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35812
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i

NASA/TP—2009–215899

Decadal Variation of the Number of El Niño 

Onsets and El Niño-Related Months and 

Estimating the Likelihood of El Niño Onset 

in a Warming World

Robert M. Wilson

Marshall Space Flight Center, Marshall Space Flight Center, Alabama

June 2009

National Aeronautics and

Space Administration

Marshall Space Flight Center • MSFC, Alabama 35812
ii

Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076 –1320

443–757–5802

This report is also available in electronic form at

<https://www2.sti.nasa.gov>
iii

TABLE OF Contents

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

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

3. SUMMARY ...................................................................................................................... 11

REFERENCES ...................................................................................................................... 12
iv

LIST OF FIGURES

1. Decadal variation of (a) NENO, (b) NENM, (c) <Dur>, and (d) <AT> .................... 5

2. Variation of (a) Dur and (b) RP by event number and (c) scatter plot of RP(i)

versus Dur(i) for moderate/strong events ..................................................................... 7

3. Superposed epoch analysis of (a) moderate/strong El Niño events and 

(b) all El Niño events ................................................................................................... 9

LIST OF TABLES

1. Listing of El Niño events and parametric values, based on ONI (ERSST.v3b) ............ 3

2. Decadal values for El Niño events and <AT> ............................................................. 4
v

LIST OF ACRONYMS AND SYMBOLS

<AT> mean decadal Armagh Observatory surface-air temperature

Dur event duration

<Dur> mean decadal duration

EN El Niño

ENSO El Niño–Southern Oscillation

ERSST.v3b extended reconstructed sea-surface temperature, version 3b

M moderate

MSFC Marshall Space Flight Center

max ONI maximum value of the ONI

N north latitude

NENM number of EN-related months

NENO number of EN onsets

No. number

NOAA National Oceanic and Atmospheric Administration

ONI Oceanic Niño Index

<ONI> mean value of the ONI over the entire event

RP recurrence period

S strong; south latitude

t elapsed time in months from EN onset

T total

W weak; west longitude
vi

NOMENCLATURE

cl confidence level

i event number

r coefficient of correlation

r2 coefficient of determination

sd standard deviation

se standard error of estimate

x independent variable

y regression equation
1

TECHNICAL PUBLICATION

DECADAL VARIATION OF THE NUMBER OF EL NIÑO ONSETS AND EL NIÑORELATED MONTHS AND ESTIMATING THE LIKELIHOOD OF EL NIÑO

ONSET IN A WARMING WORLD 

1. INTRODUCTION

About a decade ago, a number of papers were published describing the apparent ‘anomalous’ behavior during the 1990s of the El Niño-Southern Oscillation (ENSO) quasi-periodic cycle, 

suggesting, perhaps, a link with global climatic change.1–6 Research7 conducted at Marshall Space 

Flight Center (MSFC), however, indicated that the frequency and severity of El Niño events during 

the 1990s compared quite favorably with preceding decades, suggesting instead that the 1990s should 

not be regarded as anomalous, but rather as a normal fluctuation in the pattern. While true, it was 

noted at that time that the number of El Niño-related months per decade was sharply higher in the 

1990s as compared to the preceding four decades, hinting of a marginally statistically significant 

upward trend (confidence level (cl) >90%).

Nearly a decade has now passed since the earlier study was performed, so it seems appropriate to revisit the issue once again in light of nearly another decade’s worth of observations. Is the 

number of El Niño onsets and/or the number of El Niño-related months per decade increasing with 

time, possibly being related to global warming, or do the numbers better reflect normal fluctuation? 

Also, given the last occurrence of an El Niño onset, can the likelihood for the occurrence of the 

next El Niño onset be estimated? This latter aspect is of particular importance as it relates directly 

to extended forecasts of North Atlantic basin tropical cyclone activity for the 2009 hurricane season.8–11
2

2. RESULTS AND DISCUSSION

Table 1 lists the El Niño events occurring during the interval 1950–2008, some 17 events, determined from the Oceanic Niño Index (ONI). This ONI is a 3-month running mean of the extended 

sea-surface temperature (ERSST.v3b) anomalies based on the 1971–2000 base period in the Niño 

3.4 region, an area in the Pacific Ocean located between 5 ̊ N and 5 ̊ S latitude and between 120 and 

170 ̊ W longitude. El Niño refers to the warm phase and La Niña refers to the cold phase of the 

ENSO pattern. While various methodologies12–13 have been used in the past to discern the occurrence of these warm and cold events, today the ONI has become the de facto means for identifying 

them. In particular, an El Niño is said to be occurring when the ONI has at least a 5-month continuous value of 0.5  ̊C or warmer, and a La Niña is said to be occurring when the ONI has at least a 

5-month continuous value of –0.5  ̊C or cooler. When the peak ONI value is between 0.5 and 0.9  ̊C, 

the El Niño event is termed weak (W); when the peak ONI value is between 1.0 and 1.4  ̊C, the event 

is termed moderate (M); and when the peak ONI value is 1.5  ̊C or warmer, the event is termed strong 

(S). Similarly, when the peak ONI value is between –0.5 and –0.9  ̊C, the La Niña event is termed 

weak; when the peak ONI value is between –1.0 and –1.4  ̊C, the event is termed moderate; and when 

the peak ONI value is –1.5  ̊C or cooler, the event is termed strong. When the ONI fails to meet the 

threshold for establishing the occurrence of either an El Niño or La Niña event, the conditions are 

described as being ENSO-neutral. The ONI values can be found online and are updated monthly.14

In the earlier study, the listing of El Niño events was taken from the combined listings of 

Quinn et al.12 and Trenberth.13 The decadal distribution of events (58 El Niño events over 20 

decades) appeared normally distributed about a mean of three events per decade, with a range of 

one to four events per decade. In table 1, the listing is based strictly on the ERRST.v3b, which is the 

newly revised sea-surface temperature dataset introduced in December 2008 replacing the earlier version ERSST.v3, which will no longer be updated. Table 1 gives the onset and peak dates for each of 

the 17 El Niño events (month-year), the maximum ONI value (max ONI) (in degrees Celsius), the 

average ONI value (<ONI>) (in degrees Celsius) during the event, the strength (W, M, or S), and the 

recurrence period (RP) (in months) from event onset to next event onset. At the bottom of table 1 

are the mean, standard deviation (sd), median, and range for all events. Thus, on average, the 17 El 

Niño events had a duration (Dur) of about 10 months, max ONI of about 1.4  ̊C, <ONI> of about 

0.97  ̊C, and RP of about 41 months.

Inspection of onset dates reveals that all El Niño events had onsets between April and November, with most occurring in May (6 of 17 events). Weak events had onsets in June (one event), August 

(one event), and September (three events); moderate events had onsets in May (one event), July (one 

event), August (one event), and November (one event); and strong events had onsets in April (one 

event), May (five events), June (one event), and August (one event). Ten of 12 moderate/strong events 

had onsets in May–August, a timeframe associated with the yearly hurricane season (June–November).
3

Table 1. Listing of El Niño events and parametric values, based on ONI (ERSST.v3b).

Event No. Onset Date Peak Date Dur max ONI <ONI> Strength RP

1 08-1951 10-1951 5 0.8 0.70 W 68

2 04-1957 01-1958 15 1.7 0.99 S 75

3 07-1963 11-1963 7 1.0 0.86 M 23

4 06-1965 11-1965 11 1.6 1.12 S 41

5 11-1968 01-1969 8 1.0 0.79 M 10

6 09-1969 11-1969 5 0.8 0.66 W 32

7 05-1972 12-1972 11 2.1 1.32 S 52

8 09-1976 11-1976 6 0.8 0.63 W 12

9 09-1977 11-1977 5 0.7 0.64 W 56

10 05-1982 12-1982 14 2.3 1.39 S 51

11 08-1986 08-1987 19 1.6 1.11 S 57

12 05-1991 01-1992 15 1.8 1.13 S 36

13 05-1994 12-1994 11 1.3 0.83 M 36

14 05-1997 11-1997 13 2.5 1.74 S 60

15 05-2002 11-2002 11 1.5 1.03 S 25

16 06-2004 09-2004 9 0.9 0.72 W 26

17 08-2006 11-2006 6 1.1 0.83 M –

All:

mean 10.1 1.4 0.97 41.3

sd 4.2 0.6 0.31 19.5

median 11 1.3 0.86 38.5

range 5–19 0.7–2.5 0.63–1.74 10–75

Likewise, inspection of peak dates, i.e., the month when max ONI first occurred, reveals that 

all events had peaks between August and January, with most occurring in November (8 of 17 events). 

Weak events had peaks in September (one event), October (one event), and November (three events); 

moderate events had peaks in November (two events), December (one event), and January (one 

event); and strong events had peaks in August (one event), November (three events), December (two 

events), and January (two events). Of 12 moderate/strong events, 11 had peaks in November–January.

Table 2 gives the decadal counts and mean values for the number of El Niño onsets (NENO), 

the number of El Niño-related months (NENM), the mean duration (<Dur>) (in months) and the 

mean surface-air temperature (<AT>) (in degrees Celsius) as recorded by the Armagh Observatory 

(Northern Ireland) for each decade from 1950 (e.g., 1950–59, 1960–69, etc.). The 17 events over the 

past 6 decades suggest an average of about 3 events per decade, ranging between 2 and 4 events per 

decade, as was found in the earlier study. Every decade has had at least one strong event (range: 

one–two). For the current decade, only one strong event has occurred, at least, through 2008: an 

11-month event having onset in May 2002 (event 15, in table 1). The strongest event overall, in terms 

of max ONI and <ONI>, is the 13-month event having onset in May 1997 (event 14: max ONI = 2.5 
4

Table 2. Decadal values for El Niño events and <AT>.

Decade

NENO NENM <Dur>

W M S T W M S T All M/S <AT>

1950–59 1 1 2 5 15 20 10.0 15.0 9.43

1960–69 1 2 1 4 5 15 11 31 7.8 8.7 9.17

1970–79 2 1 3 11 11 22 7.3 11.0 9.15

1980–89 2 2 33 33 16.5 16.5 9.28

1990–99 1 2 3 11 28 39 13.0 13.0 9.74

2000–09* 1 1 1 3 9 6 11 26 8.7 8.5 10.11

Total 5 4 8 17 30 32 109 171

Note: * means only thru 2008.

Temperature in degrees Celsius.

and <ONI> = 1.74). The longest duration event is the 19-month strong event having onset in August 

1986 (event 11), while the shortest strong events each have had duration of 11 months (events 4, 7, 

and 15).

Figure 1 plots the decadal variation of (a) NENO, (b) NENM, and (c) <Dur> for moderate/

strong El Niño events since 1950 and of (d) <AT>. Concerning NENO (fig. 1(a)), for all decades 

since 1950 there has always been at least one event. The median is two events per decade and the 

range is one to three events per decade. Including the five weak events only slightly alters the NENO, 

yielding a median of three events per decade and range of two to four per decade. There appears 

to be no evidence for an increase in the decadal frequency of El Niño events with time. The current 

decade (2000–2009) has experienced two moderate/strong events (2002 and 2006) and one weak 

event (2004). The last El Niño onset was August 2006, peaking in November 2006, moderate in 

strength, and ending in January 2007. As of this writing (May 2009), some 33 months had elapsed 

since the last El Niño onset and some 28 months had elapsed since its end. The mean recurrence 

period, as shown in table 1, is about 41 months using all events, with a median of 38.5 months and 

a range of 10–75 months. The three moderate events with known RP were each followed by El Niño 

events after about 23 months on average (range: 10–36 months), two being followed by strong events 

and one by a weak event. Because the elapsed time between El Niño onsets is well within the range of 

known RP (in particular, the range of RP for moderate events), it seems quite possible that another 

El Niño could suddenly appear sometime during the interval June–December 2009. Mitigating this, 

however, is the National Oceanic and Atmospheric Administration’s (NOAA’s) extended forecast, 

which calls for ENSO-neutral to La Niña-like conditions to prevail during 2009.15 Table 1 shows 

that 10 of 17 El Niño events had onsets during the interval June–November, these events consisting 

of 5 weak events, 3 moderate events, and 2 strong events, with the greatest likelihood of onset being 

August–September.

Concerning NENM (fig. 1(b)), it has a median of 24 months. With the addition of the latest decade, it is now quite apparent that the former finding of a marginally statistically significant 

upward trend was premature. The number of El Niño-related months for 2000–2009 measures only
5

50s 60s 70s 80s 90s 00s 10s 

10.5

10.0

9.5

9.0

(d)

Decade

Moderate/Strong Events Only (12)

Median (9.36)

Median (12)

Median (24)

Median (2)

 <AT>

20

15

10

5 (c) <Dur>

10

20

30

40

5 (b) NENM

5

(a) NENO

Figure 1. Decadal variation of (a) NENO, (b) NENM, (c) <Dur>, and (d) <AT>.
6

17 months (through May 2009), using moderate/strong events, and only 26 months when including 

the 9-month weak event of 2004 (event 16). Certainly, the decades of the 1980s and 1990s both had 

more El Niño-related months (33 and 39, respectively) than the current decade. If another El Niño 

should happen to develop during the latter half of 2009, the greatest number of El Niño-related 

months for the current decade would still be lower than was seen in the 1980s and 1990s.

Concerning <Dur> (fig. 1(c)), its median is 12 months. The decade of the 1980s seems to have 

had the longest average duration (16.5 months), while the decade of the 1960s appears to have had 

the shortest average duration (8.7 months). The average duration of El Niño events during the current decade measures about 8.5 months using only the moderate/strong events, or about 8.7 months 

when including the lone weak event of 2004. As for NENO and NENM, no evidence appears for an 

increase with time of the average duration for El Niño events.

Concerning <AT>, its median mean decadal temperature is 9.36  ̊C. The <AT> is observed 

to have cooled during the 1960s and 1970s to about 9.17 and 9.15  ̊C, respectively, before warming to 

its highest mean decadal temperature ever (10.11  ̊C) during the current decade (the record extends 

back to 1844). Previous research has shown that the Armagh Observatory temperature dataset can 

be used as a proxy for studying trends in global temperatures.16–20

Comparisons of NENO, NENM, and <Dur> against <AT> (not shown) provide no evidence for statistically significant associations to exist between them. Thus, it cannot be claimed, as 

yet, that global warming is influencing the frequency, strength, or duration of El Niño events with 

the passage of time (i.e., over the past six decades).

Figure 2 plots the variation by event number (i) of (a) Dur and (b) RP of the 17 El Niño 

events in the interval 1950–2008 (from table 1), and shows (c) the scatter plot of RP(i) versus Dur(i), 

where i simply identifies the event number, based strictly on moderate/strong events. Concerning 

the event durations (fig. 2(a)), the median is 11 months, with strong events usually having the longest durations (11–19 months) and weak events usually having the shortest durations (5–9 months). 

Moderate events have durations between weak and strong events (6–11 months).

Concerning the event recurrence periods (fig. 2(b)), the median is 38.5 months, with strong 

events usually having RPs longer than the median (6 of 8 strong events) and weak and moderate 

events usually having RPs less than the median (7 of 8 events). The fact that the last El Niño was a 

moderate event, suggests that the next onset of El Niño likely will follow within 38.5 months from 

the onset of the August 2006 event. Thirty-eight months from the last onset corresponds to October 

2009. So, it seems that another El Niño might possibly appear during the latter half of 2009, perhaps 

in late summer or early fall of 2009.

Supporting this conjecture is the scatter plot for moderate/strong events (fig. 2(c)). The numbers beside the symbols simply refer to the event numbers and the arrow shows the known duration 

of event 17, the last observed El Niño, having a duration of only 6 months (the shortest of all moderate events). The inferred regression (y) is displayed as the diagonal line. It has a coefficient of correlation (r) equal to 0.712, a coefficient of determination (r2) equal to 0.506 (meaning that about half the 

variance can be explained by the inferred regression), a standard error of estimate (se) equal to about 
7

5 10 15 

2 y

14

7 10

11

12

15

13

4

3

5

20

20

40

60

80

(c)

Dur(i)

Event Number

EN Event (17)

Moderate/Strong Events Only (12)

RP(17) = 18 ± 14 (±1σ)

Median (38.5)

RP(i)

y = –6.007 + 3.941x

r = 0.712, r2 = 0.506

se = 13.991, cl > 98%

2 4 6 8 10 12 14 16 18 20

2 4 6 8 10 12 14 16 18 20

20

40

60

80

(b) RP (mo)

Median (11)

Strong

Moderate

Weak

5

10

15

20

(a) Dur (mo)

Figure 2. Variation of (a) Dur and (b) RP by event number and (c) scatter plot of RP(i)

versus Dur(i) for moderate/strong events.
8

14 months and cl > 98%. Based upon the inferred linear regression, the RP for event 17 (RP(17)) is 

estimated to be about 18 ± 14 months (i.e., ±1 se accuracy). The regression can be slightly improved 

if weak events 6, 8, and 16 are also included, but it becomes statistically unimportant if all weak 

events (i.e., events 1 and 9 appear to be statistical outliers) are included. Thus, event 17 is expected to 

be followed by another El Niño within about 32 months from its onset in August 2006, inferring only 

about a 16% chance of following by more than 32 months, unless, of course, event 17 is a statistical 

outlier as are events 1 and 9. Thirty-two months corresponds to about April 2009. Using the slightly 

improved regression (y = –1.158 + 3.563x, having r = 0.739, r2 = 0.545, se = 13.234, and cl >99.5%) 

yields an expected RP(17) = 20.2 ± 13.2 months, inferring only about a 16% chance of being longer 

than 33 months, or onset after about May 2009.

The ONI values are presently known only through March 2009. For December 2008 through 

March 2009 the values have been indicative of La Niña-like conditions (–0.6, –0.8, –0.7, and –0.5 

 ̊C, respectively). However, December 2008 will not be classified as an onset for La Niña unless the 

month of April 2009 also has an ONI value of –0.5  ̊C or cooler. Because the trend now appears to 

be towards warmer values, another La Niña event with onset in December 2008 seems unlikely. Previously, there has been only one La Niña onset in December during the interval 1950–2008. It was a 

weak event with onset in December 1967 and a duration of 5 months.

Figure 3 is a superposed epoch analysis of ONI values for (a) the 12 moderate/strong El 

Niño events and (b) all events, using event onset as the common epoch. ONI values tend to be, on 

average, about 0.0–0.3  ̊C just prior to onset of El Niño events, whether using all events or only the 

moderate/strong events. Moderate/strong events, on average, peak about 6 months after onset and 

fall below threshold value (0.5  ̊C) after about 11 months. Including the weak events, on average, El 

Niño events peak about 5 months after onset and fall below threshold after about 10 months. Based 

on the ±1 sd spread, moderate/strong events, on average, peak anytime between 4 and 7 months following onset and fall below threshold anytime between 7 and after 12 months. Similarly, based on the 

±1 sd spread, El Niño events (including the weak events) peak, on average, anytime between 2 and 7 

months following onset and fall below threshold anytime between 4 and after 12 months.

Presuming the sudden appearance of an El Niño in 2009, if the event is of moderate/strong 

strength, its onset is anticipated about May–August 2009 with peak in November 2009–January 

2010. If, on the other hand, the event is only of weak strength, its onset is anticipated in June–

September 2009 with peak in September–November 2009. Either way, the onset of El Niño would 

have a diminishing effect on the frequency of tropical cyclones in the North Atlantic basin during 

the 2009 hurricane season.10 This may mean that the initial early extended forecasts for the 2009 

hurricane season might be too high. In fact, while the initial December 2008 forecasts for the 2009 

hurricane season called for average-to-above average frequency (in terms of numbers of tropical 

cyclones, hurricanes, and major or intense hurricanes), the April updates of hurricane activity have 

been revised slightly downward closer to long-term average values. Such is the precarious nature of 

prediction.8–11 (On May 21, 2009, NOAA issued their 2009 Atlantic hurricane season outlook.21 

The outlook calls for a 50% chance for a near-normal season, with a 25% chance of being either an 

above-normal or below-normal season. NOAA estimates a 70% probability of having 9–14 named 

storms, 4–7 hurricanes, and 1–3 major hurricanes during the 2009 season.)
9

–3 0 3 6 9 12

–0.5

0.0

0.5

1.0

1.5

2.0

2.5

Moderate/Strong EN Events (12)

3.0

(a) t (mo)

–1 sd

+1 sd

mean

 ONI

–3 0 3 6 9 12

All Events (17)

(b) t (mo)

–1 sd

+1 sd

mean

Figure 3. Superposed epoch analysis of (a) moderate/strong El Niño events and 

(b) all El Niño events.
10
11

3. SUMMARY

This study has revisited one performed nearly a decade ago,7 updating it to include the values for the current decade (at least, 2000–2008). It is apparent that the number of El Niño onsets 

and the number of El Niño-related months display decadal frequency variations more attuned to 

being natural variations than being driven by global warming. The same appears true for the average 

decadal duration of El Niño events. About two moderate/strong El Niño onsets occur each decade, 

having a range of one to three events per decade; including weak events, the range increases to two 

to four events per decade. Every decade since 1950 has experienced, at least, one strong event. Two 

moderate/strong events have occurred in the current decade (through 2008). On average, there have 

been about 24 El Niño-related months per decade, having a range of 11–39 El Niño-related months 

per decade based on the moderate/strong events only. The fewest El Niño-related months occurred 

in the 1970s and the most occurred in the 1990s. During the current decade, thus far, some 17 El 

Niño-related months have occurred (26 months, including the lone 9-month weak event of 2004). In 

contrast to the natural variation pattern for El Niño, surface-air decadal temperatures have steadily 

increased since the 1970s, being at their highest levels in the current decade.

Individually, there have been 17 El Niño onsets since 1950, including 8 strong events, 4 moderate events, and 5 weak events. Strong events usually have the longest duration, while weak events 

usually have the shortest duration. The longest event (19 months) had onset in August 1986. During 

the current decade, the longest event persisted for 11 months (in 2002).

Moderate/strong El Niño events tend to recur about every 3 years (median). Since the last El 

Niño event of moderate/strong strength had onset in August 2006, another might be expected before 

August 2009. Certainly, for moderate/strong events, a statistically significant linear relationship is 

found to exist associating recurrence period against duration. Because the last El Niño of moderate/

strong strength had a duration of only 6 months (the shortest moderate event during the interval 

1950–2008), provided that it is not a statistical outlier, the next El Niño onset is expected to occur 

soon, probably sometime during the latter half of 2009. If true, then the 2009 North Atlantic basin 

hurricane season will be influenced by the occurrence of El Niño, thereby, causing a slight decrease in 

the expected frequencies of tropical cyclones, hurricanes, and major hurricanes from one of averageto-above average frequencies to one of near average-to-below average frequencies.
12

REFERENCES

1. Trenberth, K.E.; and Hoar, T.J.: “The 1990–1995 El Niño–Southern Oscillation Event: Longest 

on Record,” Geophys. Res. Letts., Vol. 23, p. 57, 1996.

2. Trenberth, K.E.: “What is Happening to El Niño?” in 1997 Yearbook of Science and Future, 

Encyclopedia Britannica, Inc., Chicago, p. 88, 1996.

3. Goddard, L.; and Graham, N.E.: “El Niño in the 1990s,” J. Geophys. Res., Vol. 102, p. 10423, 

1997.

4. Latif, M.; Kleeman, R.; and Eckert, C.C.: “Greenhouse Warming, Decadal Variability, or El 

Niño? An Attempt to Understand the Anomalous 1990s,” J. Climate, Vol. 10, p. 2221, 1997.

5. Rajagopalan, B.; Lall, U.; and Cane, M.A.: “Anomalous ENSO Occurrences: An Alternative 

View,” J. Climate, Vol. 10, p. 2351, 1997.

6. McPhaden, M.J.; Busalacchi, A.J.; Cheney, R.; et al.: “The Tropical Ocean–Global Atmosphere 

Observing System: A Decade of Progress, “ J. Geophys. Res., Vol. 103, p. 14169, 1998.

7. Wilson, R.M.: “El Niño during the 1990s: Harbinger of Climatic Change or Normal Fluctuation?” NASA/TP–2000–209960, Marshall Space Flight Center, Alabama, February 2000, 

available online at <http://trs.nis.nasa.gov/archive/00000513/>.

8. Klotzbach, P.J.; and Gray, W.M.: “Extended Range Forecast of Atlantic Seasonal Hurricane 

Activity and U.S. Landfall Strike Probability for 2009,” Colorado State University, Ft. Collins, 

CO, December 10, 2008, available online at <http://hurricane.atmos.colostate.edu/forecasts/>.

9. Saunders, M.; and Lea, A.: “Extended Range Forecast for Atlantic Hurricane Activity in 2009,” 

University College London, United Kingdom, December 5, 2008, available online at <http://

www.tropicalstormrisk.com/>.

10. Wilson, R.M.: “An Extended Forecast of the Frequencies of North Atlantic Basin Tropical 

Cyclone Activity for 2009,” NASA/TP–2009–215741, Marshall Space Flight Center, AL, 52 pp., 

March 2009, available at <http://trs.nis.nasa.gov/archive/00000800/>.

11. Wilson, R.M.: “North Atlantic Basin Tropical Cyclone Activity in Relation to Temperature and 

Decadal-Length Oscillation Patterns,” NASA/TP—2009–215796, Marshall Space Flight Center, 

AL, pp. 84, June 2009.

12. Quinn, W.H.; Neal, V.T.; and Antunez de Mayolo, S.E.: “El Niño Occurrences Over the Past 

Four and a Half Centuries,” J. Geophys. Res., Vol. 92, p. 14449, 1987.
13

13. Trenberth, K.E.: “The Definition of El Niño,” Bull. Am. Meteor. Soc., Vol. 78, p. 2771, 1997.

14. NOAA/Climate Prediction Center Web Page, <http://www.cpc.noaa.gov/products/analysis_

monitoring/ensostuff/ensoyears.shtml>, accessed April 2009.

15. NOAA/Climate Prediction Center Web Page, <http://elnino.noaa.gov>, accessed April 2009.

16. Butler, C.J.: “Maximum and Minimum Temperatures at Armagh Observatory, 1844–1992, and 

the Length of the Sunspot Cycle,” Solar Phys., Vol. 152, p. 35, 1994.

17. Butler, C.J.; and Johnston, D.J.: “A Provisional Long Mean Air Temperature Series for Armagh

Observatory,” J. Atmos. Terr. Phys., Vol. 58, p. 1657, 1996.

18. Wilson, R.M.: “Evidence for Solar–Cycle Forcing and Secular Variation in the Armagh 

Temperature Record (1844–1992),” J. Geophys. Phys., Vol. 103, p. 11159, 1998.

19. Butler, C.J.; García Suárez, A.M.; Coughlin, A.D.S.; and Morrell, C.: “Air Temperatures 

at Armagh Observatory, Northern Ireland, from 1796 to 2002,” J. Climatol., Vol. 25, p. 1055, 

2005.

20. Wilson, R.M.; and Hathaway, D.H.: “Examination of the Armagh Observatory Annual Mean 

Temperature Record, 1844–2004,” NASA/TP–2006—214434, 24 pp., July 2006, available online 

at <http://trs.nis.nasa.gov/archive/00000727/>.

21. NOAA Press Release, “NOAA: 2009 Atlantic Hurricane Season Outlook,” issued 21 May 2009,

<http://www.cpc.ncep.noaa.gov/products/outlooks/hurricane.shtml>, accessed May 2009.
14

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Prescribed by ANSI Std. Z39-18

Decadal Variation of the Number of El Niño Onsets and El NiñoRelated Months and Estimating the Likelihood of El Niño Onset 

in a Warming World

Robert M. Wilson

George C. Marshall Space Flight Center

Marshall Space Flight Center, AL 35812

National Aeronautics and Space Administration

Washington, DC 20546–0001

Unclassified-Unlimited

Subject Category 47

Availability: NASA CASI (443–757–5802)

Prepared by the Science and Exploration Research Office, Science and Mission Systems Office

M–1258

Technical Publication

NASA/TP—2009–215899

El Niño, ENSO, El Niño recurrence periods, ONI, climatic change

01–06–2009

UU 20

NASA

U U U

Examination of the decadal variation of the number of El Niño onsets and El Niño-related months for the interval 1950–2008 

clearly shows that the variation is better explained as one expressing normal fluctuation and not one related to global warming. 

Comparison of the recurrence periods for El Niño onsets against event durations for moderate/strong El Niño events results in 

a statistically important relationship that allows for the possible prediction of the onset for the next anticipated El Niño event. 

Because the last known El Niño was a moderate event of short duration (6 months), having onset in August 2006, unless it is a 

statistical outlier, one expects the next onset of El Niño probably in the latter half of 2009, with peak following in November 

2009–January 2010. If true, then initial early extended forecasts of frequencies of tropical cyclones for the 2009 North Atlantic basin hurricane season probably should be revised slightly downward from near average-to-above average numbers to near 

average-to-below average numbers of tropical cyclones in 2009, especially as compared to averages since 1995, the beginning of 

the current high-activity interval for tropical cyclone activity. 

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