Mendoza et al. - Historical Droughts in Central Mexico and Their Relation with El Niño

7/27/2019 Mendoza et al. - Historical Droughts in Central Mexico and Their Relation with El Nio

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Historical Droughts in Central Mexico and Their Relation with El Nio

BLANCA MENDOZA Instituto de Geofsica UNAM, Ciudad Universitaria, Del. Coyoacn, Mxico

ERNESTO JUREGUICentro de Ciencias de la Atmsfera, UNAM, Ciudad Universitaria, Del. Coyoacn, Mxico

ROSA DIAZ-SANDOVALLaboratorio de Cronobiologa Mdica, EMH, IPN, Del. Gustavo A. Madero, Mxico

VIRGINIA GARCA-ACOSTACentro de Investigaciones y Estudios Superiores en Antropologa Social, Col. Tlalpan, Mxico

VICTOR VELASCO AND GUADALUPE CORDERO Instituto de Geofsica, UNAM, Ciudad Universitaria, Del. Coyoacn, Mxico

(Manuscript received 6 October 2003, in final form 15 September 2004)

ABSTRACT

A catalog containing an unprecedented amount of historical data in central Mexico, covering almost sixcenturies (14501900), is used. This is a catalog of agricultural disasters that includes events associated withhydrometeorological phenomena, or hazards, whose effects were mainly felt in the agricultural sector, suchas droughts. An analysis of the historical series of droughts in central Mexico for the period of 14501900is performed. Periods of frequent drought centered at the years 1483, 1533, 1571, 1601, 1650, 1691, 1730,1783, 1818, and 1860 have been identified. In particular, droughts in Mexico City and northwest Mexico thatwere identified through poor tree-ring growth are included in the frequent drought periods obtained in this

work. Moreover, it was found that droughts occurred in El Nio years mainly for events of very strong andstrong strengths, at a significant level. Also, most droughts lasted for 1 or 2 yr. Last, by analyzing theperiodicities of the drought time series it was found that those that are the most conspicuous are thequasi-bidecadal frequencies of 18.9 and 21 yr.

1. Introduction

The climate of Mexico is influenced by the positionand strength of the subtropical high pressure systems of the North Atlantic and the northeast Pacific Oceans(near 30 latitude), as well as by the location of theintertropical convergence zone lying to the south of thecountry. While moist trade winds prevail during thehalf-year centered in the summer, penetration of polarcontinental air masses from North America dominatesin winter and spring, resulting in a marked drop of temperature in most of the country (Hill 1969; Juregui

1971; Klaus 1973). The effects of the intrusion of coldair are evident in the highlands of north and centralMexico, as well as on the costal plains of the Gulf of Mexico and the Yucatan Peninsula where they arecalled Nortes, producing, on occasion, intense pre-cipitation in the lowlands and rainfall on the highmountains of northcentral Mexico. The character of the rainfall regime in the central and southern parts of the country is such that there are usually two maximaone in June and the other in September. Meanwhile,July and August show a decline in convective activity,known ascancula or midsummer drought (MSD) (e.g.,Mosio and Garca 1966; Magaa et al. 1999). This bi-modal variation of rainfall seems to be the result of changes in the intensity of the low-level winds duringJuly and August blowing over a region of warm wateroff of the Pacific coast of southern Mexico, known asthe warm pool where convective activity is intense

Corresponding author address: Blanca Mendoza, Instituto deGeofisica, UNAM, Ciudad Universitaria, Del. Coyoacn 04510,Mxico.E-mail: [email protected]

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(Maga a et al. 2003). Cloud cover variations are likelyto modulate the sea surface temperature of these warmwaters. Maga a et al. (1999) note that after the onset of the summer monsoon around May June, the sea sur-face temperature over the eastern Pacific decreasesaround 1 C because of interception of solar radiationby the increasing cloudiness and stronger easterlywinds. Such sea surface temperature changes result, ac-cording to these authors, in a substantial decrease indeep convective activity during July and August in theregion.

One more clear influence on rainfall fluctuations isthe global-scale climatic disturbance that is associatedwith El Ni o phenomenon. El Ni oprecipitation rela-tionships have been examined for tropical and extrat-ropical regions (e.g., Ropelewski and Halpert 1986,1989). Given the large latitudinal extent of Mexico inthe Tropics/subtropics, Cavazos and Hastenrath (1990)have explored the role of El Ni o for various rainfallregimes in Mexico; large regions of Mexico appear to

be correlated to El Ni o in this study. Mosi o and Mo-rales (1987) have shown that for the period of 1921 85,drought conditions in central Mexico were probably as-sociated with strong El Ni o events, whereas moderateEl Nio favored abundant or normal rains. In an attemptto detect a link between the historical El Ni o series byQuinn and Neal (1992) and a historical drought series(15351987) for the whole country published by Flores-cano (1980), J uregui (1995) found that the highest fre-quency of droughts reported in some regions of Mexicofor 18221987 occurred in El Ni o years at a significancelevel 0.995 under a chi-square test.

Because the MSD occurs in both El Ni o and LaNia years, Maga a et al. (1999) concluded that, duringthe period they examined (1979 93), there were noclear links between the MSD and the two phenomena.Also, they found that the central part of Mexico is onlymarginally affected by the MSD.

Another regional-scale system affecting precipitationpatterns in northwestern Mexico is the Mexican mon-soon where negative rainfall anomalies are observedduring both, El Ni o, or La Ni a summers (Douglas etal. 1993). Precipitation anomalies in this region do notseem to be related to sea surface temperature anoma-lies in the eastern Pacific (Koster et al. 2000).

Recently, a catalog of agricultural disasters forMexico has been prepared, containing an unprec-edented amount of historical data (Garc a-Acosta et al.2003); this work includes droughts, among other disas-ters. Taking advantage of this catalog, in the present pa-per we aim to study the behavior of rainfall variations asexpressed by the secular series of droughts in centralMexico and to examine their relation with El Ni o.

2. DataWe worked with three series of data two corre-

sponding to the El Ni o phenomenon and the other todroughts in central Mexico.

a. El Ni o series

We worked with two records of El Ni o events. Thefirst one is the list given by Quinn and Neal (1992) of the Ecuador and Peru regions covering the period from1525 to 1900, with a total of 85 events of all intensities.In this list, no weak El Ni o events were recorded. Thesecond record is the compilation, provided by Ortlieb(2000), that presents a revised El Ni o series in thesame region for 1546 1900 with a total of 38 El Ni oevents of all intensities, constructed by doing a criticalanalysis of the sources used by Quinn and Neal (1992)in their various papers. This compilation questions theexistence of several El Ni o events, suggests the exclu-sion of others, and includes several previously unrec-ognized events. This list contains doubtful events thatnevertheless we took into account, otherwise, the num-ber of El Ni o events would have been too small. Fur-thermore, weak events were not considered.

The sources of information that were used to identifyan El Ni o event were of varied origin publishedbooks, reports (i.e., from missionaries, pirates, priva-teers, and historians), newspaper articles, and reviewstudies. The approach that Quinn and Neal (1992) tookin their various works for identifying, evaluating, anddetermining the strength of El Ni o events was derivedfrom a detailed study of what happened prior to, dur-ing, and after El Ni o events that occurred over thepast 140150 yr, when more data were available. Thestrength of the phenomenon was primarily based on thenumber of regional features that were activated andtheir respective intensities. In this way, El Ni o eventsare grouped by strength as being very strong (VS),strong with three levels (S , S, and S ), moderate withthree levels (M , M, and M ) and weak (W).

b. The historical drought series

Drought has been defined in several ways. The gen-eral definition of the term is a period of rainfall defi-ciency. If the occurrence of the drought affects theyields of agricultural crops to less than the expectedamount then the event is classified as an agriculturaldrought (Steila 1987). Another definition of agriculturaldrought is a condition in which sufficient soil moisture isnot available in the root zone for plant growth and devel-opment (van Bavel and Carriker 1957).

According to Florescano and Swan (1995) droughtconditions were a recurrent calamity in colonial timesin Mexico (as is still the case in the present day). Someof these droughts were linked to the arrival in centralMexico of cold polar air masses from North America,accompanied by below-freezing temperatures duringwinter and spring.

We use the historical drought data for central Mexicofrom the Garc a-Acosta et al. (2003) catalog for theperiod of 1450 1900. This catalog of agricultural disas-ters in Mexico includes events that are associated withhydrometeorological phenomena or hazards whose ef-

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fects were mainly felt in the agricultural sector, fromwhich stems its title, Agricultural Disasters in Mexico. Frost, hailstorm, and hurricane records come along withinformation about water scarcity or surplus that provokeddroughts and floods throughout almost six centuries of Mexican history. The oldest records come from picto-

graphic codexes and annals, with some of them writtenbefore the conquest (1521) (pre-Columbian times), butthe main sources were archives, chronicles, and old news-papers, as well as iconographical and bibliographical ma-terial. The historical droughts reported in the catalogwere severe enough to have seriously affected the popu-lation by causing crop failure, food and water scarcity,and, in extreme cases, famine and the spread of diseases.

With historical data, which is mainly qualitative, dat-ing the onset and termination of a drought, as well as itsspatial extent, constitutes a real challenge for the re-searcher. Droughts may last days, weeks, months, oreven years, but many times it is only if the drought islinked to its impact that the phenomenon can be actu-

ally labeled as such. In agriculture-based economies,such as that practiced in Mexico from pre-Columbiantimes up to the beginning of the twentieth century, thesuccess or failure of the main crops in 1 yr was the indi-cator of the success or failure of the whole economy. Badyears, currently associated with droughts, happened inyears that historians have identified as agricultural cri-ses, and related mainly to maize crop losses and to theincrease of its price as well as that of other basic products.The term drought ( sequ a or seca in Spanish) is not verycommon in former Mexican archival, bibliographical, or journalistic documents that were consulted to confirm theaforementioned catalog. In pre-Columbian times and upto the end of the Colonial Era (1821), primary, as well as

secondary, sources refer mainly to the scarcity, lack, ordelay of water and/or rains. The word drought began tobe commonly used from the nineteenth century onwardto refer to an extended period of time with below-normalrainfall. The drought records that appear in the catalogcorrespond then to long periods of a lack, scarcity, ordelay of rainfall, reflected as agricultural crisis.

Maga a et al. (1999) concluded that, during the pe-riod they examined (1979 93), the central part of Mexico is only marginally affected by the MSD. Thedroughts reported in the aforementioned catalog oc-curred in the central part of Mexico, away from theprominent influence of the MSD. Therefore, it is likelythat most of these droughts were anomalous phenom-ena, that is, more intense and of a longer duration,which had a greater impact on the population than didthe quasi-year-to-year occurrence of the MSD that wasobserved in eastern and southern Mexico (e.g., Mosi oand Garc a 1966; Juregui 1979).

3. Method

In Fig. 1 the area of study is shaded and comprises sixstates in the highlands of central Mexico. As expected,

the highest number of reported droughts correspondsto the most populated regions since pre-Columbiantimes that now are the Federal District (Mexico City),where the capital of the country is located, and thenearby state of Mexico. Both political entities contain59% of the whole series of reported historical droughts.

The original series presents 388 droughts reported onan annual basis for the period of 1450 1900. The actualseries of droughts that we used in the present paper wasconstructed as follows: droughts occurring in adjacentyears were counted as a single drought and assigned tothe first of the consecutive years. Also, droughts thatwere reported in the same year in different provincesthat are part of the area of study were counted as onedrought. In this way the original historical record wasreduced to 70 droughts during the period of 1450 1900.Figure 2 shows the number of droughts per decade.

We are supposing that within the area of study theclimate is homogeneous, and, therefore, that a droughtreported anywhere in the area occurred in the whole

region. We are aware of the fact that climatic inhomo-geneities happened and that our supposition in somecases is inappropriate, introducing an error in the finalnumber of droughts. On the other hand, the fact thatthe area of study has been continuously and highlypopulated along the period of study, indicates to us thatthe lack of drought reports is more likely because of thelack of the phenomenon itself than to the lack of ob-servers.

The historical data were transformed into numericaldata by means of a moving average or running mean. Arunning mean is a series of points that is obtained fromthe central value of curves that are adjusted by theminimum square method to successive groups of n

years of a given time series. A running mean is used tosmooth the fluctuations of a time series. As the lengthof the running mean is increased (i.e., as n is increased)smoother curves are obtained, allowing a better appre-ciation of the long-term behavior of the series. How-ever, if the running mean is too long, important detailsare missed. The choice of the running mean length isarbitrary and depends of the particular time series thatis used. For our case we found that a 39-yr runningmean is good enough to show the long-term trend whilestill preserving the important features of the droughtseries. The corresponding plot only indicates the rela-tive level of drought frequency.

4. Results and discussion

a. Secular drought occurrence in central Mexico

The 39-yr running mean of the drought series is plot-ted in Fig. 3. A visual inspection of this figure indicatesthat the most frequent drought occurred along theyears centered at 1571, 1691, and 1730, although thislast year belongs to a wide peak that was observed from1730 to 1750; less frequent droughts occurred around

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the years centered at 1483, 1533, 1601, 1650, 1783, 1818,and 1860. Moreover, the years with the fewest droughtsare centered at 1460, 1625, and 1804; these minima indrought frequency were increasing with time.

The historical drought data presented here includethe year 1576, when an epidemic of indigenous hemor-rhagic fever in the highlands of northern and centralMexico, aggravated by extreme drought conditions,provoked a native population collapse (Acu a-Soto etal. 2002). The data also include the years 1749 50 and178587, when wheat and corn prices reached theirmaximum cyclical prices in the eighteenth century,which was associated with droughts (Garc a-Acosta1995). A time series of tree-ring width for Mexico Cityfor the period 1560 2000, shows persistent low tree-ringgrowth in the decades of the 1590s, 1610s, 1660s, 1780

1790s, 18401860s, with the lowest growth of the ana-lyzed period, and 1890s (Villanueva-D az et al. 2003).Reconstructed rainfall events over the northwesternstate of Durango in Mexico (see Fig. 1 for location),obtained from tree-ring analysis for the period of 1450 1900, show two periods with the most conspicuous andlong-lasting droughts in the years of 154075 and185575 (Stahle et al. 2000). Another study of tree-ringchronologies across Mexico from 1780 to 1992 (Therrellet al. 2002) indicates low tree-ring growth in the yearscentered at 1785 and 1840 for the area south of theTropic of Cancer. The central part of Mexico lies in thisarea. Most of the drought periods inferred from tree-ringstudies coincide with the droughts observed in Fig. 3.

Analyzing the duration of droughts (see Table 1), welook at the unsmoothed record and found that almost

FIG . 1. The shaded area is the region under study. The numbers in parentheses are thenumber of droughts reported in the catalog of agricultural disasters (Garc a-Acosta et al.2003). The tree-ring data regions in Durango are also marked.



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half of the 70 droughts lasted 1 yr, and only one droughtcan be considered as quasi decadal. Historically,droughts have been linked to El Ni o short-durationevents (1 3 yr) (Juregui 1995). Larger-scale variationsare the changing conditions in the North Pacific atmo-sphere and ocean (e.g., Trenberth and Hurrell 1994). Ina study of the North American monsoon that affectsmainly northwestern Mexico and the southwesternUnited States, Higgins and Shi (2000) note that rela-tively dry monsoons were more frequent during thepositive phase of the North Pacific decadal oscillation(NPDO). This is consistent with the fact that El Ni oevents, which tend to favor relatively dry monsoons inMexico, occur more frequently during the positivephase of the NPDO. However, there is no evidence of any effect of the NPDO in the central part of the coun-try. The few quasi-decadal droughts in the central partof Mexico for the past five and one-half centuries con-trast with studies of droughts in the central part of theUnited States for 2000 yr, showing severe droughts of multidecadal duration in the past centuries (Wood-house and Overpeck 1998).

b. El Ni o and droughts in central Mexico

Table 2 shows the association of droughts with theQuinn and Neal (1992) El Ni o compilation. Becausewe suppose that El Ni o is causing the droughts, welook at the years where the two coincide; also, we con-sider those droughts that occurred soon after, that is,those droughts that occurred 1 yr after the El Ni oyear. We are allowing the 1 yr following because thedata are annual, and, given the uncertainties of thesources, we cannot be sure of when either the droughtor El Ni o is finished. Table 2 presents El Ni o eventsassociated with droughts grouped by El Ni o intensi-ties. We notice that the very strong El Ni o events areassociated with droughts; there is only one moderate(M ) event and, therefore, we do not consider it. To

evaluate the statistical confidence of the results, we ap-plied the chi-square test ( 2 ) to the distribution of droughts 3 yr before and 3 yr after the occurrence of ElNio. The test reveals that droughts are linked signifi-cantly to El Ni o for events of all intensities (T), withthe distribution peaking in El Ni o years. Also, verystrong and moderate (M and M ) El Ni o eventsshow an association with droughts. The blank spacesindicate a bimodal distribution and, therefore, the chi-square test was not applicable.

Table 3 corresponds to the Ortlieb (2000) compila-tion of El Ni o. Table 3 indicates a high correlation of very strong El Ni o with droughts. The chi-square testapplied to the distribution of droughts around El Ni oindicates that droughts are linked to very strong, andparticularly to strong, El Ni o. Moreover, the distribu-tion of droughts around moderate El Ni o was found tobe bimodal and, therefore, the chi-square test is notapplicable in this case.

Mosio and Morales (1987) found that for the periodof 192185 droughts in central Mexico were associatedwith strong El Ni o events, while moderate El Ni oevents favored normal rains. Here we found thatdroughts were linked significantly with all intensityevents, and in a lesser degree with very strong and mod-erate events for the Quinn and Neal (1992) compila-tion. For the Ortlieb list we found a link with verystrong and strong events. Maga a et al. (1999) pre-sented the impact of strong and moderate El Ni oevents for the years 1965, 1972, 1982, 1986, 1991, and1997, as summer rainfall anomalies in Mexico. Fromtheir map (Maga a et al. 1999, their Fig. 4.2) it is evi-dent that in extensive areas of the country, includingthe central part of Mexico, below-normal precipitationduring those years was observed. Despite the low num-ber of cases examined, this finding would support

FIG . 2. The 70 droughts used in this study plotted as number of droughts per decade. They are obtained from Garc a-Acosta et al.(2003).

FIG . 3. The 39-yr running mean of the drought series from 1450to 1900, taken from Garc a-Acosta et al. (2003) in the central partof Mexico.

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the results presented here in the sense that below-normal rainfall in parts of Mexico are linked to El Ni ophenomenon, particularly in the central region of thecountry. Moreover, the historical droughts studied byJuregui (1995) during the period of 1822 1987 werealso linked to El Ni o.

c. The main periodicities of the drought series

Having constructed a drought time series it can beuseful to investigate the periodicities, if any, that suchseries present. To identify the main frequencies of thedrought series we apply the maximum entropy method(MEM) (Burg 1972).We chose a 9-yr running mean(see section 3) in order to work with the largest possiblenumber of values, as is required if MEM is to be appliedsuccessfully, and at the same time to have a smoothtime series. Figure 4 shows the results. The 95% confi-dence level is marked in the figure by the horizontalline, only peaks above this level were considered. Anapproximation of the level of confidence was obtainedby creating a noise time series from the original data bymixing values randomly. We then obtain the MEMpower spectral density of this noise series and repeatthe process 50 times. The uncertainties of the peaks areobtained from the peak full width at half maximum,assuming that the peaks have a Gaussian shape.

The peak at 178.9 yr is not considered because the

longitude of the drought series allows only for very fewrealizations of this cycle. The most conspicuous fre-quencies are at 18.9 0.9 yr, 21.1 0.8 yr, 23.9 1 yr,and 39.8 5 yr; also, frequencies at 32.5 4 yr, andmarginally at 89.5 13 yr, appear.

We make an attempt to relate these frequencies with

climatic or other natural forcing factors. The most im-portant peaks 18.9 and 21.1 yrare quasi bidecadal.Latif and Barnett (1996) have produced a bidecadaloscillation in a coupled ocean atmosphere model,which originates from unstable ocean atmosphere in-teractions in the North Pacific. On the other hand,there are also forcings external to the climate systemwith time constants that are compatible with thedrought frequencies found in the present work. We no-tice that the 18.9 yr peak can be related to the 18.6-yrlunar saros cycle. The other quasi-bidecadal frequen-cies at 21.9 and 23.9 yr could be associated with the22-yr solar magnetic and geomagnetic cycles. Mitchellet al. (1979) found evidence of a 22-yr rhythm of

droughts in the western United States since the seven-teenth century. Later, Currie (1987, 1993) showed thatthe 22-yr drought cycle was actually 18.6 yr in lengthand is produced by the lunar saros cycle. Bell (1982)suggested that the real drought period was 20.5 yr, andwas caused by a beat between the 22 yr magnetic solarcycle and the 18.6 yr lunar nodal tidal cycle. Cook et al.(1997) also found a persistent bidecadal droughtrhythm in the western United States since 1700; theauthors also showed the possibility that solar and lunareffects could be interacting to modulate this rhythm, asBell (1982) had suggested. The peaks at 39.8 5 yr, andalso the peak at 89.5 13 yr, can be associated with the

TABLE 1. Duration of droughts for the period of 1450 1900. The first column indicates the duration of the droughts in years, andthe following columns indicate the number of droughts that presented such a duration.

Period

Duration (yr) 1450 99 150099 160099 170099 18001900 Tot1 4 10 6 7 7 342 2 4 6 7 193 1 2 2 3 84 1 3 1 55 1 16 2 27 1 1

Tot 5 14 13 21 17 70

TABLE 2. El Nio, associated with droughts in central Mexico,using the Quinn and Neal (1992) El Ni o list. Here, VS verystrong El Ni o events; S and S two levels of strong El Ni oevents; M , M, and M three levels of moderate El Ni oevents; T El Nio events of all intensities; and 2 chi-squaretest.

El Niointensity

No. of El Ni os

No. of El Ni osassociated with

droughtsPercentage

of association 2

VS 7 4 57 0.50S 9 4 44 0.25S 28 9 32 M 24 5 21 0.50M 16 7 44 M 1 1 100 0.50T 85 30 35 0.995

TABLE 3. El Nio associated with droughts in central Mexico,using the Ortlieb (2000) El Ni o list. Abbreviations are the sameas in Table 2.

El Ni ointensity

No. of El Nios

No. of El Ni osassociated with

droughtsPercentage of

association 2

VS 5 3 60 0.50S 11 5 45 0.75M 22 7 32 T 38 15 39 0.25



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long-term El Ni o occurrence: Anderson (1992) findscycles of 40, 50, and 90 yr in a composite record of ElNioSouthern Oscillation since A .D. 622. The peak at32.5 4 yr can be associated to the 30 yr sunspots andgeomagnetic activity (Cl a de Gonzalez et al. 1993;Mendoza et al. 2003). The peak at 89.5 13 yr can alsobe related to the Gleissberg cycle of 90 yr, observed in1000-yr records of solar and auroral activity (Feynmanand Fougere 1984), and also reported in 9700 yr of C 14

records from tree rings (Stuiver et al. 1991).Although it is still highly speculative to propose an

association between El Ni o and solar activity, it hasbeen found that El Ni o events are more common

when solar activity is weak (Anderson 1990; Mendozaet al. 1991). By stratifying the data according to eventstrength, Enfield and Cid (1991) found that the returnintervals for strong El Ni os are longer during epochsof high solar activity than during low solar variability. Asimilar result holds for all intensities, but with a lowersignificance. We would like to point out that these stud-ies have been carried out using the Quinn and Neal(1992) compilation of El Ni o, and, therefore, if thecompilation is either improved or changed so are theperiods found. Therefore, if the association between ElNio and solar variability exists, it would not be sosurprising to find the solar signal in phenomena that arethemselves well related to El Ni o, like the droughtspresented here.

5. Concluding remarks

We use a catalog containing an unprecedentedamount of historical data in central Mexico coveringalmost six centuries of Mexican history (1450 1900).This is a catalog of agricultural disasters that includesevents associated with hydrometeorological phenom-ena, or hazards whose effects were mainly felt in the

agricultural sector. The catalog was elaborated usingseveral sources. The oldest records come from picto-graphic codexes and annals some of them written be-fore the conquest (1521) but the main sources werearchives, chronicles, and old newspapers, as well asiconographical and bibliographical material. The his-

torical droughts reported in the catalog were severeenough to have seriously affected the population bycausing crop failure, food and water scarcity, and, inextreme cases, famine and the spread of disease.

The original series presented 388 droughts that werereported on an annual basis for the period of 1450 1900. Assuming that the same droughts were reportedin several parts of the area of study, and that onedrought could last several years, this series was reducedto 70 droughts. Then, the historical data were trans-formed into numerical data by means of a moving av-erage or running mean. For our case, a 39-yr runningmean was probed to be good enough to show the long-term trend, while still preserving the important features

of the drought series.We identified periods of frequent droughts centeredon the years 1483, 1533, 1571, 1601, 1650, 1691, 1730,1783, 1818, and 1860.

The duration of most droughts is between 1 and 2 yr;moreover, very few quasi-decadal droughts occurredduring these centuries.

We found a significant link between the Quinn andNeal (1992) El Ni o compilation and droughts for ElNio of all intensities, and, to a lesser degree, for verystrong and moderate El Ni o events. For the Ortlieb(2000) El Ni o compilation, a link with droughts isfound for very strong and strong El Ni o events.

The spectral analysis of the drought time series

showed conspicuous quasi-bidecadal drought frequen-cies. These periodicities coincide with oceanic tempera-ture oscillations, solar or solar-related activity phenom-ena, or both.

Acknowledgments. We thank Mario Casasola for thereorganization of the catalog of agricultural disastersfor Mexico (Garc a-Acosta et al. 2003) according to thedemands of the present study, and to Alfonso Estradafor the drawing of Fig. 1. This work has been partiallysupported by DGAPA-UNAM Grant IN104203-3 andCONACYT Grants 40601-F and D44201-F.

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Mendoza et al. - Historical Droughts in Central Mexico and Their Relation with El Niño