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1 2 Chapter 6 3 4 5 Cyclones in the Mediterranean Region: 6 7 Climatology and Effects on 8 the Environment 9 10 1 2 3 2 4 5 11 P. Lionello, J. Bhend, A. Buzzi, P.M. Della-Marta, S.O. Krichak, A. Jansa` , 6 7 8,9 9 12 P. Maheras, A. Sanna, I.F. Trigo, and R. Trigo 13 1 14 University of Lecce, Italy ([email protected]) 2 15 University of Bern, Switzerland ([email protected], [email protected]) 3ISAC-CNR, Italy ([email protected]) 16 4Tel Aviv University, Israel ([email protected]) 17 5INM Spain ([email protected]) 18 6University of Thessalonoki, Greece ([email protected])PROOF 19 7ARPA, Piemonte, Italy ([email protected]) 20 8Instituto de Meteorologia, Centro de Geofı´sica da, Universidade de Lisboa, 21 Portugal ([email protected]) 9 22 CGUL at University of Lisbon and Universidade Luso´fona Portugal (rmtrigo@ 23 fc.ul.pt) 24 25 26 27 28 29 6.1. Introduction 30 31 Cyclones represent the most important manifestation of the mid-latitude high- 32 frequencyUNCORRECTED variability, and play a fundamental role in the atmospheric large-scale 33 horizontal (and vertical) mixing and in modulating the air–sea interaction. 34 Cyclonic circulations, due to their frequency, duration and intensity, play an 35 important role in the weather and climate over the entire Mediterranean region 36 (Radinovic, 1987). A large spectrum of environmental variables and phenomena 37 are associated with cyclones in the Mediterranean region. Wind, pressure, 38 temperature, cloudiness, precipitation, thunderstorms, floods, waves, storm 39 surges, landslides, avalanches, air quality and even the fog and visibility in the 40 Mediterranean are influenced by the formation and passage of cyclonic 41 disturbances. Many phenomena are beneficial from the agricultural, hydrological File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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42 and economical point of view. However, some of them are damaging and 43 occasionally disastrous (MEDEX, Jansa et al., 2001). 44 Cyclones are important when information more complete than that provided 45 by the average geopotential (or sea level pressure) field is required and when 46 aspects of the probability distribution characterizing the statistics of the atmo- 47 spheric circulation besides its average pattern are investigated. Consequently, 48 statistical analysis of cyclones is important especially for the ‘‘tails’’ of probability 49 distributions, that is the part characterizing extremes values of variables such 50 as precipitation, winds, waves, storm surges. The Mediterranean area, although 51 located to the south of the main Atlantic storm track that more directly affects 52 western and northern Europe, is quite frequently subjected to sudden events of 53 extreme and adverse weather, often having high social and economic impacts. 54 The morphology of the territory, with small and steep river basins and highly 55 populated, industrialized and tourist areas, makes the Mediterranean especially 56 sensitive to the impact of weather phenomena, especially heavy rain and con- 57 sequent flooding. A report (based on 10 years of data) prepared by the Munich 58 Reinsurance Company collects 166 cases of heavy rainfall and floods and 59 104 cases of strong wind and storms producingPROOF serious damages. The total 60 number of deaths is over 1,900 and the quantified economic losses are over 61 6,000 MEuro. These figures are certainly underestimates. For Spain alone, and 62 only in four years (1996–1999), the Programme of Natural Hazards of the 63 Spanish Directorate of Civil Defence reported 155 deaths by heavy rain and 64 flood events and 28 deaths by storms and strong winds (Jansa et al., 2001a) 65 in Greece according to the data published by the Hellenic Agricultural Insurance 66 Organization (ELGA) only for the year 2002 the economic losses due to heavy 67 precipitation, floods, hail and extreme winds were over 180 MEuro. Single 68 disastrous events have been recorded, such as the storm of 4 November 1966 69 which hit central and north eastern Italy, causing more than 50 deaths and 70 widespread, huge damages in the eastern Alps, Florence and Venice (where 71 the damage produced by the surge is estimated to be equivalent to 400 KEuros 72 present-day). Over the sea, significant wave height as large as 10 or 11 m are also 73 reportedUNCORRECTED in extraordinary storms, like the 10–11 November 2001 storm in the 74 Western Mediterranean, that produced the destruction of beaches and coastal 75 flooding in northern Mallorca (Gomez et al., 2002) and the big storm at the 76 end of December 1979, that seriously damaged the port of Oran (Algeria). 77 More examples can be found in the MEDEX list of selected cases, Jansa et al. 78 (2001a). A large portion of such severe weather-related events are associated 79 with cyclones in the Mediterranean (Jansa et al., 2001a). 80 Since the capability of climate models to reproduce intense cyclones and 81 extreme weather events is limited, it is necessary to determine the links 82 between their probability distribution and large-scale patterns or/and indices. File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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83 The identification and analysis of the large-scale patterns associated to the 84 occurrence of cyclones is very important. In fact, as climate models are more 85 accurate in reproducing the large-scale structure of atmospheric circulation 86 than the statistics of cyclones and extreme events, the prediction of the behaviour 87 of large-scale patterns in future climate scenario could be a robust tool for 88 predicting changes in the intensity and characteristics of cyclones and extreme 89 events. 90 It is important to note that the link between the intensity of cyclones and 91 hazardous extreme events is not simple and different characteristics can be 92 involved as different impacts are considered. The intensity of circulation (winds), 93 precipitation (with consequent floods) and of the cyclone itself (measured as 94 the minimum value of the sea level pressure or the strength of the overall 95 associated circulation) are not necessarily related in a simple (linear) way. 96 An example is the disastrous flood which affected central and Northern Italy 97 during November 1966, characterized by very intense precipitation and high 98 winds, which but whose central minimum pressure was not remarkably low 99 (De Zolt et al., 2005). 100 Finally, Mediterranean cyclones have also an influencePROOF on regions outside their 101 region of origin. Radinovic (1987) suggests that cyclones in the Mediterranean 102 region influence the weather and climate further east in central Europe, in 103 countries such as Hungary, Romania, Ukraine and Russia, and in Asian areas, 104 like Syria, Iraq, Iran, Afghanistan or northern India. At the same time, besides 105 the large amount of cyclones entering the Mediterranean region from the middle 106 latitude storm track, there is evidence of a significant role played by tropical 107 cyclones, which can produce atmospheric circulation patterns advecting moisture 108 into the Mediterranean region and, occasionally, move into it during a later 109 stage of their life cycle after having experienced a transition to extratropical 110 systems (Pinto et al., 2001; Turato et al., 2004). 111 The introductory Section 6.1 of this chapter consists of Subsection 6.1.1 112 describing the evolution of research on cyclones in the Mediterranean region 113 and their role on climate. Section 6.2 describes the dynamics responsible for the 114 formationUNCORRECTED and evolution of cyclones in the Mediterranean region (Subsection 115 6.2.1), the datasets available for their analysis (Subsection 6.2.2) and the methods 116 for the identification of cyclones and the evaluation of the intensity of their 117 activity (Subsection 6.2.3). Section 6.3 describes the climatology of cyclones, 118 their characteristic spatial scales, seasonality, area and mechanisms of origin 119 (Subsection 6.3.1); the relation between cyclonic activity and large-scale 120 climate patterns (Subsection 6.3.2); and the observed trends (Subsection 6.3.3). 121 Section 6.4 describes the effects of cyclones on the Mediterranean environment; 122 it is divided into five subsections, describing their role on precipitation, winds, 123 storm surge, ocean waves and landslides. For each subsection, a description of File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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124 the mechanism explaining the effects of cyclones on the specific phenomenon 125 is described, and climate trends are discussed. The concluding Section 6.5 126 summarizes the available knowledge on cyclones and on their effects in the 127 Mediterranean region. The outlook section 6.6 discusses the main open research 128 issues and subjects of ongoing and future research. 129 130 131 6.1.1. ‘‘Historical’’ Notes 132 133 Pioneering studies that include climatology of cyclones and cyclogenesis in the 134 Mediterranean (in the frame of hemispheric studies) are those by Pettersen (1956) 135 and Klein (1957). They are large-scale studies and show, at even this scale, that 136 the western Mediterranean is a distinct and very active area of cyclogenesis 137 and with frequent presence of cyclones in winter, in the northern Hemisphere. 138 These studies are based on hand-made analyses and subjective detection of the 139 cyclones. Subsequent studies, using the same techniques, were able to investigate 140 smaller scales, even including mesoscale features (Radinovic and Lalic, 1959; 141 Radinovic, 1978; Genoves and Jansa, 1989). The annualPROOF total number of cyclones 142 detected in the mesoscale studies (Radinovic, 1978 or Genoves and Jansa, 1989) 143 was, as expected, much larger than the number of cyclones found in larger 144 scale analyses. Since 1990 (Alpert et al., 1990), most of the studies on climatology 145 of the Mediterranean cyclones are based on objective analyses and objective 146 techniques aimed at detecting and tracking the cyclones, but there are also 147 studies based on mixed databases, subjective and objective, like Campins et al. 148 (2000). Several studies have focused on the characteristics of the cyclones in the 149 Mediterranean area, on their dynamics, locations, frequency and temporal 150 variability of cyclogenesis (Buzzi and Tosi, 1989; Tosi and Buzzi, 1989; Alpert 151 et al., 1990a,b; Trigo et al., 1999; Campins et al., 2000; Maheras et al., 2001, 2002, 152 Lionello et al., 2002). 153 A different category of studies have analysed the link between cyclones 154 and environment. Most studies have been focused on precipitation (Trigo et al., 155 2000; JansaUNCORRECTED et al., 2001; Kahana et al., 2002; Maheras et al., 2002, 2004), but also 156 other aspects such as storm surges, (Trigo and Davies, 2002; Lionello, 2005), 157 and wind waves (Lionello et al., 2002) have been considered. 158 The impact of extreme weather conditions on landslides occurrence has been 159 attempted over different areas of the Mediterranean. Recent techniques tend to 160 favour the use of satellite and airborne imagery to assess changes in geomorphol- 161 ogy. However the use of historical data obtained form in situ analysis plus 162 local newspapers and interviews is a more reliable tool to establish precise links 163 between the timing of intense rainfall events and concurrent observed landslide 164 episodes (e.g. Zeˆ zere et al., 1999; Polemio and Petrucci, 2003; Trigo et al., 2005) File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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165 6.2. Mediterranean Cyclones: Data, Methods and Dynamics 166 167 This section contains a description of the dynamics responsible for formation 168 and evolution of cyclones in the Mediterranean region, and of data and metho- 169 dologies used for analysing their climatology. 170 171 172 6.2.1. Dynamics of Cyclones in the Mediterranean Region 173 174 The cyclones in the Mediterranean region represent a well-distinct element of 175 the global climate. A Mediterranean storm track structure has been put into 176 evidence in different studies and it has been shown that the regular cyclone tracks 177 over the Mediterranean are linked to the baroclinic waveguides (Wallace et al, 178 1988) and to high rate of alternation between cyclones and anticyclones 179 (Pettersen’s, 1956) in the Mediterranean region (Alpert, 1989). The presence of 180 a separate branch of the Northern Hemisphere storm track crossing the 181 Mediterranean region, with areas of more frequent cyclogenesis in the western 182 Mediterranean and of cyclolysis in the central and easternPROOF Mediterranean, though 183 less intense than the storm track in the Atlantic and Pacific, has been confirmed 184 in recent analysis of the Northern hemisphere (Hoskins and Hodges, 2002). 185 At the same time, there are studies showing that the Mediterranean region 186 presents the highest concentration of cyclogenesis in the world (Pettersen, 1956; 187 see also Radinovic, 1987, for a general view) during the northern hemisphere 188 winter. Some of them are so intense that they are classified as ‘‘meteorological 189 bombs’’ (Conte, 1986; Homar et al., 2000). 190 According to the conceptual model re-proposed by Hoskins et al. (1985), 191 cyclogenesis occurs when and where a high-level PV (Potential Vorticity) positive 192 anomaly overlaps a low-level potential temperature or PV positive anomaly or a 193 frontal zone. The formation of low-level shallow depressions by orography and 194 thermal contrasts is very frequent in the Mediterranean, owing to the complex 195 topography of the region. Therefore hundreds of cyclonic disturbances can be 196 subjectivelyUNCORRECTED and objectively detected in the Mediterranean every year (Campins 197 et al., 2000; Picornell et al., 2001). Actually many of them are shallow depres- 198 sions which cannot be considered deep cyclones, but, they can play a role in 199 the initiation of actual deep cyclogenesis events. There are three kinds of 200 evolution that can be identified in the Mediterranean: (a) The low-level 201 disturbance does not evolve and remains shallow, weak or moderate and 202 nearly stationary if the upper level PV anomaly is absent or too far away to 203 interact with it (Genoves and Jansa, 1991). In this case real or deep cyclogenesis 204 does not develop. (b) An upper level PV anomaly will create cyclogenesis 205 when arriving over a frontal zone, just under the maximum PV advection at File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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206 high levels, with or without the presence of a depression at low level. 207 Some Mediterranean cyclogenesis could be, at least partially of this type, 208 comprising those cyclones generated at the quasi-permanent Mediterranean 209 border front (Alpert and Ziv, 1989) or at some internal fronts. (c) The low-level 210 disturbance rapidly deepens and cyclogenesis occurs when the upper level 211 perturbation moves close enough to interact with it. 212 The Mediterranean region presents geographic factors that can substantially 213 modify the cyclogenesis mechanism. The orography of the region changes 214 quantitatively and qualitatively the baroclinic instability process, usually 215 favouring or ‘‘focusing’’ the cyclogenesis (Speranza et al., 1985). The high 216 frequency of orographically induced low-level disturbances may partially 217 explain the high frequency of real cyclogenesis in the Mediterranean (Genoves 218 and Jansa, 1991, Jansa et al., 1994). Latent heat release usually sustains 219 and intensifies most of the cyclogenesis processes. In the Mediterranean region, 220 this effect seems to be quite important in the Eastern Mediterranean, when 221 a Sharav cyclone arrives there from the desert and intensifies over the sea 222 (Alpert and Ziv, 1989). Some cases in the western Mediterranean also have 223 the same evolution (Homar et al., 2000). EnhancedPROOF baroclinic instability in 224 saturated air (Fantini, 1995) influenced by latent heat release in an environ- 225 ment convectively stable can be another process contributing to cyclogenesis. 226 In general, the role of latent heat release and diabatic processes is a key issue 227 in the Mediterranean region, though of secondary importance when intense 228 orographic cyclogenesis, both Alpine (Buzzi and Tibaldi; 1978; Dell’Osso and 229 Radinovic, 1984; Speranza et al., 1985; Tibaldi et al., 1990; Stein and Alpert, 230 1993; Alpert et al., 1995; Buzzi, 1997) or non-Alpine (Garcia-Moya et al., 1989), 231 takes place. 232 Such complicate dynamics and the potential for many different mechanisms 233 favouring cyclogenesis imply that extremely diversified classes of cyclones are 234 present in the Mediterranean region. A tentative list, based partially on the 235 mechanisms producing cyclogenesis and partially on the geographical character- 236 istics, would include lee cyclones, thermal lows, small-scale hurricane-like 237 cyclones,UNCORRECTED Atlantic systems, African cyclones and Middle East lows. 238 Lee cyclones are triggered by the passage of a major synoptic low-pressure 239 system north of the region, so that their generation is expected to be very sensi- 240 tive to the location of the storm track above Europe. Lee cyclones develop 241 south of the mountain ridges representing the northern boundary of the 242 Mediterranean region. The Gulf of Genoa is the region of most frequent intense 243 cyclogenesis in the Mediterranean, but lee cyclones are also generated in the 244 Adriatic Sea (Flocas and Karacostas, 1994; Ivanc¸an Picek, 1996), in the Cyprus 245 and Aegean Sea areas (Reiter, 1975; Alpert et al., 1990), in the Black Sea, and 246 File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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247 in several areas of the western Mediterranean sea (Jansa, 1986). The passage of 248 the same synoptic system can be responsible for successive and distinct 249 cyclogenesis in the Gulf of Genoa, Aegean and Black Sea (Trigo et al., 1999). 250 The spatial distribution of the frequency of cyclogenesis presents relative 251 maxima in the Cyprus and Aegean region, in the Adriatic, in the Palos- 252 Algerian sea, in the Catalonian-Balearic sea, and in the Gulf of Lyons 253 (Jansa, 1986). 254 Thermal lows are more frequent in spring and summer and their genesis 255 and lysis are modulated by the daily cycle of temperature. Their occurrence is 256 therefore depending on the amplitude of this cycle and on the land–sea 257 temperature contrast. Many of them remain shallow depressions confined to 258 the lower troposphere. Modeling studies suggest that they are also generated 259 over sea in autumn and winter, when the land–sea temperature gradient is 260 reversed. 261 Small-scale, hurricane-like Mediterranean Lows have been detected over sea. 262 They are a special class of Mediterranean cyclones in which the main source of 263 energy is the great amount of latent heat released in large convective cloud 264 clusters, as in tropical cyclones (Rasmussen and Zick,PROOF 1987).They are likely to 265 depend critically on the air–sea temperature difference and on the content of 266 moisture in the atmosphere. 267 Atlantic systems mostly enter in the Mediterranean region from the west 268 and northwest, and cross the Mediterranean during their attenuation phase. 269 Even when their central pressure minimum does not pass directly above the 270 Mediterranean Sea, they affect the Mediterranean weather as they can cause 271 secondary lee cyclones. 272 Northern Africa is the source of many cyclones arriving from the south 273 which often form or intensify south of the Atlas Mountains as lee cyclones. 274 Their formation is more likely to occur in spring and summer when static 275 stability is low. 276 The classification of cyclones in the south-eastern Mediterranean region 277 includes several types of cyclones, with different seasonality and origin, such as 278 the CyprusUNCORRECTED Lows, Syrian Lows and Red Sea Troughs. The Cyprus Lows 279 are mostly orographically generated or strengthened. The Syrian low (Kahana 280 et al., 2002) is a particular type of the Cyprus low system so intense that 281 penetrates into some areas of Syria. The Red Sea trough system develops during 282 spring and autumn due to topographic effects in the Red Sea area. Usually 283 it is a warm and shallow trough with a very dry southeasterly flow, but torren- 284 tial rains occur when an upper cold air trough penetrates southwards 285 above it, due to the extreme instability in this situation (Krichak et al., 1997ab; 286 Krichak and Alpert, 1998) 287 File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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288 6.2.2. Available Sets of Data 289 290 As mentioned before, pioneering studies on the climatology of cyclones in the 291 Mediterranean were based on hand-made subjective analyses. Due to their own 292 nature, the subjective analysis datasets are in general not available except in the 293 institutions where the analyses were prepared, and often not recoverable for 294 subsequent studies. The subjectivity has the advantage that a hand-made analysis 295 can be even more careful and detailed than objective analyses in some areas, 296 due to appropriate conceptual models and complementary information, etc. 297 At the same time, the subjectivity has the disadvantage of being not homo- 298 geneous, neither in space nor in time, because some areas and situations can 299 be systematically poorly or incorrectly analysed due to the lack of specific skill 300 and experience of the authors. In general, the quality of objective analyses has 301 improved to the point where their advantages outweigh those of the subjective 302 analyses. 303 The operational objective analyses are presently the main source of data for 304 performing a climatological analysis of cyclones, although the frequent changes 305 in the procedure (due to changes in the forecastingPROOF model, in its resolution, in the 306 data available for its initialization and in the data assimilation method) makes 307 the series quite temporally heterogeneous. Generally, only relatively short 308 samples of operational analyses are homogeneous and can be used for climatic 309 studies. In order to compensate for these inadequacies, homogeneous reanalysis 310 datasets, like ERA15, NCEP or ERA40, have been produced, though with 311 resolution which is coarser than most of the advanced operational analyses 312 (Gibson et al., 1996; Kalnay et al., 1996; Simmons and Gibson, 2000). These 313 homogeneous reanalysis data sets (ERA15, NCEP and ERA40) are the basis for 314 all climatological studies in general and for cyclones in particular. Without them, 315 it would be difficult or even impossible to obtain reliable fields of geopotential 316 or SLP needed for systematic analysis of trends and variability. 317 As a practical compromise, attempting to exploit both advantages of the 318 high resolution of recent operational analyses and of the homogenous multi- 319 decadalUNCORRECTED re-analyses, the MEDEX database contains lists of cyclones identified 320 both in operational analyses (HIRLAM/INM, over the Western Mediterranean, 321 and ECMWF over the whole Mediterranean) and reanalyses (ERA40). 322 Another dataset has been created to investigate the changes and variability 323 in SLP from 1850 as part of the EMULATE project (European and North 324 Atlantic daily to multi-decadal climate variability project). The dataset is a 325 combination of homogenized daily SLP station series, ship observations and 326 previously reconstructed SLP fields (Ansell et al., 2005). 327 The dataset consists of daily mean sea level data on a 5 5 grid, and it has a 328 lower spatial and temporal resolution than the reanalysis datasets. Though, File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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329 obviously, it cannot detect mesoscale systems or accurately locate the areas of 330 cyclogenesis, this dataset allows the identification and tracking of synoptic-scale 331 cyclones in the Mediterranean region from 1850. 332 Extreme cyclones can also be identified by their effects which can be measured 333 or recorded. Instrumental data on surges, waves, floods, and winds can be used 334 for reconstruction of time series of past cyclones. A peculiar example is provided 335 by the records of storm surges in Venice, from past chronicle and archives. 336 The highest surges have been reported with a precision which is sufficient to 337 reconstruct the frequency of past floods since the 8th century AD (Camuffo, 338 1993). The time series shows a succession of periods of recurrent floods, which 339 were particularly intense in the first half of the 16th and of the 18th century, 340 separated by more quiet periods, and a continuous positive trend during the 341 second half of the 20th century. This last part of the time series is very accurate 342 since the tide gauge records have begun in 1872. This example represents the 343 limits and potential of such local reconstructions. On one side, they are a tool 344 for obtaining very long time series in historical times. On the other side, in general 345 it is very difficult to associate past variability to large-scale regimes and make 346 the distinction between large-scale and local processesPROOF (the soil subsidence in 347 this case) for which additional information is needed. Also the homogeneity of 348 the time series along its whole extent can be argued, as its reconstruction often 349 involves subjective criteria which need to account for the temporal changes in 350 the level of vulnerability of society, in human activities, resources and techno- 351 logical capability. Finally, it is difficult to attribute the cause of trends without 352 supplementary information. Though floods of Venice occurred many times 353 during its history, the last 50 years represent an unprecedented period of fre- 354 quent and intense events. However, this is largely explained by the local loss 355 of relative sea level (a combination of ground subsidence and sea level rise) not 356 associated to a trend of storminess (Lionello, 2005) 357 358 359 6.2.3. Methodology 360 UNCORRECTED 361 In recent years, two distinct approaches have been used to study the storm 362 activity over the North Atlantic and Europe: storm track algorithms and analysis 363 of synoptic variability. Usually, storm track algorithms apply sophisticated 364 methods that can detect the regions of storm development (cyclogenesis) and 365 decay (cyclolysis) as well as the specific paths of each individual storm (Murray 366 and Simmonds, 1991; Serreze et al., 1997; Trigo et al., 1999, 2002; Lionello 367 et al., 2002). Analysis of synoptic variability is a simpler approach, which 368 corresponds to the identification of the synoptic variability using a band- 369 pass filter that retains mainly variability on the 2–8 day period of SLP or 850 File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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370 or 500 hPa geopotential (Buzzi and Tosi, 1989; Hoskins and Hodges, 2002). 371 This second approach has been widely applied to quantify the synoptic activity 372 associated with a high and low NAO (North Atlantic Oscillation) index 373 (e.g. Rogers, 1997; Ulbrich and Christoph, 1999; Trigo et al., 2002; Krichak 374 and Alpert, 2005a,b). However, the first technique has also been used to show 375 the areas of significant difference in storm activity between winters with high 376 and low NAO index (Serreze et al., 1997). Figure 107, A–D shows the 1,000 hPa 377 geopotental height standard deviation and associated cyclone trajectories. 378 The analysis is applied to the band-pass filtered fields with cut-off periods 379 at 1 and 7 days. Two monthly periods are considered: January 1966 and 1983, 380 which are characterized with a low and high NAO index, respectively. Both 381 approaches show the existence of a major cyclone variability mode, which is 382 strongly associated with NAO (North Atlantic Oscillation). 383 A climatology of cyclones implies a definition of cyclone and a method of 384 detecting (and describing) it. In both these steps there is a wide margin of 385 arbitrariness, which produces significant differences in the results. When 386 considering subjective analyses and a subjective detection method it was usual 387 to retain a disturbance as a cyclone when it wasPROOF a minimum of pressure 388 389 A B 390 391 392 393 394 395 396 397 398 D 399 C 400 401 UNCORRECTED 402 403 404 405 406 Figure 107: Cyclone trajectories (top panels (A) and (B)) and 1,000 hPA standard 407 deviation (bottom panels (C) and (D)) for low (Jan. 1966, left) and high (Jan 1983, 408 right) monthly NAO index values. Analysis is applied to band-pass filtered data 409 (1 to 7 day window). Only trajectories of cyclones deeper than 25 hPa and with 410 duration longer than one day are shown. File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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411 surrounded by a closed isobar. Of course, the results depend on the spacing 412 between isobars and of the scale of the map, so that the more detailed the 413 map, the higher the number of cyclones identified. These situations and problems 414 are not avoided by using objective methods. There are different conceptual 415 ways to define a cyclone. An option is to define a cyclone as a relative maximum 416 of vorticity (relative or geostrophic vorticity). Another definition states that 417 a cyclone is a relative minimum of sea level pressure (or geopotential height). 418 The choice of the definition will produce significantly different number of 419 cyclones, and will affect also other details, like their location. Not all the minima 420 of sea level pressure can be retained as cyclones, because many of them are too 421 weak or too close together, so that restrictions have to be imposed (like a 422 threshold for the minimum pressure gradient, a minimum distance between 423 centres, or other constraints), which introduce arbitrariness in the definition of 424 the cyclone and therefore in the results. Even more critical is the effect of the 425 resolution, since low-resolution analyses will permit the detection of relatively 426 large-scale cyclones, but will miss the smaller mesoscale disturbances, while, on 427 the contrary, high-resolution analyses will permit the detection of many small 428 mesoscale disturbances, but could miss the descriptionPROOF of the relatively large- 429 scale disturbances. In fact, the domain of a large-scale cyclone, defined as the 430 area of positive (geostrophic) vorticity, in a region like the Mediterranean 431 characterized by a complex orography and land–sea distribution, is often broken 432 in multiple fragments, without continuity, due to the existence of many high-low 433 small-scale disturbances, with negative–positive vorticity. The maximum detail in 434 the detection and description of small-scale cyclonic disturbances, is achieved by 435 using high-resolution objective analyses directly, but for a good description of 436 larger scale cyclones, lower resolution analyses have to be used or, alternatively, 437 the original fields of the high-resolution analyses have to be spatially smoothed. 438 The results from high-resolution unfiltered fields and from low-resolution or 439 smoothed fields are dramatically different, even with the same definition of 440 cyclones and the same restrictions to the definition. Figure 108 (Gil et al., 2002) 441 is based on a 3-year (Jun 1998 to May 2001) sample of operational analyses 442 from theUNCORRECTED ECMWF, consisting of four analyses per day at a T319 resolution, 443 which permits 0.5 latitude–longitude gridded maps. From non-smoothed 444 analyses (right-hand side in the figure), 2,248 cyclones are detected in the 445 Eastern Mediterranean (purple frame) and 2,910 in the Western Basin (blue 446 frame). From smoothed fields, using a Cressman filter with a 200 km radius of 447 influence (left side in the figure), these numbers are reduced to 353 cyclones in the 448 East and 437 in the West. Not only the number of cyclones are totally different, 449 but also some details in the distribution: the very important relative maximum 450 south of the Pyrenees obtained from the original fields disappears when the 451 smoothed fields are used. File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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452 453 454 455 456 457 458 459 460 461 Figure 108: Number of cyclones detected in a three-year (Jun 1998 to May 2001) 462 sample of operational analyses from the ECMWF at T319 resolution. Right panel 463 shows the results of analysing non-smoothed analyses: 2,248 cyclones are detected 464 in the Eastern Mediterranean (purple frame) and 2,910 in the Western Basin 465 (blue frame). Left panel refers to smoothed fields (a Cressman filter of 200 km 466 of radius of influence has been used): 353 cyclones in the East and 437 in the West (from Gil et al., 2002). 467 468 469 PROOF 470 6.3. Climatology of Cyclones in the Mediterranean 471 472 The climatology of cyclones in the Mediterranean region is highly influenced by 473 the almost enclosed Mediterranean Sea, which represents an important source of 474 energy and moisture for cyclone development; and by its complex land top- 475 ography, which plays a major role in steering and deflecting air flows. Moreover, 476 being located within the transition between the subtropical high-pressure belt 477 and the mid-latitude westerlies, the Mediterranean is also subject to strong 478 interannual variability of cyclone activity and, consequently of its precipitation 479 regime, water resources. 480 In the following section, a climatology of Mediterranean cyclones is presented, 481 which decribes the spatial distribution of cyclogenesis and associated mechan- 482 isms. The average cyclone characteristics, including their intensity and spatial 483 and temporalUNCORRECTED scales, as well as their intra- and inter-annual frequency variability, 484 will be described. 485 486 487 6.3.1. Characteristics, Sub-areas of Cyclogenesis, Seasonality 488 and Generation Mechanisms 489 490 Mediterranean cyclones are generally characterized by shorter life-cycles and 491 smaller spatial scales than extra-tropical cyclones developed in the Atlantic, as 492 shown in the analysis of storm-tracks derived from 6-hourly near surface fields File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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493 at T106 (1.125 1.125 ) resolution, available from ERA-15 (ECMWF 494 ReAnalysis) (Trigo et al., 1999). Over 65% of cyclones are within subsynoptic 495 scales, with radius of the order of 550 km or less, considerably smaller than the 496 1,000–2,000 km values, typical of Atlantic synoptic systems. If the shortest liv- 497 ing cyclones (with duration lower than 12h) are excluded, the average life of 498 cyclones in the Mediterranean region is about 28 h, compared to 3–3.5 days in the 499 Atlantic. Radius and maximum gradient tend to scale with the minimum 500 pressure. In general, cyclones are deeper and have a larger radius in the western 501 than in the Eastern Mediterranean. A recent evaluation (restricted to the western 502 Mediterranean region, Picornell et al., 2001), based on higher resolution fields 503 (computed by HIRLAM at 0.5 resolution) and including short-lived cyclones, 504 produced even smaller space and timescale values. In this dataset the radius of 505 most cyclones is within the 150–350 km range (the mean value is 255 km) and 506 the most intense cyclones have lifetimes of 18–24 h. Deepening rates are mostly 1 1 507 lower than 2 hPa (6h) , though values as high as 10 hPa(6h) can be observed. 508 Therefore, also the average deepening rates are smaller than in the Atlantic, 509 although the lower latitude at which Mediterranean cyclones develop should 510 be accounted for. Many cyclones in the MediterraneanPROOF region have null or even 511 negative deepening rate, meaning that they originated in neighbouring regions 512 and cross the Mediterranean during their attenuation phase. A very recent 513 comparison between a sample of Atlantic and West Mediterranean cyclones, 514 made by using high-resolution smoothed fields (HIRLAM/INM 0.5 latitude– 515 longitude with Cressman filter of 200 km), confirms the differences indicated 516 above. The average winter geostrophic circulation of the Mediterranean cyclones 7 2 1 7 2 1 517 is 4 10 m s whereas it is 7 10 m s for the Atlantic disturbances 518 (Campins et al., 2005). Note that the geostrophic circulation combines the size 519 (area) of the cyclones and their geostrophic vorticity, so that can be considered 520 as a measure of their total magnitude. 521 The geography of the region, namely the high orography around the 522 Mediterranean Sea and the existence of embayments and inland seas, determines 523 the relatively small areas where cyclogenesis tends to occur (Table 7; Fig. 109) 524 and theUNCORRECTED variegate mesoscale structure of Mediterranean systems (Alpert et al., 525 1990a,b; Trigo et al., 1999, 2002; Maheras et al., 2001; Picornell et al., 2001). 526 These structures correspond to the mechanisms discussed in the previous Section 527 6.2.1. Figure 109 shows that the most active areas include the Gulf of Genoa, 528 Iberia, Southern Italy, Northern Africa, Aegean Sea, Black Sea, Cyprus, Middle 529 East; the respective seasons when these areas are most active are indicated in 530 Table 7. 531 Further differentiation can be found inside such areas. In fact, studies based 532 on automated database methods (Campins et al., 2000), have resolved smaller 533 mesoscale structures and identified in the western Mediterranean 7 types of File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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534 Table 7: Cyclogenetic regions in the Mediterranean area and 535 respective seasons with significant activity (after Trigo et al., 536 1999); values represent average cyclone radius (km). 537 area seasonality radius (km) 538 539 Sahara Spring, Summer 530–590 540 Gulf of Genoa Whole year 530–380 541 Southern Italy Winter 520 542 Cyprus Spring, Summer 330–460 Middle East Spring, Summer 320–460 543 Aegean Sea Winter, Spring 500 544 Black Sea Whole year 380–400 545 Iberian Peninsula Summer 410 546 547 548 549 550 551 PROOF 552 AB 553 554 555 556 557 558 559 560 561 562 C 563 564 565 UNCORRECTED 566 567 568 569 570 571 572 Figure 109: Number of cyclogenesis events detected per 2.25 2.25 in January 573 (A), April (B), and August (C) from 1979 to 1996 in ECMWF re-analyses 574 (from Trigo et al., 1999). File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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575 cyclones, on the basis of shape and intensity of the associated circulation. 576 However, despite the use of different methodologies, selection criteria and 577 data sets, most studies (e.g. Alpert et al., 1990a,b; Trigo et al., 1999; Campins 578 et al., 2000; Maheras et al., 2001; Picornell et al., 2001) agree on the spatial 579 location of cyclone generation. 580 The overall synoptic activity over the entire basin has a well-defined annual 581 cycle, being more intense in the period from November to March which corres- 582 ponds to the so-called storm season (Fig. 110). Though the temporal and spatial 583 distributions of the Mediterranean cyclones present a large intermonthly vari- 584 ability (Alpert et al., 1990a), the analysis favours the definition of three main 585 seasons: winter, spring and summer. Autumn appears as a transitional period 586 with large interannual variability, whose months could be characterized as late 587 summer or early winter. 588 589 590 591 592 PROOF 593 594 595 596 597 598 599 600 601 602 603 604 605 606 UNCORRECTED 607 608 609 610 611 612 Figure 110: Annual cycle of the standard deviation of the 1,000 hpa geopotential 613 field. The grey lines show individual years, the black thick line the average annual 614 cycle. Values are based on the band-pass filtered fields (1–7day cut-off periods) 615 (Lionello and Zardini, 2005, personal communication) File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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616 The mechanisms typical of winter cyclogenesis in the Mediterranean exhibit 617 contrasting characteristics with those most common in spring and summer 618 seasons, with intermediate situations in spring and autumn. The 3D structure 619 of the cyclones gives a first clear indication: in the Mediterranean, only the 620 winter cyclones are mostly deep cyclones (reaching 300 hPa), while in summer 621 they are mostly shallow (reaching 850 hPa or less). The Laplacian of the temp- 622 erature at low levels, giving the thermal character of the cyclone, is always 623 negative (warm character), but double in magnitude in summer than in winter. 624 The Mediterranean cyclones are in general more similar to the Atlantic cyclones 625 in winter than in summer (Campins et al., 2005). In winter there are strong links 626 between synoptic upper-troughs and local orography and/or low-level barocli- 627 nicity observed over the northern Mediterranean coast. In spring and summer, 628 inland cyclogenesis becomes more frequent and also more sensitive to diurnal 629 forcing (Maheras et al., 2001; Picornell et al., 2001; Trigo et al., 2002). 630 Winter cyclogenesis occurs essentially along the northern coast in three major 631 areas characterized by strong baroclinicity: the lee of the Alps, when an upper- 632 trough is influenced by the mountains, and over the Aegean and Black Seas, 633 when an upper-trough moves over the relatively warmPROOF water basins (Trigo et al., 634 2002). The role of orographic cyclogenesis (Buzzi and Tibaldi, 1978) is not limited 635 to the Alps, being also fundamental in the triggering of lows in the Gulf of 636 Lyons, south of the Pyrenees, and also in Southern Italy, south of the Apennines. 637 Over the south-eastern Mediterranean region, the intensity of cyclogenetic 638 activity is to a large extent controlled by large-scale synoptic systems over 639 Europe, particularly by those characterized by mid- and upper-tropospheric 640 southward air-mass intrusions and tropopause-folding effects (Krichak and 641 Alpert, 2003). These processes are often associated with the formation of three- 642 dimensional potential vorticity structures, jet streaks and low-level jets condi- 643 tions over the region to the south of Alps (Buzzi and Foschini, 2001; Liniger and 644 Davies, 2003). 645 In spring, the strengthening of the meridional temperature gradient along 646 the northern African coast favours the development of Saharan depressions. 647 These tendUNCORRECTED to occur on the lee side of the Atlas mountains, within a region of 648 very weak static stability. Thermal forcing plays an increased role in the genesis 649 and maintenance of Mediterranean Lows in spring and, particularly, in 650 summer. As a result, the life-cycles of summer cyclones, especially those 651 developed over Northern Africa and the Iberian Peninsula, follow the diurnal 652 temperature fluctuations; maximum intensity tends to be reached by late 653 afternoon, and cyclolysis tends to occur mostly by early morning (Maheras 654 et al., 2001; Picornell et al., 2001; Trigo et al., 2002). Also the Middle East 655 trough, which is a semi-permanent feature primarily induced by the Asian 656 monsoon acting on a planetary scale (Rodwell and Hoskins, 1996), exhibits the File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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657 same kind of diurnal see-saw associated with the local thermal cycles (Trigo 658 et al., 2002). 659 There are several reported cases of very intense storms in autumn–winter and 660 spring months, with severe associated weather (intense rainfall, surges, flash 661 floods), which have either developed or re-intensified over the Mediterranean 662 Sea (e.g. Lee et al., 1988; Ramis, 1994; Lagouvardos, 1996; Lagouvardos, 1999; 663 Doswell III et al., 1999; Pytharoulis et al., 1999). A fraction of these very intense 664 events develop a hurricane-like structure, feeding on the latent heat release at the 665 sea surface; their frequency, space–time distribution, and interannual variability 666 have not been fully investigated, yet (Pytharoulis et al., 1999). 667 668 669 6.3.2. The Role of Large-scale Climate Patterns on the 670 Mediterranean Cyclones 671 672 The Mediterranean region is only partially affected by the North Atlantic 673 storm track, whose main path crosses the Northern Atlantic towards Northern 674 Europe. Consequently, the main mode of thePROOF North Atlantic storm track 675 variability, which describes its north–south shift and intensification over the 676 Atlantic, is only marginally related to the frequency and intensity of the cyclones 677 in the Mediterranean region, though Trigo et al. (2000) have demonstrated 678 that an association exists and that it depends on the structural characteristics 679 of the cyclones. 680 In fact, the analysis of low-frequency SLP variability patterns and the freq- 681 uency of cyclones in the Mediterranean region shows that there are important 682 patterns than the NAO (Krichak and Alpert (2005a,b)). The link between NAO 683 and the position and strength of the storm track in the central Atlantic implies 684 a link between NAO and the frequency of orographic cyclogenesis which is 685 triggered by the passage of Atlantic cyclones. Instead, the bulk of the variability 686 over Central and Southern Europe and over the Mediterranean region is linked 687 to low-frequency patterns, whose centres of actions are localized over Europe 688 and easternUNCORRECTED Atlantic (like the East Atlantic/Western Russia pattern (EAWR), 689 Krichak et al., 2000, 2002; Krichak and Alpert, 2005a,b). It has been demon- 690 strated that, depending on the area of the Mediterranean region, a high level of 691 SLP synoptic-scale variability is associated with the positive phases of the SENA 692 (Southern Europe Northern Atlantic) and, to a minor degree, to the SCAN 693 (SCANdinavian) patterns. Therefore, intensity of the cyclogenetic activity in the 694 eastern Mediterranean region is to a large extent controlled by the large-scale 695 synoptic processes over Europe and especially by those characterized by mid- 696 and upper-tropospheric southward air-mass intrusions and tropopause folding 697 effects (Krichak et al., 2004). The processes are often associated with the File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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698 formation of three-dimensional PV structures (PV streamers), jet streaks and 699 low-level jets conditions over the region to the south of the Alps (Buzzi and 700 Foschini, 2001; Liniger and Davies, 2003). These conditions tend to stimulate 701 development of mesoscale convective complexes and Mediterranean cyclones. 702 Intensity, location, duration and orientation of the systems as well as their 703 interdecadal trends in association with those of the main European teleconnec- 704 tion patterns appear to be important elements of the eastern Mediterranean 705 weather and climate trends. 706 Moreover, these teleconnections are defined on a monthly scale, while sub- 707 monthly large-scale features, such as the well known and relatively frequent euro- 708 Atlantic blocking (Tibaldi et al., 1997), can influence the trajectory of storm 709 tracks and their associated precipitation fields (Trigo, 2004). Winter blocking 710 episodes lasting 10–20 days are associated with a large positive anomaly of the 711 500 hPa geopotential height above the North Sea and with a higher/lower than 712 average number of cyclones in the Mediterranean/North Sea. 713 Finally, since such large-scale analyses are generally based on relatively coarse 714 resolution fields where the subsynoptic and mesoscale characteristics of the 715 cyclones in the Mediterranean region are poorlyPROOF reproduced, important 716 components of their variability might not be well described yet. 717 718 719 6.3.3. Trends 720 721 A counting of cyclone centres (without any differentiation on intensity), based 722 on the NCEP re-analysis, which covers the period 1958–1997, shows a reduction 723 of the number of cyclones in western Mediterranean and an increase in the East 724 (Maheras et al., 2001). Linear fit to the data leads roughly to a 15% increase/ 725 decrease. Changes are not seasonally homogeneous. If only the rainy period 726 (October–March) is considered, a reduction of the number of cyclones is evident 727 also in the Eastern Mediterranean. Other studies suggest a distinction between 728 the increasing trend of weak cyclones and the decreasing trend of strong 729 cyclonesUNCORRECTED in the Western Mediterranean Sea (Trigo et al., 2000). It follows 730 that the positive trend identified in the Northern Hemisphere storm track for 731 the last decades of the 20th century (Chang and Fu, 2002) is not valid in the 732 Mediterranean region. 733 The negative trend is confirmed by the analysis of longer time series 734 (Della-Marta and Bhend, 2005). A climatology of cyclone activity has been 735 created using a newly compiled dataset of the North Atlantic–European region 736 from 1850 to 2003, which was developed in the European and North Atlantic 737 daily to multi-decadal climate variability project (EMULATE). These data 738 consist of gridded daily mean sea level pressure fields, which are based on land File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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739 and island stations and have been elaborated using Reduced Space Optimal 740 Interpolation (RSOI) on a 5 5 degree grid (Ansell et al., 2005). Seasonally 741 averaged statistics of the cyclone dynamics have been computed with an 742 objective locating and tracking system developed by Murray and Simmonds 743 (1991). Commonly to all cyclone tracking algorithms, subjective decisions had 744 to be made regarding the cyclone locating and tracking algorithm parameters. 745 In order to optimize the parameters, tracking results based on EMULATE 746 data (1948–2003) have been compared to tracking results based on daily NCEP 747 data which were transferred to a 5 5 grid. The EMULATE reconstructions in 748 the Mediterranean is shown to be reliable because the RSOI error statistics 749 (see Kaplan et al., 2000) are invariant over time and space for the entire 153 years 750 of the reconstruction indicating that the number of predictors (e.g. station- 751 based observations and marine data) are dense enough to reliably reconstruct 752 SLP on the given grid. See Ansell et al. (2005) for more details on the SLP 753 fields. Significant findings are a marked decrease in winter (DJF) cyclone density 754 over most of the western Mediterranean and an increase in cyclone system 755 density in the eastern Mediterranean for the period 1950–2003 (Fig. 111,A). 756 These findings agree with the results of MaherasPROOF et al. (2001). In the longer 757 period, 1850–2003, most of the Mediterranean shows a decrease in cyclone 758 system density (Fig. 111,B). Analysis of the cyclone density time series in the 759 form of a Hovmo¨ller plot shows that the frequency of cyclones over the western 760 Mediterranean is highly variable and exhibits large interannual as well as 761 decadal variability over the last 153 years (Fig. 112). 762 763 764 765 6.4. Weather Patterns and Mediterranean Environment 766 767 As it was briefly mentioned in the introduction, cyclones have a deep influence on 768 important environmental variables and, particularly, on the timing and mag- 769 nitude of their extreme values. In general, although not all the extreme weather 770 events inUNCORRECTED the Mediterranean are related to cyclones and most of the cyclones 771 do not produce extreme weather, it is plausible to assume that Mediterranean 772 cyclones influence most of the high-impact phenomena. Moreover, the high vari- 773 ability of cyclone frequency and intensity, within the Mediterranean Sea and its 774 immediate environments, results in contrasting weather conditions in the region, 775 ranging from large arid areas (e.g., Thornes, 1998; Trigo et al., 2002) to the 776 greatest annual precipitations totals in Europe, in the Dinaric Alps (Radinovic, 777 1987; Trigo et al., 2002). This section shows the correlation of cyclones with rain, 778 winds, waves, surges and even landslides, describes the mechanisms involved 779 and discusses variability and trends of the related phenomena. File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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780 781 782 783 784 785 786 787 788 789 790 791 792 A 793 794 795 796 797 PROOF 798 799 800 801 802 803 804 805 806 807 B 808 809 Figure 111: The linear trend in the winter (DJF) cyclone density (A) 1950–2003 810 and (B) 1850–2003. Cyclone density is the average number of cyclones per unit 811 area at any one time. The trend units are the number of cyclones times 1,000 deg. 2 UNCORRECTED 812 lat. where deg. lat. is a standard length of 1/360th the circumference of the Earth 813 (from Della-Marta and Bhend, 2005, personal communications) 814 815 816 6.4.1. Precipitation 817 818 Large quantities of rain require a feeding current of warm and wet air to 819 replace the water removed by precipitation. When vertical stability is close to 820 a critical threshold, such inflow at low levels can favour or lead to instability. File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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821 822 823 824 825 826 827 828 829 830 831 832 833 Figure 112: The variability of cyclone density as a function of time and latitude 834 at the longitude of 10 W. The Hovmo¨ller plot x axis defines the latitude in degrees 835 North, the y axis defines the winter season (DJF) from 1850 (bottom) to 2003 836 (top). Cyclone density is the average number of cyclones per unit area at any given time. Units are the number of cyclones per deg. lat.2 where deg. lat. is a standard 837 length of 1/360th the circumference of the Earth (from Della-Marta and Bhend, 838 2005, personal communications).PROOF 839 840 841 Therefore, the eastward and poleward sectors of cyclones are suitable places for 842 prolonged and intense precipitation. Moreover, orographic upslope lifting is 843 also very effective for producing ascent of warm humid air and persistent rainfall. 844 In fact, in many places the coastal or inner relief intersects a moisture feeding 845 flow and can force upward motion and orographic rain. The local intensity of 846 the precipitation is very much dependent on the path followed by the cyclone 847 and by the amount of available moisture. The heaviest rain events take place 848 when the cyclone path is in such a position that it produces the local convergence 849 of moist Mediterranean air. In the Western Mediterranean, this feeding flow 850 is southerly for northern Italy and Ticino, south-easterly for France, and easterly 851 for Catalonia, the Balearics and Valencia Murcia (Jansa et al., 2001b). In the 852 EasternUNCORRECTED Mediterranean, it is mostly westerly for the Middle East countries and 853 southerly or south-westerly for Greece. 854 These dynamics explain why heavy rain events are associated with cyclones 855 (only a few events in the eastern Mediterranean and in Northern Italy can be 856 an exception) as humid Mediterranean air is advected against the slopes of the 857 mountain ridges surrounding the basin. In fact, synoptic-scale disturbances have 858 been found responsible for most of the floods both in the Western Mediterranean 859 and in the Eastern Mediterranean. Only a minority of local flash floods has been 860 associated with intense small convective cells, whose presence is not detected in 861 the standard meteorological analysis. It has been established that for most of File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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862 the cases (around 90%) of heavy rain in the Western Mediterranean there 863 was a cyclone in the vicinity (Jansa et al., 2001b, see Fig. 113, though its intensity 864 could vary from an intense and deep system to quite weak and shallow 865 depressions. In fact, considering the mean value for all cyclones identified in 866 such cases, the average vorticity in a central area of the cyclone of 400 km of 4 1 867 radius is 0.8 10 s . In approximately 80% of the events of heavy rain, the 868 location of the cyclonic centre is such that a role of the cyclone in the heavy rain 869 generation and/or location can easily be inferred. The total number of heavy rain 870 events considered is more than 900 (in 5 years, 1992–96). A heavy rain event 871 is defined here as a day with more than 60 mm/day (lowered to 30 mm/day 872 in Algeria) of precipitation in any point of a ‘‘territorial unit’’ (province, 873 department, region or island). Analogously, in Greece about the 92% of rainfall 874 during the rainy period (October–March) is produced by cyclonic patterns 875 (Maheras and Anagnostopoulou, 2003). During the cold season, precipitation 876 in the southern part of the eastern Mediterranean (EM) region is also mainly 877 associated with cyclonic systems of Mediterranean origin. A study carried out 878 for the Negev Desert identified 4 classes of synoptic disturbances responsible 879 for most of the floods, the two most important denotedPROOF as the Syrian Low and 880 the Red Sea Trough (Kahana et al., 2002). Therefore cyclones are the cause 881 of most of intense precipitation in the whole Mediterranean region. 882 From left to right and from up to down S. France, Corsica, Italy, Catalona, 883 Balearics, Sardinia, Valencia, Algeria 884 The cyclones responsible for precipitation in different areas do not share a 885 common origin and generally a single system affects only part of the 886 Mediterranean region. A study focused on Portugal, Italy and Greece has 887 shown that Atlantic Lows dominate in Portugal, where rainy months are 888 associated with an enhanced number of deep and medium cyclones between 889 Newfoundland and British Isles. Precipitation over Greece is very rarely affected 890 by Atlantic cyclones, but is associated to cyclogenesis inside the Mediterranean 891 region. Italy is influenced from both Atlantic and Mediterranean cyclones, 892 because distant lows sometimes contribute to advection of humidity and some 893 AtlanticUNCORRECTED cyclones may cross over into the Mediterranean and influence the 894 precipitation over Italy. However, the majority of precipitation sources are 895 Mediterranean cyclones (Pinto et al., 1999). During the cold season, precipita- 896 tion in the southern part of the eastern Mediterranean (EM) region is mainly 897 associated with cyclonic systems of Mediterranean origin. The cyclones produ- 898 cing intense rain in Israel usually belong to the type of Cyprus Lows or 899 Cyprus Depressions (El-Fandy, 1946; Kallos and Metaxas, 1980; Alpert et al., 900 1995, 2003; Krichak et al., 2004) and usually start their development in the 901 south-western areas of the Mediterranean Sea, and then migrate to the east. 902 During the migration process, the depressions often weaken, though a very File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 PROOF 921 922 923 924 925 926 927 928 929 930 931 932 933 934 UNCORRECTED 935 936 937 938 939 Figure 113: Most frequent location (elliptic area) of a cyclone in case of heavy 940 rain in some western Mediterranean regions (isolated dot): From left to right and 941 from up to down the locations here considered are (A) SE France, (B) Corsica, 942 (C) North Italy, (D) Catalonia, (E) Balearics, (F) Sardinia, (G) Valencia and 943 (H) Algeria (from Jansa` et al., 2001b). File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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944 significant strengthening of the cyclones is often observed in the Cyprus area, 945 where the lows regenerate in the lee of the Taurus Mountains of Turkey. 946 This phenomenology corresponds to the key role which cyclones play in the 947 internal redistribution of moisture in the Mediterranean region (Fernandez 948 et al., 2003). The transport of moisture from the Western to the Eastern 949 Mediterranean corresponds to the generation (or intensification) of cyclones in 950 the western Mediterranean and their subsequent eastward motion. Air–sea inter- 951 action and significant latent heat flux are likely to play an important role in 952 this process. 953 In Greece, for the period 1958–2000, precipitation has been analysed 954 considering the frequency of cyclones and the probability of precipitation 955 produced by them. For the majority of stations, in wintertime the decreasing 956 trends of wet-day amount and the probability of rainfall are consistent with 957 the observed changes in frequency of the various types of cyclones. During 958 autumn on the one hand, the probability of rainfall increases for a large number 959 of cyclonic circulation types, which, on the another hand, are characterized 960 by a decrease in frequency. These opposite trends partially compensate, so that 961 the autumn overall amount of precipitation showsPROOF a positive trend (Maheras 962 et al., 2004). 963 An outstanding problem in Western Mediterranean rainfall is the occurrence 964 of catastrophic torrential rains, which tend to occur in the autumn season 965 along coastlines with heavy orography, and the change of relative frequency 966 of moderate/light vs. intense precipitation events. Very little work has been 967 done to link the probability of these events to large-scale extra tropical circula- 968 tion patterns (Valero et al., 1997), although some analysis of the frequency and 969 intensity of Mediterranean cyclones (Trigo I. et al, 2002) and modeling studies 970 for individual events have been performed (Homar et al., 1999; Romero et al., 971 1999; Pastor et al., 2001). In the last decades, a tendency for more intense 972 concentration of rainfall seems to have occurred along the Mediterranean 973 coastal areas in Italy and Spain (Brunetti et al., 2001; Alpert et al., 2002; 974 Goodess and Jones, 2002), but this does not seem to be the case for inland areas. 975 In fact,UNCORRECTED over Italy, results show a negative and significant long-term trend in 976 the number of wet days and a positive one in precipitation intensity, which is 977 significant only in the northern regions (Brunetti et al., 2004). The negative trend 978 in wet days persists since the end of 19th century and is due to the marked 979 decrease in the number of low-intensity precipitation events. An increase in the 980 number of events belonging to the highest intensity interval was observed too, 981 but only in northern regions. The decrease of total precipitation during the 982 wet season in the Northern Mediterranean has been associated with the reduc- 983 tion of intense cyclones (Fig. 114) and to the northward shift of the storm track 984 File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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October - March 985 480 200 986

987 ) o 988 440 160 989 x 2.5 990 o 400 120 991 992 360 80 993 994 995 320 40 996 Rainfall mm / (3.75 997 280 0 998 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 999 Frequency of Mediterranean Cyclonic Events 1000 Figure 114: Time series, and respective linear trends, of the total amount of precipitation in the Northern Mediterranean Basin (bold curve, left axis), the 1001 total occurrence of intense Mediterranean cyclones (light curve, right axis), and 1002 of non-intense cyclones (dotted curve, right axis) forPROOF the October–March period 1003 (from Trigo and Davies, 2000). 1004 1005 1006 over Europe in the period from the 1979 onwards (Trigo et al., 2000). However, 1007 the analysis of precipitation shows different trends depending on the intensity 1008 of the events (Alpert et al., 2002). The torrential rainfall in Italy exceeding 1009 128 mm/day has increased percentage-wise by a factor of 4 during 1951–1995. 1010 In Spain, extreme categories at both tails of the distribution (light: 0–4 mm/day 1011 and heavy/torrential: 64 mm/day and up) increased significantly. No significant 1012 trends were found in Israel, Greece and Cyprus. A detailed analysis of the 1013 precipitation in the Valencia Region (Spain) suggests that land use changes in 1014 the coastal region result in surface drying, which in turn implies warmer and drier 1015 air masses over the coast and higher condensation level and fewer summer 1016 storms.UNCORRECTED It is moreover suggested that higher sea surface temperature can be the 1017 cause for the increased number of torrential rain in autumn and winter and 1018 that these two factors can be part of a climate mechanism affecting the whole 1019 western Mediterranean (Millan et al., 2004, 2005). The consequent redistribu- 1020 tion of the daily rainfall categories – torrential/heavy against moderate/light 1021 intensities – is of paramount interest particularly in the semi-arid subtropical 1022 regions for purposes of water management, soil erosion and flash floods 1023 impacts. Specific isolated regions exhibit an increase of extreme rainfall in spite 1024 of the reduction of the total amount of precipitation. 1025 File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1026 6.4.2. Strong Winds 1027 1028 Many strong winds observed in the Mediterranean belong to the category of 1029 local winds, like Mistral, Tramontana, Sirocco, Etesian, Bora, Khamsin or 1030 Sharav (see H.M.S.O., 1962,orReiter, 1975, for a general description), that is, 1031 they have repetitive location, behaviours and characteristics. Figure 115 shows 1032 the location of the main winds in the Mediterranean region. 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 PROOF 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 UNCORRECTED 1058 1059 1060 1061 Figure 115: Maps of the lagged correlation with the value in the point indicated 1062 by the dot for the 500 hpa synoptic-scale filtered geopotential height. This point 1063 is selected as a maximum of explained variance of the precipitation field associated 1064 to the EOF describing a moisture transport from western to eastern 1065 Mediterranean areas. The maps are meant to describe the evolution of the cyclonic 1066 disturbances associated with such transport (From Fernandez et al., 2002). File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1067 In general, there is a connection between the Mediterranean local winds and 1068 the Mediterranean cyclones. According to the climatology based on ship 1069 observations, the highest frequency of gale storm winds in the Mediterranean 1070 occur in the Gulf of Lyons, with large difference with other regions (see, for 1071 instance, H.M.S.O., 1962), and they can be identified as winds belonging to the 1072 Mistral local wind category. The primary cause for these winds is a cyclone 1073 located within the Mediterranean, in or near the Genoa region (together with 1074 an anticyclone in France or northwestern Europe) and the high frequency and 1075 intensity of the Mistral winds is a consequence of the high frequency and inten- 1076 sity of the Genoa cyclones. Similarly, a low pressure above Italy or west of it 1077 produces a Sirocco storms in the Adriatic Sea, where the channelling effect of 1078 Apennines and Dinaric Alps strongly intensifies a flow which would otherwise 1079 be distributed on a larger front. 1080 The interaction of air flow and orography contributes to the Mediterranean 1081 local winds, which can be partially seen as downslope flows or due to channelling 1082 effects. The Mediterranean local winds are attributable to orographic mesoscale 1083 pressure perturbation induced by the flow–mountain interaction. High- and 1084 low-pressure poles of the orographic disturbance (and/orPROOF the orographic pressure 1085 dipole as a whole) create local areas of strong pressure gradient that provide 1086 intense local acceleration, leading to the local wind generation (Campins et al., 1087 1995). The onset of a local wind is, therefore, often quite abrupt. Past the narrow 1088 accelerating zone, the winds continue blowing and spreading in an inertial way, 1089 although density gradients can contribute to a more efficient wind spreading 1090 and extension (Jansa, 1960; Alpert et al., 1982). According to this mechanism, 1091 local winds are shallow, only 1.5–2 km deep at most (Jansa, 1933; Campins et al., 1092 1995; Alpert et al., 1998) and may remain quite independent from the basic 1093 flow above the mountain. 1094 On the other hand, the frequent presence of intense cyclones is enough to 1095 explain some windstorms blowing within the Mediterranean region and the 1096 effect of orographic forcing is not always fundamental. The hot winds blowing 1097 from the desert across the Libyan and Egyptian segments of flat coast (Chili, 1098 Shimum),UNCORRECTED the strong easterlies over the Eastern Mediterranean (Saaroni et al., 1099 1998), and the Libeccio storms in the Tyrrhenian Sea, could constitute situations 1100 where such interaction is not crucial. Note that the extraordinary storms of 1101 December 1979 and November 2001, mentioned in Section 6.0, belong to the 1102 category of ‘‘intense cyclones’’. It can be added that the indirect methods of 1103 estimating winds (the scatterometers carried by satellite, like in ‘‘Quickscat’’) has 1104 given for the sustained winds in one of these storms (November 2001) values as 1105 high as 35 m/s. The synergistic combination of both mechanisms, that is the 1106 presence of intense cyclone and the generation of local winds, explains the high 1107 speed of extreme wind events. File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1108 Etesian winds blow over the Aegean Sea and belong to a larger circulation 1109 system including northerly winds over the whole Eastern Mediterranean. This 1110 Northerly circulation is produced by the combination of a high pressure 1111 over Balkan Peninsula or over central Europe and low pressure over eastern 1112 Mediterranean and Iraq, generally of thermal origin and corresponding to an 1113 extension of the summer Tibetan low to the eastern Mediterranean sea (Repapis 1114 et al., 1977). The Etesian winds can be intensified by the presence of a trough in 1115 the mid or upper troposphere over eastern Mediterranean. The temperature 1116 contrast between land and the Aegean sea can also influence (intensify or weaken) 1117 the intensity of the Etesian winds (Maheras, 1980). 1118 It is also worthy to remark that the leading edge of wind streams can act 1119 as an internal shallow front. On the other hand, associated with local wind 1120 streamers is the formation of orographically generated cyclonic and anticyclonic 1121 PV banners, characterized mainly by shear vorticity which often contributes 1122 to the Mediterranean cyclogenesis stimulating in some cases the heavy rainfall 1123 events (Ae¨bischer, 1996; Ae¨bischer and Scha¨r, 1996). 1124 1125 PROOF 1126 6.4.3. Storm Surge 1127 1128 Storm surge results from the combined action of atmospheric pressure and 1129 wind stress on the sea surface. Atmospheric pressure produces what is called 1130 the inverse barometric effect, according to which, in steady conditions a low 1131 pressure is associated with a sea level increase. Wind stress pushes horizontally 1132 the water column and tends to accumulate it at the closed end of a basin. 1133 In steady condition, the slope of the sea surface is proportional to wind stress 1134 and to the inverse of the water depth. Therefore, the action of wind stress 1135 dominates in shallow water and has a large effect in the Northern Adriatic 1136 Sea. These dynamics explain the importance of cyclones for storm surges and 1137 why the variability of cyclone regimes has an impact on the surge events which, 1138 in turn, can be considered an indicator of the cyclones characteristics. 1139 The stormUNCORRECTED surges in the northern Adriatic, and the consequent flooding of 1140 Venice, is caused by intense cyclones in the north-western Mediterranean (Trigo 1141 and Davies, 2002; Lionello, 2005). The synoptic patterns determining the surge 1142 in the Gulf of Venice present a low-pressure system with a minimum above 1143 central Europe or northern Italy (Figs. 116,117) which produces a strong 1144 Sirocco wind along the Adriatic Sea. Although these synoptic dynamics are 1145 well known, the mechanisms responsible for their frequency and intensity have 1146 not been completely understood. It appears that periods with extreme surge 1147 events are characterized by a general circulation anomaly, represented by a 1148 File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1149 1150 1151 BORA 1152 1153 MISTRAL 1154 TRAMONTANA MAESTRALE 1155 VARDARIS SIROCCO 1156 ETESIAN 1157 LIBECCIO SHARKIYA 1158

1159 VANDEVALES SIROCCO 1160 ETESIAN SIROCCO 1161 SHARAV 1162 CHILL

1163 GHIBLI KHAMSIN 1164 1165 1166 Figure 116: Location and direction of main windsPROOF in the Mediterranean region. 1167 Note that there are discrepancies between related terms in the different languages. 1168 In the French language, Mistral is the northerly wind descending along the 1169 Rhone Valley and blowing in the region of Provence, towards the western 1170 Mediterranean and Tramontane the northwesterly wind belonging to the 1171 same system and blowing in the Roussillon region. In the Italian language 1172 Tramontana is a northerly wind and Maestrale the dominant northwesterly 1173 wind blowing over most of the western Mediterranean Sea. Tramontana 1174 (or Tramuntana) is also the name of the northerly wind (belonging to the Mistral 1175 system) in Catalan and Spanish languages, blowing in the north of Catalonia and 1176 north of the Balearics. 1177 1178 1179 pattern with a negative centre of action above the Eastern Atlantic, according 1180 to whichUNCORRECTED cyclones are deviated south-eastward and penetrate into the 1181 Mediterranean Sea from North–West or generate a lee cyclone south of the 1182 Alps. This pattern is different from the NAO dipole, whose time behaviour is not 1183 correlated with that of the highest surges in the Gulf of Venice. It appears 1184 therefore that the storminess associated with the floods of Venice is not related 1185 to the NAO (Fig. 118 after Lionello, 2005). 1186 The records of floods in Venice show a clear positive trend, the cause of which 1187 is mainly the subsidence of the ground level, whose rate was particularly high in 1188 the years from the 1950s to the 1970s, rather than variations in the meteorological 1189 File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1190 A B 1191 1192 1193 1194 1195 1196 1197 1198 C D 1199 1200 1201 1202 1203 1204 1205 1206 E 1207 PROOF 1208 1209 1210 1211 1212 1213 1214 Figure 117: Evolution of the gph1000 field producing the storm surge in Venice. 1215 Panels (A) to (E) show the gph1000 composites (in hPa according to the greyscale 1216 bar) with a time lag of 48h, 24h, 0h, þ24h, þ48h with respect to the time of 1217 the surge peak. This situation is associated to a main minimum south of the Alps 1218 at the time of maximum surge (panel C). The composites are based on events 1219 with a peak surge value higher than 70 cm. 1220 1221 UNCORRECTED 1222 forcing fields. If subsidence is excluded, the residual variability, which can be 1223 associated with the meteorological forcing, shows trends which are small and 1224 dependent on the intensity of the storm surges. During the second half of the 1225 20th century, there is an indication that the frequency of moderate surges is 1226 increasing (Pirazzoli and Tomasin, 1999) while major independent surge events 1227 do not show large variation (Trigo and Davies, 2002). During the same period, a 1228 weak decreasing trend has been identified in the value of extreme levels in Trieste 1229 (Raicich, 2003). This behaviour has been shown to be consistent with the 1230 variation of the meteorological forcing in the Northern Adriatic area, that is, with File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1231 AB 1232 1233 1234 1235 1236 1237 1238 1239 C D 1240 1241 1242 1243 1244 1245 1246 1247 E 1248 PROOF 1249 1250 1251 1252 1253 1254 1255 Figure 118: Same as Figure 117, but for a situation associated with a main 1256 pressure minimum located north of the Alps at the time of maximum surge 1257 (panel C). 1258 1259 1260 more frequent moderate storms and less frequent intense storms. In Venice, 1261 there is no evidence of a correlation between extreme surge levels and the ongo- 1262 ing hemisphericUNCORRECTED warming trend. If the effect of regional sea level rise is subtracted 1263 from the data, the record of extremes is dominated by a large interdecadal 1264 variability, with respect to which an eventual residual trend is of minor 1265 importance (Fig.119, Lionello, 2005). The understanding of this variability, 1266 and of its past and future evolution, appears a very interesting scientific and 1267 practical issue. It will be important, on one side, to investigate the variability of 1268 large-scale patterns associated with it and, on the other side, to find the explana- 1269 tion for its correlation with the periodicity of the sunspot number. This last 1270 correlation points to the possible existence of a link between regional climate 1271 and external forcing which is not understood yet. File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 Figure 119: Indexes of the storm surge level time (black continuous), sunspots 1286 number (grey continuous), NAO (black dashed) during the 1940–2000 period 1287 (years on the x axis). NAO and storm surge level data include November and 1288 December only. All time series have been smoothed using a 3-year running mean. 1289 PROOF 1290 1291 1292 6.4.4. Wind Waves 1293 1294 Ocean waves are the consequences of winds, so that intense cyclones are the 1295 cause of extreme waves. In this respect storm surges and waves have a common 1296 cause. However, waves grow continuously under the action of the wind and 1297 their maximum height reflects the average intensity of the wind along the fetch. 1298 In other words, waves tend to depend on the integral of the wind stress along their 1299 travelling path, while surges are very dependent on its value over the shallow 1300 water areas near the coast. Measurements with moored buoys, and models 1301 capable of assimilating, analysing and forecasting waves, have demonstrated 1302 that high waves (with 5–7 m significant wave height) exist in the Mediterranean, 1303 in spiteUNCORRECTED of the relatively short fetches with respect to the Oceanic situations. 1304 As already mentioned, analysed SWH (Significant Wave Height) of 10–11 m were 1305 encountered in the case of the extraordinary storm of 10–11 November 2001 1306 (Gomez et al., 2002). 1307 It is not simple to reconstruct the wave climatology in the Mediterranean 1308 sea because long time-series of instrumental observations are lacking. Buoy 1309 observations are mostly available since the 1990s, when national buoy networks 1310 were installed. Satellite altimeter data are continuously available only since 1992. 1311 Therefore, the analysis of past variability is mostly based on model reconstruc- 1312 tions and ship observations. Often, model simulations under-evaluate the SWH, File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 Figure 120: Variation of the 10-year return value (y axis, cm.) during the 1327 1940–2000 period. The horizontal dashed line shows the value computed on the 1328 basis of the whole 1040–2001 period. The grey and black continuous lines show 1329 the values computed using events inside a moving 7-year and 21-year long 1330 time window, respectively.PROOF 1331 1332 1333 mostly because of lack of accuracy of the forcing wind fields. An example is 1334 Fig. 120 which results from a model integration based on the ERA40 reanalysis 1335 winds. Figure 120 represents the geographical distribution of maximum SWH 1336 values, but presumably does not show the actual height of the more intense 1337 events. 1338 There are several compilations and atlas of winds and waves derived from ship 1339 observations. There is a big difference between the Atlantic and Mediterranean 1340 situation, mainly due to different fetch, which in the Atlantic Ocean is generally 1341 larger than in the Mediterranean Sea. Therefore, the Atlantic waves are larger 1342 (longer period) than those in the Mediterranean, even when the same wind 1343 conditions are considered. Due to the low dissipation in long waves travelling 1344 out of theUNCORRECTED storm area (swell), the Atlantic waves remain relatively high for a 1345 long time, and, therefore, the average wave heights in the Atlantic are much 1346 higher than in the Mediterranean, where the swell component plays a minor role 1347 so that waves are mostly present only in correspondence to strong winds 1348 (windsea). In the Gulf of Lyons, in January, the frequency of waves above 1m 1349 of SWH is as large as 70%, and the frequency of waves above 2.5 m of SWH 1350 south-east of this zone is 20%. The growth of waves with fetch implies that 1351 the maximum frequency of high waves is displaced downstream from the 1352 maximum frequency of strong winds; for example, the highest waves in the 1353 Mediterranean are just downstream of the mistral core. Significant wave File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1354 heights of 6m or more are reached every year (on average) in this zone of the 1355 Mediterranean where their presence is related to the high frequency and intensity 1356 of the Genoa cyclones. 1357 Figure 120 confirms the presence of these maxima and shows also other 1358 features in the various basins. High waves are present over most of the 1359 Mediterranean Sea and tend to reach the highest values where strong wind and 1360 long fetch are simultaneously present. The largest maxima are located in the 1361 western Mediterranean and in the Ionian Sea, under the action of the Mistral, 1362 where the shape of the Mediterranean Sea determines the most effective 1363 combination of a long fetch and a strong wind. The Island of Crete interrupts 1364 the fetch of the Etesian winds and determines two separated maxima: one in the 1365 Aegean and another in the Levantine Basin. Sirocco produces the maximum 1366 SWH in the Northern Ionian and in the southern Adriatic. A maximum due to 1367 the Bora wind is present in the northern Adriatic Sea, and another due to the 1368 Vendavel in the Alboran Sea. In summer, wave height is small over the whole 1369 basin and a characteristic maximum is present in the Aegean sea caused by the 1370 action of the Etesian winds. 1371 Figure 121 shows the sea level pressure compositesPROOF associated with high 1372 waves in different areas of the Mediterranean Sea. Cyclones located near Cyprus 1373 are responsible for high waves in the Levantine Basin, those in the Gulf of Genoa 1374 for high waves in the western Mediterranean, while those above Tyrrenian 1375 or central Italy produce high waves in the Adriatic and central Mediterranean 1376 Sea. Obviously, waves are the result of the action of past winds, so that also 1377 1378 1379 1380 1381 1382 1383 1384 1385 UNCORRECTED 1386 1387 1388 1389 1390 1391 Figure 121: Distribution of maximum wave height as resulting from a 40-year 1392 model integration carried out using the ERA-40 re-analysis. Contour levels show 1393 the maximum SWH, arrows show the mean wave direction corresponding to 1394 SWH maximum (from Sanna and Lionello, 2005). File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1395 the past evolution of the synoptic situation contributes to the value and location 1396 of the maximum SWH. 1397 Wind wave extremes, obtained from model simulations, show little significant 1398 trends. Figure 122 shows the statistically significant variations of maximum 1399 SWH in 50 years, on the basis of linear trends derived from the ERA40 1400 reanalysis. The two main features correspond to a reduction in the Ionian and 1401 Alboran Sea, which are consistent with a reduction of cyclones in the western 1402 Mediterranean. The significant increase is limited to a very small region near 1403 the coast of France. 1404 1405 1406 A B 1407 1408 1409 1410 1411 1412 PROOF 1413 1414 1415 C D 1416 1417 1418 1419 1420 1421 1422 1423 1424 E 1425 1426 UNCORRECTED 1427 1428 1429 1430 1431 1432 1433 Figure 122: Synoptic patterns associated with extreme significant wave 1434 height in different regions of the Mediterranean Sea: (A) Tyrrenian, (B) Adriatic, 1435 (C) Balearic, (D) Ionian, (E) Levantine basin (from Sanna and Lionello 2005). File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1436 6.4.5. Landslides 1437 1438 Rainfall-induced landslides are usually directly associated with the passage of 1439 intense storms of Atlantic or Mediterranean origin. In a recent work, the link 1440 between the occurrence of Landslide episodes in Portugal and the storm tracks 1441 associated with the NAO pattern has been established (Trigo et al., 2005). 1442 Naturally, strong cyclones can produce intense rainfall events that are responsible 1443 for the rapid growth of pore pressure and for the loss of the apparent cohesion of 1444 thin soils, resulting in failure within the soil material or at the contact with the 1445 underlying impermeable bedrock. A different type of association between 1446 cyclones and landslides can be found for rainfall periods which are less intense 1447 but have a long duration. In this case long-lasting rainfall periods (from 30 days 1448 to 90 days), are responsible for the activity of deeper slope movements, such as 1449 translational slides, rotational slides and complex and composite slope move- 1450 ments. This is the group of landslide events mostly affected by the large-scale 1451 atmospheric circulation mode NAO. 1452 The western Mediterranean Basin is prone to slope instability due to geo- 1453 logical, geomorphological and climatic factors. ItPROOF is widely accepted that high 1454 duration/intensity rainfall events (associated with intense cyclone events) are the 1455 most important triggering mechanism of landslides worldwide (van Asch et al., 1456 2001). In particular, rainfall-induced landslides have been studied in Portugal 1457 (Trigo et al., 2005), Spain (Corominas and Moya, 1999), Italy (Petrucci and 1458 Poleminio, 2003) and France (Flageollet et al., 1999). Landslide consequences 1459 include damages on property, houses and particularly roads, and can be also 1460 related with the increasing human pressure related to urban development 1461 throughout the countryside (Trigo et al., 2005). 1462 1463 1464 1465 6.5. Conclusions 1466 1467 The MediterraneanUNCORRECTED region is characterized by strong morphological forcing. 1468 This characteristic is important for cyclones and their effects. The steep 1469 orography surrounding the basin and the complicated land–sea distribution 1470 introduce a rich mesoscale structure. On the one hand, these two factors produce 1471 a peculiar phenomenology internal to the basin and, on the other hand, they 1472 modulate the interaction of the Mediterranean system with global climate 1473 patterns by adding sub-regional features. As far as cyclones are concerned, 1474 steep orography and complicate land–sea distribution condition the formation 1475 and the evolution of cyclones themselves and the effects they produce on the 1476 environment. File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1477 Mountain ridges are responsible for the presence of many areas with 1478 orographic cyclogenesis, land–sea contrast for thermal lows, the presence of 1479 moist Mediterranean air with potential for strong diabatic processes conditions 1480 the development of cyclonic structures, like small-scale, hurricane-like lows. 1481 The triggering mechanism of most Mediterranean cyclones is mostly due to 1482 features external to the Mediterranean region, that is mid-latitude (primary) 1483 baroclinic waves with high-level potential vorticity anomaly, which interacts 1484 with regional structures in the Mediterranean region. The complexity of the 1485 mechanisms involved is such that many different categories of cyclones can be 1486 identified, according to the region of formation, their seasonality and dominant 1487 mechanism of formation. Beside the cyclones entering from the Atlantic sector, 1488 there are lee cyclones, thermal lows, African cyclones, mesoscale hurricane- 1489 like lows, Middle East cyclones. Further differentiations are possible, as several 1490 distinct regions of cyclogenesis exist for lee cyclones and different types of 1491 cyclones can be defined in the Middle East. Most cyclones have a radius smaller 1492 than 600 km. Seasonality is different for various categories; in general, the overall 1493 synoptic activity is higher from November to March, but there are types of 1494 cyclones, like thermal lows, Sahara cyclones andPROOF Middle East depressions, 1495 whose frequency is larger in summer. 1496 Because of this complexity, several large-scale patterns can be associated 1497 with cyclones in the Mediterranean area. While the NAO certainly plays an 1498 important role (which is dependent on the type of cyclone considered), block- 1499 ing episodes above central and northern Europe explain a large fraction of 1500 variability. Other patterns, like the East Atlantic/Western Russia pattern, exert 1501 a significant influence too. 1502 Long-term trend analysis (since 1850) shows a reduction of the cyclone 1503 activity over most of the region. During the second half of the 20th century 1504 this trend is confirmed for the western part, while an increase of cyclone activity 1505 has been observed for the eastern part. However, a very large interannual 1506 variability is superimposed on these trend. 1507 Cyclones are associated with many extreme events: precipitation, winds, waves, 1508 landslidesUNCORRECTED and surges. Several hazardous weather events take place every year 1509 in the Mediterranean region and are a relevant cause of economic losses. In all 1510 these events, the geography at regional-scale plays a fundamental role in 1511 determining the effects of cyclones on the environment. Prolonged heavy rain 1512 episodes take place when a cyclone forces surface currents of humid and warm 1513 Mediterranean air to flow over coastal mountain slopes. High waves are 1514 produced when the location of the cyclone ensures a long fetch. Strong winds 1515 occur when orography locally intensifies the cyclonic circulation around a low- 1516 pressure centre. The storm surge in the Northern Adriatic sea takes place 1517 when the south-easterly Sirocco wind is channelled along the Adriatic Sea. File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1518 This implies that the intensity of the cyclone is not the only factor responsible 1519 for its impact, but its position and evolution are also extremely important. 1520 Therefore, not only the variability of the cyclone intensity, but also that of 1521 its track contributes to the impact on the environment. 1522 In general, extremes present larger variability than average values and, 1523 consequently, it is more difficult to identify significant trends. While most of 1524 the Mediterranean region, in winter, experiences a decrease in total precipitation 1525 and average SWH (Lionello and Sanna, 2005), extremes do not show spatially 1526 and temporally coherent trends over the whole Mediterranean region. Extreme 1527 SWH levels have become smaller only in part of the Ionian and in the Alboran 1528 Sea, while are increasing in a small area close to the coast of France. The 1529 frequency of torrential rainfall has been found to increase (percentually wise) 1530 in the second half of the 20th century. 1531 1532 1533 1534 6.6. Outlook 1535 PROOF 1536 This chapter documents the large amount of research which has already been 1537 carried out and provides a well-established understanding of many aspects of 1538 the Mediterranean cyclones and their effect on the environment. However, in 1539 consideration of the importance of these processes, of their potentially damaging 1540 effects, and the need for assessing their sensitivity to climate changes, further 1541 research is recommended (see also Chapter 8). 1542 A main issue is whether the present archives of data provide an adequate 1543 database for the needed analysis. On the centennial timescales it appears that 1544 there is an imbalance between regions where phenomena are well documented 1545 (e.g. the surge of Venice, Camuffo, 1993), and regions where data are scarce 1546 and reconstruction of past events necessarily indirect (e.g. precipitation patterns 1547 and extremes on the whole African coast). For the recent decades, where 1548 meteorological observations are available worldwide and model reanalysis have 1549 been carriedUNCORRECTED out (e.g. NCEP and ERA-40 re-analysis), it has still to be fully 1550 investigated whether the subsynoptic and mesoscale characteristics of cyclones 1551 in the Mediterranean region are well represented in the available data archives. 1552 On this respect, the development of extensive, high-resolution sets of data 1553 appears extremely important for addressing unresolved scientific issues. Similar 1554 considerations apply for climate change studies. Certainly more work is needed 1555 to link modeling of selected events, long-term modeling and observation 1556 analysis, with the goal of a coherent long-term climatological perspective. 1557 In this respect, interactions between climate and meteorological projects, 1558 such as MedCLIVAR (endorsed by the WCRP) and MEDEX (a project of File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1559 1560 1561 1562 1563 1564 1565 1566 1567 1568 1569 1570 Figure 123: Statistically significant variations of maximum significant wave 1571 height in 50 years, based on linear trends evaluated from the results of a 40-year 1572 model integration carried out using the ERA-40 re-analysis. 1573 1574 1575 WMO-WWRP), national initiatives and the involvement of regional institutions 1576 are certainly important and potentially fruitful. PROOF 1577 The classifications of cyclones should involve an analysis of the sensitivity of 1578 the generation mechanisms to climate variability and change, whose identifica- 1579 tion might help in predicting future scenarios and the change in the frequency and 1580 intensity of some specific cyclone types (e.g. the hurricane-like Mediterranean 1581 Lows). The availability of regional high-resolution reanalysis, where cyclone 1582 structures are well reproduced, might be crucial for this task. Moreover, it is 1583 important to identify the deficiencies in the models that account for the 1584 inadequacies in simulating the cyclones and their variability. On this point, the 1585 connection between the climate of the cyclones in the Mediterranean region 1586 and the low-frequency large-scale variability is not sufficiently understood yet. 1587 Intensity, location, duration and orientation of the systems as well as their 1588 interdecadal trends should be analysed and put in relation with variations of 1589 the main European teleconnection patterns. The importance of regional-scale 1590 processesUNCORRECTED (e.g. latent heat release over the sea) and their variability with respect 1591 to large-scale forcing (e.g. the meridional shift and/or intensification of the 1592 storm track over northern Europe) has not been precisely quantified. 1593 The Mediterranean Sea is characterized by a complex coastline structure, 1594 with some highly vulnerable areas (for example, the Niles delta and the Gulf of 1595 Venice). Though the main danger for these areas is due to the increased coastal 1596 erosion and loss of land that would be produced by sea level rise, the change 1597 in storminess is also potentially critical. Variations of the frequency and intensity 1598 of sea storms could further increase risks and damages. The analysis of changing 1599 wind waves and surge regimes requires detailed impact studies, carried out on File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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1600 the basis of sufficiently precise forcing fields, and relies on accurate surface 1601 wind field analysis and adequate downscaling techniques. 1602 Variations of cyclone regimes affect the distribution of precipitation. It may 1603 be suggested that the variations of the precipitation observed during the last 1604 decade over the Mediterranean region are associated with relatively small 1605 variations in the transport and characteristics of the air masses. Such changes 1606 might be small and not always easily detectable; however, their impact on local 1607 climate and climate variability is likely to be large. These variations could 1608 have serious consequences for rain intensities in many Mediterranean areas. 1609 The danger is twofold. There are areas already under stress because of recurrent 1610 water shortage during summer, and areas where torrential rains have produced 1611 human casualties and large damages to properties. It is important to identify 1612 the factors responsible for the increase in rainfall extremes and reduction of 1613 total precipitation A similar understanding is also important for waves, surges 1614 and winds 1615 Finally, the links between large-scale patterns and extreme events are not 1616 simple, as extreme events are cannot easily associated to extreme values of some 1617 large-scale predictors. The characterization of patternsPROOF of cyclones, weather 1618 extremes and their link to large-scale fields is a topic on which more research is 1619 needed. 1620 1621 Acknowledgements 1622 1623 The comments by P.Malguzzi have been of great help in improving the content 1624 of this chapter. The authors are indebted to C. Zerefos, P. Alpert, J. Luterbacher 1625 and E. Xoplaki for important suggestions and to E.Elvini for his help with the 1626 graphics. 1627 1628 1629 References 1630 1631 Alpert, P.UNCORRECTED (1989). Baroclinic waveguides and rate of alternation. J. Atmos. Sci., 46, 1632 3505–3507. 1633 Alpert, P., Neeman, B. U., & Shay-El, Y. (1990a). Climatological analysis of 1634 Mediterranean cyclones using ECMWF data. Tellus, 42A, 65–77. 1635 Alpert, P., Neeman, B. U., & Shay-El, Y. (1990b). Intermonthly variability of cyclone 1636 tracks in the Mediterranean. J. Climate, 3, 1474–1478. Alpert, P., Stein, U., & Tsidulko, M. (1995). Role of sea fluxes and topography in 1637 Eastern Mediterranean cyclogenesis. The Global Atmosphere-Ocean System, 3, 55–79. 1638 Alpert, P., Osetinsky, I., Ziv, B., & Shafir, H. (2004). Semi-objective classification 1639 for daily synoptic systems: application to the Eastern Mediterranean climate change. 1640 Int. J. Climatol., 24, 1001–1011. File: {Elsevier}Lionello/Pageproofs/3d/N52170-Lionello-Ch006.3d Creator: / Date/Time: 5.11.2005/6:04pm Page: 313/358 ARTICLE IN PRESS

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UNCORRECTED