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  • View in gallery

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    (a) Economic losses from damaging hydrological events worldwide (from APFM) and (b) expected value of the expected cost as a function of different daily rainfall threshold values (Martina et al. 2006; modified).

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    Geographical setting and morphology of the Mediterranean basin. MCA (large square box) and Sicily (small box) are highlighted (base map from the International Water Management Institute; available online at http://www.iwmi.cgiar.org).

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    (top) Gumbel plot (GEV based) of the autumn precipitation vs return period over MCA in (a) 1801–40 (cold period), (b) 1881–1920 (transitional period), and (c) 1970–2009 (latest and warmer period). The interpolating curve is reported with 95% confidence intervals for the most uncertain return periods (latest interpolation and extrapolated periods). (bottom) The respective 98th percentile maps are compared [dataset sources: Pauling et al. 2006; KNMI (available online at http://www.knmi.nl/about_knmi), with updating until 2009].

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    Rainfall rate anomalies for the decade 2000–09 compared to the period 1951–99 over the MCA (from NCEP reanalysis and NOAA/ESRL): (a) autumn and (b) September. For both autumn and September, average amounts were calculated dividing total rainfall by the number of days with measurable rainfall.

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    (a) Spatial pattern of rainfall anomalies in the months of September of the decade 1998–2008 compared to the respective monthly climatology 1951–99 (arranged from the TRMM earth science data; see also Acker and Leptoukh 2007); (b) rainfall amount that occurred on 16 Sep 2009 (~2-km grid, from Meteosat Second Generation rainfall estimation; available online at http://www.cespevi.it/previs2.htm).

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    Temporal pattern of precipitation rate over Sicily: (a) September trend over 1948–2009 and (b) monthly regimen during the decade 2000–09 (gray line), compared to the climatological period 1948–99 [bold line; arranged by NCEP reanalysis and NOAA-ESRL (available online at http://www.cdc.noaa.gov/cgi-bin/PublicData/getpage.pl); Kalnay et al. 1996].

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    SPI: (a) 3-month-based SPI evolution (1973–2009) for September at Palermo and (b) map of SPI in September 2009 over Mediterranean area (arranged from the European Commission Joint Research Centre Institute for Environment and Sustainability; available online at http://desert.jrc.ec.europa.eu).

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Storminess and Environmental Changes in the Mediterranean Central Area

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  • 1 Met European Research Observatory, GEWEX-CEOP Network, World Climate Research Programme, Benevento, Italy
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Abstract

Earth ecosystems are not static, and they respond to environmental changes, particularly climatic and anthropogenic. Precipitation varying in its extremeness, with shifts to greater or lesser intensity of individual storms and/or to change in the length and frequency of wet and dry periods, can adversely affect both urban and rural ecosystems. Here, the authors review long-term precipitation records of the central Mediterranean area and employ a Web geographical information system (GIS)-based analytical approach to compare current rainfall impact with historical data on different spatial and temporal scales. Autumn (September–November) was recognized as the most hazardous season that marks the evidence of a changing climate, with a shift toward more intense rainfalls in recent times. In the first decade of the third millennium, areas of peninsular and insular Italy have been especially affected by extreme rains. A focus was put on the island of Sicily, where extraordinary rain events occurred in September 2009, discussed in the context of upcoming trends and climate histories. An improved knowledge and understanding of the scale at which changes on extremes occur is essential for dealing with the forthcoming challenges regarding soil and water conservation practices. The characteristics of changes in natural rainfall, its role on terrestrial ecosystems, and its effect on surface water erosion dynamics are discussed. It is argued that understanding these issues are major priorities for future research to promote a better understanding of the Earth interaction with water resources and related hydrological issues.

*Corresponding author address: G. Bellocchi, Met European Research Observatory, via Monte Pino, 82100 Benevento, Italy. giannibellocchi@yahoo.com

Abstract

Earth ecosystems are not static, and they respond to environmental changes, particularly climatic and anthropogenic. Precipitation varying in its extremeness, with shifts to greater or lesser intensity of individual storms and/or to change in the length and frequency of wet and dry periods, can adversely affect both urban and rural ecosystems. Here, the authors review long-term precipitation records of the central Mediterranean area and employ a Web geographical information system (GIS)-based analytical approach to compare current rainfall impact with historical data on different spatial and temporal scales. Autumn (September–November) was recognized as the most hazardous season that marks the evidence of a changing climate, with a shift toward more intense rainfalls in recent times. In the first decade of the third millennium, areas of peninsular and insular Italy have been especially affected by extreme rains. A focus was put on the island of Sicily, where extraordinary rain events occurred in September 2009, discussed in the context of upcoming trends and climate histories. An improved knowledge and understanding of the scale at which changes on extremes occur is essential for dealing with the forthcoming challenges regarding soil and water conservation practices. The characteristics of changes in natural rainfall, its role on terrestrial ecosystems, and its effect on surface water erosion dynamics are discussed. It is argued that understanding these issues are major priorities for future research to promote a better understanding of the Earth interaction with water resources and related hydrological issues.

*Corresponding author address: G. Bellocchi, Met European Research Observatory, via Monte Pino, 82100 Benevento, Italy. giannibellocchi@yahoo.com

1. Introduction

There is growing evidence at global, regional, and local scales that intra-annual precipitation regimes have already become more extreme (Easterling et al. 2000; Groisman et al. 2005; Knapp et al. 2008). However, the uncertainty of climate information poses challenges for the analysis of observed rain data because the heaviest areas of precipitation may fall between recording stations (Willmott and Legates 1991). For instance, assertions about extreme precipitation changes may be more reliable for regions with dense networks because of the small radius of correlation for many intense rainfall events (Groisman et al. 2005). Few literature sources are available worldwide regarding the extreme precipitation, especially about rainstorm effects on terrestrial ecosystems and water resources (e.g., Clarke and Rendell 2007; Curtis et al. 2007; Zolina et al. 2009). This also poses another question related to the development of dynamic hydrological models, which is hampered by incomplete understanding of spatially varying processes and the lack of adequate datasets to spatially characterize varying rain inputs.

According to Wei et al. (Wei et al. 2009), secular records of historical precipitation data are required to deal with long-term studies and cross-site comparisons. This is especially so in the Mediterranean region, where human pressure and erratic rainfall patterns with marked interannual variability expose landforms to exacerbated, damaging hydrological processes (van Leeuwen and Sammons 2003; Sánchez et al. 2004; van Rompaey et al. 2005) and also to spur the emergence of new hazards, such as coastal and urban flooding (Papathoma and Dominey-Howes 2003; Barroca et al. 2006). Mediterranean storms and cyclones tend to be characterized by short life cycles, with average radius ranging from 300 to 500 km (after Lionello et al. 2006), many of which are a combination of both frontal and convective rainstorms. Heavy flooding and storms occurring at Mediterranean sites were found to be characterized by a complex property, known as multifractality, which is the spatial distribution organized into clusters of high rainfall localized cells embedded within a larger cloud system or clusters of lower intensity (Mazzarella 1999).

For the Holocene—the geological epoch started around 10 000 BCE and continues to the present—there is continued debate about the relative impact of anthropogenic activity, but there is also increasing recognition that Mediterranean-type ecosystems should not be regarded as fragile, degraded landscapes but instead as disturbance-adapted systems (Allen 2003). Nevertheless, conservation measures face increasing challenges from contemporary human forcing and rainstorm events. The increase in sealed surfaces resulting from changes in land use together with a decrease in forest cover has increased the frequency and size of storm runoff too, causing flooding, mudflows, and landslides. These events may be grouped in some particularly rainy years or months according to storms climatic variability over interannual to century scales (García-Oliva et al. 1995; D’Odorico et al. 2001; Peterson et al. 2002; Cavazos and Rivas 2004). Rainfall variables such as depth, duration, and intensity (and its erosive power) are becoming more and more changeable (Diodato and Bellocchi 2009a) and with a regimen that would still markedly change over time and space in the following years (Richard 2007).

The environmental and economic impact of these events can be high in several regions (Kunkel et al. 1999; Alcántara-Ayala 2002), especially in agricultural and river-torrential areas (Thornes and Alcántara-Ayala 1998; Camarasa Belmonte and Segura Beltrán 2001; Ramos and Mulligan 2003). The European southernmost regions are especially more sensitive to erratic rainfalls and are currently threatened by land degradation leading to a lowering in water resource availability and agricultural productivity (Poesen and Hooke 1997). Over the 1980–90 decade, for example, flash floods caused billions of euros of damage in Europe (Gruntfest and Handmer 2001). This is also evident with more recent data that show (Figure 1a) the extent to which economic losses from worldwide hydrological disasters tend to increase [World Meteorological Organization Advanced Programme on Flood Management (APFM); available online at http://www.apfm.info/index.htm].

In Figure 1b, one can see instead how the shape of expected-value cost of disasters affected by a given cumulated volume of rainfall during a storm event (i.e., 50 mm day−1) separates decreasing cost from increasing cost. This is so because dry periods are associated with expected-value costs related to environmental drought stress, whereas excessive rains (wet periods) turn into expected-value costs associated with multiple damaging hydrological events. Threshold behavior manifests itself as an indicator of economic losses, and its characteristics are of importance for understanding and extrapolating the dynamics and stability of climate systems (Pitman and Stouffer 2006), the dynamics and resilience of geoecosystems, and the dynamics of fluxes in hydrologic systems (Zehe and Sivapalan 2009). Despite the general recognition that land degradation is a serious and widespread problem in Mediterranean countries, the same has not been quantified for many locations, and its geographical distribution and real extent are not accurately known, because prevailing studies only target subregional scales or isolated places (Clarke and Rendell 2007).

There is a need to update and estimate the current overall hazard related to rainstorms and to assess in what measure hazardous rainstorms evolved with climate variability. The present work aims at prospecting, for the central Mediterranean and for multiple spatial and temporal scales, rain-intensity trends related to recorded extreme rainfalls. To achieve this goal, precipitation data were extracted from the Web with the extension of Internet-based geographic information system (Web GIS), currently available at different spatial and temporal scales and with the support of graphical external functionalities. This review was necessary because, with the advent of weather digital networks, many datasets are indeed archived and publicly open, but often only short records (latest years) are available at subregional and local scales. Two of the most prominent reanalysis data sources that may serve the purposes of time–spatial modeling are the National Oceanic and Atmospheric Administration (NOAA)/National Climatic Data Center and National Aeronautics and Space Administration (NASA) Goddard Earth Sciences Data and Information Services Center. Review of online meteorological resources that are basic to prospective studies of climate variability is provided in the next section of this paper, together with a description of the study area. Section 3 assesses temporal variability of extreme rainfall observations (from 1801 to 2009) to reveal whether climate change may have implications on the extreme rainfall hazard. The case of Sicily (Italy), the largest island in the Mediterranean Sea and centrally situated in the basin, was examined with more detail (section 3.2) because its lands have been the theatre of storms of exceptional intensities in the autumn of 2009. Section 4 discusses possible implications between extreme rain events and their ecological impacts and concludes this paper.

2. Study area and data

2.1. Description of the study area

The study area, hereafter called the Mediterranean central area (MCA), is located between 30°–45°N, 8°–20°E (Figure 2, large square box). The area of interest is approximately centered over the Tyrrhenian Sea, between the western coast of the Italian mainland and the northern coast of Sicily. Westward, the area is delimited by southeastern France and northeastern Algeria. The Alps mountain chain and the Ligurian Sea are relevant geographic features to the north. The MCA extends eastward over two small basins (the Adriatic Sea and the Aegean Sea) up to the Balkans and includes former Yugoslavia, Albania, and a large part of Greece. The Strait of Sicily is a bridge between Tunisia and northern Libya (delimiting the southern margin of the MCA) and southern Europe, and it also subdivides the Mediterranean Sea into a western and an eastern basin. Water masses are indeed exchanged through both the Strait of Gibraltar (between Spain and Morocco) and the Strait of Sicily by eastward and westward flows. Water masses originating in the east of the basin flow westward and penetrate the Adriatic Sea and the Ionian Sea; on approaching the Strait of Sicily, part of the water mass is recirculated back within the eastern basin, whereas the remainder continues to enter the western Mediterranean basin (Rohling et al. 2009).

Two broad areas of the Mediterranean region were left out of this study for their distinct climate patterns (Lionello et al. 2006). In a western area, including the Iberian Peninsula and Morocco (with the Atlas and Rif Mountains) and forming part of the eastern Atlantic–European region, orographic cyclogenesis is often triggered by the passage of Atlantic cyclones. Instead, areas of intense cyclogenic activity occur in the eastern Mediterranean region (including parts of Greece and Turkey) around Cyprus and the Middle East, to a large extent controlled by a large-scale Atlantic–European region.

In the MCA, cyclogenesis is most frequent over the Gulf of Genoa and the rest of Ligurian Sea, but the Aegean Sea is a major center for cyclogenesis as well (e.g., Trigo et al. 1999). The majority of Genoan depressions tracks southeastward down the coast of Italy and then eastward or northeastward across the Aegean Sea. The variegated morphology of the MCA (basins and gulfs, mountainous groups, and peninsulas of various sizes) has important consequences on both sea and atmospheric circulations, which determine a nonuniform distribution of weather types (Lionello et al. 2006) and a large spectrum of associated hydrogeomorphological events (Petrucci and Polemio 2003; Sivakumer 2005). Three principal impact ways can be described for precipitation (Diodato 2004): 1) very large-scale and low-intensity continuous rainstorms, 2) large frontal heavy thunderstorms with associated large erosive phenomena, and 3) short-period and localized convective rain showers leading to flash floods and mud flows.

2.2. Generation of precipitation data

The rainfall datasets used in this study were derived from official records of data collected from a variety of national and international agencies for the Mediterranean region. The data used are mostly from the Royal Netherlands Meteorological Institute (KNMI) Web site (available online at http://climexp.knmi.nl), which is a gateway to a number of online global regional databases and supplies the Web GIS KNMI Climate Explorer (van Oldenborgh and Burgers 2005), which is applied in this study to arrange for reanalysis fields and climate change experiments. We have accessed data archived at the NOAA/National Paleoclimatology Program (available online at http://www.ncdc.noaa.gov/paleo), the NOAA/Earth System Research Laboratory (ESRL) Physical Sciences Division Web site (Boulder, Colorado; available online at http://www.esrl.noaa.gov/psd), and the NASA Goddard Earth Sciences Data and Information Services Center (available online at http://disc2.nascom.nasa.gov). NOAA provides consistent long-term weather observations and has the most complete and updated global database of online weather records of daily resolution (grid resolution of about 100 km). Paleoclimatology data with monthly resolution come from natural sources (e.g., tree rings, ice cores, corals, and ocean and lake sediments) and extend the archives back to hundreds of years (e.g., Pauling et al. 2006) with grid resolutions of about 50 km.

Specifically, reanalysis data are provided by the NOAA/ESRL Physical Sciences Division Web site. Since 1998, NASA archives and distributes high-resolution (~25 km) gridded precipitation data from the Tropical Rainfall Measuring Mission (TRMM) platform, providing satellite-based information on the intensity and distribution of the rain type on the storm depth and on the height at which the snow melts into rain (e.g., Huffman et al. 2007). For particular and localized years or events that occurred in Italy, suitable datasets are those derived from regional monitoring centers. In this study, we integrated data detected by the Sicilian Agrometeorological Information Service (SIAS; available online at http://www.sias.regione.sicilia.it) to refer precipitation data relative to the island of Sicily.

3. Extreme rainfall changes

By comparing daily satellite observations with model data during a 20-yr period, Richard Allan and his collaborators at Reading University recently confirmed that heavy precipitation events are more frequent during warm, moist periods and less frequent during cold, dry periods (press release available online at http://www.reading.ac.uk/about/newsandevents/releases/PR16410.asp). To verify if the above hypothesis could be valid for a time long enough to include the period between the last part of the Little Ice Age (period of cooling confined to approximately the sixteenth century to the mid-nineteenth century; Lamb 1977) and the current phase of warming, we have compared some particularly wet decades that have affected the Mediterranean central basin since the early nineteenth century.

3.1. Mediterranean central area

The work of Casty et al. (Casty et al. 2007) on the European pattern climatology (1766–2000), which has been the basis for the development of this study, indicates no clear trend for precipitation totals in central Mediterranean when aggregated at annual and seasonal scales. However, from reviewing and updating that study for autumn (September–November) precipitation, we turned to a different elaboration of data, capturing the rainfall amounts via generalized extreme value (GEV) distributions as Gumbel plots (e.g., Coles 2001) and comparing three time slices of about four decades each (Figures 3a–c), reflecting a cold period (1801–40), a transitional period (1881–1920), and a warmer period (1970–2009).

From these plots, it is evident a gradual exacerbation of the extreme rainfalls over fixed return periods during the last two centuries. This trend was also emphasized by the 98th percentile–based maps (Figures 3a–c, top).

Although the above illustration delineates three distinct climate periods, most of the greater hydrological impacts have occurred in recent years, which is known to be the result of unusual deluges occurred with extraordinary intensity. A more enlarged view of the Mediterranean central area over the decade 2000–09 shows that autumn seasons are in fact prone to intensified precipitation rates with positive anomalies over many zones (Figure 4a). This agrees with the results of Cislaghi et al. (Cislaghi et al. 2005), who found that the average rain rate significantly increased in the 1800–2000 period in Italy, with associated shorter durations of rain episodes and with an evident effect on rainfall extremes. This is reflected in the surprising result of the extent to which temperature rise widens precipitation extremes. According to Maheras et al. (Maheras et al. 1999), the widespread distribution of high temperatures over the Mediterranean tends to be associated with a negative sea level pressure anomaly in the Mediterranean basin that produces southwesterly flow from the ocean into the western Mediterranean, which may penetrate eastward or allow warmer air from North Africa to influence eastern parts of the basin.

Anomalous warm conditions in parts of the Mediterranean and central Europe were also recognized by Luterbacher et al. (Luterbacher et al. 2007) as related to advection of warm air masses from the eastern subtropical Atlantic as well as strong anticyclonic conditions over large parts of the continent. Increased temperatures may result in nearly unchanged (but decreasing) precipitation totals but with a shift toward heavy, intense rainfalls (Alpert et al. 2002). In this context, it is relevant to learn how the past warming affected the changes of precipitation extremes (Klein Tank and Können 2003). The major increase of precipitation rates was observed, especially affecting the month of September over peninsular Italy, where the anomaly was about +1.5 mm day−1 with an elliptical core reaching the island of Sicily (Figure 4b).

3.2. The case of Sicily

As noted in Figure 4, the rise of rainfall is not limited to Italy, but also includes the island of Sicily. A focus on this island was meant to display the influence exerted by the power of rainfall on the climate regime (Figure 5). The rainfall amount anomalies in the recent decade (1998–2008), compared to the climatological period 1951–99 (Figure 5a), show that only the central part of Sicily (includes the Etna volcano and the cities of Enna and Caltanissetta) is apparently not affected by rainfall increase.

The positive anomalies depicted in Figure 5a are often the result of rainfalls concentrated in very few days because those occurred in September 2009, when the monthly rain amount registered at the Palermo Astronomical Observatory (38°N, 13°E), 230 mm, exceeded the previous monthly record of 190 mm dating back to 1820 (V. Iuliano 2009, personal communication; http://www.astropa.unipa.it). To give an idea of the magnitude of the extreme events that characterized September 2009, we also illustrate the rainfall occurred on the 16th day of the same month, when a storm depth of about 100 mm was recorded over and around Palermo district and south of the town of Messina (38°N, 15°E), with a main storm core of 200 mm, northeast of Palermo (Figure 5b).

The extraordinary nature of the situation was the result of repeated deluges that afflicted several lands of Sicily in September 2009, combined with those occurred in the months of September of past years. For instance, deluges were recorded on 17 September 2003 at some stations of Syracuse province, reaching about 600 mm of rain, 398 mm of which fell in only 6 h. Phenomena patterns at these localities show that subgrid-scale convection and intensification are dominating the rain-producing mechanisms (Mazzarella 1999; Dünkeloh and Jacobeit 2003) and are shared with several rain showers releasing in few hours as much energy as equal to or higher than the annual amount. This is in agreement with the results of Bonaccorso et al. (Bonaccorso et al. 2005), by which subgrid-scale convection and intensification phenomena indicate for Sicily an increasing trend toward shorter rain durations (about 1 h) during the period 1927–2004.

In Figure 6a, time evolution is shown of daily precipitation rates averaged over Sicily during the September months from 1948 to 2009. The graph shows changes ramping only during the last decade according to a climate shift. This indicates an accelerated power of rainfall with severe and important impacts on ecosystems because tilled soils in the beginning of autumn are expanding with increased vulnerability of lands. Based on Figure 6b, it can be argued that the intensifying precipitation affecting the Sicilian lands could not be limited to the month of September only, but could also affect the months of November and December (arrows in Figure 6b). It is also noteworthy to mention the violent storms that struck Sicily between 1 and 2 October 2009. On this occasion, the province of Messina was the most affected, with Fiumedinisi being the rainiest site with 159 mm on 1 October (SIAS data).

To interpret the regional patterns of rainfall–climate interaction over Sicily in the context of the Mediterranean area, the evolution of the standardized precipitation index (SPI; McKee et al. 1993) was also examined (Figure 7), as arranged from the European Commission Joint Research Centre (Ispra, Italy; available online at http://www.jrc.it). SPI is based on the probability of recording a given amount of precipitation occurring over a given prior time period (which may vary from 1 to 36 months), and the probabilities are standardized so that a value of zero indicates the median precipitation amount. The index is negative for drought and positive for wet conditions, and becomes more negative or positive as the dry or wet conditions become more severe. For the month of September, the yearly evolution of SPI values calculated at Palermo over the prior 3 months (Figure 7a) indicates manifestations of climate change resulting from a gradual (although irregular) shift from dry to wet conditions. The year 2009 is remarkable after a previous peak observed in 2000. For September 2009, the map of SPI over the Mediterranean area (Figure 7b) indicates large areas of dryness at northern and central latitudes, whereas signals of wetter climate are apparent at most sites of the southern Mediterranean, including Sicily. This evidence indicates an unusual reversal of precipitation patterns between north and south in response to increased warmth.

4. Summary and environmental implications

Earth ecosystems are not static and respond to environmental changes on many scales and from a variety of causes, particularly climatic and anthropogenic, which for the Mediterranean area can be among the most severe worldwide (Allen 2003). This paper presents an approach, based on the investigation of past and current extreme rain events, to deepen the consequences of this underappreciated aspect of climate change over the Mediterranean central area. Reviewing online precipitation resources was basic to this research in view of prospective studies of climate variability. Historical datasets available for the Mediterranean area were not always updated until today. The datasets used in this study to explain over 200 years of precipitation range from rain gauges, reanalysis data, and satellite inferences. The National Centers for Environmental Prediction (NCEP) reanalysis represents an updated source providing relatively coarse spatial data (about 1° × 1°), not allowing in-depth studies at a subregional level. To overcome such a limitation, we based some analyses on the TRMM platform, a high-resolution source also updated in time.

Where possible, some comparisons between these two main sources showed substantially similar results (data not shown) that support using the two sources together in this climatological study. Other sources referenced in our study (e.g., regional monitoring centers) are minor datasets used in support to the two main sources at either early or late assessment.

Without limiting our study to investigate important rain extremes recently occurred (e.g., September 2009), attempts were made to place them in the context of the climate history of the MCA. Precipitation is, in fact, a key link to the global water cycle, and a proper understanding of its temporal and spatial character will have broad implications in ongoing climate diagnostics and predictions, global water and energy cycles, analysis and modeling, weather forecasting, freshwater resource management, and land–atmosphere–ocean interface processes (Shepherd and Burian 2003).

Our study relies on the importance of concepts such as sequence and position, as well as magnitude and frequency of storm events, stemming out of Wolman and Miller (Wolman and Miller 1960) and employed by Richards (Richards 1999). Because climatic events (such as droughts and storms) are sometimes clustered into longer-term groups, their impacts may not vary simply in relation to their size, thus complicating any simple magnitude and frequency relationship. Climatic variability is recognized as one key cause of these clusterings; thus, identification of modes of climatic variability occurring at the decadal scale going back into the Holocene may help testing the effectiveness of different rain events within real event histories and within the broad sweep of environmental and ecological history (Viles and Goudie 2003).

If the Little Ice Age is identified as the stormiest period of the last four to five centuries in the Mediterranean, important historical climate fluctuations call for high-time-resolution studies to assess seasonal changes of rain extremeness and their relation to soil conservation (e.g., Diodato et al. 2008). Based on the findings of the present study, focused on September months, alternating centuries of limited and highly hazardous rainfalls spaced out by a transitional century seem to have occurred in the recent past (three centuries) at the Mediterranean central area as a response to the changing temperatures. In a previous study on the environmental implications of erosive rainfall across the Mediterranean (Diodato and Bellocchi 2009a), large erosive rainfalls were observed to occur, especially in summer (continental areas) and autumn (along-coast and near-coast reliefs). It was also shown, from detailed explorations of temporal patterns of erosivity (Diodato and Bellocchi 2009a; Diodato and Bellocchi 2009b), that increasing the number of extreme events in autumn does not cause seasonal rainfall totals to deviate from the historical range of climate variation, but it tends to generate more disproportion between Mediterranean dry and wet periods, which could bring soil loss to higher rates (Diodato and Bellocchi 2008). If this rainfall regime were to continue, it could result in ever-increasing exacerbated erosive hazard affecting Mediterranean countries in an erratic way.

The magnitude and even the direction of climate change may differ among regions, so it appears reasonable to focus on a single region in attempting to assess probable effects of climate change when that region has some intrinsic meaning. Based on its central position in the Mediterranean, we have identified the island of Sicily as a special case for this study. Placed at the crossroads for Mediterranean flows, Sicily can be considered representative of a geographic scenario of regional climate change associated with warming, reflected in the Mediterranean cyclogenicity, and that would have contributed to the exceptional rainfall rates observed in recent times. During September, practically the whole island has experienced remarkable positive anomalies in the first decade of the twenty-first century in comparison to the monthly climatology, with a trend toward increasing daily rates. Also, similar evidence in other months indicates that precipitation patterns in this region are actually on the verge of important changes. This is also indicated by the information conveyed by the evolution of the SPI at individual Sicilian places (e.g., Palermo) and over the Mediterranean region.

Following all of this evidence, we argue that the assessment of current and future management systems should not only be based on the average rainfall for a period, but it should also include the hazard of extreme precipitation events, which likely cause accelerated urban and rural land degradation. This must be considered together with the findings that, although rainfall amounts are not always increasing, erratic spatial and temporal storm patterns in some seasons or months drive the erosive power of rain to increase its hazard. According to D’Asaro et al. (D’Asaro et al. 2007), investigations are needed, especially in terms of statistical analysis of long pluviometric series, rain intensity, and erosivity occurring at decadal and interdecadal scales, which mark substantial climatic fluctuations and changes during last centuries. More generally, identification of links between nonlinear climatic, ecological, and geomorphological systems should lead to search for increased understanding of their mutual interactions and behavior and not to use parts of them in simple cause–effect relationships. Quantitative reconstruction of the impacts of climate change on organisms and ecosystems represents an important and challenging line of enquiry. Improved conceptualization of the impacts of decadal-scale climatic variability can help interpreting the projections of the ecological impacts of future climate change. The identification of climate boundaries (areas with homogeneous climate) of rain scenarios at regional scales (as the MCA in this study) is important for understanding the climatic features and their potential impact on ecosystems.

From all the evidence presented in this paper and in the recent literature, it becomes apparent that variability of events in time will be an important part of the environmental future of the MCA. Rain intensity and erosive power on decadal and interdecadal scales remain to mark substantial climatic fluctuations and changes during the last centuries, although the perception of disasters by public domain is not only determined by objective data but also from the relevance that media give to it (Lastoria et al. 2006). This perception is central to the process of the resource allocation for the risk mitigation; in general, it somehow influences the type and entity of measures taken by authorities and people involved in flood risk management. An increased understanding of the properties of decadal and interdecadal climatic variability also puts us in a better position to understand the possibilities and limitations of extrapolating future climate events and impacts. The results gained from the present study encourage further studies on climatic change by simulation models and statistical approaches to incorporate the characteristics offered by finer time and spatial scales.

Acknowledgments

This work is part of the Hydrological Cycle in the Mediterranean Experiment (HYMEX; available online at http://www.hymex.org) aiming at hydrometeorological modeling for the whole Mediterranean basin.

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Figure 1.
Figure 1.

(a) Economic losses from damaging hydrological events worldwide (from APFM) and (b) expected value of the expected cost as a function of different daily rainfall threshold values (Martina et al. 2006; modified).

Citation: Earth Interactions 14, 5; 10.1175/2010EI306.1

Figure 2.
Figure 2.

Geographical setting and morphology of the Mediterranean basin. MCA (large square box) and Sicily (small box) are highlighted (base map from the International Water Management Institute; available online at http://www.iwmi.cgiar.org).

Citation: Earth Interactions 14, 5; 10.1175/2010EI306.1

Figure 3.
Figure 3.

(top) Gumbel plot (GEV based) of the autumn precipitation vs return period over MCA in (a) 1801–40 (cold period), (b) 1881–1920 (transitional period), and (c) 1970–2009 (latest and warmer period). The interpolating curve is reported with 95% confidence intervals for the most uncertain return periods (latest interpolation and extrapolated periods). (bottom) The respective 98th percentile maps are compared [dataset sources: Pauling et al. 2006; KNMI (available online at http://www.knmi.nl/about_knmi), with updating until 2009].

Citation: Earth Interactions 14, 5; 10.1175/2010EI306.1

Figure 4.
Figure 4.

Rainfall rate anomalies for the decade 2000–09 compared to the period 1951–99 over the MCA (from NCEP reanalysis and NOAA/ESRL): (a) autumn and (b) September. For both autumn and September, average amounts were calculated dividing total rainfall by the number of days with measurable rainfall.

Citation: Earth Interactions 14, 5; 10.1175/2010EI306.1

Figure 5.
Figure 5.

(a) Spatial pattern of rainfall anomalies in the months of September of the decade 1998–2008 compared to the respective monthly climatology 1951–99 (arranged from the TRMM earth science data; see also Acker and Leptoukh 2007); (b) rainfall amount that occurred on 16 Sep 2009 (~2-km grid, from Meteosat Second Generation rainfall estimation; available online at http://www.cespevi.it/previs2.htm).

Citation: Earth Interactions 14, 5; 10.1175/2010EI306.1

Figure 6.
Figure 6.

Temporal pattern of precipitation rate over Sicily: (a) September trend over 1948–2009 and (b) monthly regimen during the decade 2000–09 (gray line), compared to the climatological period 1948–99 [bold line; arranged by NCEP reanalysis and NOAA-ESRL (available online at http://www.cdc.noaa.gov/cgi-bin/PublicData/getpage.pl); Kalnay et al. 1996].

Citation: Earth Interactions 14, 5; 10.1175/2010EI306.1

Figure 7.
Figure 7.

SPI: (a) 3-month-based SPI evolution (1973–2009) for September at Palermo and (b) map of SPI in September 2009 over Mediterranean area (arranged from the European Commission Joint Research Centre Institute for Environment and Sustainability; available online at http://desert.jrc.ec.europa.eu).

Citation: Earth Interactions 14, 5; 10.1175/2010EI306.1

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