February

1987

T. Yasunari

67

Global Structure of the El Nino/Southern Oscillation
Part I. El Nino Composites

By TetsuzoYasunari*
Departmentof Meteorology,
FloridaStateUniversity,
Tallahassee,
FL32306(Manuscript
received
19August1986,in revised
form19December
1986)

Abstract
The three dimensional structures of sea surface temperature (SST) and atmospheric circulation
anomalies associated with ENSO are investigated over the entire globe, by using the objectively analyzed data sets of the years 1964 to 1981. The global data sets of U, V, Z and T are produced byapplying the least-square method to fit the time-filtered station data (of more than 320 stations) and
NMC tropical wind field data with truncated spherical harmonics of wavenumber 0 to 5. Global SST
and sea level pressure data are also utilized. In this paper (Part I) the composite anomaly patterns
during El Nino episodes are discussed.
Warm SST anomalies and warm upper troposphere are found to be remarkable not only over theequatorial central Pacific but also over the entire tropics, while the mid-latitudes seem to be characterized as cold SST and cold troposphere as a whole. The increasings of zonal winds in the subtropics andthe lower mid-latitudes of the two hemispheres are also prominent. These evidences apparently suggest that the intensified Hadley-type circulation in the low latitudes and the equatorward extent of thecircumpolar vortex in the high latitudes are fundamental features during El Nino episodes. A nearly
symmetric response pattern of the upper tropospheric flows with respect to the equator is also notedover the northern and the southern Pacific.
It is also suggested that the North Atlantic Oscillation (NAO) is synchronized with the SO during
the present analysis period.

1. Introduction
In recent years, the El Nino/Southern Oscillation (ENSO) has become of great interest as a
typical prototype of large-scale atmosphereocean interaction. This phenomenon was described first by Walker and Bliss(1932) and was
physically conceptualized first by Bjerknes(1966). The 1982/83 El Nino, which was one of
the largest events in this century has induced
renewed interest in this phenomenon.
Another remrkable aspect of ENSO may be
the distinct climatic anomalies over the northPacific through north America (Bjerknes, 1969;
Namias, 1976; Horel and Wallace, 1981 etc.),
associated with the extreme phases (i.e., El
Nino or anti-El Nino) over the equatorial eastern*Present

Affiliation:of Tsukuba,
1987,

IbarakiMeteorological

Institute305,

of Geoscience

Japan.

Society*of Japan, University

Pacific. These anomalies have now been understood as stationary Rossby wave responses to theanomalous heat source over the equatorial Pacific(e.g., Blackmon et al., 1983; Shukla and Wallace,1984 etc.). A theoretical background of thisproblem was given by Hoskins and Karoly(1981).
The current unresolved problem on ENSO iswhat is the physical mechanism of the ENSO
cycle with a period of several years. A numberof coupled ocean-atmosphere models have now
been proposed to simulate the tropical aspects of
ENSO, particularly the time evolution of SST
and wind field along the tropical Pacific region.
Philander et al. (1984) simulated eastward
propagation of sea surface temperature (SST)
and wind field with linear dynamics for both theocean and atmosphere. Their results seem toagree fairly well with the observations over the68

Journalof the Meteorologicalequatorial Pacific during 1982/83, though thismodel does not describe the ENSO cycle.
Anderson and McCreary (1985) proposed amodel with non-linear ocean dynamics and linear
atmosphere dynamics. They first showed thecycle, namely from El Nino to anti-El Nino andfrom anti-El Nino to El Nino in the SST andwind fields, though some features are quite different from the observations.
Zebiak and Cane (1985) proposed a moresophisticated model including the seasonal cycle
and prediction equation of SST. This modelseems to have successfully simulated the SST andwind field evolutions in the ENSO cycle withsome periodicity (but with the time scale ofseveral years), though the zonal propagation of
their wind field seems to be opposite to the observed field (Gill and Rasmusson, 1983). They
stressed that the ENSO can fundamentally bedescribed as the atmosphere-ocean interaction
over the tropical Pacific.
However, some recent observational studies
have presented distinct evidences of the ENSO
related anomalies outside the tropical Pacificregion, which precede the extreme phases overthe equatorial eastern Pacific (i.e., El Nino or
anti-El Nino events) with one or two seasons ormore. Barnett (1983, 1984a, 1984b, 1985)
offered evidences of eastward propagations ofsurface wind and pressure anomalies from theequatorial Indian Ocean toward the central Pacific. Krishnamurti et al. (1986) showed theglobal link of the surface pressure anomalies
related to the ENSO cycle. Their results on thetropical belt seem to be similar to those ofBarnett (1985).
Yasunari (1985) presented the evidences of
eastward propagations of the tropical east-west
anomalous circulation associated with ENSO.
The propagation of the anomalies of zonal anddivergent wind fields seem to be consistent withthe surface fields by Barnett and Krishnamurti
et al. Very recently, Gutzler and Harrison (1986)
showed a very similar feature to Yasunari's byusing different statistical approach. van Loon and
Shea (1985) noted the significant ENSO related
signals in the sea-level pressure, temperature andwind over the eastern Indian Ocean throughAustralia south of 15*, preceding the typical
El Nino (or anti-El Nino). These studies strongly

Societyof JapanVol.

65, No.

1suggest that the preceding signals to ENSO do
exist outside the tropical Pacific region.
In addition, a strong association of Indiansummer monsoon with ENSO (Pant and Parthasarathy, 1981; Rassmusson and Carpenter,
1983; Bhalme and Jadhav, 1984 etc.) suggestsus
some physical link (not a one-way response) between the tropics and extra-tropics with this timescale. To understand the total physical process of
the ENSO, therefore, the time evolution of atmospheric (and oceanic) parameters in the whole
ENSO cycle should be examined for the entire
globe.
The present study will forcus on this problem
based on the analysis of the objectively analyzed
atmospheric circulation parameters and SST associated with the ENSO over the global grids. This
paper (Part I) will describe the objective analysis
scheme and will assess the anomalies during El
Nino (or anti-El Nino). Part II will describe thetime evolution of SST and atmosphere during
the course of ENSO cycle and will discuss a possible physical process of the ENSO.
2.

Data sets

Main data source for wind, temperature andgeopotential height is based on Monthly Climatic
Data for the World (Upper Air Data) covering theperiod from January 1964 to December 1981.
We selected 320-340 stations which containmore than two thirds of the total records. National Meteorological Center (NMC)'s tropicaloperational wind field analysis (48.1*-48.1*)
are adopted for the period of 1968 to 1981 tocover data sparse oceanic regions. This data incorporates satellite-derived cloud winds as well as
winds observed by commerical aircrafts.
Monthly mean 10*l0*
gridded global sea
level pressure (SLP) data set compiled by Krishnamurti et al. (1986) are also utilized. Monthly
mean global SST data compiled at NOAA (Reynolds, 1983) are also used.
3.

Objective analysis of time filtered data

During the analysis period (1964-1981), El
Nino/Southern Oscillation (ENSO) cycle prominently shows the periodicity of 3 to 5 years, as
demonstrated in the Southern Oscillation Index(SOI) (Fig. 1) defined as the SLP difference ofFebruary

1987

T. Yasunari

Tahiti minus Darwin. To extract the anomalies
purely related to ENSO cycles, a recursive time
filter (M. Murakami, 1981) with the frequency
response of 30-60 month period with the maximum response of 45 month period (Fig. 2) wasapplied to all the data sets described in section 2.
Mean seasonal cycles were subtracted from theoriginal monthly data before we applied the time
filter.
Time filtered U and V components of winds
at 700mb and 200mb for 320 stations and 80
grids of NMC wind data are then objectively
analyzed by using the least-square method following Pan (1979).
For each month of the analysis period, K
points observations, e.g. [uk]:k=1, 2, * K are
given. In this case, the observation number K
are 400. The location for the K-th point is given
by * the longitude and *, the co-latitude. The
objective analysis field are defined as the function

where Pnm(cos*) represents the associated Legendre function. The coefficients Amn and
Bmn are obtained by fitting the data set [uk] insuch a way as to makeFig. 1.

Southern

Oscillation

Index(Tahiti minum

series. El Nino phases are shown with "EN".

69

a minimumwithcientsAmn

and Bmn,respect

where

0*m<M,

m*n*m+M

where

l*m<M,

m*n*m+M

that

to

each

of

the

coeffi-isandThe details to solve th equations (3) and (4)are described minutely in Pan (1979). The * areGauss' precision moduli. The function u(*, * )represents a truncated series of surface sphericalharmonics. The index m represents the zonal
wavenumber while n-m represents the meridional wavenumber, or the number of zero points
between the two poles.
In spectral numerical models, two types oftruncation have been used; i.e., the rhomboidal
and the triangular truncations. In this study therhomboidal truncation is adopted here simply
because1) it gives equal number of degree of freedom
to each zonal wavenumber2) resolution in higher latitudes are relatively
better than the triangular truncation.
There may be at least two factors for determining the truncation limit M (or the maximum
zonal wavenumber). One may be the over-deter-

Darwinsea level pressure)and its time filtered

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distribution of the original data (radiosonde
station data, NMC grid-points data and bogus
points) for U and V field is shown in Fig. 3.
4.

Fig. 2. Frequency response of time filter adopted
here. Periods of half response (30 months and
60 months period) are also indicated.

urination factor (i.e., number of data/number ofcoefficeents) for the least-square method. This
number should be as large as possible. Anotherfactor may be space-scales of anomalies thatshould be described in spherical surface. Since
the anomalies associated with the Southern
Oscillation may be of planetary-scale, the maximum zonal wavenumber M is chosen to be 5,
which totally gives 66 coefficients. The corresponding aver-determination factor is about 5,
which may be sufficient for the least-square
method.
By using A and B thus obtained, Fourier coefficients of zonal waves for each latitude arecalculated as follows:andFinally, the analyzed field of u(*, *) are obtained as follows:

Time filtered anomalies of the other parameters (v, z, T) are also produced in the same procedure. In cases of Z and T, 344 and 335 station
data are adopted, respectively. To supress artificially amplified anomalies over the data-sparse
region, some (less than 10) bogus points withzero-anomalies are also added in each case. The

Assessment of analyzed data sets

There may be some doubt about the objectively analyzed data sets which are estimatedwith the limited degree of freedom as describedin the previous section. In this section we present
some results of assessment on the obtained timeseriesand the spatial pattern.
In Fig. 4 original time-filtered data of temperature at 300mb and zonal wind at 200mb
for some stations are compared to those of theadjacent grids deduced from the objective analysis. Although the analyzed data contain only thevariances of limited zonal and meridional wavenumbers (m=0 to 5 with rhomboidal truncations), they could simulate, as a whole, the original data sufficiently. This fact may imply firstthat the anomalies of interannual time scales
such as ENSO can be substantially composedwith those of planetary-scale waves. However,
the analyzed data also contain the statistical estimate errors due to insufficiency of data samples
for the least-square method. For example, theanalyzed data near Anchorage (Alaska) could
simulate the original data substantially due tothe sufficient data in the northern middle andhigh latitudes, but those at Alice Spring (Australia) have considerable estimate errors due tolack of station data in the southern hemisphere.
The stability of the analyzed data against the
spatial truncations is also examined simply bycomparing the spatial anomaly patterns deducedwith different truncations. Fig. 5 shows theanomaly patterns of zonal wind at 200mb for
January 1973 deduced with the truncation of
zonal wavenumber 0 to 5 (upper) and from thatof zonal wavenumber 0 to 6 (lower). Thoughfrom place to place the anomaly wind speeds are
more or less different each other (generally thevalues for m=0-6 are greater than those form=0-5), the overall negative-positive patternis quite similar with each other. It seems that
only in some portions where the ENSO signals
are less significant and also the original data aresparse such as over the southern oceans, considerable changes of anomaly pattern are apparent.

February

1987

T. Yasunari71

Fig. 3. Distribution of data points for the objective analysis of U and V. Cubic marks denoteradiosonde stations, black dots denote the points adopted from NMC tropical operational
wind analysis, and white dots denote zero-anomaly bogus points.

Thus, we may conclude that the objective
analysis scheme we have employed here could
fundamentally extract the ENSO related anomalies except some data sparse region such as thesouthern middle and high latitudes. This analysis scheme seems to function more efficiently forthe time-filtered data sets.
5. Composite anomalies during El Nino
During the analysis period, one ENSO cycle
shows nearly 4-year period particularly during
1964 to 1979. In fact, El Nino occurred in 1965,
1969, 1972/73 and 1976/77 adn anti-El Nino
conditions occurred in 1964, 1967/68,1970/71,
1973/74, 1975/76 and 1978/79, as shown in Fig.
1. Although considerable decrease of SOI appeared during 1974/75, this phase did not show
any characteristics of El Nino in the SLP norwind field (Barnett, 1985; Gutzler and Harrison,
1986 etc.). Yasunari (1985) also suggested that
this minor minimum of SOI may be associatedwith the tropospheric QBO. In the present analysis, therefore, we have chosen the time filter tosmooth out this phase as possible.
We then categorized 8 phases for each ENSO
cycle during 1964 through 1979, in reference tothe smoothed SOI values. For example, category
1 (5) denotes the maximum (minimum) SOI
phase, which corresponds with the El Nino (antiEl Nino) period. Similarly, category 3 (7) corr-

spond with the intermediate stage from the El
Nino (anti-El Nino) to the anti-El Nino (El Nino)
period. One category of each ENSO cycle has a 5
to 6 months time length.
To scrutinize the circulation, temperature andheight anomalies over the whole global domain
associated with the ENSO cycle, composite
anomaly fields for each category are produced,by averaging the four (or five) cases during theanalysis period (1964-79).
In this section, the global anomaly field of 10
parameters during El Nino period (category 5)
are demonstrated.
a. SST
Composite SST anomalies during El Nino
period (category 5) are shown in Fig. 6. Large
positive anomalies exceeding 0.6* are remarkably seen over the central through the easternequatorial Pacific. Large area of negative anomalies asre also apparent over the northern Pacific,
which has been already pointed out as anothersignificant feature of SST anomalies during El
Nino (Weare et al., 1976).
It is also noteworthy to state that positive
anomalies are commonly seen over the whole
tropical oceans except the western Pacific although the maximum amplitudes appear in theeastern Pacific. This has also been noted by Pan
and Oort (1983) and Hsiung and Newell (1983).

72

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1

Fig. 4. Time-filtered data of temperature at 300mb (left) and zonal wind at 200mb (right) at four
stations (dashed line) and objectively analyzed time filtered data at grids adjacent to these statiions(solid line).

Negativeanomalies

over the southern

the south of about

25*

Pacific to

here

are

not

normalized,

also seem to be signifi-

lies

tend

to

appearcant, which may suggest a symmetric pattern of
SST anomalies
and associatedatmospheric
responses in the both sides of the equator.
b. SLP
Fig. 7 shows the composite
SLP anomalies

east-west

duringfied

El Nino.

Since

the

SLP anomalies

used

contrast

latitudes
and
which

easternisanomaliesofbetweenthesubtropicaltheanoma-

latitudes.

anomalies

Yet,

the

in thelow

Australia-Indonesian
Pacific

evidenced
overlarge-amplitudein higher

asAustralia
highregionis a predominantthemay
there,

SO.

Large

imply
while

pattern,thethepositive
intensinegativeFebruary

1987

T. YasunariFig. 5.

Filteredanomalyzonal

wind at 200mb

boidal truncationof zonal wavenumber
values are shaded. Units are 0.7ms-1.

73

for January

0 to 5 (upper)

1973 deducedwith rhom-and 0 to 6 (lower).

Negative

Fig. 6. Composite filtered anomaly distributionof sea surface temperature
at category
5 (El Nino or minimum SOI phase). Units are 0.1*
and negative values are shaded.
Data south of 40* are not showsn.

74

Journal

Fig.

7.

Same

as Fig.

6 butof the Meteorologicalforsea levelpressure.

Societyof JapanUnits

are 0.3mb

Vol.

and

negative

65,

No.

1

valuesare shaded.

anomalies over the eastern Pacific and the elongated positive anomalies from Australia toward
the southern tip of South America may suggest
the weakened South Pacific high and associatednorth-eastward displacement of South PacificConvergenceZone (SPCZ) during El Nino.
Another remarkable feature may be the northsouth contrast of anomalies over Europe throughthe north Atlantic. This may be related to theNorth Atlantic Oscillation (NAO) signals(Walker
and Bliss, 1932) though some studies (Barnett,
1985 etc.) noted that NAO and SO are not correlated each other in time series.

systematic patterns can be found. The overall
feature particularly of the zonal wind anomalies
are nearly identical to the results by Arkin(1982) and Pan and Oort (1983).
Fig. 9 shows zonal wind anomalies at 700mb.
A prominent feature in the tropics is a couplingof the westelies over the central Pacific and theeasterlies over the western Pacific through theIndian Ocean. This pattern is just opposite in the
direction of wind to that at 200mb over there(see Fig. 8(a)). That is, anomalous east-west circulation may be realized along the equatorial
belt of this region as noted by Yasunari (1985).
In the middle latitudes, in contrast, the westerlies
seem to be distributed associated with those at
200mb. In other words, baroclinic type zonal
wind anomalies are dominant along the tropicalbelt most apparently over the Pacific, but equivalent barotropic-type wind anomalies are a characteristic feature in middle and high latitudes.
Anomalous heat source during El Nino may
be located near to the westerly (easterly) maximum of 700 (200)mb over the central Pacific(180*-150*)
if we refer to simple linear theories (Matsuno,1966; Gill, 1980).
To visualize the anomalous wind field moreeasily, streamline charts are produced as shownin Fig. 10. At 200mb (Fig. l0(a)), an area ofdivergence over the central Pacific and two areasof convergence over the western Pacific and the
Amazon basin are apparent. This pattern along

February

1987

T. Yasunari75

Fig. 8. Same as Fig. 6 but for (a) zonal wind at 200mb
200mb.
Units are 0.3ms-1
and 0.2ms-1,
respectively.

the equator is consistent well with the spatial
pattern of anomalous convection (i.e., strong
convection over the former area and weak convections over the latter two areas) during 1982/
83 El Nino (Ardanuy and Kyle, 1986).
A double anticyclonic-cell structure over theequatorial central Pacific and a cyclonic-cellstructure to the north of it (i.e., over the Aleutian low area) are evident. Another cyclonic circulation can also be identified over the southern
Pacific, and these two cyclonic cells seem to havea symmetric structure with respect to the equator. This feature suggests a strong stationary
wave response of the atmosphere in the low andand (b) meridional wind at
Negative values are shaded.

middle latitudes of the two hemispheres to theanomalous heating over the equatorial Pacific.
We also notice a weak anticyclonic curvature
over Canada and cyclonic circulation to thesoutheast of it, which is identified as a so-called
PNA pattern (Horel and Wallace,1981). Anotherremarkable feature is a broad band of westerlies
over the low and middle latitudes of the Atlantic
and Afro-Eurasian region (refer to Fig. 8(a)),
which may imply the intensified subtropical jet
stream associated with El Nino events.
At 700mb (Fig. 10(b)), westerlies are prominent over the tropical Pacific as already noted inFig. 9. On either side of this westerly zone (i.e.,

76

JournalFig.

9.
valuesSame

as Fig.

6 butof the Meteorological

for

zonal

wind

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at 700mb.

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are 0.1ms-1

Vol.

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65, No.

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negativeare shaded.

over the northern and the southern Pacific), twocyclonic circulations made a pair, which may suggest the response of the middle latitude westerlies to the anomalous heat source over the central
Pacific. Over Australia, in contrast, a strong anticyclonic cell exist, which correspond to the intensified subtropical high over there and associated intensified easterlies over the Indonesian
maritime continent.
e. Temperature and geopotential height in theupper troposphere
The anomalous thermal field in the atmosphere seems to be affected by the anomalous
heating during El Nino most remarkably in theupper troposphere (e.g., Pan and Oort, 1983).
Fig. 11 shows composite anomalies of (a) temperature at 300mb and (b) geopotential heightat 200mb. A striking common feature in the twocharts is a zonally-oriented positive anomalies in
the low latitudes (roughly 30*-30*)
with ahuge maximum over the central Pacific. In the
middle latitudes of the two hemispheres, in contrast, negative anomalies are generally dominant.
Areas of minimum anomalies are located overeast Asia, northeast Pacific and western Europe
in the northern hemisphere. The negative anomalies of temperature at 300mb and 200mb geopotential height over far east Asia may correspond to relatively cool summers during El Nino
years (Kurihara, 1985). In the southern hemi-

sphere, patterns of negative and positive anomalies are rather erroneous, probably because of
insufficient data particularly in the middle andhigher latitudes. The overall feature in Fig. 11
suggests that the local Hadley-type circulation
over the Pacific is so intensified during El Ninophase that the warming of the troposphere prevails over the whole tropical belt and the subtropical jet streams are intensified over the wholesubtropics (Fig. 8(a)).
6.

Summary and discussion

The anomalies of SST, SLP, wind, temperature and geopotential height fields during El
Nino periods have been investigated over thewhole globe, by using objectively analyzed timefiltered data sets. It was recognized anew that the
ENSO should be viewed as a global-scaleclimate
system rather than a local atmosphere-ocean
coupled system over the equatorial Pacific Ocean
although the anomalies over the Pacific are remarkably large.
It is noted that the positive SST anomalies
over the equatorial Indian and Atlantic Ocean
appear nearly in phase with the warm episodeover the eastern Pacific. Warming of the upper
troposphere in the whole tropics and the increasing of zonal winds in the subtropics and the
middle latitudes of the two hemispheres are also
prominent at the same time. These evidences
apparently suggest that the Hadley-type circula-February

1987

T. YasunariFig. 10.

Same as Fig. 4 but for (a) streamline

Contours of windms-1 and 0.2ms-1

speed are shown, respectively.

77

at 200mb

with dottedtion is intensified during El Nino episodes mostsignificantly over the central Pacific but also overthe entire tropics. The results here are compatible with those by Pan and Oort (1983).
Another interesting feature is a nearly-symmetric response of the upper troposphere overthe northern and the southern Pacific with theequatorial anomalous heating. Although most of
the previous observational and theoretical studies
stressed the response in the northern mid-latitudes (e.g., Horel and Wallace, 1981), the symmetric response pattern with respect to the equator has also been suggested (e.g., Branstator,and (b) streamlines

lines.

at 700mb.

Units of wind speed

are 0.4

1985).
The NAO-type SLP anomalies over the northAtlantic seem to be consistent with the northsouth contrast of anomalies of geopotential
height (and temperature) in the upper troposphere. The negative SLP anomalies in the southern part and the positive SLP anomalies in thenorthern part imply the general weakening of
Azores high and the southward shift of Icelandic
low, which seems to be consistent with the negative anomalies of geopotential height, temperature and the positive anomalies of zonal wind in
the upper troposphere of the southern part.

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Fig. 11.
Same as Fig. 4 but for (a) temperature
at 300mb and (b) geopotential
heightat 200mb. Units are 0.9* and 4gpm, respectively. Negative values are shaded.

Barnett (1985) stressed the orthogonality of SO
mode and NAO mode based on the CEOF analysis of SLP. However, the present results have suggested a strong association of NAO with SO as
far as our analysis period (1964-79) is concerned. In fact, the two modes of Barnett (1985)
seem to be well correlated each other during thisperiod though they appeared to be independent
each other through the whole period (1951-80)
of his analysis.
In the present study, it has been revealed that
objectively-analyzed time filtered data sets are
effective to examine the global feature of ENSO
related anomalies. The application of the time

filteredseries to truncated spherical harmonics(with m=0 to 5) seems to have been efficient to
extract this mode since ENSO may be a phenomenon of planetary-scale.
In the present paper, however, we have notdiscussed how the anomalies over the whole
globe change in time associated with the evolution of ENSO cycles. This problem will be discussed in Part II of the present study.
Acknowledgements
The author would particularly thank Prof.
T.N. Krishnamurti for his great support, encour-February

1987

T. Yasunari

agement and stimulating
discussions throughout
the work. He is also indebted to Dr. Eugene M.
Rasmusson and Dr. P.A. Arkin for providing the
NMC tropical wind data set. Thansk are due to
Dr. T. Yamanouchi
of National Institute of Polar
Research for supplying some Antarctic
stationdata. The research reported here is supported by
NOAA Grant No. NA85AA-H CA019.
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Weare, B., AR. Navato and R.E.

of the Meteorological

Newell,

1976:

Em-

of JapanVol.

globalSouthern

east-west
circulation
Oscillation.
J. Met.

associatedSoc. Japan,

65, No.

Zebiak,S.E.

andsubmitted

ル ・ニ ー ニ ョ/南

with
the
63, 1013-

M.A.

to J. Atmos.

Cane,

1985:

A

model

ENSO.

Sci.

方 振 動)の 全 球 構造第1部:エル ・ニ ー ニ ョ時 の 合 成

安

成

哲

三*

(フ ロ リダ 州 立 大 学 気 象 学 教 室)

18年 間(1964〜1981)の
客 観 解 析 さ れ た 資 料 を も と に,ENSOに的 な 変 化 を調 べ た 。 客 観 解 析 は320地 点 以 上 の 高 層 観 測 資 料 と,NMC熱乗 法 で 球 面 調 和 函 数(東西 波 数5で切 断)を伴 う海 面 水 温 と大 気 構 造 の 全 球
帯 域 風 資 料 を も と に,最 小 自

求 め る手 法 を 用 い た 。

第1部 で は,エ ル ・ニ ー ニ ョ完 熟 期 に お け る海 面 水 温 と大 気 各 要 素 の 偏 差 パ ター ン を 記 述 し た 。 こ の時 期,海 面 水 温 と対 流 圏 上 層 の 気 温 は,赤 道 中 ・東 部 太 平 洋 付 近 の み な らず,全 熱 帯 域 で 高 く な る こ と,
こ れ に 対 し 中 ・高 緯 度 で は 全 般 に低 くな る こ と,中 緯 度 偏 西 風 は 両 半 球 と も 強 化 さ れ,し か も低 緯 度 側に 偏 位 す る こ とが 明 らか と な っ た 。 こ れ ら の 事 実 は,低緯 度 側 で の ハ ドレー 循 環 の 強 化,高緯度側 での

極 渦 の 拡 大 を示 唆 して い る。
南 北 両 太平 洋 域 にお け る,熱 源 に 対 す る 大 気 の 定 常 応 答 のパ タ ー ン が,赤 道 に 対 し対 称 的 で あ る こ と,
北 大 西 洋 振 動(NAO)と
南 方 振 動(SO)との 同 期 的 変 動 の 様 相 も示 され た 。

*現 在所属:筑 波大 学地 球科 学 系

1

1029.

prical orthogonal
analysis of Pacific sea surface temperatures. J. Phys. Oceangr., 6,671-678.
Yasunari, T., 1985: Zonally propagating
modes of the

ENSO(エSociety