NORTH ATLANTIC OSCILLATION CONTROL OF DROUGHTS IN NORTH-EAST
SPAIN: EVALUATION SINCE 1600 A.D.
SERGIO M. VICENTE-SERRANO1 and JOSÉ M. CUADRAT2
1
Instituto Pirenaico de Ecología, CSIC (Spanish Research Council), Campus de Aula Dei,
P.O. Box 202, Zaragoza 50080, Spain
e-mail: svicen@ipe.csic.es
2
Departamento de Geografía. Universidad de Zaragoza. Zaragoza. Spain.
Abstract. This paper analyses the role played by the North Atlantic Oscillation (NAO) in the
creation of drought conditions in a semi-arid region of North-east Spain (the middle Ebro
valley), from 1600 to the year 2000. The study used documents from ecclesiastical archives
for the seventeenth, eighteenth and nineteenth centuries. For the twentieth century,
instrumental precipitation records were used as well. A December-August drought index from
1600 to 1900 was compiled from the historical documentary sources (rogation ceremonies).
The index was validated by means of precipitation records between 1858 and 1900 and
independent precipitation data from 1600 reconstructed by means of dendrochronological
records. Using instrumental data a drought index was also calculated (Standardized
Precipitation Index, SPI) for the 1958 – 2000 period. We found that the NAO was important
in explaining the droughts identified in the study area from documents and instrumental data.
Positive values of the winter NAO index are prone to cause droughts in the middle Ebro
valley. This finding has been verified since 1600 by means of two independent reconstructions
of the winter NAO index. The same behaviour has been observed during the nineteenth and
twentieth centuries by means of instrumental records. The climatic and geographic factors that
explain the high influence of North Atlantic Oscillation on droughts in this region are
discussed in depth.
1. Introduction
Drought is a complex phenomenon which involves both human and natural factors. Although
there are different drought types: agricultural, hydrological and socioeconomic (Wilhite and
Glantz, 1985), it is usually understood to be a long and sustained period in which water
becomes scarce and the scarcity causes negative impacts on the society and/or environment
(Dracup et al., 1980; Redmond, 2002). Thus, drought can be considered to be essentially a
climatic phenomenon related to an abnormal decrease in precipitation (Oladipo, 1985; McKee
et al., 1993).
Drought is one of the main climatic hazards affecting Mediterranean regions. In Spain, it is a
frequent phenomenon due to the high spatial and temporal variability of precipitation. Several
studies have identified dry periods that have affected Spain in different centuries (Martín-Vide
and Barriendos, 1995; Rodrigo et al., 1999; Saz, 2003). Droughts were also frequently
recorded in Spain during the period when instruments were used (Pérez-Cueva, 1983; Pita,
1989; Vicente-Serrano, 2006).
The main characteristics of drought in Spain are: the spatial differences and the nonsynchrony of droughts between different regions (Martín-Vide, 2001; Vicente-Serrano,
2006b), and the severity and duration of the episodes (Pérez-Cueva and Escrivá, 1982). The
recent drought period between 1991 and 1995, which was the most intense of the twentieth
century in Spain, is an excellent example of these characteristics (Almarza et al., 1999).
Droughts in Spain are determined by atmospheric circulation variability in the North Atlantic
region (Rodriguez-Puebla et al., 1998; Trigo and Palutikof, 2001; Trigo et al., 2004; Olcina,
2001; Zorita et al., 1992; Xoplaki et al., 2004; Barriendos and Llasat, 2003; Martin-Vide and
Fernández, 2001). Atmospheric circulation in the Northern Hemisphere can be characterised
in terms of its patterns. Barnston and Livezey (1987) identified nine patterns in the Northern
hemisphere during the boreal winter and three patterns in the other seasons. One pattern,
called the North Atlantic Oscillation (NAO), occurs in all seasons though it is more intense in
winter (Rogers, 1984; Hurrell, 1995; Trigo and Palutikof, 2001).
The NAO explains much of the climatic variability in the North Atlantic areas, including the
direction and intensity of the westerlies, the trajectories of polar depressions and the position
of anticyclones (Lamb and Peppler, 1987; Hurrell et al., 2003). Changes of atmospheric
circulation in the North Atlantic are related to changes in the amount, distribution and
intensity of precipitation events (Hurrell, 1995; Hurrell et al., 2003). Positive NAO winters
usually show dry conditions in the Western Mediterranean areas (Hurrell and Van Loon,
1997; García Herrera et al., 2001; Xoplaki et al., 2004; Moses et al., 1987).
In Spain, it is well known that the NAO affects precipitation, but with important spatial
differences (Trigo et al., 2004). The effect of the NAO on precipitation is stronger in winter
but it is also detectable in other seasons such as spring and autumn (Martín-Vide and
Fernández, 2001). The NAO mainly affects precipitation in the South-Atlantic basins
(Rodríguez-Puebla et al., 1998; Martín-Vide and Fernández, 2001). In other areas of Spain
precipitation is determined by other atmospheric patterns, such as the ENSO (Rodó et al.,
1997; Vicente-Serrano, 2005), the Scandinavian (Rodríguez-Puebla et al., 1998) or the Polar
pattern (González-Hidalgo et al., 2003; Xoplaki et al., 2004). Nevertheless, the NAO not only
affects precipitation over large areas of central and South-western Spain; in the North-east as
well there is a small region in which precipitation is highly affected by it (Martín-Vide and
Fernández, 2001; Vicente-Serrano and López-Moreno, 2006). This area has semi-arid climatic
characteristics (Cuadrat, 1999), in which the temporal variability of droughts is very important
(Vicente-Serrano and Cuadrat, 2006). Moreover, droughts are very severe (it is common to
have more than 100 consecutive days with zero precipitation, Vicente-Serrano and Beguería,
2003). Droughts have an important effect on the economy and environment of the region
because drought variability determines crop production (Austin et al., 1998; Vicente-Serrano
et al., 2006) and the development of natural vegetation (Vicente-Serrano, 2004; Creus and
Saz, 2004).
The objectives of this paper are to find out how drought evolves in this semi-arid region, and
to analyse in depth the impact of the NAO on droughts. The purpose is to determine whether
the NAO influence on droughts can be recognised over wide periods. The NAO’s effect on
droughts from 1600 to 2000 was analysed by means of instrumental records and proxy data
from historical documentary sources.
The paper is structured as follows: Chapter 2 explains the characteristics of precipitation in
the middle Ebro valley. Chapter 3 describes the data and the methods used. In chapter 4 the
main results of this research are discussed. This section has been divided into two subsections: the first explains the temporal variability of droughts in the middle Ebro valley since
the Seventeenth century and the second shows the relationships between the NAO and
drought variability. Finally, section 5 provides the main conclusions of the paper.
2. The climate of the region
The principal city of the Ebro valley is Zaragoza (600,000 inhabitants) (Figure 1). This city is
in the centre of one of the most arid regions of Europe. Annual precipitation is 343.3 mm,
although the interannual variability is very high (standard deviation = 86 mm). Using
instrumental records, the minimum annual precipitation since 1858 was recorded in 1912
(182.6 mm) whilst the maximum was in 1959 (752 mm). The highest evapotranspiration rates
are recorded in the centre of the valley (> 1200 mm/year). In the whole of the region there is a
negative water balance (precipitation – evapotranspiration), which is particularly significant in
the central areas (> 900 mm).
The climate is Mediterranean with important continental characteristics (Köppen, 1936). It is
determined by various geographic factors, as several mountainous chains, which isolate the
valley from moist winds, border the Ebro valley. So in the central areas of the valley
precipitation is low (Cuadrat, 1999; Creus and Ferraz, 1995), with little differences between
months, although the dry seasons are the summer and the winter (mean winter precipitation
between 1901 and 2000 = 63.8 mm (standard deviation = 34.3), mean spring precipitation =
96.6 mm (49.0), mean summer precipitation = 65.4 mm (38.4) and mean autumn precipitation
= 93.6 mm (49.4)).
3. Data bases and methods
3.1. CLIMATIC RECORDS FROM HISTORICAL DOCUMENTARY SOURCES
Ecclesiastical documents from the sixteenth century, which have a high degree of detail and
no gaps in information, are conserved in Zaragoza (see appendix). The conservation of the
documents during this long period (four centuries) is due to the duplication of the
ecclesiastical archive because of the existence of two ecclesiastical sites: “La Seo” and “El
Pilar”. In Zaragoza there are two cathedrals as a consequence of the confrontation between the
chapters of the basilicas of El Pilar and la Seo de El Salvador. The dispute was resolved in the
17th century by a papal decision, which granted the title of cathedral to both basilicas, and the
two chapters were joined together, but for several years two ecclesiastical archives were
maintained, which were also finally combined in 1776. Each site retained one general register,
in which each document of the other site was duplicated to avoid internal conflicts in the
ecclesiastical community. This has conserved the general archive without gaps from 1557 to
the present.
Numerous documents giving information about meteorological characteristics appear in these
historical documentary sources. The records refer to rogation ceremonies taking place due to
environmental factors. The rogations “pro pluvia” and “pro serenitate” have frequently been
used in Catholic countries as “proxy” climatic data (Martin-Vide and Barriendos, 1995;
Barriendos, 1996-1997; Barriendos, 1997; Barriendos and Martín-Vide, 1998; Piervitali and
Colacino, 2001). Figure 2 gives an example from the ecclesiastical documentary sources
conserved in Zaragoza.
Rogations are a part of Catholic liturgy. They are solemn petitions by believers to ask God
specific requests (Barriendos, 1996-1997). The “pro-pluvia” rogations were made to ask for
precipitation during a drought and, therefore, they provide an indication of episodes of
drought. On the other hand, the “pro-serenitate” rogations asked for the end of the
precipitation during periods of high and/or intense precipitation, which caused floods and
damages. In the Mediterranean region the most frequent economic problem, the loss of crops,
was related to insufficient rainfall. For this reason, in a period of important religious fervour,
the rogations “pro pluvia” were the most frequent human response to climatic anomalies, and
fortunately these are well recorded in documentary sources. In this paper we focussed on the
“pro-pluvia” rogation records, which were a social response to the drought events.
The credibility of the rogations, as a manifestation of a meteorological event, is supported by
the participation of several institutions (agricultural organisations, municipal and
ecclesiastical authorities) that analysed the situation and deliberated before deciding to hold a
rogation ceremony. Agricultural organisations requested the rogations when a drop in rainfall
impeded crop development. Municipal authorities recognised the problem and discussed the
advisability of holding a rogation ceremony. The order was communicated to the religious
authorities, who placed the rogation on the calendar of religious celebrations and finally
organised and announced the rogation.
The rogation ceremonies might vary between localities (Barriendos, 1996-1997), but in all
cases had the advantage of the formality characteristic of the Catholic Church’s ceremonies. In
Zaragoza five rogation levels, depending on the seriousness of the meteorological event, are
recorded (Table 1). When the level increases, more complex ecclesiastical ceremonies were
performed and greater drought severity can be inferred. In Zaragoza, the first level of
rogations corresponded to a single petition for precipitation during the masses of different
churches. In the second level there was a direct petition for precipitation to a saint of the
church. If the precipitation did not occur the rogations gained in complexity. The third level
was characterised by different masses and processions within the church and a public
procession in the neighbouring streets with a holy image or the relics of a saint. The fourth
level consisted of litanies, masses and a public procession that usually included the churches
of El Portillo, Santa Engracia and the Hospital de Nuestra Señora de Gracia. The most
frequent relic used in the procession was the head of San Valero, patron saint of Zaragoza,
which was sometimes put into the water to ask for precipitation. Finally, the fifth level
consisted of masses in the cathedrals and a public procession with the Christ figure of La Seo
de San Salvador, which was taken from this cathedral to the cathedral of El Pilar, where the
image was left for some days in front of the image of the Virgin of El Pilar. At the same time,
several litanies and masses were celebrated in other churches.
To carry out this study, we consulted the historical documentary sources in the cathedral
archive of Zaragoza, which records rogation ceremonies from 1557 on. 98 books were
reviewed. Finally, we decided to work with the records obtained from the beginning of the
seventeenth century, as prior to 1588 rogation records are scarce.
The last rogation ceremony was recorded in 1945, but the reliability of twentieth-century
records is low. Only 14 rogations between 1900 and 1945 were recorded, although important
droughts were identified with instrumental data during this period (Vicente-Serrano, 2005b).
The decline in the social and political influence of the Catholic Church during the first
decades of the twentieth century, along with the instrumental records that are available, makes
it inadvisable to use rogation records for drought quantification during this period. Rogations
were used for drought analysis until 1900, but instrumental records from 1858 on were also
used.
3.2. CALCULATION OF DROUGHT INDEX FROM ROGATION RECORDS
Quantitative continuous monthly series from 1600 to 1900 were created from the rogation
records. We used the number of level 1, 2 and 3 rogations as a preliminary measurement of
drought occurrence, and we selected the rogations from December to August for further
analysis.
Different studies have shown that winter precipitation is the most important for the final crop
productions in the dry-farming areas of the middle Ebro valley (wheat and barley) (Austin et
al., 1998; McAneney and Arrúe, 1993) because water is stored in the soil as a consequence of
the low evapotranspiration rates during the winter (McAneney and Arrúe, 1993; Austin et al.,
1999). Therefore, it is reasonable to assume that the winter droughts would not just have
consequences on the total number and the level of the rogations recorded in winter (December
to March). The socio-economic effects would be more evident during the period of vegetation
growth (March-May) and also during the harvesting period (June-July). For this reason, the
rogations recorded between December and August were used to calculate an annual drought
index and to relate it to the winter NAO.
We created a continuous drought index (DI) by grouping the rogations at various levels. We
followed a simple approach, similar to that of Martín-Vide and Barriendos (1995). We did not
use level 4 and 5 rogations because they are very scarce. They are concentrated in specific
periods (consecutive years) and usually occur in years in which there were no lower-level
rogations (79% of level 5 rogations were recorded in years with no level 1 rogations – 33% or with only one level 1 rogation – 46%). This arouses suspicions as to the reliability of level
4 and 5 rogations for quantifying droughts, because they may have been held because of other
social, economic or religious factors.
Annual values of the drought index (DI) were obtained by means of the weighted average of
the number of level 1, 2 and 3 rogations recorded in the period between December and
August. The weight for each level was 1, 2 and 3, respectively.
3.3. VALIDITY OF ROGATION RECORDS FOR DROUGHT QUANTIFICATION
Numerous authors attest to the validity of rogations in quantifying droughts (e.g. Pfister, 1999;
Barriendos, 1997; Piervitali and Colacino, 2001; Barriendos, 2005; Luterbacher et al., 2006
and references therein). In general, the information is highly credible due to the strict control
of the process by the Catholic Church and the costs involved in holding the ceremonies, borne
by the civil authorities (Martín-Vide and Barriendos, 1995).
In this paper we also quantitatively analysed the validity of the rogation records for drought
quantification by comparing the rogation records with the precipitation series available in the
Zaragoza observatory. Precipitation records start in 1858. Between this year and 1900, we
have 39 years of complete records. The years 1863, 1864, 1886 and 1887 are missing.
Also we compared the DI with an independent reconstruction of past climate.
Dendrochronological reconstructions of annual precipitation (October to September), which
start in 1570 were used (Saz, 2003). The precipitation records were available in two
observatories of the centre of the Ebro valley: Haro (190 km west of Zaragoza) and Pallaruelo
de Monegros (85 km North-East). A series was generated from average of annual records in
each observatory, weighted for the distance to Zaragoza.
Figure 3 shows the mean December-August precipitation between 1858 and 1900, years in
which a varying number of rogations of each level were recorded. For level 1 rogations mean
December-August precipitation was lower in the years with two rogations (175 mm) than in
the years with only one rogation (193.6 mm). Moreover, mean December-August precipitation
in years without level 1 rogation records was higher (231 mm). With level 2 rogations there
are also differences in December-August precipitation values between the years without
rogations of this level (221.2 mm) and years that did record a rogation of this level (190 mm).
Differences are more important in the analysis of level 3 rogations. The December-August
mean precipitation during the years in which one rogation of this level was recorded is only
169.7 mm.
In addition, DI values were compared with the December-August precipitation in Zaragoza
(Figure 4). Results show that the years with high DI values, such as 1870, 1876, 1878 and
1896, coincide with a major decrease in precipitation, whereas in years with DI values of 0
and 1 annual precipitation was, in general, higher. This can be seen in the years 1858, 1862,
1871-1874 and 1883-1892. The correlation between the two time series is negative and
statistically significant (R = -0.34, p < 0.05). Although the amount of shared variance between
the DI and the precipitation is only a few percent, the characteristic of the rogation records
does not allow a continuous quantitative comparison between the DI and the precipitation
because the DI is only indicative of the drought events but it is not sensitive to high
precipitation values. The DI is bounded in 0. This value would be recorded the years with
normal precipitation but also in some years with high precipitation values, which would not be
accompanied with a decrease of the DI.
Figure 5 shows the average annual precipitation (October to September) corresponding to
dendrochronological records as a function of the DI (December to August) between 1600 and
1900. Although comparison considers different months, there is a good agreement between
the average annual precipitation and the DI (R = 0.77). In general, low precipitation values
correspond to high DI values.
Therefore, drought quantification by means of rogation records in the semi-arid region of the
middle Ebro valley is quite feasible. The sample tested between 1858 and 1900 and compared
to annual precipitation in Zaragoza, and the comparison of the whole series with independent
dendrochronological reconstructions of precipitation, confer greater reliability on the DI
values calculated from rogations between 1600 and 1900.
3.4. INSTRUMENTAL RECORDS
Drought during the twentieth century was analysed from the instrumental records of the
Zaragoza observatory. We obtained the monthly data set between 1858 and 2000 from the
National Institute of Meteorology (Spain). The period between 1858 and 1900 overlaps with
the drought index obtained from rogation records, which makes it possible to compare both
drought indices.
Temporal homogeneity of precipitation series was tested by statistical techniques. During the
twentieth century there was a change in the location of the observatory, which was moved in
1941 from the centre to outside the city. To test whether this change or other factors caused a
lack of homogeneity in the series, we used precipitation series from other weather stations in
the middle Ebro valley with precipitation records from the beginning of the twentieth century,
such as the observatories of Fraga, Huesca, Tarazona, Alcorisa, Calatorao and Monzón, all
within a radius of less than 100 km from Zaragoza. These observatories’ records gave us a
reference series by means of the weighted average of the monthly precipitation, following
Peterson and Easterling (1994). Relative homogeneity was tested by the Standard Normal
Homogeneity Test (Alexandersson, 1986). For this purpose we used Anclim software
(Štìpánek, 2004). We did not find any significant lack of homogeneity in the precipitation
series for Zaragoza during the twentieth century.
3.5. DROUGHT INDEX CALCULATION USING INSTRUMENTAL RECORDS
To quantify and analyse droughts with the precipitation data, we used the Standardized
Precipitation Index (McKee et al., 1993) at different time scales from the monthly
precipitation series of Zaragoza.
SPI calculation starts with precipitation calculation over a range of time scales. The total
k
precipitation X i , j in a given month j and year i depends on the time scale chosen, k. For
example, the total precipitation for one month in a particular year i with a 12-month
precipitation total in the month j of year i and in consecutive months, with k=12, is calculated
by (Paulo et al., 2003):
X
k
i, j

X ik, j 
j
12
w
l 13 k  j
i 1, l
 wi ,l
l 1
, if j<k, and
j
w
l  j  k 1
i ,l
, if j=k
where wi,l is precipitation in the lst month of year i [mm].
We calculated precipitation at time scales of 9 months to agree with the time scale used to
analyse droughts from rogation records (9 months from December to August). Details about
the index calculation procedure according to Pearson III distribution and the L-moments
method can be consulted in Vicente-Serrano (2006).
3.6. NORTH ATLANTIC OSCILLATION DATA
In this paper we used two indices to measure the North Atlantic Oscillation (NAO). There are
different NAO indices obtained from instrumental records for the nineteenth and twentieth
centuries. The NAO index has been measured by means of multivariate analysis using a
surface pressure grid (Portis et al., 2001 and references therein) in some cases, whilst on other
occasions the time series of surface pressures, particularly at sea level weather stations, was
used (Rogers, 1984; Hurrell, 1995; Jones et al., 1997).
Following the latter approach, the NAO index is calculated from the surface pressure gradient
between observatories in Iceland and the west of the Iberian Peninsula including the Azores.
Here, we used the NAO index of Jones et al. (1997), who calculated the difference between
the monthly standardised surface pressures in Gibraltar (in the southwest of the Iberian
Peninsula) and in Southwest Iceland. The data is available from 1821, being not continuous at
the beginning and it is updated to recent months (http://www.cru.uea.ac.uk/ftpdata/nao.dat)
(Vinther et al., 2003).
The NAO is mainly active in winter, the season in which the NAO determines the climate of
extensive areas of Europe (Hurrell and Van Loon, 1997). Therefore, we used winter data
(December, January and February), but March was also included in the winter NAO index to
take into account early spring precipitation, which is very important for the central Ebro
valley.
The period with data available on the NAO index (1821-2000) is not long enough to
determine the NAO’s impact on droughts in the middle Ebro valley from the seventeenth
century. There are different approaches to modelling the NAO index from “proxy” data so as
to have records from earlier periods (e.g., Garcia et al., 2000; Rodrigo et al., 2001; Cullen et
al., 2001; Luterbacher et al., 1999 and 2002; Glueck and Stockton, 2001). Here, to analyse the
NAO influence on droughts from the seventeenth century, we used an NAO index obtained
from “proxy” data that has been tested in depth and shown its general good quality (Cook et
al., 2002). These authors reconstructed the winter NAO index from 1400 A.D. by means of
tree-ring chronologies, ice-core records in North Europe and principal-component regression.
The results were verified by early European data from instruments (Jones et al., 1999;
Luterbacher
et
al.,
2002)
with
good
results.
The
index
is
available
at:
http://www.ngdc.noaa.gov/paleo/pubs/cook2002/cook2002.html. To verify that our results are
independent of the selected NAO reconstruction, we also repeated the analysis with the
reconstructed NAO index by Lutterbacher et al. (2002), which starts in 1659. There is another
reconstruction of the winter NAO index made by the same authors considering the months of
December, January and February, which starts in 1500. These indices are available at
http://www.cru.uea.ac.uk/cru/data/paleo/naojurg.htm.
4. Results and discussion
4.1. DROUGHT EVOLUTION BETWEEN 1600 AND 2000 FROM ROGATIONS AND
INSTRUMENTAL DATA
Figure 6 shows the evolution of the December-August number of level 1, 2 and 3 rogations
between 1600 and 1900. The years that recorded the highest number of level 1 rogations (4)
were 1680, 1769 and 1781. Level 2 rogations are rare, and the annual maximum is 2. Level 3
rogations are similar: the maximum recorded in a year is 3, which occurred in 1765.
Temporal analysis of the three rogation levels shows few entries between 1600 and 1675.
There was an important increase in the last quarter of the seventeenth century. At the
beginning of the nineteenth century the number of rogation ceremonies dropped, which could
be due to the Napoleonic wars. The rogation ceremony series can be considered homogeneous
between the 17th and the 20th century because Catholic liturgy was maintained during this
period. Nevertheless, during the Napoleonic wars, Zaragoza was besieged and the major part
of the city destroyed. The city lost 78% of its population (From 55,000 in 1808 to 12,000 after
the sieges in 1810), and rogations could have been affected as a consequence of this dramatic
episode. During the nineteenth century there were fewer rogations than in the last quarter of
the eighteenth century, although more than in the seventeenth century.
As there were few rogations between 1600 and 1675, we included as a complementary source
the number of level 4 and 5 rogations recorded in the documentary sources. During this
period, Spanish municipalities suffered considerable economic problems because of inflation,
and the reduction in the number of rogations could have been due to efforts to economise.
Following this line of argument, only when the drought was very advanced and crops were
lost, were high-level rogations held (Barriendos, personal communication). Figure 7 shows the
evolution of the number of level 4 and 5 rogations. Their patterns of temporal evolution are
very similar to other rogation levels, and few rogations between 1600 and 1675 were
recorded, which reinforces the climatic explanation for the evolution of the number of level 1,
2 and 3 rogations during this period.
Figure 8 shows the evolution of the DI between 1600 and 1900. The grey vertical bars indicate
the annual value of the index and the black line is the result of the application of a Gaussian
filter (10 years) to the original series to retain the most general characteristics. Different dry
periods are identified, mainly during the eighteenth century. Between 1725 and 1755, DI
values over 3 are recorded in many of the years. Between 1755 and 1765 there was a clear
spell of years with DI = 0. Nevertheless, 1765 to 1800 saw the highest concentration of
extreme droughts. Between 1800 and 1814 there was a significant decrease in the drought
index values and then a new increase until 1825. During the second half of the nineteenth
century high values on the drought index were less frequent than in the eighteenth century.
Figure 9 shows the evolution of the SPI in Zaragoza at a time scale of 9 months from 1858 to
2000. Negative values indicate dry conditions. The most important droughts were recorded in
the decades beginning in 1870, 1900, 1920, 1940 and 1960, and then, even more, in the 1980s
and 1990s. In the period of data common to DI and the SPI, it is shown that temporal
resolution is very different, and the DI provides few temporal details in relation to the SPI.
Nevertheless DI values other than 0 always correspond to negative SPI values, which are
indicative of drought conditions.
Results for drought evolution in the middle Ebro valley coincide with the results of other
authors who analysed “proxy” data in Western European Mediterranean regions (Martín-Vide
and Barriendos, 1995; Grove, 2001). Barriendos (1997) also showed, by means of rogation
records in Catalonia (East of Spain) and in the South of the Iberian Peninsula, that major
drought episodes were recorded there in the second half of the eighteenth century, just as they
were in the middle Ebro valley. The drought increase recorded between 1635 and 1650 in the
middle Ebro valley was also identified in Catalonia. Nevertheless, temporal non-synchrony of
drought occurrence in the Iberian Peninsula was also noted by Barriendos (1997), on
comparing Catalonia with other regions in the South of the Peninsula.
Rodrigo et al. (1999) used various historical records to reconstruct precipitation in the South
of the Iberian Peninsula from 1500. They showed moist conditions at the beginning of the
seventeenth century and normal precipitation between 1650 and 1725, although from 1700 on
some dry periods were recorded, equivalent to those observed in the middle Ebro valley and to
those found by Barriendos (1997) in Catalonia. The most intense droughts in the South of the
Iberian Peninsula, according to Rodrigo et al. (1999), were in the first half of the eighteenth
century, whereas the most intense droughts in the middle Ebro valley and, according to
Martín-Vide and Barriendos (1995), in Catalonia, were during the last quarter of the
eighteenth century, but were not recorded in the South of the Iberian Peninsula.
The non-synchrony of droughts within the Iberian Peninsula is a general characteristic of the
geographic diversity and the different atmospheric patterns that control precipitation in each
region (Vicente-Serrano et al., 2004; Rodríguez-Puebla et al., 1998; Martín-Vide, 2001).
Nevertheless, although spatial and temporal differences in droughts are a normal characteristic
of the Iberian Peninsula, the period 1775-1800 is also recognised as extremely dry and
anomalous in other regions. This period has been called the “Maldá” anomaly, and was
characterised by major climatic variability, with a rapid succession of droughts and floods
(Barriendos and Llasat, 2003). This anomaly affected extensive areas in the North of the
Iberian Peninsula (Saz, 2003) and also other Mediterranean regions such as Italy (Camuffo et
al., 2000) and the Balkans (Xoplaki et al., 2001).
The decrease in the number of rogations during the second half of the nineteenth century
could be associated with the declining influence of the Catholic Church. In addition, during
the second half of the nineteenth century, palliative measures, such as agrarian insurance and
the subsidies paid by provincial authorities since the end of the 1840s, reduced the need to
hold rogation ceremonies. Nevertheless, the close relationship between the DI and SPI in
Zaragoza between 1858 and 1900 validates the results obtained. Moreover, climatic evolution
during these years coincides with the findings of other authors. The second half of the
nineteenth century is a period of great climatic stability in the North of Spain. Saz (2003) has
shown, by means of tree-ring chronologies, that the period between 1850 and 1950 was the
most climatically stable period between the fifteenth and the twentieth centuries. This is
confirmed by the instrumental data. The Coefficient of Variation (CV) of the DecemberAugust precipitation for 1871-1900 was 0.30 and 0.25 for 1901-1930. For 1931-1960 and
1961-2000, the CV values were higher (0.38 and 0.37, respectively).
Also during the twentieth century, the temporal patterns coincided with drought evolution in
other areas of the Iberian Peninsula, although some differences were found. For example, the
episodes of 1920 and 1960 only occurred in the North and East of the Iberian Peninsula
(Vicente-Serrano, 2006b). Raso (1993) indicated that the drought observed at the beginning of
the century was also recorded in other observatories, such as those in Barcelona, La Coruña,
Salamanca and Soria. Olcina (1994) also identified the dry period between 1949 and 1950 in
the South and South-east of Spain. The drought observed in the middle Ebro valley between
1978 and 1985 also affected the Mediterranean littoral (Pérez-Cueva, 2001), and the dry
period between 1990 and 1995 was also very dry in most of the Iberian Peninsula (Almarza et
al., 1999).
In addition, the evolution of droughts from 1950 to 2000 coincides with the observed
evolution recorded in other Western Mediterranean areas. Delitala et al. (2000) indicated two
general dry periods: 1942-1956 and 1980-1996. Moreover, in Europe an increase in the
succession of dry periods was found, so dry years are concentrated consecutively (Hisdal et
al., 2001; Szinell et al., 1998; González-Rouco, 2000). This was also observed in the middle
Ebro valley (i.e. 1994, 1995 and 1998, the driest years of the century, all fell in the last
decade). Therefore, the persistence of the dry years seems to increase.
4.2. NAO INFLUENCE ON DROUGHTS: ANALYSIS FROM ROGATION DOCUMENTS
(1600-1900) AND INSTRUMENTAL RECORDS (1858-2000)
Figure 10 shows the average December to March (DJFM) NAO index values between 1600
and 1900 as a function of the number of level 1, 2 and 3 rogations recorded each year
(December-August). It can be seen that the years that recorded the highest number of level 1
rogations had positive average values on the NAO index of over 0.5.
The effect of the NAO on rogation records becomes clearer on analysis of the number of level
2 and 3 rogations. The years with the greatest number of rogations coincide with the highest
average values on the DJFM NAO index. This is clearly observed in level 3 rogations. There
is an increase in the average values on the NAO index, which coincides with the increase in
the number of rogations, which in turn is related to more drought. For this reason, years with
positive values of the DJFM NAO index tended to reflect more drought in the middle Ebro
valley between 1600 and 1900.
We also analysed the relationship between the DJFM NAO index and the DI obtained from
rogation records (Figure 11). The correlation between the two series is positive and
statistically significant, R = 0.23 (p< 0.01). Nevertheless, there is a high dispersion of NAO
index values as a function of DI values, mainly due to low values of the DI.
The amount of shared variance between the DI and the NAO index is only a few percent.
However, it must be taken into account that the high dispersion of the DJFM NAO index
values during the years in which low values of DI are recorded significantly affects the
relationship between both indicators because low values are predominant. The years in which
DI has 0 value coincide with years that record normal precipitation, but also with years of high
precipitation. These latter years probably coincide with negative values on the DJFM NAO
index, years in which there were no religious ceremonies. For this reason, analysis of the
relationship between the DI and the NAO index is biased due to the presence of years with
negative NAO index values.
To avoid this problem, and to better identify the role of NAO in droughts, we used the average
of the DJFM NAO index as a function of DI values. Figure 12 shows this relationship. The
correlation between both indices is positive and statistically significant (R = 0.86, p < 0.01)
with a strong linear relationship between them.
Figure 13 shows the same analysis but using the Luterbacher et al. (2002) NAO index
(December, January and February). Although there is also a good agreement (R = 0.78), the
relationship is better by means of the Cook et al. (2002) NAO index. This reinforce the
inclusion of March to create the winter NAO index, which would explain better the drought
variability in the middle Ebro valley.
The analysis was repeated from 1659 using the Lutterbacher et al. (2002) and the Cook et al.
(2002) winter NAO indices, which are comparable between 1659 and 1900 since they include
March to calculate the NAO index. Figure 14 shows very close results considering both
indices, guaranteeing the robustness of our major findings.
There are few studies about the role of the NAO on precipitation and droughts during preinstrumental periods. Among the few examples, in the Canary Islands, García et al. (2003)
found that the inter-annual variability of crop production during the past five centuries was
greatly affected by the NAO. Barriendos and Llasat (2003) indicated that during the “Maldá”
anomaly (last quarter of the eighteen century), the winter NAO was very irregular and had
anomalous values. This could explain the great climatic variability and the quick and
considerable alternation of dry and wet periods recorded in some Mediterranean areas during
this period, also identified in this paper.
The use of a drought index, obtained from precipitation series, permits more detailed analysis.
Figure 15 shows the relationship between the DJFM NAO index and the SPI at the time scale
of 4 months in March, which summarises precipitation between December and March, and
also the SPI at the time scale of 9 months in August. The relationship is negative and
statistically significant (p < 0.01) in both cases (R = -0.55 and R = -0.40, for the SPI at the
time scales of 4 and 9 months, respectively). This indicates that even in August the drought
index calculated at the time scale of 9 months, which summarised the precipitation between
December and August, is significantly affected by the DJFM NAO index.
The evolution of the two indicators (DJFM NAO and SPI) was the opposite during the
twentieth century (Figure 16). Years with positive DJFM NAO values usually coincided with
drought conditions in the middle Ebro valley. The opposite was so during winters with
negative DJFM NAO index values, which usually corresponded with positive values of SPI.
The response of precipitation to the inter-annual variations of the winter NAO index has also
been recorded in other regions of the Iberian Peninsula, but mainly in the Southwest
(Rodríguez-Puebla et al., 1998; Rodó et al., 1997; Martín-Vide and Fernández, 2001), where
winters with negative values of the NAO are related to storms and perturbations to the west of
the Iberian Peninsula (Trigo and DaCamara, 2000), which cause the highest precipitation in
the Southwest.
In this paper we provide evidence that in the middle Ebro valley, the NAO has an important
effect on droughts. This was indicated previously by Esteban et al. (2002), and may be linked
to the presence of the Pyrenees mountain range in the north, with peaks rising to over 3,000
m. The Pyrenees, running W-E, act as a barrier to flows from the Southwest. SW flows, which
are usually dry due to the precipitation discharges in the Southwest of the Iberian Peninsula
and the crossing of the inland regions, are reactivated because of the ascent of the air mass on
reaching the mountain range. This climatic process has been confirmed by the analysis of a
great many precipitation series in the Ebro valley for shorter periods than those considered in
this paper. The results show that the NAO effect increases to the north, on contact with the
Pyrenees (Vicente-Serrano, 2004; López-Moreno, 2005).
4. Conclusions
This paper analyses drought evolution in the semi-arid lands of the Northeast of the Iberian
Peninsula (middle Ebro valley) between 1600 and 2000. For this purpose, historical
documents (rogation ceremonies) and data from instruments (precipitation) were used. The
influence of the North Atlantic Oscillation (NAO) on the evolution and severity of droughts
was also studied. Documentary data were carefully selected and analysed, and the drought
index obtained from them was tested by means of the instrumental precipitation data and
dendrochronological reconstructions.
Results indicate that drought evolution during the past four centuries often coincides in time
with the evolution recorded in other Mediterranean areas. Between the sixteenth and
nineteenth centuries the most important droughts were recorded in the last quarter of the
eighteenth century, which coincided with a period of high climatic variability known as the
“Maldá” anomaly (Barriendos and Llasat, 2003). In general, the eighteenth century was drier
than the seventeenth and nineteenth centuries. In the twentieth century (analysed by means of
instrumental records), the main dry periods also coincided with droughts in other regions of
the Iberian Peninsula, mainly in the 1940s, 1980s and 1990s.
It has been concluded that the temporal variability in the NAO plays a major part in
explaining droughts in the study area. Considering instrumental data from 1858, droughts
were closely related to the winter (DJFM) NAO index, with a negative and statistically
significant correlation. This confirms the importance of the NAO in explaining the interannual
variation of drought in the study area.
Moreover, the influence of the NAO on drought in the middle Ebro valley has also been
observed between the 17th and the 19th centuries by means of rogation ceremonies. The
December to August number of level 1, 2 and 3 rogations and the December-August drought
index (DI), obtained from rogations between 1600 and 1900, show an increase as a
consequence of the increase in the winter NAO index. Therefore, positive values of this
atmospheric circulation pattern have tended to cause dry conditions. This result has been
verified by means of two different and well-verified independent reconstructions of the NAO
index.
We have shown that the DJFM NAO index is closely related to the total number of rogations
recorded between December and August. Until the twentieth century the economy and the
subsistence of the inhabitants of the middle Ebro valley were based, essentially, on rain-fed
agriculture (mainly on winter cereals). Analysis by means of the SPI showed that the greatest
influence of the NAO on drought occurs during the first months of the year, coinciding with
the greatest vulnerability of cereals to drought. This explains the close relationship found
between the December-August number of rogations and the DJFM NAO index, since
precipitation in the first months of the year would condition the subsistence of the population
in pre-industrial times.
In the middle Ebro valley the great influence of the NAO on droughts was identified from
instrumental records for the twentieth century, and this influence was also confirmed during
the three previous centuries by means of historical documents. Therefore, the robustness of
the results makes it possible to plan the usefulness of the NAO index for climatic prediction
and drought early warning, as indicated in other regions (Thompson et al., 2002; Trigo et al.,
2004).
Acknowledgements
This work has been supported by the projects: STRIVER financed by the European
Commision (VI framework programme), CGL 2005-04508/BOS financed by the Spanish
Commission of Science and Technology and FEDER, PIP176/2005 and “Programa de grupos
de investigación consolidados” (BOA 48 of 20-04-2005) financed by the Aragón Government.
We would like to thank to Dr. Mariano Barriendos and the 3 anonymous reviewers for their
helpful comments.
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Figure 1. Location of the Ebro valley and Zaragoza.
Figure 2: Ecclesiastical document of the archive of Zaragoza (a “pro pluvia” rogation of the
seventeenth century). “Libro de Actas del Archivo Metropolitano de La Seo de Zaragoza, vol.
23, from 1681 to 1687, pag 248”.
Table 1. Rogation levels recorded in the historical documentary sources of Zaragoza (see
Appendix).
1
2
3
4
5
Petition in church
Exposure of the intercessor in church
Masses and processions with the intercessor within the church
Processions with the intercessor outside the church
Pilgrimage to the intercessor of another sanctuary or church
220
200
180
160
-1
0
1
2
3
Dec-Aug precipitation (mm)
Dec-Aug precipitation (mm)
240
240
220
200
180
160
-1
1
2
Number of Rogations (Level 2)
Number of Rogations (Level 1)
Dec-Aug precipitation (mm)
0
240
220
200
180
160
-1
0
1
2
Number of Rogations (Level 3)
Figure 3. December-August precipitation in the observatory of Zaragoza in the years
(December to August) with different numbers of level 1, 2 and 3 rogations. 1858-1900.
450
400
5
350
DI
4
300
3
250
200
2
150
1
100
0
1850
1860
1870
1880
1890
50
1900
December-August precipitation (mm)
6
Figure 4. Temporal evolution of the drought index (DI) obtained from rogations (vertical bars)
and December-August precipitation in Zaragoza (black line).
Precipitation (October to September)
450
440
430
420
410
400
390
380
0
2
4
6
8
10
12
DI
Figure 5. Average values of annual precipitation (October to September) obtained from
dedrochronological records (Saz, 2003) as a function of the December-August DI (16001900).
Nº Rogations
Nº Rogations
Nº Rogations
5
LEVEL 1
4
3
2
1
0
1600
1625
1650
1675
1700
1725
1750
1775
1800
1825
1850
1875
1900
1650
1675
1700
1725
1750
1775
1800
1825
1850
1875
1900
1650
1675
1700
1725
1750
1775
1800
1825
1850
1875
1900
5
LEVEL 2
4
3
2
1
0
1600
1625
5
LEVEL 3
4
3
2
1
0
1600
1625
Figure 6. December-August number of level 1, 2 and 3 rogations. 1600-1900.
Nº rogations
3
2
1
0
1600
1625
1650
1675
1700
1725
1750
1775
1800
1825
1850
1875
Figure 7: December-August number of level 4 and 5 rogations. 1600-1900.
1900
Drought Index (DI)
12
10
8
6
4
2
0
1600
1625
1650
1675
1700
1725
1750
1775
1800
1825
1850
1875
1900
Figure 8. Evolution of the December-August drought index from rogation documents (16001900). Grey bars: DI. Black line: Gaussian filter to the original series (10 years).
6
2
5
1
4
0
3
-1
2
-2
1
DI
Standardized Precipitation Index
3
0
-3
1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Figure 9. Evolution of the Standardized Precipitation Index (SPI) at the time scale of 9 months
in Zaragoza (1858-2000). Also the DI obtained from rogation records (December-August) is
shown each year (triangles)
1.0
0.6
0.8
Mean DJFM NAO
Mean DJFM NAO
0.8
0.4
0.2
0.0
-0.2
-0.4
-1
0
1
2
3
4
0.6
0.4
0.2
0.0
-0.2
-0.4
5
-1
Number of rogations (Level 1)
0
1
2
3
Number of rogations (Level 2)
Mean DJFM NAO
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
-1
0
1
2
3
4
Number of rogations (Level 3)
Figure 10. Average values of the DJFM NAO index (1600-1900) as a function of the number
of level 1, 2 and 3 rogations recorded between December and August.
3
DJFM NAO Index
2
1
0
-1
-2
-3
0
2
4
6
8
10
12
DI
Figure 11. Relationship between the DJFM NAO index and the December-August DI (16001900)
12
10
DI
8
6
4
2
0
-0.5
0.0
0.5
1.0
Mean DJFM NAO
Figure 12. Average values of the DJFM (December, January, February, March) NAO obtained
from Cook et al. (2002) as a function of the December-August DI (1600-1900).
12
10
8
DI
6
4
2
0
-0.4 -0.2 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Mean DJM NAO
Figure 13. Average values of the DJF NAO index obtained from Luterbacher et al. (2002) as a
function of the December-August DI (1600-1900).
DI (December to August) (1659-1900)
DI (December to August) (1659-1900)
12
R = 0.86
10
8
6
4
2
0
-0.5
0.0
0.5
1.0
1.5
Average DJFM Cook et al. (2002) NAO index
12
R = 0.88
10
8
6
4
2
0
-0.5
0.0
0.5
1.0
1.5
2.0
Average DJFM
Lutterbacher et al. (2002) NAO index
Figure 14. Comparison of the average values of the DJFM NAO indices obtained from Cook
et al. (2002) and Luterbacher et al. (2002) as a function of the December-August DI (16591900).
3
SPI (9 months)-August
SPI (4 months)-March
3
2
1
0
-1
-2
2
1
0
-1
-2
-3
-3
-3 -2 -1
0
1
2
3
-3 -2 -1
4
0
1
2
3
4
DJFM NAO index
DJFM NAO index
SPI (4 months)-March
3 the DJFM NAO index and the SPI in Zaragoza at the time
Figure 15. Relationship between
scales of 4 months (March) and 9 months (August). 1858-2000.
2
1
0
-1
-2
-3
-3
-2
-1
0
1
2
3
3
2
2
0
0
-1
-2
-2
-4
-3
1860
1870
1880
1890
1900
1910
1920
1930
4
DJFM NAO Index
SPI
1
3
2
2
1
0
0
SPI
DJFM NAO Index
4
-1
-2
-2
-4
-3
1930
1940
1950
1960
1970
1980
1990
2000
Figure 16. Evolution of the DJFM NAO (white squares) index and the SPI (black circles) in
Zaragoza (time scale of four months in March).