HYDROLOGICAL PROCESSES
Hydrol. Process. 16, 1247– 1260 (2002)
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.1060
El Niño–Southern Oscillation and aspects of western
South American hydro-climatology
Peter Waylen1 * and Germán Poveda2
1
2
Department of Geography, University of Florida, Gainesville, Florida, 32611-7315, USA
Programa de Posgrado en Approvechamiento de Recursos Hidráulicos, Facultad de Minas, Universidad Nacional de Colombia sede
Medellı́n, AA 1027, Medellı́n, Colombia
Abstract:
The countries of western South America are heavily dependent upon their water resources and are subjected to
considerable interannual variability in precipitation and streamflow. Many of these extremes, both high and low, are
shown to be associated with the various aspects of atmosphere–ocean behaviour in the eastern and central Pacific, which
are collectively termed El Niño–Southern Oscillation (ENSO). ENSO itself constitutes a complex of environmental
changes that have differing influences throughout the study area, but which are loosely associated, statistically and
physically, with the major regional precipitation-generating mechanisms. As the result of a non-linear chaotic system,
no two ENSO events are the same, nor are their interactions with other regional climatic factors and the state of
the continental hydrologic system, both of which may act to dampen or amplify any changes. However, statistical
evidence suggests that probabilistic forecasts are possible based either upon anticipated state of the atmosphere–ocean
arbitrarily classified according to some a priori classification scheme or upon forecasted sea surface temperatures.
Examples of the changing probability distributions of hydro-climatological variables in the study area suggest that
some practical information of value may be extracted. Copyright  2002 John Wiley & Sons, Ltd.
KEY WORDS
El Niño–Southern Oscillation; annual precipitation; western South America; floods
INTRODUCTION
Knowledge of the interannual variability of hydro-climatic variables, and of any potential for reducing the
uncertainty associated with the occurrence of hydrologic events of practical importance in the near- to mediumterm future, is of considerable value in western South America. A list of water-related environmental hazards
and the resulting human and economic costs that are frequently experienced in this region includes: flood
damage, disruption of freshwater supply by both floods and droughts, reduction in the quantity of hydroelectricity produced (and the ensuing importation of fossil fuel alternatives), irrigation for agriculture, loss
of lives and infrastructure through devastating mass movement phenomena, and environmental and social
conditions favourable to the spread of diseases (see Table I for some representative figures). The geographic
range of the study area (13 ° N to 56 ° S) encompasses a variety of hydro-climatic environments from amongst
the driest (Atacama Desert) to the wettest (NW Colombia) places in the world, and from coastal plains to the
second largest mountain chain and sustained high-altitude plateau in the world. It includes climate types from
every one of the major Köppen classifications excluding D (snowy forests). The proximity of the area to the
seat of the El Niño–Southern Oscillation (ENSO) phenomenon may result in a clearer picture of its effects,
but the latitudinal range furnishes a continental-scale picture of variations in responses. A brief review of
the major precipitation and stream flow regimes of the area is followed by a discussion of major changes in
circulation patterns associated with ENSO. Examples are provided to illustrate the way in which the statistical
* Correspondence to: P. Waylen, Department of Geography, University of Florida, Gainesville, FL 32611-7315, USA.
E-mail: [email protected]
Copyright  2002 John Wiley & Sons, Ltd.
Received 31 April 1999
Accepted 15 October 2001
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P. WAYLEN AND G. POVEDA
Table I. Significance of water resources to countries within the study area; source: Roberts (1998)
Variable
Chile
Colombia
Ecuador
Peru
Internal renewable water resources (km2 )
Internal renewable water resources (m3 per capita)
Annual withdrawals (% of available resources)
National energy from hydroelectricity (%)
Agricultural land under irrigation (%)
Cases of cholera, 1991–95 (per 100 000)
Cases of malaria, 1991–95 (per 100 000)
468
31 570
4
62
29
1070
28 393
<0.1
76
14
21
458
314
25 791
2
45
9
230
417
40
1613
15
82
43
782
341
properties of various hydro-climatic variables change in association with the ENSO phase, and the potential
for their forecast is investigated.
STUDY AREA AND DATA AVAILABILITY
The study area, defined by its political boundaries, encompasses Colombia, Ecuador, Peru and Chile. Bolivia
was excluded because of the paucity of available records. In reality, most observations are restricted very
much to the Andes themselves (as in the case of Colombia) or to their western flanks.
Monthly precipitation totals are primarily based upon records from the Global Historic Climate Network,
although these, and various discharge records, have been supplemented extensively: in Colombia, by data
from the Instituto de Hidrologı́a, Meteorologı́a y Estudios Ambientales (IDEAM); in Chile by records from
the Air Force, Ministry of Works and Professor Hugo Romero; and in Peru, by records obtained from Servicio
Nacional de Meteorologı́a e Hidrologı́a (SENAMHI). Only the 314 stations possessing at least 20 complete
calendar years of records are employed in these analyses. The distribution of mean annual precipitation and
the wettest monthly triad are shown in Figure 1. Isohyets in Figure 1a are interpolated using kriging and
should be interpreted with caution in areas of sparse or absent observations (indicated in Figure 1b).
Several dominant regional controls on hydro-climatology can be identified. The inter-tropical convergence
zone (ITCZ) of the eastern equatorial Pacific generally migrates from a most southerly average position
of about 3 ° N in March to approximately 10 ° N in September (Alpert, 1945; Hastenrath, 1988). This broad
band is associated with unstable rising air and extensive precipitation, as can be noted by the dominance of
the September–November (SON) triad in Colombian rainfall in, and to the west of, the Andes (Velasco and
Fritsch, 1987). Areas to the south of the ITCZ in the Northern Hemisphere experience cross-equatorial westerly
winds that form a low-level jet, which, in combination with the topography of the Andes, are responsible for
some of the wettest regions in the world along the Pacific coast of Colombia (mean annual precipitation in
excess of 7000 mm; Poveda and Mesa, 2000). To the south of about 3 ° N the peak rainfall season changes to
March–May (MAM), and eventually, December–February (DJF), marking the annual southward progression
of the sun.
The Andes vary considerably in both altitude and topographic complexity, frequently exceeding 4000 m
and attaining almost 7000 m of maximum elevation (Caviedes and Knapp, 1995). The number of constituent
cordilleras varies from three in northern Colombia to one in Chile, isolating generally narrow coastal plains
to their west. Locally, position with respect to the cordilleras may have a considerable effect on the patterns
of precipitation, as witnessed by both the complexity of the isohyets and the timing of the maximum triaddescribing stations in the Colombian Cordilleras (Poveda and Mesa, 1997). Stations to the east of the cordillera
derive most of their moisture from the Caribbean or Amazon basin, as illustrated by the June–August (JJA)
maximum corresponding to the period of maximum intensity of the northeast trades over the Caribbean
(Hastenrath, 1988).
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Hydrol. Process. 16, 1247– 1260 (2002)
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EL NIÑO–SOUTHERN OSCILLATION
(a)
(b)
10
10
0
200
0
300000
4
0
300
0
00
10
0
2000
2000
500
250
−10
−10
10
0
50 0
0
250
−20
Annual
Precipitation
(mm)
−20
Triad of Max.
Precipitation
DJF
MAM
−30
−30
JJA
SON
−40
20
00
2000
−40
−50
−50
−80
−70
−80
−70
Figure 1. (a) Mean annual precipitation totals and (b) the triad of maximum precipitation based upon the available station records
The South Pacific sub-tropical anticyclone is generally located to the west of the continent at about 25–30 ° S.
It is partially responsible for the aridity at these latitudes and for sustaining the cold Humboldt/Chile/Peru
Current along the coastal waters south of the equator. The subtropical jet associated with the release of
latent heat at the upper levels of the troposphere over the western equatorial Pacific brings moisture in to
central Chile (Aceituno, 1988) during the austral winter (JJA maximum), whereas precipitation to the south
is dominated by the polar jets, westerlies and frontal activity that move northwards with the weakening of
the anticyclone (JJA).
ENSO
Using the established definitions, El Niño is the warm ocean current frequently (3–4 years) observed in the
eastern equatorial Pacific off the coast of Ecuador, and the Southern Oscillation refers to the varying bi-polar
nature of atmospheric pressures in the western Pacific (low) and the South Pacific sub-tropical anticyclone. The
two are now seen as two aspects of the same Pacific-wide atmosphere–ocean system, and the term El Niño
has popularly taken on a much broader meaning of global environmental disruption (Philander, 1990). The
lower atmospheric pressure over Indonesia/Australia, and a possible jet over the eastern Pacific (Hastenrath,
1999) induce generally easterly surface winds across the Pacific. There is a return airflow at high altitudes and
descending air at the sub-tropical anticyclone. This pattern of Walker cell circulation pushes warmer water
Copyright  2002 John Wiley & Sons, Ltd.
Hydrol. Process. 16, 1247– 1260 (2002)
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P. WAYLEN AND G. POVEDA
towards the western Pacific, driving down the thermocline (and pycnocline) to about 100 m, while raising
it to about 40 m and producing equatorial upwelling of cold waters in the east. A balance may be viewed
as existing between the surface wind stress and the gradient of the thermocline. Should the wind stress for
any reason be reduced in the central Pacific, the gradient of the thermocline will also be reduced locally.
This local change will be propagated west to east along the thermocline by the rotation of the Earth, as a
Kelvin wave (Harrison and Larkin, 1998). In terms of sea surface temperatures (SSTs) and their interaction
with the atmosphere, the effects of these Kelvin waves are most pronounced where the thermocline is at its
shallowest depth beneath the surface. The Kelvin wave ultimately moves sufficiently eastward to be reflected
northwards and southwards off of the coast of equatorial western South America. Equatorial easterlies abate
and even reverse, producing two areas of shallower thermocline depth west of the international dateline, north
and south of the equator, and the ocean–atmosphere enters a new mean, unstable state.
These areas of shallower thermocline affect the strength and positions of the sub-tropical anticylones and
thereby the planetary, or Rossby, waves. Away from the equator, evaporation and turbulent mixing are the
dominant mechanisms of energy transfer. The stronger winds in mid-latitudes increase evaporation (thereby
increasing latent heat releases to the atmosphere), and mixing. The extra-tropical ocean waters then cool,
reducing the equatorward flow of the ocean currents down the west coasts of mid-latitude continents and
permitting the poleward transfer of the reflected equatorial waters. These delayed changes away from the
equator portend the end of the current unstable mean state and herald a series of positive feedbacks towards
the opposing mean condition. In this way the atmosphere and ocean are in a permanently unstable condition,
and tend to move alternately between two end conditions (Vallis, 1986). Given the intrinsic non-linear nature of
the climate system, each change of mean condition is unique. The general suite of environmental conditions
associated with the two mean states has become known as El Niño (deepened thermocline in the eastern
Pacific) and La Niña, anti-El Niño or El Viejo (shallower thermocline in eastern Pacific). As the exact state
of each unstable mean condition varies, no universally accepted definition of El Niño and La Niña can exist
in terms of the dominant factors of SSTs and atmospheric pressures.
However, in order to facilitate the generalization of effects, certain canonical characteristics of warm phases
(deeper eastern thermocline) and cold phase (shallower eastern thermocline) have been extracted (e.g. Harrison
and Larkin, 1998), but the non-linear chaotic nature of the atmosphere–ocean system requires that each event
will vary in its time of initiation, severity and spatial extent, and suggests that a probabilistic representation
of the results is the appropriate one. Two basic approaches exist to the definition of the relationship between
continental hydro-climatic variables and the state of the atmosphere–ocean system. The first is to establish
associations with indices of the dominant components of the atmospheric and oceanic systems. The Southern
Oscillation index (SOI), a standardized measure of the differences between surface pressures at Darwin,
Australia, and Tahiti, or measures of the oceanic component, such as SSTs in various areas throughout the
Pacific, are commonly used in this way. In this paper SSTs in regions Niño3.4 (5 ° N–5 ° S, 170 ° W–120 ° W)
and Niño1C2 (0° –10 ° S, 90 ° W–80 ° W) are employed (see http://www.cpc.ncep.noaa.gov, for sources of
these data). The fact that existing climate models appear to have a reasonable capacity to forecast SSTs 3
to 9 months in advance (Barnston et al., 1999), and are thereby potentially translatable into similar hydroclimatic forecasts, may be an advantage of employing SSTs (particularly in the Niño3.4 region). None of the
three series shows any significant (0Ð05) autocorrelation at lag 1.
A second approach has been to subdivide years according to some a priori classification scheme (generally
El Niño, La Niña and normal) based on a variety of environmental indicators. Many of these are fairly
subjective, and vary with investigator and region of application; they are extensively reviewed by Rossel
(1997). Various statistical properties of the hydro-climatic variables can be compared between sub-samples.
For the sake of objectivity, the classification of the Japanese Meteorological Society is used throughout this
work. This classification is based on an index calculated as a 5 month running mean of spatially averaged
SST anomalies over the tropical Pacific: 4 ° S–4 ° N, 150 ° W–90 ° W. If the index values exceed 0Ð5 ° C for six
consecutive months, then the ENSO year of October through to the following September is categorized as El
Copyright  2002 John Wiley & Sons, Ltd.
Hydrol. Process. 16, 1247– 1260 (2002)
EL NIÑO–SOUTHERN OSCILLATION
1251
Niño; a La Niña has index values less than 0Ð5 ° C; all other values are considered neutral. A full listing of
years and explanation is given at http://www.coaps.fsu.edu/¾legler/jma index1.shtml.
EMPIRICAL RESULTS
Annual precipitation
Maps of simple simultaneous correlations between annual precipitation and SOI, Niño3.4 and Niño1C2 are
shown in Figure 2, indicating areas of positive and negative associations. Slightly over 50% of all the stations
exhibit statistically significant (0Ð05 level) correlations between the SOI and annual precipitation, dropping to
20% for correlations with SST in Niño1C2, and 28% for Niño3.4. These figures may not reflect the true areal
coverage, owing to the spatial bias towards stations in central Chile and the Colombian Andes (Figure 1b).
Although the geographic limits of the affected areas vary depending upon the indicator (independent) variable
employed, a common pattern emerges. Colombia and northern Ecuador, from the Andes westward, experiences
increased positive associations with the SOI, and negative ones with measures of SST. During years of warmer
Figure 2. Synchronous (year 0) correlations of annual precipitation total to three major indices of the state of the atmosphere– ocean system
in the equatorial Pacific
Copyright  2002 John Wiley & Sons, Ltd.
Hydrol. Process. 16, 1247– 1260 (2002)
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P. WAYLEN AND G. POVEDA
than normal SST (lower than normal SOI) in the eastern Pacific (warm-phase years) the descending limb of
the Walker cell circulation moves eastward over these regions subduing convection, and the ITCZ moves
further south. During El Niño there is a decrease in water vapour advection as a result of the weakening of
the low-level westerly jet that normally penetrates from the Pacific Ocean to inland Colombia. This slackening
results form a diminished temperature gradient between Colombia and the cold tongue off the Peruvian coast
(Poveda and Mesa, 2000). In addition, the number of tropical easterly waves decreases throughout the tropical
North Atlantic and the Caribbean (Gray and Sheaffer, 1992). Although such events have no direct impact
upon South America, their presence in the Caribbean basin induces a regional reversal of the normal pressure
gradient and enhances vapour transport from the Pacific (Vargas and Trejos, 1994). Southern Ecuador and
northern coastal Peru demonstrate the opposite signal, reflecting the reduction of off-shore upwelling, and
the spatial extent of the ‘cold tongue’ in general, during warm phase years, which permits the southward
migration of the ITCZ and associated precipitation in the eastern Pacific (Rossel, 1997). The other zone of
marked correlation is found in central Chile, which experiences increased (decreased) precipitation during
warm (cold) phases. This pattern reflects the northward displacement of the sub-tropical jet-stream and the
concomitant increase in frontal activity (Aceituno, 1988). The differences in patterns of statistically significant
correlations with the two measures of SST are indicative of these varying physical processes. The fact that
temperatures in Niño3.4 are more closely related physically to the location of the Walker cell circulation
and the sub-tropical jet is reflected in the concentrations of significant associations in Colombia and central
Chile, whereas stations in Ecuador and northern Peru are more directly affected by temperatures in the waters
immediately offshore (Niño1C2). There is also a slight indication of a potential negative correlation with SST
in Altiplano of southern Peru and northern Chile (Tapley and Waylen, 1990).
The ability to forecast precipitation based on the value of last year’s independent variables would be of
considerable practical use. Figure 3 shows the results of similar lag cross-correlations with a lead of one year.
The potential for such forecasting looks slim, with only 4% of the stations reporting statistically significant
associations with preceding values of SOI and Niño1C2, and 10% with Niño3.4. 4% is within the number
that one would expect under the null hypothesis of no correlation at the selected significance level, and the
10% probably reflects, in part, the spatial bias/clustering of the stations (Wolter, 1987). However, SST in
Niño3.4 changes more gradually than the other two indices, and this persistence may account for the higher
percentage of correlations. In many of the regions in which precipitation appears most strongly correlated with
ENSO the sign of the correlation changes between Figures 2 and 3, indicating the tendency for alternating
phases (warm/cold) to follow one another (Rasmusson et al., 1990); this is particularly marked in central
Chile.
Similar regional patterns are observed when the annual precipitation records of each station are sub-divided
according to selected ENSO classification, and the l-means and l-variances (Hosking, 1990) of the El Niño
and La Niña years are compared (Figure 4). L-moments can be interpreted in a similar fashion to their product
moment counterparts, but have been shown to be less sensitive to small sample sizes (Vogel and Fennessy,
1993). Given the tremendous variability in annual precipitation totals over the region (Figure 1a), these
differences (El Niño La Niña) are expressed as percentages of the La Niña figures. Statistical comparisons
of the observed variances (F-test) and means (t-test) are carried out, but it should be noted that the assumption
of normally distributed sub-samples in annual rainfall totals will probably be violated in the arid and semiarid regions, therefore the significance level is, in reality, larger in these areas than the 0Ð05 level used.
Significant differences in variances were detected in 19% of the stations, which were fairly evenly spread
throughout the principal areas affected by ENSO. Increased variances are shown throughout Chile and littoral
Peru and Ecuador during El Niño years, whereas the signal is more mixed in Colombia. The method of
calculating percentage changes will highlight more strongly locations at which statistics in El Niño years are
larger. Almost 40% of the stations reported significant differences in mean annual precipitation. Percentage
changes are most marked in the arid and semi-arid areas, although what appear to be smaller negative changes
(10–50%) in Colombia correspond to very large absolute changes.
Copyright  2002 John Wiley & Sons, Ltd.
Hydrol. Process. 16, 1247– 1260 (2002)
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EL NIÑO–SOUTHERN OSCILLATION
Figure 3. As in Figure 2, but correlations are based upon indices in the preceding year (year 1)
Probability distributions of hydrologic variables
Given the chaotic nature of the atmosphere–ocean system, and the frequently non-linear way in which outputs from it are translated through the land-based portion of the hydrologic cycle (Poveda and Mesa, 1997),
the response of terrestrial hydrologic systems to ENSO are not deterministic and must best be approached
stochastically. Figure 5 separates the empirical cumulative probability distributions (calculated using Weibull
plotting position) of annual precipitation totals at Valparaiso, Chile (33° 020 S, 71° 400 W), and Guayaquil,
Ecuador (2° 150 S, 79° 520 W), both of which appear as having significant differences in their means under
differing atmosphere–ocean conditions (Figure 4). This illustrates the changing probabilities of various rainfall totals under each set of conditions, with larger rainfall totals generally associated with El Niño years at
both stations, but with some notable exceptions. A number of probability distributions could be fit to these
sub-samples, and the distributions of each set of conditions combined in a mixture model using the historic
relative frequencies of each type of condition (or those defined by the entire classification scheme employed)
as weightings. These plots suggest that a log-normal might be appropriate (Tapley and Waylen, 1990), in
which case the comparisons of means and variances discussed above have broader implications than merely
one of simple descriptive statistics, as they would completely specify the probabilities of extremely wet or
dry years under each set of circumstances.
Copyright  2002 John Wiley & Sons, Ltd.
Hydrol. Process. 16, 1247– 1260 (2002)
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P. WAYLEN AND G. POVEDA
Figure 4. Percentage changes in the mean and variance of annual precipitation totals between El Niño and La Niña years
A current approach to the probabilistic representation of forecasted annual and seasonal rainfalls is simply
to divide the entire historic record into terciles or ‘above normal’, ‘normal’ and ‘below normal’, and to
provide forecasts conditioned upon anticipated ocean–atmosphere state. For instance, if an El Niño year
was certain to occur, then, based on the empirical records of similarly classified years at Valparaiso, there
Copyright  2002 John Wiley & Sons, Ltd.
Hydrol. Process. 16, 1247– 1260 (2002)
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EL NIÑO–SOUTHERN OSCILLATION
0.5 1 2
5
10
20 30
50
70 80
90 95
98 99
0.51
Valparaiso
2
5
10
20 30
50
70 80
90 95
98 99
Valparaiso
1000
above
normal
800
400
Annual Precipitation (mm)
Annual Precipitation (mm)
1200
Historic
Upper Tercile
Historic
Lower Tercile
normal
Neutral Years
EI Niño Years
below
normal
La Niña Years
100
0
4000
10000
Guayaquil
3000
above
normal
2000
1000
Historic
Upper Tercile
1000
normal
Historic
Lower Tercile
below
normal
100
0
0.5 1 2
5
10
20 30
50
70 80
90 95
Cumulative Probability (*10−1)
98 99
Annual Precipitation (mm)
Annual Precipitation (mm)
Guayaquil
0.5 1 2
5
10
20 30
50
70 80
90 95
98 99
Cumulative Probability (*10−1)
Figure 5. Empirical distributions of historic annual precipitation data at Valparaiso, Chile, and Guayaquil, Ecuador, plotted on normal and
log-normal scales. Data are subdivided according to an ENSO classification scheme. Horizontal lines represent historic tercile boundaries
for the undifferentiated records
is a probability of about 0Ð68 (17/25) of the annual total precipitation being ‘above average’, 0Ð20 (5/25)
of ‘normal’ precipitation, and about 0Ð12 (3/25) of being ‘below average’. If the next year were forecast to
be a La Niña, the comparable probabilities would be 0Ð18, 0Ð18 and 0Ð64. These estimates could be made
more precisely if probability distributions were fit to the empirical data. In a similar vein, the uncertainties
concerning the forecast of ocean–atmosphere state can be incorporated into the model by weighting each
condition’s probability of being in the desired tercile by the forecast probabilities of that condition and the
products then summed. Waylen and Caviedes (1990), Quesada and Caviedes (1992) and Poveda and Mesa
(1997) have constructed similar probability distributions of annual stream flows in Colombia and central
Chile
Potential for forecasting hydrologic extremes
Given the strength of some of the associations between indices of ENSO and hydrologic extremes in western
South America, there appears to be some potential for forecasting. The poor lag cross-correlations (Figure 3)
indicate that the previous values of observed SOI or SST cannot themselves be used as the basis for forecasts.
This may partially result from the complex lead/lag relationships, which vary regionally depending upon the
phasing of the hydrologic cycle and the ENSO events (note the spatial variation in the triad of maximum
precipitation in Figure 1). However, forecasts of SST in Niño3.4 are made as standard by several agencies
and are publicly available through such documents as the Climate Diagnostics Bulletin (US Department of
Copyright  2002 John Wiley & Sons, Ltd.
Hydrol. Process. 16, 1247– 1260 (2002)
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P. WAYLEN AND G. POVEDA
Commerce, 1999) and the World Wide Web at http://www.cpc.noaa.gov. The ‘canonical correlation ENSO
forecast’ (Barnston and Ropelewski, 1992) employs a variety of environmental indicators to forecast seasonal
SST in Niño3.4, up to a year into the future. The forecasts, which include error estimates, might be used as
the basis for the forecasting of hydrologic variables.
To illustrate this potential, the links between the annual flood series of the Piura River (1926–96)
in northern coastal Peru and SST in Niño3.4 are investigated more fully. Figure 1 shows that the triad
of maximum precipitation in this desert region is MAM, which is also historically the period of high
flows. Because of this region’s proximity to the equatorial Pacific and the cold tongue, it often experiences devastating floods associated with El Niño (Waylen and Caviedes, 1986). The results of an
analysis of lag cross-correlations (Figure 6) reveal significant (0Ð05 level) positive associations between
annual flood magnitudes and monthly SSTs in Niño3.4 during the period April through July, implying that reasonable forecasts of SST for these months could yield estimates of the annual flood. Higher
correlations exist with SST in Niño1C2 (Figure 2), but because of this region’s proximity to the continent, forecasts of atmosphere–ocean conditions are less reliable. The lack of correlation with SST in
preceding months echoes the findings of the cross-correlations with annual precipitation, and the significant negative correlations with SST 2 years later provides further indication of the tendency for the two
opposing atmosphere–ocean extremes to follow one after the other (Philander, 1990; Rasmusson et al.,
1990).
A simple linear relationship is established between the historic seasonal (MAM) mean of the SST in
Niño3.4 and annual floods (r D 0Ð36, significant at 0Ð05). Estimates of MAM SST are taken from the Climate
Diagnostic Bulletin for the years 1998 (El Niño) and 1999 (La Niña) during the preceding Novembers (a
5 month lead to the middle of the rainy/flood season). Both the mean anticipated SST and the SSTs that
represent plus/minus one and two standard deviations either side of this mean are extracted. Derived flood
estimates represent the anticipated mean flood size corresponding to each measure of SST. There is also a
standard error of forecast associated with the formulation of the regression model, which is the anticipated
standard deviation of forecasts about this forecast mean. According to the assumptions of the regression
model, the forecasts will be normally distributed and, therefore, completely characterized by their respective
means and standard errors of forecast.
Figures 7 and 8 depict the probability distribution of historic annual floods on the Piura River and the
fitted generalized extreme value distribution. In the absence of any forecast ability, this distribution (or
some other distribution) would provide the best estimate of the risks of various flood sizes in any future
year. For the sake of comparison, annual upper and lower tercile levels are shown. Five diagonal lines
represent the probability distributions of forecast flood sizes based upon the various (mean and plus/minus
one and two standard deviations) SSTs. Flood sizes forecasted by means of all five functions would have
placed the predicted the extremely large 1998 event (Figure 7) as being well within the upper tercile of
historic events. It can be seen that, even with the lowest SST forecast (two standard deviations below
the anticipated SST), the probability that the coming annual flood would be in the upper tercile was
about 0Ð65. Such high probabilities of anticipated above average flood sizes would have given ample
opportunity at least to prepare for the devastation that was to strike the area in 1998 (estimated peak of
3367 m3 s1 on 1 April 1998). Similar forecasts (Figure 8) for the La Niña year of 1999 reveal that the mean
anticipated SST would have yielded an annual flood with a 0Ð55 probability of being in the lowest tercile
and about a 0Ð27 probability of being in either of the other two terciles. This also reveals the limitations
of this very simplistic approach, in that the forecasted mean temperatures minus one and two standard
deviations yield negative forecasted mean floods. However, information may still be extracted concerning
likely flood size and each tercile. Employing the forecasted SST two standard deviations below the mean
yields estimates of roughly 0Ð75, 0Ð18 and 0Ð07 of the annual flood in 1999 being below normal, normal and
above normal respectively. The observed peak (based on unofficial data current until June 1999) provided
by Dr Norma Ordinola of the Universidad de Piura) was 2447 m3 s1 on 25 February. Actual SSTs in
March 1999 (about two standard deviations below the mean) were very similar to those forecasted, yet the
Copyright  2002 John Wiley & Sons, Ltd.
Hydrol. Process. 16, 1247– 1260 (2002)
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EL NIÑO–SOUTHERN OSCILLATION
Jan
SST 2 Yr.
later
Jul
Jan
SST 1 Yr.
later
Jul
Jan
Synchronous
Jul
Jan
SST in
preceeding
year
Jul
Jan
SST 2 Yrs
previous
Jul
Jan
SST 3 Yrs
Previous
Jul
Jan
−0.6
−0.4
−0.2
0.0
0.2
0.4
0.6
Lag-Cross Correlation
Coefficient
Figure 6. Lag cross-correlations of monthly SSTs in Niño3.4 and annual flood sizes on the Piura River, Peru. Statistically significant figures
at the 0Ð05 level are shaded
flood size was totally unexpected under ‘La Niña’ conditions and appears to be amongst the largest ever
observed.
Summary
There is a wealth of evidence to suggest that ENSO is associated with considerable interannual variability
in precipitation throughout western South America. Linkages differ spatially with various indices of the ENSO
phenomenon, and their sign and strength. Although the physical linkages between these facets of ENSO and
the dominant regional precipitation mechanisms are becoming clearer, the intrinsic non-linear nature of the
atmosphere–ocean system in the Pacific, and its interaction with other hemispheric and global climatological
phenomena, makes reliable deterministic forecasts of both the future state of the atmosphere–ocean system
Copyright  2002 John Wiley & Sons, Ltd.
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Figure 7. Forecast distributions of the annual flood on the Piura River for 1998 based on forecasts of SSTs in Niño3.4 made in November
1997. Historic flood distribution and tercile levels are shown for comparative purposes
and regional precipitation problematic. In terms of forecasting the effects upon aspects of the continental
hydrologic system, the task is made more complex by potential positive and negative feedbacks. There is
evidence that, within those portions of the region where the hydro-climatology can be shown to be most closely
associated (statistically and physically) to ENSO (i.e. northwestern Colombia, coastal Ecuador and Peru, and
central Chile), there also exists a difference in the probabilistic characteristics of hydro-climatic variables
conditioned upon either the classification of ENSO events or one of the ENSO indicator variables. Resultant
probabilistic statements permit the incorporation and expression of the inherent uncertainties witnessed
in the atmosphere–ocean and hydrologic systems in such way as to facilitate their incorporation by risk
managers into computations of potential long-term gains and losses in taking actions based on the forecasts.
Such statements are conditioned upon the assumption of some form of stationarity in both the index of
ENSO activity and hydro-climatic variable, and in the relationship between the two. There is mounting
evidence (e.g. see Trenberth, 1990; Wang, 1995) to question this assumption of longer run (inter-decadal)
stationarity.
Copyright  2002 John Wiley & Sons, Ltd.
Hydrol. Process. 16, 1247– 1260 (2002)
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Figure 8. Same as Figure 7, except that forecasts for annual floods in 1999 are based on November 1998 forecasts of SSTs
ACKNOWLEDGEMENTS
This work was supported in part by a grant from the Inter-American Institute, ISP 3-022, and by travel funds
from National Oceanic and Atmospheric Administration. We are grateful for the thoughtful comments of the
two anonymous reviewers.
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