Wilson 1° and

herein, a scheme based upon whether the 10-yr moving average is above or below the long-term median

rate, is largely arbitrary; yet, clearly, it serves as a convenient descriptor for differentiating seasonal rates 32

and the t-statistic indicates that such a division is statistically important.)

5
(a) 10

z,,,

"" 5

o

_D

N(MH)

m = 2.26

sd = 1.66

n=113

0 I I

1950 1960

V

1970 1980 1990

Median

Year

2OOO

(b)

10

E_5

Z

N= 44

I I

-1.00

•_70% _ r(2000) lO-yr ma (1995) fd(1994)

0 2.45 0.10

2 2.55 0.20

7 2.80 0.45

,I, II1,,,,

-0.50 0 0.50 1.00

fd[N(MH)llO-yr ma

Figure 4. Same construction as employed in figure 1 except now it is

for major (or intense) hurricanes, where N(MH) is the number

of major hurricanes.

Figure 4(b) shows the spread offd for major hurricanes. One finds that 70 percent of the time, the

10-yr moving average for the following year has been within +0.10 units of the last known year's value.

Hence, the 10-yr moving average for 1995 was expected to be =2.35 + 0.10. This range, however, was

expected to be exceeded on the positive side (i.e., >0.10) because a rate of 2.45 for 1995 implied a seasonal

rate of zero for the year 2000 season (see tabular insert). While a zero rate is possible for any given season,

such a rate has occurred only once during the past 50 yr when the season was classified as NENR. More

often than not, when the season has been classified as a "more active" NENR season, clearly one expects

the seasonal rate for major hurricanes to be _>3.10 Presuming this to be the case, one expected the 10-yr

moving average for 1995 to be _>2.60 (although <2.80, since such a rate implied that the seasonal rate for

the year 2000 season would be 7, the highest seasonal rate yet observed; as it turned out, the actual 10-yr

moving average for 1995 measured 2.60).

Table 1 compares the observed seasonal rate distributions, obtained from figures 1-4, against the

associated Poisson probability distributions using the combined data set (i.e., not differentiating between
various climatic conditions). The Poisson distribution is quite useful for measuring the probability

of occurrence in positively skewed discrete distributions, where knowledge of the mean number of random

events that occur within a given time interval is plainly known. 36 (In tables 1-6, the terms denoted N(S), n,

and m refer to the number of seasons, the number of individual storms, and the mean rate, respectively.)

Table 1. The distribution of the seasonal rate of selected groupings of Atlantic

basin cyclones for 1950-1999 and the associated Poisson distributions

for the combined data set.

r

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

N(S)

/7

m

N(NS)

Obs P(r)

0 0.0001

0 0.0013

0 0.0057

0 0.0167

2 0.0368

3 0.0652

7 0.0960

8 0.1212

7 0.1339

2 0.1316

5 0.1163

7 0.0935

5 0.0689

1 0.0468

2 0.0296

0 0.0174

0 0.0096

0 0.0050

0 0.0025

1 0.0011

50

442

8.84

N(TS) N(H) N(MH)

Obs P(r) Obs P(r) Obs P(r)

1 0.0442

7 0.1378

12 0.2149

11 0.2235

13 0.1743

0 0.1088

3 0.0566

2 0.0252

1 0.0098

0 0.0034

0 0.0011

0 0.0003

0 0.0001

0 0.0000

0 0.0000

0 0.0000

0 0.0000

0 0.0000

0 0.0000

0 0.0000

50

156

3.12

50

286

5.72

0.0033

0.0188

0.0537

0.1023

0.1463

0.1674

0.1595

0.1304

0.0932

0.0592

0.0339

0.0176

0.0084

0.0037

0.0015

0.0006

0.0002

0.0001

0.0000

0.0000

5

13

15

9

1

4

2

1

0

0

0

0

0

0

0

0

0

0

0

0

50

113

2.26

0.1044

0.2358

0.2665

0.2008

0.1134

0.0513

0.0193

0.0062

0.0018

0.0004

0.0001

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

As an example, for N(NS), the number of named storms, one notes that the fewest number of named

storms that has occurred during the 50-yr interval of 1950-1999 is 4, having occurred only twice (1972 and

1983, both ENR seasons during the "less active" interval). 10 Hence, one should always expect that at least

4 named storms will be seen during any given season (the probability of obtaining <3 named storms during

a season is easily computed to be 2.4 percent, obtained by summing the individual probabilities, P(r), for r

= 0, 1, 2, and 3). On average, one expects =9 named storms (442 named storms in 50 yr), although, clearly

the average rate appears to have risen with the progression of time. Also, one should note that the largest

number of named storms that has been seen so far during any given season has been 19, having occurred

only once (1995, an NENR season during the second, more active interval). Hence, realistically, one should

not anticipate a seasonal rate higher than 19 during a season (the probability of obtaining 20 or more named

storms during a season is easily computed to be <0.1 percent, computed as 1 - ZP(r) for r = 0-19).

7
Previously, Gray 33 demonstrated that E1 Nifio is one of the strongest climatic factors for determining how active a hurricane season ultimately will be. 10,23,24,34,37_40 Tables 2 and 3 compare the observed

seasonal rate distributions against the associated Poisson probability distributions, differentiating between

ENR and NENR seasons, respectively. For ENR seasons (table 2), one finds, continuing the example, that

N(NS) has spanned 4-12, averaging =7 per season (138 named storms in 20 ENR seasons). The probability

of obtaining <3 named storms or of obtaining >13 named storms during an ENR season is 8.8 and 2.4

percent, respectively. For NENR seasons (table 3), one finds that N(NS) has spanned 5-19, averaging =10

per season (304 named storms in 30 NENR seasons). The probability of obtaining <4 named storms or of

obtaining >20 named storms during an NENR season is 2.7 and 0.4 percent, respectively. (Previously,

Wilson 1° noted that the differentiation of seasonal rates for major hurricanes on the basis of the "ENRNENR" criterion was statistically important at >99.9-percent level of confidence, having a t-statistic equal

to -3.61.)

Table 2. The distribution of the seasonal rate of selected groupings of Atlantic basin cyclones

for 1950-1999 and the associated Poisson distributions for ENR seasons.

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

N(S)

/7

m

N(NS)

Obs P(r)

0 0.0010

0 0.0070

0 0.0240

0 0.0552

2 0.0952

2 0.1314

5 0.1511

4 0.1489

5 0.1284

0 0.0985

1 0.0679

0 0.0426

1 0.0245

0 0.0130

0 0.0064

0 0.0029

0 0.0013

0 0.0005

0 0.0002

0 0.0001

20

138

6.90

N(TS) N(H) N(MH)

Obs P(r) Obs P(r) Obs P(r)

0 0.0863

5 0.2114

5 0.2590

6 0.2115

4 0.1295

0 0.0635

0 0.0259

0 0.0091

0 0.0028

0 0.0008

0 0.0002

0 0.0000

0 0.0000

0 0.0000

0 0.0000

0 0.0000

0 0.0000

0 0.0000

0 0.0000

0 0.0000

20

49

2.45

20

89

4.45

0.0117

0.0520

0.1156

0.1715

0.1908

0.1698

0.1260

0.0801

0.0445

0.0220

0.0098

0.0040

0.0015

0.0005

0.0002

0.0000

0.0000

0.0000

0.0000

0.0000

20

26

1.30

0.2725

0.3543

0.2303

0.0998

0.0324

0.0084

0.0018

0.0003

0.0001

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000
Table 3. The distribution of the seasonal rate of selected groupings of Atlantic basin cyclones

for 1950-1999 and the associated Poisson distributions for NENR seasons.

N(NS) N(TS) N(H) N(MH)

Obs P(r) Obs P(r) Obs P(r) Obs P(r)

0 0

1 0

2 0

3 0

4 0

5 1

6 2

7 4

8 2

9 2

10 4

11 7

12 4

13 1

14 2

15 0

16 0

17 0

18 0

19 1

N(S) 30

n 304

rn 10.13

0.0000

0.0004

0.0020

0.0069

0.0175

0.0354

0.0598

0.0866

0.1096

0.1234

0.1250

0.1151

0.0972

0.0757

0.0548

0.0370

0.0234

0.0140

0.0079

0.0042

3O

107

3.57

0.0282

0.1005

0.1794

0.2135

0.1906

0.1361

0.0810

0.0413

0.0184

0.0073

0.0026

0.0008

0.0003

0.0001

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

3O

197

6.57

0.0014

0.0092

0.0303

0.0663

0.1088

0.1430

0.1566

0.1470

0.1207

0.0881

0.0579

0.0346

0.0189

0.0096

0.0045

0.0020

0.0008

0.0003

0.0001

0.0000

3O

87

2.90

0.0550

0.1596

0.2314

0.2237

0.1622

0.0940

0.0455

0.0188

0.0068

0.0022

0.0006

0.0002

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

Wilson 1° showed that in addition to E1 Nifio, another important influencing factor, at least for

major hurricanes, is an apparent two-tier level of activity ("more" versus "less"), inferred to be inherent in

the record of major hurricanes. Prior to the mid-1960's and subsequent to the early 1990's, the 10-yr

moving average of major hurricanes is found to have always exceeded the long-term median rate, while it

was always below the median rate between the mid-1960's and the early 1990's. This pattern of behavior,

if an inherent characteristic of major hurricanes, suggests that the activity level for major hurricanes may

oscillate between two preferred states of activity ("more" versus "less") every 25 yr or so, inferring an

oscillatory period of =50 yr, a length that has been associated with climatic regime shifts found over

the North Pacific, North America, and tropical oceans. 2° Table 4 compares the observed distributions and

associated Poisson probability distributions for the two cases of climatic conditions that should now be in

vogue, specifically, the "more-NENR" season and the "more-ENR" season, following the categorization

scheme employed in Wilson. 1° (Previously, Wilson 1° noted that the differentiation of seasonal rates for

major hurricanes on the basis of the "more-less" criterion was statistically meaningful; specifically, it has

a t-statistic measuring -2.78, inferring that the division by level of activity is statistically important at

>99-percent level of confidence.)

9
Table4. Thedistributionof theseasonalrateof selectedgroupingsofAtlantic basincyclones

for 1950-1999andfor selectedclimatic conditionsandtheassociatedPoissonprobability

distributions.

More-NENR(16) More-ENR(9)

N(NS) N(TS) N(H) N(MH) N(NS) N(TS) N(H) N(MH)

r 0bs P(r) 0bs P(r) 0bs P(r) 0bs P(r) 0bs P(r) 0bs P(r) 0bs P(r) 0bs P(r)

0 0 0.0000 1 0.0529 0 0.0007 1 0.0235 0 0.0007 0 0.0773 0 0.0094 1 0.2369

1 0 0.0004 2 0.1554 0 0.0051 0 0.0882 0 0.0053 2 0.1979 0 0.0438 4 0.3412

2 0 0.0019 4 0.2285 0 0.0187 4 0.1654 0 0.0191 3 0.2533 0 0.1022 3 0.2456

3 0 0.0066 3 0.2239 1 0.0451 3 0.2067 0 0.0459 1 0.2162 3 0.1591 1 0.1179

4 0 0.0169 5 0.1646 2 0.0818 1 0.1938 0 0.0829 3 0.1383 3 0.1857 0 0.0424

5 1 0.0344 0 0.0968 0 0.1185 4 0.1453 1 0.1196 0 0.0708 0 0.1735 0 0.0122

6 2 0.0584 0 0.0474 3 0.1432 2 0.0908 2 0.1440 0 0.0302 1 0.1350 0 0.0029

7 1 0.0850 0 0.0199 3 0.1484 1 0.0487 2 0.1485 0 0.0111 1 0.0901 0 0.0006

8 0 0.1082 1 0.0073 2 0.1344 0 0.0228 3 0.1340 0 0.0035 1 0.0526 0 0.0001

9 2 0.1226 0 0.0024 2 0.1083 0 0.0095 0 0.1075 0 0.0010 0 0.0273 0 0.0000

10 4 0.1249 0 0.0007 1 0.0785 0 0.0036 1 0.0776 0 0.0003 0 0.0127 0 0.0000

11 1 0.1157 0 0.0002 2 0.0518 0 0.0012 0 0.0510 0 0.0001 0 0.0054 0 0.0000

12 2 0.0982 0 0.0000 0 0.0313 0 0.0004 0 0.0307 0 0.0000 0 0.0021 0 0.0000

13 1 0.0770 0 0.0000 0 0.0174 0 0.0001 0 0.0170 0 0.0000 0 0.0008 0 0.0000

14 1 0.0560 0 0.0000 0 0.0090 0 0.0000 0 0.0088 0 0.0000 0 0.0003 0 0.0000

15 0 0.0381 0 0.0000 0 0.0044 0 0.0000 0 0.0042 0 0.0000 0 0.0001 0 0.0000

16 0 0.0242 0 0.0000 0 0.0020 0 0.0000 0 0.0019 0 0.0000 0 0.0000 0 0.0000

17 0 0.0195 0 0.0000 0 0.0008 0 0.0000 0 0.0008 0 0.0000 0 0.0000 0 0.0000

18 0 0.0082 0 0.0000 0 0.0003 0 0.0000 0 0.0003 0 0.0000 0 0.0000 0 0.0000

19 1 0.0044 0 0.0000 0 0.0001 0 0.0000 0 0.0001 0 0.0000 0 0.0000 0 0.0000

N(S) 16 16 16 16 9 9 9 9

n 163 47 116 60 65 23 42 13

m 10.19 2.94 7.25 3.75 7.22 2.56 4.67 1.44

Over the past 50 yr, the climatic condition designated "more-NENR" has been experienced

16 times. On average, during more-NENR seasons, there have been =10 named storms, including 3 tropical storms and 7 hurricanes, of which slightly more than half were major hurricanes. The observed range

has been 5-19 named storms, 0-8 tropical storms, 3-11 hurricanes, and 0-7 major hurricanes. The probability of >11 named storms is 44.1 percent, the probability of >4 tropical storms is 33.9 percent, the

probability of >8 hurricanes is 43.9 percent, and the probability of >4 major hurricanes is 51.6 percent.

Thus, during a typical more-NENR season, one anticipates activity levels that could be significantly higher

than average, certainly higher than what was experienced during the 25-yr lull that immediately preceded

the current, more active mode.

Likewise, over the past 50 yr, the climatic condition designated "more-ENR" has been experienced

9 times. On average, during more-ENR seasons, there have been =7 named storms, including 2-3 tropical

storms and 4-5 hurricanes, of which about 1-2 were major hurricanes. The observed range has been 5-10

named storms, 1-4 tropical storms, 3-8 hurricanes, and 0-3 major hurricanes. The probability of 11 or

more named storms is only 11.5 percent, the probability of 4 or more tropical storms is only 25.5 percent,

the probability of 8 or more hurricanes is only 10.1 percent, and the probability of 4 or more major hurricanes is only 5.8 percent. Thus, during a typical more-ENR season, one anticipates activity levels that are

diminished, especially as compared to the typical more-NENR season, with little likelihood of seeing

abnormally high seasonal rates.

10
Tables5 and6 givetheobserveddistributionsandassociatedPoissonprobabilitydistributionsfor

themore-NENRandmore-ENRclimaticconditions,respectively,differentiatingbetweeneven-andoddnumberedyears(thebasisfor aneven-oddsplit is purely arbitrary,yet it servesas anothersimpleand

convenientway of differentiatingthe data,perhapsbeing an overtmanifestationof the quasi-biennial

oscillation(QBO).33,34While nomechanismis demonstratedherethat explainsthis differencein storm

formationbetweenevenandoddyears,if suchamechanismweredemonstratedin futurework,this relationshipcouldhavepredictiveability. Comparingeven-more-NENRandodd-more-NENRseasons,surprisingly onefinds that even-numberedyearscomprisethebulk of the more-NENRclimatic condition:

11versus5 seasons(table5). Comparingeven-more-ENRandodd-more-ENRseasons,againsurprisingly,

one finds that odd-numberedyearscomprisethe bulk of the more-ENRclimatic condition: 7 versus

2 seasons(table 6). Comparingeven-more-NENRand even-more-ENRseasons,one finds that evennumberedyearsaremorelikely more-NENRseasonsthanmore-ENRseasons:11versus2 seasons.Finally,comparingodd-more-NENRandodd-moreENRseasons,onefindsneithergroupingtobedominant

(i.e.,thetwo groupsaremoreevenlysplit: 5versus7 seasons,respectively).Whatthis seemsto suggestis

thatbecausetheyear2000seasonwasevennumbered,it appeared=5.5timesmorelikely to be amoreNENR seasonthanamore-ENRseason.Indeed,thelackof occurrenceofE1Nifio onsetin late1999-early

2000seemedto substantiatethattheyear2000seasonwouldbeamore-NENRseason,25,26and,it was.

Table5. Thedistributionof the seasonalratesof selectedgroupingsofAtlantic basincyclones

for 1950-1999andfor themore-NENRclimatecondition,differentiatingeven

andoddyears.

Even-More-NENR(11) Odd-More-NENR(5)

N(NS) N(TS) N(H) N(MH) N(NS) N(TS) N(H) N(MH)

r Obs P(r) Obs P(r) Obs P(r) Obs P(r) Obs P(r) Obs P(r) Obs P(r) Obs P(r)

0 0 0.0001 I 0.0781 0 0.0012 I 0.0347 0 0.0000 0 0.0224 0 0.0002 0 0.0101

1 0 0.0009 1 0.1991 0 0.0080 0 0.1167 0 0.0001 1 0.0850 0 0.0019 0 0.0462

2 0 0.0040 3 0.2539 0 0.0271 3 0.1961 0 0.0004 1 0.1615 0 0.0079 1 0.1063

3 0 0.0125 3 0.2158 1 0.0607 3 0.2196 0 0.0015 0 0.2046 0 0.0222 0 0.1631

4 0 0.0290 3 0.1376 2 0.1021 1 0.1845 0 0.0046 2 0.1944 0 0.0466 0 0.1875

5 1 0.0537 0 0.0702 0 0.1374 1 0.1240 0 0.0113 0 0.1477 0 0.0784 3 0.1725

6 2 0.0830 0 0.0298 2 0.1542 1 0.0694 0 0.0230 0 0.0936 1 0.1097 1 0.1323

7 1 0.1100 0 0.0109 3 0.1482 1 0.0333 0 0.0402 0 0.0508 0 0.1317 0 0.0869

8 0 0.1274 0 0.0035 0 0.1247 0 0.0140 0 0.0612 1 0.0241 2 0.1382 0 0.0500

9 2 0.1312 0 0.0010 1 0.0932 0 0.0052 0 0.0830 0 0.0102 1 0.1290 0 0.0255

10 1 0.1217 0 0.0003 1 0.0627 0 0.0018 3 0.1013 0 0.0039 0 0.1084 0 0.0118

11 1 0.1025 0 0.0001 1 0.0384 0 0.0005 0 0.1123 0 0.0013 1 0.0828 0 0.0049

12 1 0.0792 0 0.0000 0 0.0215 0 0.0002 1 0.1142 0 0.0004 0 0.0579 0 0.0019

13 1 0.0565 0 0.0000 0 0.0111 0 0.0000 0 0.1072 0 0.0001 0 0.0374 0 0.0007

14 1 0.0374 0 0.0000 0 0.0054 0 0.0000 0 0.0934 0 0.0000 0 0.0225 0 0.0002

15 0 0.0231 0 0.0000 0 0.0024 0 0.0000 0 0.0759 0 0.0000 0 0.0126 0 0.0001

16 0 0.0134 0 0.0000 0 0.0010 0 0.0000 0 0.0579 0 0.0000 0 0.0066 0 0.0000

17 0 0.0073 0 0.0000 0 0.0004 0 0.0000 0 0.0416 0 0.0000 0 0.0033 0 0.0000

18 0 0.0038 0 0.0000 0 0.0001 0 0.0000 0 0.0282 0 0.0000 0 0.0015 0 0.0000

19 9 0.0018 0 0.0000 0 0.0000 0 0.0000 1 0.0181 0 0.0000 0 0.0007 0 0.0000

N(S) 11 11 11 11 5 5 5 5

n 102 28 74 37 61 19 42 23

m 9.27 2.55 6.73 3.36 12.20 3.80 8.40 4.60

11
Table 6. The distribution of the seasonal rate of selected groupings of Atlantic basin cyclones

for 1950-1999 and for the more-ENR climate condition, differentiating even and odd years.

Even-More-ENR(2) Odd-More-ENR(7)

N(NS) N(TS) N(H) N(MH) N(NS) N(TS) N(H) N(MH)

r Obs P(r) Obs P(r) Obs P(r) Obs P(r) Obs P(r) Obs P(r) Obs P(r) Obs P(r)

0 0 0.0015 0 0.0498 0 0.0302 1 0.6065 0 0.0006 0 0.0880 0 0.0067 0 0.1809

1 0 0.0098 0 0.1494 0 0.1057 1 0.3033 0 0.0044 2 0.2139 0 0.0337 3 0.3093

2 0 0.0318 1 0.2240 0 0.1850 0 0.0758 0 0.0164 2 0.2599 0 0.0842 3 0.2644

3 0 0.0688 0 0.2240 1 0.2158 0 0.0126 0 0.0406 1 0.2105 2 0.1404 1 0.1507

4 0 0.1118 1 0.1680 1 0.1888 0 0.0016 0 0.0753 2 0.1279 2 0.1755 0 0.0644

5 0 0.1454 0 0.1008 0 0.1322 0 0.0002 1 0.1119 0 0.0622 0 0.1755 0 0.0220

6 1 0.1575 0 0.0504 0 0.0771 0 0.0000 1 0.1386 0 0.0252 1 0.1462 0 0.0063

7 1 0.1462 0 0.0216 0 0.0385 0 0.0000 1 0.1471 0 0.0087 1 0.1044 0 0.0015

8 0 0.1188 0 0.0081 0 0.0163 0 0.0000 3 0.1366 0 0.0027 1 0.0653 0 0.0003

9 0 0.0858 0 0.0027 0 0.0066 0 0.0000 0 0.1128 0 0.0007 0 0.0363 0 0.0001

10 0 0.0558 0 0.0008 0 0.0023 0 0.0000 1 0.0838 0 0.0002 0 0.0181 0 0.0000

N(S) 2 2 2 7 7 7 7 7

n 13 6 7 1 52 17 35 12

m 6.50 3.00 3.50 0.50 7.43 2.43 5.00 1.71

Unfortunately, looking ahead to the year 2001 season (an odd-more year), because neither grouping (NENR or ENR) seems to be the more dominant for odd-more years, one must be content to wait and

see whether or not E1 Nifio returns during the 2001 hurricane season. 26 (A 2 × 2 contingency table, not

shown, constructed by comparing even-odd years against ENR-NENR seasons, is found to be marginally

statistically significant, having a probability of obtaining the observed result, or one more suggestive of a

departure from independence, P = 7.4 percent; i.e., the likelihood that the result is due entirely to chance is

only 7.4 percent.)

Figure 5(a)(1) depicts the percentage of seasons over the past 50 yr that had onsets of named storms

during each month of the year (i.e., their genesis or when the storms were first tracked). Although the

interval running from June-November is the officially recognized "hurricane season," perhaps surprisingly, one finds that onsets of named storms have actually occurred over the broader interval of MayDecember. In particular, 5 of the past 50 seasons (10 percent) have had a tropical cyclone to form in the

Atlantic basin that attained tropical storm or hurricane status during the month of May and 2 seasons

(4 percent) have had onsets in December. During June, the first official month of the hurricane season, 19

of the past 50 seasons (38 percent) have had named storms. This number increases to 25 (50 percent) for

July, with the overwhelming bulk of seasons having onsets of named storms during August-October: 48

seasons (96 percent) having had onsets in August, 49 seasons (98 percent) in September, and 38 seasons

(76 percent) in October. For November, the number of seasons having a named storm drops to 14

(28 percent). The percentage of seasons having onsets of named storms, differentiating between even- and

odd-numbered years, ENR and NENR seasons, and less and more active seasons are plotted in (a), subpanels

(2)-(4).

12
(a)

(b)

(1)

100 -

==50

el.

N(NS)= 50

/

I I I I I I

MJ J ASO ND

Month

(2)

• Even (25)

[] Odd (25) (3)

MJ J ASOND MJ J AS0

Month Month

• ENR (20)

[] NENR (30) (4)

ND MJ

• Less (25)

[] More (25)

J ASO ND

Month

(1) (2) (3)

100 - N(NS)= 442 • Even (218) • ENR (138)

[] Odd (224) [] NENR (304)

"8

" 50

g 0

MJ J ASOND MJ J ASOND MJ J ASOND

Month Month Month

(4)

• Less (214)

[] More (228)

MJ J ASO ND

Month

Figure 5. (a) Histograms of the percentages of seasons for the number of named storms

per calendar month (May-December) for the combined data set and for even-odd,

ENR-NENR, and less-more differentiated data sets. (b) Same construction

as panel (a) except now they are histograms of the percentages of the total

number of named storms.

Figure 5(b)(1) displays the percentage of named storms that had onsets during each month of the

year for the past 50 yr. Thus, while 5 of the seasons had onsets of named storms in the month of May, they

only accounted for 5 individual storms (3 tropical storms and 2 hurricanes, or only =1 percent of all named

storms, 2 percent of all tropical storms, and 0.7 percent of all hurricanes). Similarly, while 2 of the seasons

had onsets of named storms in the month of December, these 2 seasons accounted for only 2 individual

storms (both hurricanes). For June, one finds 22 individual storms, including 13 tropical storms and

9 hurricanes, or approximately 5 percent of all named storms, 8 percent of all tropical storms, and 3 percent

of all hurricanes. For July, one finds 43 individual storms, including 20 tropical storms and 23 hurricanes,

or approximately 10 percent of all named storms, 13 percent of all tropical storms, and 8 percent of all

hurricanes. In contrast, the months of August-October account for 80 percent of all named storms (353),

74 percent of all tropical storms (116), and 83 percent of all hurricanes (237). November has accounted for

only 17 individual storms, including 4 tropical storms and 13 hurricanes, or about 4 percent of all named

storms, 2.5 percent of all tropical storms, and 4.5 percent of all hurricanes. The percentage of named

storms, differentiating between even- and odd-numbered years, ENR and NENR seasons, and less and

more seasons are plotted in (b), subpanels (2)-(4).

13
Figure6(a)plots themonthly countsof namedstormsand (b)plots themonthly countsof hurricanesfor even-moreandodd-moreyears,differentiatingbetweenENR(thefilled histograms)andNENR

seasons(theunfilled histograms).During even-more-NENRseasons,aclassificationthat canbeassociatedwith theyear2000season,11of the 13even-moreyearshavebeensodescribed.Furthermore,they

accountedfor 102of the 115namedstormsand74of the81hurricanesthat occurredduringthe13evenmoreyears.During the 11 even-more-NENRseasons,nonehad an onsetof a namedstormin May,

althoughone occurredin December(a hurricane).Only 15namedstormsoccurredin JuneandJuly

(including 9 hurricanes).AugustandSeptember,together,accountedfor 66 namedstorms(including

47hurricanes),while OctoberandNovemberaccountedfor 20namedstorms(including17hurricanes).

Duringodd-moreyears,7of theseasonswereENRwhile5 wereNENR.Also,whilefewerseasons

weredenotedNENR duringodd-moreyears,theyaccountedfor thebulk of thenamedstormsandhurricanes,actuallyhavinga higheraverageseasonalratethanwas seenduringeven-more-NENRyears(see

table5). Theoneaspectthat seemsapparentfor odd-moreyearsis thatwhentheseasonis denotedENR,

thehurricaneseasonappearsto abruptlyendin October,whereasit continuesonfor anothermonthwhen

theodd-moreseasonis reckonedasNENR. (Theodd-moreyearis whatoneexpectsfor theyear2001

season.)

14
(a)

4O

3O

.a

E

20

"6

e_

E

Z

Even-MoreYears (13) Odd-More Years (12)

mENR (2) r--INENR (11) _ ENR(7) r--J NENR (5)

(n = 13) (n = 102) (n = 52) (n = 61)

F

10

M J J A S 0 N D

Month

M J J A S 0 N D

Month

(b)

2O

"E

_10

.=

0

ENR(2)_ NENR(11)

(n = 7) (n = 74)

- n

M J J A S 0 N D

Month

ENR(7) _ NENR(5)

(n = 35) (n = 42)

F

M J J A S 0 N D

Month

Figure 6. (a) Histograms of the percentages of the total number of named storms for ENR

and NENR seasons during even-more and odd-more years. (b) Same construction

as panel (a) except now they are histograms of the percentages of the total number

of hurricanes.

15
3. DISCUSSION AND SUMMARY

It can be said that when the water is warm enough and the winds are right (i.e., little or no vertical

shear and proper steering currents), one should be especially wary of the heightened potential for the

formation of tropical cyclones in the Atlantic basin. These heat engines, once unleashed, should they

encounter populous land areas, can literally wreak havoc on the infrastructure of both the prepared and the

unprepared.3,5,6,9, 41

Simulations by Emanue142 suggest that once tropical cyclones have reached tropical storm status,

their intensity evolution is controlled mostly by the thermodynamic properties of the atmosphere and the

upper ocean through which they move. 43,44 How the intensity and frequency of occurrence of tropical

cyclones relate specifically to climatic change, however, remains somewhat ambiguous. 3°

Today, there is considerable concern that now, in this present epoch, a change in climate is underway. This change may be a normal fluctuation or, on the other hand, it may be related to the important

issues of global warming and the relative frequency of occurrence of E1 Nifio events. 16,25,26,4532

In this study, it has been shown that the 10-yr moving averages for named storms, tropical storms,

hurricanes, and major hurricanes are all currently at or near record levels. Thus, from the perspective of

tropical cyclones in the Atlantic basin, it is quite apparent that the current epoch can be described correctly

as one of enhanced activity.

This enhanced level of (persistent) activity may continue for many years to come (perhaps

1-2 decades if indeed it is cyclic, or longer if it is a manifestation of global warming), with actual seasonal

rates modulated by the random occurrences of E1 Nifio. Therefore, given that a particular year is ENR or

NENR, one can approximately gauge the expected level of activity for that specific season; i.e., whether or

not seasonal rates will be below (when E1 Nifio is present) or above average (when E1 Nifio is absent).

For hurricanes and major hurricanes, the change from a less active to a more active (enhanced)

environment appears to have occurred rather suddenly. For example, prior to 1995, the seasonal rates for

hurricanes and major hurricanes were relatively flat and depressed (generally below their long-term

median rates). Then, beginning with the 1995 season, a marked increase in the seasonal rates was seen.

Landsea et al. 53 noted that the increase coincided with the return of hurricane activity to the deep tropical

latitudes, attributing the increase to the "juxtaposition of virtually all of the large-scale features over the

tropical North Atlantic that favor tropical cyclogenesis and development." In fact, except for the 1997

season (which was an ENR season), all seasons from 1995 have been more active than normal, as has been

shown in this study, 54_7 strongly indicating perhaps that a return to the more active phase of major hurricanes reminiscent of the1940's and 1950's has taken place.

On the basis of the behavior of the 10-yr moving averages, it was shown that the seasonal rates for

named storms, tropical storms, and hurricanes for the year 2000 hurricane season should have been equal

to or higher than respective long-term median rates. Also, because the year 2000 was an even-numbered

16
year,statistically,it would likely beanNENR-relatedseason.Both of theseperceptionsturnedoutto be

true.Theyear2000hurricaneseasonwasindeedaneven-more-NENRseason,accountingfor 14named

storms,6tropical storms,and8hurricanes(of which 3weremajorhurricanes).So,thetechniqueappears

to haveworkedratherwell for thispastseasonandmeritscontinuedconsideration(for theyear2001and

futureseasons).

For theyear2001season,it will becategorizedaseitheranodd-more-NENRseasonor anoddmore-ENRseason.As yet,onecannotunequivocallystatewhichdesignationwill apply.If E1Nifio should

clearlymanifestitself laterthisyear,26,58especiallyduringtheyear2001hurricaneseason,thenplainly the

seasonwill becategorizedasodd-more-ENR.Presumingthistobetrue,a slightdiminutionin theseasonal

rateswould beexpected.On the otherhand,if anotherNENR seasonprovesmorelikely (certainlynot

unprecedented), thenoneshouldexpectacontinuationof enhancedactivityfor all classesofAtlanticbasin

tropicalcyclonesin 2001.

Onthebasisof thepaststatisticalbehaviorof the 10-yrmovingaverages,fortheyear2001season

oneexpectsapproximately7 + 3 named storms, 4 + 2 tropical storms, and 3 + 3 hurricanes, inferring <3

major hurricanes. Thus, either the year 2001 season will be a statistical outlier (if seasonal rates equal to or

higher than the median rates occur) or it will have diminished rates, probably because E1 Nifio unexpectedly recurs. This makes for an interesting season.

17
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23
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May 2001 Technical Publication

4. TITLE AND SUBTITLE

Decadal Trends of Atlantic Basin Tropic al Cyclones (195 (_ 1999)

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

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M 1017

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AGENCY REPORT NUMBER

NASA/TP 2001 210991

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Prepared by Space Science Department, Science Directorate

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13. ABSTRACT (Maximum 200 words)

Ten-yr moving averages of the seasonal rates for "named storms," tropical storms, hurricanes, and major

(or intense) hurricanes in the Atlantic basin suggest that the present epoch is one of enhanced activity,

marked by seasonal rates typically equal to or above respective long-term median rates. As an example, the

10-yr moving average of the seasonal rates for named storms is now higher than for ally previous year over

the past 50 yr, measuring 10.65 in 1994, or 2.65 units higher than its median rate of 8. Also, the 10-yr

moving average for tropical storms has more than doubled, from 2.15 in 1955 to 4.60 in 1992, with 16 of

the past 20 yr having a seasonal rate of 3 or more (the median rate). For hurricanes and major hurricanes,

their respective 10-yr moving averages turned upward, rising above long-term median rates (5.5 and 2,

respectively) in 1992, a response to the abrupt increase in seasonal rates that occurred in 1995. Taken

together, the outlook for future hurricane seasons is for all categories of Atlantic basin tropical cyclones to

have seasonal rates at levels equal to or above long-term median rates, especially during non-E1 Nifiorelated seasons. Only during E1 Nifio-related seasons does it appear likely that seasonal rates might be

slightly diminished.

14. SUBJECT TERMS

climatic change, climate, hurricanes, Atlantic basin, E1 Nifio

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