JOURNAL OF GEOPHYSICAL

E1 NifioRESEARCH, VOL. 92, NO. C13, PAGES 14,189-14,196, DECEMBER 15, 1987

Events and Their

Relationto the Southern

Oscillation'

1925-1986

CLARA DESER AND JOHN M. WALLACE
Department of AtmosphericSciences,University of Washington,Seattle

Relationshipsamong sea surfacetemperatures(SSTs) at the coast of Peru and offshore,river discharge
in northern Peru, and sea level pressureat Darwin, Australia, during the period 1925-1986 are investigated using time seriesplots, frequencydistributions,and a simple statisticalanalysis.It is shown that
SSTs undergo a larger seasonalcycle offshorethan at the coast,exhibit more interannual variability
during the warm than the cool season,are positively skewed during much of the year, and exhibit
greatest month-to-month persistenceduring the cool season. Many, but not all, episodesof above
normal coastalSSTs are accompaniedby enhancedriver dischargein northern Peru. Comparisonof the
Darwin pressureand coastalSST recordsduring the past 60 yearsshowsthat El Nifio events(episodesof
above normal SSTs along the coastof Peru) have occurredboth in advanceof and subsequentto major
negativeswingsof the Southern Oscillation (and associatedclimatic changesin the central equatorial
Pacific). In addition, El Nifio events and negative swings of the Southern Oscillation have occurred
separately.Hence El Nifio and the Southern Oscillation are more loosely coupled than other studies
would imply.

1.

INTRODUCTION

The E1 Nifio phenomenon along the coast of northern Peru
and southern

Ecuadorand the Pacific-wide

Southern

Oscil-

lation phenomenonwere discoveredindependently,and until
the 1960s, they were considered unrelated. Carillo [1892]
ascribed the term E1 Nifio

to Peruvian

mariners

who used thetitle to refer to a warm narrow countercurrentflowing southward along the northern coast of Peru shortly after
Christmastime.As such,E1 Nifio was originally viewed as anannual event, although the strength and southernmostextent
of this countercurrentwere observedto vary from year to year
[Eguiguren, 1894]. In his investigation of the interannual
variability of the rains at Piura in northern coastal Peru,
Eguiguren hypothesizedthat the enhancedprecipitation observedduring some rainy seasonsis due to an unusuallywelldeveloped countercurrent. He noted that weakened coastal
southerlysurfacewinds would aid the developmentof the E1
Nifio current. Murphy [1926] documentedthe torrential rains
and unusually strong countercurrentalong coastal Peru andsouthwestern Ecuador during the first few months of 1925.

coast, and allowing the coastal countercurrent to penetrate
further south. Accordingto this hypothesis,"a strong E1 Nifio
would representnothing but an amplification of the [normal]
summer warming" [Wyrtki, 1975]. Wyrtki subsequentlydisproved this hypothesisby showing that the coastal southerlies
actually tend to be slightly stronger than normal during E1
Nifio events.Quinn et al. [1978] provided the first comprehensive documentation on past E1 Nifio events, including statistics on intensity, duration, and frequencyof occurrence;thiswork has recently been revised and updated [see Quinn, thisissue].
The

Southern

Oscillationrefers to the seesaw in surfacepressure anomalies between the Indian Ocean-Australianregion and the southeastern tropical Pacific on seasonal timescales. The Southern

Oscillation

was first documented

in theworks of Hildebrandsson[1897] and Lockyer and Lockyer[1904] and was clearly identified in the landmark paper ofWalker and Bliss [1932], who recognized its relation to acomplex pattern of pressure,temperature, and rainfall anomalies encompassingmuch of the globe. Subsequentstudies byLobell[1942] foundanomalouslywarm surfacetemperatures Troup [1965], Berlage [1966], Kidson [1975], Trenberthin the Peruvian coastal waters during early 1941 but no evi- [1976], Barnett [1985], and many others have confirmed thedence of a southward flowing countercurrent.He concluded relationshipsidentified by Walker and Bliss and have addedthat warm water was advectedonshore,a condition brought many important details. The idea of alternating strengthening
about by a shift in the prevailing winds. Schweigger[1945] and weakeningof the southeasttrades and the equatorial eastproposed that large-scale southeastward advection of warm erlies in the Pacific, first noted by Brooks and Braby [1921],
water from around the GalapagosIslands is responsiblefor has come to be regarded as central to the concept of thethe major climatic changes previously attributed to the E1

Southern

Oscillation.

Bjerknes[1966] postulatedthat weakeningof the equatorial

Nifio current. However, the use of the term E1 Nifio to de-scribe the occasionalappearanceof excessivelywarm water

easterlies

along the coast of northern Peru and southern Ecuador ex-association with negative swings of the Southern Oscillation(i.e., pressurerises over Australia and Indonesia and falls overthe subtropicalsoutheastPacific) reducesequatorial upwellingin the eastern Pacific and thereby leads to an increaseof SST.
Bjerknes used the term E1 Nifio, which had previously beenused exclusivelyin associationwith the SST increasesalong

tendingwestwardto the GalapagosIslandswas already widespread [see Schott, 1931; Gunther,1936; Meats 1943]. Wooster [1960] and Bjerknes['1961] hypothesizedthat warming of
the coastal waters is due to a weakening of the northward
componentof the wind stressalong the coast, reducing upwelling, allowing warmer waters offshore to approach the
Copyright 1987 by the American GeophysicalUnion.
Paper number 7C0605.
0148-0227/87/007C-0605$05.00

across the eastern half of the Pacificthat occurs in

the Peru and Ecuador coast, as a label for this much broader-scale phenomenon, and this usage has been subsequently
adopted by Wyrtki [1975] and many other authors. In thepresentstudy, we adhere to the classicaldefinition of E1 Nifio
as a warming of the coastal waters off Peru and Ecuador.
Wyrtki [1975] postulated that the initial increase of SST
14,189

14,190

DESERAND WALLACE:EL N]•O AND THE SOUTHERNOSCILLATION

along the coast of northern Peru is a remote responseto asudden relaxation of the equatorial winds in the central andwestern Pacific. The increasein SST in the eastern equatorial
Pacific leads to a reduction of the atmosphericsurfacepressure gradient across the Pacific which further weakens theequatorial zonal winds.The dynamic couplingof E1 Nifio and
the Southern Oscillation has been confirmed by the results ofequatorialoceanmodels(for example,seethe reviewby Cane[1983]), which have proved to be capable of simulatingmany
aspectsof the observedchangesin sea level, sea surfacetemperature (SST), equatorial currents, and thermocline depththat occur in association

with E1 Nifio

events.

Rasrnusson
and Carpenter[1982] carried the synthesisof El
Nifio and the Southern Oscillation phenomenona step further
by presentinga Pacific-wide compositebased on six E1 Nifio
events along the Peru coast during the period 1950-1975.
Their study has provided what has come to be known as the
canonical E1 Nifio-Southern

Oscillation event, characterizedby (1) strongpositiveSST anomaliesextendingfrom the Galapagos Islands eastward to the south American coast during
January-May and weaker but spatially coherentwarming of
SST extending across the entire equatorial Pacific east of170øE peaking around October, (2) heavy rainfall along thePeru coast and over the central equatorial Pacific, and (3) apronouncednegativeswing of the Southern Oscillation.The
conceptof E1 Nifio and the Southern Oscillation as a coupledphenomenonhas gained such wide acceptancethat the acronym ENSO (El Nifio-Southern Oscillation), used extensively
in the Rasmusson and Carpenter paper, has been widelyadopted as a label for large scale atmosphere-oceaninteraction in the tropical Pacific, and it has even been used as aname for specificepisodesof positiveSST anomalies[Fu et al.,lation coefficientsbetween monthly SST anomalies(smoothedwith a three-point binomial filter) at Puerto Chicama andTalara (4.5øS)and at Puerto Chicama and Callao (12øS)are
0.84 and 0.90, respectively,for the period 1950-1979. Puerto
Chicama is the only coastal site between4øS and 12øSproviding SST data sincethe 1920s;other coastalSST recordsbegin
in the late 1940s.In this section,we compare Puerto Chicama
and offshore SSTs by examining their frequency distributions
as a function of month (Figures 1 and 2). Table 1 provides
more quantitative information on the frequencydistributions,
i.e., means, standard deviations, and skewnesscoefficients as afunction of calendar month. Unless otherwise noted, the results for Puerto Chicama SSTs are corroborated by the shorter SST records at other coastal sites between 4øS and 12øS.

SST is generally lower at Puerto Chicama than offshore
throughout the year, despitethe fact that the mean latitude of
the ship track coincideswith the location of the coastalstation(8øS).In fact, the annual mean SST is higher offshore than at
any coastal station between 4øS and 12øS [Lagos et al., thisissue].The averagedifferencebetweenSST at Puerto Chicama
and offshore ranges from around 2.5øC during JulySeptember to over 5.0øC during the warm season,JanuaryApril. The relative coolnessof the nearshore surface water is1986].

In the presentstudy we will take a more critical look at therelationshipsinvolved in the E1 Nifio phenomenon along thecoast of Peru. Specifically,we will considerthe increaseof SST
and the enhancement of rainfall along the northern Peru
coast. In contrast to the works of Wyrtki [1975] and Rasrnusson and Carpenter [1982], which focussedon E1 Nifio events
during the 1950-1975 period, we examine 60 years of SST
data from ship reports and a coastal station. We also makeuse of a newly available time seriesof river dischargein northern Peru. We will show that when the full 60-year record istaken into account,the relationshipsbetweenE1 Nifio and theSouthern Oscillation are not as strong and as straightforward
as might be inferred from previousstudiesencompassingthe
two phenomena.
2.

COASTAL VERSUS OFFSHORE SSTTime seriesof monthly mean SST for Puerto Chicama (8øS,80øW)and the shippinglane along the coastof northern Peru
were provided by E. Rasmussonof the University of Maryland, CollegePark. The ship track data, hereafterreferredto
as "offshore",representthe averageSST for latitudes4ø-12øS,
betweenthe coastand approximately5ø of longitudeoffshore.
The offshore SST series for the periods of 1921-1938 and1949-1976 were presented by Rasmusson and Carpenter[1982] (their SST1); the serieswe are usinghas beenupdated
to July 1986.The SST recordfor Puerto Chicamacoversthe
period January 1925 throughJune 1986. SST anomaliesat
PuertoChicama22

are well correlatedwiththose at othercoastal sites between 4øS and 12øS;in particular, the corre-

oø111111
N

D

J

F

M

A

M

J

J

I'1'1'1ø111/
A

5;

0

N

D

J

F

Fig. 1. (top) Distribution of SST (in degreesCelsius)at Puerto
Chicama(8øS,80øW)during 1931-1986,stratifiedby month. Each dot
marks the mean SST for an individual year-month. Each column ofdots within a given calendar month representsa particular decade;i.e., the first (last) column of dots containsdata for the 1930s(1980s).
November through February are repeated.Data for November 1982
through July 1983 are marked by open circles.(bottom) Standard
deviation (in degreesCelsius)of SST anomaliesat Puerto Chicama,
stratifiedby month. Dots representvaluesbasedon the period 19311986;opencirclesdenotevaluesbasedon the periods1931-1981and
1984-1986.

DESERANDWALLACE:
EL NIlqOANDTHESOUTHERNOSCILLATION
coast of Peru relative

14,191to that several hundred

kilometers

off-

shore may be, in part, a direct consequenceof the narrowhorizontal scale of the coastal upwelling. The surface water
within the narrow coastal upwelling zone originatesfrom between 20-m and 40-m depth where the annual temperature
variation is relatively weak.
It is apparent from Figures 1 and 2 that the shape of themean annual SST curve is markedly different at Puerto
Chicama than offshore.The mean seasonalcycle of offshore
SST can be closelyapproximatedby a singleharmonic,withwarming from Septemberto March and cooling during therest of the year. However, at Puerto Chicama the mean seasonal cycle is asymmetric,with rapid warming from November to February and more gradual cooling from February to
October.

SSTs at other coastal stations between 2øS and 15øS

do not exhibit such pronounced asymmetry of the annual
cycle [Lagos et al., this issue].
As can be seen from Figures 1 and 2, SSTs tend to exhibit
larger dispersionabout the monthly meansat Puerto Chicama
than offshore.In particular, the 1982-1983 temperatureanomalies, highlighted in the figures, were far more extreme at
Puerto Chicama than offshore. The monthly standard deviations of the SST anomalies, which are a measure of interan-

N

D

J

F

M

A

M

J

J

A

S

0

N

D

J

nual variability, are shown in the bottom panels of Figures 1
and 2. (The standard deviations of the SST anomalies computed without data for 1982-1983 are also shown in Figures 1
and 2.) Both time seriesexhibit pronounced seasonalvariations in standard deviation, with a maximum in May and a

F

Fig. 2. As in Figure 1 but for offshoreSST during 1921-1939 and
1948-1986. (Offshore SST is the average SST in the region 4øS to12øS,within 5ø of longitude of the coast.) Standard deviations are
based on the periods 1921-1939 and 1948-1986 (dots) and 1921-1939,
1948-1981, and 1984-1986 (open circles).

primarily due to coastal upwelling, which along the coast ofPeru between 4øS and 12øS is confined to within

about 50 kmof the shore [Smith, 1981; Guill•n and Calienes,1981].
The annual range of SST averages about 6øC offshore,
nearly twice the amplitude at Puerto Chicama. In fact, offshore SSTs exhibit a substantiallylarger annual cyclethan do
SSTs from any coastal station between 2øS and 15øS [seeLagoset al., this issue].This differencein amplitudeis particularily pronounced for the cold limit of the SST "envelope"which undergoesonly a small (2øC) annual variation at Puerto
Chicama versusa 5øC annual variation offshore.(In 1950 and1981 the annual range of SST at Puerto Chicama was only
1.8øC.)The reduction of the mean annual range of SST at the
TABLE

1.

Momentminimum around October. In addition, Puerto Chicama SST

exhibits a second maximum in standard deviation during
January-February, when the standard deviation of offshore
SST is near a minimum.

This distinction

between the two SST

seriesis even apparent in the frequencydistributions themselves.Rasmussonand Carpenter [1982] noted that the behavior of SST offshore and at Puerto Chicama was markedly
different during the tail end (January-February) of major El
Nifio events: a secondanomaly maximum during JanuaryFebruary was prominent at Puerto Chicama but was barely
detectable

in offshore

SST.

We have computed the monthly standard deviations of SST
for non-E1 Nifio years separately (not shown) and found asimilar, albeit lesspronounced,pattern of seasonalvariation.
In particular, for Puerto Chicama SST, the maximum standard deviationis reduced from 1.8øC to 1.3øC while the mini-mumdeviation

standard

falls from

0.6øC to 0.5øC when

Nifio years are excludedfrom the calculation.

Statistics for SST Anomalies

Jan.

Feb.

March

April

May

Chicama
Offshore

17.4
23.3

18.9
24.3

18.6
24.5

17.9
23.4

17.5
21.9

Chicama
Offshore

2.0
1.0

2.0
1.0

1.6
1.2

1.8
1.4

Chicama

1.2

0.7

1.5

Offshore

0.7

0.7

0.9

2.1
0.5*
1.2

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

16.7
19.4

16.2
18.8

15.8
18.3

15.5
18.7

15.6
19.8

16.2
21.2

Standard Deviations
2.0
1.8
1.2
1.5
1.5
1.4

0.8
1.2

0.7
0.9

0.9
0.9

1.1
0.9

1.7
0.9

Means

17.2
20.6

SkewnessCoefficients
2.3
0.9*

2.5
0.8*

1.1

0.9

0.2

0.5

2.2

1.7

1.2

1.3

1.3

1.1

0.6

0.6

0.8

0.8

Statistics are based on 1931-1986 for Puerto Chicama and on 1920-1939*Values

exclude data from 1982-1983.

and 1949-1986 for offshore.

El14,192

DESER
ANDWALLACE:
EL NnqoANDTHESOUTHERNOSCILLATIONTABLE 2. Lag-AutocorrelationCoefficientsx 100for PuertoChicamaSST as a Functionof Month, Basedon the Period 1931-1986
Feb. March April May June July Aug. Sept. Oct. Nov. Dec. Jan. Feb. March April May June July Aug. Sept. Oct. Nov.
Jan.
Feb.
March

62

47
76

36
56
82

April
May

13
-9
-16
-12

11
-15
-22
-17

-12
-11
-1

30
45

0
25
44

-19
--19
-10
-12
-7
20
41

43

32

28

35
61
81

35
61
75

14
55
63

10
45
58

20
51
61

25
40
49

19
19
32

4
23
34

16
18
26

7
4
13

-6
-8

93

85
94

68
77

59
66

54
66

50
62

33
42

32
42

14
27

14
11

--4
--5

85

71

66

61

37

40

22

5

-6

89

81
89

71
74
83

49
50
61

59
58
64

35
34
49

18
16
19

7
1
4

84

76
85

63
75

32
45

81

48

June

July
Aug.
Sept.
Oct.
Nov.
Dec.

5

-5

-20
-28
-19

7

--1
-24
-34
-19

--20
--31
--18

-17
-22
-11

-19
-19
-12

--4
-17
-14
-11

-21
-21
-14

-25
-23
-19

-24
-22
--16

-15
-15
-4

-13
-12
-8

-12
-9
--6

-16

-18

-14

0

-6

-4

-13
18
38

-17

--13

3
23
20

5
19
11

11

7

24

0

0

4
8

3
3

5
20
19

3

0

11

6

The reduction of the SST anomalies during the cold season calendar month for SST anomalies at Puerto Chicama andrelative to the warm seasonmay be related to the annual cycle offshore,respectively.These correlation functionsare basedonof mixed layer depth. The mixed layer is deepest (around unsmoothed monthly mean anomalies.The 1-month lag auto40-60 m) and the stratificationin the upper 100 m is weakest correlations for Puerto Chicarea SSTs exhibit a pronounced
during the cold season[Wyrtki, 1964]. Thus SSTs should be seasonalvariation, with maximum values in April and May,
relatively insensitiveto anomaloussurfaceheating and vertical and minimum valuesin January and February. In a statisticalmixing during the cold season.However, during the warm sense,the time series of January SST anomalies accounts forseason,when the mixed layer is shallow (around 20 m) and the only about 40% of the variance of the February SST anomastratification below the mixed layer is strong, anomalous net lies, whereas the SST anomalies in May explain over 90% of
surfaceheating and vertical mixing can causelarge changesin the variance of those in June. Warm season (February-June)
SST. It should be noted that during major El Nifio eventssea anomalies tend to persistthrough the subsequentcool season
level height anomaliesalong the Peru coast diminish during (July-November), but there is much less "memory" from thethe cold season [Wyrtki, 1975], suggestinga more dynam- cool seasoninto the subsequentwarm season.It is evident
ically orientedinterpretationof the correspondingSST anom- that the January-February period is critical for the subsealies.
quent evolution of the Puerto Chicarea SSTs. The overall
The monthly coefficientsof skewnessfor Puerto Chicarea pattern of the autocorrelationsis similar for offshoreSST, but
and offshore SST are listed in Table 1. (The coefficient of the 1- to 3-month lag correlationsare substantiallyhigher at
skewness(k) is definedas the third moment coefficientdivided all seasonsand the "lapse in memory" is not as sharply conbythecuberootof thevariance).
If k exceeds
0.64,thedistri- centratedin the January-February interval. It shouldbe notedbution is significantlydifferent (at the 95% confidencelevel) that these statisticsdo not necessarilycharacterize all years
from a normal distribution for a sample size of 60. The posi- individually: in particular, coastal SST anomalies during two
tive skewnessexhibited by the SST seriesis readily apparent of the strongest E1 Nifio events (1982-1983 and 1940-1941)
in the frequencydistributionsshown in Figures 1 and 2. It is began in the cold season and persisted into the following
evident from Table 1 that the positive skewnessis highly sig- warm season.We have computedautocorrelationstatisticsfor
nificant during November-January and March-August at non-E1 Nifio years separately(not shown) and found that the
Puerto Chicarea and April-August offshore.(If we disregard autocorrelation coefficientsare similar during the cold season
the extreme anomalies associated with the 1982-1983 E1 Nifio,
but are generally lower at all lags during the warm season
when El Nifio years are excludedfrom the calculation.
the skewnessfor Chicarea drops to around 0.8 during AprilFigure 3 shows the time series of monthly mean SSTs at
June but is still highly significant.)In contrast,the frequency
distribution for SST in the central equatorial Pacific exhibits Puerto Chicama during 1925-1986, together with the meanannual cycle for the period 1950-1979. SST anomalieslarger
negligibleskewness[Wri•lht et al., 1987].
Tables 2 and 3 show the autocorrelations as a function ofthan IøC are shaded. The shape of the mean annual cycle

TABLE 3. Lag-Autocorrelation
Coefficientsx 100forOffshore
SSTasa Functionof Month,BasedonthePeriods1921-1939
and
1948-1986

Feb. March April May June July Aug. Sept. Oct. Nov. Dec. Jan. Feb. March April May June July Aug. Sept. Oct. Nov.
Jan.
Feb.
March

April
May
June

July
Aug.
Sept.
Oct.
Nov.
Dec.

86

72
91

53
75
90

48
70
85

44
66
78

36
59
71

32
56
67

34
53
61

35
55
61

31
46
50

27
43
45

16
36
36

12
24
21

5
10
7

94

88
96

82
91

76
86

68
77

64
71

53
61

45
54

34
42

24
29

13
16

96

91

82

75

63

55

45

32

17

94

84
90

76
87
94

64
71
85

56
65
77

47
56
67

35
42
49

18
25
35

92

86
95

76
83
93

57
67
77

40
51
58

4

1
-3
2
6
7
8
13
25
28
38
43

-4

-8
-8
-7
-11
-13
-12
-17
-19
--7 --11
--12
0
--4
--6
0
--3
--5
0
0
--2
6
4
--1
14
11
5
18
14
5
33
30
22
39
37
29

-16

--5
-21
-11
-25
-12
--19
--11
--15
--8
--15
--7
--14
--7
--10
--7
--2
0
-- 1
0
16
16
24
26

--7
-12
-11
--7
--6
--5
--6
--4
4
5
20
30

1
--6
-1
2
3
4
2
1
10
10
27
32

DESERAND WALLACE'EL Nl•qO AND THE SOUTHERNOSCILLATION
24-

1925 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

14,193successivepeaks in the coastal SST anomalies, the first typically in February through June and the second in December
through February. However, if the monthly SST anomalies
are normalized by their standard deviations, this double peakstructure becomes less pronounced. Hence significant positive
SST anomalies are nearly always present during the coldseasonof E1 Nifio years, but their magnitudes are usually less
than

IøC.

The time seriesof offshore SST [see Rasmussonand Carpenter, 1982, Figure 11] lacks the pronouncedsecond(December1200

1946 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67

IOOO

8OO

196768 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88

T

Fig. 3. Time series of monthly SST at Puerto Chicama during
1925-1986 (solid curve) superimposedupon the mean annual cycle ofPuerto Chicama SST based on the period 1950-1979 (dashed curve).
Monthly anomalies in excessof iøC are shaded. Anomalies have beensmoothedwith a three-pointbinomial filter. Vertical lines mark January of each year.

E 600

curve would not be noticeably altered if E1 Nifio years had
been left out of the monthly averages. We have reason tosuspect that the temperatures during 1925-1930 are biased
toward the high side: The offshore SST anomalies, which
otherwise agree well with Puerto Chicama values, run about
IøC lower than the Puerto Chicama SST anomalies during
this period. (The correlation coefficient between monthly

200

400

N
600

D

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F

M

A

M

J

J

_

A

['[][I

S

0

N

D

][[[

J

F

[[[[l•

anomalies of Puerto Chicama and offshore SSTs, smoothedwith a three-point binomial filter, is 0.86 for the period 19311986.) Despite these reservations, we have confidence in the
short-term E1 Nifio signal superimposedon this probable bias.
All periods of SST anomalies above IøC (the shaded intervals in Figure 3), except 1927-1928, 1942, and 1949, correspond to E1 Nifio events as identified by Quinn et al. [1978].
(Quinn et al.'s criteria for E1 Nifio events consist of a wide
variety of climatic indicators, including above normal SSTs
and rainfall along the coastsof Ecuador and Peru and in thecentral and eastern equatorial Pacific, low Southern Oscillation index, and disruption of the marine ecosystemoff thecoast of Peru.) The well-known irregularity of E1 Nifio occurrencesis apparent in Figure 3:E1 Nifio events may occur
as closeas 1 or 2 years apart, or longer than 6 years apart asin the decade of the 1930s. The durationof E1 Nifio

October (1982, 1940, and 1930).Theseaspectsof past E1 Nifio
eventshave been noted by previousauthors [e.g., Quinnet al.,1978].
coastalSSTanomalies

associated

with

L

E1 Nifio

are

ql 72

-

[

[

[

I•s •
I

I
I

400

[

3OO

73
2OO

zt965

2539
••

I
I

ti3

7 33
I • ?• •:lI•' • I••I00

ß' : : " ' ' I

events isalso variable: an event may last from a few months (e.g., in
1932, 1943, and 1949) to a full year (e.g.,in 1957, 1965, 1972,
and 1982-1983). It can be seen from Figure 3 that E1 Nifio
frequentlysetsin abruptly during February (e.g.,in 1932, 1953,
1957, 1965, and 1972); E1 Nifio has also begun in January(1939 and 1943), March (1953), and April (1969) and around

The

5OO

J

F

M

A

M

J

Fig. 4. (a) Distribution of discharge(in cubic meters per second)from the Piura River at Piura (5øS,81øW) during 1925-1983, stratified
by month. Each dot marks the median discharge for an individualyear-month. Each column of dots within a given calendar month
representsa particular decade; i.e., the first (last) column of dots
contains data for the 1920s (1980s). November through February are
repeated.Data for November 1982 through July 1983 are marked byopen circles. (b) As in Figure 4a but for January through June only,

1925-1981.Dischargesgreaterthan 100 m3 S-1 are identifiedby theinvariably most pronounced during the warm months, De- last two digits of the year in which they occurred. Data are missing
cember through June (see Figure 1). Therefore those events for March and April 1925 and for 1950-1951. Note that data forthat begin around February and last 1 year appear as two 1982-1983 have been omitted.

14,194

DESERAND WALLACE:EL NifiO AND THE SOUTHERNOSCILLATION
4OO

300

•

200

.•

•oo

E

o

o

........
,......
•............

1925 26 27

28 29 30',,'-..,'....... ,"• !i.,".,,......

",,,_.,:,..,.,.,...
,.:"',,,/.....
-,f",,.,,,;',,,.
.... 31 32 33 34

35

36 37

38 39

,,......

40 41

42

45 44

45

400

300

7
•

200

.•

•oo

•

o

6

•

400

-

5
4

300 •
200 rq

•

0

3

IO0

•=

I

-2

1965 66 67

68""':':-' "'""

69 70 71

72

73 74

75 76

'

77 78 79 80'"""" -81 82

83

84

85

Fig. 5. Time seriesof monthly discharge(in cubicmetersper second)of the Piura River at Piura (solidcurve)and SST
anomalies(in degreesCelsius)at Puerto Chicama (dashedcurve) during 1925-1983. The data have been smoothedwith athree point binomial filter. Vertical linesmark Januaryof each year. Note that dischargevaluesare missingfor March andApril 1925, and for 1950-1951; data offscaleduring 1983 are omitted.

February) peaks present in the Puerto Chicama record. Offshore SST remains above normal during thesemonths toward
the end of E1 Nifio events, but the anomalies tend to be muchsmaller than on the coast (seeFigure 2).
3.

PIURA RIVER RUNOFF AND ITS RELATION
TO PUERTO CHICAMASSTRiver dischargefor January-June only, 1925-1981; months ofhigh dischargeare labeled by year.
It is interestingto note that the onset of the rainy seasoninnorthern coastal Peru occurs when SSTs are already at theirseasonal maximum. Precipitation is much heavier in April
than in January; yet SSTs along northern Peru are nearly
identical for these 2 months. Other factors besides SST, such

We use the median discharge of the Piura river at Piura(5øS,81øW) as a measure of rainfall integrated over a drainage

as low-level wind convergenceand static stability, must play astrong role in determining the timing of the rainy seasonin

basinroughly 7500 km2 in extent.The low-lyingPiura Rivernorthern

receivesalmost no snowmelt from the Andes (R. Mugica, personal communication, 1987). This dischargerecord covers theperiod 1925-1983 and was kindly provided by R. Mugica of
the Universidad de Piura. The streamflow recorder was swept
away by floods during March 1925 [Murphy, 1926-1,and data
are also missing for 1950-1951 (see Horel and Cornejo-Garrido[1986] for further discussionof this record). It should be noted

Peru.

Figure 5 shows the time series of Puerto Chicama SST
anomalies and Piura River dischargeat Piura, smoothed witha three-point binomial filter, during 1925-1983. It can be seenthat many years of above (below) normal SST are accompanied by above (below) normal streamflow. Notable exceptions
are 1939 (very high runoff but only small positive SST anomalies), 1948 and 1969 (negligible runoff but substantial positivethat annual rainfall totals at individual
stations in the lowland
SST anomalies), and 1933 and 1934 (below normal SSTs but
plains of Peru and Ecuador are only moderately well corre- above averagerunoff). The prolonged E1 Nifio eventsof 1925,
lated with annual runoff of the Piura River. The correlation
1957, and 1972 appear to have enhanced the runoff in thecoefficients between anomalies of annual totals of Piura River
subsequentcalendar year, but the prolonged event of 1965 did
runoff and precipitation at Guayaquil (2.1øS, 80øW), Talara not produce substantial runoff in 1966. Piura River discharge(4.5øS,81øW), Lambayeque (6.6øS,80øW), and Chiclayo (6.8øS, in 1943 was one of the largest on record, yet Puerto Chicama80øW) are 0.77, 0.23, 0.64, and 0.45, respectively,based on the SST was above normal for only the first 2 months of the year.
In contrast, SST was above normal from May to December inperiod 1950-1979.
Figure 4a showsthe distribution of the median dischargeof 1976, but runoff was only slightly enhanced.
the Piura River at Piura as a function of month during 1925It appearsthat the precipitation responseis sensitiveto the1983. On average, the largest flows of the Piura River occur timing of the SST anomaly associatedwith El Nifio. Figure 6
from February through May. There is a lack of appreciable showsthe runoff for years with above normal Puerto Chicama
streamflow during August-December, even in E1 Nifio years. SST during December-February but not later (1926, 1943,
1958, 1966, 1973, and 1977) and years with above normal
Record flows occurred in January 1983 and from Marchthrough July 1983. Figure 4b showsthe distribution of Piura Puerto Chicama SST during February-June but not before

DESERAND WALLACE' EL NIlqO AND THE SOUTHERNOSCILLATION

14,195

SST than to offshore SST, Darwin, Australia, sea level pressure (SLP), or Tahiti SLP. The correlation coefficients be-

tween anomalies of January-June averages of Piura Riverrunoff and seasonalaveragesof Puerto Chicama SST, offshore

x

500

SST, Darwin SLP, and Tahiti SLP are 0.74, 0.58, 0.40, and-0.39, respectively,for the period 1931-1983.

600
400

4.

EL NIfO

AND THE SOUTHERNOSCILLATION

Seasonal anomalies of Darwin SLP are highly correlated(correlation coefficientsnear 0.9 for the period 1950-1979)

300

xø¸ø
øo
øøø
øx

with seasonal anomalies of SST, rainfall, and zonal wind in200

the central equatorial Pacific (160-170øW) and with the conventional Southern Oscillation indices (e.g., Tahiti minus
Darwin SLP) [Wright et al., 1987]. Therefore Darwin SLP

anomalies

I00

can

be used as an indicatorof the stateof theSouthern Oscillation and the associated large-scale climaticchangesin the central equatorial Pacific.
Smoothed monthly anomalies of Darwin SLP and Puerto
Chicama SST for the period 1925-1985 are shown in Figure 7.
J
F
M
A
M
J
SST anomalies are plotted only when they exceed IøC. The
Fig. 6. Distribution of discharge (in cubic meters per second) monthly SST and SLP anomalies have been smoothed withfrom the Piura River at Piura during January-June, 1925-1981. Years single and double applications of a 3-point binomial filter
with above normal SST at Puerto Chicama during December- respectively.It is evident that E1 Nifio events (indicated by
February but not later (1926, 1943, 1958, 1966, 1973, and 1977) aremarked by crosses;years with above normal SST at Puerto Chicama shading) occur in a variety of juxtapositions with episodesof
during February-June but not before (1932, 1948, 1949, 1953, 1957,
1965, 1969,and 1972)are marked by open circles.

(1932, 1948, 1949, 1953, 1957, 1965, 1969, and 1972). The lattergroup of years shows a greater enhancement of rainfall toward
the end of the wet season (March-April), while the formergroup exhibits a more modest enhancement of runoff (with the
exception of 1943) near the onset of the wet season.
Finally, it should be noted that on interannual time scales,
Piura river runoff is more closely related to Puerto Chicama

abovenormalDarwinSLP.

E1 Nifio

events have occurred

3

E0--tO_.

1945•

•V

V'-"
.

47 48 49

50 51 52 53 54 55 56 57 58 59 60 61 6•I

63 64 65

•'•'

2

S0
-2

19656667 68 69 70

71 72

73 74

75

in

advance of episodes of positive Darwin SLP anomalies (as in
1925, 1951, 1953, 1957, 1965, and 1972) and just subsequentto
them (as in 1930-1931, 1940--1941, and 1982-1983). It is notable that the 1950-1975 period, which was the basis for the
Rasmussonand Carpenter [1982] ENSO composite,was dominated by eventsbelongingto the former group.
It is also evident from Figure 7 that E1 Nifio events haveoccurred in the absence of pronounced positive Darwin SLP
anomalies (as in 1932 and 1943). The E1 Nifio events of 1932

76

77 78

79

80 81

82 83

84 85

Fig. 7. Time seriesof monthly SLP anomalies(in millibars)at Darwin (solidline) and SST anomaliesin excessof I•C
at Puerto Chicarea(shaded)during 1925-1985.The SST and SLP anomalieshave beensmoothedwith singleand double
applicationsof a three-pointbinomial filter respectively.Note that the origin of the ordinatefor the SST anomaliesis IøC.
Verticallinesmark Januaryof eachyear.Valuesoffscaleduring 1982-1938are not plotted.

14,196

DESERAND WALLACE:EL Nifoand 1943 were associatedwith heavy rains and severeflooding
along the coastal plain of northern Peru [Sheppard, 1933;
Alpeft, 1946] and thus rank as being comparablein importance to the other major (and perhapsmore familiar) eventsof
1925-1926, 1940-1941, 1957-1958, and 1972-1973. Offshore

AND THE SOUTHERNOSCILLATION

Center, Natl. Meteorol. Center, Natl. Oceanic and Atmos. Admin.,Washington,D.C., July 1984.Available from translator.)Fu, C-G., H. F. Diaz, and J. O. Fletcher, Characteristics of the re-sponseof sea surfacetemperaturein the central Pacific associatedwith warm episodesof the Southern Oscillation, Mon. Weather
Rev., 114, 1716-1738, 1986.

SSTs were well above normal during 1932; the offshore SST

Guil16n, O., and R. Calienes, Upwelling off Chimbote, in Coastal
Upwelling,Coastaland EstuarineSci., vol. 1, edited by F. A. Richindex was not calculated in 1943 for lack of observations. The
ards, pp. 312-326, AGU, Washington, D.C., 1981.
lack of a Darwin SLP signal in 1932 and 1943 is consistent
Gunther, E. R., A report on oceanographicalinvestigationsin thewith observedsurfacepressuresin these years at other AusPeru Coastal Current, DiscoveryRep., 13, 107-276, 1936.
tralian and Indonesian stations and with the lack of above
Hildebrandsson,H. H., Quelquesrecherchessur les centresd'action
de l'atmosphere,K. Sven.Vetenskapsakad.
Handl.,29, 33 pp., 1897.
normal SST and rainfall in the central equatorial Pacific (notHorel, J. D., and A. G. Cornejo-Garrido, Convection along the coast
shown).
of northern Peru during 1983: Spatial and temporal variation ofConversely,it is also evidentfrom Figure 7 that the major
clouds and rainfall, Mon. Weather Rev., 114, 2091-2105, 1986.
negative swings of the Southern Oscillation that occurred in Kidson, J. W., Tropical eigenvectoranalysis and the Southern Oscillation, Mon. Weather Rev., 103, 1'87-196, 1975.
1963 and 1977 were not accompaniedby E1 Nifio events.A
similar lack of correspondencewas observedin 1923, when Lagos, P., T. P. Mitchell and J. M. Wallace, Remote forcing of sea
surfacetemperaturein the El Nifio region, d. Geophys.Res., thisDarwin SLP was well above normal (not shown).The pronouncedpeaks in Darwin SLP anomaliesduring 1923, 1963,
and 1977 were attended by above normal SSTs and rainfall in
the central equatorial Pacific and below normal SLPs at
Tahiti (not shown),yet eastern Pacific SSTs (coastaland offshore)and rainfall were about average.

issue.

Lobell, M. J., Some observations on the Peruvian coastal current, Eos
Trans. AGU, 332-336, 1942.

Lockyer, N., and W. J. S. Lockyer, The behaviour of the short-periodatmosphericpressurevatioation over the earth's surface,Proc. R.
Soc.London,73, 457-470, 1904.

are more

Mears, E.G., The ocean current "The Child," Smithson. Inst. Annu.
Rep., 245-251, 1943.

looselycoupledthan the studiesof Bjerknes[1966] and Rasmussonand Carpenter [1982] would imply. Evidently, there
are strong local controlson the climate of the easternequa-

Murphy, R. C., Oceanic and climatic phenomena along the west coastof South America during 1925, Geogr.Rev., 16, 26-54, 1926.
Quinn, W. H., V. T. Neal, and S. E. Antunez de Mayolo, E1 Nifio
occurrencesover the past four and a half centuries,d. Geophys.

HencetorialE1 Nifio

and the Southern

Oscillation

Pacific that sometimes transcend the influence of theSouthern Oscillation. Year-to-year variations in coastal SSTs
may not necessarilybe reflected in swings of the Southern
Oscillation and associatedlarge-scaleclimatic changesin thecentral equatorial Pacific, but they could have an important
influenceon local rainfall. An understandingof the local climatic controls in the eastern equatorial Pacific would constitute an important step toward a full explanation of the ENSO
phenomenon.

Res., this issue.

Quinn, W. H., D. O. Zopf, K. S. Short, and R. T. W. Kuo Yang,
Historical trends and statistics of the Southern Oscillation, E1
Nifio, and Indonesian droughts,Fish. Bull., 76, 663-678, 1978.
Rasmusson,E. M., and T. H. Carpenter, Variations in tropical sea
surface temperature and surface wind fields associated with theSouthern Oscillation/E1 Nifio, Mon. Weather Rev., 110, 354-384,
1982.

Schott, G., Der Peru-Strom und seine nordlichen Nachbargebietein
normaler und anormaler Ausbildung, Ann. Hydrogr. Mar. Meteo-rol.,59,161-252,1931.
Schweigger, E., La "legitima" corriente del Nifio, Bol. Cia. Adm.

Acknowledgments.We would like to thank Kay Dewar for her
Guano, 21, 255-296, 1945.
help in preparing the figures.This work was supportedunder grant Sheppard, G., The rainy season of 1932 in southwesternEcuador,
8318853from the National ScienceFoundation'sClimate Dynamics
Geogr.Rev., 23, 210-216, 1933.
Program Office. The suggestionsand commentsof the anonymous Smith, R. L., A comparisonof the structure and variability of the flow
reviewershelpedimprove the manuscript.
field in three coastal upwelling regions: Oregon, northwestAfrica,and Peru, in Coastal Upwelling, Coastal and Estuarine Sci., edited
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C. Deser and J. M. Wallace, Department of AtmosphericSciences,
AK-40, University of Washington, Seattle, WA 98195.

(Received March 2, 1987;
acceptedMarch 25, 1987.)