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On Cyclonic Tracks over the Eastern Mediterranean

HELENA A. FLOCAS

Department of Environmental Physics and Meteorology, Faculty of Physics, University of Athens, Athens, Greece

IAN SIMMONDS

School of Earth Sciences, University of Melbourne, Melbourne, Victoria, Australia

JOHN KOUROUTZOGLOU

Department of Environmental Physics and Meteorology, Faculty of Physics, University of Athens, Athens, Greece

KEVIN KEAY

School of Earth Sciences, University of Melbourne, Melbourne, Victoria, Australia

MARIA HATZAKI, VICKY BRICOLAS, AND DEMOSTHENES ASIMAKOPOULOS

Department of Environmental Physics and Meteorology, Faculty of Physics, University of Athens, Athens, Greece

(Manuscript received 10 September 2009, in final form 23 May 2010)

ABSTRACT

In this study, an updated and extended climatology of cyclonic tracks affecting the eastern Mediterranean

region is presented, in order to better understand the Mediterranean climate and its changes. This climatology

includes intermonthly variations, classification of tracks according to their origin domain, dynamic and ki-

nematic characteristics, and trend analysis. The dataset used is the 1962–2001, 2.58 3 2.58, 40-yr European

Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The identification and

tracking of the cyclones was performed with the aid of the Melbourne University algorithm. It was verified

that considerable intermonthly variations of track density occur in the eastern Mediterranean, consistent with

previous studies for the entire Mediterranean, while further interesting new features have been revealed. The

classification of the tracks according to their origin domain reveals that the vast majority originate within the

examined area itself, mainly in the Cyprus area and the southeastern Aegean Sea, while the tracks that

originate elsewhere most frequently enter from the west. Deeper cyclones follow the southwest track origi-

nating from the area between Algeria and the Atlas Mountains. A greater size characterizes the westerly

tracks (southwest, northwest, and west), while the northwest tracks propagate faster over the study area. A

negative trend of the track frequency was found on an annual basis that can be mostly attributed to the winter

months, being associated with variations in the baroclinicity. This negative trend is more prominent for the

westerly and northeasterly tracks, as well as for those originating in the northern part of the examined area.

1. Introduction

The source and path of extratropical cyclones, namely,

the cyclonic tracks, strongly influence the climate in

midlatitudes, especially the precipitation regime. Con-

sequently, detailed knowledge of cyclonic tracks is

essential in forecasting weather and understanding the

atmospheric environment. Modifications of the cyclonic

tracks caused either by anthropogenic effects or by long-

term natural variability are important sources of in-

formation in understanding the atmospheric dynamics

and determining the impact on regional climates in the

future.

The eastern Mediterranean, extending between 208

and 388E to include the Ionian, Aegean, and Levantine

Seas (see Fig. 1), is an area of great interest with respect

to cyclone behavior, because of its location between the

Corresponding author address: Helena A. Flocas, Building

PHYS-5, Department of Environmental Physics and Meteorol-

ogy, Faculty of Physics, University of Athens, University Campus,

157 84 Athens, Greece.

E-mail: efloca@phys.uoa.gr

1 OCTOBER 2010 F L O C A S E T A L . 5243

DOI: 10.1175/2010JCLI3426.1

� 2010 American Meteorological Society

subtropics and midlatitudes and also its complex to-

pography (HMSO 1962). In addition, the Mediterra-

nean Basin is considered to be particularly vulnerable to

climate change (Solomon et al. 2007), and it is of value to

diagnose the changes in cyclone behavior over an ex-

tended period.

Several studies have attempted to depict the charac-

teristics of the cyclonic tracks in the Mediterranean by

employing objective methods for cyclone detection and

tracking (Alpert et al. 1990a,b; Trigo et al. 1999; Campins

et al. 2000; Picornell et al. 2001; Bartholy et al. 2009). In

particular, Alpert et al. (1990b) used European Centre

for Medium-Range Weather Forecasts (ECMWF) oper-

ational analyses every 12 h for a short period of 6 yr

(1982–87) and examined intermonthly variations of cy-

clonic tracks in the entire Mediterranean. Trigo et al.

(1999), employing higher-resolution data (1.1258 3 1.1258)

for an 18-yr period (1979–96), examined cyclonic tracks

as part of a complete climatological analysis of Mediter-

ranean cyclones. Trigo (2006) performed a comparison of

tracks in the Euro-Atlantic sector, including the Med-

iterranean, as derived by two different resolution re-

analysis datasets: the 2.58 3 2.58 National Centers for

Environmental Prediction–National Center for Atmo-

spheric Research (NCEP–NCAR) reanalysis and the

1.1258 3 1.1258 40-yr ECMWF Re-Analysis (ERA-40).

Furthermore, Maheras et al. (2001) examined cyclonic

centers in the Mediterranean region with the aid of an

objective method, without distinguishing between cy-

clones that form at a specific grid and those migrating at

a subsequent development stage. Campins et al. (2000),

Picornell et al. (2001), and Bartholy et al. (2009) focus

on the western Mediterranean tracks.

Studies related to the cyclonic tracks in the eastern

Mediterranean refer to specific types of cyclones, such as

frontal (Flocas 1988), and in specific areas, such as the

Aegean Sea and Cyprus (Maheras 1983; Nicolaides et al.

2004), employing synoptic analyses of mean sea level

pressure (MSLP). Some of these studies (Maheras 1983;

Flocas 1988; Alpert et al. 1990b; Trigo et al. 1999; Maheras

et al. 2001) have demonstrated the importance of exam-

ining Mediterranean cyclonic tracks on a monthly basis.

Over the last 20 yr several numerical algorithms have

been developed to objectively identify cyclones and

their tracks, using different approximations in the defi-

nition of cyclonic centers and the strength of the cy-

clones detected (e.g., Le Treut and Kalnay 1990; Alpert

et al. 1990a; Murray and Simmonds 1991a,b; Konig et al.

1993; Hodges 1994; Serezze 1995; Haak and Ulbrich

1996; Blender et al. 1997; Sinclair 1994; Lionello et al.

2002). Cyclone centers have been defined in terms of

pressure minima at sea level or minima in 1000-hPa

geopotential height (e.g., Le Treut and Kalnay 1990;

Alpert et al. 1990a). Alternatively, cyclones can be de-

fined in terms of maxima in low-level vorticity (e.g.,

Hodges 1994; Sinclair 1994). Methods searching for

pressure minima tend to overestimate deep and mature

cyclones, while they miss small-scale systems that are

better identified from their local maxima in relative

vorticity, for example, fast-moving systems or cyclones

in the early and late stages of their life cycle (Hoskins

and Hodges 2002). On the other hand, vorticity maxima

are not always connected with local pressure minima.

For this reason, Murray and Simmonds (1991a,b) and

Konig et al. (1993) employed a combination of both

criteria.

Despite the considerable research that has been per-

formed on Mediterranean cyclonic tracks, there is a

strong need to update their climatology for longer pe-

riods and further examine important cyclonic features.

This is particularly so for the eastern Mediterranean,

resulting from more limited research as compared to the

western Mediterranean. In line with these considerations,

the objectives of this study are (i) to investigate the

FIG. 1. Geographical chart of the Mediterranean region where the sectors are displayed

corresponding to the origin of the tracks (see text for more details). The target area of the

eastern Mediterranean is covered by EM12 (shaded area).

5244 J O U R N A L O F C L I M A T E VOLUME 23

intermonthly variations of cyclonic tracks affecting the

eastern Mediterranean, (ii) to examine the tracks ac-

cording to their origin domain, (iii) to investigate their

fundamental dynamic/kinematic characteristics, and (iv)

to explore possible trends in their frequency and intensity.

2. Methodology

The dataset used in this study comprises 6-hourly

analyses of MSLP on a 2.58 3 2.58 regular latitude–

longitude grid for the period of 1962–2001, as derived

from ERA-40. It should be noted that global analyses

significantly improved after 1979, because of the major

improvement of the overall observing system. However,

the observational coverage in the Mediterranean basin

was very good prior to that time, and hence the ERA-40

dataset there before 1979 can be used with confidence

(Courtier et al. 1998; Uppala et al. 2005). The domain of

study includes the eastern Mediterranean area, extend-

ing from 208 to 388E and from 308 to 458N (Fig. 1). The

resolution of the input dataset means that the identified

tracks do not include smaller-scale cyclones that form in

the Mediterranean region, mainly during the warm pe-

riod by thermal forcing (Trigo et al. 1999, 2002).

The cyclone identification and tracking was performed

with the algorithm developed at Melbourne University,

according to the Lagrangian perspective (MS algorithm;

see Murray and Simmonds 1991a,b; Simmonds and

Murray 1999). The MS algorithm ‘‘finds’’ a cyclone only

if an open or closed depression can be associated with

a vorticity maximum. This approach is considered to be

crucial, because open lows are also incorporated into the

storm life cycle, preventing possible time series breaks,

if a temporary weakening to an open, low state occurs

(Simmonds et al. 2008).

The performance of the MS algorithm has been as-

sessed (Leonard et al. 1999; Pinto et al. 2005; Mesquita

et al. 2009) and proved to be a very powerful tool, not

only in the generation of cyclone climatologies, but also

in the assessment of individual tracks. The algorithm

was found to be capable of identifying cyclones in a range

of locations and with different characteristics, including

small-scale systems over secondary storm-track regions

and fast-moving storms that produce extreme events in

Europe (Pinto et al. 2005). Especially for the Mediter-

ranean region, Pinto et al. (2005) demonstrated that their

results are in good agreement with those of Trigo et al.

(1999), while for individual cases there is good agreement

with hand-analyzed synoptic weather maps. Leonard et al.

(1999) pointed out that the MS algorithm is able to detect

a larger number of cyclones in the Southern Hemisphere,

as compared to other two algorithms developed by Konig

et al. (1993) and Terry and Atlas (1996), and, moreover,

it can track these cyclones and maintain their continuity

over a long time. Furthermore, Leonard et al. (1999)

concluded that the MS algorithm is most applicable for

research purposes. Mesquita et al. (2009) demonstrated

that the MS algorithm, since accounting for open, low

systems, produces more reliable storm climatology results,

as compared to the National Oceanic and Atmospheric

Administration (NOAA)/Climate Prediction Center

(CPC) current operational algorithm.

In our study, a bicubic spline interpolation was em-

ployed to a 121 3 121 polar stereographic grid with

a resolution of 1.98 latitude at the northern pole, de-

creasing to half of this value (0.958) at the equator. The

control parameters of the MS algorithm have been mod-

ified, as compared to the application in the entire North-

ern Hemisphere (Lim and Simmonds 2002), in order to

better capture the individual characteristics of cyclones in

a closed basin with complex topography, such as the

Mediterranean. In particular, (i) diffusive smoothing to

the original data has not been applied; (ii) all cyclones

identified at grid points with surface heights of more

than 1500 m are excluded; (iii) an averaging radius

equivalent to 48 in latitude is selected, considering that

Mediterranean cyclones have a typical horizontal scale

of 300 km (Trigo et al. 1999); (iv) during the detection

stage of the algorithm, no strength criteria are set in the

beginning of the life span of the cyclones, because many

Mediterranean cyclones at the beginning of their life are

weak and unimportant, but during the next steps they

turn into strong systems (Trigo et al. 1999); and (v)

a minimum life span of 24 h is imposed in order to ex-

clude short-lived systems and to enable the calculation

of time derivatives of the velocity and pressure tendency

(Simmonds and Murray 1999).

Every track entering the eastern Mediterranean basin

for at least one analysis time step was considered as an

eastern Mediterranean track. The frequency of tracks

entering the eastern Mediterranean was determined for

each month of the year. Then, the frequency of tracks

for each month was calculated according to their origin

domain, which is represented by the first step of the

track. More specifically, the following six sectors were

distinguished (see Fig. 1): (i) the northwestern sector,

(ii) the western sector, (iii) the southwestern sector, (iv)

the northeastern sector, (v) the northern part of the ex-

amined area of eastern Mediterranean (EM1), and (vi)

the southern part of the examined area (EM2). Based on

this classification, our target area for the eastern Medi-

terranean is represented by the sectors EM1 and EM2,

which hereafter will be referred to as EM12 area.

For each sector, the following dynamic and kinematic

parameters of the cyclonic tracks are calculated for the

period that they remain within the EM12 area: (i) the

1 OCTOBER 2010 F L O C A S E T A L . 5245

effective cyclone radius R [in degrees of latitude, (8lat)],

which is defined as

R2 5

�N

i51r 2

i

N,

where ri is the distance of the radial line from the cyclone

center to the points at which the Laplacian of MSLP is

zero around the edge of a cyclone and N is the number of

the radial lines drawn at azimuthal spacings of 208 (Lim

and Simmonds 2007), that is, N 5 18, in this case; (ii) the

Laplacian of the central pressure =2P [hPa (8lat)22],

representing an effective measure of cyclone intensity

(Petterssen 1956); (iii) the cyclone depth D (hPa), which

combines the cyclone size and intensity by the rela-

tionship and, in the case of an axisymmetric parabolic

cyclone, can be written as

D 5R2=2p

4

[this represents the ‘‘pressure deficit’’ of the cyclone,

i.e., the difference between the pressure at the ‘‘edge’’

of a cyclone and at the center, and it is also related to

the total kinetic energy of the cyclone (Simmonds et al.

1999; Simmonds and Keay 2000, 2009)]; and (iv) the av-

erage propagation velocity Uc (m s21; Lim and Simmonds

2007).

3. Intermonthly variations

Figure 2 displays the tracks that enter the EM12 area

for each month during the 40-yr period, covering their

whole lifetime from their formation until their dissipa-

tion (viz., track density). It can be seen that there are

considerable intermonthly variations of track density in

the target area EM12, even for months belonging to the

same season, which are results that are consistent with,

or complementary to, previous studies (e.g., Alpert et al.

1990a,b).

More specifically, in December the track density is

reduced, as compared to the other winter months, es-

pecially for cyclones originating in the western Medi-

terranean, as was found by Bartholy et al. (2009) for the

winter period. In January, the maritime tracks increase

substantially in the entire Mediterranean, suggesting the

role of the enhanced sea–land temperature difference

on cyclone formation and movement (Lolis et al. 2004).

The tracks originating from southern Italy form mainly

in January. Similarly, the track density over the Black

Sea increases in January. In February, the tracks migrate

northward along the northern Mediterranean coast,

where the major topographic barriers are located, from

the Gulf of Lions and Gulf of Genoa to the Adriatic,

Ionian, and Aegean Seas, which are mainly associated

with the lee origin of the cyclones affecting the exam-

ined area (Trigo et al. 2002; Bartholy et al. 2009).

In spring, the most notable feature is the increase of

the North African tracks, forming in the south of the

Atlas Mountains, reflecting the low-level baroclinicity

increase (Trigo et al. 1999), as was also found by Alpert

et al. (1990b). The tracks from North Africa differenti-

ate after entering the eastern Mediterranean, in accor-

dance with Prezerakos (1985). More specifically, in March

they follow a northward/northeastward direction toward

the Ionian and Adriatic Seas, while in April and May a

more eastward route is prominent toward the Levantine

Sea and Cyprus. In April, the northward cyclonic tracks

become more common, contributing to the substantial

density increase over the Balkan Peninsula and the Black

Sea. In May the track density of the North African cy-

clones reduces, becoming very small in the following

summer months. Furthermore, the density of the tracks

originating in the western Mediterranean decreases, as

compared to the other spring months, this being most

likely related with the corresponding remarkable re-

duction of the number of cyclogenetic events in the

Gulfs of Genoa and Lions, and the Ligurian and Adri-

atic Seas during this month (Trigo et al. 1999).

During boreal summer months, there are no substantial

intraseasonal differences of the tracks passing over the

eastern Mediterranean, in accordance with Alpert et al.

(1990b), except that the track density is lower in July and

August. The cyclonic tracks are mainly concentrated over

the land, reflecting their thermal origins (Trigo et al.

2002). Moreover, the cyclones originate in the eastern

Aegean and Cyprus (EM2), along the Turkish coast, be-

cause the subtropical high prevails over the whole Med-

iterranean region and the Pakistan low extends over the

eastern Mediterranean (Ziv et al. 2004). Furthermore, the

cyclones are not migratory, because of their thermal

character and small life span (Trigo et al. 2002). It becomes

evident that the tracks from the western Mediterranean

are limited in summer, although the number of cyclo-

genetic events is increased in this region, because of the

short distance that they cover (Picornell et al. 2001).

In September, similar features are depicted as for Au-

gust, except that the number of tracks originating in the

North African coastal region is slightly increased, because

of the reduced influence of the subtropical high during

this month. In October, the distribution of tracks changes

considerably. The density of maritime tracks increases,

especially for those starting from the Adriatic and Ionian

Seas, which is probably related to sea surface temperature

(SST) changes during this month, as was demonstrated by

5246 J O U R N A L O F C L I M A T E VOLUME 23

FIG. 2. Cyclonic tracks passing over or originating within the eastern Mediterranean region on a monthly basis. The

tracks cover the whole lifetime of the cyclones from their formation to dissipation and are represented.

1 OCTOBER 2010 F L O C A S E T A L . 5247

Marullo et al. (1999) and Skliris et al. (2010). Also, there

are cyclones that form in the south of Crete. It should be

noted that the thermal effect prevailing during summer

months and September is almost eliminated during Oc-

tober and November. In November, the cyclonic routes

increase in the western Mediterranean, mainly along the

northern coast, where the low-level baroclinicity in-

tensifies (Trigo et al. 2002).

In summary, as one can see from Fig. 3, the cyclonic

tracks passing through EM12 are most numerous from

December to April; their number decreases during the

warm period and tends to increase again in October. The

maximum number of cyclonic tracks over the target area

is observed in January (11.2% of the annual total) and

March (10.3%). The minimum number of tracks occurs

in July (5.3%).

4. Track classification

The classification of the tracks passing through the

EM12 area, according to their origin domain, during

the period of 1962–2001 and for all months, reveals that

the greatest number (36.8%) originate in the southern part

of the examined area (EM2), that is, the Ionian Sea, the

southeastern Aegean Sea, Crete, Cyprus, and the Levan-

tine Sea (Fig. 4), representing a major cyclonic center in

Mediterranean (Maheras et al. 2001). The number of

these tracks is particularly high in the boreal summer

and fall (Fig. 5), reflecting the influence of the thermal

Pakistan low extension in the eastern Mediterranean

during this period (Bitan and Saaroni 1992).

A considerable proportion of systems (26.8%) origi-

nate in the northern part of the examined area (EM1),

including northern Greece, the north Aegean, the Bal-

kans, and a major part of the Black Sea. The tracks

originating in the Aegean Sea prevail during winter, and

mainly in January (Fig. 5), consistent with the cyclo-

genesis frequency distribution (Flocas and Karacostas

1996). In the boreal summer, the number of cyclonic

tracks originating in EM1 sector drops dramatically

(Fig. 5), although the southeastern Black Sea is a cyclo-

genetic area throughout the entire year (Trigo et al.

1999).

Apart from these tracks that originate within the ex-

amined area, the next most common cyclonic tracks

originate in the western sector (13.8%), which incor-

porates the Adriatic Sea and parts of the western

Mediterranean and the Atlantic. Moreover, these tracks

exhibit their maximum frequency in January and Feb-

ruary (Fig. 5), complying with the consideration that the

FIG. 3. Intermonthly variation of the frequency of the tracks

passing through the EM12 target area.FIG. 4. Relative frequency of tracks passing through the EM12

target area, originating in each sector.

FIG. 5. Intermonthly variation of the frequency of tracks originating

in each sector.

5248 J O U R N A L O F C L I M A T E VOLUME 23

Gulf of Genoa is the greatest winter cyclogenetic area

(Trigo et al. 1999) and that southern Italy is a major

cyclonic center in winter (Maheras et al. 2001). It should

be noted that the westerly track coincides with a com-

mon path of frontal depressions passing over Greece

during winter (Flocas 1988) and of baroclinic depressions

affecting Cyprus during the cold season (Nicolaides et al.

2004).

The tracks from North Africa and the Alboran Sea,

being attributed to the southwest category, comprise

a significant portion of the total track number (13%), in

agreement with previous studies (Maheras 1983; Alpert

et al. 1990b; Trigo et al. 1999). The tracks of Saharan

depressions predominate during March and April (Fig. 5;

see also Prezerakos 1985), while tracks from the Alboran

Sea are evident during the boreal summer (Picornell et al.

2001).

A limited number of tracks (4.9%) originate in the

northwest sector throughout the entire year, since the

northwest and EM12 sectors share an infinitesimally

short border. However, the northwest tracks are found

to enter the target area mainly through the west sector.

The northwest tracks predominate in December, with,

however, no significant intermonthly variations (Fig. 5).

This is the favored path for frontal depressions originat-

ing in the Atlantic near the British Isles from January to

April (Flocas 1988). However, depressions originating in

this sector are also apparent in the boreal summer (Fig. 5),

as was also noted by Maheras (1983) for depressions

passing over the Aegean Sea. This could be associated

with the northward displacement of the subtropical jet,

which does not allow the westerly or southwesterly tracks

to enter the examined region (HMSO 1962).

The tracks originating in the northeast sector are also

limited in number (4.7%), being related with cyclones,

forming mainly in the northern part of the Balkan

Peninsula, eastern Europe, and part of the Black Sea.

These tracks do not exhibit any intra-annual variations

(Fig. 5), in agreement with the cyclogenesis frequency in

this area (Trigo et al. 1999).

From Fig. 5 it is evident that there are significant in-

termonthly variations of the track frequency originating

from the same sector among the spring months and au-

tumn months, suggesting corresponding variations in the

atmospheric circulation responsible for cyclone forma-

tion during these months, as reflected in the distribution

of the cyclogenesis events during these months (Trigo

et al. 1999).

5. Trends of track frequency

The trends of the total frequency of cyclonic tracks

entering the EM12 region are investigated for each month

separately during the examined 40-yr period. The trends

are calculated with the aid of linear regression analysis on

a monthly basis, and the statistical significance of the

trends at a 5 0.10 and a 5 0.05 was examined with the

Student’s t test (Table 1).

Figure 6 outlines the interannual variation of the num-

ber of tracks entering the EM12 region. The total number

of tracks passing over EM12 for the 40-yr period is 10 461,

suggesting an average number of about 260 tracks per

year. In the beginning of the period the number of the

tracks is quite high during the cold months. The year 1969

is characterized by a peak (286) in the total annual number

of tracks.

In the Mediterranean, the cyclone development and

motion is greatly affected by low-level baroclinicity,

topography, and sea surface fluxes, as well as by upper-

level dynamics (Trigo et al. 2002; Maheras et al. 2002).

Any observed changes in the frequency of Mediterra-

nean cyclonic tracks are related principally with changes

in baroclinicity and upper-level circulation. Enhanced sea

surface fluxes associated with a warmer sea and mirroring

increased SSTs play a more important role in the intensi-

fication of Mediterranean cyclones (Alpert et al. 1990a;

Lionello et al. 2002; Solomon et al. 2007; Lagouvardos

et al. 2007), and will be further examined in the trends of

track intensity in section 6.

Taking into account that the low-level baroclinicity is

proportional to the temperature gradient (Hoskins and

Valdes 1990),

gradTj j5

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi›T

›x

� �2

1›T

›y

� �2s

,

TABLE 1. Monthly linear trends of the number of tracks passing

through EM12 region, their intensity and depth, and of SST in the

eastern Mediterranean Sea; (*) denotes the trend is significant at

a 5 0.10, and (**) denotes the trend is significant at a 5 0.05.

Month

Trend of

tracks

(decade21)

Intensity [hPa

(8lat)22

decade21]

Depth (hPa

decade21)

SST (8C

decade21)

January 21.3 20.012 20.07 20.07

February 21.4** 20.007 20.03 20.07

March 20.2 20.006 20.01 20.06

April 0.6 20.009* 20.05 0.02

May 20.9 0.005 0.05* 0.02

June 0.5 0.007** 0.04** 0.05

July 20.1 0.009** 0.02 0.07

August 21.3** 0.010** 0.02 0.17**

September 1.2** 0.001 0.01 0.12

October 0.1 0.004 0.03 0.06

November 1.1* 0.009 0.06* 20.05

December 20.9 20.003 0.01 0.00

Annual 20.2 20.001 20.00 0.02

1 OCTOBER 2010 F L O C A S E T A L . 5249

we examine the extent to which the cyclone changes can

be seen as associated with changes in the 850-hPa tem-

perature gradient magnitude (Wang et al 2009).

The isobaric level of 850 hPa is selected as being rep-

resentative of the lower troposphere; it reflects the effect

of the free-tropospheric flow and the surface character-

istics, and thus best represents the position of the large-

scale baroclinic zone. For each monthly field and for each

year the magnitude of the temperature gradient over the

greater Mediterranean region was calculated, following

the approach presented in Simmonds and Lim (2009).

Then, for the first (1962–81) and the second (1982–2001)

half of the study period the average monthly magnitude is

computed along with the difference between the average

of the second half minus the corresponding average dur-

ing the first half (see Fig. 7).

Changes in the upper-level circulation are not the

main interest in the present study. However, an attempt

FIG. 6. Interannual variations of the frequency of the tracks passing through the EM12

examined area for each month; the contours are frequency isopleths for every four tracks.

FIG. 7. Spatial distribution of the differences in the magnitude of the temperature gradient at 850 hPa that correspond to the second half

(1982–2001) minus the first half (1962–81) of the examined period, over the greater Mediterranean region for (a) December, (b) January,

(c) February, and (d) August, (e) September, and (f) November. The contour interval is 2 K (1000 km)21.

5250 J O U R N A L O F C L I M A T E VOLUME 23

is made to relate our results with the findings of previous

studies, dealing with the seasonal trends of the anticy-

clonic and cyclonic types at 500 hPa over the Greek area

during the period of 1958–2000 (Maheras et al. 2004,

2006) and the seasonal trends of the 500-hPa geopoten-

tial height over Europe for the period of 1958–2002

(Hatzaki and Flocas 2004).

For the entire 40-yr period, a negative trend (20.2

decade21) of the frequency of the cyclonic tracks was

found, which is significant at a 5 0.10. However, from

Table 1 it becomes evident that the sign and the sig-

nificance of the trends vary considerably, not only in-

terseasonally, but also intraseasonally, following the

intermonthly variations of frequency. In the following,

we focus on the statistically significant monthly trends.

It can be seen that there is a negative trend of the

frequency during all three winter months, from December

to February. This is consistent with the overall negative

trend of cyclonic tracks in winter over the Mediterranean,

which is statistically significant at a 5 0.1 over the north-

ern Balkans and Turkey, found by Trigo (2006), and the

decreasing trend of cyclonic occurrences in eastern

Mediterranean during the rainy period found by Maheras

et al. (2001). Furthermore, the negative trend during the

winter months agrees well with the downward trend of

surface cyclonic types prevailing over the greater Greek

area in winter as well as the corresponding upward trend

of surface anticyclonic types (Maheras et al. 2000). This

result is further supported by the statistically significant

positive trend values of the 500-hPa geopotential height

over the EM12 target area in winter (Hatzaki and Flocas

2004), and the decreasing trend of cyclonic types at

500 hPa over the greater Greek area (Maheras et al. 2006).

More specifically on a monthly basis, the most sig-

nificant negative trend of track frequency in winter was

found in February (21.4 tracks per decade). In December

(Fig. 7a) and January (Fig. 7b), the negative trends are

well related with a remarkable decrease of baroclinicity

over the entire Mediterranean and European area, sup-

porting the statement of Trigo (2006) for a northward

shift of storm tracks in the Euro-Atlantic sector in winter.

In both months, the peak of the change in the magnitude of

the gradients is found in north Europe as well as northern

Africa [24 K (1000 km)21]. By contrast, in February

(Fig. 7c), the major part of the target area EM12 is

characterized by a baroclinicity increase, which seems

contradictory to the apparent track decrease. However,

a careful inspection of the track origin during this month

(Figs. 2 and 5) demonstrated that a significant proportion

of the tracks originate in the west, southeast, and north-

east sectors, where a substantial magnitude of the tem-

perature gradient decrease was indeed found, with peak

values smaller by 4 K (1000 km)21. Therefore, it is appears

that during February the remote baroclinic influences

are more important than the local influences acting within

the target area.

During spring, the trend values vary, being negative in

March and May and positive in April, but they are not

statistically significant and are not accompanied by any

noticeable change of the baroclinicity (not shown). The

trend differences among the spring months could be

attributed to the profound intermonthly variations of

track frequency (see section 3) and the significant in-

termonthly variations in the cyclogenetic mechanisms

acting in this period (Trigo et al. 2002). This is also re-

flected in the different sign of trend among the cyclonic

types affecting Greece in spring, following changes in

the atmospheric circulation prevailing over the greater

European area during this season (Maheras et al. 2000,

2004). It should be noted that the overall spring trend of

500-hPa geopotential height over EM12 is (significantly)

positive, with, however, lower values, as compared to

winter (Hatzaki and Flocas 2004).

Despite the overall increase of the frequency of sur-

face anticyclonic types in summer (Maheras et al. 2000),

the decrease of frequency of 500-hPa cyclonic types

(Maheras et al. 2004) in Greece, and the significant

positive trend values of 500-hPa geopotential height over

EM12 (Hatzaki and Flocas 2004), only in August does

a significant negative trend of cyclonic tracks become

evident. This trend is consistent with the increasing trend

of the frequency of the surface mixed-weather-type

‘‘Mb’’ (Maheras et al. 2000) during this month, which is

characterized by a weak surface pressure gradient over

the Aegean Sea and zonal circulation at the upper levels,

resulting in calm weather conditions and surface warming

(Good et al. 2008). This is also consistent with an overall

decrease in the magnitude of the gradient of temperature

(or baroclinicity) over the entire Mediterranean region,

peaking [at 26 K (1000 km)21] in the Iberia and south-

eastern Aegean Seas (Fig. 7d), representing strong cy-

clonegetic centers during August (Trigo et al. 1999).

In fall, the overall trend of frequency is positive, in

general agreement with the positive trend values at

500 hPa over EM12, although these are not statistically

significant in the northern sector EM1 (Hatzaki and

Flocas 2004). On a monthly basis, a significant positive

trend of cyclonic tracks is observed in September, which

agrees well with a substantial baroclinicity increase in the

major part of the Mediterranean and southern Europe

(Fig. 7e), peaking in the area of Italy [6 K (1000 km)21].

For November, there is also a statistically significant

positive trend (a 5 0.1) that is consistent with the increase

of surface cyclonic types over the greater Greek area and

the associated tropospheric cooling in the 1000–500-hPa

layer during this month (Good et al. 2008). Furthermore,

1 OCTOBER 2010 F L O C A S E T A L . 5251

the track increase in the target area agrees with baro-

clinicity increase in the western Mediterranean, the At-

lantic, and northwest Europe (Fig. 7f), suggesting the role

of the baroclinic effects acting in these areas.

For each origin sector, the trends of track density were

calculated at each grid point within the EM12 target

area and for the entire examined period, using linear

regression. The statistical significance of the trend at each

grid point of the target area was examined at a 5 0.05.

The spatial distribution of the density trends for the

tracks originating in the west sector (Fig. 8a) shows

a significant negative trend in the entire target area,

except over the Black Sea. Greater values are found in

western Greece and the Ionian Sea, as well as in Cyprus.

A significant negative trend is also observed for the

northeast tracks, but over the northern part of the EM12

area, namely, the Balkans, and the northern Aegean and

Black Seas (Fig. 8b). On the contrary, a positive trend

characterizes the southwest tracks, which are significant

over Crete, Cyprus, and the Middle East (Fig. 8c). No

significant trend was found for the northwest tracks (not

shown). The tracks that originate within EM1 exhibit a

FIG. 8. Spatial distribution of the density trends of tracks entering EM12 for each sector: (a) west, (b) northeast,

(c) southwest, (d) northwest, (e) EM1, and (f) EM2. The dots represent the grids where the trend is statistically

significant at a 5 0.05.

5252 J O U R N A L O F C L I M A T E VOLUME 23

negative trend that becomes significant over the south-

ern Ionian Sea, the southern Turkish coast, and the

eastern part of the Black Sea (Fig. 8d). The tracks origi-

nating within EM2 exhibit a significant increasing trend

over Cyprus and the Middle East and a decreasing one in

the remaining area, but mainly over the east Black Sea

and the Ionian Sea (Fig. 8e).

6. Dynamic and kinematic parameters

In this section, in order to obtain a comprehensive

picture of their overall behavior and impact on climate

according to the different origin domains, insightful di-

agnostic dynamic and kinematic parameters are de-

scribed that are associated with cyclones passing through

the eastern Mediterranean. Furthermore, these parame-

ters are very important for understanding changes in cy-

clonic activity in eastern Mediterranean.

The estimation of the following four parameters were

performed only for the part of each relevant track within

the EM12 target area and for each origin sector, as de-

scribed in section 2: (i) the Laplacian of central cyclone

pressure =2P, (ii) the cyclone depth D, (iii) the cyclone

radius R, and (iv) the propagation velocity Uc. Table 2

summarizes the value range, the mean value, and the

standard error of the four parameters.

It can be seen that the mean Laplacian =2P has the

same mean value [0.3 hPa (8lat)22] for the cyclones

generating within the examined area (EM1 and EM2),

as well as for those that originate in the northeast. The

mean Laplacian value increases to 0.35–0.38 hPa (8lat)22

when the cyclones originate in the northwest, southwest,

and west. The maximum mean intensity of 0.38 hPa (8lat)22

characterizes the southwesterly tracks.

The average cyclone radius exhibits significant spatial

variation in the target area, with values increasing from

the north to the south, particularly in winter (not shown),

in agreement with Simmonds and Keay (2002), for cy-

clones in the Northern Hemisphere oceans. The radius

is at least 2.58–2.78lat for all categories. Larger cyclones

are associated with the southwest, west, and northwest

categories, with average radii of 38, 2.958, and 2.948lat,

respectively.

The cyclone depth similarly exhibits larger values for

the tracks originating from the southwest, northwest,

and west, a fact that seems reasonable for systems de-

veloping outside the target area. For the northwest tracks,

the cyclone depth changes significantly between the ori-

gin domain and the target area: 5.5 hPa in the Atlantic

area reduces to 1.5–3.3 hPa over the EM12 area. Fur-

thermore, the depth of the west cyclones exhibits sub-

stantial spatial variations: 4.50 hPa in the Atlantic and

2.50–2.90 hPa in the western Mediterranean, reducing to

1.7–2.5 hPa over EM12. The depth is almost uniform for

the entire target area (2.5 hPa) for the southwest tracks.

The propagation velocity also differs substantially

between the origin sectors, with the lowest values being

associated, as one might have expected, with those orig-

inating within EM1 and EM2 (3.6 and 3.9 m s21, re-

spectively). By contrast, the northeast cyclones propagate

comparatively faster, especially at the southern bound-

ary. Larger velocities of 5 m s21 in the whole area ex-

amined characterize the west and southwest categories.

The northwest cyclones appear as the maximum velocities

within the examined area at 7.5 m s21, while their mean

velocity in the Atlantic region was as high as 12 m s21.

The trends of the above-mentioned parameters were

calculated on a monthly basis for the whole target area

and for the 40-yr period. Negative trends were found for

=2P, D, and R, but these were statistically significant (at

a 5 0.05) only for radius (20.018lat decade21). On a

monthly basis (see Table 1), the trend signs of track in-

tensity and depth indicate considerable changes through

the year. Following the overall decreasing tendency of

intensity, the trend is negative from December to April,

but at a statistically significant level (at a 5 0.1) only in

April. Then, from May to November, the monthly trends

of intensity are positive and are particularly noteworthy

(and statistically significant at a 5 0.05) during the sum-

mer months of June–August. On the contrary, the trend

of depth is negative from January to April and positive

from May to December, with statistically significant

values in May, June, and November.

Considering the relationship of the SST increase with

cyclone intensification (see section 5), we have explored

the link between the monthly trends of the SST in the

eastern Mediterranean region with corresponding trends

of track intensity (Table 1). For this purpose, the Met

Office Hadley Centre’s SST dataset (HadSST2) is em-

ployed, which is available on a monthly global field on a

58 latitude 3 58 longitude grid (Rayner et al. 2006). In our

case, the average of six grid boxes that cover the 308–

408N, 208–358E region was calculated for the 1962–2001

TABLE 2. Mean values of the dynamic/kinematic parameters for

the tracks within eastern Mediterranean region (EM12) for dif-

ferent origin sectors. The standard error of the mean value of each

parameter refers to the whole population.

Origin

domain

=2P 6

0.02 hPa (8lat)22R 6

0.05 8lat

DP 6

0.014 hPa

Uc 6

0.5 m s21

EM1 0.31 2.88 2.15 3.7

EM2 0.31 2.87 2.17 4.0

Northeast 0.30 2.84 2.12 4.3

Northwest 0.35 2.94 2.46 7.1

Southwest 0.38 3.00 2.54 4.9

West 0.36 2.95 2.44 5.4

1 OCTOBER 2010 F L O C A S E T A L . 5253

period. It was found that the increasing trend of track

intensity in June–August corresponds well with the SST

increasing trend, which is significant in August. This is in

accord with Alpert et al. (1990a), who demonstrated the

major positive thermal effect of the sea on eastern Med-

iterranean cyclones, particularly in summer.

We next calculated the trend of each parameter by

linear regression at each grid point of the target area for

the examined period and for each sector separately. The

spatial distribution of the intensity trend reveals that

there are no statistically significant changes for any sec-

tor, except for a negative trend in the Middle East for the

west track (Fig. 9a). The radius of the tracks exhibits an

overall decreasing trend over the whole target area for

all sectors, which is significant only over the Aegean Sea,

Greece, and Turkey for the EM2 tracks (Fig. 9b), and for

the Black Sea for the northeast tracks. Only for the

northwest and southwest tracks, does a (non significant)

positive trend appear in the eastern part of EM2, namely,

around the area of Cyprus and the Middle East (not

shown). Similarly, for propagation velocity, no significant

trend was found in the E12 area.

With respect to the spatial distribution of the depth’s

trend, it is demonstrated that the tracks originating in

EM1 present a decreasing trend over western Greece

and the Black Sea that is statistically significant only

over the Ionian Sea (Fig. 10a). In the remaining area, the

trend becomes positive, though not significant. The

southwest and EM2 track depths exhibit the (not sig-

nificant) positive trend of depth over southern Aegean

Sea, Levantine Sea, and Cyprus, and the (not significant)

negative trend over the Black Sea (not shown). On the

contrary, the significant decreasing trend of the depth of

the northwest (not shown) and west tracks (Fig. 10b)

appears in the eastern sector of EM2, that is, Lebanon,

Syria, and Israel.

7. Conclusions

In this study the main features of cyclonic tracks af-

fecting the eastern Mediterranean region have been

explored for a period of 40 yr (1962–2001) with the aid

of the Melbourne University tracking algorithm. This

long period of quality reanalysis has allowed us a new

and more comprehensive and robust view of the eastern

Mediterranean cyclone behavior.

It was verified that considerable intermonthly varia-

tions of track density occur over the eastern Mediter-

ranean, consistent with the results of previous studies for

the entire Mediterranean. As was expected for Medi-

terranean cyclones, the track frequency decreases after

May and tends to increase in October. Peaks were found

in January and March and a minimum was found in July.

Moreover, this study has exposed interesting features of

the intraseasonal track density variations. The track

density is reduced in December, as compared to other

winter months, especially for cyclones originating in the

western Mediterranean. Similarly, in May the number of

tracks originating in the western Mediterranean is re-

duced, as compared to the other spring months. In Sep-

tember, the distribution of the track density is similar to

that of the summer months. The maritime tracks increase

during January and October, while the tracks migrate

northward to the northern Mediterranean coast in Feb-

ruary and November, reflecting shifts in low-level baro-

clinicity. Although it is well known that the number of

tracks originating in northern Africa increases during the

summer months, it was demonstrated that those af-

fecting the eastern Mediterranean follow a northward/

northeastward route in March and an eastward one in the

other spring months, April and May.

FIG. 9. Spatial distribution of the trend of the (a) intensity for

the west tracks and (b) the radius for the EM2 tracks. The dots

represent the grids where the trend is statistically significant at

a 5 0.05.

5254 J O U R N A L O F C L I M A T E VOLUME 23

The classification of the tracks according to their ori-

gin domain has been very revealing, and has shown that

the vast majority originate within the examined area

itself, mainly in the Cyprus area and over the southeast-

ern Aegean Sea, which is a very important finding for the

cyclone climatology in the eastern Mediterranean region.

The most common cyclonic tracks that do not originate

in the examined area start from the western sector with

a maximum frequency in January and February. The

tracks from North Africa (the southwest sector) are most

common in the spring months of March and April. The

number of tracks from the northwestern sector is rather

small throughout the whole year.

The study of the kinematic and dynamic parameters

of tracks according to their origin demonstrated that

deeper cyclones follow the southwest track. Greater size

characterizes the westerly tracks (southwest, northwest,

and west), while the northwest tracks propagate faster

over the examined area.

The analysis of the frequency trends of the cyclonic

tracks exhibit significant intermonthly variations of sign

and statistical significance, following intermonthly var-

iations of frequency. In general, there is a statistically

significant negative trend on an annual basis. This is

mostly attributed to the negative sign during the winter

months that can be explained in terms of a decrease of

the baroclinicity, while supported by geopotential height

changes at the upper levels. On the contrary, significant

positive trends were found in September and November,

which could be associated with the increases in SST and

tropospheric cooling in the 1000–500-hPa layer, respec-

tively, as well as changes in the baroclinicity.

Furthermore, the negative trend of frequency char-

acterizes the tracks originating in the western and

northeastern sector and within the northern part of the

examined area (EM1) that is statistically significant at

specific subareas. On the contrary, positive trend char-

acterizes the southwesterly tracks.

There is no statistically significant trend of intensity of

tracks affecting the eastern Mediterranean for any sec-

tor, except for a negative trend in the Middle East for

the west tracks. The size of the tracks tends to decrease

over the whole target area for all of the sectors. Con-

cerning the depth, a negative trend prevails over the

examined area, which is significant for EM1, northwest,

and west tracks but only at specific subregions.

We comment that the relatively coarse-resolution

dataset employed in this study is not fully capable of

representing smaller-scale cyclones in specific regions of

the eastern Mediterranean, such as thermal lows de-

veloping during the warmer months around Cyprus and

the Middle East (Trigo et al. 1999) and secondary cen-

ters within complex systems (Trigo 2006). Therefore, in

our study the frequency of the tracks could be seen as

being underestimated, especially during the period from

June to September. Perhaps a more serious effect of

the coarse resolution is related to an underestimate of

cyclone intensity (Lionello et al. 2002; Trigo 2006).

Nevertheless, the ERA-40 dataset seems to represent

larger-scale cyclonic tracks that cover an extensive area

encompassing the examined region and have consider-

able impact on the temperature and precipitation re-

gime. In this sense, the small-scale features that our

dataset cannot pick up may not be all that important in

the overall picture of cyclonic influence in this region.

The constraints imposed by the low resolution does not

seem crucial for the trend analysis of the track frequency,

because Trigo (2006) demonstrated the agreement be-

tween two different resolution datasets (2.58 3 2.58 and

1.1258 3 1.1258) on the sign and location of trends over the

Mediterranean region, with, however, some discrepancies

regarding their strength and significance.

FIG. 10. Spatial distribution of the trend of depth for (a) EM1

tracks and (b) west tracks. The dots represent the grids where the

trend is statistically significant at a 5 0.05.

1 OCTOBER 2010 F L O C A S E T A L . 5255

The MS algorithm has proven to be a very valuable

tool for examining cyclonic tracks in a smaller-scale in-

land sea with complex topography, such as the Medi-

terranean. Moreover, it has been used to generate an

extended climatology of cyclonic tracks that verifies

results obtained in previous related studies based on

other algorithms, and has revealed valuable additional

insights related to cyclone frequency, kinematics, and

dynamics. This is most likely related to the fact that the

MS scheme accounts for both closed and open systems,

and also performs well in maintaining cyclone temporal

consistency (see, e.g., Pinto et al. 2005; Mesquita et al.

2009).

The study has expanded our knowledge of the structure

and variability of Mediterranean cyclones and, in turn,

contributes significantly to our understanding of physical

processes associated with Mediterranean climate and its

variability. The results postulated by this study highlight

the changes in synoptic activity, which are not in-

consistent with the vulnerability of the Mediterranean

climate to global warming (Solomon et al. 2007).

Acknowledgments. This work was supported by the

project KAPODISTRIAS 2009, which is funded by the

Special Account for Research Grants of the University

of Athens. Parts of the work were made possible by a

grant from the Australian Research Council.

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