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Chapter 6
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Cyclones in the Mediterranean Region:
Climatology and Effects on
the Environment
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University of Lecce, Italy ([email protected])
University of Bern, Switzerland ([email protected], [email protected])
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ISAC-CNR, Italy ([email protected])
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Tel Aviv University, Israel ([email protected])
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INM Spain ([email protected])
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University of Thessalonoki, Greece ([email protected])
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ARPA, Piemonte, Italy ([email protected])
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Instituto de Meteorologia, Centro de Geofı´sica da, Universidade de Lisboa,
Portugal ([email protected])
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CGUL at University of Lisbon and Universidade Lusófona Portugal (rmtrigo@
fc.ul.pt)
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P. Lionello,1 J. Bhend,2 A. Buzzi,3 P.M. Della-Marta,2 S.O. Krichak,4 A. Jansà,5
P. Maheras,6 A. Sanna,7 I.F. Trigo,8,9 and R. Trigo9
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6.1. Introduction
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Cyclones represent the most important manifestation of the mid-latitude highfrequency variability, and play a fundamental role in the atmospheric large-scale
horizontal (and vertical) mixing and in modulating the air–sea interaction.
Cyclonic circulations, due to their frequency, duration and intensity, play an
important role in the weather and climate over the entire Mediterranean region
(Radinovic, 1987). A large spectrum of environmental variables and phenomena
are associated with cyclones in the Mediterranean region. Wind, pressure,
temperature, cloudiness, precipitation, thunderstorms, floods, waves, storm
surges, landslides, avalanches, air quality and even the fog and visibility in the
Mediterranean are influenced by the formation and passage of cyclonic
disturbances. Many phenomena are beneficial from the agricultural, hydrological
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and economical point of view. However, some of them are damaging and AQ: please
occasionally disastrous (MEDEX, Jansa et al., 2001).
clarify whether
Cyclones are important when information more complete than that provided it is 2001a or b
by the average geopotential (or sea level pressure) field is required and when
aspects of the probability distribution characterizing the statistics of the atmospheric circulation besides its average pattern are investigated. Consequently,
statistical analysis of cyclones is important especially for the ‘‘tails’’ of probability
distributions, that is the part characterizing extremes values of variables such
as precipitation, winds, waves, storm surges. The Mediterranean area, although
located to the south of the main Atlantic storm track that more directly affects
western and northern Europe, is quite frequently subjected to sudden events of
extreme and adverse weather, often having high social and economic impacts.
The morphology of the territory, with small and steep river basins and highly
populated, industrialized and tourist areas, makes the Mediterranean especially
sensitive to the impact of weather phenomena, especially heavy rain and consequent flooding. A report (based on 10 years of data) prepared by the Munich
Reinsurance Company collects 166 cases of heavy rainfall and floods and
104 cases of strong wind and storms producing serious damages. The total
number of deaths is over 1,900 and the quantified economic losses are over
6,000 MEuro. These figures are certainly underestimates. For Spain alone, and
only in four years (1996–1999), the Programme of Natural Hazards of the
Spanish Directorate of Civil Defence reported 155 deaths by heavy rain and
flood events and 28 deaths by storms and strong winds (Jansa et al., 2001a)
in Greece according to the data published by the Hellenic Agricultural Insurance
Organization (ELGA) only for the year 2002 the economic losses due to heavy
precipitation, floods, hail and extreme winds were over 180 MEuro. Single
disastrous events have been recorded, such as the storm of 4 November 1966
which hit central and north eastern Italy, causing more than 50 deaths and
widespread, huge damages in the eastern Alps, Florence and Venice (where
the damage produced by the surge is estimated to be equivalent to 400 KEuros
present-day). Over the sea, significant wave height as large as 10 or 11 m are also
reported in extraordinary storms, like the 10–11 November 2001 storm in the
Western Mediterranean, that produced the destruction of beaches and coastal
flooding in northern Mallorca (Gomez et al., 2002) and the big storm at the
end of December 1979, that seriously damaged the port of Oran (Algeria).
More examples can be found in the MEDEX list of selected cases, Jansa et al.
(2001a). A large portion of such severe weather-related events are associated
with cyclones in the Mediterranean (Jansa et al., 2001a).
Since the capability of climate models to reproduce intense cyclones and
extreme weather events is limited, it is necessary to determine the links
between their probability distribution and large-scale patterns or/and indices.
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The identification and analysis of the large-scale patterns associated to the
occurrence of cyclones is very important. In fact, as climate models are more
accurate in reproducing the large-scale structure of atmospheric circulation
than the statistics of cyclones and extreme events, the prediction of the behaviour
of large-scale patterns in future climate scenario could be a robust tool for
predicting changes in the intensity and characteristics of cyclones and extreme
events.
It is important to note that the link between the intensity of cyclones and
hazardous extreme events is not simple and different characteristics can be
involved as different impacts are considered. The intensity of circulation (winds),
precipitation (with consequent floods) and of the cyclone itself (measured as
the minimum value of the sea level pressure or the strength of the overall
associated circulation) are not necessarily related in a simple (linear) way.
An example is the disastrous flood which affected central and Northern Italy
during November 1966, characterized by very intense precipitation and high
winds, which but whose central minimum pressure was not remarkably low
(De Zolt et al., 2005).
Finally, Mediterranean cyclones have also an influence on regions outside their
region of origin. Radinovic (1987) suggests that cyclones in the Mediterranean
region influence the weather and climate further east in central Europe, in
countries such as Hungary, Romania, Ukraine and Russia, and in Asian areas,
like Syria, Iraq, Iran, Afghanistan or northern India. At the same time, besides
the large amount of cyclones entering the Mediterranean region from the middle
latitude storm track, there is evidence of a significant role played by tropical
cyclones, which can produce atmospheric circulation patterns advecting moisture
into the Mediterranean region and, occasionally, move into it during a later
stage of their life cycle after having experienced a transition to extratropical
systems (Pinto et al., 2001; Turato et al., 2004).
The introductory Section 6.1 of this chapter consists of Subsection 6.1.1
describing the evolution of research on cyclones in the Mediterranean region
and their role on climate. Section 6.2 describes the dynamics responsible for the
formation and evolution of cyclones in the Mediterranean region (Subsection
6.2.1), the datasets available for their analysis (Subsection 6.2.2) and the methods
for the identification of cyclones and the evaluation of the intensity of their
activity (Subsection 6.2.3). Section 6.3 describes the climatology of cyclones,
their characteristic spatial scales, seasonality, area and mechanisms of origin
(Subsection 6.3.1); the relation between cyclonic activity and large-scale
climate patterns (Subsection 6.3.2); and the observed trends (Subsection 6.3.3).
Section 6.4 describes the effects of cyclones on the Mediterranean environment;
it is divided into five subsections, describing their role on precipitation, winds,
storm surge, ocean waves and landslides. For each subsection, a description of
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the mechanism explaining the effects of cyclones on the specific phenomenon
is described, and climate trends are discussed. The concluding Section 6.5
summarizes the available knowledge on cyclones and on their effects in the
Mediterranean region. The outlook section 6.6 discusses the main open research
issues and subjects of ongoing and future research.
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6.1.1. ‘‘Historical’’ Notes
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Pioneering studies that include climatology of cyclones and cyclogenesis in the
Mediterranean (in the frame of hemispheric studies) are those by Pettersen (1956)
and Klein (1957). They are large-scale studies and show, at even this scale, that
the western Mediterranean is a distinct and very active area of cyclogenesis
and with frequent presence of cyclones in winter, in the northern Hemisphere.
These studies are based on hand-made analyses and subjective detection of the
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cyclones. Subsequent studies, using the same techniques, were able to investigate list these
smaller scales, even including mesoscale features (Radinovic and Lalic, 1959; references.
Radinovic, 1978; Genoves and Jansa, 1989). The annual total number of cyclones
detected in the mesoscale studies (Radinovic, 1978 or Genoves and Jansa, 1989)
was, as expected, much larger than the number of cyclones found in larger
scale analyses. Since 1990 (Alpert et al., 1990), most of the studies on climatology
of the Mediterranean cyclones are based on objective analyses and objective
techniques aimed at detecting and tracking the cyclones, but there are also
studies based on mixed databases, subjective and objective, like Campins et al.
(2000). Several studies have focused on the characteristics of the cyclones in the AQ: Please
Mediterranean area, on their dynamics, locations, frequency and temporal list these
variability of cyclogenesis (Buzzi and Tosi, 1989; Tosi and Buzzi, 1989; Alpert references.
et al., 1990a,b; Trigo et al., 1999; Campins et al., 2000; Maheras et al., 2001, 2002,
Lionello et al., 2002).
A different category of studies have analysed the link between cyclones AQ: please
and environment. Most studies have been focused on precipitation (Trigo et al., clarify whether
2000; Jansa et al., 2001; Kahana et al., 2002; Maheras et al., 2002, 2004), but also it is 2001a or b
other aspects such as storm surges, (Trigo and Davies, 2002; Lionello, 2005),
and wind waves (Lionello et al., 2002) have been considered.
The impact of extreme weather conditions on landslides occurrence has been
attempted over different areas of the Mediterranean. Recent techniques tend to
favour the use of satellite and airborne imagery to assess changes in geomorphology. However the use of historical data obtained form in situ analysis plus
local newspapers and interviews is a more reliable tool to establish precise links AQ: Please
between the timing of intense rainfall events and concurrent observed landslide list these
episodes (e.g. Zêzere et al., 1999; Polemio and Petrucci, 2003; Trigo et al., 2005) references.
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6.2. Mediterranean Cyclones: Data, Methods and Dynamics
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This section contains a description of the dynamics responsible for formation
and evolution of cyclones in the Mediterranean region, and of data and methodologies used for analysing their climatology.
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6.2.1. Dynamics of Cyclones in the Mediterranean Region
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The cyclones in the Mediterranean region represent a well-distinct element of
the global climate. A Mediterranean storm track structure has been put into
evidence in different studies and it has been shown that the regular cyclone tracks
over the Mediterranean are linked to the baroclinic waveguides (Wallace et al,
1988) and to high rate of alternation between cyclones and anticyclones
(Pettersen’s, 1956) in the Mediterranean region (Alpert, 1989). The presence of
a separate branch of the Northern Hemisphere storm track crossing the
Mediterranean region, with areas of more frequent cyclogenesis in the western
Mediterranean and of cyclolysis in the central and eastern Mediterranean, though
less intense than the storm track in the Atlantic and Pacific, has been confirmed
in recent analysis of the Northern hemisphere (Hoskins and Hodges, 2002).
At the same time, there are studies showing that the Mediterranean region
presents the highest concentration of cyclogenesis in the world (Pettersen, 1956;
see also Radinovic, 1987, for a general view) during the northern hemisphere
winter. Some of them are so intense that they are classified as ‘‘meteorological
bombs’’ (Conte, 1986; Homar et al., 2000).
According to the conceptual model re-proposed by Hoskins et al. (1985),
cyclogenesis occurs when and where a high-level PV (Potential Vorticity) positive
anomaly overlaps a low-level potential temperature or PV positive anomaly or a
frontal zone. The formation of low-level shallow depressions by orography and
thermal contrasts is very frequent in the Mediterranean, owing to the complex
topography of the region. Therefore hundreds of cyclonic disturbances can be
subjectively and objectively detected in the Mediterranean every year (Campins
et al., 2000; Picornell et al., 2001). Actually many of them are shallow depressions which cannot be considered deep cyclones, but, they can play a role in
the initiation of actual deep cyclogenesis events. There are three kinds of
evolution that can be identified in the Mediterranean: (a) The low-level
disturbance does not evolve and remains shallow, weak or moderate and
nearly stationary if the upper level PV anomaly is absent or too far away to
interact with it (Genoves and Jansa, 1991). In this case real or deep cyclogenesis
does not develop. (b) An upper level PV anomaly will create cyclogenesis
when arriving over a frontal zone, just under the maximum PV advection at
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high levels, with or without the presence of a depression at low level.
Some Mediterranean cyclogenesis could be, at least partially of this type,
comprising those cyclones generated at the quasi-permanent Mediterranean AQ: Please
border front (Alpert and Ziv, 1989) or at some internal fronts. (c) The low-level list this
disturbance rapidly deepens and cyclogenesis occurs when the upper level reference.
perturbation moves close enough to interact with it.
The Mediterranean region presents geographic factors that can substantially
modify the cyclogenesis mechanism. The orography of the region changes
quantitatively and qualitatively the baroclinic instability process, usually
favouring or ‘‘focusing’’ the cyclogenesis (Speranza et al., 1985). The high
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frequency of orographically induced low-level disturbances may partially list these
explain the high frequency of real cyclogenesis in the Mediterranean (Genoves references.
and Jansa, 1991, Jansa et al., 1994). Latent heat release usually sustains
and intensifies most of the cyclogenesis processes. In the Mediterranean region,
this effect seems to be quite important in the Eastern Mediterranean, when
a Sharav cyclone arrives there from the desert and intensifies over the sea
(Alpert and Ziv, 1989). Some cases in the western Mediterranean also have
the same evolution (Homar et al., 2000). Enhanced baroclinic instability in
saturated air (Fantini, 1995) influenced by latent heat release in an environment convectively stable can be another process contributing to cyclogenesis.
In general, the role of latent heat release and diabatic processes is a key issue
in the Mediterranean region, though of secondary importance when intense
orographic cyclogenesis, both Alpine (Buzzi and Tibaldi; 1978; Dell’Osso and
Radinovic, 1984; Speranza et al., 1985; Tibaldi et al., 1990; Stein and Alpert,
1993; Alpert et al., 1995; Buzzi, 1997) or non-Alpine (Garcia-Moya et al., 1989),
takes place.
Such complicate dynamics and the potential for many different mechanisms
favouring cyclogenesis imply that extremely diversified classes of cyclones are
present in the Mediterranean region. A tentative list, based partially on the
mechanisms producing cyclogenesis and partially on the geographical character- AQ: Please
istics, would include lee cyclones, thermal lows, small-scale hurricane-like list these
references.
cyclones, Atlantic systems, African cyclones and Middle East lows.
Lee cyclones are triggered by the passage of a major synoptic low-pressure
system north of the region, so that their generation is expected to be very sensitive to the location of the storm track above Europe. Lee cyclones develop
south of the mountain ridges representing the northern boundary of the
Mediterranean region. The Gulf of Genoa is the region of most frequent intense
cyclogenesis in the Mediterranean, but lee cyclones are also generated in the
Adriatic Sea (Flocas and Karacostas, 1994; Ivançan Picek, 1996), in the Cyprus
and Aegean Sea areas (Reiter, 1975; Alpert et al., 1990), in the Black Sea, and
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in several areas of the western Mediterranean sea (Jansa, 1986). The passage of
the same synoptic system can be responsible for successive and distinct
cyclogenesis in the Gulf of Genoa, Aegean and Black Sea (Trigo et al., 1999).
The spatial distribution of the frequency of cyclogenesis presents relative
maxima in the Cyprus and Aegean region, in the Adriatic, in the PalosAlgerian sea, in the Catalonian-Balearic sea, and in the Gulf of Lyons
(Jansa, 1986).
Thermal lows are more frequent in spring and summer and their genesis
and lysis are modulated by the daily cycle of temperature. Their occurrence is
therefore depending on the amplitude of this cycle and on the land–sea
temperature contrast. Many of them remain shallow depressions confined to
the lower troposphere. Modeling studies suggest that they are also generated
over sea in autumn and winter, when the land–sea temperature gradient is
reversed.
Small-scale, hurricane-like Mediterranean Lows have been detected over sea.
They are a special class of Mediterranean cyclones in which the main source of
energy is the great amount of latent heat released in large convective cloud
clusters, as in tropical cyclones (Rasmussen and Zick, 1987).They are likely to
depend critically on the air–sea temperature difference and on the content of
moisture in the atmosphere.
Atlantic systems mostly enter in the Mediterranean region from the west
and northwest, and cross the Mediterranean during their attenuation phase.
Even when their central pressure minimum does not pass directly above the
Mediterranean Sea, they affect the Mediterranean weather as they can cause
secondary lee cyclones.
Northern Africa is the source of many cyclones arriving from the south
which often form or intensify south of the Atlas Mountains as lee cyclones.
Their formation is more likely to occur in spring and summer when static
stability is low.
The classification of cyclones in the south-eastern Mediterranean region
includes several types of cyclones, with different seasonality and origin, such as
the Cyprus Lows, Syrian Lows and Red Sea Troughs. The Cyprus Lows
are mostly orographically generated or strengthened. The Syrian low (Kahana
et al., 2002) is a particular type of the Cyprus low system so intense that
penetrates into some areas of Syria. The Red Sea trough system develops during
spring and autumn due to topographic effects in the Red Sea area. Usually
it is a warm and shallow trough with a very dry southeasterly flow, but torrential rains occur when an upper cold air trough penetrates southwards
above it, due to the extreme instability in this situation (Krichak et al., 1997ab;
Krichak and Alpert, 1998)
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6.2.2. Available Sets of Data
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As mentioned before, pioneering studies on the climatology of cyclones in the
Mediterranean were based on hand-made subjective analyses. Due to their own
nature, the subjective analysis datasets are in general not available except in the
institutions where the analyses were prepared, and often not recoverable for
subsequent studies. The subjectivity has the advantage that a hand-made analysis
can be even more careful and detailed than objective analyses in some areas,
due to appropriate conceptual models and complementary information, etc.
At the same time, the subjectivity has the disadvantage of being not homogeneous, neither in space nor in time, because some areas and situations can
be systematically poorly or incorrectly analysed due to the lack of specific skill
and experience of the authors. In general, the quality of objective analyses has
improved to the point where their advantages outweigh those of the subjective
analyses.
The operational objective analyses are presently the main source of data for
performing a climatological analysis of cyclones, although the frequent changes
in the procedure (due to changes in the forecasting model, in its resolution, in the
data available for its initialization and in the data assimilation method) makes
the series quite temporally heterogeneous. Generally, only relatively short
samples of operational analyses are homogeneous and can be used for climatic
studies. In order to compensate for these inadequacies, homogeneous reanalysis
datasets, like ERA15, NCEP or ERA40, have been produced, though with
resolution which is coarser than most of the advanced operational analyses
(Gibson et al., 1996; Kalnay et al., 1996; Simmons and Gibson, 2000). These
homogeneous reanalysis data sets (ERA15, NCEP and ERA40) are the basis for
all climatological studies in general and for cyclones in particular. Without them,
it would be difficult or even impossible to obtain reliable fields of geopotential
or SLP needed for systematic analysis of trends and variability.
As a practical compromise, attempting to exploit both advantages of the
high resolution of recent operational analyses and of the homogenous multidecadal re-analyses, the MEDEX database contains lists of cyclones identified
both in operational analyses (HIRLAM/INM, over the Western Mediterranean,
and ECMWF over the whole Mediterranean) and reanalyses (ERA40).
Another dataset has been created to investigate the changes and variability
in SLP from 1850 as part of the EMULATE project (European and North
Atlantic daily to multi-decadal climate variability project). The dataset is a
combination of homogenized daily SLP station series, ship observations and
previously reconstructed SLP fields (Ansell et al., 2005).
The dataset consists of daily mean sea level data on a 5 5 grid, and it has a
lower spatial and temporal resolution than the reanalysis datasets. Though,
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obviously, it cannot detect mesoscale systems or accurately locate the areas of
cyclogenesis, this dataset allows the identification and tracking of synoptic-scale
cyclones in the Mediterranean region from 1850.
Extreme cyclones can also be identified by their effects which can be measured
or recorded. Instrumental data on surges, waves, floods, and winds can be used
for reconstruction of time series of past cyclones. A peculiar example is provided
by the records of storm surges in Venice, from past chronicle and archives.
The highest surges have been reported with a precision which is sufficient to
reconstruct the frequency of past floods since the 8th century AD (Camuffo,
1993). The time series shows a succession of periods of recurrent floods, which
were particularly intense in the first half of the 16th and of the 18th century,
separated by more quiet periods, and a continuous positive trend during the
second half of the 20th century. This last part of the time series is very accurate
since the tide gauge records have begun in 1872. This example represents the
limits and potential of such local reconstructions. On one side, they are a tool
for obtaining very long time series in historical times. On the other side, in general
it is very difficult to associate past variability to large-scale regimes and make
the distinction between large-scale and local processes (the soil subsidence in
this case) for which additional information is needed. Also the homogeneity of
the time series along its whole extent can be argued, as its reconstruction often
involves subjective criteria which need to account for the temporal changes in
the level of vulnerability of society, in human activities, resources and technological capability. Finally, it is difficult to attribute the cause of trends without
supplementary information. Though floods of Venice occurred many times
during its history, the last 50 years represent an unprecedented period of frequent and intense events. However, this is largely explained by the local loss
of relative sea level (a combination of ground subsidence and sea level rise) not
associated to a trend of storminess (Lionello, 2005)
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6.2.3. Methodology
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In recent years, two distinct approaches have been used to study the storm
activity over the North Atlantic and Europe: storm track algorithms and analysis
of synoptic variability. Usually, storm track algorithms apply sophisticated
methods that can detect the regions of storm development (cyclogenesis) and
decay (cyclolysis) as well as the specific paths of each individual storm (Murray
and Simmonds, 1991; Serreze et al., 1997; Trigo et al., 1999, 2002; Lionello
et al., 2002). Analysis of synoptic variability is a simpler approach, which
corresponds to the identification of the synoptic variability using a bandpass filter that retains mainly variability on the 2–8 day period of SLP or 850
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or 500 hPa geopotential (Buzzi and Tosi, 1989; Hoskins and Hodges, 2002).
This second approach has been widely applied to quantify the synoptic activity
associated with a high and low NAO (North Atlantic Oscillation) index
(e.g. Rogers, 1997; Ulbrich and Christoph, 1999; Trigo et al., 2002; Krichak
and Alpert, 2005a,b). However, the first technique has also been used to show
the areas of significant difference in storm activity between winters with high
and low NAO index (Serreze et al., 1997). Figure 107, A–D shows the 1,000 hPa
geopotental height standard deviation and associated cyclone trajectories.
The analysis is applied to the band-pass filtered fields with cut-off periods
at 1 and 7 days. Two monthly periods are considered: January 1966 and 1983,
which are characterized with a low and high NAO index, respectively. Both
approaches show the existence of a major cyclone variability mode, which is
strongly associated with NAO (North Atlantic Oscillation).
A climatology of cyclones implies a definition of cyclone and a method of
detecting (and describing) it. In both these steps there is a wide margin of
arbitrariness, which produces significant differences in the results. When
considering subjective analyses and a subjective detection method it was usual
to retain a disturbance as a cyclone when it was a minimum of pressure
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Figure 107: Cyclone trajectories (top panels (A) and (B)) and 1,000 hPA standard
deviation (bottom panels (C) and (D)) for low (Jan. 1966, left) and high (Jan 1983,
right) monthly NAO index values. Analysis is applied to band-pass filtered data
(1 to 7 day window). Only trajectories of cyclones deeper than 25 hPa and with
duration longer than one day are shown.
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surrounded by a closed isobar. Of course, the results depend on the spacing
between isobars and of the scale of the map, so that the more detailed the
map, the higher the number of cyclones identified. These situations and problems
are not avoided by using objective methods. There are different conceptual
ways to define a cyclone. An option is to define a cyclone as a relative maximum
of vorticity (relative or geostrophic vorticity). Another definition states that
a cyclone is a relative minimum of sea level pressure (or geopotential height).
The choice of the definition will produce significantly different number of
cyclones, and will affect also other details, like their location. Not all the minima
of sea level pressure can be retained as cyclones, because many of them are too
weak or too close together, so that restrictions have to be imposed (like a
threshold for the minimum pressure gradient, a minimum distance between
centres, or other constraints), which introduce arbitrariness in the definition of
the cyclone and therefore in the results. Even more critical is the effect of the
resolution, since low-resolution analyses will permit the detection of relatively
large-scale cyclones, but will miss the smaller mesoscale disturbances, while, on
the contrary, high-resolution analyses will permit the detection of many small
mesoscale disturbances, but could miss the description of the relatively largescale disturbances. In fact, the domain of a large-scale cyclone, defined as the
area of positive (geostrophic) vorticity, in a region like the Mediterranean
characterized by a complex orography and land–sea distribution, is often broken
in multiple fragments, without continuity, due to the existence of many high-low
small-scale disturbances, with negative–positive vorticity. The maximum detail in
the detection and description of small-scale cyclonic disturbances, is achieved by
using high-resolution objective analyses directly, but for a good description of
larger scale cyclones, lower resolution analyses have to be used or, alternatively,
the original fields of the high-resolution analyses have to be spatially smoothed.
The results from high-resolution unfiltered fields and from low-resolution or
smoothed fields are dramatically different, even with the same definition of
cyclones and the same restrictions to the definition. Figure 108 (Gil et al., 2002)
is based on a 3-year (Jun 1998 to May 2001) sample of operational analyses
from the ECMWF, consisting of four analyses per day at a T319 resolution,
which permits 0.5 latitude–longitude gridded maps. From non-smoothed
analyses (right-hand side in the figure), 2,248 cyclones are detected in the
Eastern Mediterranean (purple frame) and 2,910 in the Western Basin (blue
frame). From smoothed fields, using a Cressman filter with a 200 km radius of
influence (left side in the figure), these numbers are reduced to 353 cyclones in the
East and 437 in the West. Not only the number of cyclones are totally different,
but also some details in the distribution: the very important relative maximum
south of the Pyrenees obtained from the original fields disappears when the
smoothed fields are used.
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324 Mediterranean Climate Variability
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Figure 108: Number of cyclones detected in a three-year (Jun 1998 to May 2001)
sample of operational analyses from the ECMWF at T319 resolution. Right panel
shows the results of analysing non-smoothed analyses: 2,248 cyclones are detected
in the Eastern Mediterranean (purple frame) and 2,910 in the Western Basin
(blue frame). Left panel refers to smoothed fields (a Cressman filter of 200 km
of radius of influence has been used): 353 cyclones in the East and 437 in the West
(from Gil et al., 2002).
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6.3. Climatology of Cyclones in the Mediterranean
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The climatology of cyclones in the Mediterranean region is highly influenced by
the almost enclosed Mediterranean Sea, which represents an important source of
energy and moisture for cyclone development; and by its complex land topography, which plays a major role in steering and deflecting air flows. Moreover,
being located within the transition between the subtropical high-pressure belt
and the mid-latitude westerlies, the Mediterranean is also subject to strong
interannual variability of cyclone activity and, consequently of its precipitation
regime, water resources.
In the following section, a climatology of Mediterranean cyclones is presented,
which decribes the spatial distribution of cyclogenesis and associated mechanisms. The average cyclone characteristics, including their intensity and spatial
and temporal scales, as well as their intra- and inter-annual frequency variability,
will be described.
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6.3.1. Characteristics, Sub-areas of Cyclogenesis, Seasonality
and Generation Mechanisms
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Mediterranean cyclones are generally characterized by shorter life-cycles and
smaller spatial scales than extra-tropical cyclones developed in the Atlantic, as
shown in the analysis of storm-tracks derived from 6-hourly near surface fields
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at T106 (1.125 1.125 ) resolution, available from ERA-15 (ECMWF
ReAnalysis) (Trigo et al., 1999). Over 65% of cyclones are within subsynoptic
scales, with radius of the order of 550 km or less, considerably smaller than the
1,000–2,000 km values, typical of Atlantic synoptic systems. If the shortest living cyclones (with duration lower than 12h) are excluded, the average life of
cyclones in the Mediterranean region is about 28 h, compared to 3–3.5 days in the
Atlantic. Radius and maximum gradient tend to scale with the minimum
pressure. In general, cyclones are deeper and have a larger radius in the western
than in the Eastern Mediterranean. A recent evaluation (restricted to the western
Mediterranean region, Picornell et al., 2001), based on higher resolution fields
(computed by HIRLAM at 0.5 resolution) and including short-lived cyclones,
produced even smaller space and timescale values. In this dataset the radius of
most cyclones is within the 150–350 km range (the mean value is 255 km) and
the most intense cyclones have lifetimes of 18–24 h. Deepening rates are mostly
lower than 2 hPa (6h)1, though values as high as 10 hPa(6h)1 can be observed.
Therefore, also the average deepening rates are smaller than in the Atlantic,
although the lower latitude at which Mediterranean cyclones develop should
be accounted for. Many cyclones in the Mediterranean region have null or even
negative deepening rate, meaning that they originated in neighbouring regions
and cross the Mediterranean during their attenuation phase. A very recent
comparison between a sample of Atlantic and West Mediterranean cyclones,
made by using high-resolution smoothed fields (HIRLAM/INM 0.5 latitude–
longitude with Cressman filter of 200 km), confirms the differences indicated
above. The average winter geostrophic circulation of the Mediterranean cyclones
is 4 107 m2 s1 whereas it is 7 107 m2 s1 for the Atlantic disturbances
(Campins et al., 2005). Note that the geostrophic circulation combines the size
(area) of the cyclones and their geostrophic vorticity, so that can be considered
as a measure of their total magnitude.
The geography of the region, namely the high orography around the
Mediterranean Sea and the existence of embayments and inland seas, determines
the relatively small areas where cyclogenesis tends to occur (Table 7; Fig. 109)
and the variegate mesoscale structure of Mediterranean systems (Alpert et al., AQ: please clarify
1990a,b; Trigo et al., 1999, 2002; Maheras et al., 2001; Picornell et al., 2001). whether
These structures correspond to the mechanisms discussed in the previous Section it is 2002a or b
6.2.1. Figure 109 shows that the most active areas include the Gulf of Genoa,
Iberia, Southern Italy, Northern Africa, Aegean Sea, Black Sea, Cyprus, Middle
East; the respective seasons when these areas are most active are indicated in
Table 7.
Further differentiation can be found inside such areas. In fact, studies based
on automated database methods (Campins et al., 2000), have resolved smaller
mesoscale structures and identified in the western Mediterranean 7 types of
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326 Mediterranean Climate Variability
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Table 7: Cyclogenetic regions in the Mediterranean area and
respective seasons with significant activity (after Trigo et al.,
1999); values represent average cyclone radius (km).
area
seasonality
radius (km)
Sahara
Gulf of Genoa
Southern Italy
Cyprus
Middle East
Aegean Sea
Black Sea
Iberian Peninsula
Spring, Summer
Whole year
Winter
Spring, Summer
Spring, Summer
Winter, Spring
Whole year
Summer
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Figure 109: Number of cyclogenesis events detected per 2.25 2.25 in January
(A), April (B), and August (C) from 1979 to 1996 in ECMWF re-analyses
(from Trigo et al., 1999).
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cyclones, on the basis of shape and intensity of the associated circulation.
However, despite the use of different methodologies, selection criteria and
data sets, most studies (e.g. Alpert et al., 1990a,b; Trigo et al., 1999; Campins
et al., 2000; Maheras et al., 2001; Picornell et al., 2001) agree on the spatial
location of cyclone generation.
The overall synoptic activity over the entire basin has a well-defined annual
cycle, being more intense in the period from November to March which corresponds to the so-called storm season (Fig. 110). Though the temporal and spatial
distributions of the Mediterranean cyclones present a large intermonthly variability (Alpert et al., 1990a), the analysis favours the definition of three main
seasons: winter, spring and summer. Autumn appears as a transitional period
with large interannual variability, whose months could be characterized as late
summer or early winter.
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Figure 110: Annual cycle of the standard deviation of the 1,000 hpa geopotential
field. The grey lines show individual years, the black thick line the average annual
cycle. Values are based on the band-pass filtered fields (1–7day cut-off periods)
(Lionello and Zardini, 2005, personal communication)
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The mechanisms typical of winter cyclogenesis in the Mediterranean exhibit
contrasting characteristics with those most common in spring and summer
seasons, with intermediate situations in spring and autumn. The 3D structure
of the cyclones gives a first clear indication: in the Mediterranean, only the
winter cyclones are mostly deep cyclones (reaching 300 hPa), while in summer
they are mostly shallow (reaching 850 hPa or less). The Laplacian of the temperature at low levels, giving the thermal character of the cyclone, is always
negative (warm character), but double in magnitude in summer than in winter.
The Mediterranean cyclones are in general more similar to the Atlantic cyclones
in winter than in summer (Campins et al., 2005). In winter there are strong links
between synoptic upper-troughs and local orography and/or low-level baroclinicity observed over the northern Mediterranean coast. In spring and summer,
inland cyclogenesis becomes more frequent and also more sensitive to diurnal
forcing (Maheras et al., 2001; Picornell et al., 2001; Trigo et al., 2002).
AQ: please
Winter cyclogenesis occurs essentially along the northern coast in three major clarify
areas characterized by strong baroclinicity: the lee of the Alps, when an upper- whether it is
trough is influenced by the mountains, and over the Aegean and Black Seas, 2002a or b
when an upper-trough moves over the relatively warm water basins (Trigo et al.,
2002). The role of orographic cyclogenesis (Buzzi and Tibaldi, 1978) is not limited
to the Alps, being also fundamental in the triggering of lows in the Gulf of
Lyons, south of the Pyrenees, and also in Southern Italy, south of the Apennines.
Over the south-eastern Mediterranean region, the intensity of cyclogenetic
activity is to a large extent controlled by large-scale synoptic systems over
Europe, particularly by those characterized by mid- and upper-tropospheric
AQ: Please
southward air-mass intrusions and tropopause-folding effects (Krichak and list these
Alpert, 2003). These processes are often associated with the formation of three- references.
dimensional potential vorticity structures, jet streaks and low-level jets conditions over the region to the south of Alps (Buzzi and Foschini, 2001; Liniger and
Davies, 2003).
In spring, the strengthening of the meridional temperature gradient along
the northern African coast favours the development of Saharan depressions.
These tend to occur on the lee side of the Atlas mountains, within a region of
very weak static stability. Thermal forcing plays an increased role in the genesis
and maintenance of Mediterranean Lows in spring and, particularly, in
summer. As a result, the life-cycles of summer cyclones, especially those
developed over Northern Africa and the Iberian Peninsula, follow the diurnal
temperature fluctuations; maximum intensity tends to be reached by late
afternoon, and cyclolysis tends to occur mostly by early morning (Maheras
AQ: please
et al., 2001; Picornell et al., 2001; Trigo et al., 2002). Also the Middle East
clarify
trough, which is a semi-permanent feature primarily induced by the Asian
whether it is
monsoon acting on a planetary scale (Rodwell and Hoskins, 1996), exhibits the 2002a or b
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same kind of diurnal see-saw associated with the local thermal cycles (Trigo
et al., 2002).
There are several reported cases of very intense storms in autumn–winter and
spring months, with severe associated weather (intense rainfall, surges, flash
floods), which have either developed or re-intensified over the Mediterranean
Sea (e.g. Lee et al., 1988; Ramis, 1994; Lagouvardos, 1996; Lagouvardos, 1999;
Doswell III et al., 1999; Pytharoulis et al., 1999). A fraction of these very intense
events develop a hurricane-like structure, feeding on the latent heat release at the
sea surface; their frequency, space–time distribution, and interannual variability
have not been fully investigated, yet (Pytharoulis et al., 1999).
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The Mediterranean region is only partially affected by the North Atlantic
storm track, whose main path crosses the Northern Atlantic towards Northern
Europe. Consequently, the main mode of the North Atlantic storm track
variability, which describes its north–south shift and intensification over the
Atlantic, is only marginally related to the frequency and intensity of the cyclones
in the Mediterranean region, though Trigo et al. (2000) have demonstrated
that an association exists and that it depends on the structural characteristics
of the cyclones.
In fact, the analysis of low-frequency SLP variability patterns and the frequency of cyclones in the Mediterranean region shows that there are important
patterns than the NAO (Krichak and Alpert (2005a,b)). The link between NAO
and the position and strength of the storm track in the central Atlantic implies
a link between NAO and the frequency of orographic cyclogenesis which is
triggered by the passage of Atlantic cyclones. Instead, the bulk of the variability
over Central and Southern Europe and over the Mediterranean region is linked
to low-frequency patterns, whose centres of actions are localized over Europe
and eastern Atlantic (like the East Atlantic/Western Russia pattern (EAWR),
Krichak et al., 2000, 2002; Krichak and Alpert, 2005a,b). It has been demonstrated that, depending on the area of the Mediterranean region, a high level of
SLP synoptic-scale variability is associated with the positive phases of the SENA
(Southern Europe Northern Atlantic) and, to a minor degree, to the SCAN
(SCANdinavian) patterns. Therefore, intensity of the cyclogenetic activity in the
eastern Mediterranean region is to a large extent controlled by the large-scale
synoptic processes over Europe and especially by those characterized by midand upper-tropospheric southward air-mass intrusions and tropopause folding
effects (Krichak et al., 2004). The processes are often associated with the
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6.3.2. The Role of Large-scale Climate Patterns on the
Mediterranean Cyclones
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whether it is
2002a or b
AQ: Please
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references.
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formation of three-dimensional PV structures (PV streamers), jet streaks and
low-level jets conditions over the region to the south of the Alps (Buzzi and
Foschini, 2001; Liniger and Davies, 2003). These conditions tend to stimulate
development of mesoscale convective complexes and Mediterranean cyclones.
Intensity, location, duration and orientation of the systems as well as their
interdecadal trends in association with those of the main European teleconnection patterns appear to be important elements of the eastern Mediterranean
weather and climate trends.
Moreover, these teleconnections are defined on a monthly scale, while submonthly large-scale features, such as the well known and relatively frequent euroAtlantic blocking (Tibaldi et al., 1997), can influence the trajectory of storm AQ: please
tracks and their associated precipitation fields (Trigo, 2004). Winter blocking list the
episodes lasting 10–20 days are associated with a large positive anomaly of the reference
500 hPa geopotential height above the North Sea and with a higher/lower than
average number of cyclones in the Mediterranean/North Sea.
Finally, since such large-scale analyses are generally based on relatively coarse
resolution fields where the subsynoptic and mesoscale characteristics of the
cyclones in the Mediterranean region are poorly reproduced, important
components of their variability might not be well described yet.
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6.3.3. Trends
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A counting of cyclone centres (without any differentiation on intensity), based
on the NCEP re-analysis, which covers the period 1958–1997, shows a reduction
of the number of cyclones in western Mediterranean and an increase in the East
(Maheras et al., 2001). Linear fit to the data leads roughly to a 15% increase/
decrease. Changes are not seasonally homogeneous. If only the rainy period
(October–March) is considered, a reduction of the number of cyclones is evident
also in the Eastern Mediterranean. Other studies suggest a distinction between
the increasing trend of weak cyclones and the decreasing trend of strong
cyclones in the Western Mediterranean Sea (Trigo et al., 2000). It follows
that the positive trend identified in the Northern Hemisphere storm track for
the last decades of the 20th century (Chang and Fu, 2002) is not valid in the
Mediterranean region.
The negative trend is confirmed by the analysis of longer time series
(Della-Marta and Bhend, 2005). A climatology of cyclone activity has been
created using a newly compiled dataset of the North Atlantic–European region
from 1850 to 2003, which was developed in the European and North Atlantic
daily to multi-decadal climate variability project (EMULATE). These data
consist of gridded daily mean sea level pressure fields, which are based on land
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reference.
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and island stations and have been elaborated using Reduced Space Optimal
Interpolation (RSOI) on a 5 5 degree grid (Ansell et al., 2005). Seasonally
averaged statistics of the cyclone dynamics have been computed with an
objective locating and tracking system developed by Murray and Simmonds
(1991). Commonly to all cyclone tracking algorithms, subjective decisions had
to be made regarding the cyclone locating and tracking algorithm parameters.
In order to optimize the parameters, tracking results based on EMULATE
data (1948–2003) have been compared to tracking results based on daily NCEP
data which were transferred to a 5 5 grid. The EMULATE reconstructions in
the Mediterranean is shown to be reliable because the RSOI error statistics
(see Kaplan et al., 2000) are invariant over time and space for the entire 153 years
of the reconstruction indicating that the number of predictors (e.g. stationbased observations and marine data) are dense enough to reliably reconstruct
SLP on the given grid. See Ansell et al. (2005) for more details on the SLP
fields. Significant findings are a marked decrease in winter (DJF) cyclone density
over most of the western Mediterranean and an increase in cyclone system
density in the eastern Mediterranean for the period 1950–2003 (Fig. 111,A).
These findings agree with the results of Maheras et al. (2001). In the longer
period, 1850–2003, most of the Mediterranean shows a decrease in cyclone
system density (Fig. 111,B). Analysis of the cyclone density time series in the
form of a Hovmöller plot shows that the frequency of cyclones over the western
Mediterranean is highly variable and exhibits large interannual as well as
decadal variability over the last 153 years (Fig. 112).
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As it was briefly mentioned in the introduction, cyclones have a deep influence on
important environmental variables and, particularly, on the timing and magnitude of their extreme values. In general, although not all the extreme weather
events in the Mediterranean are related to cyclones and most of the cyclones
do not produce extreme weather, it is plausible to assume that Mediterranean
cyclones influence most of the high-impact phenomena. Moreover, the high variability of cyclone frequency and intensity, within the Mediterranean Sea and its
immediate environments, results in contrasting weather conditions in the region,
ranging from large arid areas (e.g., Thornes, 1998; Trigo et al., 2002) to the
greatest annual precipitations totals in Europe, in the Dinaric Alps (Radinovic,
1987; Trigo et al., 2002). This section shows the correlation of cyclones with rain,
winds, waves, surges and even landslides, describes the mechanisms involved
and discusses variability and trends of the related phenomena.
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332 Mediterranean Climate Variability
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Figure 111: The linear trend in the winter (DJF) cyclone density (A) 1950–2003
and (B) 1850–2003. Cyclone density is the average number of cyclones per unit
area at any one time. The trend units are the number of cyclones times 1,000 deg.
lat.2 where deg. lat. is a standard length of 1/360th the circumference of the Earth
(from Della-Marta and Bhend, 2005, personal communications)
U
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C
807
O
806
814
815
816
6.4.1. Precipitation
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Large quantities of rain require a feeding current of warm and wet air to
replace the water removed by precipitation. When vertical stability is close to
a critical threshold, such inflow at low levels can favour or lead to instability.
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O
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Figure 112: The variability of cyclone density as a function of time and latitude
at the longitude of 10 W. The Hovmöller plot x axis defines the latitude in degrees
North, the y axis defines the winter season (DJF) from 1850 (bottom) to 2003
(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
length of 1/360th the circumference of the Earth (from Della-Marta and Bhend,
2005, personal communications).
D
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F
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847
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849
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Therefore, the eastward and poleward sectors of cyclones are suitable places for
prolonged and intense precipitation. Moreover, orographic upslope lifting is
also very effective for producing ascent of warm humid air and persistent rainfall.
In fact, in many places the coastal or inner relief intersects a moisture feeding
flow and can force upward motion and orographic rain. The local intensity of
the precipitation is very much dependent on the path followed by the cyclone
and by the amount of available moisture. The heaviest rain events take place
when the cyclone path is in such a position that it produces the local convergence
of moist Mediterranean air. In the Western Mediterranean, this feeding flow
is southerly for northern Italy and Ticino, south-easterly for France, and easterly
for Catalonia, the Balearics and Valencia Murcia (Jansa et al., 2001b). In the
Eastern Mediterranean, it is mostly westerly for the Middle East countries and
southerly or south-westerly for Greece.
These dynamics explain why heavy rain events are associated with cyclones
(only a few events in the eastern Mediterranean and in Northern Italy can be
an exception) as humid Mediterranean air is advected against the slopes of the
mountain ridges surrounding the basin. In fact, synoptic-scale disturbances have
been found responsible for most of the floods both in the Western Mediterranean
and in the Eastern Mediterranean. Only a minority of local flash floods has been
associated with intense small convective cells, whose presence is not detected in
the standard meteorological analysis. It has been established that for most of
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334 Mediterranean Climate Variability
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the cases (around 90%) of heavy rain in the Western Mediterranean there
was a cyclone in the vicinity (Jansa et al., 2001b, see Fig. 113, though its intensity
could vary from an intense and deep system to quite weak and shallow
depressions. In fact, considering the mean value for all cyclones identified in
such cases, the average vorticity in a central area of the cyclone of 400 km of
radius is 0.8 104 s1. In approximately 80% of the events of heavy rain, the
location of the cyclonic centre is such that a role of the cyclone in the heavy rain
generation and/or location can easily be inferred. The total number of heavy rain
events considered is more than 900 (in 5 years, 1992–96). A heavy rain event
is defined here as a day with more than 60 mm/day (lowered to 30 mm/day
in Algeria) of precipitation in any point of a ‘‘territorial unit’’ (province,
department, region or island). Analogously, in Greece about the 92% of rainfall
during the rainy period (October–March) is produced by cyclonic patterns
(Maheras and Anagnostopoulou, 2003). During the cold season, precipitation
in the southern part of the eastern Mediterranean (EM) region is also mainly
associated with cyclonic systems of Mediterranean origin. A study carried out
for the Negev Desert identified 4 classes of synoptic disturbances responsible
for most of the floods, the two most important denoted as the Syrian Low and
the Red Sea Trough (Kahana et al., 2002). Therefore cyclones are the cause
of most of intense precipitation in the whole Mediterranean region.
From left to right and from up to down S. France, Corsica, Italy, Catalona,
Balearics, Sardinia, Valencia, Algeria
The cyclones responsible for precipitation in different areas do not share a
common origin and generally a single system affects only part of the
Mediterranean region. A study focused on Portugal, Italy and Greece has
shown that Atlantic Lows dominate in Portugal, where rainy months are
associated with an enhanced number of deep and medium cyclones between
Newfoundland and British Isles. Precipitation over Greece is very rarely affected
by Atlantic cyclones, but is associated to cyclogenesis inside the Mediterranean
region. Italy is influenced from both Atlantic and Mediterranean cyclones,
because distant lows sometimes contribute to advection of humidity and some
Atlantic cyclones may cross over into the Mediterranean and influence the
precipitation over Italy. However, the majority of precipitation sources are
Mediterranean cyclones (Pinto et al., 1999). During the cold season, precipitation in the southern part of the eastern Mediterranean (EM) region is mainly
associated with cyclonic systems of Mediterranean origin. The cyclones producing intense rain in Israel usually belong to the type of Cyprus Lows or
Cyprus Depressions (El-Fandy, 1946; Kallos and Metaxas, 1980; Alpert et al.,
1995, 2003; Krichak et al., 2004) and usually start their development in the
south-western areas of the Mediterranean Sea, and then migrate to the east.
During the migration process, the depressions often weaken, though a very
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904
905
AQ: Please
specify A-G in this
figure as per
legend.
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907
908
909
910
911
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913
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O
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Figure 113: Most frequent location (elliptic area) of a cyclone in case of heavy
rain in some western Mediterranean regions (isolated dot): From left to right and
from up to down the locations here considered are (A) SE France, (B) Corsica,
(C) North Italy, (D) Catalonia, (E) Balearics, (F) Sardinia, (G) Valencia and
(H) Algeria (from Jansà et al., 2001b).
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336 Mediterranean Climate Variability
957
958
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960
961
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967
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971
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D
952
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R
949
AQ: Please
clarify whether
it is Trigo et al
2002a or b
R
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significant strengthening of the cyclones is often observed in the Cyprus area,
where the lows regenerate in the lee of the Taurus Mountains of Turkey.
This phenomenology corresponds to the key role which cyclones play in the
internal redistribution of moisture in the Mediterranean region (Fernandez
et al., 2003). The transport of moisture from the Western to the Eastern
Mediterranean corresponds to the generation (or intensification) of cyclones in
the western Mediterranean and their subsequent eastward motion. Air–sea interaction and significant latent heat flux are likely to play an important role in
this process.
In Greece, for the period 1958–2000, precipitation has been analysed
considering the frequency of cyclones and the probability of precipitation
produced by them. For the majority of stations, in wintertime the decreasing
trends of wet-day amount and the probability of rainfall are consistent with
the observed changes in frequency of the various types of cyclones. During
autumn on the one hand, the probability of rainfall increases for a large number
of cyclonic circulation types, which, on the another hand, are characterized
by a decrease in frequency. These opposite trends partially compensate, so that
the autumn overall amount of precipitation shows a positive trend (Maheras
et al., 2004).
An outstanding problem in Western Mediterranean rainfall is the occurrence
of catastrophic torrential rains, which tend to occur in the autumn season
along coastlines with heavy orography, and the change of relative frequency
of moderate/light vs. intense precipitation events. Very little work has been
done to link the probability of these events to large-scale extra tropical circulation patterns (Valero et al., 1997), although some analysis of the frequency and
intensity of Mediterranean cyclones (Trigo I. et al, 2002) and modeling studies
for individual events have been performed (Homar et al., 1999; Romero et al.,
1999; Pastor et al., 2001). In the last decades, a tendency for more intense
concentration of rainfall seems to have occurred along the Mediterranean
coastal areas in Italy and Spain (Brunetti et al., 2001; Alpert et al., 2002;
Goodess and Jones, 2002), but this does not seem to be the case for inland areas.
In fact, over Italy, results show a negative and significant long-term trend in
the number of wet days and a positive one in precipitation intensity, which is
significant only in the northern regions (Brunetti et al., 2004). The negative trend
in wet days persists since the end of 19th century and is due to the marked
decrease in the number of low-intensity precipitation events. An increase in the
number of events belonging to the highest intensity interval was observed too,
but only in northern regions. The decrease of total precipitation during the
wet season in the Northern Mediterranean has been associated with the reduction of intense cyclones (Fig. 114) and to the northward shift of the storm track
N
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references.
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October - March
985
988
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440
160
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120
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80
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40
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O
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58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96
1003
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Figure 114: Time series, and respective linear trends, of the total amount of
precipitation in the Northern Mediterranean Basin (bold curve, left axis), the
total occurrence of intense Mediterranean cyclones (light curve, right axis), and
of non-intense cyclones (dotted curve, right axis) for the October–March period
(from Trigo and Davies, 2000).
TE
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1000
F
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Rainfall
o
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480
o
987
mm / (3.75 x 2.5 )
986
1005
1011
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1016
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1021
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reference.
N
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over Europe in the period from the 1979 onwards (Trigo et al., 2000). However,
the analysis of precipitation shows different trends depending on the intensity
of the events (Alpert et al., 2002). The torrential rainfall in Italy exceeding
128 mm/day has increased percentage-wise by a factor of 4 during 1951–1995.
In Spain, extreme categories at both tails of the distribution (light: 0–4 mm/day
and heavy/torrential: 64 mm/day and up) increased significantly. No significant
trends were found in Israel, Greece and Cyprus. A detailed analysis of the
precipitation in the Valencia Region (Spain) suggests that land use changes in
the coastal region result in surface drying, which in turn implies warmer and drier
air masses over the coast and higher condensation level and fewer summer
storms. It is moreover suggested that higher sea surface temperature can be the
cause for the increased number of torrential rain in autumn and winter and
that these two factors can be part of a climate mechanism affecting the whole
western Mediterranean (Millan et al., 2004, 2005). The consequent redistribution of the daily rainfall categories – torrential/heavy against moderate/light
intensities – is of paramount interest particularly in the semi-arid subtropical
regions for purposes of water management, soil erosion and flash floods
impacts. Specific isolated regions exhibit an increase of extreme rainfall in spite
of the reduction of the total amount of precipitation.
AQ: Please
list this
reference.
U
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Frequency of Mediterranean Cyclonic Events
Cyclones in the Mediterranean Region: Climatology and Effects on the Environment 337
AQ: Please
list these
references.
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338 Mediterranean Climate Variability
1026
6.4.2. Strong Winds
1027
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Many strong winds observed in the Mediterranean belong to the category of
local winds, like Mistral, Tramontana, Sirocco, Etesian, Bora, Khamsin or
Sharav (see H.M.S.O., 1962, or Reiter, 1975, for a general description), that is,
they have repetitive location, behaviours and characteristics. Figure 115 shows
the location of the main winds in the Mediterranean region.
1033
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Figure 115: Maps of the lagged correlation with the value in the point indicated
by the dot for the 500 hpa synoptic-scale filtered geopotential height. This point
is selected as a maximum of explained variance of the precipitation field associated
to the EOF describing a moisture transport from western to eastern
Mediterranean areas. The maps are meant to describe the evolution of the cyclonic
disturbances associated with such transport (From Fernandez et al., 2002).
AQ: please list the
reference
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In general, there is a connection between the Mediterranean local winds and
the Mediterranean cyclones. According to the climatology based on ship
observations, the highest frequency of gale storm winds in the Mediterranean
occur in the Gulf of Lyons, with large difference with other regions (see, for
instance, H.M.S.O., 1962), and they can be identified as winds belonging to the
Mistral local wind category. The primary cause for these winds is a cyclone
located within the Mediterranean, in or near the Genoa region (together with
an anticyclone in France or northwestern Europe) and the high frequency and
intensity of the Mistral winds is a consequence of the high frequency and intensity of the Genoa cyclones. Similarly, a low pressure above Italy or west of it
produces a Sirocco storms in the Adriatic Sea, where the channelling effect of
Apennines and Dinaric Alps strongly intensifies a flow which would otherwise
be distributed on a larger front.
The interaction of air flow and orography contributes to the Mediterranean
local winds, which can be partially seen as downslope flows or due to channelling
effects. The Mediterranean local winds are attributable to orographic mesoscale
pressure perturbation induced by the flow–mountain interaction. High- and
low-pressure poles of the orographic disturbance (and/or the orographic pressure
dipole as a whole) create local areas of strong pressure gradient that provide
intense local acceleration, leading to the local wind generation (Campins et al.,
1995). The onset of a local wind is, therefore, often quite abrupt. Past the narrow
accelerating zone, the winds continue blowing and spreading in an inertial way,
although density gradients can contribute to a more efficient wind spreading
and extension (Jansa, 1960; Alpert et al., 1982). According to this mechanism,
local winds are shallow, only 1.5–2 km deep at most (Jansa, 1933; Campins et al.,
1995; Alpert et al., 1998) and may remain quite independent from the basic
flow above the mountain.
On the other hand, the frequent presence of intense cyclones is enough to
explain some windstorms blowing within the Mediterranean region and the
effect of orographic forcing is not always fundamental. The hot winds blowing
from the desert across the Libyan and Egyptian segments of flat coast (Chili,
Shimum), the strong easterlies over the Eastern Mediterranean (Saaroni et al.,
1998), and the Libeccio storms in the Tyrrhenian Sea, could constitute situations
where such interaction is not crucial. Note that the extraordinary storms of
December 1979 and November 2001, mentioned in Section 6.0, belong to the
category of ‘‘intense cyclones’’. It can be added that the indirect methods of
estimating winds (the scatterometers carried by satellite, like in ‘‘Quickscat’’) has
given for the sustained winds in one of these storms (November 2001) values as
high as 35 m/s. The synergistic combination of both mechanisms, that is the
presence of intense cyclone and the generation of local winds, explains the high
speed of extreme wind events.
N
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references.
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1112
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Etesian winds blow over the Aegean Sea and belong to a larger circulation
system including northerly winds over the whole Eastern Mediterranean. This
Northerly circulation is produced by the combination of a high pressure
over Balkan Peninsula or over central Europe and low pressure over eastern
Mediterranean and Iraq, generally of thermal origin and corresponding to an
extension of the summer Tibetan low to the eastern Mediterranean sea (Repapis
et al., 1977). The Etesian winds can be intensified by the presence of a trough in
the mid or upper troposphere over eastern Mediterranean. The temperature
contrast between land and the Aegean sea can also influence (intensify or weaken)
the intensity of the Etesian winds (Maheras, 1980).
It is also worthy to remark that the leading edge of wind streams can act
as an internal shallow front. On the other hand, associated with local wind
streamers is the formation of orographically generated cyclonic and anticyclonic
PV banners, characterized mainly by shear vorticity which often contributes AQ: Please
to the Mediterranean cyclogenesis stimulating in some cases the heavy rainfall list these
events (Aëbischer, 1996; Aëbischer and Schär, 1996).
references.
O
1109
PR
1108
1124
1125
6.4.3. Storm Surge
D
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Storm surge results from the combined action of atmospheric pressure and
wind stress on the sea surface. Atmospheric pressure produces what is called
the inverse barometric effect, according to which, in steady conditions a low
pressure is associated with a sea level increase. Wind stress pushes horizontally
the water column and tends to accumulate it at the closed end of a basin.
In steady condition, the slope of the sea surface is proportional to wind stress
and to the inverse of the water depth. Therefore, the action of wind stress
dominates in shallow water and has a large effect in the Northern Adriatic
Sea. These dynamics explain the importance of cyclones for storm surges and
why the variability of cyclone regimes has an impact on the surge events which,
in turn, can be considered an indicator of the cyclones characteristics.
The storm surges in the northern Adriatic, and the consequent flooding of
Venice, is caused by intense cyclones in the north-western Mediterranean (Trigo
and Davies, 2002; Lionello, 2005). The synoptic patterns determining the surge
in the Gulf of Venice present a low-pressure system with a minimum above
central Europe or northern Italy (Figs. 116,117) which produces a strong
Sirocco wind along the Adriatic Sea. Although these synoptic dynamics are
well known, the mechanisms responsible for their frequency and intensity have
not been completely understood. It appears that periods with extreme surge
events are characterized by a general circulation anomaly, represented by a
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1149
1150
1151
BORA
1152
1153
MISTRAL
1154
TRAMONTANA
MAESTRALE
1155
VARDARIS
SIROCCO
1156
ETESIAN
1157
LIBECCIO
1158
1159
SHARKIYA
VANDEVALES
SIROCCO
1160
O
CHILL
1163
GHIBLI
1165
1173
1174
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1176
D
TE
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R
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Figure 116: Location and direction of main winds in the Mediterranean region.
Note that there are discrepancies between related terms in the different languages.
In the French language, Mistral is the northerly wind descending along the
Rhone Valley and blowing in the region of Provence, towards the western
Mediterranean and Tramontane the northwesterly wind belonging to the
same system and blowing in the Roussillon region. In the Italian language
Tramontana is a northerly wind and Maestrale the dominant northwesterly
wind blowing over most of the western Mediterranean Sea. Tramontana
(or Tramuntana) is also the name of the northerly wind (belonging to the Mistral
system) in Catalan and Spanish languages, blowing in the north of Catalonia and
north of the Balearics.
C
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1180
1181
1182
1183
1184
1185
1186
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1188
1189
pattern with a negative centre of action above the Eastern Atlantic, according
to which cyclones are deviated south-eastward and penetrate into the
Mediterranean Sea from North–West or generate a lee cyclone south of the
Alps. This pattern is different from the NAO dipole, whose time behaviour is not
correlated with that of the highest surges in the Gulf of Venice. It appears
therefore that the storminess associated with the floods of Venice is not related
to the NAO (Fig. 118 after Lionello, 2005).
The records of floods in Venice show a clear positive trend, the cause of which
is mainly the subsidence of the ground level, whose rate was particularly high in
the years from the 1950s to the 1970s, rather than variations in the meteorological
U
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1178
KHAMSIN
PR
1164
1167
SHARAV
O
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1166
F
ETESIAN
SIROCCO
1161
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342 Mediterranean Climate Variability
1190
A
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Figure 117: Evolution of the gph1000 field producing the storm surge in Venice.
Panels (A) to (E) show the gph1000 composites (in hPa according to the greyscale
bar) with a time lag of 48h, 24h, 0h, þ24h, þ48h with respect to the time of
the surge peak. This situation is associated to a main minimum south of the Alps
at the time of maximum surge (panel C). The composites are based on events
with a peak surge value higher than 70 cm.
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forcing fields. If subsidence is excluded, the residual variability, which can be
associated with the meteorological forcing, shows trends which are small and
dependent on the intensity of the storm surges. During the second half of the
20th century, there is an indication that the frequency of moderate surges is
increasing (Pirazzoli and Tomasin, 1999) while major independent surge events
do not show large variation (Trigo and Davies, 2002). During the same period, a
weak decreasing trend has been identified in the value of extreme levels in Trieste
(Raicich, 2003). This behaviour has been shown to be consistent with the
variation of the meteorological forcing in the Northern Adriatic area, that is, with
AQ: Please
list this
reference.
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Figure 118: Same as Figure 117, but for a situation associated with a main
pressure minimum located north of the Alps at the time of maximum surge
(panel C).
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more frequent moderate storms and less frequent intense storms. In Venice,
there is no evidence of a correlation between extreme surge levels and the ongoing hemispheric warming trend. If the effect of regional sea level rise is subtracted
from the data, the record of extremes is dominated by a large interdecadal
variability, with respect to which an eventual residual trend is of minor
importance (Fig.119, Lionello, 2005). The understanding of this variability,
and of its past and future evolution, appears a very interesting scientific and
practical issue. It will be important, on one side, to investigate the variability of
large-scale patterns associated with it and, on the other side, to find the explanation for its correlation with the periodicity of the sunspot number. This last
correlation points to the possible existence of a link between regional climate
and external forcing which is not understood yet.
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344 Mediterranean Climate Variability
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Figure 119: Indexes of the storm surge level time (black continuous), sunspots
number (grey continuous), NAO (black dashed) during the 1940–2000 period
(years on the x axis). NAO and storm surge level data include November and
December only. All time series have been smoothed using a 3-year running mean.
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6.4.4. Wind Waves
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Ocean waves are the consequences of winds, so that intense cyclones are the
cause of extreme waves. In this respect storm surges and waves have a common
cause. However, waves grow continuously under the action of the wind and
their maximum height reflects the average intensity of the wind along the fetch.
In other words, waves tend to depend on the integral of the wind stress along their
travelling path, while surges are very dependent on its value over the shallow
water areas near the coast. Measurements with moored buoys, and models
capable of assimilating, analysing and forecasting waves, have demonstrated
that high waves (with 5–7 m significant wave height) exist in the Mediterranean,
in spite of the relatively short fetches with respect to the Oceanic situations.
As already mentioned, analysed SWH (Significant Wave Height) of 10–11 m were
encountered in the case of the extraordinary storm of 10–11 November 2001
(Gomez et al., 2002).
It is not simple to reconstruct the wave climatology in the Mediterranean
sea because long time-series of instrumental observations are lacking. Buoy
observations are mostly available since the 1990s, when national buoy networks
were installed. Satellite altimeter data are continuously available only since 1992.
Therefore, the analysis of past variability is mostly based on model reconstructions and ship observations. Often, model simulations under-evaluate the SWH,
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Figure 120: Variation of the 10-year return value (y axis, cm.) during the
1940–2000 period. The horizontal dashed line shows the value computed on the
basis of the whole 1040–2001 period. The grey and black continuous lines show
the values computed using events inside a moving 7-year and 21-year long
time window, respectively.
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mostly because of lack of accuracy of the forcing wind fields. An example is
Fig. 120 which results from a model integration based on the ERA40 reanalysis
winds. Figure 120 represents the geographical distribution of maximum SWH
values, but presumably does not show the actual height of the more intense
events.
There are several compilations and atlas of winds and waves derived from ship
observations. There is a big difference between the Atlantic and Mediterranean
situation, mainly due to different fetch, which in the Atlantic Ocean is generally
larger than in the Mediterranean Sea. Therefore, the Atlantic waves are larger
(longer period) than those in the Mediterranean, even when the same wind
conditions are considered. Due to the low dissipation in long waves travelling
out of the storm area (swell), the Atlantic waves remain relatively high for a
long time, and, therefore, the average wave heights in the Atlantic are much
higher than in the Mediterranean, where the swell component plays a minor role
so that waves are mostly present only in correspondence to strong winds
(windsea). In the Gulf of Lyons, in January, the frequency of waves above 1m
of SWH is as large as 70%, and the frequency of waves above 2.5 m of SWH
south-east of this zone is 20%. The growth of waves with fetch implies that
the maximum frequency of high waves is displaced downstream from the
maximum frequency of strong winds; for example, the highest waves in the
Mediterranean are just downstream of the mistral core. Significant wave
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heights of 6m or more are reached every year (on average) in this zone of the
Mediterranean where their presence is related to the high frequency and intensity
of the Genoa cyclones.
Figure 120 confirms the presence of these maxima and shows also other
features in the various basins. High waves are present over most of the
Mediterranean Sea and tend to reach the highest values where strong wind and
long fetch are simultaneously present. The largest maxima are located in the
western Mediterranean and in the Ionian Sea, under the action of the Mistral,
where the shape of the Mediterranean Sea determines the most effective
combination of a long fetch and a strong wind. The Island of Crete interrupts
the fetch of the Etesian winds and determines two separated maxima: one in the
Aegean and another in the Levantine Basin. Sirocco produces the maximum
SWH in the Northern Ionian and in the southern Adriatic. A maximum due to
the Bora wind is present in the northern Adriatic Sea, and another due to the
Vendavel in the Alboran Sea. In summer, wave height is small over the whole
basin and a characteristic maximum is present in the Aegean sea caused by the
action of the Etesian winds.
Figure 121 shows the sea level pressure composites associated with high
waves in different areas of the Mediterranean Sea. Cyclones located near Cyprus
are responsible for high waves in the Levantine Basin, those in the Gulf of Genoa
for high waves in the western Mediterranean, while those above Tyrrenian
or central Italy produce high waves in the Adriatic and central Mediterranean
Sea. Obviously, waves are the result of the action of past winds, so that also
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Figure 121: Distribution of maximum wave height as resulting from a 40-year
model integration carried out using the ERA-40 re-analysis. Contour levels show
the maximum SWH, arrows show the mean wave direction corresponding to
SWH maximum (from Sanna and Lionello, 2005).
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the past evolution of the synoptic situation contributes to the value and location
of the maximum SWH.
Wind wave extremes, obtained from model simulations, show little significant
trends. Figure 122 shows the statistically significant variations of maximum
SWH in 50 years, on the basis of linear trends derived from the ERA40
reanalysis. The two main features correspond to a reduction in the Ionian and
Alboran Sea, which are consistent with a reduction of cyclones in the western
Mediterranean. The significant increase is limited to a very small region near
the coast of France.
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Figure 122: Synoptic patterns associated with extreme significant wave
height in different regions of the Mediterranean Sea: (A) Tyrrenian, (B) Adriatic,
(C) Balearic, (D) Ionian, (E) Levantine basin (from Sanna and Lionello 2005).
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Rainfall-induced landslides are usually directly associated with the passage of
intense storms of Atlantic or Mediterranean origin. In a recent work, the link
between the occurrence of Landslide episodes in Portugal and the storm tracks
associated with the NAO pattern has been established (Trigo et al., 2005).
Naturally, strong cyclones can produce intense rainfall events that are responsible
for the rapid growth of pore pressure and for the loss of the apparent cohesion of
thin soils, resulting in failure within the soil material or at the contact with the
underlying impermeable bedrock. A different type of association between
cyclones and landslides can be found for rainfall periods which are less intense
but have a long duration. In this case long-lasting rainfall periods (from 30 days
to 90 days), are responsible for the activity of deeper slope movements, such as
translational slides, rotational slides and complex and composite slope movements. This is the group of landslide events mostly affected by the large-scale
atmospheric circulation mode NAO.
The western Mediterranean Basin is prone to slope instability due to geological, geomorphological and climatic factors. It is widely accepted that high
duration/intensity rainfall events (associated with intense cyclone events) are the
most important triggering mechanism of landslides worldwide (van Asch et al., AQ: Please
2001). In particular, rainfall-induced landslides have been studied in Portugal list these
(Trigo et al., 2005), Spain (Corominas and Moya, 1999), Italy (Petrucci and references.
Poleminio, 2003) and France (Flageollet et al., 1999). Landslide consequences
include damages on property, houses and particularly roads, and can be also
related with the increasing human pressure related to urban development
throughout the countryside (Trigo et al., 2005).
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6.5. Conclusions
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The Mediterranean region is characterized by strong morphological forcing.
This characteristic is important for cyclones and their effects. The steep
orography surrounding the basin and the complicated land–sea distribution
introduce a rich mesoscale structure. On the one hand, these two factors produce
a peculiar phenomenology internal to the basin and, on the other hand, they
modulate the interaction of the Mediterranean system with global climate
patterns by adding sub-regional features. As far as cyclones are concerned,
steep orography and complicate land–sea distribution condition the formation
and the evolution of cyclones themselves and the effects they produce on the
environment.
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Mountain ridges are responsible for the presence of many areas with
orographic cyclogenesis, land–sea contrast for thermal lows, the presence of
moist Mediterranean air with potential for strong diabatic processes conditions
the development of cyclonic structures, like small-scale, hurricane-like lows.
The triggering mechanism of most Mediterranean cyclones is mostly due to
features external to the Mediterranean region, that is mid-latitude (primary)
baroclinic waves with high-level potential vorticity anomaly, which interacts
with regional structures in the Mediterranean region. The complexity of the
mechanisms involved is such that many different categories of cyclones can be
identified, according to the region of formation, their seasonality and dominant
mechanism of formation. Beside the cyclones entering from the Atlantic sector,
there are lee cyclones, thermal lows, African cyclones, mesoscale hurricanelike lows, Middle East cyclones. Further differentiations are possible, as several
distinct regions of cyclogenesis exist for lee cyclones and different types of
cyclones can be defined in the Middle East. Most cyclones have a radius smaller
than 600 km. Seasonality is different for various categories; in general, the overall
synoptic activity is higher from November to March, but there are types of
cyclones, like thermal lows, Sahara cyclones and Middle East depressions,
whose frequency is larger in summer.
Because of this complexity, several large-scale patterns can be associated
with cyclones in the Mediterranean area. While the NAO certainly plays an
important role (which is dependent on the type of cyclone considered), blocking episodes above central and northern Europe explain a large fraction of
variability. Other patterns, like the East Atlantic/Western Russia pattern, exert
a significant influence too.
Long-term trend analysis (since 1850) shows a reduction of the cyclone
activity over most of the region. During the second half of the 20th century
this trend is confirmed for the western part, while an increase of cyclone activity
has been observed for the eastern part. However, a very large interannual
variability is superimposed on these trend.
Cyclones are associated with many extreme events: precipitation, winds, waves,
landslides and surges. Several hazardous weather events take place every year
in the Mediterranean region and are a relevant cause of economic losses. In all
these events, the geography at regional-scale plays a fundamental role in
determining the effects of cyclones on the environment. Prolonged heavy rain
episodes take place when a cyclone forces surface currents of humid and warm
Mediterranean air to flow over coastal mountain slopes. High waves are
produced when the location of the cyclone ensures a long fetch. Strong winds
occur when orography locally intensifies the cyclonic circulation around a lowpressure centre. The storm surge in the Northern Adriatic sea takes place
when the south-easterly Sirocco wind is channelled along the Adriatic Sea.
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This implies that the intensity of the cyclone is not the only factor responsible
for its impact, but its position and evolution are also extremely important.
Therefore, not only the variability of the cyclone intensity, but also that of
its track contributes to the impact on the environment.
In general, extremes present larger variability than average values and,
consequently, it is more difficult to identify significant trends. While most of
the Mediterranean region, in winter, experiences a decrease in total precipitation
and average SWH (Lionello and Sanna, 2005), extremes do not show spatially
and temporally coherent trends over the whole Mediterranean region. Extreme
SWH levels have become smaller only in part of the Ionian and in the Alboran
Sea, while are increasing in a small area close to the coast of France. The
frequency of torrential rainfall has been found to increase (percentually wise)
in the second half of the 20th century.
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6.6. Outlook
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This chapter documents the large amount of research which has already been
carried out and provides a well-established understanding of many aspects of
the Mediterranean cyclones and their effect on the environment. However, in
consideration of the importance of these processes, of their potentially damaging
effects, and the need for assessing their sensitivity to climate changes, further
research is recommended (see also Chapter 8).
A main issue is whether the present archives of data provide an adequate
database for the needed analysis. On the centennial timescales it appears that AQ: Please
there is an imbalance between regions where phenomena are well documented list this
(e.g. the surge of Venice, Camuffo, 1993), and regions where data are scarce reference.
and reconstruction of past events necessarily indirect (e.g. precipitation patterns
and extremes on the whole African coast). For the recent decades, where
meteorological observations are available worldwide and model reanalysis have
been carried out (e.g. NCEP and ERA-40 re-analysis), it has still to be fully
investigated whether the subsynoptic and mesoscale characteristics of cyclones
in the Mediterranean region are well represented in the available data archives.
On this respect, the development of extensive, high-resolution sets of data
appears extremely important for addressing unresolved scientific issues. Similar
considerations apply for climate change studies. Certainly more work is needed
to link modeling of selected events, long-term modeling and observation
analysis, with the goal of a coherent long-term climatological perspective.
In this respect, interactions between climate and meteorological projects,
such as MedCLIVAR (endorsed by the WCRP) and MEDEX (a project of
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Figure 123: Statistically significant variations of maximum significant wave
height in 50 years, based on linear trends evaluated from the results of a 40-year AQ: please cite the
figure
model integration carried out using the ERA-40 re-analysis.
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WMO-WWRP), national initiatives and the involvement of regional institutions
are certainly important and potentially fruitful.
The classifications of cyclones should involve an analysis of the sensitivity of
the generation mechanisms to climate variability and change, whose identification might help in predicting future scenarios and the change in the frequency and
intensity of some specific cyclone types (e.g. the hurricane-like Mediterranean
Lows). The availability of regional high-resolution reanalysis, where cyclone
structures are well reproduced, might be crucial for this task. Moreover, it is
important to identify the deficiencies in the models that account for the
inadequacies in simulating the cyclones and their variability. On this point, the
connection between the climate of the cyclones in the Mediterranean region
and the low-frequency large-scale variability is not sufficiently understood yet.
Intensity, location, duration and orientation of the systems as well as their
interdecadal trends should be analysed and put in relation with variations of
the main European teleconnection patterns. The importance of regional-scale
processes (e.g. latent heat release over the sea) and their variability with respect
to large-scale forcing (e.g. the meridional shift and/or intensification of the
storm track over northern Europe) has not been precisely quantified.
The Mediterranean Sea is characterized by a complex coastline structure,
with some highly vulnerable areas (for example, the Niles delta and the Gulf of
Venice). Though the main danger for these areas is due to the increased coastal
erosion and loss of land that would be produced by sea level rise, the change
in storminess is also potentially critical. Variations of the frequency and intensity
of sea storms could further increase risks and damages. The analysis of changing
wind waves and surge regimes requires detailed impact studies, carried out on
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the basis of sufficiently precise forcing fields, and relies on accurate surface
wind field analysis and adequate downscaling techniques.
Variations of cyclone regimes affect the distribution of precipitation. It may
be suggested that the variations of the precipitation observed during the last
decade over the Mediterranean region are associated with relatively small
variations in the transport and characteristics of the air masses. Such changes
might be small and not always easily detectable; however, their impact on local
climate and climate variability is likely to be large. These variations could
have serious consequences for rain intensities in many Mediterranean areas.
The danger is twofold. There are areas already under stress because of recurrent
water shortage during summer, and areas where torrential rains have produced
human casualties and large damages to properties. It is important to identify
the factors responsible for the increase in rainfall extremes and reduction of
total precipitation A similar understanding is also important for waves, surges
and winds
Finally, the links between large-scale patterns and extreme events are not
simple, as extreme events are cannot easily associated to extreme values of some
large-scale predictors. The characterization of patterns of cyclones, weather
extremes and their link to large-scale fields is a topic on which more research is
needed.
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The comments by P.Malguzzi have been of great help in improving the content
of this chapter. The authors are indebted to C. Zerefos, P. Alpert, J. Luterbacher
and E. Xoplaki for important suggestions and to E.Elvini for his help with the
graphics.
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Acknowledgements
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References
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AQ: please update the status of
the references which are in press, in
preparation, submitted, in review etc.,
if any in the reference list
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Alpert, P., Neeman, B. U., & Shay-El, Y. (1990a). Climatological analysis of
Mediterranean cyclones using ECMWF data. Tellus, 42A, 65–77.
Alpert, P., Neeman, B. U., & Shay-El, Y. (1990b). Intermonthly variability of cyclone
tracks in the Mediterranean. J. Climate, 3, 1474–1478.
Alpert, P., Stein, U., & Tsidulko, M. (1995). Role of sea fluxes and topography in
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Alpert, P., Osetinsky, I., Ziv, B., & Shafir, H. (2004). Semi-objective classification
for daily synoptic systems: application to the Eastern Mediterranean climate change.
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Alpert, P., Ben-Gai, T., Baharad, A., Benjamini, Y., Yekutieli, D., Colacino, M.,
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paradoxical increase of Mediterranean extreme daily rainfall in spite of decrease in total
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