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FRONTAL ATLANTIC RAINFALL COMPONENT IN THE WESTERNMEDITERRANEAN
BASIN. VARIABILITY AND SPATIAL DISTRIBUTION
Mara J. Estrela Navarro2, Javier Mir Prez
1, Francisco Pastor Guzman
1and Milln Milln Muoz
1
Laboratorio de Meteorologa-Climatologa, Unidad Mixta CEAM-UVEG.
E-mail: [email protected]. Fundacin Centro de Estudios Ambientales
del Mediterrneo 2. Departament de Geografa, Universitat de
Valencia
Recent climate change studies indicate a positive trend in
precipitation fororthern Europe whereas the trend could be negative
in Southern Europeanountries. However, these results for total
precipitation values can hide wethere variations are connected with
changes in the type of weather conditionsroducing precipitation.
The authors have developed a novel technique for thesaggregation of
the precipitation series depending on its synoptic setting orrigin.
The results of their application in Valencia, while confirming
theownward trend in total precipitation, show important differences
regarding thepe of precipitation, whether coming from Atlantic
frontal systems, convective
torms or Mediterranean advections.Subsequently, we have extended
this methodology to the Westernediterranean basin to analyze the
current situation and future behaviour ofater regimes in the area.
The Western Mediterranean basin has been dividedto four main
sectors (east of the Iberian Peninsula, Southern France,
Italianeninsula and Northern Africa) where we have applied the
methodologyeveloped for the Valencian Region.This paper presents
the results obtained for precipitation associated to fronts
f Atlantic origin; results show a very marked decrease in this
type of rainfall fore whole Western Mediterranean basin.
ntroduction
Bibliography
Milln M, Estrela MJ, Mir J. 2005. Rainfall components:
variability and spatial distribution in a mediterranean area
(Valencia Region). J. Cl(14): 2682-2705
Pearrocha D, Estrela MJ, Milln M. 2002. Classification of daily
rainfall patterns in a mediterranean area with extreme intensity
levels: theRegion. Int. J. Climatology. 22: 677-695. D-14070
Alexandrov T. (2006): Software package for automatic extraction
and forecast of additive components of time series in the
frameworCaterpillar-SSA approach. PhD thesis, St.Petersburg State
University.
Alexandrov, T. and Golyandina, N. (2005): Thresholds for methods
of automatic extraction of time series trend and periodical
components withof the Caterpillar-SSA approach. In: Proc. 4th Conf.
System 8 Identification and Control Problems. Inst. of Control
Sci., Moscow, 1849 1
Schnwiese, C.D.; Rapp, J. (1997). Climate trend atlas of Europe
based on observations 1891-1990. Kluwer Academic Publishers.
DordrechtEstrela, M.J., Mir, J., Pastor, F., Milln, M. (2004).
Precipitaciones por frentes atlnticos en la Comunidad Valenciana:
cambios y tendencia
ltimas dcadas.XXVIII Jornadas Cientficas de la Asociacin
Meteorolgica Espaola. 11-13 Febrero 2004. Badajoz.Elsner, J.B.,
Tsonis, A.A (1996). Singular Spectral Analysis. A New Tool in Time
Series Analysis. Plenum Press.
Acknowledgments
This work has been funded by the Spanish Ministerio de Educacin
y Ciencia through the projects GRACCIE (Programa CONSOLIDER-2010
CSD2007-00067), NIEVA (CGL2008-04550) and complementary action
CGL2007-29847-E/CLI and by the European Comission througIntegrated
project CIRCE (Proj. No. 036931-2). Fundacin CEAM is funded by the
Generalitat Valenciana and Fundacin Bancaja.
Data base
A database on daily precipitation in coastal and near
coastalareas of the western Mediterranean basin has been
collectedfor the period 1950-2007 (except North Africa). The
selectedstations belong to the South of France, Italy, North of
Tunisia,Northern Algeria and Eastern Spain. In the Valencian Region
amore comprehensive rainfall series from AEMET is availablewhile
for the rest of the basin we have used data from theGlobal Surface
Summary of Day Data (GSOD), managed bythe National Climatic Data
Center (NCDC). This database,albeit with lower resolution, offers
daily resolution data sets longenough for the Western
Mediterranean. A total of 125 stationson the Mediterranean sector
of the Iberian peninsula (mostly inthe Valencia Region), 45
stations in Italy, 15 in South of France,
12 in northern Algeria, 6 in North Tunisia and 2 in
NortheasternMorocco have been incorporated into the database.For
the extraction of trends the authors have used the
procedure of Singular Spectrum Analysis. This methodsusefulness
lies in its ability to extract information from theseries,
separating into different fields the trend component,oscillatory
components, and random noise. Thus the series isdecomposed in basic
additive patterns using singular filtersmatched to the data by the
covariance matrix.
Regionalization and synoptic analysisThe Western Mediterranean
basin has been split into areas where precipitation is
caused by a given synoptic setting, bearing in mind the windward
or leeward orientationof the main flow and the coastal or inland
characteristics of the area. As a result we haveobtained a first
division of the basin in four main areas for which the synoptic
pattern ofeach input is the same:
1. Eastern Iberian Peninsula and Balearic Islands2. Southern
France and Northeastern extreme of the Iberian Peninsula3. Italian
Peninsula, Corsica, Sardinia and Southwestern half of Sicily4.-
Northern Africa (Northeast Morocco, North of Algeria and
Tunisia).
Each sector is divided into areas where statistical analysis is
applied and where
correlations between synoptic settings and daily precipitation
data are investigated.These subdivisions are:
Sector 1: 1a) Iberian Mediterranean coastal strip (except
northeastern Catalonia), 1b) Iberianinterior sector, 1c) Balearic
Islands
Sector 2: 1a) French Mediterranean coast and central and North
Catalonia coast, 1b) InlandSoutheastern France and central
Pyrenees
Sector 3: 3a) Western Italian coast and western side of Corsica
and Sardinia, 3b) Central ItalianPeninsula strip. 3c) Adriatic
coast of Italy, 3d) Eastern seaboard of the islands of Corsica
andSardinia, 3rd) sector of Sicily and southern Italy in contact
with the Ionian Sea.
Sector 4: 4a) West coast of Algeria and Morocco's Mediterranean
coast. 4b) East coast ofAlgeria, northern Tunisia and
North-Northeast facade of Sicily. 4c) Transition band to the Sahara
inthe interior of North Africa and southern Tunisia.
Main fluxes by area
esults
Italian PeninsulaIn most sectors of this area a cleardecreasing
trend is observed forprecipitation coming from westernfrontal
systems. Only the southernsector shows evidence of a veryweak, not
so clear, decrease,while in the central area (3b) thereis a weak
decline for much of thedata period but a rise in recent
years.
Stations map
Regionalization of the Western Mediterranean basin
Eastern Iberian Peninsula: A decreasing trend isobserved in
precipitation coming from Atlantic frontalsystems in both coastal
and interior sectors.
Southern France:No significant trend is found in the interior
areas of this sector; nevertheless, a decreasein western frontal
systems in the coastal areas is observed.
orthern frica: In the western sector of the North African coast
aght downward trend is observed while in the eastern and
interioreas there is a slight upward trend.
ONCLUSIONS
the framework of various research projects the authors have
studied the precipitation regimes in the Western Mediterranean
basindisaggregating daily data series by synoptic weather
situations that cause precipiation. For this purpose, the Western
Mediterranean
s been divided in four main areas, which in turn are subdivided
into sectors according to their character as coastal or
inlandatersheds and according to their location upwind or downwind
of the major wind flows. The four main areas are defined as East of
theerian Peninsula, Southern France, Italian Peninsula and islands
and North Africa.
his paper shows the results for the zonal advections, rainfall
related to the western component coming from Atlantic frontal
systems.ased on the overall results, a decrease in both rainfall
amounts and regional precipitation peaks is found for the whole of
the Westernediterranean basin. This downward trend can be seen in
most areas (and sectors within the areas) throughout the whole
westernediterranean basin to a greater or lesser extent, except in
the central sectors of Italy and North Africa.
the case of North Africa, a slight increase in precipitation
caused by situations from the West is found in its eastern and
interior partshile its western area shows a similar behaviour to
the general decline in the whole basin. It should be noted that
both spatial andmporal coverage of data sets in this area may
question these results so it is necessary to get longer data series
to review andepen this study. The case of the central sector of
Italy is more remarkable because precipitation in this area shows a
clear rise in this
pe of rainfall at the end of the study period although during
the previous years it had remained unchanged or went down slightly,
itould thus be of interest to study the mechanisms that produced
the precipitation rise in this area.
1a
1b
4a 4b 4c 3d 3e
3c
3b
3a2a 2b
