February

1987

T. Yasunari

81

Global Structure of the El Nino/Southern Oscillation
Part II. Time Evolution

By Tetsuzo

Yasunari*

Department of Meteorology, Florida State University, Tallahassee, FL32306(Manuscript received 19 August 1986, in revised form 19 December 1986)

Abstract
Time evolutions of anomalies of SST and atmosphericparameters
over the global domain were
investigated
for the several ENSO cycles during the period 1964-79, using the data set described inPart I. The principal results are summarized as follows:
1) the eastward propagationof SLP and zonal wind anomalies
from the Indian Ocean toward theeastern Pacific is a fundamental
nature of ENSO along the tropics and the southern subtropics.
2) the significant anomalies of SLP and circulation
field which first appear over the Indian Ocean
seem to originate from central Asia or Eurasia.
3) the verticalstructuresof these anomalies

over central Asia at the intermediatestages of the cycle(i.e., about one year before the SST maximum (minimum)over the equatorial Pacific) are suggested to represent large-scale cold (warm) air outbreak associated with more (less) than normalsnow cover over there.
4) a time-lag teleconnection
between the north Pacific and central Asia is found in the circulation
field, which seems to have a key role on the mechanism of ENSO cycle.
The first two results have substantiallyconfirmed the evidences of ENSO signals from Eurasia
and the Indian Ocean toward the equatorial Pacific, which were noted in the surface fields by Barnett(1984,1985a)
and Krishnamurti
et al. (1986).
The third result suggests the importantrole of Asian monsoon as a connector of the process inthe extra-tropics
and that in the equatorial Pacific. The fourth result seems to be associated with theinteraction
between the PNA and the EU (Eurasian) pattern with a time lag of half a year or more.
The links between the tropics and the extra-tropics
and between the ocean and the continentdescribed above strongly suggest that the ENSO should be understoodas a global scale land (cryosphere)-atmosphere-ocean
coupled system rather than an atmosphere-ocean
coupled system over theequatorial Pacific.

1. Introduction
In the earlier study (Yasunari, 1987: hereafter referred to as Part I) we first demonstrated
the objective analysis scheme for time-filteredstation data by using the least-square method(Pan, 1979). We then investigated the global
distributions of anomalies in the sea surface
temperature (SST) and the atmospheric parameters for the El Nino phases by compositing thefour events. The anomalies over and around the*Present

Affiliation:of Tsukuba,

Ibaraki

1987, Meteorological

Instituteof Geoscience

305, Japan.
Society*of Japan, University

tropical Pacific region proved to be consistent
with the previous observational studies (Arkin,
1982; Pan and Oort, 1983 etc.). It has also been
noted that the atmospheric as well as the oceanic
anomalies associated with the El Nino event are
not confined within the tropical Pacific but are
distributed over the global domain.
The current paper will examine the time evolution of the whole ENSO cycle (i.e., from the
El Nino to anti-El Nino and again from anti-E1
Nino to El Nino) by compositing the time-filteredanomalies of the four or five ENSO cycles. A
possible physical process for the ENSO cycle will
finally be discussed.

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Journalof the Meteorological

Methods
As described in section 4 of Part I, the timefiltered anomalies of SST and objectively analyzed atmospheric parameters (zonal and meridional
wind, temperature, geopotential height) in the
troposphere and the sea level pressure (SLP) are
categorized to 8 phases for each ENSO cycle
during 1964 through 1979 in reference to thesmoothed Southern Oscillation Index (SOI)
values. Category 1 (5) denotes the maximum(minimum) SOI phase, which corresponds to theanti-El Nino (El Nino) period. Similarly, category
3 (7) corresponds to the intermediate stage fromthe anti-El Nino (El Nino) to the El Nino (anti-El
Nino) period. One category of each ENSO cycle
has a 5 to 6 months time length. To scrutinize
the circulation, temperature and height anomalies over the whole global domain associatedwith the ENSO cycle, composite anomaly fields
for each category are produced, by averagingthe
four (or five) cases during the analysis period(1964-79). Longitude-time sections of the filtered data are also produced to examine the stationarity or non-stationarity of the anomalies foreach ENSO cycle.

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2.

3.

Evolution of SST

It has been already noted that the positive
SST anomalies appear nearly simultaneously inthe whole tropics except the equatorial western
Pacific during the El Nino events (refer to Fig. 6of Part I). This is clearly shown in the longitudetime section of filtered SST along the equator(Fig. 1). Although the amplitudes of anomalies
in the Indian Ocean and the Atlantic are far
smaller than that in the eastern Pacific, thephases of the maximum (minimum) anomalies
appear nearly in phase with (or slightly lag
behind) the eastern Pacific. These anomalies havebeen interpreted as responses to anomalous circulation associated with ENSO (Reiter, 1983).
In the central through eastern Pacific, SST seems
to change with a slight westward component, but
this may be due to the combined effect of large
amplitudes of the seasonal cycle in the eastern
Pacific and the strong east-west gradient in themean seasonal SST distribution. The actual
expansion of warm SST area associated withmajor El Nino is mostly eastward (Gill andFig. 1. Longitude
time sectionof filteredsea
surface temperature(SST) anomalies along theequator(7.5*N-7.5*S).
Units are 0.2*
andnegativevaluesare shaded.
Hatchedarea(100*E-150*E)
denotes Indonesian
maritime
continent
region. The areas of black colors
denotes the continents.
El Nino phases are also
indicated with "EN".

Rassmusson,1983).
The cold SST anomaly in the northern Pacific is another dominant feature during El Nino(see Fig. 6 of Part I). To examine the associationof the anomalies there with those in the equatorial Pacific in time sequence, the latitude-time
section of the filtered anomalies in the central
Pacific (160*E-130*W) is produced (Fig. 2). A
standing-type north-south oscillation is predominant in the middle and low latitudes (40*N40*S) with out of phase relation between theequator and the northern (and the southern)
mid-latitudes. This has been already noted in theearlier studies (Hsiung and Newell, 1983 etc.). It
is worth noting, however, that particularly inthe northern latitudes there seems to be sometime lag between the maximum (minimum) SST
in the equator and the minimum (maximum)
SST in the mid-latitudes (30*N-40*N) with one
season or so. The positive correlation between

February

1987

Fig. 2.

T. Yasunari

Latitude

Units are 0.2*

time sectionof filtered

SST anomalies

and negative values are shaded.

the northern high latitudes (50*N-60*N) andthe equator with a phase lag of about one year
is also noticeable.
The anomalies at mid-latitudes have been
postulated as a result of atmospheric anomalies
which may be a response of the equatorial heating anomalies (Newell et al., 1982). The anomalies at higher latitudes (50*N-60*N) may berelated to warmer (colder) air advection associated with more cyclonic (anticyclonic) condition at mid-latitudes, as Niebauer (1986)
pointed out. He showed that the maximum SST
minimum sea ice and maximum air temperature
over Bering Sea lag the minimum SOI with 3 to
4 seasons. The anomalies at higher latitudes(50*N-60*N) in Fig. 2 are consistent with his results. Thus, the SST anomalies in the northern
Pacific seem to be fundamentally explained by
the atmospheric anomalies produced as a linear

Fig. 3.

83

along central

Pacific (160*E-130*W).

El Nino phases are also shown with "EN".

response of the equatorial anomalous heating.
However, a positive feedback of SST anomalies
to circulation anomalies is also possible over thenorth Pacific (Reiter, 1983). We also notice aslow northward propagation of the anomalies
from the equator to the mid-latitudes and asouthward propagation from the subpolar latitudes to the mid-latitudes. These evidences may
suggest that a slow advective process in the ocean
surface layer via the subtropical and the subpolar
gyre is partly responsible for the SST anomalies
in the northern Pacific (Reiter, 1978; Kawamura,1986).
In the western Pacific region (120*E-150*E),
the fluctuation of the SST anomalies in thelatitude-time domain (Fig. 3) appears to be quite
different from that in the central Pacific (Fig. 2).
Although the anomalies at the equator are verysmall compared to the eastern Pacific, those inSame as Fig. 2 but for the western

Pacific (120*E-150*E).

84

Journalof theMeteorological

the subtropics (20*N-40*N) are considerablylarge. It is worth noting that the phases of maximum (or minimum) in the subtropics seem to bein quadrature with those at the equatorial eastern
Pacific. That is, the maximum anomalies there
seem to lag behind the El Nino (the maximum inthe equatorial eastern Pacific) with about a halfor one year. If we note the atmospheric process,a diminished cloud amount over there associatedwith El Nino condition in the western Pacific
may account for the high SST (Newell et al.,1982). If we note the oceanic process, the slow
advection of warm water in the mixed layer via
the north equatorial current may be more
important. Along this line Yamagata et al. (1985)
showed some evidences of lagged relation between the Kuroshio extension and the northequatorial current. In any case, further study isneeded for this problem.
4.

Evolution of atmospheric anomalies

a. SLP
SinceWalker and Bliss (1932), SLP has been
one of the important parameters relevant to theSouthern Oscillation (SO). Recently, Barnett(1985) and Krishnamurti et al. (1986).independently examined the time-evolution of SLP
anomalies associated with the SO over the global
domain, by utilizing different data sets andmethods. Although their emphasis points are
somewhat different each other, they bothclaimed the significantly large anomalies outside
the SO region in a narrow sense (i,e., the Pacificthrough the Indian Ocean in the tropics), which
precede the extreme phases of the ENSO overthe equatorial eastern Pacific. Particularly, theappearance of anomalies in the extratropics ofthe northern and/or the southern mid-latitudes in
the Indian Ocean sector and the eastward propagation of these anomalies toward the Pacificbasin is a prominent common feature noted inthese two papers. Here, we re-examine the global
evolution of SLP anomalies associated with the
ENSO cycle.
Fig. 4 shows the composite SLP anomalies
from category 1 to category 5. As mentioned inPart I, category 1 denotes the maximum SOI(anti-El Nino) phase and category 5 denotes theminimum SOI (El Nino) phase, respectively. At
category 1 the anomalies relevant to the SO (i.e.,

Societyof JapanVol.

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1

positive anomalies over Australia and negative
anomalies over the eastern Pacific) is dominatedin the low latitudes. East- or southeast-ward extension of negative anomalies from Australia to
the southern Pacific may correspond to morethan normal cumulus convection along the ITCZ
in the western Pacific and SPCZ. The area of
positive anomalies in the eastern Pacific is nearly
identical with that of the cold SST anomalies. In
the northern extratropics, the large north-south
anomaly contrast is significant over the western
Europe through the Atlantic. This may be closely
related to the North Atlantic Oscillation (NAO)
as described in Part I. In the southern extratropics, the positive anomalies over the southwestern Pacific and the southern Atlantic near
Antarctica are noted. Since the anomalies described here are not normalized, the amplitudes
are generally large in the extratropics than in thetropics. At category 2 the overall feature is similar to that at category 1, but the east-west
anomaly contrast between Australia and theequatorial eastern Pacific are considerably diminished.
At category 3, which corresponds to theintermediate stage from anti-El Nino to El Ninophase, some drastic changes are apparent especially over Eurasia and the Pacific region. The
east-west anomaly contrast over Australia andthe eastern Pacific are completely diminishedwhile the large positive anomaly area appear overEurasia and the tropical Indian Ocean. In thenorthern high latitudes the anomalies are generally reversed from negative to positive values. In
the southern extratropics, the negative anomalies
over the Pacific subtropics and mid-latitudes andthe small positive anomalies to the south ofAustralia should be noted, which suggest an"infant stage" of the typical weak SO (or El
Nino) phase. At category 4, which roughly correspond to a half year before the mature El Ninophase, the positive anomalies over Eurasia
through the Indian Ocean seem to expand toward Australia, and at the same time the negative
anomalies are developed over the eastern south
Pacific. At this stage, the north-south SLP contrast over the Atlantic through western Europe
is reversed in sign. If we refer to the composite
SST anomaly maps (not shown here), warm SST
anomalies are widely developed over the equato-FebruaryFig. 4.

1987

T. Yasunari

Composite

diagram.

Negative

filteredvaluesanomalies
are shaded.

of SLP for

category

1 to 4. Units

85

are indicatedon the rightupside of each

86

Journalof the Meteorologicalrial central and eastern Pacific. At category 5 (ElNino phase) the overall anomaly pattern becomes
nearly identical to the pattern of category 1, butreversedin sign.
Thus, the anomalies at category 3 and 4
strongly suggest that the El Nino signals first
appear over Eurasia through the Indian Ocean
about one year before the extreme event over theeastern Pacific. Surprisingly, the overall pattern
at category 3 and 4 are very similar to theanomaly patterns of a half- and one-year before
the El Nino of 1972 and 82 produced by Barnett(1985). (Note that one category roughly denotes
half a year as mentioned in Part I), the small
positive anomalies to the south of Australia at

Fig. 5.

Longitude-category

sectionSocietyVol.

65, No.

1

category 3 develop and expand rapidly overAustralia through category 4 and 5. This feature
may agree with the result by van Loon and Shea
(1985), where they noted this phenomenon as aprecursor to the El Nino.
Fig. 5 shows the longitude-time section of thecomposite anomalies of the whole ENSO cycle(category 1 to 8) for the northern and thesouthern subtropics. The SO has been recognized
as a standing-type oscillation along the southern
subtropics (Troup, 1965; Kidson, 1975 etc.).
However, these diagrams show that the eastward
propagation of anomalies from the Indian Ocean
to the Pacific is a salient feature in the southern
subtropics, while the standing-type oscillation be-of filtered SLP anomalies

40*N) and (b) southern
subtropics
Negative values are shaded.

of Japan(20*S-40*S).

Units

for(a)

northern(mb) are shown

subtropics(20*N-

on the right up.

February

1987

T. YasunariFig. 6.

Same

as Fig. 4 but

for

zonal

wind

87

at 200mb.

Units

are 0.3 ms-1.

88

Journalof theMeteorologicaltween the Eurasia-Atlantic region and the Pacificregion is prominent in the northern subtropics.
b. zonal wind
It has been elucidated that the zonal wind
anomalies associated with the ENSO considerablychange in the subtropics and mid-latitudes (refer
to Fig. 8 and 10 in Part I) as well as in the tropics(Barnett, 1984;Yasunari, 1985 etc.).
Fig. 6 shows the composite zonal wind anomalies at 200 mb from category 1 to category 4.
Category 5 (El Nino phase) is nearly the same as
category 1 but with opposite sign (refer to Fig. 6of Part I). At category 1 (anti-El Nino phase)easterly anomalies are dominated in the tropics
and mid-latitudes except the equatorial central
Pacific, while the westerly anomalies are domi-

Fig.

7.
valuesSame

as Fig.

are shaded.

5 but

for

zonal

wind

Societynantof Japanin thestronger

high

than

suggested

latitudes.
normalwithIndonesian

Vol.

the

upwardmaritime

portionoverslightchangeof pattern

At
be

the

anomalies

to be shiftedoverslightlycategorychangedtheequator,portiontheeastern

Pacific.
at category

central3 theanomalypatternseems

the tropi-

anomalies

overnoticeable.

The

is
the

eastern

shiftedeastwardandAmerica.

In themid-latitudes

lies

over

the

are

reduced.

northern
At category

are shownisto

2. The

Ocean

Units

seem

categoryof westerly

at 200mb.

a

2. The

from

cal

over

Only

Pacific

appearance

anomalies

the

down-

eastward.

considerably

Indian

a
is

over

and

is seenthe1

circulation

continent

ward

westerly

Alongeast-west

65, No.

Pacific
extendedover

the easterly

andwesterlyisfurthersouthanoma-

thesouthern

Pacific

4 theanomalypatternon the rightup.

NegativeFebruary

1987

T. Yasunari

over the whole globe is completely changed to
the El Nino pattern (Fig. 8 of Part I). The
westerly anomalies are now dominated over thetropics and the mid-latitudes, and the westerlies
in the polar latitudes at category 1 to 3 are replaced with the easterlies.
The eastward propagation of zonal wind anomalies in the upper and the lower tropospherealong the equatorial belt noted by Yasunari(1985) is confirmed in the longitude-time sectionof the composite anomalies as shown in Fig. 7.
The anomalies at 200mb and 700mb particularly
over the Indian Ocean through the central Pacificconsist of the upper and lower part of the anomalous east-west circulation along the equatorial
plain. The eastward propagation of these cells(e.g., note the zero lines between the easterlies
and the westerlies at 200mb) are nearly consistent with the eastward propagation of SLP anomalies in the southern low latitudes (Fig. 5 (b)).
To examine whether the vertical structure ofanomalies is equivalent barotropic or baroclinic,
the correlation coefficients of zonal wind anomalies between 200mb and 700mb are computedfor every 10* lat. * 30* long, blocked area (Fig.
8). As postulated from Fig. 8, a baroclinic-type
structure (negative correlation) is dominant along
the equatorial belt over the Indian Ocean throughthe central Pacific. It is noteworthy that even in
the tropics baroclinic-type response is not apparent over south America through the Atlantic.
This may suggest that the anomalous diabatic
heating responsible for the ENSO is confinedover the Indian Ocean through the Pacific. The
negative correlations, though not so significant,Fig. 8.

Distribution

are 0.2 (multipliedof correlation

coefficients89

over the northern Atlantic may be related tolarge variability of the upper westerly jet associated with the activity of baroclinic disturbances. The reason for negative correlations overthe southern Indian Ocean is not clear. In otherparts of the world positive correlations (equivalent barotropic-type anomalies) are dominated.
Since the anomalous heating in the tropics
during the El Nino is so global as shown in Fig. l lof Part I, the zonal mean wind change should beconsiderably large associated with the ENSO
cycle. Actually, Rosen et al. (1984) found thatthe total atmospheric angular momentum andlength of day increased during 1982/83 El Nino.
If we refer to the global distribution of zonal
wind anomalies (see Fig. 6), we can postulatethat the general increase of westerly anomaly inthe lower latitudes may contribute the increaseof total westerly angular momentum of theatmosphere. Fig. 9 shows longitude time sectionof zonal mean wind at 200mb and 700mb. A
prominent feature is a poleward propagation of
anomalies, particularly in the northern hemisphere. The westerly (easterly) maximum in thelower latitudes (centered in the subtropics) nearlycorrespond with the El Nino (anti-El Nino) phase.
The westerly (easterly) anomalies at 200mb and700mb are nearly in phase each other except
around the equator. This may imply the barotropic-type response of the global atmosphere as awhole to the anomalous heating in the tropics.
From these diagrams we easily speculate the increase (decrease) of the total atmospheric angularmomentum during El Nino (anti-El Nino) as
Rosen et al. (1984) suggested. It is noteworthy

betweenby 102). Negative values are shaded.

U at 200mb

and U at 700mb.

Units

90

JournalFig. 9.

of the Meteorological

averagedSocietyof Japan

Latitudetime sectionof zonally

Units (ms-1)

are indicatedon the right up. Negative values are shaded.

that the phase speed of poleward propagation isdifferent each other in the northern and thesouthern hemisphere. This may be partly due tothe difference of accuracy of data between thenorthern and southern hemisphere but most probably due to the difference of response process
in each hemisphere.
c. temperature
Thetemperature anomaly associated with the
ENSO appears most significantly in the upper
troposphere (Pan and Oort,1983). Fig. 10 shows
composite temperature anomalies at 300 mb
from category 1 to category 4. Category 5 (ElNino phase) is shown in Fig. 11(a) of Part I. At
category 1 (anti-El Nino phase) and 2 negative
anomalies are predominant in the tropics withthe minimum over the central Pacific, while positive anomalies are distributed along mid-latitudes.
At category 3 the positive anomalies at midlatitudes are diminished particularly in thenorthern hemisphere. Negativeanomalies seem to
be intensified over south and central Asia. At
category 4, about half a year before the mature
El Nino phase (category 5), anti-El Nino pattern
has completely disappeared. The negative ano-

zonal wind anomaliesVol.

for (a) 200mb

65, No.

1and (b) 700mb.

malies over south and central Asia further
developed, while positive anomalies have rapidly
evolved over the central Pacific region. At category 5 positive anomalies are extended over thewhole tropical belt with the maximum over theequatorial central Pacific, while negative anomalies are elongated along mid-latitudes as described in Part I. The large negative anomalies
over south and central Asia developed at categories 3 and 4 may presumably be associated withstrong cold air outbreak in winter and/or weak
monsoon in summer which precede the El Nino
event over the equatorial Pacific. We will discuss
further about this problem.
d. circulations
Finally,the global non-divergent and divergent circulation changes during the ENSO cycle
are described by using streamfunction andvelocity potential anomalies computed from U
and V anomalies. Fig. 11(a) shows the composite streamfunction (*) field at 200mb fromcategory 1 to 4. At category 1 (anti-El Ninophase) anticyclonic conditions are dominant athigh and middle latitudes of the two hemispheres, which are consistent with the easterly

February

1987

Fig. 10.
Same as Fig. 4 but for temperature
values are shaded.

T. Yasunariat 300mb.

91

Units (*)

are indicatedon the right up. Negative

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Journalof theMeteorologicalSocietyof JapanVol.

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1

February

1987

T. YasunariFig. 11.
Same as Fig. 4 but for streamfunction
at (a) 200mb and (b) 700mb. Units (m2s-1) are shown on theright up. Negative values are shaded. Note that L and H denotes center of cyclonic (anticyclonic)
and anticyclonic (cyclonic) circulation in the northern (southern) hemisphere.

93

94

Journalof theMeteorologicalanomalies over there as shown in Fig. 6. In the
Pacific region double cyclonic anomalies (L inthe northern Pacific and H in the southern Pacific) spread to the north and the south of theequator and anticyclonic anomalies polewards
are salient features. Over the north Pacific areversedPNA pattern is noted. Along the equator,
easterly anomalies over the Indian Ocean and thewesterly anomalies over the central Pacific are
noticeable. At category 2 the overall feature isvery similar to category 1.
A drastic change of the anomaly * field isnoticed again at category 3 as well as SLP field(Fig. 4). The meridionally-oriented wave-likestructure over the Pacific noted at category 1 and
2 are completely diminished. In the southern low
latitudes a strong anticyclonic cell appear overAustralia, while cyclonic cells appear over theeastern Pacific, the Atlantic and the Indian
Ocean. In the northern low latitudes a cyclonic
circulation is intensified especially over northAfrica through southern Eurasia. At category 4
and 5 the overall feature is analogous to that ofcategory 1 or 2 but with the opposite sign. At
category 5 (El Nino phase) a double anticycloniccell structure over the equatorial central Pacificand PNA pattern are explicitly shown.
In the lower troposphere (700mb) the anomalous * field (Fig. 11(b)) seems to be similar
to that at 200mb in most parts of high andmiddle latitudes, but in most parts of tropics thepattern seems to be similar but with opposite insign. At category 1 and 2, the anticyclonic circulation is dominant over the northern Pacific,
which is barotropically consistent with that at
200mb. In the southern low latitudes, thecyclonic circulation over Australia and theanticyclonic circulation over the central Pacific is
noticeable, which is consistent with the SLP
anomalies (Fig. 4). The twin cyclonic cell structure is also apparent over the Indonesian maritime continent, which may represent stationary
Rossby-wave response to heat source over there(Matsuno,1966, Gill, 1980).
At category 3, the overall circulation patternchanges considerably over the entire Pacificdomain. In the northern Pacific anticyclonicanomalies over the Aleutian Low area is weakened and alternately, the cyclonic anomalies spreadeastward over the whole northern Pacific region

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from the Asian continent. In the southern hemisphere, the east-west contrast of circulation
anomalies over Australia through the Pacific has
completely disappeared. Instead, a cyclonic anomalies appeared over the eastern Pacific, which
implies the weakening of the south Pacific highand/or the equatorward extent of mid-latitude
westerly trough. It is noteworthy to state thatat this stage the anomalous westerlies are dominated over the entire equatorial Pacific. At
category 4, the cyclonic anomalies over thesouthern Pacific furthern developed and the anticyclonic anomalies appeared over Australia,
which nearly correspond to the pattern at El
Nino phase.
Fig. 12 shows the composite anomalies ofvelocity potential (X) at 700mb from category
1 to 4. Here, X is defined as such that minimum
values denote centers of convergence. At category 1 (anti-El Nino phase) a center of convergence is located over the Indonesian maritime
continent and a center of divergence is locatedover the equatorial eastern Pacific. Anothercenter of convergence and diverdgenceis locatedover north Africa and over the Caribbean Sea
respectively, which may consist of another eastwest circulation cell over the Atlantic. The overall feature shows a wavenumber-one structure,
which suggests the east-west (or Walker) circulation along the equatorial Pacific is a dominant
divergent circulation over the globe. At category
2 the overall pattern resemble with that ofcategory 1, but the center of convergence overIndonesia seems to be slightly shifted eastward
and a small center of divergence appear over the
Indian Ocean.
A drastic change occurs again at category 3.
The center of divergence over the Indian Ocean
develops and is extended over the eastern hemisphere while the center of convergence over the
maritime continent is shifted further eastward
and is replaced with the center of divergence overthe central Pacific. That is, the wavenumber-one
structure is reversed in sign. At category 4 and 5(El Nino phase) the total feature is fundamentally the same as category 3, but the main centersof divergence and convergence seem to be moved
further to the east, (i.e., over the maritime continent and the eastern Pacific).
At 200mb (not shown) the anomaly patterns

February

1987

T. YasunariFig. 12. Same as Fig. 4 but for velocity potential
Negative values (convergence) are shaded.

at 700mb.

95

Units

(m2s-1)are indicatedon the right up.

96

Journalof the Meteorologicalof velocity potential particularly over the tropics
are roughly consistent with those at 700mb with
an opposite sign. However, unfortunately, theglobal patterns seem to be considerably distorted
because of large errors in divergent wind fieldover the southern middle and high latitudes.
5. Mechanism of evolution
a. process in the tropics
Some remarkable features have been noted in
the time evolutions of global SST and atmospheric anomalies associated with the ENSO cycle.
The eastward propagations of some atmospheric parameters (SLP, zonal wind, velocity
potential) along the equator and the southern
subtropics have been confirmed as most prominent aspects of ENSO in the low latitudes. As
described in Yasunari (1985), this phenomenon
is characterized as a zonally propagating mode of
the tropical east-west (or Walker) circulation,
though the standing mode also occupies a considerable portion of variance (Barnett, 1985b).
This eastward propagation in the wind field has
also been noted recently by Gutzler and Harrison(1986). In addition, the present results suggestthat this eastward propagation in the tropics is
coupled with the eastward propagation of SLP
anomalies in the southern subtropics.
It should be emphasized here that the significant changes of anomalies in SST and atmospheric anomalies appear at the intermediate stage
from anti-El Nino to El Nino (i.e., category 3) orvice versa (i.e., category 7). As has been noted
earlier, at category 3 the upper level anticyclonicanomalies over Australia is considerably intensified and slightly shifts eastward (refer toFig. 11(a)), while at the surface the negative
SLP anomalies are extended from northeast
Australia southeastward along SPCZ. Simultaneously, the south Pacific high is considerably
weakened (Fig. 4). At 700mb the center of convergence moves further eastward from northern
Australia and the westerly anomalies are extended eastward along the equator. These features
totally suggest that the center of convection over
the western Pacific has moved eastward from its
normal position. As has been pointed out bymany authors (Luther et al., 1983 etc.), thissituation over the equatorial western throughcentral Pacific is a necessary, or otherwise, fa-Societyof JapanVol.

65, No.

1vourable condition for triggering the El Nino.
Once the trade wind over the central Pacific is
weakened, or the equatorial westerlies over thewestern Pacific is intensified associated with thissituation, some sort of unstable atmosphere/
ocean interaction (e.g., by Philander et al. (1984))
may trigger eastward propagation of anomalies.
What, then, causes the eastward shift of convection and wind field? van Loon and Shea
(1985; 1987) emphasized the role of SPCZ andthe mid-latitude circulation in the Australiasouth Pacific sector. However, our analysis
strongly suggests that the preceding ENSO signals come further from the west, i.e., the Indian
Ocean side. The scenario by van Loon and Shea
may possibly be a part of our scenario if we include the downstream effect of atmosphericprocess over the southern Indian Ocean.
b. process over Eurasia and Indian Ocean
The positive SLP anomalies, negative temperature and streamfunction anomalies in the upper
troposphere and the positive (divergent) anomalies of velocity potential at 700mb overEurasia and the northern Indian Ocean at category 3 suggest us, as a whole, that the cold air
out-break associated with the southward extended trough over this region about one year
before the mature El Nino phase may have a keyrole to the phase change from anti-El Nino to E1
Nino condition.
Here, we should recall the negative correlation
between the Eurasian snow cover extent and thefollowing Indian monsoon rainfall amountpointed out first by Blanford (1884) and recently
re-examined with satellite data by Hahn andShukla (1976), Dey and Banu Kumar (1981) andDickson (1984). The anomalies over southernEurasia at category 3 mentioned above should be
responsible for more than normal snow cover
over there. To confirm this inference, composite
snow cover anomalies for category 3 and 7 areproduced as shown in Fig. 13. At category 3 a
positive anomaly area is largely extended overEuarasia with the maximum over Tibet. Interestingly, negative anomalies are noticeable overnorth America. In the streamfunction anomalies(Fig. 11) cyclonic and anti-cyclonic circulation
anomalies exist over the west coast and the east
coast of north America, respectively. Since a

February

1987

T. Yasunari97

Fig. 13. Composite
snow cover anomalies for (a) category 3 and (b) category 7. Unit of contour
is 0.2 (Full cover of each grid is 9.0). Negative values (of less than
-0.2)
are shown withdotted lines, and positive values (of more than 0.2) are shaded.

stationary ridge (trough) is located over theRockies (west coast) in a mean state, this anomaly pattern implies a more zonal flow overnorth America, which may be responsible for theless snow cover over there. At category 7, theanomalous condition is nearly the opposite tothat at category 3, that is, less than normal snow
cover over south Asia and more than normalsnow cover over north America. At category 4(and 3) the atmospheric anomalies over south
Asia (i.e., positive SLP, positive U at 200mb,
negative U at 700mb, negative T at 300mb,
negative * at 200mb and positive X at 700mb)
totally characterize the weak summer monsoon
condition. That is, the cold-surgetype anomalies
over central and south Asia with more thannormal snow cover extent in cold seasons and theweak monsoon over south Asia in the following
summers seem to be distinct signalsprior to the
El Nino events in the equatorial eastern Pacificin the following autumn and winters. These results have fundamentally confirmed the evidences of the preceding ENSO signals in SLP
anomalies from Eurasia and the Indian Ocean
toward the equatorial Pacific, first noted byBarnett (1985). Shukla and Paolino (1983) also
noted that the anomalous summer monsoon precedes the extreme phase of ENSO in autumn
through winter. In other words, the two factors,i.e., the ENSO indices (SOI or SST in the equatorial Pacific) and the snow cover over Eurasia andTibet, which are highly or moderately correlatedwith the Indian summer monsoon rainfall (Pantand Parthasarathy, 1981; Bhalme and Jadhav,
1984; Hahn and Shukla, 1976; Dickson, 1984etc.) should be inferred as parts of one climate
system of global ENSO rather than independent
external parameters.
c, a time-lag teleconnection
As scrutinized above, there seem to be twocenters of actions in the northern mid-latitudes
associated with ENSO. One is the northern Pacific area where distinct anomalies appear during
the extreme phases (category 1 or 5) of ENSO
cycle. These anomalies have been noted inmany previous studies (Horel and Wallace,1981;
Chen, 1982 etc.). The other may be central Asia
or Eurasia at around 60*E, where significant
anomalies appear during the intermediate phases(category 3 or 7) as noted here. The problem
may be, then, how the significant anomalies ofthese two areas evolve and inter-relate each otherduring the course of ENSO cycle. To approach
this problem, the longitude-category sectionof composite streamfunction anomalies along
40*-60*N are produced as shown in Fig. 14.
Large anomalies are noticeable over central

98Journalof the MeteorologicalSocietyof JapanVol.

65, No.

1

Fig. 14.
Same as Fig. 5 but for (a) streamfunction
at 200mb and (b) streamfunction
at 700mb
along 40*N-60*N.
Zonally averaged values are subtracted.
Units are indicated on the rightup, and negative values are shaded.

Eurasia and northern Pacific but with one or two
category lags. Particularly at 700mb (Fig. 14 (b))
only these two areas have appeared to have distinct anomalies after compositing the several
ENSO cycles. This may be interpreted as a teleconnection with some time lags. This teleconnection between the north Pacific and central Asia
(or Eurasia) have not been noted in other previous studies except Chen (1982). He noted weaknegative simultaneous correlation between the
SOI and 700mb height anomalies over central
Asia, though the correlations of other areas are
more significant (e.g., north Pacific through the
Atlantic). But once the time lag is taken to half ayear or one year, this time-lag teleconnection

seems to be dominated.
Another feature to be noted may be the eastward propagation of anomalous circulation from
Eurasia toward the northern Pacific, thoughstanding-type anomalies between the two areas
seem to be more prominent. Interestingly, theeastward propagation of anomalous circulation
along this latitude (40*-60*N) seems to be nearly synchronized with that of zonal wind along
the equator as shown in Fig. 7. For example, theeastward propagation of cyclonic circulation at
200 mb (negative anomalies in Fig. 14 (a)) seems
to be in phase with that of westerly anomalies
at 200mb along the equator (positive anomalies
in Fig. 7 (a)). If so, the large negative anomalies

February

1987

T. Yasunari

over north Pacific associated with El Nino events,
which are noted as part of PNA pattern (Horeland Wallace, 1981), cannot be interpreted solely
as a response of anomalous heating over theequatorial Pacific.
The overall features shown in Fig. 14 are notresulted from the `time and space filtering' and`composite' effects . To examine this, the longitude section of time-lag correlations of 500mb
geopotential height anomalies are computed
along 40*-60*N. In this case only 11-monthmoving average is applied to height anomalies(from normal). Fig. 15 shows the result for thereference point over central Asia (60*E). The
eastward movement of positive correlations withpositive time lags (each point lags behind thereference point) from central Asia toward northPacific is noted. In this case, the statistical significance level of 95% correspond with *= 0.2.
This correlation pattern implies that about one
year after the positive (negative) anomalies
appear over central Asia the anomalies of thesame sign tend to appear over the north Pacific.
Another feature to be noted may be that thenorth Pacific (120*E -180*) leads central Asiawith 6 to 12 months lag with significant negative
correlations. Nearly contemporaneous positive
correlation between central Asia and 100*-

Fig. 15. Lag correlationsof 500mb
geopotential
height anomalies along 50*N with reference to60*E. Units are 0.2. Positive values (>0.2) are
hatchedand negativevalues (<-0.2)
areshaded.

99

120*W (around the Rockies) are also noticeable.
Thus, the composite anomaly pattern as shownin Fig. 14 can fundamentally be reproduced in
the lag-correlation diagram by using low-passfiltered 500mb height anomalies.
The overall feature as noted in Fig. 14 (orFig. 15) may be inferred as follows: the anticyclonic anomalies over north Pacific associatedwith anti-El Nino condition may cause cylonicand cold anomalies over central Asia after a halfor one year. This anomalous circulation overcentral Asia may be responsible for more thannormal snow cover extent, which further leads
weak summer monsoon over India. The weak
monsoon over India may be associated with weaktropical east-west circulation over the Indian
Ocean through the equatorial Pacific, which may
trigger the El Nino event via the weaking of the
Pacific trades or the burst of equatorial westerlies
over central Pacific. The anomalous circulation in
the southern Indian Ocean through the southern
Pacific may be involved in this process throughsome inter-hemispheric response of the southernwesterlies to that in the northern hemisphere.
This process along the equator may also be manifested as the eastward propagation of the anomalous east-west circulation along the equator
as noted by Yasunari (1985) and others. Oncethe El Nino condition is established, the anticyclonic anomalies over north Pacific is changed tocyclonic anomalies, which in turn may lead thechange of circulation anomalies over central Asia
via the time-lag teleconnection mentioned above.
The anti-cyclonic anomalies over central Asia
may be favourable for strong summer monsoon
over India, which in turn may intensify thetropical east-west circulation. Thus, anti-El Nino
condition is again established via the circulation
change over the north Pacific through Eurasia.
A big question may be, then, what is a physical mechanism of the 'time-lag teleconnection'
between the north Pacific and central Asia. One
possible mechanism may be a slow downstream
amplification which is initiated as PNA (or antiPNA) pattern over the north Pacific through thenorth Atlantic by anomalous heating over theequatorial Pacific, which finally causes the large
amplification of anomalies over Eurasia. The
downstream anomaly pattern from the northPacific certainly seems to show some kind of

100

Journalof the Meteorologicalwave train with slight time lags. Reiter (1982)
also suggested similar mechanism. However, someproblems may arise such as why only the anomalies over central Asia are so amplified, eventhough central Asia is located at the most downstream side from the north Pacific.
Another possibility may be more direct coupling of these two areas via some land-atmosphereocean interaction. For example, the warm SST
anomalies in the north Pacific and the northAtlantic during anti-El Nino phase (refer to Fig. 6of Part I) may set up the warm troposphere along
mid-latitudes (30*N-70*N). This feature isclearly demonstrated in the 300mb temperature
anomaly field at categories 1, 2 and 3 (Fig. 10).
This warm condition in the middle latitudes
implies the more zonal westerly flow (i.e., highindex circulation) at high latitudes associatedwith the shrinkage of the cold polar vortex
toward the arctic region. Under this condition,
the thermal contrast between the warm oceans
and the continents may be seasonally strongly
intensified during winter when the continents arecooled. In response to this thermal contrast,
anomalous ridge and trough may be intensified
over the oceans and the continents, respectively.
Particularly over Eurasia, where the cooling
effect may be far stronger than that over northAmerica, this anomalous trough seemingly formsa real stationary trough with the cold air massover the interior of Eurasia. In fact, the long-term
Eurasian pattern (Blackmon et al., 1984) withnegative height anomalies over Eurasia seems tocorrespond well to this anomalous circulation
pattern in the northern winter. Very recently,
Morinaga and Yasunari (1986) has found that
this Eurasian pattern is greatly responsible forthe large-extent of snow cover over central Asia.
The snow cover may further strengthen the landocean thermal contrast. However, if this condition develops and persists even in the beginningof warmer seasons, it may weaken the following
summer monsoon most probably via snow covereffect, which in turn weakens the east-west circulation along the tropics. The El Nino condition
may be, thus, triggered following the process asmentioned earlier.

Society6.

of JapanVol.

65, No.

1Conclusion

Time evolutions of anomalies of SST andatmospheric parameters over the global domain
were investigated for the several ENSO cycles
during the period from 1964 to 1979 by using
objectively analyzed time-filtered data sets. These
anomalies were subjected to composite techniquerelevant to each phase of the ENSO cycle as wellas simple time sectional analysis along longitudes
and latitudes. The evolution of the anomalies
from anti-El Nino to El Nino and vice versa were
described and summarized as a scenario of thequasi-periodic global climate system including
oceanic and land-surface process. Some plausible
physical mechanisms of this system were also
discussed. The principal results are summarized
as follows;
1) The eastward propagation of SLP and zonal
wind field from the Indian Ocean toward theeastern Pacific is a fundamental nature of
ENSO along the equator and the southern
subtropics. Although SO has been traditionally understood as a standing oscillationof SLP between the Indian Ocean and theeastern Pacific, this propagating feature may
also be an important facet of ENSO.
2) The significant anomalies of SLP and circulation field which first appear over the Indian
Ocean seem to originate from central Asia or
Eurasia. The vertical combination of SLP,
temperature and streamfunction anomalies
suggest that these anomalies over central
Eurasia at the intermediate stages (category
3 and 7) are related to cold (or warm) air
outbreak with more (or less) snow coveranomalies there. The structure of SLP evolution presented here has substantiated theevidences of global ENSO previously shownby Barnett (1985) and Krishnamurti et al.
(1986).
3) A time-lag teleconnection between the northPacific and central Asia was found in thecirculation anomalies, which seems to have adominant role on the ENSO cycle, namely
the phase change from anti-El Ninno to El
Nino and vice versa.
The links among the tropical and the extratropical oceans, and the Eurasian continent described above lead us to the idea that ENSO

February

1987

T. Yasunari

should be considered to be a land-atmosphereocean coupled system as a whole rather than aplain atmosphere-ocean coupled system over theequatorial Pacific. In this system, the Asiansummer monsoon seems to play an importantrole as a connector of oceanic process and land
process. In this context, Normand's viewpoint(Normand, 1953) on the monsoon/SO relationthat "The Indian monsoon stands out as anactive, not a passive feature in world weather,more efficient as a broadcasting tool than as an
event to be forecast" should be reevaluated.
Some critical problems are still remained for
future studies. One problem may be the physicalmechanism of the time-lag teleconnection as
described here. In the present study, the ENSO
cycles were examined for the near-periodic phase(1964-79) with a periodicity of 3 to 4 years.
However, the ENSO also has considerable variances in aperiodic and non-stationary portions in
the long-term period. This problem also deserves
to be studied as an interaction of climate systems
among different time scales. Another problem
may be a more exact assessment of the role ofthe southern hemisphere on ENSO, which could
not be made sufficiently because of the qualityof the data sets. This may be one of the important subjects for the forthcoming TOGA.
Acknowledgements
The author is greatly indebted to Prof. T.N.
Krishnamurti for his great support, encouragement and stimulating discussions throughout thework. The research reported here is mainly supported by NOAA Grant No. NA85AA-HCA019.
The research is partly supported by Grant-In-Aid
for 1985 (No. 26) from the Toray Science Foundation, and by the project research at the University of Tsukuba.
References
Arkin,P.A.,1982:The relationshipbetweeninterannual
variabilityin the 200mb tropicalwindfieldand theSouthernOscillation.Mon. Wea.Rev., 110, 13931404.
Barnett, T.P., 1984: Interaction of the monsoon andPacifictrade wind systemat interannualtime scales.
Part III: A partial anatomyof the SouthernOscillation.Mon.Wea.Rev., 112,2388-2400.
1985a: Variations-,in near-globalsea levelpressure,J. Atmos.Sci.,42, 478-501.

101-, 1985b: Three-dimensional structure of low
frequency pressure variations in the tropical atmosphere. J. Atmos. Sci., 42, 2798-2803.
Bhalme, H.N. and S.K. Jadhav, 1984: The southern oscillation and its relation to the monsoon rainfall. J.
Climat., 4, 509-520.
Blackmon, ML., Y.-H. Lee and J.M. Wallace, 1984:
Horizontal structure of 500mb height fluctuations
with long, intermediate and short time scales. J.
Atmos. Sci., 41, 961-979.
Chen, W.Y., 1982: Fluctuations in Northern Hemisphere
700mb height field associated with the Southern
Oscillation. Mon. Wea. Rev., 110, 808-823.
Dey, B. and Bhanu Kumar, O.S.R.U.,1982: An apparent
relationship between Eurasian spring snow cover andthe advance period of the Indian summer monsoon.
J. App. Met., 21,1929-1933.
Dickson, R.R., 1984: Eurasian snow cover versus Indiansummer monsoon rainfall- An extension of the
Hahn-Shukla results. J. App. Met., 23,171-173.
Gill, A.E., 1980: Some simple solutions for heat-induced
tropical circulation. Quart. J. Roy. Met. Soc., 106,447-462.
-, and E.M. Rassmusson, 1983: The 1982-83
climate anomaly in the equatorial Pacific. Na.ture,306, 229 -234.
Gutzler, D.S. and D.E. Harrison, 1986: The structureand evolution of seasonal wind anomalies over thenear-equatorial eastern Indian and western Pacific
Oceans. Submitted to Mon. Wea. Rev.
Hahn, D.G, and J. Shukla, 1976: An apparent relationship between Eurasian snow cover and Indiansummer monsoon rainfall. J. Atmos. Sci., 33, 24612462.
Horel, J.D. and J.M. Wallace, 1981: Planetary-scaleatmospheric phenomena associated with the Southern Oscillation. Mon. Wea. Rev., 109, 813-829.
Hsiung, J. and R.E. Newell, 1983: The principal nonseasonal modes of variations of global sea surface
temperature. J. Phy. Oceangr., 13,1957-1967.
Kawamura, R., 1986: Seasonal dependency of atmosphere-ocean interaction over the north Pacific. J.
Met. Soc. Japan, 64, 363-371.
Kidson, J.W., 1975: Tropical eigenvector analysis andthe Southern Oscillation. Mon. Wea. Rev., 103,187194.
Krishnamurti, T.N., S.H. Chu and W. Iglesias,1986: On
the sea level pressure of the Southern Oscillation.
Arch. Met. Geoph. Biocl., Ser. A, 34, 385-425.
Luther,D.S.,D.E. Harrison and R.A. Knox,1983: Zonal
winds in the central equatorial Pacific and El Nino.
Science, 222, 327-330.
Matsuno, T., 1966: Quasi-geostrophic motions in theequatorial area. J. Met. Soc. Japan, 44, 25-43.
Morinaga, Y, and T. Yasunari, 1986: Interactions between the large-scale snow cover and the general circulation in the northern hemisphere. Abstract ofannual meeting of Japanese Soc. Snow and Ice, 255.
(in Japanese).
Newell, R.E., R. Selkirk and W. Ebisuzaki, 1982: The

102

Journalof the MeteorologicalSouthern Oscillation: Sea surface temperature andwind relationships in a 100-year data set. J. Climat.,2, 357-373.
Niebauer, H.J., 1986: El Nino-Southern Oscillation
teleconnections
with the subarctic Bering Sea.
Submitted to J. Geophy. Res.
Normand, C., 1953: Monsoon seasonal forecasting.
Quart. J. Roy. Met. Soc., 79, 463-473.
Pan, H-L., 1979: Upper tropospheric tropical circulations during a recent decade. Florida State University
Report No. 79-1. Department of Meteorology, FSU.
141pp.
Pan, Y-H. and A.H. Oort, 1983: Global climate variations connected with sea surface temperature anomalies in the eastern equatorial Pacific Ocean for the
1958-73 period. Mon. Wea. Rev., 111,1244-1258.
Pant, G.B. and B. Parthasarathy,1981: Some aspects ofan association between the Southern Oscillation andIndian summer monsoon. Arch. Met. Geoph. Biokl.,
Ser. B, 29, 245-252.
Philander, S.G.H., T. Yamagata and R.C. Pacanowski,
1984: Unstable air-sea interactions in the tropics.
J. Atmos. Sci., 41, 604-613.
Reiter, E.R., 1978: The interannual variability of theocean-atmosphere system. J. Atmos. Sci., 35, 349370.
-,1982: Where we are and where we are goingin mountain meteorology. Bull. Ame. Met. Soc., 63,
1114-1122.
-,1983: Teleconnections with tropical precipitation surges. J. Atmos. Sci., 40,1631-1647.
Rosen, R.D., D.A. Salstein, T.M. Eubanks, JO. Dickey

ENSO(エSociety of Japan

Vol. 65, No. 1

and J.A. Steppe, 1984: An El Nino signal in atmospheric angular momentum and earth rotation.
Science, 225,411-414.
Shukla, J. and D.A. Paolino, 1983: The Southern Oscillation and long-range forecasting of the summer
monsoon rainfall over India. Mon. Wea. Rev., 111,
1830-1837.
Troup, A.J., 1965: The `Southern Oscillation'. Quar. J.
Roy. Met. Soc., 81, 490-506.
van Loon, H, and D.J. Shea, 1985: The Southern Oscillation. Part IV: The precursors south of 15°S to
the extremes of the oscillation. Mon. Wea. Rev., 113,
2063-2074.
-, and -,1987: The southern oscillation. Part VI: Anomalies of sea level pressure on thesouthern hemisphere and of Pacific sea surface temperature during the development of a warm event. In
press in Mon. Wea. Rev., 116, January issue.
Walker, G.T. and E.W. Bliss, 1932: World weather V.
Mem. Roy. Met. Soc., 4, 5 3-84.
Yamagata, T., Y. Shibao and S. Umatani,1985: International variability of the Kuroshio Extension and its
relation to the Southern Oscillation/E1 Nino. J.
Oceangr. Soc. Japan, 41, 274-281.
Yasunari, T., 1985: Zonally propagating modes of the
global east-west circulation associated with theSouthern Oscillation. J. Met. Soc. Japan, 63,10131029.
-, 1987: Global structure of El Nino/Southern
Oscillation. Part I. El Nino composites. J. Met. Soc.
Japan, 65,

ル ・ニーニョ/南方 振 動)の全 球構 造第2部:ENSOサ

イ ク ル に 伴 う変 化

安

成

哲

三*

(フ ロ リダ州立 大学 気象学教 室)
第1部

で 記 述 さ れ た デ ー タ に よ り,ENSO

サ イ ク ル に 伴 う海 面 水 温 及 び 大 気 構 造 の 全 球 的 な 変 化 を

解 析 し,以 下 の よ う な結 果 が 得 ら れ た 。
1)熱 帯 お よび 南 半 球 亜 熱 帯 で は,地 上 気 圧 と大 気 循 環 の ア ノマ リー に,イン ド洋 上 に 出 現 し,東 部 太

平 洋 上 へ と伝 播 す る東 進 の 位 相 が 顕 著 で あ る 。
2)イ ン ド洋 上 に 出現 す る こ れ らの ア ノ マ リー は,反 対 エ ル ・ニーニ ョ期 か ら エ ル ・ニ ー ニ ョ期(又そ の 逆)に

3)ユ

移 行 す る 中 間 段 階 に,ユはー ラ シ ア 大 陸 上 に 現 わ れ るア ノマ リー と深 い 関 連 が あ る。 ユ ー

ラ シ ア 大 陸 上 で の ア ノマ リー は,とくに 下 部 対 流 圏 で傾 圧 的 な 鉛 直 構造 を 示 し,積 雪 分 布 の ア ノマ

リー と も対 応 して い る こ とか ら,イ
応 して い る こ とが 示 唆 さ れ る 。

ン ド洋 方 面 へ の 大 規 模 な 寒 気 吹 き出 しの 強 化(又ー ラ シ ア 大 陸 上 の 大 気 循 環 の ア ノマ リー と北 太 平 洋 上 の ア ノマ リー との 間 に,時た テ レ コ ネ ク シ ョン が 認 め ら れ る 。 こ の テ レ コ ネ クシ ョ ン は ENSO

割 を 果 して い る と推 測 さ れ る 。
以 上 の 結 果 は,ENSO
を 全 球 的 な 大 陸(雪してい る。

*現 在所 属:筑 波 大学地球 科学 系氷)一は 弱 化)と対

間 の ズ レ を持 っサ イ クル に お い て,重要 な役

大 気 一 海 洋 相 互 作 用 と し て 把 え るべ き こ と を示 唆