JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 92, NO. C13, PAGES 14,291-14,296, DECEMBER
15, 1987
Remote Forcing of Sea Surface Temperature in the El Nifio Region
PABLO LAGOS1
Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle
TODD
P. MITCHELL
Department of Atmospheric Sciences, University of Washington, Seattle
JOHN M. WALLACE 2
Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle
The relationship between oceanic Kelvin waves forced over the western equatorial Pacific and
seasonaland nonseasonalsea surfacetemp9rature(SST) variability at the South American coast is
investigatedusing'harmonic dial analysis and lag correlation statistics. The seasonalcycle of coastal
SSTisadequately
described
bythefirsttwoharmonics
andisveryregular
inbothEl NifioandnonEl
Nifio years. In contrast, the seasonalcycle of zonal wind over the western equatorial Pacific varies
from year to year and is dominatedby years of large negativeswingsof the SouthernOscillationindex.
Hence it seemsunlikelythat the windsover the westernequatorialPacificcontributestronglyto the
seasonalcycleof coastalSST. For nonseasonal
variabilitythe winds,with periodsshorterthan 15
monthsprecedefluctuationsin coastal'SST by 2 to 4 months,consistentwith the remote forcing
hypothesis,
whileforthelowerfrequencies
theSSTleadsthewindsby 2 to 3 months,
similarto El
Nifiocomposite
zonalwindsof Rasmusson
andCarpenter
(1982).
1.
eastern Pacific. A common feature of these latter studies is
INTRODUCTION
A recurrent theme in the recent El Nifio literature is the
role of remote forcing in producing variations in sea surface
temperature (SST) along the South American coast. Wyrtki
[1975] was the first to propose that remotely forced,
downwelling oceanic Kelvin waves initiate the rapid rise of
South American coastal SST •ssociated with El Nifio. Ac-
that they emphasizethe role of fluctuations in the central and
western equatorial Pacific winds in the nonseasonalvariability of the eastern Pacific Ocean.
In the present study we will use simple, empirical techniquesto make someinferencesconcerninghow much of the
climatological mean seasonal cycle and nonseasonal vari-
by fluctuations
in the
cordingto Wyrtki's hypothesiS,
the oceanicKelvin waves abilityof coastalSSTcanbeexplained
that initiateEl Nifio are the ocean'sresponseto a relaxation zonal winds over the western and central equatorial Pacific.
of abnormally
strong
easterlies
overthewestern
andcentral The study will proceed in several steps. First we will
equatorial Pacific in the year before the warming. The
downwelling waves act to warm the eastern Pacific and
examinethe climatologicalmonthly mean SST along the
South American coast. Then the climatological"
monthly
SouthAme{ican
coastby adveCting
warmerwaterfromthe mean zonal winds over the equatorial Pacific will be pre-
west and by depressing
the thermocline,therebyreducing sented, essentially replicating Meyers' analysis of shipthe coolingeffectof upwelling(seeCane [1983]for a review). derivedzonalwindstress,butfor zonalwindonlyandfor a
'basedondataat islandstations.
We are
Subsequent
observational
studies
of ship-derived
zonalwind longerdatarecord
stressby Meyers [1979] and experimentswith linear and
nonlinear reduced-gravity equatorial ocean models by
Busalacchi and O'Brien [1980] and Kindle [1979] have
emphasized the role of the climatological semiannualharmonic of zonal wind stressover the central equatorial Pacific
in forcingthe-climatological
semiannualharmonicin the
easternPacificpycnoclinedepth.For the nonseasonal
variability, Busalacchi and O'Brien [1981] and Busalacchi et al.
[1983] hfive forced a linear equatorial reduced-gravity ocean
model with the observed equatori• Pacific wind stress.for
indebted
to D. E. Harrison
of the NOAA
Pacific
Marine
Environmental Laboratory (Seattle, Washington)for preparing the island wind data set for the period 1948-1980 and
making it available to us for this study. Consistentwith
Meyers'work,thephaselagsbetween
the climatolo•gical
monthlymeanwind and SST harmonicsare approximately
correctfor the Kelvinwavehypothesis.
As a morerigorous
testof whether
theclimatological
windsforcetheclimatological SST, we will then examine the regularity of the
annualharmonicsin PuertoChicamaSST and•selectedwind
theperiods
1961-1970
and1961-1978,
re•spectively,
to sim- records.Finally, the relationbetweenthe nonseasonalvari-
ulate the nonseasonalvariations of pycnocline height in the
abilityo(the windsandcoastalSSTwill be addressed
using
lag correlationsof filtered time series.
• Permanently
at InstitutoGeofisicodel Perti, Lima.
2.
2 Alsoat Department
of Atmospheric
Sciences,
Universityof
WaShington,
Seattle.
•
Copyright!987 by the AmericanGeophysicalUnion.
Paper number 7C0704.
0148-0227/87/007C-0704505. O0
CLIMATOLOGICAL
COASTAL SST AND WESTERN
EQUATORIAL PACIFIC ZONAL WIND
.. •'
Climatological mean SST, as a function of calendar
month, is shown in Figure 1 for a selection of Ecuadorian
and Peruvian coastal stations. The grand mean has been
14,291
14,292
LAGOS ET AL.' REMOTE FORCING OF SEA SURFACE TEMPERATURE
Station
•FMAM•
•AS0ND•
FMAM•
•ASOND
•FMAM•
•AS
0ND
Grand
Amplitude
(øC)and
Mean
Day(s) of Maximum
(o•
Annual
Semiannual
1
-1
1.8
0.2
Mar. 7
Mar./Sep. 13
1
o
_l
Talara
4ø5
19.1
1.8
0.6
Mar. 5
Feb./Aug. 24
!
0•--f••
.....
o•i'
-zo
•.
_•o
_•o
!•-••
..........
2.3
0.5
Mar. 2
Feb./Aug.25
• Paita18.3
_j
$o$
Pueyto16.8Apr. 2
Isla
Don
Martin,
11øS 16.9Mar. 13
Callao
16.4
12os
San
15øSJuan14.?
Mar. 15
1.4
-•
Chicarea,
1.5
_
0.5
Fcb./Aug.22
0.3
Feb./Aug.21
1.3
0.2
Apr. 2
Mar./Sep. 4
1.2
0.2
Feb./Aug.2
J FMAMJJASONDJFMAMJ J A$OND J FMAMJJAS OND
Fig. 1. Climatologicalmonthlymean SST (in degreesCelsius)for stationsalongthe southAmericancoastbetween
2øand 15øS.The grandmeanhasbeen removedfrom each seriesand is listedin a separatecolumn.Also shownare the
amplitudeand phaseof the first two harmonics.The climatologyhasbeen repeatedtwice for continuity.
removed from each series, and the climatologyis repeated
twice for the sake of continuity. All the seriesexhibit a warm
season centered abut February-March and a cold season
centered around September.The annual variation of climatological mean SST is simple and similar at all stationsand
can be adequately described by the first two harmonics,
whose amplitudesand phasesare shown in a tabular format
in Figure 1. The phasesof the harmonicsare fairly uniform
samplesthe regionfrom 4øto 12øSand out to approximately
5øof longitudeoffshore.The grandmeantemperaturefor this
region (21.2øC) is 2ø to 5øC warmer than the coastal stations
in this latitude belt, and the peak to peak amplitude is
substantiallylarger than the peak to peak amplitude for the
correspondingcoastal stations(see Deser and Wallace [this
issue] for further discussion of the differences between
coastal and offshore temperatures). The amplitudes and
phases of the first and second harmonics of this series are
also shown in tabular form in Figure 2. The phasesof the
February-early March and late August-early September. harmonicsare very similar to those of the coastal series. The
The relative phasingof the semiannualand annualharmonics amplitudeof the first harmonicis roughly 10times as largeas
is consistentwith the relatively rapid transitionfrom coldest that of the second harmonic.
to warmest temperatures seen at Talara (4øS), Paita (5øS),
The existenceof a large annualcycle, with larger ampliand Puerto Chicama (8øS). The amplitude of the annual tudesin the deeptropicsthan in the subtropics,suggeststhat
harmonic is largest at Paita, where meridional excursionsof dynamicalprocessesmust play an important role in the SST
the equatorial front may be a factor. Southward of Paita the climatology. Locally forced coastal upwelling may in part
amplitude decreaseswith increasinglatitude except at Isla explainthe coldertemperaturesalongthe coastand a portion
Don Martin (11øS)which is approximately 1 km off the South of the seasonal cycle [Brink et al., 1983, and references
American coast. The semiannual harmonic amplitude is therein], but it cannot account for the even larger annual
strongest between Talara and Puerto Chicama, where it is cycle offshore.
almost as large as the annual harmonic.
Figure 3 displays the climatological zonal wind for a
The same simple structure is also evident for SST, derived selection of stations in the equatorial western and central
from ship observations, just off the northern Peru coast Pacific. The first two series are derived from ship-of(Figure 2). This series,which was kindly providedby E. M. opportunity observationscontained in the Comprehensive
Rasmussonof the University of Maryland, College Park, Ocean Atmosphere Data Set [Fletcher et al., 1983] for the
along the coast with maxima of the annual harmonic in
March-early April and maxima of the semiannual in late
Station
JFMAMJJASONDJFMAMJJASONDJFMAMJJASOND
Grand
Mean
Amplitude(øC)and
Day(s) of Maximum
(øC)
3
Annual
-
Semiannual
2
1
0
ß
Offshore
4-1 2øS
-1
21.2
3.1
0.3
Mar.10
Feb./Aug.
19
-2
-3
J FMAMJ J ASOND JFMAMJ J ASONDJ FMAMJ JAS OND
Fig. 2. As in Figure 1 but for SST for a shiptrackbetween4øand 12øScenteredapproximately5ølongitudeoffshore.
LAGOS ET AL.' REMOTE FORCING OF SEA SURFACE TEMPERATURE
Station
JFMAMJ JA. SObIDJFMAMJ
Grand Amplitude
( ms' 1) and
Mean
J A$OND J FIL•.MJ JA.SOND
Day(s)of weakesteasterlies
(ms-1)
Annual
0
0
-1
150-160øE
0
-1
-
01•
.•••[
___
0.5
Jan.3
0.3
May/Nov.26
1.0
Dec.12
0.6
Jun./Dec.
16
Beru
176øE
-3.9
O.9
Dec.21
O.5
JuneJDec.
14
0 -1
Canton
172øW
-4.7
0.3
Jan.8
0.6
May/Nov.
20
0 ......
_!
Christmas -5.1
157øW
O.3
Jan.
23
O.2
Apr./Oct.
24
J A$ON'D JFMAM J J ASOND J FIIA•J
JA.S OI•ID
As in Figure 1 but for climatological monthly mean zonal wind (in meters per second)for regions in the western
and central equatorial Pacific.
region just to the north and east of New Guinea, and the
remaining series are for island stations roughly equally
spaced along the equator from 169øE (Ocean Island) to
157øW (Christmas Island). The structure of the zonal wind
climatology, like the coastal series examined above, is fairly
simple and can adequately be described by the first two
harmonics, whose amplitudes and phasesare listed in tabular form in Figure 3. The relative amplitudesand phasesare
qualitatively similar to Meyers' [ 1979] results for zonal wind
stressand Horel's [ 1982] results for ship-derivedzonal wind
with the amplitude of the annual harmonic roughly a factor
of 2 strongerthan the semiannualamplitudeover the western
Pacific, half as strong in the vicinity of Canton Island
(172øW), and comparable at Christmas Island.
Figure 4 shows the climatological zonal winds for a
northwest
0
-1
-
o
-1
-
JA. SOIgDJFMAMJ
J A.$OND J FIIAMJ
ot-
Day(s)of weakesteasterlies
-3.8
1.8
0.4
Sep. 11
Mar./Sep.27
Butaritari -3.8
3TM
0.7
Oct. 7
0.2
Jun./Dec. 4
Tarawa
IøN
0.8
Oct. 8
0.5
JunJDec.5
0.9
Dec. 17
0.5
Jun./Dec.
14
1.0
Dec. 25
0.6
Jun./Dec.
9
1.5
Jan. 13
0.5
Jun./Dec.
25
2.3
Jan. 20
0.5
Jun./Dec.
30
2.6
Jan. 24
0.3
Jun./Dec. 31
2.3
0.3
Feb. 3
Aug./Feb. 15
-3.6
••Arorae
-3.4
6ø$
0•.............
-11
Nanumea
-2.9
0
-•1
0••
_o,
J I•A•!J
Amplitude
( ms' 1) and
Grand
Mean
Beru
-3.9
1øS
01
in the Gilbert
(ms-1)
Majuro
7TM
-1
line of islands
••Nui
-2.7
9øS
Funafuti
-2.9
11øS
JASOlgD JFMAM J J ASOND J FltAMJ
JAS OI•ID
As in Figure 1 but for climatological monthly mean zonal winds (in meters per second) for a northwest to
southeast
chain
Inspection of the phasesof the wind harmonics relative to
the phases of the SST suggestthat the seasonal variations in
the winds in the western and central Pacific might contribute
to the remote forcing of the annual and semiannual harmonics of coastal SST. As a more rigorous test of this hypothesis, let us examine the seasonal cycles of coastal SST and
western Pacific winds in each year to see how regular they
J•S OlqD
0
-1
to southeast
(7øN, 171øE to 11øS, 180ø). The annual and semiannual
harmonic amplitudes vary smoothly with latitude for these
records, lending confidence in the credibility of island wind
data as a proxy for the winds over the open ocean (see also
Luther and Harrison [1984]. The amplitudes and phases are
similar to those computed by Meyers [1979] and Hotel
[1982].
Station
JF•A•J
Fig. 4.
0.5
May/Nov.25
Tarawa
-3.60.8
0.5
173øE
Oct.8
Jun./Dec.
5
-
J FRA•J
Fig. 3.
-0.6
OceanIsland -3.1
169øE
_!
0
-1
Semiannual
• 140-150øE
-0.3 Jan.
1.4
23
-1
14,293
line of island stations in the Gilbert
chain.
14,294
LAGOS ET AL.: REMOTE FORCING OF SEA SURFACE TEMPERATURE
January
I
January
I
ary 1
Novernberl/
/ •••
• '•,tarchl
Novera
October
I
March1
April
I October
I
I
April
1
Septemb
1
September
I
5
August1
•
•
----6
•
June1
July1
July1
Fig. 5. Harmonic dial representation of the contribution of
individual years to the annual harmonic of historical Puerto Chicama
SST (1925-1985). The year is taken from July to the following June
(see text). A dot is plotted for each year with the phasedenotingthe
day of warmest annual harmonic SST and the distance from the
origin proportional to the amplitude of the harmonic in that year.
Years with large amplitudeharmonicshave the dot replacedwith the
last two digits of the year of the January.
are. Figure 5 shows the amplitude and phase of the annual
harmonic for each year of the Puerto Chicama SST historical
record (1925-1986). In order to emphasize the year to year
variability, which tends to be largest in the months from
December to May, we have defined the year from July to
June instead of the conventional January to December. A
dot is plotted for the phase and amplitude of each year,
except for years with large amplitudes,for which the dot has
been replaced with the last two digits of the year, defined in
terms of the January. As can be immediately seen, the years
with large amplitude correspondto the major E1 Nifio events
during the first half of the calendar year in 1941, 1951, 1953,
January1
Fig. 7. As in Figure 5 but for the zonal wind for an average of
four Gilbert Islands stationsfor the period 1961-1979. The year is
taken from April to the following March, and the phasedenotesthe
day of weakest easterlies in the annual harmonic.
1957, 1965, 1972, and 1983. The amplitude of the year
extendingfrom July 1982 through June 1983is 5.2øC, 3 times
the amplitude of the climatologicalman (Figure 1), with a
maximum in annual harmonic SST on March 20. The phase
of the annual harmonic is quite regular in both E1 Nifio years
and non-E1 Nifia years: roughly 90% of the values fit within
an envelope with phasesbetween February 1 and April 31.
The years with very small amplitude, 1950, 1952, and 1954,
were notably cold at the coast during the warm season
[Deser and Wallace, this issue]. The constancy of the phase
of the annual harmonic from year to year underscores the
regularity of the seasonalcycle in coastal SST. The annual
harmonic of historical offshore SST (Figure 6) displays an
even more regular annual harmonic, with the maximum
temperatures of the harmonic always occurring between
February 15 and April 15 and the amplitude always larger
than IøC. Horel [1982] has also noted the regularity of the
annual harmonic of SST at Talara (4øS) on the northern Peru
December
I
Febtomy
1
coast and at Baltra (0ø, 90øW) in the GalapagosIslands. The
semiannual harmonic
November I
rch 1
2
October
1
65
for the Puerto Chicama
SST historical
record (not shown), with its smaller amplitude, exhibits less
regularity with respect to phase, but the points on the scatter
plot still show a distinct tendency for a maximum in semiannual harmonic SST between January 15 and March 15 and
betweenJuly 15 and September 15. For the remainderof this
section we will focus on the annual harmonic.
April
1
Figure 7 shows the annual harmonic amplitude and phase
for each year for the zonal wind averaged over four equatorial stationsin the Gilbert Islands. The "year" for this series
has been chosen to begin 3 months earlier than the "year"
September
I
•• / May
1
used for the Puerto Chicama series, consistent with the
August
I
6
JulyI
Fig. 6.
As in Figure 5 but for the historical offshore SST record
(1921-1938 and 1949-1986).
notion that equatorially trapped oceanic Kelvin waves are
instrumentalin the remote forcing of coastal SST. The winds
for this series, which are typical of the western Pacific
stations(Figures 3 and 4), do not exhibit as nearly as regular
a phasingas the Puerto Chicama and offshore SST series.
The years strongest easterlies are observed as frequently in
July as in January. The years 1963, 1965, 1972, and 1977, in
LAGOS ET AL.: REMOTE FORCING OF SEA SURFACE TEMPERATURE
which the Southern Oscillation index (defined as the pressure difference between Tahiti and Darwin, Australia) was
very low, were obviously highly influential in determining
the mean phases of the first harmonic in Figure 4. Meyers
[1982] performed a similar analysis for sea level height at
Truk (7øN, 152øE) in the western Pacific, and also found an
irregular phasing of the annual harmonic, with strong contributions from years with strong swings of the Southern
14,295
TABLE 2. Lag Correlation Coefficients x 100 for Tarawa (IøN,
173øE)Zonal Wind and Puerto Chicama SST Stratified by Month
J
Apr
F
M
Month of Puerto Chicama SST
AM
J
J A S O N D
J
F
M
40
35
....
%}}}}}}}}}iiii•i•
.......43 42
Oscillation.
3.
NONSEASONAL
VARIABILITY
Jun
OF COASTAL SST AND
Jul
......
%}}}ii}}i}}i}iiii:}:8
43 45
......
•ii!::.4•iii•:•
.....
WESTERN EQUATORIAL PACIFIC ZONAL WIND
In the preceding section we examined the regularity of the
annual
harmonic
of SST and winds
in order
to draw infer-
ences concerningthe importance of remote forcing in determining the average annual harmonic of coastal SST. As was
pointed out by Wyrtki [1975], the Nifio signal in coastal SST
can be interpreted as an amplificationand modificationof the
regular seasonal cycle. To test the relationship between the
winds and SST for these nonseasonal variations, i.e., for the
departures of the annual and semiannual harmonics in a
given year from the grand mean harmonics, and also for
lower frequency fluctuations, we have calculated lagged
correlations between anomalies of the wind and SST (Table
1). The use of anomalies removes the average annual,
semiannual, and higher harmonicsas well as the grand mean.
The first row
of the table
shows the correlation
between
Ocean Island zonal wind and Puerto Chicama SST, the first
column being the correlation coefficientfor the winds leading
the SST by 4 months. To simplify the tables, only those
correlations stronger than 0.33 in absolute value are shown.
TABLE 1. Lag Correlation Coefficients x 100 for Western
EquatorialPacific Wind Seriesand Puerto Chicama SST, Based
on the Available
Data for the Period 1950-1979
Lag, months
-4
-3
-2
-1
0
1
2
3
4
48
45
46
45
45
47
47
48
47
44
44
47
49
46
47
43
41
43
48
46
46
42
39
40
49
45
46
44
40
42
Unfiltered
Ocean Island
Butaritari
Tarawa
Beru
Arorae
Canton Island
Ocean Island
Butaritari
Tarawa
Beru
Arorae
Canton Island
Ocean Island
Butaritari
Tarawa
Beru
Arorae
Canton
Island
Ocean Island
Butaritari
Tarawa
Beru
Arorae
Canton
Island
35
49
40
48
58
52
34
33
51
56
52
40
40
38
Periods Between
40
36
50
50
49
48
43
42
45
3 and 9 Months
-35
47
Pe•odsBetween
45
43
47
48
40
50
43
38
46
41
9 and 15Months
PeriodsGreater
43
54
64
55
64
72
47
57
66
40
51
39
50
36
47
than 15Months
71
76
79
77
79
79
72
75
76
60
67
72
60
68
73
57
65
71
-34
35
44
43
-33
-40
79
76
75
74
76
75
76
71
71
74
76
76
-43
71
64
65
71
73
75
Oct
Nov
Each row contains correlation
coefficients between a different
month of Tarawa wind and each month of Puerto Chicama SST.
Shading indicates the month of simultaneous correlation.
The correlations for these time series are only moderately
strong and do not exhibit a consistent tendency for the
western Pacific winds to lead or lag the SST. The power
spectra of Puerto Chicama SST (not shown) shows distinct
peaks for periods centered on the semiannual and annual
periodicities and for periods greater than 15 months. In
recognition of this frequency structure, the anomaly series
was filtered with a four-pole recursive filter [Kaylot, 1977] to
produce series retaining the variability in each of these
frequency bands, with cutoffs at 3 and 9 months (9 and 15
months) to retain variability near the semiannual (annual)
period. For the series containing variability around the
semiannual periodicity (second group of correlations, Table
1), westerly wind anomalies over the western Pacific tend to
lead the warmer than normal coastal SST, and easterly wind
anomalies lead colder than normal coastal SST by 3 to 4
months, in agreement with the Kelvin wave hypothesis.
Although these correlations explain only a small part of the
variance in coastal SST, they are significant at the 90% a
priori confidence limits. In a similar manner the anomaly
seriesfiltered to retain variability near the annual cycle (third
group of correlations, Table 1) exhibit significant correlations, with the western Pacific winds leading the coastal SST
by 3 to 4 months. For all periods shorter than 15 months,
there is also a tendency at some of the stations for above
(below) normal coastal SST to be accompanied or followed
by stronger (weaker) than normal easterlies over the central
and western equatorial Pacific. The second wind pulse,
which is of opposite polarity, forces oceanic Kelvin waves
which tend to reduce the original SST anomaly. Finally, the
anomaly series filtered to retain only periods longer than 15
months (fourth group of correlations, Table 1) show the SST
fluctuations preceding the wind fluctuations by 2 to 3
months, consistent with the composite zonal winds described by Rasmusson and Carpenter [1982] for six E1 Nifio
events during the period 1950-1979.
The relationship between the higher-frequency zonal
winds and SST can be further explored by stratifying the lag
correlation calculation by calendar month. Table 2 shows lag
correlations between the Tarawa (IøN, 173øE) zonal wind
anomaly series and Puerto Chicama SST. The data were
filtered to retain periods shorter than 15 months, thus
14,296
LAGOS ET AL.: REMOTE FORCING OF SEA SURFACETEMPERATURE
combining the variability near the annual and semiannual
periods. The first row of Table 2 contains correlation coefficients between the January Tarawa zonal winds and
Chicama SST during each of the calendar months. Zero lag
correlations lie along the diagonal in the table, with wind
leading SST to the right of the diagonal. It can be seen that
there is not a strong seasonal preference for the higherfrequency winds to influence the coastal SST. Correlation
tables for other wind records for the western equatorial
Pacific (not shown) yield similar results.
REFERENCES
Brink, K. H., D. Halpern, A. Huyer, and R. L. Smith, The physical
environmentof the Peruvian upwelling system, Prog. Oceanogr.,
12,285-305,
1983.
Busalacchi, A. J., and J. J. O'Brien, The seasonal variability in a
model of the tropical Pacific, J. Phys. Oceanogr., 10, 1929-1951,
1980.
Busalacchi, A. J., and J. J. O'Brien, Interannual variability of the
equatorial Pacific in the 1960's, J. Geophys. Res., 86,
10,901-10,907, 1981.
Busalacchi, A. J., K. Takeuchi, and J. J. O'Brien, Interannual
variability of the equatorial Pacific--revisited, J. Geophys.Res.,
88, 7551-7562, 1983.
4.
CONCLUSIONS
In this study the seasonal cycle of SST for the Ecuador
and Peru coast has been documented. The climatology
exhibits a simple temporal structure with a regular annual
component and a smaller and less regular, albeit significant,
semiannualcomponent. Consideringthat the E1 Nifio signal
at the coast is, in some sense,an amplification of the regular
seasonalcycle [Wyrtki, 1975], it remains an important part of
E1 Nifio problem to understandthe processesthat determine
the annual cycle of coastal SST. We have shown that the
annual cycle of zonal wind over the central and western
equatorial Pacific is highly irregular from year to year. It is
not obvious to us how such an irregular forcing can contribute substantially to the much more regular annual cycle in
SST along and off the South American coast. These results
are in agreement with recent numerical simulations with a
primitive equation ocean model by Philander et al. [1987]
which indicate that the observed annual cycle in SST in the
eastern pacific is mainly a response to the annual cycle in
zonal winds in the eastern equatorial Pacific.
For nonseasonalvariability, however, it has been shown
that the higher-frequencywind fluctuationsover the western
and central equatorial Pacific do precede coastal SST fluctuations, consistent with the mechanism of forced oceanic
Kelvin waves. The processes that determine the observed
coastal SST are probably fairly complex, but the successof
the simple analysis scheme employed here suggeststhat at
least some of the higher-frequency variability can be understood in terms of remote forcing from the western and
central equatorial Pacific.
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P. Lagos and J. M. Wallace, Joint Institute for the Study of the
Atmosphere and Ocean, University of Washington, Seattle, WA
98195.
T. P. Mitchell, Department of Atmospheric Sciences, AK-40,
University of Washington, Seattle, WA 98195.
Acknowledgments. This work was supported under grant
8318853 from the National Science Foundation's Climate Dynamics
Program Office.
(Received March 30, 1987;
accepted May 15, 1987.)