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A Study of Historical Droughts in Southeastern Mexico BLANCA MENDOZA
AND
VICTOR VELASCO
Instituto de Geofisica, UNAM, Ciudad Universitaria, México, Mexico
ERNESTO JÁUREGUI Centro de Ciencias de la Atmósfera, UNAM, Ciudad Universitaria, México, Mexico (Manuscript received 21 March 2005, in final form 15 September 2005) ABSTRACT A catalog containing an unprecedented amount of historical data in the southeastern part of Mexico covering almost four centuries (1502–1899) is used to construct a drought time series. The catalog records information of agricultural disasters and includes events associated with hydrometeorological phenomena or hazards whose effects were mainly felt in the agricultural sector, such as droughts. An analysis of the historical series of droughts in southeastern Mexico for the period 1502–1899 is performed. The highest drought frequency occurred around the years 1650, 1782, and 1884; no droughts were reported around 1540, between 1630 and 1640, along the largest time lapse of 1672–1714, and between 1740 and 1760. From 1760 until the end of the period of study droughts definitively occur more often than they did from ⬃1550 to 1760. In addition, most droughts lasted for 1–2 yr. Analyzing the frequencies of the drought time series it is found that the most conspicuous cycles are ⬃3–4 and 7 yr, although cycles of ⬃12, 20, 43, and 70 yr are also evident. The relation between droughts and El Niño events indicates that 38% of droughts are associated with El Niño. Sea surface temperature changes, the Southern Oscillation index, and solar activity leave their signals in the southeastern part of Mexico, with the signs in Oaxaca clearer than in the Yucatan Peninsula. However, the dominance of some phenomena over others depends on the time scales considered.
1. Introduction Historical climate studies on time scales of centuries and millennia provide a very valuable basis for interpreting the present climate behavior and for assessing future effects of climate change. Instrumental records of temperature in most places have been available since approximately the last century. Then proxy climate indices such as tree-ring chronologies, ice cores, lake sediments, etc. have been used in order to reconstruct past climates. Other types of proxy climate information used are diaries, archives, chronicles, old newspapers, as well as iconographical and bibliographical material. In Mexico, instrumental temperature records are available from the late nineteenth century (see review by Jáuregui 1997). Recently a catalog of agricultural
Corresponding author address: Blanca Mendoza, Instituto de Geofisica, UNAM, Ciudad Universitaria, México DF 04510, Mexico. E-mail:
[email protected]
© 2006 American Meteorological Society
JCLI3726
disasters for Mexico has been prepared containing an unprecedented amount of historical data over almost six centuries of Mexican history (Garcia-Acosta et al. 2003). This work includes droughts among other disasters. Taking advantage of this catalog, in a previous study using the Quinn and Neal (1992) and the Ortlieb (2000) El Niño series, Mendoza et al. (2005) found that historical droughts recorded from 1450 to 1899 were significantly correlated to strong and very strong El Niño events for the highlands of central Mexico. In the present paper we explore the behavior of historical droughts in southeastern Mexico and attempt to study their possible association to natural phenomena such as El Niño events and solar activity.
2. Present climate The climate of Mexico is influenced by the position and 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 the intertropical convergence zone lying to the south of the country. While moist trade winds prevail during the
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half year centered in summer, penetration of polar continental air masses from North America dominates in winter and spring resulting in a marked drop of temperature in most of the country (Hill 1969; Jáuregui 1971; Klaus 1973). The effects of the intrusion of cold air are evident in the highlands of north and central Mexico, as well as on the costal plains of the Gulf of Mexico and the Yucatan Peninsula where they are associated with strong northerly winds called nortes, occasionally producing intense precipitation in the southeast as well as on the high mountains of northern– central Mexico. The character of the rainfall regime in the central and southern parts of the country is such that there are usually two maxima: one in June and the other in September. Meanwhile July and August show a decline in convective activity known as canícula or midsummer drought (MSD) (e.g., Mosiño and García 1966; Magaña et al. 1999). This bimodal variation of rainfall seems to be the result of changes in the intensity of the low-level winds during July and August blowing over a region of warm waters off the Pacific coast of southern Mexico, known as the “warm pool” where convective activity is intense (Magaña et al. 2003). Cloud cover variations are likely to modulate the sea surface temperature of these seasonally warm waters. Magaña et al. (1999) note that after the onset of the summer monsoon around May–June the sea surface temperature over the eastern Pacific decreases around 1°C due to interception of solar radiation by the increasing cloudiness and stronger easterly winds. Such sea surface temperature changes result, according to these authors, in a substantial decrease in deep convective activity during July and August in the region. The variability of temperature is largely dependent on the minimum temperature. Supan (1931) has shown that the interior of North America is one of the centers of maximum interdiurnal variability decreasing toward the coasts. The dependence of large interdiurnal variability of winter temperature upon synoptic disturbances, particularly cold waves, has been suggested by Ward and Brooks (1936). Rouney (1968) describes the formation of these waves as follows: as the continental interior of North America receives less warmth from the lowering sun, the land becomes steadily cooler and ground frost creeps gradually southward. Polar continental air masses enlarge, intensify, and repeatedly surge toward the equator bringing strong blasts of frigid air as noted by Bryson and Hare (1974). Since no significant mountain barriers impede the flow east of the Rockies the arctic air from Canada may cross the entire length of North America over Mexico to the Isthmus of Tehuantepec in a few days time arriving there much colder than the tropical air. This intrusion of arctic air
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sweeps southward with high winds mainly over the coastal plains of the Mexican portion of the Gulf of Mexico and the Yucatan Peninsula as well as over the mountains of Chiapas and the eastern Oaxaca states, for example, the Isthmus of Tehuantepec. These polar outbreaks are at the origin of major fluctuations of temperature in the region and therefore constitute a characteristic feature of the winter climate there. One more clear influence on rainfall fluctuations is the global-scale climatic disturbance associated with the El Niño phenomenon. El Niño–precipitation relationships have been examined for tropical and extratropical regions (e.g., Ropelewski and Halpert 1986, 1989). Given the large latitudinal extent of Mexico in the Tropics–subtropics, Cavazos and Hastenrath (1990) have explored the role of the El Niño for various rainfall regimes in Mexico; large regions of Mexico appear to be correlated to El Niño in this study. In an attempt to detect a link between the historical El Niño series by Quinn and Neal (1992) and a historical drought series (1535–1987) for the whole country published by Florescano (1980), Jáuregui (1995) found that the highest frequency of droughts reported in some regions of Mexico for 1822–1987 occurred in El Niño years at a 0.5% significance level under a chi-square test. Since the MSD occurs in both El Niño and La Niña years, Magaña et al. (1999) concluded that, during the period they examined (1979–93), there were no clear links between the MSD and the two phenomena. Also, they found that while the central part of Mexico is only marginally affected by the MSD, southeastern Mexico is under the clear influence of this phenomenon, most notably in the states of Oaxaca and Chiapas as may be seen in Fig. 4 of their paper.
3. Data We worked with three series of data: two corresponding to El Niño phenomenon and the other to droughts in central Mexico.
a. The El Niño series We used two records of El Niño events; the first one is the list given by Quinn and Neal (1992) of the Ecuador and Peru regions covering the period from 1525 to 1900 with a total of 85 events of all intensities. In this list, no weak El Niño events were recorded. The second record is the compilation provided by Ortlieb (2000) that presents a revised El Niño series in the same region for 1546–1900 with a total of 38 El Niño events of all intensities, constructed by doing a critical analysis of the sources used by Quinn et al. in their various papers. This compilation questions the existence of several El Niño events, suggests the exclusion of others, and in-
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cludes several previously unrecognized events. This list contains doubtful events that we, nevertheless, took into account; otherwise, the number of El Niño events would have been too small for either statistical or power spectra studies. Furthermore, weak events were not considered. The sources of information used to identify an El Niño event were of varied origin: published books, reports (from, for instance, missionaries, pirates, privateers, and historians), newspaper articles, and review studies. The approach that Quinn and Neal took in their various works for identifying, evaluating, and determining the strength of El Niño events was derived from a detailed study of what happened prior to, during, and after El Niño events that occurred over the past 140–150 yr when more data were available. The strength of the phenomenon was primarily based on the number of regional features activated and their respective intensities. In this way El Niño events are grouped by strength as very strong (VS), strong with three levels (S⫹, S, and S⫺), moderate with three levels (M⫹, M, and M⫺), and weak (W).
b. The historical drought series Drought has been defined in several ways. The general definition of the term is a period of rainfall deficiency. If the occurrence of the drought affects the yields of agricultural crops to less than the expected amount, then the event is classified as an agricultural drought (Steila 1987). While droughts clearly involve a shortage of water, they really can only be defined in terms of a particular need (Lindsley 1982). Droughts can be also defined (Steila 1987) as the time interval generally of the order of months or years in duration during which the actual moisture supply at a given place rather consistently falls short of the climatologically expected moisture supply. During the period under study, Mexico was a predominantly agricultural country and, therefore, a drought caused an impact mostly on agricultural activities. In other words, the reported agricultural droughts usually had an origin in below normal precipitation (or meteorological drought) conditions, which in turn are associated with a reduction in cloudiness and consequently with high irradiation and higher than normal temperatures. However, drought identification based solely on precipitation seems to be inadequate. Numerous studies have shown that a region’s moisture status is constituted by more than precipitation receipts alone (Steila 1987). Van Bavel and Carricker (1987) define an agricultural drought as “a condition in which sufficient soil moisture is not available in the root zone for plant growth and development, in addition to high crop evapotranspiration rate.” Given
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their impact on society, agricultural droughts reported by Garcia-Acosta et al. (2003) must have been characterized by well below normal precipitation and consequently warm or hot conditions that most likely led to crop failure and therefore to an agricultural disaster. According to Florescano and Swan (1995) drought conditions were a recurrent calamity in colonial times in Mexico (and still are in current times). Some of these droughts were linked to the arrival in the highlands of central Mexico of cold polar air masses from North America, accompanied by below freezing temperatures during winter and spring. We use the historical drought data for the southeast part of Mexico from the Garcia-Acosta et al. (2003) catalog for the period 1502–1899. This catalog of agricultural disasters includes events associated with hydrometeorological phenomena or hazards whose effects were mainly felt in the agricultural sector, from which stems its general title “Agricultural Disasters in Mexico: Historical Catalogue.” Frost, hailstorm, and hurricane records come along with information about the water scarcity or water surplus that provoked the drought and flood conditions throughout almost six centuries of Mexican history. The oldest records come from pictographic codexes and annals, some of them written before the Spanish conquest (1521) in preColumbian times, but the main sources were archives, chronicles, and old newspapers as well as iconographical and bibliographical material. The historical droughts reported in the catalog were severe enough to have seriously affected the population by causing crop failure, food and water scarcity, and, in extreme cases, famine and the spread of diseases. With historical data, mainly qualitative, dating the onset and termination of a drought, as well as its spatial extent, constitutes a real challenge for the researcher. Droughts may last days, weeks, months, or even years. It was mostly when the drought was linked to its impact that the phenomenon was actually labeled as such. In agriculture-based economies such as that practiced in Mexico from pre-Columbian times up to the beginning of the twentieth century, the success or failure of the main crops in one year was the indicator of the success or failure of the whole economy. “Bad years,” currently associated with droughts, happened in years that historians have identified as “agricultural crises,” related mainly to maize crop losses and to the increase of its price as well as that of other basic products. The term drought (sequía or seca in Spanish) is not very common in former Mexican archival, bibliographical, or journalistic documents consulted to conform the aforementioned catalog. In pre-Columbian times and up to the
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FIG. 1. Location of states in southeastern Mexico and number of droughts registered during the period 1502–1899 from the “Catalogue of Agricultural Disasters” (Garcia-Acosta et al. 2003).
end of the colonial era (1821), primary as well as secondary sources refer mainly to scarcity, lack, or delay of water and/or rains. The word drought began to be commonly used from the nineteenth century onward to refer to an extended period of time with below normal rainfall. The drought records that appear in the catalog correspond then to long periods of the lack, scarcity, or delay of rainfall reflected as agricultural crisis. It is likely that most of these droughts were anomalous phenomena, that is, more intense and of longer duration, which had a greater impact on the population than did the quasi-year-to-year occurrence of the MSD observed in eastern and southern Mexico (e.g., Mosiño and García 1966; Jáuregui 1979).
4. Method Figure 1 shows the area of study comprising six states: Tabasco, Campeche, Yucatan, Quintana Roo, Chiapas, and Oaxaca. As expected the highest number of reported droughts correspond to the most populated regions since pre-Columbian times: the Yucatan Peninsula with the 48% of the reports and Oaxaca with the 35% of the reports. The original series present 80 droughts reported on an annual basis for the period 1502–1899; Table 1 shows the list together with other meteorological phenomena.
The actual series of droughts that we used in the present paper was constructed as follows: droughts occurring in adjacent years were counted as a single drought and assigned to the first of the consecutive years. Also droughts reported in the same year in different provinces that are part of the area of study were counted as one drought. In this way the original historical record was reduced to 42 droughts during this period. We are supposing that within the area of study the climate is homogeneous and, therefore, that a drought reported anywhere in the area occurred in the whole region. We are aware of the fact that climatic inhomogeneities happened and that our supposition in some cases is inappropriate, introducing an error in the final number of droughts. On the other hand, the fact that the area of study has been continuously occupied and also highly populated along the period of study indicates that the lack of droughts reported is more likely due to the lack of the phenomenon itself rather than to a reduced number of observers. The drought historical data were transformed into a series of the pulse width modulation types (PWM: 1, drought; 0, no drought) (Sergyenko 2003) shown in Fig. 2a as vertical lines. The PWMs are used widely for instance to solve problems concerning the binary systems of radiolocalization, remote sensing, and informa-
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TABLE 1. Historical droughts and some meteorological phenomena associated with them: 1, flood; 2, drought; 3, frost; 4, snow; 5, hail; 6, windstorm; 7, plague; 8, scarcity; 9, famine; 10, epidemic; 11, death; Nueva España and all, all of the Mexico. Year
Location
1535 1551 1556 1563 1564 1564 1571 1575 1576 1586 1595 1607 1619 1648 1651 1661 1725 1739 1765 1770 1778 1780 1783 1790 1791 1797 1800 1805 1809 1809 1810 1817 1822 1834 1834 1834 1836 1837 1837 1842 1849 1854 1854 1856 1858 1863 1864 1868 1868 1868 1877 1881 1882 1882 1882 1884 1884 1885
Yucatán Yucatán Oaxaca Chiapas Yucatán Chiapas Yucatán Yucatán Nueva España Oaxaca Nueva España Oaxaca Oaxaca Yucatán Yucatán Yucatán Yucatán Oaxaca Oaxaca Oaxaca Oaxaca Oaxaca Oaxaca Oaxaca Oaxaca Oaxaca Yucatán Yucatán Oaxaca Yucatán Yucatán Yucatán Yucatán Yucatán All Oaxaca Oaxaca Yucatán Oaxaca Yucatán All Tabasco Oaxaca All Tabasco Oaxaca All Oaxaca Chiapas All All Yucatán Oaxaca Yucatán Campeche Oaxaca All Oaxaca
1
⫻
⫻
⫻ ⫻
⫻
⫻
2 ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻
3
4
5
6
7
8
9
⫻
⫻
⫻ ⫻ ⫻ ⫻
⫻ ⫻ ⫻ ⫻ ⫻
⫻ ⫻ ⫻ ⫻
⫻
⫻ ⫻
⫻ ⫻ ⫻
⫻
⫻
⫻
⫻
⫻ ⫻ ⫻ ⫻
⫻
⫻
⫻
⫻ ⫻
⫻ ⫻ ⫻
⫻ ⫻ ⫻
⫻ ⫻ ⫻
⫻ ⫻
⫻
⫻ ⫻ ⫻
⫻ ⫻ ⫻
10
⫻
⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻
⫻
⫻
⫻ ⫻
⫻
⫻
⫻ ⫻
⫻
⫻ ⫻
⫻
⫻
⫻ ⫻
⫻
⫻ ⫻
⫻
⫻ ⫻ ⫻
⫻
⫻
⫻ ⫻
⫻
⫻ ⫻ ⫻
⫻
11
⫻
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Year 1887 1887 1887 1888 1889 1889 1890 1890 1891 1891 1892 1892 1892 1893 1895 1895 1896 1896 1896 1899 1899 1900
Location Yucatán All Chiapas Chiapas Campeche Yucatán Oaxaca Yucatán All Tabasco Oaxaca Carmen All All Yucatán Chiapas Yucatán Oaxaca Tabasco Chiapas Oaxaca Oaxaca
1
2
⫻ ⫻
⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻
⫻
⫻ ⫻
⫻ ⫻ ⫻
3
4
5
6
7 ⫻ ⫻ ⫻ ⫻
⫻
⫻ ⫻
8 ⫻
11
⫻
⫻ ⫻
⫻
⫻
⫻ ⫻
tion transmission (Bolosyuk 1997). The dashed curve in Fig. 2a represents the energy distribution of droughts in time (Torrence and Compo 1998). Having constructed a PWM drought time series, it
10
⫻
⫻ ⫻
⫻ ⫻
9
⫻ ⫻
⫻
⫻
can be useful to investigate the frequencies, if any, that such series presents. To analyze localized variations of power within a PWM time series at different frequencies, we apply the wavelet method using the Morlet
FIG. 2. (a) Historical drought time series. (b) Morlet wavelet power spectrum. The interval of confidence is marked by the curved black lines that cross the whole figure. (c) Global wavelet spectrum. The uncertainties of the peaks are obtained from the peak full width at half maximum, assuming that the peaks have a Gaussian shape, and the dashed curve indicates the power of the red noise level.
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wavelet (Daubechies 1990; Lau and Weng 1995; Grinsted et al. 2004). The disadvantages of the various methods of spectral analysis, such as the fast Fourier transform or the maximum entropy method, are overcome in the wavelet transform (WT; Astaf’eva 1996). In contrast the WT offers the two-dimensional expansion for a time-dependent signal with the scale and translation parameters that are interpreted physically as the inverse of frequency and time, respectively. As a basis of the WT, we employ the mother wavelet (in our case the Morlet wavelet), which is localized in both frequency and time domain. The WT expansion is carried out in terms of a family of wavelets which is made by dilation and translation of the mother wavelet. The time evolution of the frequency pattern can be followed with an optimal time-frequency resolution. The WT appears to be an ideal tool for analyzing signals of a nonstationary nature. Furthermore, in order to identify frequency bands, time intervals, and the phase with which two time series are covarying, we use the wavelet squared coherence (Torrence and Webster 1999, see their appendix; Grinsted et al. 2004). Only coherences of 0.5 or larger appear in the corresponding figures; however, we shall discuss mainly those frequencies that are at the 95% confidence level. The arrows in the coherence spectra show the phase between the phenomena: arrows at 0° (horizontal right) indicate that both phenomena are in phase and arrows at 180° (horizontal left) indicate that they are in antiphase; arrows at 90° and 270° (vertical up and down, respectively) indicate in general an out-of-phase situation. We would like to point out that the wavelet coherence is especially useful in highlighting the time and frequency intervals where two phenomena have a strong interaction.
5. Results and discussion a. Secular drought occurrence in southeastern Mexico A visual inspection of Fig. 2a indicates that the years of most frequent droughts were around 1650, 1782, and 1884; very few droughts were reported around 1540, and droughts were conspicuously absent between 1630 and 1640, during 1672–1714, and between 1740 and 1760. From 1760 until the end of the period of study, droughts are definitively more frequent than from ⬃1550 to 1760. Analyzing the duration of droughts in Table 2 we found that among the 42 droughts, 74% lasted 1 yr, 24% lasted 2 yr, and only one drought (2%) lasted 6 yr. Historical droughts in the central part of Mexico lasted a few years (mostly 1 yr) (Mendoza et al. 2005).
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TABLE 2. Duration of droughts for the period 1502–1899. The first column indicates the duration of the droughts in yr; columns 2–6 indicate the number of droughts that presented such duration. Duration (yr)
1500–99
1600–99
1700–99
1800–99
Total
1 2 6 Total
6 2 0 8
5 0 0 5
8 1 0 9
12 7 1 20
31 10 1 42
b. El Niño and droughts in southeastern Mexico Table 3 shows the association of droughts with the Quinn and Neal (1992) El Niño compilation. As we suppose that El Niño is causing the droughts, we look at the years where the two coincide; we also consider those droughts that occurred soon after, that is, those droughts that occurred 1 yr after the year of an El Niño. We are allowing 1 yr to pass in these cases because the data are annual and given the uncertainties of the sources, we cannot be sure of when either the drought or the El Niño finished. Table 3 presents El Niño events associated with droughts grouped by El Niño intensities. We notice that the highest association is with high moderate (M⫹) events with 38% of association; the moderate (M⫺) events are only one event and therefore we do not consider it. To evaluate the statistical confidence of the results, we applied the chi-square test (2) to the distribution of droughts 3 yr before and 3 yr after the occurrence of El Niño. In Table 4 for the Quinn and Neal (1992) El Niño list, the 2 test reveals that droughts around very strong (VS), strong (S⫹), and all (T) El Niño events peak 3 yr before the event. Then, regardless of the significance of such peaks, droughts are not linked to these El Niño events. For the strong (S) and moderate (M⫹ and M) El Niño events, there
TABLE 3. El Niño associated with droughts in southeastern Mexico, using the Quinn and Neal (1992) list of El Niños: VS, very strong El Niño events; S⫹ and S, two levels of strong El Niño events; M⫹, M, and M⫺, three levels of moderate El Niño events; T, all intensities of El Niño events.
El Niño intensity
Total No. of El Niños
No. of droughts associated with El Niño
Percentage of association
VS S⫹ S M⫹ M M⫺ T
7 9 28 24 16 1 85
1 2 6 9 5 1 24
14 22 21 38 31 100 28
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TABLE 4. Frequency distribution of droughts around different intensities of El Niño (Quinn and Neal 1992). The boldface numbers correspond to the peaks of the distributions: VS, very strong El Niño events; S⫹ and S, two levels of strong El Niño events; M⫹, M, and M⫺, three levels of moderate El Niño events; T, all intensities of El Niño events; and 2, chi-squared test. El Niño intensity
⫺3
⫺2
⫺1
0
1
2
3
2
VS S⫹ S M⫹ M M⫺ T
2 3 3 5 4 0 17
0 0 2 4 1 0 7
1 1 2 2 4 0 10
1 1 4 5 2 0 13
0 1 2 4 3 1 11
0 0 1 3 4 0 8
1 2 4 2 2 0 11
⬎0.50 ⬎0.25 — — — ⬎0.25 ⬎0.25
are bimodal or even trimodal distributions (the 2 test was not applicable), further indicating that there is not a clear association between droughts and El Niño. Table 5 corresponds to the Ortlieb compilation of El Niño. Table 5 indicates that the highest correlation is between droughts and strong (S) and moderate (M) El Niño events, with 27% associated. The 2 test applied to the distribution of droughts around El Niño in Table 6, indicates that droughts around VS, M, and T El Niños peak 3 yr before the event. Again, regardless of the significance of such peaks, droughts are not linked to these El Niño events. Even more so, for the strong (S) El Niño events, there is a bimodal distribution peaking 3 and 1 yr before the events. Historically, droughts have been linked to El Niño short-duration events (1–3 yr) (Jáuregui 1995). In a study of the North American monsoon that affects mainly northwestern Mexico and southwestern United States, it is noted that relatively dry monsoons were more frequent during the positive phase of the North Pacific decadal oscillation (PDO). This is consistent with the fact that El Niño events, which tend to favor relatively dry monsoons in Mexico, occur more frequently during the positive phase of the PDO.
TABLE 5. El Niño associated with droughts in southeastern Mexico, using the Ortlieb (2000) list of El Niño: VS, very strong El Niño events; S, strong El Niño events; M⫺, moderate El Niño events; T, all intensities of El Niño events.
El Niño intensity
Total No. of El Niños
No. of droughts associated with El Niño
Percentage of association
VS S M⫺ T
5 11 22 38
1 3 6 10
20 27 27 26
TABLE 6. Frequency distribution of droughts around different intensities of El Niño (Ortlieb 2000). The boldface numbers correspond to the peaks of the distributions: VS, very strong El Niño events; S, strong El Niño events; M, moderate El Niño events; T, all intensities of El Niño events; and 2, chi-squared test. El Niño intensity
⫺3
⫺2
⫺1
0
1
2
3
2
VS S M T
2 3 5 10
0 1 2 3
0 3 3 6
1 2 3 6
0 1 3 4
0 0 4 4
1 1 2 4
⬎0.25 — ⬎0.90 ⬎0.25
c. The main periodicities of the drought series The global wavelet spectrum is shown in Fig. 2c, in which eight peaks are evident. The lowest-frequency peak close to 112 yr is not considered because the longitude of the drought time series allows only for very few realizations of this cycle. The other frequencies are around 2.7 ⫾ 0.3 yr, 4 ⫾ 0.5 yr, 6.7 ⫾ 0.5 yr 12 ⫾ 1 yr, 20 ⫾ 2 yr, 42.6 ⫾ 4 yr, and 69.3 ⫾ 8.5. The power is mainly distributed in the frequency band of ⱖ8 yr (see Figs. 2b and 2c). There are several natural sources of regional climate modulation with time scales compatible with those of droughts. The short-term El Niño events have as main frequencies 3–4 and ⬃7 yr (Enfield and Cid 1991). The historical drought frequencies of 2.7, 4, and 6.7 yr are compatible with these cycles. The long-term El Niño occurrence presents cycles of ⬃40, 50, and 90 yr, in a composite record of El Niño–Southern Oscillation since A.D. 622 (Anderson 1992). Also the drought peaks at 42.6 ⫾ 4 yr and 69.3 ⫾ 8.5 yr could be associated with these El Niño cycles. Other large-scale variations are the changing conditions in the North Pacific atmosphere and ocean (e.g., Biondi et al. 2001) that present decadal and bidecadal modes, or the Atlantic multidecadal oscillation with primary modes in the band of ⬃40–128 yr (Gray et al. 2004). These ocean frequencies are also compatible with those of the historical droughts. Hodell et al. (2001) studied lake-sediment cores from the Yucatan Peninsula over the past 2600 yr, with spectral analysis of sediment proxies and found conspicuous cycles of ⬃50 ⫾ 10 yr and ⬃39 ⫾ 6 yr. Considering the uncertainties, the first cycle coincides with the 69.3 ⫾ 8.5 yr drought peak and the second one with the 42.6 ⫾ 4 yr peak found here. Other natural sources of climate modulation are discussed in section 5g.
d. Comparison of historical droughts in southeastern Mexico and droughts in other areas Tree-ring chronologies have been widely used to study the long-term history of drought and precipita-
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tion. A study of tree-ring data over North America, reaching as far south as Durango (northwestern Mexico) (Stahle et al. 2000) shows that the sixteenthcentury drought was the most severe and sustained from 1500 to the present in Missouri, Colorado, New Mexico, and Durango. In particular in Durango droughts are evident from 1540 to 1580, and in the 1860s and 1950s (Stahle et al. 2000; Therrell et al. 2002). While no droughts were reported in the Garcia-Acosta et al. (2003) drought series around 1540, for southeastern Mexico, a drought period is evident between 1550 and 1620; also around 1860 there is a period of frequent droughts (see Fig. 2a). This would suggest that synoptic circulation mechanisms leading to drought development are likely to be out of phase for both regions; this was also noticed by Therrell et al. (2002) when comparing northern and southern regions (as far as Oaxaca and excluding the Yucatan Peninsula) in Mexico. Using the Garcia-Acosta et al. (2003) catalog Mendoza et al. (2005) constructed a drought time series for central Mexico. Figure 3a shows that in general droughts in Oaxaca, the Yucatan Peninsula, and all of southeastern Mexico do not coincide with droughts in central Mexico. As it is not easy to identify a pattern of behavior by simple inspection of Fig. 3a, we constructed the coherence spectrum of the central and southeastern drought time series. Figure 3b shows that the strongest coherences occur for frequencies of ⬃7, 16, and 64 yr. However, only for frequencies ⱕ7 yr do we have coincidences in phase (0°) at ⬃1550, 1600, 1660, 1770, and 1850–1870; for the other two frequencies we either have droughts in one region but not in the other (antiphase, 180°) or they are out of phase. A recent study of tree-ring chronologies across Mexico from 1780 to 1992 has been carried out reaching as far south as Oaxaca and excluding the Yucatan Peninsula (Therrell et al. 2002). For southern Mexico the study shows severe growth reductions in the 1830s and 1930s. The droughts of the 1830s reported by Therrell et al. (2002) coincide with the historical droughts in Fig. 2a around 1830. Below we shall consider in some detail the two areas of the region of study with higher drought reports: Oaxaca and the Yucatan Peninsula.
e. Oaxaca Considering only the Oaxaca state, the original series present 41 droughts (see Table 1) but the actual series, constructed as described in section 3, has 29 droughts. Five droughts lasted 2 yr, 1 drought lasted 4 yr, and the remaining 23 lasted 1 yr. According to the coherence spectra of Fig. 4, the Oaxaca droughts occurred between 1556 and 1619, then not until 1739, between 1765
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and 1810, around 1830, and between 1850 and 1900. Between 1619 and 1759 no droughts were reported. Figures 4a–c also compare the droughts with indices such as the Palmer drought severity index (PDSI) reconstructed from tree-ring chronologies for Oaxaca (Cook et al. 1999), the PDO (Biondi, et al. 2001), and the Southern Oscillation index (SOI; Jones et al. 2001), respectively. Table 7 presents a summary of results. From Fig. 4a (top panel) it is very clear that around 1700 the tree-ring reconstruction indicates an episode of severe drought that is not present in the historical data. After 1739 and up to 1765 and after 1810 and up to 1830 the PDSI indicate periods of few or no droughts coinciding with the historical reports. If we attend to the coherence spectrum between droughts and the PDSI (Fig. 4a, bottom panel), it is easier to see the coincidence of droughts and positive (in phase) or negative (antiphase) values of the PDSI for brief lapse times. A sustained coherence between droughts and PDSI appears for periods ⬃2 yr, as the total power spectrum at the right in Fig. 4a shows a coincidence with negative PDSI is evident after 1750. Also, high coherence appears for periods ⬃11 and 24 yr, and at ⬃64 yr we have the strongest coherence between 1650 and 1800; in general an out-of-phase behavior is evident, except for the frequency ⬃24 yr where a coincidence of droughts and negative PDSI is seen between 1700 and 1800. Figure 4b shows the behavior of droughts and the PDO. Droughts tend to coincide with the cold phase of the PDO (antiphase according to the arrows in the figure) between 1760 and 1810 at frequencies of ⬃7 yr and between 1740 and 1810 for frequencies ⬃14 yr. However, for the highest power at frequencies ⬃24 yr, the coincidence is with the warm phase of the PDO from 1800 until the end of the period of study and they are out of phase from 1700 to 1760. In Fig. 4c for droughts and the SOI we notice the strongest coherence at frequencies ⬃24 yr from 1830 until the end of the period of study presenting a coincidence between the negative mode of the SOI and droughts (antiphase).
f. The Yucatan Peninsula Here, we consider only the Yucatan Peninsula (the states of Campeche, Yucatan, and Quintana Roo in Fig. 1). The original series present 41 droughts (see Table 1) but the actual series, constructed as described in section 3, has 28 droughts. Only 4 droughts lasted 2 yr and one drought lasted 5 yr; the remaining 23 lasted 1 yr. According to Fig. 5a (top panel), the Yucatan droughts occurred between 1535 and 1576, then not again until 1595, then between 1640 and 1661, around 1725, and
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FIG. 3. (a) Drought time series in Mexico: 1) central region (Mendoza et al. 2005), 2) The Yucatan Peninsula, 3) Oaxaca, and 4) the southeastern region. (b) (top) The central and southeastern drought time series. (b) (bottom) Their coherence spectra. The direction of the arrows in the coherence spectrum indicates the phase between the two phenomena involved: horizontal right is 0° and corresponds to an in-phase situation, horizontal left is 180° and corresponds to an antiphase situation; both vertical up (90°) and vertical down (270°) correspond to an out-ofphase situation. The arrows are plotted every 12 yr. The top panel of the coherence spectrum shows both series of data.
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FIG. 4. Coherence spectrum between droughts in Oaxaca and (a) tree-ring chronologies, (b) PDO, and (c) SOI. The key for the arrows is the same as in Fig. 3. The top panel in each figure shows both time series of data.
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TABLE 7. Summary of coherence spectra results for Oaxaca. The boldface text corresponds to the frequencies with the maximum power. Frequency (yr) 2 7 11 14 24 28 32 64
PDSI
PDO
SOI
TSI
10
Be
Out of phase after ⬃1750 Antiphase 1760–1810 Out of phase 1700–1850 Antiphase 1700–1800
Antiphase 1740–1810 In phase 1800, out of phase 1700–60
Antiphase 1830 onward Out of phase 1760–1850
Out of phase
between 1800 and 1900. Therefore, time lapses with no droughts reported were after 1576 and 1595, after 1595 and 1640, after 1640 and 1661, after 1661 and 1725, and after 1725 and 1800. Figures 5a–c also compare the historical droughts with droughts inferred from multiple sediment cores in a Yucatan lake (SC) (Hodell et al. 2004), the AMO (Gray et al. 2004), and the SOI (Jones et al. 2001). Table 8 shows a summary of results. Figure 5a shows the coherence between droughts and lake sediment cores (Hodell et al. 2004). We are using the results of gypsum concentration. Strong coherence is evident for frequencies of ⬃28, 48, and 64 yr. Coincidence at 0° is strong between 1600 and 1700 at a frequency of 64 yr. Droughts occurred from 1800 to 1870, in this time lapse coherence at roughly 0° (increases of gypsum concentration) and an out-of-phase condition is observed at the frequency band from ⬃28 to 48 yr. From Fig. 5b it is very clear that ⬃32 yr droughts and the cold phase of the AMO nearly coincide throughout all of the time of study. Less strong coincidence is manifested for droughts and the warm AMO phase around 1600, at the frequency centered ⬃6 yr, and around 1730 at a frequency of ⬃14 yr As the region of study is narrow (longitudinally speaking), the effect of the SOI could be felt even in the Yucatan Peninsula, therefore a comparison with the SOI is adequate. In Fig. 5c for droughts and the SOI we notice that the strongest power is for the frequency of ⬎64 yr with an out-of-phase coherence, but the confidence lapse time is shorter than this frequency and, therefore, we do not discuss it. The second strongest peak is along the time lapse between 1785 until the end of the period of study for a frequency ⬃28 yr, presenting an out-of-phase coincidence between the SOI and droughts. At the frequency ⬃14 yr, between 1700 and 1740 and ⬃1890, there is a tendency for antiphase between droughts and
Antiphase 1650–1750 In phase all period
Out of phase 1650–1790
SOI and also around 1800–1830 for the frequency of ⬃6 yr.
g. Droughts and solar activity There are also natural forcings external to the climate system with time constants compatible with the drought frequencies found in the present work. The historical droughts quasi-decadal peak around 12 ⫾ 1 yr can be associated with the well-known sunspot cycle. The droughts quasi-bidecadal frequency at 20 ⫾ 2 yr could be associated with the 22-yr solar magnetic and geomagnetic cycles. The drought peaks at 69.3 ⫾ 8.5 yr can be related to the Gleissberg cycle of ⬃80 yr observed in 1000-yr records of solar and auroral activity (Feynman and Fougere 1984) and also reported in 9700 yr of C14 records from tree rings (Stuiver and Braziunas 1993). Furthermore, the drought quasi-bidecadal peak may also be related to the 18.6-yr lunar Saros cycle (van den Bergh 1955). Mitchell et al. (1979) found evidence of a 22-yr rhythm of droughts in the western United States since seventeen century. Later Currie (1987, 1993) showed that the 22-yr drought cycle was actually 18.6 yr in length and is produced by the lunar Saros cycle. Bell (1982) suggested that the real drought period was 20.5 yr and was caused by a beat between the 22-yr magnetic solar cycle and the 18.6-yr lunar nodal tidal cycle. Cook et al. (1997) also found a persistent bidecadal drought rhythm in the western United States since 1700, the authors also showed the possibility that solar and lunar effects could be interacting to modulate this rhythm as Bell (1982) suggested. The final answer awaits further study. Although it is premature to propose an association between El Niño and solar activity there is evidence of this relation: it has been found that El Niño events are more common when solar activity is weak (Anderson
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FIG. 5. Coherence spectrum between droughts in the Yucatan Peninsula and (a) sediment lake cores, (b) AMO, and (c) SOI. The key for the arrows is the same as in Fig. 3. The top panel in each figure shows both time series of data.
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TABLE 8. Summary of coherence spectra results for the Yucatan Peninsula. The boldface text corresponds to the frequencies with the maximum power. Frequency (yr) 6 10 14 28
SC
64
SOI
In phase ⬃1600
Antiphase 1800–30
In phase ⬃1730
Antiphase 1700–40, ⬃1890 Out of phase 1785 onward
TSI
10
Be
In phase all period Out of phase 1800–70
32 48
AMO
Antiphase all period Out of phase 1800–70 In phase 1600–1700
1990; Mendoza et al. 1991). Stratifying the data according to event strength, Enfield and Cid (1991) found that the return intervals for strong El Niño events are longer during epochs of high solar activity than during low solar variability; a similar result holds for all intensities but with a lower significance. We would like to point out that these studies have been carried out using the Quinn and Neal (1992) compilation of El Niño data, and therefore if the compilation is either improved or changed, so are the periods found. Therefore, if the association between El Niño and solar variability exists, it would not be surprising to find the solar signal in phenomena that are themselves well related to El Niño, like the droughts presented here. Hodell et al. (2001) studied lake sediment cores from the Yucatan Peninsula over the past 2600 yr, with spectral analysis of sediment proxies and found conspicuous cycles of ⬃50 ⫾ 10 yr and ⬃39 ⫾ 6 yr. They indicate that the peak at 50 yr may be a harmonic of the peak at 208 yr, which they have as the most prominent one in their series; this peak is prevalent in cosmogenic nucleid production of carbon 14 isotope (C14) and beryllium 10 isotope (10Be), which are produced by the cosmic rays modulated by solar activity (Beer 2000). In Fig. 6a the coherence spectrum between droughts in southeastern Mexico and the total solar irradiance (TSI; Lean 2000), a proxy of solar variability, is plotted. Coherence is sustained throughout all of the period for the frequency ⬃32 yr, presenting a tendency for an antiphase coincidence and for the band frequency band of ⬃64 yr, showing a tendency for an in phase coincidence. In addition to this, conspicuous antiphase coherence appears in the short lapse time (1630–1665) at a frequency of ⬃20 yr. Cosmic rays are highly energetic charged particles from outside the solar system. Their fluxes present a very well known overall anticorrelation with the sun-
Antiphase 1600–1750; in phase 1810–1860
Antiphase 1700–1800
spot number and are considered to be another proxy of solar activity. Figure 6b shows the coherence between 10 Be, a proxy of cosmic rays (Beer 2000), and droughts. Strong in-phase coherence at the frequency band of ⬃6–16 yr is observed along all the period of study. Sustained coherence is also observed out of phase for the band of ⬃64 yr. We notice that irradiance and 10Be are both strong for ⬃64 yr and tend to be opposite, which is consistent with the fact that during increased solar irradiance there is increased solar activity and decreased cosmic ray flux and therefore lower 10Be deposition. The relation of droughts in Oaxaca with solar activity is analyzed in Fig. 7. Figure 7a shows the coherence between droughts and total solar irradiance is strong along all the period of study for a frequency of ⬃64 yr, and the phase indicates that droughts tend to occur for increased solar irradiance. Also for 32 yr between ⬃1650 and 1750 a high coherence is observed to be roughly in antiphase. Figure 7b shows coherence for droughts and 10Be, with brief strong time lapses of outof-phase coherence between 1760 and 1850 for the frequency of ⬃28 yr; however, for the frequency ⬃64 yr between 1650 and 1790, although the coherence is not as strong as with irradiance, an out-of-phase situation is noticed. Again at ⬃64 yr, we have increased solar irradiance, increased solar activity, and decreased cosmic ray flux, thus lowering the 10Be deposition. The relation of droughts in the Yucatan Peninsula with solar activity is analyzed in Fig. 8. Figure 8a shows that the strongest coherence between droughts and TSI is in antiphase at frequencies ⬃64 yr for the time lapse of 1700–1800; this coherence is weaker than the coherence for droughts and 10Be (see Fig. 8b). Figure 8b shows strong coherence ⬃10 yr in phase over all the period of study with increases of cosmic rays (i.e., decreases of solar activity and therefore of irradiance).
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FIG. 6. (a) Coherence spectrum for southeastern droughts and total solar irradiance. (b) Coherence spectrum for southeastern droughts and 10Be. The key for the arrows is the same as in Fig. 3. The top panel in each figure shows both time series of data.
Centered at a frequency of ⬃32 yr there is an antiphase coherence from 1600 to 1750 and in-phase coherence from 1810 to 1860. Then it is clear that in Fig. 6, which corresponds to the whole area of study, it is the Yucatan Peninsula that is contributing with the 10Be in-phase coherence between droughts and solar activity (Fig. 6b) at a frequency ⬃10 yr, and it is Oaxaca that is contributing in
Fig. 6a to the coherence with solar irradiance at a frequency ⬃64 yr with a tendency for an in-phase coincidence.
6. Concluding remarks We use a catalog containing an unprecedented amount of historical data in southeastern Mexico covering almost four centuries of Mexican history (1502–
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FIG. 7. (a) Coherence spectrum for Oaxaca droughts and total solar irradiance. (b) Coherence spectrum for droughts and 10Be. The key for the arrows is the same as in Fig. 3. The top panel in each figure shows both time series of data.
1899). This is a catalog of agricultural disasters and includes events associated with hydrometeorological phenomena or hazards whose effects were mainly felt in the agricultural sector. The catalog was elaborated upon using several sources. The oldest records come from pictographic codexes and annals, some of them written before the conquest (1521), but the main sources were archives, chronicles, and old newspapers
as well as iconographical and bibliographical material. The historical droughts reported in the catalog must have been characterized by well below normal precipitation and consequently warm or hot conditions that most likely led to crop failure and therefore to an agricultural disaster. For the southeastern part of Mexico, the original series presented 80 droughts reported on an annual basis
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FIG. 8. (a) Coherence spectrum for the Yucatan Peninsula droughts and total solar irradiance. (b) Coherence spectrum for droughts and 10Be. The key for the arrows is the same as in Fig. 3. The top panel in each figure shows both time series of data.
for the period 1502–1899. Assuming that the same droughts was reported in several parts of the area of study, and that one drought could last several years, this series was reduced to 42 droughts. Then the historical data were transformed into numerical data. We identified periods of frequent droughts around the years 1650, 1782, and 1884; few droughts were reported around 1540, and few or no droughts were ob-
served between 1630 and 1640, along the long time interval of 1672–1714 and between 1750 and 1760. From 1760 until the end of the period of study droughts are definitively more frequent than between ⬃1550 and 1760. The duration of most droughts is between 1 and 2 yr; moreover, only one longer-duration drought occurred during these centuries.
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The spectral analysis of the drought time series showed the most conspicuous peaks at ⬃3–4 and 7 yr; less important frequencies appear at ⬃12, 20, 43, and 70 yr. These periodicities are compatible with oceanic temperature oscillations, short and longer El Niño events, and solar or solar-related activity phenomena. At most 38% of droughts are related to El Niño events. Since the precipitation regime in this region is mostly determined by tropical disturbances, for example, easterly waves, tropical storms, and hurricanes, during the rainy season (Jáuregui 1995), it is possible that the impact of variability of these weather systems on the rainfall regime of southeastern Mexico may be generally less influenced by the El Niño phenomenon than in, for instance, the center of the country. Other natural sources are also modulating droughts. For Oaxaca the warm PDO and the negative SOI dominate at 24-yr periodicities from 1830 to the end of the period of study. As mentioned in the introduction, a warm phase of the PDO and a negative SOI phase are related with El Niño events, which in turn are linked to droughts in Oaxaca. For the Yucatan Peninsula the cold phase of the AMO and an out-of-phase SOI dominate at frequencies ⬃32 yr, modulating droughts from 1785 on. For the southeastern part of Mexico droughts at ⬃64 yr have the highest coherence with increased TSI and decreased 10Be. For the frequency band of ⬃28 yr, droughts and 10Be share a signal in a stronger way than 10 Be and solar irradiance do over time. In particular, droughts in Oaxaca coincide with increased TSI and decreased 10Be at ⬃64 yr, with the TSI signal being stronger than the 10Be signal. Droughts in the Yucatan Peninsula coincide with low TSI at ⬃64 yr, and coincide with 10Be at ⬃32 yr but the phase changes with time and dominates over irradiance. The above results show that the dominance of some phenomena over others depends on the time scales considered. Sea surface temperatures (PDO, AMO), SOI, and solar activity leave a clearer signal in Oaxaca than in the Yucatan Peninsula. Acknowledgments. We thank Mario Casasola for the reorganization of the Catalogue of Agricultural Disasters for Mexico according to the demands of the present study, to Alfonso Estrada for the drawing of Fig. 1, and to Chistopher, Gilbert P. Compto, Aslak Grinsted, John Moore, and Svetlana Jevrejeva for software. This work has been partially supported by DGAPA-UNAM Grants IN104203-3 and IN 116705 and CONACYT Grants 40601-F and 44201-T.
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