INTERNATIONAL JOURNAL OF CLIMATOLOGY
Int. J. Climatol. 26: 1455–1475 (2006)
Published online in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/joc.1388
THE WESTERN MEDITERRANEAN OSCILLATION AND RAINFALL IN THE
IBERIAN PENINSULA
JAVIER MARTIN-VIDE* and JOAN-ALBERT LOPEZ-BUSTINS
Group of Climatology, University of Barcelona, Baldiri Reixac, s/n, 08028-Barcelona, Catalonia, Spain
Received 24 May 2005
Revised 19 September 2005
Accepted 11 April 2006
ABSTRACT
Seasonal precipitation variability in the east of the Iberian Peninsula is weakly linked to the North Atlantic Oscillation
(NAO) during autumn and winter. For the purpose of improving the study of its performance, low-frequency variability
patterns specific to the Mediterranean basin have been searched for. In this way, the Western Mediterranean Oscillation
(WeMO) has been defined by means of the dipole composed, in its positive phase, by the anticyclone over the Azores
and the depression over Liguria, and its index (WeMOi), as a result of the difference of the standardised values in
surface atmospheric pressure in San Fernando (Spain) and Padua (Italy). This new index allows the detection of the
variability relevant to the cyclogenesis next to the western Mediterranean basin, which determines in a predominant way
the types of rainfall in the Gulf of Valencia. In this area, the WeMO is significantly better than the NAO to explain the
monthly pluviometric anomalies during these seasons. Also, a daily resolution of the WeMOi can provide a useful tool to
forecast torrential rainfall events in the north-western zones of the Mediterranean (eastern part of the Iberian Peninsula
and the south of France), and such significantly daily rainfall frequencies for different thresholds. Copyright  2006
Royal Meteorological Society.
KEY WORDS:
correlation; Iberian Peninsula; NAO; frequency of a rainy day; torrential rainfall; Western Mediterranean Oscillation
(WeMO)
1. INTRODUCTION
The atmospheric circulation regimes in the Mediterranean basin show a seasonal cycle, linked to the wet
temperate circulation in winter and to the strictly subtropical one in summer. This produces rainy winters and
dry summers. The Mediterranean climate thus shows the geographical transition situation between the wet mild
domain of the mid-latitudes and the arid and desertic area of the tropical anticyclone belt. However, the highly
geographic complexity of the Mediterranean basin composed of three peninsulas with a remarkable surface in
its northern versant, and its singularity, almost closed over the Atlantic, diversifies its general Mediterranean
climate, yielding a great variety of atmospheric behaviours, mainly pluviometric. When geographical factors
interfere in the circulation dynamics, they cause a seasonal distribution of rainfall in certain scopes of the
basin, which differs from the typically Mediterranean one. Specifically, in the eastern façade of the Iberian
Peninsula, leeward of the Atlantic influence, autumn is the rainiest season, whereas winter is relatively dry
(Martı́n-Vide and Olcina, 2001). Especially illustrative is the pluviometric calendar, on a daily resolution,
obtained from observatories located on the eastern coast of the Iberian Peninsula (Figure 1) showing an
annual regime that differs from the typically Mediterranean one. Therefore, the maxima are equinoctial both
in frequency and in quantity, with autumn standing out in terms of quantity and winter barely surpassing
summer (Soler and Martı́n-Vide, 2002).
* Correspondence to: Javier Martin-Vide, Group of Climatology, University of Barcelona, Baldiri Reixac, s/n, 08028-Barcelona,
Catalonia, Spain; e-mail: [email protected]
Copyright  2006 Royal Meteorological Society
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J. MARTIN-VIDE AND J. LOPEZ-BUSTINS
0.6 (a)
0.5
0.4
0.3
0.2
0.1
0
1-Jan
18 (b)
16
14
12
10
8
6
4
2
0
1-Jan
7 (c)
6
5
4
3
2
1
0
6
5
4
3
2
1
0
1-Feb
1-Mar
1-Apr
1-May
1-Jun
1-Jul
1-Aug
1-Sep
1-Oct
1-Nov
1-Dec
1-Feb
1-Mar
1-Apr
1-May
1-Jun
1-Jul
1-Aug
1-Sep
1-Oct
1-Nov
1-Dec
(d)
Figure 1. Pluviometric calendar of Sant Feliu de Guı́xols (NE Spain) (1927–1995). (a) Relative frequencies of rainy days. (b) Daily
amount averages (mm). (c) Daily amount averages in 5 days (mm). (d) Daily amount averages in 10 days (mm)
The amplitude of the Mediterranean basin favours the presence of a synchronised but opposed behaviour
by atmospheric dynamics between its eastern and western sub-basins. This is defined as the Mediterranean
Oscillation (MO) concept by Conte et al. (1989). The MO is a low-frequency variability pattern producing
opposing barometric, thermal and pluviometric anomalies between the extremities of the basin. Strongly linked
in winter to the Northern Hemisphere teleconnection modes of the Arctic Oscillation (AO) and the North
Atlantic Oscillation (NAO) (Dünkeloh and Jacobeit, 2003), the MO has been considered the most important
regional low-frequency pattern influencing rainfall in the Mediterranean basin by some studies (Dünkeloh
and Jacobeit, 2003; Kutiel et al., 1996; Douguédroit, 1998; Maheras et al., 1999a). The influence of the
MO on climate variability has also been analysed in other studies (Corte-Real et al., 1995; Palmieri et al.,
2001; Palutikof et al., 1996; Piervitali et al., 1997; Kutiel and Paz, 1998; Maheras et al., 1999b; Brunetti
et al., 2002; Xoplaki et al., 2003; Baldi et al., 2004). In a simple way, the MO can be interpreted by two
opposite surface pressure (and geopotential height) configurations, in its positive mode by an anticyclone
(or an anticyclonic ridge in mid-tropospheric levels) in the western Mediterranean and Iberia, and a low (or
a trough) in the eastern Mediterranean. The original Mediterranean index was defined as the difference of
standardised geopotential height anomalies at Algiers and Cairo (Conte et al., 1989). Other similar indexes are
calculated as the difference of standardised pressure anomalies at Gibraltar and the Israelian meteorological
station of Lod (Palutikof, 2003), or as the difference between standardised pressure anomalies at Marseilles
and Jerusalem (Brunetti et al., 2002).
Using canonical correlation analysis to identify the main coupled circulation–rainfall patterns in the
Mediterranean basin, Dünkeloh and Jacobeit (2003) suggest the existence of a Mediterranean meridional circulation (MMC) pattern. It is defined by two opposite pressure centres, one west of Great
Britain, near 45–50 ° N and 20–25 ° W, and the other in the Italian Peninsula. This pattern produces
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
WESTERN MEDITERRANEAN OSCILLATION AND RAINFALL IN THE IBERIAN PENINSULA
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meridional flows over the Iberian Peninsula, northerly in its positive mode and southerly in its negative one.
There have been other recent contributions to the research of the relationship between the Mediterranean
atmospheric dynamics, mainly that in the eastern basin, and that in the rest of Europe. The surface pressure
and the 500 hPa geopotential heights over Greece have been particularly correlated with the standard weather
types (Lamb’s, and Hess and Brezowsky’s types) for western and central Europe (Anagnostopoulou et al.,
2004; Hatzaki et al., 2004). Some hydrographic works link the MO with the temperature, salinity and density
of the waters in North Adriatic (Grbec et al., 2003; Supić et al., 2004). They introduce an MO index expressly
defined by means of the sea-level pressure differences between the best-correlated areas in mid-North Atlantic
and south-east Mediterranean with water density in the North Adriatic.
One of the main objectives is to define a secondary oscillation form in the Western Mediterranean basin,
which is able to explain the pluviometric variability in the eastern fringe of the Iberian Peninsula, an area
weakly or not related to the NAO pattern. This might have practical value in downscaling methods. A second
one is to use the index of the new pattern as a hazard indicator for the occurrence of heavy rains in the
above-mentioned Spanish area.
The definition of a new low-frequency pattern over the Western Mediterranean basin is presented in
Section 2. Its relationships, by means of its index, with other patterns, especially with NAO and AO, are
shown in Section 3. The correlations between the new pattern and the monthly precipitation in the Iberian
Peninsula are analysed in Section 4, and, finally, its use on a daily resolution is proposed for the case of
torrential rainfalls over the north-western Mediterranean coast in Section 5.
2. THE WESTERN MEDITERRANEAN OSCILLATION (WEMO): DEFINITION
The different definitions of the MO have always intended to cover mainly the atmospheric dynamics of the
whole Mediterranean basin. However, in the new proposal, the Western Mediterranean Oscillation (WeMO)
is defined only within the synoptic framework of the western Mediterranean basin and its vicinities. The
suggested areas are the Po plain, in the north of the Italian peninsula, an area with a relatively high barometric
variability due to the different influence of the central European anticyclone and the Liguria low; and the Gulf
of Cádiz, in the south-west of the Iberian Peninsula, often subject to the influence of the Azores anticyclone
and, episodically, to the cut off of circumpolar lows or to its own cyclogenesis. The transect linking both
areas matches more or less to the coastline of the basin’s north-western quadrant, that is to say, a large
part of the eastern façade of the Iberian Peninsula. In this way, the identification of the basic atmospheric
circulations will be carried out on the basis of the segment’s south-west–north-east orientation. Moreover, in
the WeMO definition and the corresponding Western Mediterranean Oscillation index (WeMOi) the surface
level has been chosen because, for purposes of application to calculate the precipitation on the eastern façade
of Iberian Peninsula, the surface flow direction is a determining factor in the case of torrential rainfall (Estrela
et al., 2002; Azorı́n Molina and López-Bustins, 2004).
The positive phase of the WeMO corresponds to the anticyclone over the Azores enclosing the southwest Iberian quadrant and low-pressures in the Liguria Gulf; and its negative phase coincides with central
European anticyclone located north of the Italian peninsula and a low-pressure centre, often cut off from
northern latitudes, in the framework of the Iberian south-west. A neutral phase will apply in the case of
the low-pressure gradient over the western Mediterranean basin and the surrounding areas, or whenever a
north-east advection with the same isobar is established, linking both areas of the dipole (Figure 2).
The mean sea-level pressure maps for winter months, December to March (1951–2000), show a
clear opposed pressure fields between Cádiz Gulf and north Italy (NOAA-CIRES plots for 20 ° W–20 ° E,
30 ° N–50 ° N, not shown). Esteban et al. (2005), using a daily surface pressure data-grid and multivariable analysis, detected for an area in the East of Iberia, located in the middle of the transect, seven
synoptic circulation patterns, four of them clearly showing a WeMO positive phase, and one of them a
negative one.
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
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J. MARTIN-VIDE AND J. LOPEZ-BUSTINS
Phase (index)
Surface pressure Surface pressure Synoptic pattern
over the western
in Padua
in San Fernando
Mediterranean
Positive (+)
high
low
N, NW, WNW
and W advections
Negative (-)
low
High
ENE, E, ESE
and SE advections
Surface synoptic map (real case)
NE advection
Neutral (∼ 0)
=
=
Low gradient
Figure 2. Definition of the Western Mediterranean Oscillation phases by means of synoptic real maps (the transect Gulf of Cadis–Po
plain is drawn)
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
WESTERN MEDITERRANEAN OSCILLATION AND RAINFALL IN THE IBERIAN PENINSULA
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3. THE WEMO INDEX: CONSTRUCTION AND CHARACTERISTICS
The quality and length of the surface atmospheric pressure series available in the areas forming the pattern’s
dipole have been considered when creating the WeMOi. Following the classical methods used to create
other variable low-frequency pattern indexes, such as NAO, it was decided to use only two specific
observation points, one for each dipole area, with some of the longest barometric series in Europe, Padua
(45° 24 N–11° 47 E) in the north of Italy and San Fernando (Cádiz) (36° 17 N–6° 07 W) in the south-west of
Spain. Both barometric series with a daily resolution were collected within the framework of the European
project IMPROVE (Camuffo and Jones, 2002), with the availability of adequately detailed metadata (Cocheo
and Camuffo, 2002). The series cover jointly and continuously the 1821–2000 period. Nevertheless, certain
inhomogeneities in both series were corrected later on. A homogenised daily Padua series was taken from a
barometric variability analysis for the Po plain (Maugeri et al., 2004). Similarly, the last decade of the San
Fernando series was corrected by the authors for this article, using as a reference series that of Gibraltar
(Jones et al., 1997). Even so, the San Fernando series contains potential measuring errors for the 1821–1869
period, which are difficult to correct. The mentioned period was covered by the Urrutia brothers in the nearby
city of Cádiz, but there is no metadata available (Barriendos et al., 2002). A comparison with Gibraltar infers
a certain underestimate of the pressure registered in Cádiz, which is difficult to quantify. This fact will not
affect the results of further analyses, or it will be carefully taken into consideration.
After several tests (not shown), the standardisation used to create the WeMOi consisted of the independent
normalisation of the monthly mean barometric series of each meteorological station, obtaining the corresponding monthly mean and standard deviation for the 1961–1990 period, and the later difference of the San
Fernando and Padua z values.
As in most of the variability patterns of the Northern Hemisphere, such as NAO, AO, East Atlantic (EA),
Eurasia-1 (EU-1) or Eurasia-2 (EU-2), WeMO shows its most relevant dynamics during the cold half of the
year. The trend analysis of WeMO in winter, covering December to March, shows phases with a different
sign through the use of a slight third grade polynomial (Figure 3(a)). During the entire nineteenth century
the WeMO presented a negative phase with predominantly negative WeMOi values, even outside the Urrutia
brothers’ registration period, whereas since the beginning of the twentieth century up to late 1960s the phase
was positive. In the course of the last three decades of the twentieth century, year groups alternated between
positive and negative WeMOi, generally showing a certain tendency to drop.
The WeMOi trend in winter (December to March) for the 1870–2000 period is null; if the Urrutia brothers’
initial period is included (1821–2000), it will be slightly positive, with no statistical significance using T -test
(Štepánek, 2004) at the 95% confidence level (Figure 3(b)).
As mentioned earlier, the MO is strongly modulated in winter by other more northerly patterns, such
as NAO or AO, and is therefore linked to the polar vortex (Dünkeloh and Jacobeit, 2003). The WeMO
shows a greater independence from the external dynamics of the Mediterranean basin than the MO in winter.
Even though the correlation found with the NAO index in winter (December to March) is positive, it is not
significant (Table I). The same happens with the EA, which also locates its poles in the North Atlantic but
with a strong subtropical link. The different behaviour between the WeMO and the AO and NAO is worthy
of attention because of the high correlation coefficient between these two last patterns. During the winter
periods in the second half of the twentieth century, the WeMOi was negatively correlated with the AO index
(r = −0.39, p-value = 0.0055), and also, but slightly, with that of the EU-1 (r = −0.30, p-value = 0.0379)
(Table I). The opposing relationship between the eastern Mediterranean basin and the EU-2 pattern (Krichak
and Alpert, 2005) loses importance in the scope of our research, the western basin, with the correlation
between its index and the WeMOi not being significant at level 0.05 (Table I).
The El Niño Southern Oscillation (ENSO) shows a slightly negative relationship which is not significant,
and which recall of a possible positive correlation between El Niño and the precipitation in areas of the
Iberian Peninsula, which are more sheltered from the Atlantic (Rodó et al., 1997). Lastly, the Quasi Biennial
Oscillation (QBO) shows no definite relationship (Table I).
The previous results show a western Mediterranean basin that is weakly connected to the most common
nearby patterns, thus giving sense to the search for its own pattern. This can explain, among other things,
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
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J. MARTIN-VIDE AND J. LOPEZ-BUSTINS
(a) 1.5
1
Urrutia Brothers Period
N
0.5
0
18211822
-0.5
18491850
18991900
19491950
19992000
-1
-1.5
-2
(b)
Trend (Z/ 100
years)
1821-2000
1870-2000
December
+0.20
-0.13
January
February
March
+0.24
+0.08
+0.21
+0.12
-0.07
+0.01
Winter (D,
J, F and M)
+0.15
0.00
Figure 3. (a) Temporal evolution of the WeMOi in winter (December, January, February and March) for the period 1821–2000 with a
3-grade polynomial adjustment. (b) Trends in winter and in winter months for the whole period and for the 1870–2000 period
Table I. Pearson’s coefficients of correlation between the WeMOi and other low-frequency variability patterns indices
for winter (December to March) during the period 1950/1951 to 1999/2000. (p-values are shown. The bold ones mean
significance at 0.05). Data sources: NAO (Jones et al., 1997), AO (Thompson and Wallace, 2000), EU-1 (NOAA), EU-2
(NOAA), EA (NOAA), ENSO (Trenberth, 1984) y QBO (Marquardt and Naujokat, 1997) (QBO data start in 1953)
WeMO
Pearson’s correlation
coefficient
p-value
NAO
AO
EU-1
EU-2
EA
ENSO
QBO
+0.1220
−0.3866
−0.2945
−0.2424
+0.1332
−0.0971
−0.0583
0.3988
0.0055
0.0379
0.0898
0.3566
0.5023
0.6972
the pluviometric variability of the east of the Iberian Peninsula, which is very weakly correlated to the NAO
(Rodó et al., 1997; Martı́n-Vide and Fernández, 2001).
To exclude any eventualities in the significance of the existing correlation between the AO index (AOi)
and the WeMOi during the winter period, the analysis has been increased, doubling the period, in the entire
twentieth century. The results show a maintenance of the relationship, even strengthening its significance
(r = −0.2956, p-value = 0.0031). The dot cloud, even though hardly aligned, shows an inversely proportional
relationship, as marked by the straight regression line, WeMOi = −0.165AOi + 0.022 (Figure 4). Only the
1995–1996 winter moves away from the prediction limit as a result of a prolonged meridional circulation of
the polar jet-stream (Halpert and Bell, 1997).
In this way, opposed phases are expected in the temporal evolution of both indexes. In the course of the
twentieth century seven different phases with different lengths have been established in the evolution of the
winter AOi, as the independent variable modulating the WeMOi behaviour. WeMOi responds with totally
opposed phases of similar periodicities during the second half of the twentieth century, while the opposed
behaviour is hardly visible in the first half (Figure 5). Therefore, an insignificant negative correlation is hoped
for the first half of the twentieth century (r = −0.1993, p-value = 0.1745).
Getting to know the WeMO better and its relationship with the AO, the maximum entropy spectral analysis
(MESA) applied to the WeMOi values of the winter period establishes periodicities of 5 and 22 years, with
a 0.05 significance; these cycles are also found in the AOi (Figure 6(a) and (b)). Even though they have no
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
WESTERN MEDITERRANEAN OSCILLATION AND RAINFALL IN THE IBERIAN PENINSULA
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WeMOi=-0.164754*AOi+0.0220072
1.5
Prediction limit
1
Correlation Coefficient = -0.295639
R-squared = 8.74024 percent
R-squared (adjusted for d.f.) = 7.78962 percent
Standard Error of Est. = 0.520741
Mean absolute error = 0.404168
Durbin-Watson statistic = 1.78187 (P=0.1348)
Lag 1 residual autocorrelation = 0.105885
WeMOi
0.5
95% confidence
0
interval
-0.5
Prediction limit
-1
1995-96
-1.5
-3
-2
-1
0
1
2
3
AOi
Figure 4. Linear regression between WeMOi and AOi in winter (from December to March) for the period 1900/1901 to 1999/2000
AOi
2.0
1.0
Z
0.0
-1.0
-2.0
-3.0
-4.0
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
1
2
3
4
5
6
7
2.0
Z
1.0
0.0
-1.0
-2.0
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
WeMOi
Figure 5. WeMO and AO phases in winter for the 1900/1901 to 1999/2000 period (there is a gap in WeMOi in 1921). Low-pass
Gaussian filter, with an input wavelength period of 10 years, of WeMOi and AOi
statistical significance, other 2- and 8-year peaks appear in the WeMOi. They are also present in the AOi and
the NAOi, in which Pozo-Vázquez et al. (2000), by means of a cross-spectral analysis since the beginning
of the nineteenth century, found 6-month and 1-, 2- and 8-year peaks. Therefore, as far as cyclicities are
concerned, the WeMO has a behaviour remarkably similar to that of the AO, and only slightly similar to that
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
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J. MARTIN-VIDE AND J. LOPEZ-BUSTINS
96 24
Value
6.0
12
6
5
4
3
2.5
(a)
2
Period
5.0
4.0
3.0
2.0
95%
1.0
Frequency
0.0
0.0
0.1
96 24
12
0.2
6
5
0.3
4
3
0.4
0.5
2.5
2
1.0
(b)
Value
Period
0.8
0.6
95%
0.4
0.2
Frequency
0.0
0.0
0.1
96 24
4.0
Value
3.5
12
(c)
0.2
6
5
0.3
4
3
0.4
0.5
2.5
2
Period
8 years
5 years
3.0
22 years
3,5 years
2.5
2,40 years
2.0
1.5
1.0
0.5
Frequency
0.0
0.0
0.1
0.2
0.3
0.4
0.5
Figure 6. (a) Maximum entropy spectral analysis (MESA) output of AOi winter (from December to March) values for the 1901–2000
period. (b) Same as in (a) for WeMOi. (c) Comparison of AOi and WeMOi peaks
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
WESTERN MEDITERRANEAN OSCILLATION AND RAINFALL IN THE IBERIAN PENINSULA
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of the NAO. Cyclic variations common to WeMO and AO are approximately those of 2–2.5, 3.5, 5, 8 and
22 years (Figure 6(c)).
4. CORRELATION BETWEEN THE WEMOI AND THE PRECIPITATION IN THE IBERIAN
PENINSULA
Once the WeMO and its index are defined, its application to the precipitation of the whole of the Iberian
Peninsula will be tested using Spanish and Portuguese observatories. A comparison between the correlations
offered by this pattern and the NAO will also be established to regionalise the territory on the basis of the
best correlation. The NAO index used has been the one based on the series from the south-west of Iceland
and Gibraltar (Jones et al., 1997) as, according to Pozo-Vázquez et al. (2000), it is the NAO’s southern pole
station that defines more accurately the index in winter. The chosen analysis period ranges from 1910 to 2000,
a temporal interval that is long enough to obtain the statistical characteristics of Mediterranean pluviometry.
The previous studies related to the winter influence of the NAOi on the Iberian precipitation show a clearly
negative relationship in the centre and the south-west of the peninsula, with correlation coefficients between
−0.5 and −0.7 from December to March (Martı́n-Vide and Fernández, 2001). The rest of the Iberian Peninsula,
both on the northern strip and the eastern versant, shows a weak or null negative correlation with the North
Atlantic pattern.
The 51 meteorological stations spread throughout the entire peninsula and used by Vicente-Serrano and
Beguerı́a-Portugués (2004) have been considered at a monthly resolution. Nine of these series are from the
Sistema National de Informação de Recursos Hı́dricos of Portugal, while the rest were obtained from the
Instituto National de Meteorologı́a of Spain (Figure 7). The series were checked using a quality control
process, which identified the anomalous registrations, and then homogenised according to the Standard
Normal Homogeneity Test (SNHT) (Alexandersson and Moberg, 1997), with the help of the AnClim Program
(Štepánek, 2004).
The 51 stations were used to calculate the Pearson’s correlation coefficients on a monthly basis, between the
NAOi and the WeMOi, and the total monthly precipitation. The most consistent results and the best correlations
were found during the cold half of the year, as expected, i.e. from October to March (Figure 8). The WeMOi
is negatively correlated with the precipitation of the eastern Mediterranean façade of the Iberian Peninsula.
The maximum extension in the signification of correlation takes place in December, the same as the highest
correlation, peaking in the centre and south of the Gulf of Valencia, which is less than −0.6. The negative
correlation is consistent with the incidence of eastern Mediterranean storms under the negative phase of the
WeMO. However, the WeMO also explains the precipitation variability outside the Iberian Mediterranean
scope, in the coastal areas of the Bay of Biscay, where the precipitation is above normal during the pattern’s
positive phase. This phase is associated with advections from the north-west, and therefore with maritime
component on the east Cantabrian coast. Correlation coefficient values up to +0.6 are reached in January.
The tight layout of the isolines between the Bay of Biscay and the Gulf of Valencia shows very clearly the
transition between the mild Atlantic climate and the Mediterranean area.
The NAO, on the other hand, shows a good negative correlation with the central and south-western area in
the Iberian Peninsula, as already mentioned, with the correlation coefficient reaching its highest values in January. The maps of differences between the correlation coefficients in absolute value |r(WeMOi/precipitation)|
minus |r(NAOi/precipitation)| show positive values, that is to say, a better correlation with the WeMO pattern
than with the NAO on the eastern façade of the Iberian Peninsula and in the Bay of Biscay (Figure 8). In the
east, curiously enough, it is in October (with a moderate influence of the NAO pattern and with some very
active pluviometric mechanisms linked to the positive thermal anomaly of the Mediterranean waters) on the
Iberian surface, with better correlation with the WeMO than with the NAO, which is at its greatest. It engulfs
a wide eastern strip, from Catalonia to eastern Andalusia, with the maximum difference favourable to the
WeMO towards the Gulf of Valencia. The regionalisation obtained is very similar to that found by means of
the concentration index (CI), which evaluates the relative weight in the pluviometric total of more abundant
daily amounts of rain (Martin-Vide, 2004). The same area is maintained in the remaining months, though
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
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J. MARTIN-VIDE AND J. LOPEZ-BUSTINS
N
FRANCE
22
34
15
23
36
7
39
PORTUGAL
46
30
21
47
35
31
A
B
Catalonia
20
19
18
24
49
Costa
48
Brava
Cantabrian Sea
50
4
40
5
45
38 41 43
6
Atlantic
Ocean
26
29
37
44
Gulf of
Biscay
32
Iberian
Mediterranean
Peninsula
Sea
27
12
SPAIN 28
51
8
10
Gulf of
Valencia
1
2
11
42
16
Andalusia
14
Gulf of
Cádiz
C
13
9
AFRICA
33
17
25
3
D
Gibraltar
0
EUROPE
Padua
Datum_European 1950
100 200
400
600
Group of Climatology UB
800
kilometers
Monthly data
Daily data
Sant Feliu de Guíxols
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Albacete
Alicante
Almería
Ávila
Badajoz
Bilbao
Burgos
Cáceres
Cádiz
Ciudad Real
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
Córdoba
Cuenca
Granada
San Fernando
A Coruña
Huelva
Jaén
Lleida
Zaragoza
Huesca
San Sebastián
Santander
Santiago
Segovia
Sevilla
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
Soria
Teruel
Toledo
Valladolid
Zamora
Logroño
Madrid
Murcia
Oviedo
Pamplona
Pontevedra
Salamanca
Lisbon
Coimbra
Chouto
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
A.
B.
C.
D.
Coruche
Sao Bras
Extremos
Idanha
Pavia
Rio Torto
Canfranc
Palafrugell
Barcelona
Tortosa
Valencia
Marseilles
Perpignan
Torrevieja
Málaga
Figure 7. Map of locations of the 51 Iberian meteorological stations (bold circles), the other meteorological stations analysed and their
names used in the text
with lesser extension, like the one whose pluviometric variability is better explained by the Mediterranean
pattern than by the North Atlantic pattern. In January, the WeMO pattern does not improve substantially the
correlation with respect to the NAO except in a very reduced sector in the south-east.
For the entire 6-month period, from October to March, the Gulf of Valencia and the Alicante sector, south
of the former, is the area with the greatest WeMO influence with respect to the NAO. This is consistent with
the fact that they are the Iberian regions that are most sheltered from the Atlantic dynamics. The weight of
the WeMO along the southern coast of the Iberian Peninsula decreases fast due to the crisp influence of the
NAO through the Strait of Gibraltar. This explains the high gradient in the influence of the WeMO existing
between Murcia, in the Iberian south-east and Andalusia. On the other hand, within the area of influence of
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
1465
WESTERN MEDITERRANEAN OSCILLATION AND RAINFALL IN THE IBERIAN PENINSULA
-0.2
-0.1-0.2
-0.3
0
-0.2
2
.3
-0
-6
-4
-2
0
-0.4
42
-4
42
40
0
1
-0.4
-0.
5
-0.5
-6
-4
.4
-0
0
40
.3
-2
0
-0
.2
-0.2
-8
2
0.1 0 -0.1 -0.2 .3
-0
-0.4
42
-0.4
-0
-6
42
36
-8
-6
-4
0.1
42
-2
0
-8
2
0.2 0.1 0 -0.1
-0.2
-0.3
0
-6
2
2
36
-8
-6
-2
0
0
5
0.
-0.6
-0.1
40
-0.2
0
2
-6
-4
-2
0.3
0
-8
2
-0.1
-0.2
-6
-4
-0.2
42
-0.4
-0.3
-2
-0.3
0
2
0
-0.4
0
0
-0.5
0.1
40
-0.1
-0.2
40
40
-0.5
-0.5
-4
-2
0
2
0
36
36
36
.4
-0
-0.4
38
-0.5
-0.2
0
38
-0.1
-0.5
-0.4
-0.3
-0.6
38
-6
-0.10
-0.4
38
0.2 0.1 0
0.2
-8
0
-0.4
36
-8
42
42
2
0.1
-2
.3
-0
0
0
-0.5
-0.4
-0.3
-0.2
-4
-2
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.2
36
-6
-4
40
-0.3
38
36
-8
42
-6
-0.2
-0.5
-0.1
40
-8
2
0.2 0.1 0 .1
-0 2
.
-0
-0.3
-0.4
42
0
0
.2
-00.3
-
-
-4
0.1
-0.3
-0.3
-0.2
-0.1
0
-0.2
-0.2
-0.5
-0.4
.3
.4
0
-0
-2
.4
-0
-0
42
-4
-0.4
-0.3
-6
-0.2
-0.3
38
36
-8
38
-0.3
.3
-0
36
-0.3
2
40
-0.2
38
0
-0.1
-0.6
38
-2
-0.
42
-0
-
40
5
0.
-4
.3
-0
.5
-0.5
-0.1
-0
2
-0
.1
40
-0
.1
-0.3
-0.2
-0.1
0
0.1
-0.5
-0.4
0
2
0.2
-2
-0.2
-4
-0.4
42
-0.4
0
.6
38
36
-6
-2
-0
.2
4
36
-8
-4
40
-0
38
-0.
.6
-0
38
6
0.
-0.3
-
-0.2
-0.1
0
36
-8
-0.1
40
2
-0.2
-0.5
0
-0.4 -0
.3
-2
-0.1
-4
-0.4
-6
-0
.3
42
March
-0.1
-0.2
2
0.
38
36
-8
February
0
0
36
January
-2
-0.2
-0.3
-0.
2
-0.3
-0
.3
-0
.4
-0.2
December
-6
2
38
-0.1 -0.2
-8
.
-0
38
0
-0.2
-0.1
40
2
-0.3
-0.4
-0.4
.3
0
-0.3
-0.2
-0.1
-0
40
0
36
-8
-0.1
42
0.1
-0.1
-2
0.2
.1
.2
.5
0
-0.4
-0.3
-0.6
.5
-0
-4
-0
38
36
-6
-0.1
0
-0.2
40
38
36
-8
-0.1
42
.4
-0
-0
-0.5
38
November
-0.4
40
40
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.
1
October
-0.3
0.1
-0.4
-0.3
0.3
-0.3
42
-0.4
-0.3
-0.4
42
|r(WeMOi/precipitation)|
minus
|r(NAOi/precipitation)|
r(WeMOi/precipitation)
-0.1
r(NAOi/precipitation)
-8
-6
-4
-2
0
2
-8
-6
-4
-2
0
2
Figure 8. Distribution of the Pearson’s coefficient of correlation between NAOi and WeMOi, and precipitation, for October, November,
December, January, February and March. (values greater than +0.30 or less than −0.30 (thick line) are significant (confidence level
>99%)). In the maps of the third column, the differences between the absolute values of the two coefficients are shown (0-value is
drawn as a bold line)
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
1466
J. MARTIN-VIDE AND J. LOPEZ-BUSTINS
the WeMO, the coasts oriented south have a relatively better Atlantic dependence (NAO) than those oriented
east (Martin-Vide and Lopez-Bustins, 2004). This explains the weakening of the Barcelona coast, facing
south-east, as opposed to the Costa Brava, more towards the north, oriented to the east (Figure 8). The spatial
model of WeMO and NAO influences on Iberian precipitation is summarised in Figure 9, which provides a
tripartite regionalisation of the Iberian Peninsula with: (1) a wide central and south-western area negatively
well correlated with the NAO; (2) the eastern façade, with a negative correlation with the WeMO and (3) the
eastern Cantabrian strip, positively correlated with the WeMO. Apart from these three areas, the Iberian
north-western corner lacks a clear correlation with both patterns.
The incidence of the WeMO pattern on the precipitation variability in the months of winter in the Gulf of
Valencia is clearly visible if the months with the most extreme index are chosen (higher than +1 and lower
than −1, thresholds used for NAO (Hurrell, 1995)). Thus, in the first case the precipitation stays at barely
20–40% of normal in November, December, February and March, whereas in the second case it can be as
high as 200% in November and December, in the most critical sectors (Figure 10). The eastern end of the
north of the Iberian Peninsula coast, even though with a lower variability, behaves opposite to the Gulf of
Valencia, as in the months with a WeMOi higher than +1 the precipitation may be as much as 140–160%
of normal, and in the months with an index lower than −1 it can be reduced to 60%. On the other hand,
variabilities in the south-west are also remarkable in October, November and December.
Summarizing, there are three areas with the greatest variability in precipitation based on the WeMOi extreme
values: the Bay of Biscay area with a direct proportion between pluviometry and the WeMOi; the Iberian
eastern façade, especially the area of the Gulf of Valencia, whose precipitation varies inversely to WeMOi;
and the Gulf of Cádiz, with a behaviour similar to the latter (Figure 11).
The meteorological stations whose precipitation reaches the highest correlation with the WeMOi are
Valencia, with −0.66 in December, and Bilbao, with +0.60 in January (Table II). Therefore, these points
constitute the extremes of a transect along which the maximum gradient lies in the correlation coefficient
43
42
WeMO +
WeMO -
41
40
39
oo
NAO -
38
37
36
-8
-6
-4
-2
0
2
Figure 9. Regionalisation of Iberian Peninsula following a pluviometric rise according to WeMO and NAO phases from October to
March
Table II. Pearson’s coefficients of correlation between the WeMOi, and monthly precipitation at Valencia and at Bilbao,
for the months of October to March, and absolute differences between them. (All the coefficients are significant at 0.01%,
see Figure 8 above)
r (Pearson’s
correlation coefficient)
October
November
December
January
February
March
Valencia
Bilbao
||
−0.36
+0.45
0.81
−0.45
+0.38
0.83
−0.66
+0.47
1.13
−0.37
+0.60
0.97
−0.48
+0.50
0.98
−0.36
+0.51
0.87
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
WESTERN MEDITERRANEAN OSCILLATION AND RAINFALL IN THE IBERIAN PENINSULA
WeMOi > +1
42
WeMOi < -1
80
100
42
16
140
160
0
180
80
80
1467
60
40
200
180
38
200
0
60
16
40
180
160
0
18
October
140
60
40
38
36
36
-6
-4
-2
0
-8
-4
-2
0
2
80
40
0
12
140
120
40
120
42
140
80
60
40
80
12
0
140
38
160
14
0
16180
0
38
60
November
-6
100
42
2
100
10
0
-8
36
36
-8
-6
-4
-2
0
100
42
80
-8
2
60
80
-6
42
-4
-2
0
12
0
140
2
140
160
18
0
40
40
60
80
140
0
16
160
0
36
18
38
60
38
180
0
2 2 00
2
140
December
12
0
40
36
-8
-6
-4
-2
0
100
42
120 100
80
-8
2
42
80
60
-6
-4
-2
100
0
0 14
12
100
2
0
40
12
0
60
60
80
140
160
140
38
38
36
36
-6
-4
-2
0
0
12
0
10
-8
42
-4
-2
60
0
2
120
140
160
80
100
40
80
20
80
40
36
100
180
160
140
120
38
80
38
-6
60
40
80
100
100
42
2
80
-8
February
120
40
0
10
36
-6
-4
-2
42
0
-8
2
120 100
140
-8
80
-6
-4
-2
0
80
42
10
80
January
160
80
0
2
120
60
0
12
0
0
40
10
140
10
80
40
100
12
60
0
March
40
10
0
100
38
80
0
10
38
36
36
-8
-6
-4
-2
0
2
-8
-6
-4
-2
0
2
Figure 10. Distribution of the pluviometric anomalies (in %) of the months with a WeMOi greater than +1.0 and less than −1.0 for
the 1910–2000 period
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
1468
J. MARTIN-VIDE AND J. LOPEZ-BUSTINS
Negative phase WeMO
Positive phase WeMO
43
42
41
40
39
38
37
36
43
42
41
40
39
38
37
36
-8
-6
-4
-2
0
2
-8
-6
-4
-2
0
2
Figure 11. Summary map of the main pluviometric anomalies (+, positive; −, negative) following the two phases of WeMO from
October to March
values. The months with the largest gradient are, from greater to lesser, December, February, January, March,
November and October. Consequently, winter is when the greatest pluviometric contrasts occur on the basis
of the WeMO influence.
For the entire winter period (December to March) the WeMOi has a high negative correlation with the
precipitation of Valencia, and a positive one with that from Bilbao. Besides, both precipitation series present
a slightly negative correlation (Figure 12(a)). The previous correlations are consistent during its temporary
evolution throughout the 1910–2000 period, in such a way that the negative linear trend of the WeMOi in
the winter period coincides with an increase of rainfall in Valencia (both are significant), and with a decrease
in Bilbao (Figure 12). Similarly, the mentioned WeMOi trend can be related to the increase in the surface
atmospheric pressure on the Po plain (Maugeri et al., 2004).
5. THE WEMOI AND THE TORRENTIAL RAINFALL ON THE MEDITERRANEAN COASTLINE
The low-frequency variability patterns are a result of temporary behaviours, which greatly surpass the day as a
unit of time. Therefore, its application at a daily resolution is hardly orthodox or feasible. At the most, periods
of approximately 10 days have been considered (Baldwin and Dunkerton, 1999, 2005), the temporary interval
from which atmospheric variability in the medium latitudes can no longer be explained using baroclinic,
intrinsically meteorological, processes, since they are more linked to low-frequency behaviours. In this section
and as a result of some satisfactory previous tests with daily pluviometric typologies in some Iberian regions
(López-Bustins and Azorı́n Molina, 2004), WeMO at a daily resolution is applied.
To check the functionality of the WeMOi at a daily resolution, seven observatories on the coast have been
chosen; five of them are located in the area of maximum influence of the WeMO, within the Iberian Peninsula
or its limits, Perpignan (France), Barcelona, Tortosa, Valencia and Torrevieja, and two towards the extremes
of the influence area, Marseilles in the north, and Málaga in the south. This covers the entire coastline of the
north-western Mediterranean basin, which is a critical strip in terms of occurrence of short torrential rainfall,
particularly in autumn.
The first analysis consisted in identifying the WeMOi for the dates on which at least one of the seven
meteorological stations registered rainfall higher than 100 mm during the 1951–2000 period. The second
analysis determined the probabilities of occurrence of 1 day with rainfall >0.1 mm and >10.0 mm with
extreme WeMOi values, both negative and positive, based on the same 50-year period.
5.1. Analysis of the values reached by the WeMOi on days with >100 mm
Fifty-six days have been detected where at least one of the meteorological stations involved registered
rainfall higher than 100 mm. This threshold has been used to distinguish a torrential episode in the Spanish
Mediterranean region (Armengot Serrano, 2002). The distribution of frequencies of the 56 corresponding
values of the WeMOi, show, as the most relevant result, the absence of cases in the positive value classes
(Figure 13). In other words, a WeMOi positive value excludes the risk of torrential rainfall, with the threshold
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
WESTERN MEDITERRANEAN OSCILLATION AND RAINFALL IN THE IBERIAN PENINSULA
1469
(a)
1910-2000 Winter (D-J-F-M)
WeMOi/Valencia precipitation
WeMOi/Bilbao precipitation
Valencia precip./Bilbao precip.
Pearson Coefficients
-0.6009
+0.5063
-0.3090
p-value
0.0000
0.0000
0.0030
(b)
Trends 1910-2000
Z/ 10 years
T-test for coefficient b1
(y = b0+b1x)
WeMOi
-0.049
Valencia pluviometry
+0.042
Bilbao pluviometry
-0.024
T= /-2.111/ > 1.988 (95%)
T=/+2.117/ > 1.987 (95%)
T=/-1.108/ <1.987 (95%)
(c)
2.0
1.5
1.0
0.5
y = +0.2175-0.0049x
0.0
z
WeMOi
-0.5
-1.0
-1.5
-2.0
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
2.0
1.5
1.0
y = -0.1827+0.0042x
0.5
z
Valencia
0.0
-0.5
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
2.0
1.5
1.0
0.5
z
Bilbao
y = +0.1033-0.0024x
0.0
-0.5
-1.0
-1.5
-2.0
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Figure 12. (a) Pearson’s coefficient of correlation between WeMOi and winter (from December to March) precipitation for Bilbao and
Valencia, and between their precipitation series (1910–2000 period). (b) Temporal trends of WeMOi, and of Valencia and Bilbao winter
precipitations (1910–2000 period). (c) Temporal evolution of WeMOi, and of Valencia and Bilbao precipitation for the same period
used for its definition, on the north-western coastline in the Mediterranean basin. As an example, three real
cases with more than 100 mm in Perpignan, Tortosa and Valencia, and with strong negative WeMOi, have
been selected (Figure 14). The class with the largest absolute number of cases is (−2, −1), even though this
does not mean a higher risk of occurrence of torrential rainfall for this interval of WeMOi values than with
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
1470
Number of the days with > 100 mm at least
in 1 of the 7 meteorological stations
J. MARTIN-VIDE AND J. LOPEZ-BUSTINS
25
20
15
10
5
0
<-4
[-4, -3]
[-3, -2]
[-2, -1]
[-1, 0]
[0, +1] [+1, +2] [+2, +3] [+3, +4]
>+4
Daily WeMOi
Figure 13. Histogram of WeMOi values of days with more than 100.0 mm in at least one of the seven meteorological stations with
daily data for the 1951–2000 period
110,3 mm in Tortosa with WeMOi –2.32
222,0 mm in Perpignan with WeMOi –2.96 165,0 mm in Valencia with WeMOi –2.27
Figure 14. Examples of synoptic real cases of days with heavy rains and a remarkable negative daily WeMOi value. (sea-level pressure
isobars are shown on the maps)
a value lower than −4. On the other hand, half of the cases in class (−1, 0) are associated with north-east
advections with WeMOi values not far from 0 (Figure 2) and they can cause abundant rainfall in the Valencia
region.
5.2. Analysis of the observed frequencies of occurrence in the 1951–2000 period of a rainy day with extreme
WeMOi values
Contrary to that carried out in the previous subsection, dates with extreme WeMOi values during the
1951–2000 period were chosen, for which values +4 and −4 have been set as the threshold, above and
under, respectively, an extreme daily WeMOi. This period contains 29 cases with a WeMOi lower than −4,
and 44 cases with a WeMOi higher than +4. The most relevant result is that the frequency of occurrence
of an amount of rainfall greater than or equal to 0.1 mm in 1 day with a WeMOi lower than −4, surpasses
82% in five of the seven meteorological stations subjected to analysis. The general frequencies of days with
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
WESTERN MEDITERRANEAN OSCILLATION AND RAINFALL IN THE IBERIAN PENINSULA
1471
rainfall equal to or greater than 0.1 mm are fourfold in Tortosa, Valencia, Torrevieja and Málaga (Table IIIa
and c). Equally, the frequency is maintained high, especially in Málaga and Perpignan, where it surpasses
62%, for an amount of rainfall greater than 10 mm. In the case of Torrevieja, the null value is related to the
scarce number of rainy days in the south-east of Spain.
For the WeMOi positive extreme values (> + 4.0), there is a dramatic reduction of the frequency of rainfall
with respect to the general, in the southern points (Valencia, Torrevieja and Málaga), until it becomes null in
Málaga with a threshold of 0.1 mm, and with 10 mm in this last city as well as in Barcelona, Valencia and
Torrevieja. On the other hand, the frequency of a day with rainfall equal to or greater than 0.1 mm increases
remarkably with respect to the general in the two French meteorological stations (Marseilles and Perpignan),
probably due to the still active pass of fronts through the Aquitaine plain in the south-west of France. Even
in the case of Perpignan, it is slightly superior to the frequency found with the negative extreme values.
However, these rainfall are nearly always weak, as the frequency of occurrence of a day with rainfall greater
than 10 mm is greatly reduced, becoming in general very inferior to those found for the days with WeMOi
negative extreme values in the seven points (Table IIIb and c).
The highest frequency for WeMOi < −4 and exceeding the pluviometric threshold of 0.1 mm has occurred
in Tortosa (93.1%), an observatory located at a central latitude between the seven stations studied, having,
together with Valencia, the winter precipitation most influenced by the WeMO, as seen in Section 4. Furthermore, Tortosa has the highest number of days with rainfall greater than 50 mm (77 episodes). The distribution
of the rainfall days according to the corresponding daily WeMOi in this meteorological station (Figure 15)
shows the neat influence of the negative index values in the occurrence of rainfall, both for all the amounts and,
very specially, for the highest ones, greater than 50 and 100 mm. Therefore, 69.8% of the 4071 rainfall days
for the 1951–2000 period were registered with negative WeMOi values (clearly surpassing 52.9% of the total
number of days for the period with a negative index). Nothing less than 96.1% of the daily rainfall registers
greater than 50 mm coincided with negative WeMOi values, and 100% of the amounts greater than 100 mm.
Table III. (a) Frequencies of occurrence of a rainy day (≥0.1 mm) and of a day with >10 mm at the seven meteorological
stations with daily data, in the cases of daily WeMOi less than −4 (period 1951–2000). (b) Same as in (a) with daily
WeMOi greater than +4. (c) General frequencies of occurrence of a rainy day (≥0.1 mm) and of a day with >10 mm
(a)
Daily
WeMOi < −4
≥0.1 mm
≥10 mm
Marseilles
(%)
Perpignan
(%)
Barcelona
(%)
Tortosa
(%)
Valencia
(%)
Torrevieja
(%)
Málaga
(%)
51.7
3.4
82.8
62.1
86.2
41.4
93.1
34.5
82.8
20.7
44.8
0
89.7
65.5
Marseilles
(%)
Perpignan
(%)
Barcelona
(%)
Tortosa
(%)
Valencia
(%)
Torrevieja
(%)
Málaga
(%)
40.9
2.3
84.1
13.6
4.5
0
29.5
2.3
2.3
0
2.3
0
0
0
Marseilles
(%)
Perpignan
(%)
Barcelona
(%)
Tortosa
(%)
Valencia
(%)
Torrevieja
(%)
Málaga
(%)
22
5.0
25
4.2
22
5.2
22
4.2
19
3.3
10
1.8
15
4.6
(b)
Daily
WeMOi > +4
≥0.1 mm
>10 mm
(c)
≥0.1 mm
>10 mm
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
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J. MARTIN-VIDE AND J. LOPEZ-BUSTINS
180
Precipitation (mm)
150
120
100 mm
90
60
50 mm
30
0
-6
-4
-2
0
2
Daily WeMOi
4
6
Figure 15. Statistical distribution of the daily WeMOi values for the rainy days of the 1951–2000 period at Tortosa
6. DISCUSSION AND CONCLUSIONS
The regions surrounding the Mediterranean Sea have a high and very complex orography that creates a basin
weakly connected with the Northern Hemisphere atmospheric dynamics in some specific territories, such as
the east strip of the Iberian Peninsula, leeward in respect to the Atlantic western flows. The NAO pattern, for
instance, has little influence on the climate variability in that area of Spain. The precipitation is much more
related to the Mediterranean systems of pressure and flows than to the Atlantic ones. Moreover, the weight
of the local factors on climate is very marked, due to the geographical factors (high altitudes, steep slopes,
etc.) as well as the latitude. The Mediterranean basin is under the subtropical anticyclone domain from late
spring to early autumn, producing low gradient sea-level pressure fields. In this context, some years, even
mesoescalic lows, account for much more precipitation than frontal depressions in many places of eastern
Spain.
The relative disconnection from general circulation and the importance of the local factors give marked
regional variations as well as a high interannual variability (coefficients of variation as high as 40% for
annual precipitation in south-eastern Spain). The application of downscaling methods are essential in the
Mediterranean area to get to know the climate variability at a good detailed resolution (Goodess and Jones,
2002; Palutikof et al., 2002). The search for specific low-frequency variability patterns over the Western
Mediterranean basin and the Iberian Peninsula, where Atlantic (zonal) and Mediterranean Sea influences are
met, can improve the knowledge about their climatic characteristics in detail.
The mean sea-level pressure maps for winter months in Western Europe show a very marked gradient
between Azores-Cádiz Gulf area and northern Italy-Adriatic-Central Europe region, drawing two centres of
action clearly. The eastern façade of the Iberian Peninsula, oriented towards the Mediterranean basin, is located
almost in the middle between these two regions. There the weak correlation of NAO with the precipitation is
the reason for the search for a new low-frequency variability pattern known as WeMO. Its definition is based
on the above-mentioned dipole covering the north-western sector of the Mediterranean basin. It is formed, in
its positive phase, by the high in Azores and the low in Liguria. The existence of good-quality long barometric
series in San Fernando and Padua allows the standard definition of its index (WeMOi) between 1821 and
2000.
The WeMOi shows a negative phase throughout the nineteenth century and a positive one in the twentieth
century up to its last third. This behaviour is consistent with the decreasing precipitation trend in San Fernando
and Gibraltar since the late nineteenth century (Wheeler and Martin-Vide, 1992). The WeMOi is negatively
correlated with the AO index, but not with the NAO’s. The MESA analysis also emphasises coincidences
between the WeMOi and the AOi regarding common and significant 5 and 22-year periodicities. More research
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
WESTERN MEDITERRANEAN OSCILLATION AND RAINFALL IN THE IBERIAN PENINSULA
1473
is needed in the future to explain this different relationship or influence of AO and NAO on WeMO. On the
other hand, the opposed phases of similar periodicities during the second half of the twentieth century between
AO and WeMO, hardly visible in the first half, seem to show an increasing modulation of the Mediterranean
pattern by the Arctic one, probably due to the reinforcement of the western flow in mid- and high-latitudes in
the last decades. Furthermore, a significant increasing sea-level pressure in northern Italy has been detected
during the twentieth century (Maugeri et al., 2004), which might contribute to justify the WeMO negative
trend.
Using data obtained in 51 meteorological stations during the 1910–2000 period, it has been proved by
means of a correlation analysis that winter precipitation in the eastern slope of the Iberian Peninsula, especially
in the Gulf of Valencia, increases significantly with WeMOi negative values, whereas the same happens with
that in the area close to the Bay of Biscay with positive values. This matches synoptically with the existence
of humid flows from the east and north-west, respectively. In any case, the WeMOi is better correlated than
the NAOi with the rainfall of a good part of the eastern coastline of the Iberian Peninsula, from October
to March. Therefore, whatever analysis of the precipitation variability in that part of Spain has to take into
account that the component from the Mediterranean Sea, expressed here by means of the WeMOi, contributes
more than the NAO dipole. The previous correlations allow a regionalisation of the Iberian territory in three
areas: (1) a large central and south-western area, negatively well correlated with the NAO; (2) the eastern
façade, with a negative correlation with the WeMO and (3) the eastern Cantabrian façade, positively correlated
with the WeMO. This result could be worthwhile for water management in Spain because the last two areas
have a very contrasted annual mean precipitation, from less than 300 mm in south-eastern Iberia, to more than
1500 mm in eastern Cantabria. On the other hand, it is worth noting that the winter precipitation variability
of the south-west of the Iberian Peninsula is mainly explained by the NAO. So the weight of the geographical
factors – here the position towards the west or the east – determines very much the characteristics of the
precipitation, although the influence of the mid- and high-troposphere is the same in the two southern Spanish
regions.
The WeMOi at daily resolution has allowed the achievement of interesting results in the analysis of torrential
rainfall, used as a hazard indicator. Therefore, the positive daily values of the WeMOi exclude the possibility
of occurrence of daily amounts higher than 100 mm on the coastline extending from Málaga to Marseilles.
The WeMOi with very negative values, lower than −4, shows remarkably high probabilities of a rainy day
and a day with rainfall greater than 10 mm, of over 80 and 60%, respectively, in some stations. This result
shows clearly that heavy rains in eastern Spain require a surface Mediterranean flow, as the negative values
of WeMOi indicate. So this index at daily resolution could contribute to a better public understanding of the
torrential rainfall in the above-mentioned area, given its simplicity.
The WeMOi values at monthly resolution are available in the web page of the Climatology Group, University
of Barcelona: http://www.ub.edu/gc/menu/htm.
ACKNOWLEDGEMENTS
Thanks are due to Dr Dario Camuffo, from the University of Padua (IMPROVE Project coordinator), who
suggested the idea of relating the atmospheric pressures of San Fernando and Padua. We thank the Institute
of General Applied Physics from the University of Milan, especially to Dr Fabio Monti and Dr Maurizio
Maugeri, for the supply of the Padua data homogenised at daily resolution. Thanks are also due to Dr Sergio
Vicente-Serrano, from the University of Zaragoza, who has provided the homogenised monthly precipitation
data from 51 observatories in the Iberian Peninsula. We also thank César Azorı́n Molina, from the University of
Alicante, for his suggestions to relate pluviometric typologies and daily WeMOi. This work has been carried
out within the framework of the Group of Climatology (2005SGR 01034, Regional Catalan Government)
and the Spanish IPIBEX Project REN2001-2865-C02-01/CLI, CGL2005-07664-C02-01/CLI. One of the coauthors has been granted a pre-doctoral scholarship by the Ministerio de Educación y Ciencia (Spain). Thanks
are also due to the anonymous reviewers for their useful comments.
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
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J. MARTIN-VIDE AND J. LOPEZ-BUSTINS
REFERENCES
Alexandersson H, Moberg A. 1997. Homogenization of Swedish temperature data. Part I: homogeneity test for linear trends. International
Journal of Climatology 17: 25–34.
Anagnostopoulou C, Flocas H, Maheras P, Patrikas I. 2004. Relationship between atmospheric circulation types over Greece and
Western-Central Europe during the period 1958–1997. International Journal of Climatology 24: 1745–1758.
Armengot Serrano R. 2002. Las lluvias intensas en la Comunidad Valenciana. Ministerio de Medio Ambiente: Spain; 263.
Azorı́n Molina C, López-Bustins JA. 2004. Catálogo sinóptico manual y causas atmosféricas de la precipitación en la provincia de
Alicante. Boletı́n de la AGE 38: 279–310.
Baldi M, Cesarone F, Carella F, Crisci A, Dalu GA. 2004. Mediterranean winter and fall climate: trends and mechanisms. EMS Annual
Meeting Abstracts 1: 00266.
Baldwin M, Dunkerton T. 1999. Propagation of the Artic Oscillation from the stratosphere to the troposphere. Journal of Geophysical
Research 104: 30.937–30.946.
Baldwin M, Dunkerton T. 2005. The solar cycle and stratosphere-troposphere dynamical coupling. Journal of Atmospheric and SolarTerrestrial Physics 67: 71–82.
Barriendos M, Martı́n-Vide J, Peña JC, Rodrı́guez R. 2002. Daily meteorological observations in Cádiz-San Fernando. Analysis of the
documentary sources and the instrumental data content (1786–1996). Climatic Change 53: 151–170.
Brunetti M, Maugeri M, Nanni T. 2002. Atmospheric circulation and precipitation in Italy for the last 50 years. International Journal
of Climatology 22: 1455–1471.
Camuffo D, Jones P. 2002. Improved Understanding of Past Climatic Variability from Early Daily European Instrumental Sources.
Kluwer Academic Publishers: Dordrecht, The Netherlands; 392.
Cocheo C, Camuffo D. 2002. Corrections of systematic errors and data homogenisation in the daily temperature Padova series
(1725–1998). Climatic Change 53: 77–100.
Conte M, Giuffrida S, Tedesco S. 1989. The Mediterranean oscillation: impact on precipitation and hydrology in Italy. In Proceedings
of the Conference on Climate and Water, Vol. 1. Publications of Academy of Finland: Helsinki; 121–137.
Corte-Real J, Zhang X, Wang X. 1995. Large-scale circulation regimes and surface climatic anomalies over the Mediterranean.
International Journal of Climatology 15: 1135–1150.
Douguédroit A. 1998. Que peut-on dire d’une oscillation Méditerraneénne? In Climate and Environmental Change, Alcoforado MJ (ed.).
Proceedings of the Conference of the IGU Commission on Climatology: Evora, Portugal; 135–136.
Dünkeloh A, Jacobeit J. 2003. Circulation dynamics of Mediterranean precipitation variability 1948–98. International Journal of
Climatology 23: 1843–1866.
Esteban P, Jones PD, Martin-Vide J, Mases M. 2005. Atmospheric circulation patterns related to heavy snowfall days in Andorra,
Pyrenees. International Journal of Climatology 25: 319–329.
Estrela MJ, Millán M, Peñarrocha D, Pastor F. 2002. De la gota frı́a al frente de retroceso. Las precipitaciones intensas en la Comunidad
Valenciana. CEAM: Valencia; 260.
Goodess CM, Jones PD. 2002. Links between circulation and changes in the characteristics of Iberian rainfall. International Journal of
Climatology 22: 1593–1615.
Grbec B, Morović M, Zore-Armanda M. 2003. Mediterranean Oscillation and its relationship with salinity fluctuation in the Adriatic
Sea. Acta Adriat 44: 61–76.
Group of Climatology. 2005. WeMO Monthly Data. University of Barcelona: Barcelona, Spain; http://www.ub.es/gc/English/wemo.htm.
Halpert MS, Bell GD. 1997. Climate Assessment for 1996. Bulletin of the American Meteorological Society 78: S1–S49.
Hurrell JW. 1995. Decadal trends in the North Atlantic Oscillation: regional temperatures and precipitation. Science 269: 676–679.
Hatzaki M, Flocas H, Maheras P, Asimakopoulos D. 2004. On the study of teleconnection patterns affecting the Mediterranean region.
EMS Annual Meeting Abstracts 1: 00105.
Jones PD, Jonsson T, Wheeler D. 1997. Extension to the North Atlantic Oscillation using early instrumental pressure observations from
Gibraltar and south-west Iceland. International Journal of Climatology 17: 1433–1450.
Krichak SO, Alpert P. 2005. Decadal trends in the East Atlantic-West Russia pattern and mediterranean precipitation. International
Journal of Climatology 25: 183–192.
Kutiel H, Paz S. 1998. Sea level pressure departures in the Mediterranean and the relationship with monthly rainfall conditions in Israel.
Theoretical and Applied Climatology 60: 93–109.
Kutiel H, Maheras P, Guika S. 1996. Circulation indices over the Mediterranean and Europe and their relationship with rainfall conditions
across the Mediterranean. Theoretical and Applied Climatology 54: 125–138.
López-Bustins JA, Azorı́n Molina C. 2004. Aplicación del ı́ndice diario de la Oscilación del Mediterráneo Occidental al estudio de la
tipologı́a pluviométrica en Alicante. El Clima, entre el Mar y la Montaña. AEC: Santander; 333–345.
Maheras P, Xoplaki E, Kutiel H. 1999a. Wet and dry monthly anomalies across the Mediterranean basin and their relationship with
correlation, 1860–1990. Theoretical and Applied Climatology 64: 189–199.
Maheras P, Xoplaki E, Davies T, Martin-Vide J, Barriendos M, Alcoforado MJ. 1999b. Warm and cold monthly anomalies across the
Mediterranean basin and their relationship with circulation; 1860–1990. International Journal of Climatology 19: 1697–1715.
Marquardt C, Naujokat B. 1997. An update of the equatorial QBO and its variability. 1st SPARC Gen Assembly, Vol. 814. WMO/TD:
Melbourne; 87–90.
Martin-Vide J. 2004. Spatial distribution of a daily precipitation concentration index in peninsular Spain. International Journal of
Climatology 24: 959–971.
Martı́n-Vide J, Fernández D. 2001. El ı́ndice NAO y la precipitación mensual en la España peninsular. Investigaciones Geográficas 26:
41–58.
Martı́n-Vide J, Olcina J. 2001. Climas y tiempos de españa. Spanish, Climates and Weathers in Spain. Alianza Editorial: Madrid.
Martin-Vide J, Lopez-Bustins JA. 2004. The Western Mediterranean Oscillation (WeMO) and its influence in the Eastern Spanish
rainfall. A methodological approach. EMS Annual Meeting Abstracts 1: 00166.
Maugeri M, Brunetti M, Monti F, Nanni T. 2004. Sea-level pressure variability in the Po plain (1765–2000) from homogenized daily
secular records. International Journal of Climatology 24: 437–455.
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc
WESTERN MEDITERRANEAN OSCILLATION AND RAINFALL IN THE IBERIAN PENINSULA
1475
NCEP/NCAR. 2005. Reanalysis Project at the NOAA-CIRES Climate Diagnostics Center. (http://www.cdc.noaa.gov/cdc/reanalysis/
reanalysis.shtml).
NOAA. 2005. National Weather Service. (http://www.cpc.ncep.noaa.gov/data/teledoc/telecontents.shtml).
Palmieri S, Siani AM, Casale GR, Meloni D. 2001. Climate fluctuations in the mediterranean. 22th ISODARCO Summer Course on:
“Global Climate Changes and Impact on Natural Resources”: Candriai, Trento; (contributed paper).
Palutikof JP. 2003. Analysis of Mediterranean climate data: measured and modelled. In Mediterranean Climate-Variability and Trends,
Bolle HJ (ed.). Springer-Verlag: Berlin; 133–153.
Palutikof JP, Goodess CM, Watkins SJ, Holt T. 2002. Generating rainfall and temperature scenarios at multiple sites: examples from
the Mediterranean. Journal of Climate 15: 3529–3548.
Palutikof JP, Conte M, Mendes JC, Goodess CM, Esprito Santo F. 1996. Climate and climatic change. In Mediterranean ClimateVariability and Trends, Bolle HJ (ed.). Springer Verlag: Berlin; 133–153.
Piervitali E, Colacino M, Conte M. 1997. Signals of climatic change in the central-western mediterranean basin. Theoretical and Applied
Climatology 58: 211–219.
Pozo-Vázquez D, Esteban-Parra MJ, Rodrigo FS, Castro-Dı́ez Y. 2000. An analysis of the variability of the North Atlantic Oscillation
in the time and the frequency domains. International Journal of Climatology 20: 1675–1692.
Rodó X, Baert E, Comin FA. 1997. Variations in seasonal rainfall in Southern Europe during the present century: relationships with the
North Atlantic Oscillation and the El Niño-Southern Oscillation. Climate Dynamics 13: 275–284.
Soler X, Martı́n-Vide J. 2002. Los calendarios climáticos. Una propuesta metodológica. El Agua y el Clima. AEC: Mallorca; 577–586.
Štepánek P. 2004. AnClim-Software for Time Series Analysis. Department Of Geography, Faculty of Natural Sciences, Masaryk
University: Brno.
Supić N, Grbec B, Vilibć I, Ivančić I. 2004. Long term changes in hydrographic conditions in northern Adriatic and its relationship to
hydrological and atmospheric processes. Annales Geophysicae 22: 733–745.
Thompson WJ, Wallace JM. 2000. Annular modes in the extratropical circulation. Part I: month-to-month variability. Journal of Climate
13: 1000–1016.
Trenberth KE. 1984. Signal versus Noise in the Southern Oscillation. Monthly Weather Review 112: 326–332.
Vicente-Serrano SM, Beguerı́a Portugués S. 2004. Analysis of the spatial patterns of drought magnitude in the Iberian Peninsula by
means of the Standardized Precipitation Index (SPI) and the Generalized Pareto distribution. EMS Annual Meeting Abstracts 1: 00175.
Wheeler D, Martin-Vide J. 1992. Rainfall characteristics of Mainland Europe’s most Southerly Stations. International Journal of
Climatology 12: 69–76.
Xoplaki E, González-Rouco JF, Luterbacher J, Wanner H. 2003. Mediterranean summer air temperature variability and its connection
to the large-scale atmospheric circulation and SSTs. Climate Dynamics 20: 723–739.
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 1455–1475 (2006)
DOI: 10.1002/joc