Changes in seasonal precipitation in the Iberian Peninsula during 1946–2005
Introduction
The global hydrological cycle in the region of the Mediterranean basin is dominated by very high spatial and temporal variability. Thus, despite the fact that Mediterranean climate types are characterized by a summertime dry season and wintertime maximum rainfall, in certain areas of the Western Mediterranean basin, the seasonal rainfall regimes become equinoctial ones, with maximum precipitation in spring and autumn (Lionello et al., 2006).
Seasonal rainfall regimes in the Iberian Peninsula (IP) have been described as showing great contrast, with this variability being explained by factors such as the western position of the IP, its location between two contrasting water masses, the orography or the storm tracks (Martin-Vide and Gil Olcina, 2001). Thus, De Castro et al. (2005) and De Luis et al. (2008) describe how, from the total of 24 possible permutations among seasonal dominant precipitation seasons, 12 are found over Spanish conterminous land. The spatial distribution of such regimes is as follows: autumn is the main precipitation season along the East Mediterranean coastland, the north and west of the IP is under a winter maximum, transitional inland areas present a maximum in spring time and summer is the main precipitation season in two specific areas of the eastern Pyrenees and the highland plateau of the Iberian mountains. This complex map of seasonal rainfall regimes is basically accounted for by the concurrence of (1) an oceanic component from the Atlantic, which becomes more notable as it moves westwards, and which brings winter rains associated with circulation from the west; (2) a component from the Mediterranean, which is more evident as it moves towards the east, bringing autumn rains resulting from the cyclogenetic processes of this sea; and (3) an Iberian component, inland on the Peninsula, which strengthens rainfall in spring, and under certain conditions relating to the relief, also in summer, due to convection in the warm half of the year (Martin-Vide and Gil Olcina, 2001).
In the IP, this spatial variability in seasonal precipitation regimes is overlapped by clear, but complex, patterns of temporal variability and traditionally described space domains of seasonal rainfall regimes in Spain may, therefore, change over time (Esteban-Parra et al., 1998, Romero et al., 1998; González-Hidalgo et al., 2001; De Luis et al., 2009a).
Exploring spatial and temporal changes in seasonal precipitation regimes in areas of high spatial and temporal variability requires high-data spatial coverage in order to define transitions between sectors accurately. The new Spanish database, MOPREDAS (Monthly Precipitation Database of Spain) (González-Hidalgo et al., 2010), constitutes a suitable tool for the purpose because of its temporal record and its spatial density. The MOPREDAS precipitation series covers a 60-year period (1946–2005) and therefore enables comparison between two independent 30-year periods (1946–1975 and 1976–2005). Furthermore, MOPREDAS has a high spatial density, with 2670 complete and homogeneous stations, i.e., a global average of 1 station per 185 km2 (Fig. 1).
The objectives of this paper involve evaluating the seasonal precipitation trends over Spanish conterminous land with the use of the MOPREDAS database and determining whether these trends are modifying seasonal rainfall regimes in the study area.
The new MOPREDAS database was set up following an exhaustive quality control process designed to detect suspicious data and inhomogeneous series. Thus, a total of 2670 series was checked, filled and reconstructed from December 1945 to November 2005 (60 years), in which the percentage contribution of the original data is 69.2 %; 21.7% of the data were reconstructed from information from neighbours less than 10 km apart; finally 9.1% of data were reconstructed using information from neighbours lying between 10 and 25 km from the candidate series (details on database construction can be found in González-Hidalgo et al., 2009, González-Hidalgo et al., 2010).
The mean distance to the nearest neighbouring station is less than 10 km (i.e., 1/10 degree longitude and latitude) with minor variations among the different elevation intervals, except for areas above 1500 m a.s.l. As a consequence, MOPREDAS is the densest database for monthly precipitation ever created on the IP for the December 1945 to November 2005 period (González-Hidalgo et al., 2010).
To facilitate spatial analysis of precipitation, stations were interpolated onto a regular grid. The grid resolution chosen was equal to the mean distance between the 2670 selected stations (10 km, i.e., 1/10 degree longitude and latitude). Before interpolating the station data onto the grid cells, we converted each seasonal series into multiplicative anomalies, normalizing each seasonal value by its average estimated over the entire period.
We used a radial weight to weight the stations involved in the estimation of each grid point. Apart from the radial weight, we also used an angular weight, which accounts for the geographical separation among the sites with available station data. Since the time series averaged are, in general, unevenly distributed in space, the angular separation between the sites of the interpolated time series is taken into account by avoiding over-weighting of information from the same sector: well-separated time series are more strongly weighted than nearby ones. This leads to the information that converges into a grid cell being more spatially isotropic and less affected by areas with a higher density of stations. The final gridded data set consists of 5334 cells covering the whole of Spain. Previous analyses have confirmed the internal consistency and coherence of the database. Details of this procedure can be also found in González-Hidalgo et al. (2010).
In order to highlight any change in the seasonality of precipitation, we also investigated the evolution of precipitation distribution over the year. For this purpose, we created a second grid. The first step involved converting all the seasonal precipitation station series into de-trended and normalized series, by dividing each seasonal value by the current-year total amount. Thus, the sum of the seasonal values is always 1 for each year of the series, and the changes in the seasonal relative contributions to total annual amount can be studied independently from the trend in the yearly precipitation amount. Consequently, these data series were interpolated on the same 0.1° resolution grid, following the same technique used for construction of the previous grid.
Seasonal analyses were performed following the common procedure for winter (December–January–February), spring (March–April–May), summer (June–July–August) and autumn (September–October–November) (De Luis et al., 2009a). We evaluated the trends of seasonal precipitation series and their percentage contribution to total annual precipitation using the Mann–Kendall test with different levels of probability (see Table 1). Finally, in order to explore the stability of the traditionally described seasonal regimes, we calculated the seasonal precipitation regimes for two 30-year independent periods (1946–1975 and 1976–2005).
Section snippets
Seasonal precipitation trends (1946–2005)
The global results of seasonal precipitation trends (expressed as a percentage of the land affected) are shown in Table 1, along with different levels of probability. The trends are mostly negative in winter, spring and summer and positive in autumn.
Winter precipitation decreased in the whole of conterminous Spain during the study period, except along the Mediterranean coast and in isolated areas of the northern plateau, which returned a positive signal (Fig. 2). The areas with the strongest
Discussion
We found spatial variability in precipitation trends in Spanish conterminous land, but the different temporal periods and spatial density coverage hinders comparison with previous research.
Thus, significant trends have not been detected over the whole Iberian Peninsula, either in winter or autumn, by Sotillo et al., 2006, Valero et al., 2009 between 1961 and 2003. In Mediterranean regions, however, de Luis et al. (2009a) observed an increase in winter precipitation in coastal areas during the
Acknowledgements
The Spanish Government, Contract Grant Sponsor CGL2008-05112-C02-01 and DGA (Aragón Regional Govt.), Consolidated Research Group “Clima. Agua, Cambio Global y Sistemas Naturales” (BOA 69, 11-06-2007) supported this study. This research was also conducted within the framework of the Spanish RECABA Project CGL2008-06129-C02-01 and the Climatology Group of the University of Barcelona(2009 SGR 443, Catalonia Regional Govt.). We wish to thank AEMET (Spanish National Meteorological Agency) for
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