Palaeogeography, Palaeoclimatology, Palaeoecology 281 (2009) 309–319

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Palaeogeography, Palaeoclimatology, Palaeoecology

j ourna l homepage: www.e lsev ie r.com/ locate /pa laeo

El Niño-Southern Oscillation signal in the world's highest-elevation tree-ringchronologies from the Altiplano, Central Andes

Duncan A. Christie a,b,⁎, Antonio Lara a,b, Jonathan Barichivich a, Ricardo Villalba c,Mariano S. Morales c, Emilio Cuq a

a Laboratorio de Dendrocronología, Facultad de Ciencias Forestales, Universidad Austral de Chile, Casilla 567, Valdivia, Chileb Forest Ecosystem Services under Climatic Fluctuations (Forecos), Universidad Austral de Chile, Valdivia, Chilec Departamento de Dendrocronología e Historia Ambiental, Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales, IANIGLA, C.C. 330, 5500 Mendoza, Argentina

⁎ Corresponding author. Laboratorio de DendrocroForestales, Universidad Austral de Chile, Casilla 567, Vald221566; fax: +56 63 221230.

E-mail address: duncanchristieb@gmail.com (D.A. ChURL: http://www.dendrocronologia.cl.

0031-0182/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.palaeo.2007.11.013

a b s t r a c t

a r t i c l e i n f o

Article history:

El Niño-Southern Oscillatio Received 23 June 2007Accepted 19 November 2007Available online 9 September 2008

Keywords:ENSO variabilityDendroclimatologyTropical AndesHighest elevation treelineAltiplanoPolylepis tarapacana

n (ENSO) is the largest source of inter-annual variability operating in the earth'sclimate system with enormous ecological, social, and economic impacts. As the instrumental record of ENSOvariability is limited to the past 150 yr, the search for proxies sensitive to ENSO has become a priority task inrecent years. It is vital to expand the sparse network of annually resolved and exactly dated ENSO-sensitiveproxies to entirely capture the spatial variability of the ENSO coupled ocean-atmospheric phenomenon.Several tree-ring records have been used to reconstruct past ENSO variability, however, none of them arefrom tropical South America. The Polylepis tarapacanawoodlands form the world's highest elevation treeline.Trees grow on the Altiplano plateau between 4000–5200 m elevation (16°–23° S). The climate variability inthis climatically transitional tropical–subtropical region is strongly modulated by ENSO. Two chronologies ofP. tarapacana along the Western Cordillera in the tropical central Andes were analyzed to determine thestrength of the ENSO signal present in these tree-ring records.The growth of P. tarapacana has a strong common signal amongst trees in a single site and between sitesacross the Altiplano. Ring formation is induced by climatic conditions during the previous and currentgrowing season (December–February), at the time of the strongest ENSO influences on the Peruvian Coast.Tree growth variations show opposite relationships with climate between consecutive growing seasons.During the current growing season, ring-width variations show positive and negative relationships withtemperature and precipitation, respectively. Both tree-ring chronologies are significantly correlated withaustral spring–summer (August–February) SST in the Niño3.4 region and show oscillatory modes within theclassical ENSO bandwidth. Our results support the idea that P. tarapacana chronologies from the south-central tropical Andes provide high-resolution records extremely sensitive to ENSO in the tropical Pacific, andrepresent an important component to be considered in future multiproxy ENSO reconstructions.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

El Niño-Southern Oscillation (ENSO) is one of the most prominentsources of interannual variability operating in the earth's climatesystem (Trenberth and Caron, 2000). ENSO is associated worldwidewith extreme weather conditions such as heavy snowstorms, floods,droughts and cyclone activities, having large ecological (Holmgrenet al., 2001), social (Bouma et al., 1997) and economic impacts (Chenet al., 2001). As the instrumental record of ENSO variability is limitedto the past 100–150 yr, the search for proxies sensitive to ENSO has

nología, Facultad de Cienciasivia, Chile. Tel.: +56 63 293791

ristie).

l rights reserved.

become a priority task in the scientific community in recent decades.Long records of ENSO variability are needed to characterize the non-stationary behavior of ENSO prior to the instrumental period, tocompare previous with recent-century ENSO variability, and todevelop predictive models of ENSO in the future.

A number of proxy records have been used to reconstruct ENSOvariability including documentary archives (Quinn and Neal, 1983;Ortlieb, 2000), corals (Evans et al., 2002), ice cores (Thompson et al.,1984), and tree rings (Stahle et al., 1998; D'Arrigo et al., 2005). A fewmultiproxy-based reconstructionshavealsobeenconducted (Mannet al.,2000; Gergis and Fowler, 2006). However, as Stahle et al. (1998) andGergis et al. (2006) indicate, the network of ENSO-sensitive proxies is stillin its infancy with regards to its temporal length, exactly annual dating-resolution, and spatial distribution, particularly within South America.

In order to successfully achieve high-quality ENSO reconstructionsin the future, it is a priority to increase the geographical distribution ofproxy records to include new areas around the tropical Pacific. Tree-

Fig. 1. Polylepis tarapacana woodlands at 4750 m on the slope of the Guallatiri volcano(6071 m) in the Altiplano.

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rings are one of the best high-resolution proxy climate indicators andexhibit some of the strongest statistical relationships with instru-mental climate records (Jones et al., 1998). However, the search forENSO-sensitive tree species and sites is still ongoing. At present,highly sensitive ENSO tree-ring chronologies have been developed forIndonesia (Berlage, 1931; D'Arrigo et al., 1994), Thailand (Buckley etal., 1995), New Zealand (Fowler et al., 2000), and North America(Stahle et al., 1998; Cleaveland et al., 2003). Although the potential toinfer past ENSO behavior has been explored in tree-ring records fromextratropical South America (Lough and Fritts, 1985; Villalba, 1994),no ENSO-sensitive tree-ring chronologies have been produced for theeastern tropical Pacific, where ENSO variability has a direct impact onthe local climate (Garreaud et al., 2009-this issue).

During the past few decades, the study of ENSO influences on theclimate of the tropical Central Andes in South America has become amajor issue for the local and international scientific community due tothe negative effects of El Niño events onwater resources in the region(Aceituno, 1988; Vuille, 1999; Vuille et al., 2000; Garreaud andAceituno, 2001; Francou et al., 2003). Glacier mass balance, rain andsnowfall variability in the Central Andes are strongly modulated byENSO (Vuille et al., 2000; Francou et al., 2004). Most water for humanconsumption, agriculture, and mining come from the Andes, con-ferring on these mountains the role of regional “Water Towers”(Viviroli et al., 2003; Messerli et al., 2004). Under the future scenariosof increasing temperatures (Bradley et al., 2006) and the lack ofknowledge about interactions between global warming and ENSO,additional information on ENSO behavior under different air tem-perature regimes is urgently required (Trenberth and Hoar, 1997).

In the tropical central Andes, and particularly in the WesternCordillera in Bolivia and Chile, are located the world's highestelevation woodlands formed by Polylepis tarapacana trees (Braun,1997). This unique species grows up to 5200m, reaches an age ofmorethan 600 yr, and dead wood remains on the ground for centuries dueto the arid dominant climate (Argollo et al., 2004) (Fig. 1). Thesedendrochronological characteristics, in combination with its particu-lar location in a directly ENSO-influenced tropical region, make P.tarapacana particularly suitable for reconstructing past ENSO varia-bility. The main goals of this study are 1) to develop two tree-ringchronologies of P. tarapacana at high elevations along the WesternAndean Cordillera on the Altiplano, 2) to investigate the relationshipbetween ENSO and the Altiplano local climate, 3) to determine thestrength of ENSO signal on the tree-ring chronologies, and 4) toevaluate the potential of P. tarapacana proxy records in estimatingpast changes in the coupled ocean–atmospheric phenomenon of thetropical Pacific.

2. Setting and climate of the Altiplano

The tropical central Andes act as a formidable obstacle for thetropospheric circulation which generates two contrasting regions, thetropical lowlands to the east and the Pacific coastal deserts to thewest(Garreaud et al., 2003). The Altiplano is a high-altitude arid plateau atabove ~4000 m, on average 300 kmwide and located at the southernsector of the tropical central Andes between the Eastern and WesternAndean Cordillera (15° S–23° S). This high-altitude region separatesthe dry Pacific from the wet Atlantic influences (Fig. 2). The WesternAndean Cordillera is characterized by the presence of high mountainsand volcanoes that reach more than 6500 m above sea level (ASL).

The Altiplano is located within the “Arid Diagonal”, a narrow bandof scarce precipitation spanning South America from the east-coast ofPatagonia up to northern Peru (Abraham et al., 2000). The climate ofthe Altiplano is characterized by a reduced seasonality in temperaturebut a marked seasonality in precipitation with cool–dry winters andcool–wet summers (Garreaud et al., 2003; Vuille et al., 2000).Precipitation over the Altiplano is highly related to the upper-aircirculation with an easterly (westerly) zonal flow favoring wet (dry)

conditions (Garreaud et al., 2003). Summer (DJF) rainfall is of aconvective nature and represents more than 75% of the total annualprecipitation (Aceituno, 1988). Seasonal climate variability dependson the intensity and location of the Bolivian High, an upper-air highpressure system which develops in the upper troposphere south andsoutheast of the Altiplano (Lenters and Cook, 1997; Garreaud et al.,2009-this issue). DJF precipitation is driven by the southernmigrationof the maximum convection zone across the Amazon Basin and alongthe Andes in response to the South American monsoon circulation(Vera et al., 2006). Precipitation events are associated with local-convective storms resulting from moisture advection from thecontinental Amazon Basin into the Altiplano (Aceituno, 1993;Garreaud, 2000).

Interannual climate variability is mainly related to changes in themean zonal wind over the Altiplano largely modulated by sea-surfacetemperature (SST) changes in the tropical Pacific (Vuille et al., 2000;Garreaud and Aceituno, 2001; Bradley et al., 2003). Marked climaticanomalies occur during the ENSO events (Vuille, 1999; Vuille et al.,2000; Garreaud and Aceituno, 2001; Bradley et al., 2003). During thewarm phase of ENSO (El Niño), the Bolivian High is displacednorthward leading to reduced precipitation and increased tempera-tures. Conversely, during the cold ENSO (La Niña) events, the easterlyflow is enhanced resulting in increased cloud-cover and convectiveprecipitation, especially during afternoons and evenings (Aceituno,1993; Garreaud and Wallace, 1997).

With regard to current Altiplano temperature trends, instrumental-based measurements indicate that temperatures have been increasingat a rate of 0.10 °C–0.11 °C per decade since 1939. This rate of warminghas more than tripled over the last 25 yr (0.32°–0.34 °C per decade),and temperatures during the 1997–98 El Niño event were thewarmestduring the last six decades (Vuille and Bradley, 2000). Predictiveatmospheric circulation models suggest that maximum temperatures

Fig. 2. Location of the northern (Volcán Guallatiri 18°28′ S/69°04′W) and southern (Cerro Granada 22°32`S / 66°35`W) study sites in theWestern Andean Cordillera in the Altiplano.

Table 1Meteorological stations in the Altiplano used for comparision with Sea SurfaceTemperature (SST) in the tropical Pacific and Polylepis tarapacana tree growth.

Climatic variable Location Elevation (m) Country Period

SST Niño 3.4 region – – 1870–2001a

TemperatureOruro 17°57′ S/67°08′ W 3706 Bolivia 1953–1998b

Charaña 17°35′ S/69°26′ W 4059 Bolivia 1951–1998b

Sajama 18°06′ S/68°53′ W 4220 Bolivia 1975–1981b

Pacollo 18°10′S/69°29′ W 4050 Chile 1978–1993b

Chungará 18°17′ S/69°07′ W 4500 Chile 1986–1993b

La Quiaca 22°07′ S/65°36′ W 3442 Argentina 1911–1990c

PrecipitationCharaña 17°35′ S/69°26′ W 4059 Bolivia 1948–1998b

Isla Blanca 17°36′ S/69°36′ W 4500 Chile 1970–1989b

Oruro 17°57′ S/67°08′ W 3706 Bolivia 1948–1998b

Cotakotani 18°11′ S/69°14′ W 4450 Chile 1962–1992b

Chucuyo 18°13′ S/69°20′ W 4200 Chile 1960–1994b

Parinacota 18°14′ S/69°12′ W 4390 Chile 1952–1994b

Chungará 18°17′ S/69°07′ W 4500 Chile 1962–1994b

Guallatiri 18°30′ S/69°10′ W 4280 Chile 1969–1994b

La Quiaca 22°07′ S/65°36′ W 3442 Argentina 1903–1993c

Source: a Hurrell et al. (2008); b Vuille et al. (2000); c Servicio Meteorológico Nacional,Argentina.

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in the lower troposphere in this region will substantially increaseduring the next 100 yr (Bradley et al., 2006).

3. Methods

3.1. ENSO and local climate

To asses the relationships between ENSO and climate variability inour study sites (Fig. 2), we utilized a combination of meteorologicalstation records across the Altiplano and sea surface temperatures(SSTs) from the Niño-3.4 region (5° N/5° S, 170°/120° W) (Table 1).Long, continuous and homogeneous meteorological records in thecentral Andes are sparse, especially for temperature. For the northernsector of our study area, precipitation variations at Charaña, Chucuyo,Chungará, Cotakotani, Guallatiri, Isla Blanca and Parinacota (Fig. 2; seealso Solíz et al., 2009-this issue) were combined to create a regionalrainfall record.Monthly standard deviationswere determined for eachstation and then averaged among stations to produce a regionalrecord. Most temperature records close to the northern tree-ringchronology were short and discontinuous. However, as the spatialvariability of air temperature anomalies over the Altiplano is not verylarge (Vuille et al., 2000), we used the Oruro record for comparisonwith SSTs. Short and fragmentary temperature records from Pacollo,Sajama, Charaña, and Chungará, (Fig. 2) which are closer to thenorthern tree-ring site, were pooled together to characterize themonthly temperature variations shown in Fig. 6. For the southernchronology, we utilized the long, continuous, precipitation andtemperature records from La Quiaca.

Relationships between Niño-3.4 SST with temperature andprecipitation records were evaluated on a monthly basis usingcorrelation analysis. The effect of ENSO events on local temperatureand precipitation variations was assessed using Superposed Epochanalysis – SEA – (Mooney and Duval, 1993). SEA compares precipita-tion and temperature time series with a list of ENSO events (El Niñoand La Niña years). For each event, a five-year windowwas consideredwhich includes 2 yr leading up to and 2 yr following the event. Thefive-year windows for all the events were superimposed and averagedto obtain themean pattern of temperature and precipitation related toENSO events. Confidence limits were calculated using 1000 MonteCarlo simulations.

We objectively defined ENSO-year events following the criteriaoutlined by Trenberth (1997). Consequently, El Niño (La Niña) eventswere defined as those showing, from October to March, 5-monthrunning means of the SST anomalies in the Niño-3.4 region exceeding±0.4 °C. We choose the October to March season to include the time

of the year which encompasses the peak of ENSO, the seasonwith thelargest influences of tropical Pacific SST on the Altiplano climate andthe time concurrent with P. tarapacana growing season (December toMarch). The Southern Hemisphere tree-ring dating convention(Schulman, 1956) assigns to annual rings the date of the year inwhich radial growth begins. Thus, we assigned to ENSO events thedates of the year in which the corresponding growing season began.For example, the 5-month running means exceeding +0.4 °C wereregistered from October 1997 to March 1998 and assigned to the 1997ENSO event. Table 2 shows the list of ENSO events used in this study.

3.2. Tree-ring chronology development

The genus Polylepis comprises 26 species distributed along theAndes Cordillera from Venezuela to northern Argentina (8° N–32° S).P. tarapacana grows in the Altiplano between 16° and 23°S (Kesslerand Schmidt-Lebuhn, 2006). We sampled two P. tarapacana popula-tions separated latitudinally by about 500 km, located on thewest andeast slopes of the Western Andean Cordillera. The Guallatiri volcanowoodland is in Chile at 4600–4700 m on the western slope of theCordillera (18°45′ S 69°10′ W), whereas the Cerro Granadas popula-tion is located in Argentina at 4500–4750 m on the opposite slope

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(22°32′ S 66°35′ W). In this paper, we will refer to the Guallatiri andGranada records as the north and south chronologies, respectively(Fig. 2).

Due to the eccentric patterns of radial growth and the presence oftwisted trunks, we collected wedges and cross-sections from branchesof living individuals and dead wood, respectively. Samples wereprepared following standard dendrochronological techniques as out-lined in Stokes and Smiley (1968). Growth rings were visually cross-dated to the year of ring formation (Fritts, 1976). As mentioned above,we followed the Schulman (1956) convention for the SouthernHemisphere, which assigns dates of annual rings to the year in whichradial growth started. Annual rings were measured under a binocularstereoscope with a Bannister-type measuring machine (0.001 mmaccuracy) connected to a computer (Robinson and Evans, 1980). Thecross-dating quality was checked with the computer programCOFECHA (Holmes, 1983), which calculates correlation coefficientsbetween individual tree-ring series as a tool for identification of absentor false rings.

Ring-width measurements were standardized to remove varia-bility in the time series not related to climate such as tree ageing orforest disturbances (Cook et al., 1990). Prior to standardization, thevariance of each series was stabilized using a data adaptive powertransformation based on the local mean and standard deviation ofeach tree-ring series (Cook and Peters, 1981; Cook and Peters, 1997).Standardization was accomplished by fitting cubic smoothing splineswith a 50% frequency-response cutoff width equal to 67% of the serieslength for each individual tree-ring series. During the standardizationprocess indices were obtained by differences between powertransformed ring width measurements and fitted splines (Cook andPeters, 1997).

Tree-ring chronologies were produced with the ARSTAN40cprogram (Cook and Krusic, 2006). Residual instead of standardchronologies were used in our analysis. By using autoregressivemodeling to remove the serial autocorrelation in the original tree-ringseries (Cook et al., 1990), the residual chronologies satisfy thestatistical assumptions required for establishing the statisticalsignificance of the climate–growth relationships. Residual chronolo-gies also emphasize the high-frequency domain contained in the timeseries, which could be related to ENSO (Gilman et al., 1963).

Finally, the quality of the tree-ring chronologies was assessed byusing the statistic Expressed Population Signal (EPS). To calculate EPS,we use a statistically demanding 10-year window with an overlap of5 yr between adjacent windows. The EPS measures the strength of thecommon signal in a chronology over time and quantifies the degree towhich a particular chronology portrays the hypothetically perfectchronology (Cook et al., 1990). While there is no level of significancefor EPS per se, values above 0.85 are generally accepted as showing a

Table 2El Niño and La Niña events for the period 1870–1999 following Trenberth (1997).

El Niño La Niña

1876 1918 1965 1870 1903 19551877 1919 1968 1871 1906 19561880 1923 1969 1872 1908 19641884 1925 1972 1873 1909 19671885 1929 1976 1874 1910 19701887 1930 1977 1875 1916 19711888 1935 1979 1878 1917 19731895 1939 1982 1879 1920 19741896 1940 1986 1882 1922 19751899 1941 1987 1886 1924 19831900 1947 1990 1889 1933 19841902 1951 1991 1890 1938 19851904 1952 1992 1892 1942 19881905 1953 1993 1893 1945 19951911 1957 1994 1894 1949 19981913 1958 1997 1897 1950 19991914 1963 1898 1954

good level of common signal fidelity between trees (Wigley et al.,1984). If a given period in the tree-ring chronology drops below thisthreshold, it should be viewed more critically when trying to makeinferences about climate.

3.3. Tree-rings and climate

To identify the relationships between climatic factors and treegrowth we computed correlation functions between tree-ring widthindices and monthly mean temperature, monthly mean Niño-3.4 SSTand total monthly precipitation for each study site (Blasing et al.,1984). Tree radial growth is influenced by climatic conditions severalmonths prior to ring formation, therefore, we examined the climategrowth relationships over the period starting with June from the priorgrowing season to May in the current year of ring formation (Fritts,1976). The effects of warm (El Niño) and cold (La Niña) ENSO eventson tree growth were evaluated using the SEA.

For the common period between the tree-ring chronologies, theirfrequency domains were determined using Maximum EntropySpectral Analysis – MESA – (Ghil et al., 2002). In order to expandthe tree-ring chronologies into time-frequency space and localizeintermittent periodicities in the records, we performed ContinuousWavelet Transform analysis – WT – (Torrence and Compo, 1998).Additionally, causal relationships in the time-frequency spacebetween ENSO variability and P. tarapacana growth were examinedemploying Cross Wavelet Transform analysis – XWT – (Grinsted et al.,2004). XWT shows the common power and relative phase betweentime-series in the time-frequency space, to determine whether theypresent a consistent phase relationship, and consequently whetherthey are physically related (Jevrejeva et al., 2003). Monte Carlomethods were used to assess the statistical significance against rednoise backgrounds (Grinsted et al., 2004).

Finally, to determine the atmospheric and oceanic features moreclosely related to P. tarapacana growth we calculated correlationmapsbetween the tree-ring chronologies and monthly averaged 2.5°×2.5°gridded SST, Sea Level Pressure (SLP), and 250 hPa zonal wind (U250)time series derived from the NCEP reanalysis global database (Kistleret al., 2001). In order to determine the strongest temporal relation-ships between P. tarapacana growth and the variables of interest, wecompared several seasonal combinations of SST, SLP, and UA using themap correlation routines available at the Climate Diagnostics Center,National Oceanic and Atmospheric Administration (CDC, NOAA).

4. Results and discussion

4.1. ENSO influences on local climate

At our study sites, temperature (T°) and precipitation (Pp) co-varyin anti-phase on seasonal and annual time-scales. Correlation betweenmonthlymean temperature andmonthly total precipitation is stronglynegative especially during the summer season (DJF), with values up to−0.71 and −0.61 during January at the north and south study site,respectively (Pb0.001; n=46 and 80, respectively). At annual time-scales, the relation betweenmean T° and total Pp is−0.65 and−0.6 atthe north and south study site, respectively (Pb0.001; n=46 and 80,respectively). These results are consistent with previous studies onlarge-scale spatial patterns in the Altiplano using these climaticparameters (Vuille et al., 2000; Vuille and Keimig, 2004).

At monthly time-scales, relationships between Niño-3.4 SST,temperature and precipitation are variable. During the australsummer period Niño-3.4 SST is positively correlated with temperatureand negativelywith precipitation (Fig. 3). It is in this season that about85% of the total annual precipitation occurs at our study sites. Theregional atmospheric circulation in summer favors the advection ofwater vapor from the Amazon basin at times when the annual rings ofP. tarapacana formed (Garreaud, 2000).

Fig. 3. Sliding monthly correlations between the Niño-3.4 SST and lag1 monthprecipitation andmean temperature from the north and south study sites in the Altiplano(n=number of correlatedpairs). Dashedhorizontal lines indicate statistical significance atthe 95% confidence level. The 12monthyearwas set according to theP. tarapacana growingyear (June–May). The grey vertical bar indicates the tree growing season.

Fig. 4. Superposed epoch analysis (SEA) comparing summer season (DJFM) temperature(a), and precipitation (b) at the north and south study sites during El Niño and La Niña years.The X-axis represents a set of 5 yr, from 2 yr before the ENSO event to 2 yr following it. Thedotted horizontal lines represent the 95% confidence levels after 1000Monte Carlo iterations.Theperiod from1953–97wasused for thenorth, and in the south1991–89 and1911–97wereused for temperature and precipitation, respectively. The ENSO events utilized are listed inTable 2. About 90% of annual rainfall occurs over this region during the DJFM season.

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Significant correlations between tropical Pacific SST and summertemperature on both sides of the Western Cordillera reflect the spatialuniformity in the temperature variability across the Altiplano (Vuilleet al., 2000). Conversely, the relationships between Niño-3.4 SST andprecipitation areweaker (Fig. 3) and somewhat different between sites.However, Vuille et al. (2000) who identified the main spatiotemporalmodes of interannual summer precipitation from the Altiplano andtropical Pacific based on principal components analysis concluded thatprecipitation variability in this region is related primarily to ENSO.

At the interannual time-scale, there is a clear modulation of ENSOevents on summer climatic conditions in the Altiplano (Fig. 4).Temperature anomalies are closely related to ENSO events at bothstudy sites, whereas precipitation has a more modest response. Ingeneral, El Niño events tend to causewarm and dry summers, whereasLa Niña events are related to cold and rainy summers (Aceituno, 1988;Vuille et al., 2000; Garreaud and Aceituno, 2001). During El Niñoevents (warm-ENSO phase), the westerly-dry flow on the Altiplanoincreases in intensity forcing the Bolivian High to northern latitudes.In consequence, the eastern-wet influences are reduced over theentire region leading to generally warm-dry summers (Vuille et al.,1998; Garreaud et al., 2003). Conversely, when there are negativeanomalies in the tropical Pacific SSTs (cold-ENSO phase), the easterlyflow increases the moisture transport from continent producing cold–cloudy–wet summers dominated by afternoon convective storms(Aceituno, 1993).

4.2. Tree-growth response to local climate

The tree-ring width variations of P. tarapacana show a strongcommon signal between individuals at a single site and between sites

across the Altiplano (Fig. 5; Table 3). The EPS values remain above thethreshold of 0.85, evenwhen a statistically-demanding 10-yearwindowcomparison is used to evaluate the common signal between trees in asingle site. The two P. tarapacana chronologies, at 500 km apart onopposite sides of theWestern Andean Cordillera, are remarkably similarin their oscillations (r=0.6; pb0.001, period 1830–1999). The similarityin growth patterns between the two P. tarapacana chronologies suggestthat the records share large-scale environmental patterns, as it is shownin a companion study based on a network of 14 chronologies of P.tarapacana across the Altiplano (Solíz et al., 2009-this issue).

The north and south chronologies show contrasting responses totemperature in the previous (negative) and the current (positive)growing seasons (Fig. 6). This opposite relationships between treegrowth and climate for consecutive growing seasons is difficult toexplain on a biological basis. Argollo et al. (2004) and Morales et al.(2004) suggested that P. tarapacana growth is largely influenced by thewater balance during the previous growing season. They also indicatedthat the reversal in the response of growth to temperature between twoconsecutive years is due, to some extent, to the properties of thetemperature variations in the Altiplano, characterized by a persistentoscillation centered at 2.3 yr. In consequence, if cool (warm) summersare consistently followed in most cases by warm (cool) summers,opposite relationships between radial growth of P. tarapacana andtemperature are expected for consecutive growing seasons. Thecounter–intuitive relationship between tree growth and climate forconsecutive growing seasons may be also partly due to the semiarid

Fig. 5. The prewhitened Polylepis tarapacana ring-width chronologies from Volcán Guallatiri (north) and Cerro Granada (south). The Expressed Population Signal (EPS) statisticcalculated for 10-year windows and sample depth trough time are shown for each chronology. The r value indicates the correlation coefficient between chronologies.

Table 3Descriptive statistics for the tree-ring chronologies from Polylepis tarapacana used inthis study.

Site Correlation within series Mean sensitivity Variance in first eigenvector (%)

North 0.67 0.277 52.2South 0.57 0.307 38.1

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climate and the extreme high elevation of the P. tarapacanawoodlands.However, additional studies are needed to gain insight on this particularresponse of P. tarapacana radial growth to temperature in the Altiplano.

In this study, we related P. tarapacana radial growth during thecurrent growing season with SST in the tropical Pacific. In thecompanion study from Solíz et al. (2009-this issue), the potential ofP. tarapacana chronologies to reconstruct spatial precipitation patternsacross the Altiplano is assessed based on the positive relationshipbetween precipitation and radial growth during the previous growingseason. As summer variations in precipitation and temperature in theAltiplano are strongly anti-correlated and vary in anti-phase, the radialgrowth of P. tarapacana is simultaneously recording seasonal changesin both climatic parameters. In consequence, tree growth reflectschanges in both temperature and precipitation during summer.

4.3. The ENSO signal on P. tarapacana growth

Monthly tropical Pacific SSTs from the Niño-3.4 region andP. tarapacana growth are significantly correlated from July previousto the growing season to March during the current year of ringformation. This period encompasses the peak ENSO season (November–February), when warm and cold events typically mature in theequatorial Pacific (Fig. 6). The strong relationship betweenP. tarapacana chronologies and ENSO is consistentwith the documentedassociation between variations in the tropical Pacific SSTs and those ofsummer temperature and precipitation across the Altiplano. Correlationcoefficients reach higher significance when SST variability in theequatorial Pacific precedes temperature and precipitation variations inthe Altiplano by 1–2 months (Fig. 3; Vuille et al., 2000).

El Niño and La Niña events are well recorded in our P. tarapacanachronologies. Summer warm and cold conditions during El Niño andLa Niña events are related to above- and below-average ring-widthindexes during the current growing season (Fig. 7). However, in theinstrumental records there are some warm-dry (cold-wet) summersin the Altiplano not directly related to El Niño (La Niña) events(Garreaud et al., 2003). In consequence, not all the wide (narrow)rings represented in our chronologies should be inferred as past El

Niño (La Niña) events. During the common period between theAltiplano tree-rings and Niño-3.4 SST time series (130 yr; Fig. 8), 87%of the 15 highest tree-ring indices of each chronology correspond to ElNiño events. In the case of the 15 lowest tree-rings indices, 80% and87% correspond to La Niña events at the north and south study sites,respectively.

The P. tarapacana chronologies capture most of the high-frequencyvariability present in the Niño-3.4 SSTs (Fig. 8). Strong El Niño eventssuch as 1877, 1888, 1896, 1918, 1925, 1965, 1972, 1987 and 1997 arecoincident in both chronologies with tree-ring indices above themean. Strong and sustained El Niño conditions as observed during1904–05 and 1940–41, which were associated with sustaineddroughts in the Altiplano, negatively affected growth of P. tarapacanaresulting in ring widths below the long-term mean. Differences inchronology responses to El Niño during the 1900, 1912, 1982 events,registered as higher indices at the north chronology, show the majorsensitivity of the west slope climate of the Western Cordillera to ElNiño induced climate anomalies, a feature previously described byGarreaud et al. (2003). Strong La Niña events such as those of 1874,1886, 1892, 1910, 1916, 1942, 1971, 1998 and 1999 are clearly identifiedin both chronologies by indices well below the mean. Nevertheless,this relationship was not observed in all cases such as the 1950 La Niñaevent.

Correlation coefficients between the P. tarapacana chronologiesand Niño-3.4 SSTare similar, in some cases higher, than those reportedbetween the Texas–Mexico (Tex–Mex) chronologies and the winterSOI (Stahle et al., 1998). The Tex–Mex chronologies register one of thestrongest ENSO signal detected in tree-ring data worldwide. Correla-tions with SOI are significantly higher during January and February,

Fig. 6.Meanmonthly temperature (°C) and precipitation (mm)±2 standard error and correlations functions based on prewhitened chronologies showing the relationships betweenP. tarapacana radial growth and monthly precipitation, temperature, and Niño-3.4 SST from the prior to the current year of ring formation (n and n′=length in years of thetemperature and precipitation records, respectively. N=number of years used in the correlation analyses). Dashed horizontal lines indicate statistical significance at the 95%confidence level. Grey vertical bars indicate the previous and current P. tarapacana growing season, respectively.

Fig. 7. A superposed epoch analysis (SEA) is illustrating comparing tree-ring indexdepartures for northern and southern P. tarapacana residual (prewhitened) chron-ologies during El Niño and La Niña years for the period 1870–1999. The ENSO eventsutilized are listed in Table 2. The X-axis represents a set of 5 yr from 2 yr before to 2 yrfollowing the ENSO event. We use as ENSO event years 50 El Niño and 50 La Niñacorresponding to the 1870–1999 period according to Trenberth (1997). The dottedhorizontal lines represent the 95% confidence levels resulting from 1000 Monte Carloiterations. The large (small) tree-ring indices during El Niño (La Niña) years aresignificant different at the 95% confidence level for both chronologies.

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which temporally coincided with the season of higher correlationsbetween P. tarapacana chronologies and Niño-3.4 SST (August toFebruary, Fig. 8). This feature suggest that spatial patterns of pastENSO variability and teleconnections can be explored using tree-ringchronologies from North and South America showing similarseasonal-response to climate anomalies from ENSO.

The significant oscillatory modes in both P. tarapacana chronolo-gies occur within the high frequency domain. MESA indicates that alarge percentage of the variance is centered within the “classical”ENSO bandwidth with peaks that exceed the 95% confidence limit at2.1–3.1, 3.6–4.1, and 6.1–6.3 yr. A multi-decadal peak at 36 yr isregistered in the north chronology. The WT, which allows theexpansion of the tree-ring chronologies in the time-frequency spaceand therefore can reveal intermittent periodicities, shows similarspectral peaks to those identified by MESA (Fig. 9). The WT alsorevealed strong temporal similarities within the ENSO band betweenthe chronologies, and a multi-decadal peak of 36 yr in the interval1890–1930 for the north chronology. In the WT of the chronologies,the oscillatory periods capturing the largest variance are in agreementwith the significant common spectral power between the chronolo-gies and the instrumental Niño-3.4 SST revealed by the XWT (Fig. 9).The XWT indicates large covariance between the P. tarapacanachronologies and the Niño-3.4 SST within the classical ENSObandwidth. The north and south XWT are remarkably similar,demonstrating again the strong regional signature of ENSO on the P.tarapacana chronologies. Significant intervals within the north andsouth XTW show mostly consistent in-phase behavior between theNiño-3.4 SST variability and P. tarapacana growth, as indicated by thevector directions (Grinsted et al., 2004). These results are consistentwith the previous SEA and correlations analysis between thechronologies and Niño-3.4 SST. In general, the results demonstratethat the tree-ring chronologies contain tropical Pacific forced compo-

nents represented by signals within the ENSO band, and that P.tarapacana growth is physically linked to tropical Pacific SST variability.

The spatial correlation patterns between the P. tarapacana chron-ologies and the instrumental gridded SST, U250 and SLP provide insightsonhowENSO-related large-scale climatic variabilityaffectsP. tarapacana

Fig. 8. Comparison between the northern and southern prewhitened P. tarapacana chronologies from the Altiplano and the instrumental 5-month running mean of monthly SSTanomalies for the Niño-3.4 region from 1870 to 1999. Black (grey) shading shows tree-ring indices above (below) the long-term mean, and for the Niño-3.4 SST record the + (−)0.4 °C threshold indicating El Niño (La Niña) events according to Trenberth (1997). Correlation coefficients between the tree-ring chronologies and the Niño-3.4 SST anomalies(August to February) are indicated.

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growth in both study sites (Fig. 10). These spatial patterns mostlyresemble El Niño-like conditions, supporting the relationships betweenENSO conditions and P. tarapacana growth. The spatial correlation fieldsfor August–February SST show the classical ENSO pattern with warmeranomalies in the central and eastern equatorial Pacific related to P.tarapacana above-average growth.With regard toU250, the patterns are

Fig. 9. The wavelet power spectrum (Morlet) of the northern and southern tree-ring chronoland Niño-3.4 SST (August–February period). Black thick contours indicate the 95% significanVectors indicate the relative phase relationship between the Niño-3.4 SST and the tree-ring crelationships respectively, and pointing upmeans the second series lag the first by 90°. All sigP. tarapacana chronologies.

characterized by a reduction of thewesterly flow at 250 hPa in a narrowequatorial band over the central Pacific and an enhancement between10°–25° across the Pacific domain, which in turn reduces the wet-easterly influences over the Altiplano during ElNiño events (Vuille et al.,2000). Finally, the spatial structure of the December–February SLP ischaracterized by the well-known dipole over the eastern equatorial

ogies (WT), and the cross-wavelet transform (XWT) between each tree-ring chronologyce level using the red noise model, and the cone of influence is shown as a lighter shade.hronologies (a horizontal arrow pointing right and left implies in-phase and anti-phasenificant sections in the XTW show in-phase relationships between the Niño-3.4 SST and

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Pacific and thewestern Pacific-eastern IndianOcean (the tropicalWarmPool). The spatial correlation patterns between SST, U250 and SLP andthe tree-ring chronologies are consistent with previous SEA andcorrelation analyses between the tree-ring records and Niño-3.4 SSTs(Figs. 7 and 8), which indicate that the P. tarapacana growth isinfluenced by atmospheric circulationmodulated by the tropical Pacific.

5. Conclusions and prospects

Wehave identified the ENSO signal in theworld's highest elevationP. tarapacana tree-ring chronologies. The chronologies have beendeveloped in the tropical south-central Andes, a region where theinterannual climatic variability is strongly modulated by ENSO. Thegrowth of P. tarapacana is influenced by the December–Februaryanomalies in temperature and precipitation, which in turn arestrongly connected with the ENSO cycles. The P. tarapacana tree-ringrecords show a large common signal within individuals in a single site,and between sites across the Altiplano. These records are significantlycorrelated with instrumental ENSO-related records such as the Niño-

Fig. 10. Spatial correlation fields between the P. tarapacana prewhitened chronologies andDecember–February Sea Level Pressure (SLP), and August–February Sea Surface Temperareanalysis. Study sites are indicated by red dots.

3.4 SST, and they capture the dominant spectral oscillations within theclassical ENSO bandwidth.

P. tarapacana is a long-lived tree species whose individuals reachmore than 600 yr of age. Abundant death and sub-fossil material arepresently being used to extend the chronologies for the past 1000 yr.As P. tarapacana grows 800 km along the Andes from 16°S to 23°S, itoffers the unique opportunity to develop a large multi-centurynetwork of tree-ring chronologies all above 4000 m in the tropics(Argollo et al., 2004; Solíz et al., 2009-this issue).

Our results indicate that the P. tarapacana chronologies from theAltiplano represent high-resolution proxies extremely sensitive toENSO in the Pacific basin. These new ENSO-sensitive tree-ring recordsfrom the tropical Andes offer a unique opportunity to geographicallyexpand the sparse network of annually resolved and exactly datedENSO proxies. Our records should be considered in the developmentof new multi-proxy ENSO reconstructions in order to adequatelycapture the past spatial variability of this coupled ocean–atmosphericphenomenon. More research is needed to gain insights into thecomplexity of the climate–P. tarapacana growth relationships, and to

2.5°×2.5° gridded monthly averaged December–February 250 hPa zonal wind (U250),ture (SST). U250, SLP and SST data for the interval 1949–1999 are from NCEP–NCAR

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increase the spatial distribution and temporal extent of the tree-ringrecords which will provide consistent patterns of climate variability inthe Altiplano and their teleconnections worldwide.

Acknowledgements

This work was carried out with the aid of a grant from the Inter-American Institute for Global Change Research (IAI) CRN I # 03 and CRNII # 2047 which is supported by the US National Science Foundation(Grant GEO-0452325), the ICM project P04-065-F Chilean Ministry ofPlanning, the Argentinean Agency for Promotion of Science (PICTR 123)and CONICET. The first author was also supported by a doctoralscholarship from the Chilean Science Research Council (CONICYT). Weare grateful to M. Vuille from the U. of Massachusetts for providingmeteorological data records, D. Shea from the NCAR Climate and GlobalDynamics Division for the Niño-3.4 region SST record extracted fromHurrell et al. (2008), and P. Štěpánek and M. Masiokas for AnClimprogram and RespoSum_Avg time series analysis routine, respectively. J.P. Francois, G. Lara and J. Moya are thanked for “high altitude” fieldassistance. We acknowledge the comments from D. Stahle, R. Garreaudand one anonymous reviewer which significantly improved this manu-script, and CONAF (Chilean Forest Service) for permission to collect P.tarapacana samples in Lauca National Park-UNESCO Biosphere Reserve.

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