April

1993

T. Tomita

On

By

the

Tomohikoand

273

T. Yasunari

Two

Types

Tomitaandof ENSO

Tetsuzo

Yasunari

Institute of Geoscience, University of Tsukuba, Tsukuba, Ibaraki 305, Japan(Manuscript received 8 November 1991, in revised form 15 February 1993)

Abstract

From 1950 to 1988, 10 ENSO (El Nino-Southern Oscillation) events have occurred. These ENSO
events are defined by using the Southern Oscillation index (SOI) and the sea surface temperature(SST) anomaly in the central equatorial Pacific. The ENSO events are classified based on the positive
anomaly length of the SST time series: about a year with one northern winter, or two or more years
with two northern winters. The shorter one (BO-ENSO) terminates in the following year of theoccurrence, and the longer one (LF-ENSO) terminates in the two years after the occurrence. The
occurrence and termination of both types have strong phase preference to the seasonal cycle of themean field, i. e., the seasons between northern winter and northern summer. The occurrence years ofeach type are as follows; the years of BO-ENSO are 1951, '53, '63, '65, '72, and 1982 (6 cases) andthe years of LF-ENSO are 1957, '68, '76, and 1986 (4 cases). Since the occurrence, development, andtermination of the both types of ENSO have a seasonal phase preference, composite analysis is used
to explain the difference between the two types of ENSO clearly. In the northern winter immediately
after the BO-ENSO occurrence, the wind stress anomaly field has a specificclockwiseeddy off the east
coast of the Philippines. This eddy affects the ocean through Ekman pumping and plays an importantrole in ENSO termination. The most significant difference between a BO-ENSO and an LF-ENSO is
whether this eddy appears or not.

1. Introduction

The phenomenon called ENSO (El Nino-Southern
Oscillation) has attracted many researchers as a
typical large-scale air-sea interaction since Bjerknes(1966, 1969). This phenomenon has the interannual
time scale of 2 to 6 years or so, and has been noted asa major cause of the extreme weather conditions allover the world (e.g. Ropelewski and Halpert, 1987;
Halpert and Ropelewski, 1992). However,the characteristics of each ENSO seem to be quite different
from one to another. It is very important to identify accurately the strength and duration of each
ENSO, and the seasonal timing of the occurrence
and termination, thereby to understand the mechanism and the effect of ENSO on the global circulation. The long-term forecast also requires physicalunderstanding. One of the most essential purposesof this study is to evaluate and classify with certainty the duration of each ENSO.
The years of ENSO since the 1500's were investigated in detail by Quinn (1978, 1987) from the pastrecords of catastrophic floods in northwestern SouthAmerica and of cruises in the adjacent sea area, documented in several languages. In addition, Thompson (1984) and Lough and Fritts (1985) determined
the past ENSO years from the stratigraphy of the(c)1993, MeteorologicalSociety of Japan

Quelccaya ice cap and tree-ring analysis, respectively. Wang (1992) surveyed the ENSO chronology
for the last 600 years from proxy data such as Nilefloods, Peruvian floods, Australian draughts, numbers of typhoons landing on China, cooler summers
in East Asia, and so on. The ENSO years in this century have already been determined by van Loon and
Madden (1981),Rasumusson and Carpenter (1983),
and Trenberth (1984), mainly from SST (sea surface
temperature) and SOI (the Southern Oscillation index) data.
In terms of the time interval of ENSO, Wyrtki(1975, 1985) pointed out that it was a consequenceof the piling up process of warm waters in the western tropical Pacific. Graham and White (1988) andWhite and Pazan (1989)showed, on the other hand,that the off-equatorialRossby wave of the ENSO delayed oscillator played an essential role in fixing theperiod. Enfield and Cid S. (1991) surveyed the relationship between the interval of ENSO and solar
activity, by using Quinn's (1987) data, showing thatthe interval is about 5 years when the solar activity is strong, while it is 3.5 years shorter when theactivity is weak.
The past ENSO events were sorted in four levels
according to strength by Quinn (1987) and Wang
(1992). There is also the classificationof ENSOwith
respect to the interval, such as by Enfield and Cid

274

Journalof theMeteorologicalS. (1991). Fu et al. (1986) classified ENSO events
into three types from the zonal gradient patternof SST in the equatorial Pacific. Yasunari (1985)
showed that the major and minor ENSO events
could be explained by the superposition of two
dominant modes (20-30 and 40-60-month periods),
which were detected in the interannual variabilityof tropical Walker circulation. Barnett (1991) advanced the idea of Yasunari (1985) and classified
ENSO events into three categories of strength, according to the relation between amplitude and phaseof the two predominant periodic components in interannual variability mentioned above. Lau andSheu (1988) predicted the two types of the ENSO cycle from the conceptional model: one is the two-year
cycle, and the other is the four-year cycle. In thecycle of the conceptional model, the period depends
on the positive or negative correlation between the
air-sea interaction.

Although ENSO events are classified in various
manners as listed above, they have never been classified in terms of the duration. In this study, we
propose two types of ENSO, by classifying the duration of the individual events. This classification
contributes to an explanation of the relationship between the dominant modes in interannual variability such as that of Rasumusson et al. (1990), more
concretely. In addition, the difference of the impact on the global weathers, depending upon thetype of ENSO, is important, especiallyregarding thepersistence and variability of the global circulation
anomalies. The evolution and its seasonality of eachtype of ENSO are also investigated.
Section2 of this paper describes
the data used.
The classifications
based on the durationof ENSOand the seasonal evolutionof each type are describedin Section 3 and Section
4, respectively.
The summary and discussionof this study is given in Section 5.
2. DataIn this study, the following three datasets wereused.
Sea surface temperature (SST). These data,
edited by the United Kingdom Meteorological Office, includes monthly data for the period from January

1854

lat. ‑lon.

to
grids

February
from

80°N

1989,

and

is given

to

80°S

for

on

5° ×5°

all longitudes.

Missing data were interpolated and extrapolated
further as follows. The missing data in the space
field were linearly interpolated if the number ofneighboring missing data was less than three. The
missing data with gaps of more than two grid points
were extrapolated by using the regression line andthe correlation coefficient between the time series ofthis grid point and the adjoining grid points. The
1950-1989 data were used for analyses because anumber of missing data were noticed before 1950.

Societyof JapanVol. 71, No. 2

The Southern Oscillation index (SOI). The normalized sea-level pressure difference between themaritime continent (Darwin) and the central tropical Pacific (Tahiti) was used. "Monthly Climatic
Data for the World", published by the National
Oceanic and Atmospheric Administration Environmental Data Service of the USA during the period
from January 1950 to June 1989, was the original
data source.
The wind stress (WS) data. These data, which
are based on ship reports from the National Climatic Data Center, were produced at Florida State
University from January 1961 through June 1989
by the month. It covers the area over the tropical

Pacific(30°N‑30°S,

120°E‑70°W)

with

2° ×2°

lat. -lon. grids. These data do not have any missing
values, but the qualityof some data for 1988 and1989 is questionable,
since they are produced
fromthe "quick-look"
product.
3. Classification
FigureSST

inof ENSO

1 shows
thethe time-longitude

equatorial

Pacific

cross section(5°N‑5°S)

from

of

Jan‑

uary 1950 to February
1989.
This figure displays
the different features of dominant
fluctuationof SST
from region to region in the equatorial
Pacific.
Inthe eastern Pacific, the annual cycle is evident.
The
SST rises from northernsummerto northern
winter and falls from the northern
winter to the next
northernsummer.
Hereafter,
we adopt the seasonsof the northernhemisphere.
In the central
Pacific,
the interannual
variability
is prominent,
so that thewarm water in the west spreads
eastward
and retreats westward.
In the western region, SST is usually

more

than

28.5℃

throughout

the

year.

The oceanographical component of ENSO,
namely the El Nino event, is defined as the positive
large anomaly in the interannual fluctuation which
domains in the central equatorial Pacific (e.g. Ramage, 1975; Weare, 1976; Wyrtki, 1975, 1985). The
Southern Oscillation is the most representative meteorological component of ENSO. This phenomenon
is strongly coupled with the large-scale interannual
fluctuation of SST mentioned above (e.g. Bjerknes,
1966). Hereafter, we observed the interannual fluctuations of SST in the central equatorial Pacific and
SOI as indicators of ENSO.
Looking at this figure more carefully,we note thatthe warm water expansion from the west, for example around 1987,obstructed the cold SST expansion
from the east over two successive years. A similarcase is found in the '68 ENSO. On the other hand,in the '82 ENSO, the largest one in this century,
the warm SST from the west migrated only for one
year, though it spread to the farthest east to reachthe Peruvian coast. In this case, followingthe cold
SST, expansion from the east was restored normally.
A similar case is noted in the '72 ENSO. Hence, weAprilT. Tomita

1993

and

275

T. Yasunaridefined by van Loon and Madden (1981), Rasumusson and Carpenter (1983), Trenberth (1984), Quinn(1987) and others.
By examining the time series of Fig. 2a, we defined the timings of the occurrence and terminationof ENSO, when the anomalies turned over from negative to positive and from positive to negative, respectively. The duration of ENSO was, then, deduced as a period between the occurrence and thetermination. We also referred to the time seriesof Fig. 2b as support for the definition. Then we
considered the fact that the dispersion of Fig. 2b in
the high-period band is larger than that of Fig. 2a,though the correlation coefficientbetween the twotime series is 0.81.
It is noteworthy
to state that ENSO events are
identifiable
as two types, i. e., the longer one, withtwo maximum
peaks, and the shorter one with one
maximumpeak.
The longer events correspondto'57, '68, '76, and '86 ENSOs.
The '57 ENSO
seems
to be the exception,
since this event does not have
two clear maximum
peaks in SST. However, we estimatedthe event as the longer type because of its
long duration.
The shorter events are '51, '53, '63,
'65
, '72, and '82 ENSOs.
We did not regard '51 and'53 ENSOs
as one event from Fig .2b, though theseparationFig.

1. Time-longitude
along

the

equatorFebruary
1.5℃

sectionofJanuary1950

tointerval

is

1989.

The

contour

the

areas

more

and

28.5℃

cross
from

(25.5℃)are(less)

lightlySST

than(darkly)

hatched.

classified ENSO events based upon these contrasting features,
i. e. the durationof events by using thetime series of SST in the central equatorial
Pacific.
Figure 2 shows the time series of interannual
fluctuationsof SOI and SST in the central
equatorial
Pacific(5°N‑5°S,

160°E‑120°W)

from

1950

to

1989.

Here SOI was multiplied
by a factor of -1 to make it
agree in sign with SST. The interannual
variability
related to ENSO stands out in the central equatorial
Pacific, as shown in Fig. 1. In this study, ENSO was
definedas the simultaneous
swing phase with positive

anomalies

of

each

time

seriesexceeding+1σ(the standard deviation). The following 10 ENSO
events were identified as shown with the year of theoccurrence, i.e. 1951, '53, '57, '63, '65, '68, '72, '76,
'82

, and

'86.

These

years

were

identical

tois subtle in Fig. 2a.

It should be noted that the durationof the negative anomaly
before and after each ENSO seems to
be closely associatedwith the durationof the ENSO.
For example,
the preceding
negativephases before
the longer ENSO events are longer than those before
the shorter
ones.
Hence we call the longer one an
LF-ENSO,
which contains
the low frequency
oscillation, while the shorter one is termed a BO-ENSO,
which demonstrates
the biennial nature of the swing.
These features are more quantitatively
discussedinthe following.

those

Table 1 shows the seasons of occurrence and termination, the durations of each type, and the duration of the preceding negative anomalies. The duration of an LF-ENSO is significantlylonger than thatof a BO-ENSO. An LF-ENSO terminates two years
after the occurrence, while a BO-ENSO terminatesin the year followingthe occurrence. The mean durations of an LF-ENSO and the preceding negative
phase are 26 and 31 months, while that of a BOENSO and its preceding negative phase are 17 and13 months. The total duration of LF-ENSOs andtheir preceding negative phases (57 months) is about
twice as long as that of BO-ENSOsand their preceding negative phases (30 months). The differencesof
mean durations between the two types of ENSO arestatistically significantwith a 99% confidentlevel ormore. The differencebetween the mean durations ofnegative phases followedby the two types, however,is not significant.
The seasons of initiation and termination of each276

Journal

Fig.

2.

Time

multiplied
each

line

seriesofnormalized

by a factorwassmoothedof -1of theanomalies

to makeby theMeteorologicalof

it agreeweighted(a)SST

in signrunning

Society(5°N‑5°S,

with

mean.

SST.
These

of Japan

160°E‑120°W)

Thelong-term

weights

were

Vol.

71, No. 2

and

(b)

SOI.

SOI

was

trend

was

removed,andcalculated

froma normaldistribution.

Table 1. The seasonal timing of the initiation and termination of a positive phase and preceding negative
phase, and the duration. The unit of duration is one month. Y(0) indicates the year that the ENSO
occurred.

type are concentrated to winter through summer foralmost all events (cf. Philander et al., 1984). In particular, strong phase preferences to spring are notedat the initiation of the BO-ENSO and at the termination of LF-ENSO. The seasons of the terminationof the LF-ENSO are fixed, though the durations aredifferent by about one year from the durations of
the BO-ENSO. The seasons of the initiation of thepreceding negative anomalies are also concentrated
to winter through summer.
Figure 3 shows composite time series of the two
types of ENSO in the time series of Fig. 2. Note thehighly coherent fluctuation between SST and SOI.
This suggests that the oceanographical and atmospheric components of ENSO evolve coherently on
the interannual time scale through the air-sea interaction. It should be emphasized that the BOENSO (Fig. 3a) is associated with the biennial-type
oscillation from the preceding negative phase to thenegative phase after the event. The LF-ENSO, incontrast, seems to occur embedded in the low frequency oscillation of longer time scale from the pre-cedingresults
ENSO.

negativephase, as shown in Fig. 3b. These
give us ample evidence
for the two types of

4. Evolutionof thetwo

typesof ENSO

In order to reveal the differences between the spatial and time evolutions of the two types of ENSO,
we examine the seasonal (winter and summer) composites of SST and WS anomalies in the tropical
Pacific(30°N‑30°S,

120°E‑80°W).

Rasumusson

andCarpenter (1982) illustrated similar composite diagrams of unclassified ENSO, which included the
6 cases of '51, '53, '57, '65, '68, and '72 ENSOs.
However,since our purpose is to see the differences
between the two types of ENSO, it is pointless tocompare with the result of Rasumusson and Carpenter (1982), which treats the two types combined
together. In this section, we show the composite
evolution of the two types of ENSO.
Figure 4 shows the spatial evolution of the BOENSO from the winter (WINTER (0) in Fig. 4)
immediately before the occurrence to the summer

AprilT. Tomita

1993

and

277

T. Yasunari

MER (0) and WINTER (1), however, are found.
One is the diminution of the warm region off thewest coast of South America and the other isthe meridional expansion of the warm region near
160°W.

Corresponding

to

this

expansion,

the

con‑

vergencein the central equatorial
Pacific enlarged.
The north or northeasterly
anomalous
flows on thenorthcentralPacific(180‑160°W,

5°N‑25°N)

arealso strengthened. The other is the remarkablechange of anomalous flows from SUMMER (0) to
WINTER (1) which occurs in the western tropical
Pacific. The large westerly anomalous flows appeared before summer (SUMMER (0)), but turnnortheasterly and a large clockwiseeddy whose center

Fig. 3. The composite diagrams of (a) a BOENSO and (b) an LF-ENSO using the timeseries of Fig. 2. Solid (dashed) line indicates

the

mean

SST

(‑1×SOI).

(SUMMER (.1)in Fig. 4) in the next year. This period includes the life of the BO-ENSO.
In WINTER (0) (Fig. 4a), large positive anomalies of SST (SSTAs) do not appear. The strong
WS anomalies (WSAs), however, have southwesterlycomponents

140°W,
inthein the5°N‑25°N)
easterncentralandtropicalnorthPacific

northwesterly
Pacific(160°E‑

components(130°W‑80°W,

In SUMMER (0) (Fig. 4b), large positive SSTAs
appear in the central through eastern equatorial Pacific in the shape of a tongue. To the west of the dateline, particularly to the north of Australia, large negative SSTAs are noticeable. The large zonal gradientof SSTAs in the central through western tropical Pacific seem to be coupled with the large WSAs in thewestern tropical Pacific. Convergence is found in theequatorial

This patternactivities

Pacific

represents

relating

tothe(180‑150°W,

10°N‑10°S).

the expansionof convective

eastward

spreadof 28℃at

140°E,

10°N

appears(cf. Kutsuwada,

In WINTER (0) (Fig. 5a), large negative SSTAs
appeared in the central through eastern equatorial

EQ‑15°S).

centrallocates1991). This eddy seems to contribute to the termination of the BO-ENSO through the modulationof the oceanic "Mindanao Dome" (Masumoto andYamagata, 1991). Positive SSTAs appear off thesoutheast coast of the Eurasian Continent and northof Australia (cf. Hanawa et al., 1989). This patternsuggests that the eddy depends on the heat contrast between the western tropical Pacific and either
the Eurasian continent or the east equatorial Indian
ocean.
In SUMMER (1) (Fig. 4d), the positive SSTAs
in the central through eastern tropical Pacific have
obscured and have even reversed their signs. The
northeasterly WSAs in the tropical western Pacificdo not change, though the clockwiseeddy does notappear clearly. In the region of the negative SSTAs,
weak divergence appears, especially in the central
equatorial Pacific.
Figure 5 shows the spatial evolution of seasonal
anomalies of SST and WS for the LF-ENSO. The
duration is from the winter (WINTER (0)) immediately before the occurrence to the summer (SUMMER (2)) two years after the occurrence. This duration includes the life of the LF-ENSO.
Pacific(170°W‑110°W,

10°N‑10°S).

Aregionlikethis does not appear in WINTER (0) of the BOENSO (Fig. 4a), though there is a possibility thatthe difference depends on the time lag of about one
season between the occurrences of the two types of
ENSO (see Table 2). To the north and south of
this region, the northern or southern WSAs appear,
respectively.

In SUMMER (0) (Fig. 5b), the region of the positive SSTAs of ENSO appears around the equatororwarmer water in the west (cf. Gadgil et al., 1984;
Graham and Barnett, 1987).
In WINTER (1) (Fig. 4c), which corresponds tothe mature phase of the BO-ENSO, the large positive SSTAs still persist with a similar pattern to
SUMMER (0). Two differences between SUM-

to

the

east

of 160°W,

but

the

regionis smallerthanthat for the BO-ENSO (Fig. 4b). Furthermore, theSSTAs around the maritime continent are slightly
positive. The zonal gradient of SSTAs in the western tropical Pacific is smaller than that of Fig. 4b.
The westerly WSAs in the western equatorial Pacificseem to be small for this reason. The positive SSTAs

278

Journalof theMeteorologicalSocietyof JapanVol. 71, No. 2

Fig. 4. Evolution of the seasonal composites (winter and summer) of SST and WS anomalies for a BOENSO.

The

contourinterval

is

0.5℃.

Areas

over

0.5℃

(under

‑0.5℃)

areshadedlightly(darkly).

The arrows indicate the direction and the relative size of the normalized WS anomalies. WINTER (0)indicates the winter immediately before the occurrence of a BO-ENSO.
in theregionstrikingly

around145°W‑125°W,

20°N‑35°N

aredifferent.

In WINTER (1) (Fig. 5c), the region of the positive SSTAs in the central through eastern tropical Pacific expand westward and to higher latitudes.
The positive SSTAs around the maritime continent
have changed to negative from SUMMER (0), andthe westerly WSAs have appeared over the westernequatorial Pacific. The large clockwiseeddy over thewestern Pacific shown in WINTER (1) of BO-ENSO(Fig. 4c) has not appeared.
In SUMMER (1) (Fig. 5d), which correspondswith the terminal season of the BO-ENSO, the positive SSTAs are still as large as in WINTER (1) andexpandfurther

to

the

west

by

about

20°

of

longi‑

tude. However, the region of positive SSTAs off thewest coast of South and Central America diminishes.
The anomalous flow toward the equator is strengthened over the western through central tropical Pacific, and the westerly WSAs shown in WINTER (1)(Fig. 5c) on the western tropical Pacific disappear.

In WINTER (2) (Fig. 5e), the positive SSTAs
spread from the central tropical Pacific to the west
coast of North America. The negative SSTAs stayat almost the same location in the north and south
at latitude 20 degrees since WINTER (1). The large
positive SSTAs in the eastern equatorial Pacific disappear completely. The northwesterly WSAs haveappeared again in the western equatorial Pacific,
and southeasterly WSAs are shown in the easternequatorial Pacific. These anomalous flows turned to
the region of the positive SSTAsin the central tropical Pacific. The anomalous flows to the equator
shown in SUMMER (1) (Fig. 5d) were weakened.
In SUMMER (2) (Fig. 5f), large negative SSTAs
are shown in the easternequatorial
Pacific.
Theslight positive SSTAs appeared
widely in the rest of
the tropical
Pacific.
This figure shows the state after the terminationof the LF-ENSO.
In the central
equatorial
Pacific,
an obvious divergenceis foundover the negative
SSTAs.
WSAs
on the western tropical
Pacific have turned northeasterlyfromApril

1993

T. Tomita

and

279

T. YasunariFig. 4. (Continued)

northwesterly. This change of the direction seems
to terminate the LF-ENSO, though the clockwise
eddy does not appear in WINTER (2) in the western tropical Pacific.
The fundamental differencebetween the two typesof ENSO is summarized as follows. The clockwise
eddy which appears in the western tropical Pacificin WINTER (1) of BO-ENSO is not found in thatof the LF-ENSO. This eddy is likely to let the surrounding warm water converge under it through Ekman pumping, and affects greatly the ENSO termination. In the LF-ENSO, with no such eddy, the
ENSO state remains unchanged and a warm-water
anomaly persists centered in the central equatorial
Pacific. This state continues into the next year, but
the clockwiseeddy, which appeared in WINTER (1)immediately before the BO-ENSO termination, is
not as clear in WINTER (2) as in WINTER (1) of
the LF-ENSO.
It is also clear that the negative
SSTAs spreadin the central and the easternequatorial
Pacific are
more specific before and after the LF-ENSO
than forthe BO-ENSO.
This fact implies a relation with thelong-duration
negativeanomaly
precedingthe LF-

ENSO (see Fig. 3), but further studies are needed.
5. Summary

anddiscussion

This study classifies ENSO events into two types
with respect to their duration,and specifies the differences between
the two types of ENSO.

First, we select ENSO events in the time series
anomalies of SST in the central equatorial Pacificand SOI, with the criterion whether the positive
anomalies of the two time series exceed concurrently
the standard deviation or not. With this criterion,
10 ENSO events are selected during the period from1950 to 1988. These occurrence years are 1951, '53,
'57

, '63, '65, '68, '72, '76, '82, and'86.

The selected
ENSO events are classified into two
types according
to the durationof the event. We defined the durationby the continuous
period of thepositiveanomaliesin the two time series used forthe selectionof ENSO events.
10 ENSO events after 1950 are classified into two types: one is the biennial oscillatory
event, namely the BO-ENSO,
andthe other is the lower
LF-ENSO.
The average duration

frequency

event,

of a BO-ENSO

thatis theis about

17

280

Journalof theMeteorologicalSocietyof JapanVol.

71, No.

2

Fig. 5. Evolution of the seasonal composites (winter and summer) of SST and WS anomalies for an LFENSO.

The

contourinterval

is

0.5℃.

Areas

over

0.5℃

(under

‑0

.5℃)

are

shadedlightly(darkly).

The arrows indicate the direction and the relative size of the normalized WS anomalies . WINTER (0)indicates the winter immediately before the occurrence of an LF-ENSO .
monthsin thewith one northern
year following
thewinter,andoccurrence.

terminates
The yearsof occurrence
are 1951, '53, '63, '65, '72, and '82.
The average
durationof an LF-ENSO
is about
26
monthswith two northern
winters,
and terminatesin the two years after the occurrence.
The occurrence years of the LF-ENSO
are 1957, '68, '76, and'86. The numberof BO‑ENSOs
is six and that of
LF-ENSOs
is four, among the 10 ENSO events. The
difference
between the average durationsof the two
types of ENSO is statisticallysignificant
with a 99% confident level or more. The significant difference
betweenthe mean durationsof negative
anomalies
precedingthe two types of ENSO
is also noticed,
i. e., about 13 months for the BO-ENSO,
and about
31 months
for the LF-ENSO.
The total mean duration of the negative
phase before the LF-ENSO
to the following positive phase is about 57 months,
which is almost
two times longer than that for a
BO-ENSO,i. e., about 30 months.
Althoughthe duration of an ENSO differs significantly
between the

two types, both ENSOs tend to initiate and terminate with seasonal phase preference,that is, mostly
in the northern
winter throughnorthernsummer.
By applying
a composite
analysis,the differencesof the time and spatial
evolutionsof the two typesof ENSO are elucidated
as follows: Firstly, the positive SST anomalies
in the central
througheasternequatorial
Pacific
in the LF-ENSO,
rial Pacific, while
disappearby thecurrenceyear.

appear for three successive
years
especiallyin the central
equatoin the BO-ENSO
these anomalies
next northernsummerof the oc-

Secondly,in the northern winter of a BO-ENSO,
that is, during the mature phase of a BO-ENSO,
a clockwiseeddy of wind-stress anomalies appears
in the western tropical Pacific, and in the following
northern summer, easterly anomalies prevail in thewestern equatorial Pacific (cf. Kutsuwada, 1991).
No eddy appears, on the other hand, in the corresponding northern winter of an LF-ENSO immediately after the occurrence. The easterly anoma-April

1993

T. Tomita

andT. Yasunari281

Fig. 5. (Continued)

lies are found in the northern summer two years
after the occurrence, although the clockwiseeddy
does not appear in the northern winter before. The
eddy seems to be the most significant phenomenon
that characterizes a BO-ENSO. This clockwiseeddyof wind-stress anomalies affects the ocean with themodulation of the oceanic "Mindanao Dome" in thewestern tropical Pacific and plays an important rolein an ENSOtermination (Masumotoand Yamagata,1991). In other words, the eddy steadily restores thewarm water spread over the western tropical Pacificby Ekman pumping. The observations of wind fieldsuggest that the eddy plays a major role in keeping the biennial oscillation constant, and also suggest that this eddy occurrence is closely related tothe anomalous north-south
flow off the east coastof the Eurasian Continent in the northern winter.
The Asian monsoon system, that is, the air-sealand system seems to control the termination of an
ENSO through this anomalous flow strength, whichis strongly affected by the pressure gradient between
the Siberian high and the Aleutian low and thesecenter positions. The Asian winter as well as sum-mer monsoon activity is, therefore, suggested to beimportant for ENSOs (cf. Yasunari, 1990, 1992).
Thirdly, before and after an LF-ENSO, the negative SST anomalies in the central through the eastern equatorial Pacific appear more prominentlythan
those before and after a BO-ENSO. The time series
analysis clarified that the negative phase before an
LF-ENSO is longer than that before a BO-ENSO.
Regarding the occurrence of each type of ENSO,
the results suggest the importance of the condition
before the occurrence, that is, the depth and distribution of the oceanic mixing layer related to theheat content in the western tropical Pacific (cf. Masumoto and Yamagata, 1991).
The two types of ENSO classified in this study
may be composed into the two types of ENSO suggested by Lau and Sheu (1988) in the conceptionalmodel: one is the two-year cycle, and the other isthe four year cycle. The fact that, within the timeseries of real ENSO factors such as SST or SOI,
the two types of ENSO exist together, and that
ENSOevent can be classifiedclearly into either type,shows that correlation of the air-sea interaction is in282

Journalof theMeteorologicalSocietyof JapanVol.

71, No. 2

Fig. 5. (Continued)

the ENSO/monsoon system, sometimes positive andsometimes negative. This study strongly suggeststhat the clockwiseeddy in the wind-stress anomaly
field over the western tropical Pacific plays the roleof a switch of this correlation, and that this switch
changes the anomalous condition in the equatorial
Pacific, which controls the termination or continuation of an ENSO event.
In the future, these results must be further evaluated quantitatively, with observational or modelstudies. The scientific results of TOGA (tropicaloceans and global atmosphere) are expected to give
us further evidence and new ideas about the two
types of ENSO.

the equatorial
172.
Enfield,

D. B.

changesT. P.,

1991:

in thetropicalscalesThe

interaction
climateof multiple

system.

J. Climate,

time
4,

269-285.

Bjerknes, J., 1966: A possible response of the atomospheric Hadley circulation to the equatorial anomalies of ocean temperature. Tellus, 18, 820-829.
Bjerknes,J., 1969:

Atmosphericteleconnections

from

L.

Cid

S.,

Wea.

Rev.,

1991:

Low-frequency

Oscillation.

97,

163-

J. Climate,4, 1137-1146.
Fu, C., H. F. Diaz

andJ. O. Fletcher,

1986:

Characteris-tics of the responseof sea surface
temperature
centralPacific associatedwith warm episodes

in theof theSouthern
1738.

1716-

Gadgil,Oscillation.

S.,

P. V.

atmosphere
312,
Graham,

Mon.

Wea.

Rev.,

Josephand

N. V. Joshi,

coupling

overmonsoonN. E. andT. P. Barnett,

1987:

N. E.

and

W. B. White,AnaturaloscillatorofScience,240,

atmospheresystem.

M. S. andperaturecillation.
Hanawa,1984:

Ocean-

regions.

Nature,Sea surface

perature,surfacewind divergence,over tropicaloceans.
Science,
238,

Halpert,114,

141-143.

cycle:
Barnett,

and

Mon.

in El Nino-Southern

Graham,References

Pacific.

1988:

The

El Ninothe

Pacific

ocean-

C. F. Ropelewski,

1293-1302.
1992:

Surfacetem-theSouthernOs-patterns
associatedwith
J. Climate,
5, 577-593.
K.,Y.

YoshikawaCompositeanalyses

fieldsrespect

with

and

T.

of wintertime
to SSTtem-and convection
657-659.

anomalies

Watanabe,
wind

1989:

stressvectorin thewesternApril

1993

north

T. Tomita

Pacific

composite.

and

the

ENSO

J. Meteor.

eventsSoc.

Part

Japan,

67,

andII. ENSO

833-845.

Kutsuwada, K., 1991: Quasi-periodic variabilities ofwind-stress fields over the Pacific ocean related to
ENSO events. J. Meteor. Soc. Japan, 69. 687-700.
Lau,

K. -M.

andP. J. Sheu,

1988:

biennial

oscillation,

andglobal10988.

precipitation.

J.

Lough,

AnnualH. G. Fritts,

1985:

reings,1600-1961.

Meteor.,

tree

24,

of themonsoon:tropicalThe

J. Phys.

generation

Oceanogr.,

Philander,

21,

Trenberth,

ern Oscillation.
van Loon, H. andTheto theAsianandresponse

Mindanao

C. S., 1975:

rologySoc.,of the56,

Preliminary

1972-73

El Nino.

Bull.

Sinica,6, 47-57.

relationshipandbetweenT. H.

Carpenter,easternequatorialsurfacetempertureandrainfalloverLanka.

Mon.

Rev.,

111,

517-528.

Rasumusson,

Wea.

E. M., X. Wang

B. C., A. R. Navato
orthogonal

of

600

with
Mon.

presWea.

El

Nino

event

Acta

and R. E. Newell,

1976:

Em-

sea surface

tem-of Pacificpropagating

Rosbbywaves

incal pacificduring1982-83and

1986-87

events.
the

J. Phys.

Oceanogr.,

1975:

El Nino-TheK.,

equatorial

J. Phys.
Wyrtki,

Pacific

Oceanogr.,

K.,

1985:

and thegenesis

90,

the

19,

the

tropiENSO

1397-1406.
dynamic

ocean

Me-

6, 671-678.
1989: Off-equatorial

westward

Wyrtki,South-

period.

analysisyear

peratures.
J. Phys. Oceanogr.,
White,
B. W. Y. He and S. E. Pazan,

Meteor.

234-242.

E. M.

Reconstruction

teorologicaJ.

in the1150-1162.
1992:

Dome.

piricalnoise

Globalassociations
in northern
winter.

last

Rasumusson, E. M. and T. H. Carpenter, 1982: Variations in tropical sea surface temperature and surface wind fields associated with the Southern Oscillation/El Nino. Mon. Wea. Rev., 110, 354-384.
Rasumusson,

S.,versusfor theof meteo-Amer.

SignalMon.
Wea. Rev., 112, 326-332.
R. A. Madden,
1981:
The Southernchronology

tropics.

discussion

1984:

winter

Weare,
in the

109,

Wang,

Quinn, W. H., D. O. Zopf, K. S. Short and
R. J. W. K. Yang, 1978: Histrical trends and statisticsof the Southern Oscillation, El Nino, and Indonesiandroughts. Fish. Bull., 76, 663-678.
Quinn, W. H. and V. T. Neal, 1987: El Nino occurrences
over the past four and a half centuries. J. Geophys.
Res., 92, 14449-14461.
Ramage,K. E.,

Oscillation.
Part
1:
sure and temperature

R. C. Pacanowski,

1984: Unstableair-sea interactions
Atmos.
Sci., 41, 604-613.

J.

Appl.

1386-1398.

S. G. H., T. Yamagata

variability.

Oscil-

1991:of theENSO

Southern

in

Rev.,

PacificofJ. Climate

The69, 197-207.
and T. Yamagata,

component
1, 71-96.

Thompson, L.G. et al., 1984: El Nino-Southern Oscillation events recorded in the stratigraphy of the tropical Quelccaya ice cap, Peru. Science, 226, 50-53.

Res.,

952-966.

western

Sci.,

Ropelewski, C.F. and M.S. Halpert, 1987: Global
and regional scale precipitation associated with El
Nino/Southern Oscillation. Mon. Wea. Rev., 115,
1606-1626.

Masumoto,
Y. and T. Yamagata,
1991:
On the originof a model ENSO
in the western
pacific.
J. Meteor.
Soc. Japan,
Masumoto,Y.

biennialMar.

10975-

Oscillation

Geophys.

andquasi-

The

93,

Southern

J. M. andlationcycle,

283

T. Yasunari

response

to atmosphericofforcing.

5, 572-584.

Waterdisplacementsof El Nino

cycles.

J.

in thePacific

Geophys.

Res.,

7129-7132.

Yasunari,

T.,

1985:

the

grobaleast-west

ZonallySouthern
1013-1029.

Osillation.

propagating

circullation
J.

modes

of

with

the

Japan,

63,

associatedMeteor.

Soc.

1983:

The

Pacific

sea

Yasunari, T., 1990: Impact of Indian monsoon on the
coupled atmosphere/ocean system in the tropical Pacific. Meteor. & Atmos. Phys., 44, 29-41.

Sri

Yasunari,

India

and

T.,

Interannual

and C. F. Ropelewski,

1990:

tem.

1992:

J. Meteor.

2つ の タ イ プ のENSOに

Role

Variability
Soc.

of theAsianof theJapan,

70,

MonsoonGlobalon theClimateSys-

177-189.

つ いて

冨 田智彦 ・安成 哲三(筑波 大 学地球 科 学 系)

1950年 以 降 の10例のENSO(エル ニ ー ニ ョ/南 方 振 動)に

つ い て 、期 間 に 着 目 した分 類 を試 み た 。ENSO

は 、 期 間 に着 目す る と、 発 生 年 の 翌 年 に終 息 す る タ イ プ(BO‑ENSO)とプ(LF‑ENSO)の2つ

そ の さ らに 翌 年 まで 継 続 す る タ イの タ イ プ に 分 類 で き る 。 そ して ど ち ら の タ イ プ のENSOに

つ いて もその発 生、終 息の 時 期 は 、 北 半 球 の 冬 か ら夏 に か け て の 季 節 に集 中 して い る。 発 生 年 で 両 者 を示 す とBO‑ENSOは、1951,'53,'63,'65,'72,'82年の6例 、LF‑ENSOは、1957,'68,'76,'86年の4例 で あ っ た 。 次 に両 者 の 発 生 、 発
達 、 終 息 に は 明 瞭 な 季 節 固 定 が あ る こ と よ り、 合 成 解 析 を行 い 両 者 の 違 い を 明 ら か に した 。 風 応 力 の 偏 差284

Journal

場 に お い てBO‑ENSOのLF‑ENSOのof theMeteorologicalSocietyof JapanVol. 71, No. 2

発 生 直 後 の 冬 季 、 フ ィ リ ピ ン 東 岸 沖 に 時 計 回 りの 明 瞭 な 渦 が 見 られ る。 対 応 す る

発 生 直 後 の 冬 季 に は 、 こ の 様 な 渦 は 出 現 して い な い 。 この 渦 は 、EkmanPumpingを

洋 に作 用 しENSOの

終 息 に 大 き く寄 与 して い る と考 え られ る。BO‑ENSOとLF‑ENSOを

つ と し て こ の 渦 の 出現 の 有 無 を あ げ る 。

介 して 海分 け る要 因 の 一