Seasonal Aspects of an Objective Climatology of Anticyclones Affectingthe Mediterranean

MARIA HATZAKI AND HELENA A. FLOCAS

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

KEVIN KEAY AND IRINA RUDEVA

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

(Manuscript received 6 March 2014, in final form 21 September 2014)

ABSTRACT

An objective climatology of anticyclones over the greater Mediterranean region is presented based on the

Interim ECMWFRe-Analysis (ERA-Interim) for a 34-yr period (1979–2012) and the Melbourne University

automatic identification and tracking algorithm. The scheme’s robustness and reliability for the transient

extratropical propagation of anticyclones, with the appropriate choices of parameter settings, has been es-

tablished and the results obtained here present new research perspectives on anticyclonic activity affecting the

Mediterranean. Properties of Mediterranean anticyclones, such as frequency, generation and dissipation,

movement, scale, and depth are investigated. The highest frequency of anticyclones is found over continental

areas, while the highest maritime frequency occurs over closed basins exhibiting also maxima of anticyclo-

genesis. There is a significant seasonality in system density and anticyclogenesis maxima, this being associated

with the seasonal variations of the larger-scale atmospheric circulation that affect the greater Mediterranean

region.

1. Introduction

Short-termweather variations in extratropical regions

are largely determined by cyclones and anticyclones.

Along with cyclones, anticyclonic transports contribute

to the maintenance of the general circulation (Wallace

et al. 1988; Trenberth 1991). There have been many

studies of cyclones and these have been prompted by the

direct link of these systems to active weather and rapid

development. By contrast, climatological studies of an-

ticyclones are few, even though anticyclonic activity can

be regarded as equally important as cyclonic activity in

determining surface climate conditions. High pressure

systems are associated with important weather phe-

nomena such as droughts, cold weather spells, blowing

snow events, clear-sky conditions, and heat waves, and

these play important roles in affecting the radiation

budget, hydrological cycle, and seasonal variability

(Leroux 1998). The extratropical surface anticyclonic

activity has been largely overlooked in the literature on

climate change as well (Favre and Gershunov 2006),

Anticyclones are governed by different atmospheric

dynamics according to their scale. Planetary-scale anti-

cyclones, such as the semipermanent subtropical anti-

cyclones, are associated with the descending branch of

the Hadley cell. Intermediate-scale anticyclones, as ex-

emplified by blocking highs, are persistent slow-moving

systems that grow due to the transport of vorticity and

Corresponding author address: Maria Hatzaki, Department of

Environmental Physics–Meteorology, Faculty of Physics, Building

PHYS-5, University of Athens, University Campus, 157 84 Athens,

Greece.

E-mail: marhat@phys.uoa.gr

9272 JOURNAL OF CL IMATE VOLUME 27

DOI: 10.1175/JCLI-D-14-00186.1

� 2014 American Meteorological Society

energy by transient eddies. The extratropical migratory

anticyclones belong to synoptic-scale features induced

by baroclinic instability, while mesoscale anticyclones

can be triggered by uplift of air masses over a mountain

barrier or intense low-level cooling over continental

regions during winter (Bluestein 1992; Ioannidou and

Yau 2008).

Anticyclones can be also classified according to their

vertical thermal structure into cold core, warm core, or

mixed. Cold-core anticyclones are of thermal origin,

developing over continental interiors. They result from

persistent radiative cooling of the land surface and

chilling of the overlying layers, which leads to air density

increase in the lower troposphere and a subsequent

surface pressure rise. In contrast, warm-core anticy-

clones result from convergence in the upper troposphere

and air subsidence beneath (Musk 1988). This produces

warmer than normal temperatures in the middle and

lower troposphere and, because of this, such deep highs

intensify with height (Kurz 1998). Warm-core anticy-

clones develop in the subtropics and midlatitude regions

with the subtropical high pressure cells to be located at

approximately 308N/S latitude. In the midlatitudes,

warm-core anticyclones form beneath the leading side of

ridges in the upper westerlies and may be associated

with blocking action.

Although anticyclones have been studied theoreti-

cally (e.g., Rodwell and Hoskins 2001), it is mainly their

role in blocking that has been extensively investigated

(e.g., Pelly and Hoskins 2003; Barriopedro et al. 2006;

Tyrlis and Hoskins 2008). Climatological studies of

transient anticyclones based on objective methods have

been performed in the Southern Hemisphere (SH) by,

for example, Jones and Simmonds (1994), Sinclair (1994),

and Pezza and Ambrizzi (2003) and in the Northern

Hemisphere (NH) by Ioannidou and Yau (2008). More-

over, some climatologies have been assembled for spe-

cific domains over the NH, but few have explored the

nature of themoving anticyclones affecting the European

and Mediterranean areas (Godev 1971; Makrogiannis

and Giles 1980; Katsoulis et al. 1998).

The Mediterranean is an area of great interest with

respect to synoptic system behavior because of its lo-

cation between subtropics and midlatitudes and also its

complex topography (Meteorological Office 1962). In

addition, the Mediterranean basin is considered to be

particularly vulnerable to climate change (Solomon

et al. 2007; Navarra and Tubiana 2013). The Mediter-

ranean basin and southern Europe experience extensive

cyclonic activity in winter and spring (Maheras et al.

2001; Flocas et al. 2010). It is also influenced by semi-

permanent large-scale anticyclones, the Azores anticy-

clone in the west during summer and the cold Siberian

anticyclone in the northeast during winter. In addition,

the Mediterranean is affected by moving anticyclones

generated over Scandinavia, the Atlantic Ocean, or

North Africa (HMSO 1962; Makrogiannis and Giles

1980) as well as by local anticyclogenesis (Godev 1971).

These moving anticyclones also exert an influence in

daily weather variations, especially if they become

quasi-stationary and create blocking in more northern

latitudes.

In recent times many studies have attempted to depict

the characteristics of the cyclonic tracks in the Medi-

terranean by employing objective methods for cyclone

detection and tracking (e.g., Trigo et al. 1999; Bartholy

et al. 2009; Flocas et al. 2010), while the effort devoted to

the study of anticyclones in the Mediterranean con-

tinues to lag that of cyclones even though, as we have

pointed out above, anticyclones are an important com-

ponent of the Mediterranean climate system. The

tracks of moving anticyclonic systems in the Mediter-

ranean have been studied by Makrogiannis and Giles

(1980) and Katsoulis et al. (1998), both on the basis of

daily surface synoptic charts, while blocking anticy-

clones affecting the Mediterranean have been studied

by Quadrelli et al. (2001) and Trigo et al. (2004).

However, there are no studies that describe the char-

acteristics of anticyclonic systems in theMediterranean

employing objective methods.

Different automated schemes have been developed

for cyclone tracking (e.g., Le Treut and Kalnay 1990;

König et al. 1993; Hodges 1994; Serreze 1995; Sinclair

1997; Hoskins and Hodges 2002; Zolina and Gulev

2002). In our study, for the construction of a compre-

hensive climatology of the anticyclonic systems affecting

the Mediterranean, the Melbourne University auto-

matic tracking algorithm (MS algorithm) is applied

(Murray and Simmonds 1991a), based on the Interim

European Centre forMedium-RangeWeather Forecasts

(ECMWF) Re-Analysis (ERA-Interim) of 6-h mean sea

level pressure (Dee et al. 2011). This algorithm has been

extensively applied for cyclone studies worldwide and it

has been also successfully applied on the Mediterranean

cyclones, although its use for anticyclone tracking has up

to now been limited to the SH (Jones and Simmonds

1994; Sinclair 1994; Pezza et al. 2007).

This study will assemble a new climatology of anticy-

clones affecting the Mediterranean, employing, for the

first time, an objective identification and tracking scheme.

Specifically, the study focuses on 1) the performance of

the MS tracking algorithm for the Mediterranean region,

which poses considerable challenges for an automatic

scheme, and 2) the spatial and temporal distribution of the

frequency and properties of anticyclonic systems, as

density, genesis, movement, dissipation, depth, and scale.

15 DECEMBER 2014 HATZAK I ET AL . 9273

2. Data, tracking algorithm, and methods

Themean sea level pressure (MSLP) global reanalysis

datasets from the ERA-Interim Project have been used

for the period from 1979 to 2012. The forecast model,

data assimilation method, and input datasets for ERA-

Interim are described in Dee et al. (2011), where the

performance of the system is also discussed. For the

present analysis, the spatial resolution of 1.58 3 1.58longitude/latitude is used, as derived from the T255L60

assimilation model output.

The domain encompassed in this study includes a wide

region around the Mediterranean, made up of the Eu-

ropean, northern African, eastern Atlantic, and Middle

East areas (Fig. 1). We chose to do this rather than re-

strict ourselves only to the Mediterranean region, in

order to take into account the large-scale anticyclones

and the major routes of the anticyclonic systems that

participate in the formation of the synoptic conditions

that characterize the Mediterranean. For the part of our

analysis that is based on the specific tracks that are

considered as ‘‘Mediterranean tracks,’’ only the Medi-

terranean region is considered that is bound by the area

extending from 108Wto 408E and between 258N to 508N,

indicated by the black box in Fig. 1.

A separation of the Mediterranean into two parts was

effected in order to discriminate the different processes

involved. The east/west separation was made about the

208E longitude, and thus the western Mediterranean

(WM) extends from 108W to 208E and the eastern

Mediterranean (EM) from 208 to 408E. This division

arises from the fact that the synoptic conditions are

different (e.g., the cold northern invasions in the EM do

not extend to the west). In addition, the Mediterranean

was separated in a northern and a southern part, with the

dividing latitude set at 408N; thus, the northern Medi-

terranean (NM) extends from 408 to 508N and the

southernMediterranean (SM) from 258 to 408N, to allow

expression for the thermodynamic differences between

cold-core (generated mainly in the northern part of the

examined region) and the warm-core anticyclones (ex-

isting mainly in the west and south). At this point, it is

important to note that the anticyclone tracking was

performed for the entire NH in order to retain the

continuity of the tracks at the boundaries.

As mentioned above, the anticyclone identification

and tracking was performed with the algorithm de-

veloped at the Melbourne University (the MS algo-

rithm; see Murray and Simmonds 1991a). One of the

most important advantages of the MS algorithm lies in

its spatially continuous representation of MSLP field as

an analytical function based on the representation of the

pressure field on a polar stereographic (PS) projection.

This is done by bicubic spline fitting, allowing the

identification of both closed (i.e., surrounded by closed

isobars) and open (i.e., associated with extrema of rel-

ative vorticity) synoptic systems, using both pressure

and surface geostrophic relative vorticity fields. More

details about the identification and tracking procedure

can be also found in Simmonds et al. (1999), Pinto et al.

(2005), Lim and Simmonds (2007), and Simmonds and

Keay (2009).

The algorithm has been widely employed for both SH

and NH and has demonstrated its reliability and effi-

ciency in capturing the weather patterns and synoptic

climatology of the transient activity and providing

FIG. 1. Geographical chart of the study area. The Mediterranean region is indicated by the

black box. The vertical line at 208E and the horizontal line at 408N denote the separation

between the western and eastern Mediterranean and the northern and southern Mediterra-

nean, respectively.

9274 JOURNAL OF CL IMATE VOLUME 27

objective climatologies, while it has also proved to be

powerful tool in the analysis of cyclone case studies

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

2009). The scheme was seen to perform well in a recent

intercomparison of state-of-the-art cyclone tracking

schemes (Neu et al. 2013). Specifically, for the Mediter-

ranean, the algorithm was found to be capable of identi-

fying cyclones in a range of locations and with different

characteristics (e.g., Pinto et al. 2005; Flocas et al. 2010;

Kouroutzoglou et al. 2011a). However, Neu et al. (2013)

showed that two methods (M02 and M10; see the paper

for the details), which were based on the MS algorithm

but used different parameter settings, identified some-

what different of cyclone tracks, particularly in the

Mediterranean. This emphasizes that parameter settings

of an algorithmmay have an impact on the climatology of

synoptic features and, therefore, much attention has been

given to the parameters during algorithm’s setup.

Thus, in order to better capture the individual char-

acteristics of anticyclones in a closed basin with complex

topography, such as the Mediterranean, the control

parameters of the MS algorithm were modified. Only

systems tracked at five or more consecutive analysis

time steps (i.e., track duration$ 1 day) were eventually

retained, in order to exclude short-lived systems and to

enable the calculation of time derivatives of the velocity

and pressure tendency (Simmonds and Murray 1999).

From the resulting collection of anticyclonic tracks,

every track spending at least one analysis time in the

Mediterranean region (black box in Fig. 1) was consid-

ered as a Mediterranean track, thus constituting the

total anticyclonic population studied in the present

work. For some of the following analysis, from this total

population (further divided into NM, SM, WM, and

EM tracks), the following subset populations were also

created: a) the tracks that had their genesis in the

Mediterranean region, constituting ;75% of the total

population (further divided into NM, SM,WM, and EM

generated tracks), and b) the tracks that spend their

entire life cycle in the Mediterranean region, constitut-

ing ;46% of the total population.

Finally, the following properties of the anticyclonic

tracks were calculated: (i) the system density [systems

(deg.lat)22], where 1 degree of latitude (hereinafter,

deg.lat) ’ 111 km, presenting the average number of

systems per unit area at any one time (Murray and

Simmonds 1991b); (ii) the anticyclogenesis, that is,

the number of new systems generating [systems

(deg.lat)22 day21]; (iii) the anticyclolysis, that is, the num-

ber of systems disappearing [systems (deg.lat)22 day21];

(iv) the mean central pressure of the systems (hPa);

(v) the Laplacian of the central pressure =2P [hPa

(deg.lat)22], representing an effective measure of synoptic

system intensity (Petterssen 1956); (vi) the anticyclone

radius R (deg.lat), taken as the weighted mean distance

from the anticyclone center to the points at which =2P is

zero around the edge of an anticyclone; and (vii) the

system depth DP (hPa), which combines the size and

intensity of the systems and is also related to the total

kinetic energy of the cyclone (see, e.g., Simmonds and

Keay 2009). (We point out that cyclone system ‘‘depth’’

has been defined as the difference between the pressure

at the edge of a cyclone and the pressure at the central

point. We apply this same concept to anticyclones and

strictly speaking it should be spoken of as the ‘‘height.’’

However, for ease of nomenclature we also refer to depth

in the case of anticyclones.) Also calculated within the

scheme are (viii) the migration velocity (ms21) and (ix)

the zonal or meridional anticyclone propagation (flux)

component [systems (deg.lat)21 day21], defined as the

average number of anticyclones crossing a unit length of

parallel (northward flux) or meridional (eastward flux),

where the unit of length is 1 deg.lat, normal to themotion

per unit time and then weighted by the anticyclone ve-

locity. In this last case, in order to avoid multiple counts

for a given grid cell the components of anticyclone

propagation were taken proportional to their velocity

components and, thus, were calculated by weighting the

frequency counts by the sampled velocity. The statistical

summations are normalized for area and for number of

sampling periods but, owing to the velocity weighting, are

effectively scaled only linearly for distance (Murray and

Simmonds 1991a).

3. Modification and assessment of the trackingalgorithm for use onMediterranean anticyclones

When anticyclonic tracks are determined with an

objective method, one should bear in mind that anticy-

clonic formation and propagation exhibit characteristics

that differ from those of extratropical cyclones. Al-

though cyclones mainly affect a specific area or areas

through their movement during their life cycle, anticy-

clones on the other hand are likely to affect an area

through their ridges. Thus, we were conscious of the

importance of the appropriate modification of the al-

gorithm in order to efficiently capture the individual

characteristics of tracks affecting the Mediterranean.

The anticyclones generated inside the Mediterranean

and the extensions of the major anticyclones located

outside the area are often weak and small-scale systems,

and therefore their detection or otherwise is strongly

influenced by the filtering imposed by some crucial

parameters of the algorithm. For example, in areas

such as North Africa and the greatest part of the cen-

tral Mediterranean, which frequently host anticyclonic

15 DECEMBER 2014 HATZAK I ET AL . 9275

systems, a stricter filtering, as an attempt to minimize the

existence of possible insignificant centers, can also lead

to the loss of other smaller scale anticyclonic centers,

which can be often synoptically significant, since they

allow the assessment of the impact of the large-scale

semipermanent anticyclonic centers in Mediterranean

anticyclonic activity. A possible loss of systems is im-

portant for the North African area, where these highs

are connected with the heat waves in theMediterranean

basin, and also for the EM where the extensions of an-

ticyclones from the north are connected to the Etesian

winds that strongly influence the weather variations in

the EM during summer.

Consequently, our objective was to eliminate in-

significant features while retaining meteorologically

important small-scale systems. Such a balance is not easy

to attain, but we believe that our selected parameter

setting results in an optimum representation of the

synoptic behavior of the Mediterranean anticyclones.

Specifically, in determining the optimum choice of the

parameters, we took into consideration the following

factors:

d The parameterization was assessed on the basis of

the intercomparison with previous climatological

studies on anticyclonic activity of the Mediterranean

(e.g., Makrogiannis and Giles 1980; Prezerakos 1985;

Katsoulis et al. 1998), while being aware that new

insights will be revealed by the present work.d The identified systems and tracks were evaluated

according to the known synoptic behavior of anticy-

clonic systems affecting the Mediterranean region,

specifically the moving anticyclones that follow the

passage of frontal systems over central Europe, the

extensions of permanent and semipermanent systems

either from north or west of the Mediterranean, the

moving anticyclones that cross the Balkans from north

to south/southeast, and the moving systems with

tracks parallel to the NorthAfrican coast. The systems

and tracks were also validated with the aid of synoptic

charts. The comparison between automatic and man-

ual identification showed that for the stronger systems

the two analyses are very similar, while the algorithm

tends to identify a greater number of smaller systems

than the manual analysis, as the weaker systems may

be difficult to perceive on a synoptic chart. A range of

uncertainties associated with manual identification of

synoptic systems has been reported (e.g., Simmonds

et al. 2012), while identification with automatic algo-

rithms is now overwhelmingly being regarded as best

practice (Neu et al. 2013).d The parameters were also based on, and validated

against, the parameterization applied on the SH

anticyclones (Pezza et al. 2007) and the parameteri-

zation used for the Mediterranean cyclones (e.g.,

Pinto et al. 2005; Flocas et al. 2010; Kouroutzoglou

et al. 2011a) and for high-resolution identification

(e.g., Kouroutzoglou et al. 2011b; 2014).

a. The identification procedure

The first step of the identification procedure is the

transformation of MSLP grid fields to a PS grid via

bicubic spline interpolation, which greatly helps in the

localization of the centers (Murray and Simmonds

1991a,b; Haak and Ulbrich 1996; Pinto et al. 2005).

Second, the algorithm searches for maxima/minima (for

a cyclonic/anticyclonic center) of the Laplacian on the

derived field by requiring that the Laplacian at a certain

grid point is greater/less than the surrounding grid

points (defined by the user).When amaximum/minimum

of the Laplacian is found at a specific point, then the

algorithm iteratively (the number of iterations is also

prescribed by the user) searches for a minimum/

maximum of pressure (for closed systems) or pressure

gradient (for open systems) in the vicinity of the

Laplacian extremum in order to associate the latter with

an existing minimum/maximum of pressure or pressure

gradient. Thus, the Laplacian of pressure is used to

screen the pressure field for a possible center and then the

algorithm iteratively searches for a pressure minimum/

maximum. In other words, the algorithm uses both the

pressure and its Laplacian in the identification procedure.

More details can be found in, for example, Murray and

Simmonds (1991a), Simmonds et al. (1999), and Pinto

et al. (2005).

In the final step, the identification procedure involves

a process of system removal on the basis of thresholds.

The chosen values of the most important parameters of

this procedure are discussed below, while a specific ex-

ample revealing the impact of different parameter se-

lections is presented for illustration. In preparing the

scheme for application to the Mediterranean region, the

identification response to a wide range of parameter

settings was extensively tested before settling on a set

that considered close to optimum.

High-resolution analyses tend to identify rather too

many centers, and often this is associated with noise in

the analysis. Pressure diffusive smoothing is effective in

reducing the number of systems and coalescing closely

spaced extrema. It has been found to produce more re-

liable results than simply interpolating from a coarser-

resolution analysis (Simmonds et al. 1999; Lim and

Simmonds 2007). The optimum amount of smoothing

required will depend upon the type of system being

sought and the characteristics and resolution of the

9276 JOURNAL OF CL IMATE VOLUME 27

dataset. Here, the radius of 2.0 deg.lat was chosen, as it

was found that larger diffusive smoothing exclude many

open highs that form in the extensions of the large high-

pressure systems, while a smaller radius leads to the

identification of artificial centers. The elimination of

systems under increased pressure diffusive smoothing is

shown in Figs. 2a and 2b (for 1.5 and 2.0 deg.lat diffusive

smoothing radii, respectively).

In midlatitudes the central pressure tends not to be

a particularly robust measure of system intensity, as it is

greatly influenced by climatological pressure in its vi-

cinity. By contrast, a good measure for the strength or

intensity of a system is the Laplacian of the pressure

(=2P). To remove insignificant systems, their minimum

intensity is examined. The average of the Laplacian is

calculated over a certain radius around each anticyclone

center. The appropriate selection of the averaging ra-

dius ensures that small-scale systems in the initial stage

of their life cycle are kept. Increased radii can remove

insignificant features, although higher values will elim-

inate system that should be retained. Here, the selected

averaging radius is 4.0 degrees of latitude. The effect of

the averaging radius can be seen between Figs. 2b and 2c

(for 4 and 5 deg.lat, respectively).

Most numerical analyses tend to contain many shal-

low and meteorologically weak systems. These can be

eliminated by applying a criterion of minimum strength

(i.e., a minimum value of the Laplacian magnitude).

Higher thresholds can remove systems with small sep-

aration, since the curvature of the pressure field and,

thus, the calculated Laplacian will be weaker in such

areas (Figs. 2b,d). In terms of anticyclone location, an

increased threshold should lead to fewer retained anti-

cyclones provided there are systems with Laplacian

lower than the set threshold. In this study, a threshold of

0.15 hPa (deg.lat)22 for closed and 0.1 hPa (deg.lat)22

for open systems is applied. The effect of these thresh-

olds can be seen comparing Figs. 2b and 2d [with 0.1/0.0

and 0.15/0.1 hPa (deg.lat)22, respectively].

It should be added at this point that the radii for the

pressure diffusive smoothing and the Laplacian aver-

aging militate against retaining small-scale systems,

without, however, eliminating systems with radius

smaller than the chosen values of these parameters; that

is, a small but intense anticyclone can have large enough

Laplacian values in the vicinity of its center that can still

exceed the thresholds while being relatively small.

b. The tracking procedure

During the tracking procedure, the algorithm ‘‘pre-

dicts’’ a subsequent position and a central pressure. The

identified centers at the next analysis time, located in

the vicinity of the predicted position, are examined and

the most likely candidate is chosen.

FIG. 2. TheMSLPcontours and the identified anticyclonic centers at 0600UTC27Sep 2011 for (a) pressure diffusive smoothing 1.5 deg.lat,

Laplacian averaging radius 4 deg.lat, and Laplacian thresholds 0.1/0.0 hPa (deg.lat)22 for closed/open systems; (b) pressure diffusive

smoothing 2.0 deg.lat, Laplacian averaging radius 4 deg.lat, and Laplacian thresholds 0.1/0.0 hPa (deg.lat)22 for closed/open systems;

(c) pressure diffusive smoothing 2 deg.lat, Laplacian averaging radius 5 deg.lat, and Laplacian thresholds 0.1/0.0 hPa (deg.lat)22 for closed/

open systems; and (d) pressure diffusive smoothing 2 deg.lat, Laplacian averaging radius 4 deg.lat, and Laplacian thresholds 0.15/0.1 hPa

(deg.lat)22 for closed/open systems.

15 DECEMBER 2014 HATZAK I ET AL . 9277

The projected displacement of a center in the scheme

may be based on one or more velocity measures: a con-

tinuation of the movement of the center during the

previous analysis interval, the climatological velocity at

the certain latitude and longitude and a scaled steering

velocity. If more than one predictor is used, the pre-

diction velocity is a weighted combination of them.

Here, the prediction velocity is deduced solely from the

previous displacement.

The matching calculation finds the possible combi-

nations of associations such that any center shall occur in

only one association within the combination. The com-

bination which has the greatest probability gives the

matching for the group. Unmatched anticyclones are

deemed to have been born or to have decayed.

A characteristic example revealing the effective per-

formance of the scheme for this study is presented in

Fig. 3. Synoptic MSLP analyses of the ERA-Interim for

the period 6–9 January 2011 reveal the southward in-

trusion of the extension of the Eurasian anticyclone to-

ward the EM and finally over the North African region

(centered on Libya) between 6 and 7 January 2011. Al-

though the synoptic examination of the MSLP analyses

between 7 and 9 January 2011 suggested an extended

almost stationary anticyclonic surface circulation, theMS

algorithm detected a short-lived anticyclonic track within

this extended circulation in the region between the mar-

itime area north of Libya and its northern parts. This

example demonstrates that the use of objective methods

and gridded data for the identification of anticyclonic

tracks allow a more detailed detection compared to the

synoptic charts that suffer from lack of observations, es-

pecially in maritime areas. This structure can be consid-

ered as the result of both (a) the use of gridded data of the

advanced ERA-Interim reanalysis dataset (Dee et al.

2011), especially in areas of sparse surface stations ob-

servations, and (b) our new parameter settings for theMS

algorithm on the basis of including smaller-scale and

short-lived mobile anticyclones inside theMediterranean

area and excluding insignificant systems of small separa-

tion within a larger-scale extension of a major anticy-

clonic system.

4. Average motion and size of anticyclones

An analysis of motion was performed in order to re-

veal the features of the mobility of the systems. The

systems employed in this analysis are theMediterranean

tracks, as described in section 2.

The peak of the frequency distribution of 6-h dis-

placement for the total population occurs at about 200–

250 km (Fig. 4a), in accord with the mean propagation

velocity proposed by Leighton (1994) and Katsoulis

et al. (1998). From the same distribution, it is demon-

strated that the 0.02% of the tracks have mean 6-h

displacement less than 20 km (which corresponds to

about the 10th percentile of the average displace-

ment), indicating that this study is confined to systems

that migrate.

To provide further insight into the behavior of anti-

cyclone tracks, the ratio between the net anticyclone

displacement (i.e., the distance between the first and the

last point of a track) and the length of a track was cal-

culated (Fig. 4b). This ratio will range between 0 and 1,

reflecting the linearity of a track. For the majority of the

tracks the ratio is above 0.5, which means that most of

tracks tend to propagate in space.

We remark that the above results allow us to consider

this anticyclonic population to be made up of moving

systems. Specifically, the majority of the tracks move

progressively mainly toward the east/northeast and the

south. The anticyclonic population includes also the ex-

tensions of the semipermanent large-scale anticyclonic

centers that affect the examined area. These extensions

are considered asmoving systems, since they are captured

as tracks that move progressively or as tracks that retro-

grade toward their initial position before their dissipa-

tion. It can be regarded as an advantage of employing an

automated tracking scheme that this propagation of the

extensions can be identified quite clearly, as compared

to the manual methods. Finally, systems of subsynoptic

scale, mainly over North Africa, have been retained that

seem to move near their initial position, without partic-

ular migration toward a specific direction exhibiting small

linearity. The above tracks have been also considered as

moving systems.

Regarding the scale of the systems, their radius is

examined as a function of their life cycle, separating the

tracks by their total lifespan (Fig. 5). The average radii

and the lifespan indicate that the anticyclonic systems

belong to the synoptic and subsynoptic scale. It can be

seen for the total population of anticyclones affecting

the Mediterranean (Fig. 5a) that independently of their

lifespan, the initial radius is greater than the radius at the

time of dissipation, but the longer-lived anticyclones

have greater maximum radius, reaching 8 deg.lat for

anticyclones lasting up to 13 days. Moreover, the nega-

tive skewness of the curves indicates that the rate at

which the radius decreases after its maximum exceeds

the rate of increase during the growth phase. It is worth

noting that both NH and SH cyclones exhibit a very

similar behavior (Simmonds 2000; Rudeva and Gulev

2007).

The anticyclones that have one time step in the NM

and the SM are further examined (Figs. 5b and 5c, re-

spectively). In the NM, the systems exhibit significantly

9278 JOURNAL OF CL IMATE VOLUME 27

greater lifespan and radius than in the SM, indicating

that the NM tracks are more persistent due to the

stronger low-level subsidence over the colder conti-

nental areas, mainly during the cold season (Sutcliffe

1960; Trigo et al. 2002; Lolis et al. 2004). In addition, it

was found that the systems that spend their entire life-

time in the Mediterranean (not shown) are short-lived

and of smaller size compared to the total population,

when considering the radii for anticyclones of the same

lifespan.

Moreover, apart from the dependency of the radius on

the lifetime, an analogously strong relationship was

found between the depth and the lifetime (not shown),

while longer-lived systems exhibit depths greater than

7 hPa. The systems that spend their entire lifetime in the

Mediterranean are notably weaker.

FIG. 3. MSLP maps from 1800 UTC 7 Jan 2011 to 0000 UTC 10

Jan 2011 and the corresponding anticyclonic track. The black dot

on the MSLP maps denotes the position of the anticyclonic center

for the respective time step. The blue dot and the red dot on the

track map denote the genesis and the lysis of the anticyclone,

respectively.

15 DECEMBER 2014 HATZAK I ET AL . 9279

5. Intra-annual variations

To gain an overall impression of the temporal and

spatial variability of the anticyclonic tracks the mean

seasonal variations of the number of anticyclonic tracks,

as well as the spatial distribution of the anticyclonic

properties are examined. The monthly variations in the

number of anticyclonic tracks with at least one synoptic

time and of those generated in the Mediterranean region

and its subregions indicate that themajority of anticyclones

affecting the Mediterranean have been generated within

examined area (Fig. 6). It becomes evident that the anti-

cyclonic tracks affect the Mediterranean throughout the

year, though there are seasonal variations in the number of

anticyclones with higher frequencies in December and

January, and fromMarch to June and lower frequencies

from August to October. This variation of anticyclonic

tracks presents an updated picture from that shown by

Makrogiannis and Giles (1980) and Katsoulis et al.

(1998) (since they found increased frequency in August

and September, especially along the northern Mediter-

ranean coast), while these frequencies are in line with

the monthly anticyclonic activity presented by Godev

(1971) even for a smaller region.

Another interesting feature is the high frequency of

anticyclones generated in theMediterranean as compared

to the frequency of the total population of Mediterranean

tracks (;150 and ;200 tracks per year, respectively).

Figure 6b shows monthly ratios between the number of

tracks generated and those that hadone analysis step in the

Mediterranean and its subregions, as well. For the entire

region, it can be seen that the ratio is high throughout the

year,more than 0.7, reaching peak values fromMay to July

and having a minimum in October. These ratios are

much lower for the northern and the eastern parts of the

Mediterranean, which suggests that a great percentage

of the tracks penetrate the northern Mediterranean

from the west, while more than half of systems are

passing through the eastern Mediterranean rather than

generating in it.

Specifically, the total number of anticyclonic tracks

that have at least one step in the WM is slightly higher

than the corresponding number in the EM (;135 and

;115 tracks per year, respectively). The generating

systems are twice as numerous in the WM: ;100 tracks

FIG. 4. Frequency distribution of (a) the mean 6-h displacement

(km) and (b) the ratio between the net displacement and the track

length, for the anticyclonic tracks with at least one step in the

Mediterranean.

FIG. 5. Mean evolution of anticyclone radius for systems binned

according to their lifetime (in days) for (a) the total anticyclone

population of the Mediterranean, (b) the anticyclone population

with at least one step in the NM, and (c) the anticyclone population

with at least one step in the SM.

9280 JOURNAL OF CL IMATE VOLUME 27

per year generate in theWMversus;50 tracks in the EM,

which agrees well with the findings of Godev (1971) and

Katsoulis et al. (1998). Comparing the NW (;125 passing/

;75 generating per year) and the SM (;105 passing/;75

generating), it can be seen that there are more systems

having at least one point in the NM compared to the SM,

while the number of systems generated in both regions is

the same. These findings are further confirmed on sea-

sonal basis in the following analysis, while the reduced

frequency of anticyclones and anticyclogenesis during

autumn will be further discussed.

The next step in the analysis is the examination of the

anticyclonic properties on a seasonal basis. First, the

routes of the tracks of the anticyclones from their ap-

pearance till their dissipation are presented in Fig. 7. In

winter, the tracks begin preferentially in three regions:

a) from the northeastern Atlantic or western Europe

following an eastward/northeastward route toward

eastern Europe, b) over the Balkans following a south-

eastward route toward the EM, or c) over central North

Africa following an eastward route toward the EM. In

summer, the number of maritime tracks increases over

the southern Mediterranean, while there is a reduction

along the northern Mediterranean coast and Europe. In

addition, there is a high track density over the Black Sea,

with a pronounced eastward path toward the Caspian

Sea. Spring and autumn present a transitional phase

between cold and warm periods. During spring (Fig. 7b),

the anticyclonic activity is spread over wider areas,

mainly over Africa and the eastern Mediterranean

maritime regions, while during autumn (Fig. 7d), the

anticyclonic activity over the southern parts is largely

reduced.

The abovementioned migration patterns of the anti-

cyclones can be appreciated in terms of the spatial var-

iations of the anticyclonic occurrence described by the

seasonal system density (Fig. 8). In winter (Fig. 8a), the

anticyclonic activity is confined in the continental re-

gion along the Mediterranean coast, mainly along the

northern part, as the warmer sea during this season

prevents the generation and the persistence of anticy-

clonic systems (Katsoulis et al. 1998). This is also in ac-

cord to the results of Flocas et al. (2001), where an

extended low-level positive geostrophic vorticity maxi-

mum covers the greatest part of the Mediterranean Sea

during the cold period of the year, favoring low-level

convergence (Kurz 1998) and upward motion over the

maritime areas. The system density peaks over the

Iberian Peninsula and the northern African coast, fol-

lowing the extensions of the subtropical ridge during this

period of the year (Katsoulis et al. 1998). A remarkable

winter maximum is found over the Balkans. This is likely

due to the extensions of winter cold persistent anticy-

clones originating from central Europe and Siberia that

can also cause anticyclogenesis (Makrogiannis 1976) (as

will be seen in Fig. 12a). The major path of the cyclones

generating in the main cyclogenetic regions of the north-

ern strong baroclinic regions of the western and central

Mediterranean (Trigo et al. 1999) follow a route toward

the Balkans and eastern Europe (Kouroutzoglou et al.

2011a). This synoptic behavior favors the low-level cold

advections and the generation of cold-core anticyclones in

the northernBalkans, affecting theEMandmainlyGreece

and the Aegean Sea with strong north/northeast winds

(Metaxas and Bartzokas 1994).

From spring to summer, the number of anticyclones

tends to increase over the sea. Specifically in spring

(Fig. 8b), the density maximizes along the northern

African coast, following the intensification of theAzores

anticyclonic center (Flocas et al. 2001). The eastward

shift of this peak along the northernAfrican coast results

from the generation of the northernAfrican depressions

(Hannachi et al. 2011; Ammar et al. 2014) and the

FIG. 6. (a) Intermonthly frequency distribution of the systems

having at least one step in the examined area and the systems

generating in the examined area. (b) Ratios of the systems gener-

ated in and having at least one step in the Mediterranean and the

western, eastern, northern, and southern subregions of the exam-

ined area, respectively.

15 DECEMBER 2014 HATZAK I ET AL . 9281

subsequent increase of the cyclonic tracks toward the

Mediterranean (Flocas et al. 2010; 2013). The notable

peak over the Black Sea during spring reflects the gradual

establishment of the Pakistan low over the Middle East

(Bitan and Saaroni 1992) that causes spatial confinement

of the northerly and westerly anticyclonic tracks.

In summer (Fig. 8c), the Black Sea peak strengthens,

following the further intensification of the Pakistan low

(Bitan and Saaroni 1992). The North African peak be-

comes more localized over Libya and a new peak ap-

pears over northern Spain, being associated with the

predominance of Azores anticyclone and the strength-

ening of the warm-core anticyclones at southern lati-

tudes (Katsoulis et al. 1998) that can be probably

attributed to the thermal ridge identified over the North

African area extending to the coastal areas of southern

France (Prezerakos 1978).

The increased anticyclonic frequency over central

Europe and the northern Balkans could be explained by

the cyclonic activity that remains during summer af-

fecting the European region from the northwest to the

east (Trigo et al. 1999; Campins et al. 2010). Even in the

absence of cyclonic activity in summer, the thermal in-

stability in the continental parts of central Europe and

the Balkans can lead to convective activity during the

daytime, while the respective low-level cooling can en-

hance the generation and building of surface anticy-

clones which affect the EM basin (Makrogiannis 1976;

Katsoulis et al. 1998).

In autumn, the reduction in system density over the

entire area cannot be related solely to the increase of

cyclonic tracks over the entire Mediterranean (Flocas

et al. 2010) but occurs in conjunction with other factors

(Fig. 8d). The decrease of anticyclonic activity during

this season can be attributed to a southward displace-

ment of the subtropical jet over the Mediterranean at

around 308N at 250–200 hPa, resulting in the subsequent

reduction of 1000–500-hPa thickness and 500-hPa geo-

potential heights (Prezerakos 1978), discouraging anti-

cyclogenesis over North Africa. Furthermore, a gradual

transition of the atmospheric circulation over Europe

from high to low index [i.e., the transition of the atmo-

spheric circulation from predominantly zonal (high) to

meridional (low) flow, representing the displacement of

the anticyclones] enhances themeridional component of

the atmospheric flow and allows the southward dis-

placement of the cyclonic activity (Makrogiannis et al.

1981) and a weakening of the upper-tropospheric

FIG. 7. Anticyclonic tracks (increment by 10) for (a) winter, (b) spring, (c) summer, and (d) autumn.

9282 JOURNAL OF CL IMATE VOLUME 27

blocking overAtlantic and western Europe (Wiedenmann

et al. 2002). Moreover, the strong semipermanent anti-

cyclones of northeast Europe and the Atlantic that exhibit

frequent southward extensions with accompanying

cold invasions during winter, extending from western Eu-

rope to Siberia, are not yet strong in autumn, due to the

warmer low-level environment (Musk 1988), while the

MediterraneanSea cools less rapidly than the adjacent land

and this does not favor anticyclogenesis (see Fig. 12d), even

behind cold fronts (Palmén andNewton1969; Flocas 1988).The areas of maximumdensity of moving anticyclones

also constitute centers of anticyclogenesis with similar

seasonal characteristics (Fig. 9), in accordance with

Godev (1971). During winter (Fig. 9a), the anticyclones

FIG. 8. Anticyclone system density for (a) winter, (b) spring, (c) summer, and (d) autumn. The contour interval is 0.53 1023 anticyclones

(deg.lat)22.

FIG. 9. Anticyclogenesis for (a) winter, (b) spring, (c) summer, and (d) autumn. The contour interval is 0.2 3 1023 anticyclones

(deg.lat)22 day21.

15 DECEMBER 2014 HATZAK I ET AL . 9283

preferentially have their genesis over land, mainly in the

WM. The northern Mediterranean acts also as anti-

cyclogenetic area, since the frequent passage of cold

fronts and frontal depressions in winter (Flocas 1984)

favors the generation of anticyclones behind them

where the pressure rises. This could be also attributed to

the effect of themajor topographic barriers of theAlps and

the Pyrenees; when a cold front impinges on these areas

with a movement perpendicular to the mountains, the

conservation of potential vorticity dictates a pressure rise

and tendency of anticyclogenesis on the windward side

(Buzzi andTibaldi 1978;Holton 2004). In spring (Fig. 9b),

the number of generating systems increases over north-

western Africa, while the wide maximum of winter over

the northwestern Mediterranean is disrupted. During

summer (Fig. 9c), the number of generated anticyclones

increases. It is noteworthy that summer anticyclogenesis

also occurs over the sea, mainly over theWM, theGulf of

Syrte, and the Black Sea. The decrease over the conti-

nental areas of northern Africa and the greatest part of

the Iberian Peninsula during summer can be attributed to

the convection due to the surface heating favoring the

generation of heat lows over these areas rather than an-

ticyclones (Trigo et al. 1999). In autumn, there is an

overall reduction in the number of the generated systems

(Fig. 9d), in line with the reduction in density that was

discussed previously.

The difference between the average anticyclogenesis

and anticyclolysis gives a picture of the regions that are

net generators or net dissipaters of systems. In all sea-

sons, the northern parts of the Mediterranean and the

western Africa act as a source, while the southeastern

Mediterranean acts as a sink. Specifically, in winter

(Fig. 10a) the western North African region acts as a net

anticyclonic source, while the areas of Egypt andGreece

act as sinks. In summer, the main source is found over

the Atlantic near the northern Iberian coast, consistent

with the respective maximum of anticyclogenesis (see

Fig. 10c), while the WM source is shifted over the

Alboran Sea following the maritime movement of anti-

cyclogenesis. Similarly, the EM sink is now found mainly

over the sea.

The above findings are further supported by the dis-

tribution of the vector propagation fluxes (Fig. 11). Fo-

cusing on the zonal component, it can be seen that the

surface anticyclones tend to move eastward in all sea-

sons, in accordance to the mean behavior of the derived

tracks (see Fig. 7). In winter, the meridional component

prevails over most of the Mediterranean basin and

northwestern Africa (Fig. 11a). The anticyclonic flux is

characterized by a substantial southward component

over the Balkans, while a northward component is

established over western Europe and the Atlantic.

In spring and summer, the anticyclones tend to move

northeastward over northern Europe and southeast-

ward over Libya and the Gulf of Syrte (Figs. 11b,c).

The strengthening of meridional over zonal propaga-

tion over the Azores area during summer and autumn

(Figs. 11c,d) reflects the northward extension of the

Azores anticyclone toward western and central Europe

(Makrogiannis 1976).

The migration velocity completes the picture of the

evolution and movement of the anticyclones. During

winter, it increases from the north to the south, with the

systems moving in a southeastward direction with high

values over the sea and maximum over North Africa

(Fig. 12a), while this maximum disappears during sum-

mer (Fig. 12c). The increased westward migration ve-

locities over theNorthAtlantic can be possibly attributed

to respective stronger eastward frontal cyclone move-

ment in the above areas toward the European region and

theMediterranean region (Kouroutzoglou et al. 2012). In

spring, summer, and autumn the maximum velocities are

found over central Europe with average values reaching

10ms21 (Figs. 12b–d). In general, the anticyclones ex-

hibit greater velocities inwinter than summer (Figs. 12a,c),

which could be attributed to the fact that during winter

the development of minor anticyclonic systems be-

tween frontal depressions increase due to the relevant

increase of the cyclonic activity (Musk 1988).

In addition the evolution and migration properties of

the anticyclones, the scale and strength properties add

considerably to the characterization of these features.

FIG. 10. Anticyclone genesis minus lysis for (a) winter and

(b) summer. The contour interval is 0.1 3 1023 anticyclones

(deg.lat)22 day21.

9284 JOURNAL OF CL IMATE VOLUME 27

The spatial distribution of the mean central pressure of

the anticyclones (Fig. 13) exhibits a substantial degree

of seasonality. Consistent with the above discussion,

the central pressure is higher during winter (Fig. 13a).

The minimum central pressure is found in summer over

the southeastern edge of the examined area (Fig. 13b).

The spatial distribution of the mean anticyclone ra-

dius (Fig. 14) shows that for both cold andwarm seasons,

the radius in the Atlantic area from the Azores up to the

British Isles is large, while two maxima are established

over northeastern Europe and the eastern parts of

North Africa that exhibit lower values during summer

(Fig. 14b). Moreover, the anticyclones assume their

smallest size over theMediterranean Sea itself. For most

of the year the mean radii lie between 4.5 and 5 deg.lat,

but in summer fall to less than 4.5 deg.lat.

As indicated earlier, the depth reflects both the size

and intensity of the systems. In line with the radius and

the central pressure, themaximum of depth also appears

over the Atlantic (Fig. 15). Strong gradients of depth are

FIG. 11. Vector propagation fluxes for (a) winter, (b) spring, (c) summer, and (d) autumn. The contour interval is 2 3 1023 anticyclones

(deg.lat)21 day21.

FIG. 12. Migration velocity for (a) winter, (b) spring, (c) summer, and (d) autumn. The contour interval is 1m s21.

15 DECEMBER 2014 HATZAK I ET AL . 9285

observed along the northern Mediterranean coast dur-

ing winter (Fig. 15a), spring, and autumn, highlighting

the temperature differences between the cooler north-

ern continental areas and warmer the sea and the Afri-

can coast. During summer (Fig. 15b), the maxima of

depth decrease, while the gradients weaken due to the

warming as well of the northern continental areas and

due to enhanced local baroclinicity within the strong

baroclinic regions of northern Mediterranean maritime

and coastal areas (Trigo et al. 1999).

6. Concluding remarks

For the first time an objective and comprehensive cli-

matology of many aspects of the behavior of anticyclones

affecting the Mediterranean region has been assembled

for the period 1979–2012. The constructed climatology

presents an updated and more detailed picture from that

shown in the previous climatologies of Mediterranean

anticyclones (Godev 1971; Makrogiannis and Giles 1980;

Katsoulis et al. 1998).

The climatology was constructed using the ERA-

Interim reanalysis and the Melbourne University auto-

matic identification and tracking algorithm. The scheme

has been tuned to appropriately capture Mediterranean

anticyclone behavior, and the results, in the main, are

consistent with (manually based) climatologies con-

structed heretofore. It was shown that the number of the

detected anticyclonic centers and the resulting tracks

has some dependence on the search radius and the

curvature strength criteria applied. It was also found

that different scale anticyclones and secondary centers

that lie within larger anticyclone structures can be well

represented, which is important, since the extensions of

major anticyclonic systems affect the Mediterranean

basin throughout the year.

The frequency analysis of the anticyclonic tracks

revealed that most of the systems are generated in the

examined area. This is particularly true for the WM,

while the EM acts mainly as anticyclonic sink. From the

mean 6-h displacement of the anticyclonic systems we

confirmed that the studied population consists of the

migratory systems affecting the Mediterranean. Addi-

tionally, a dependency of the size and depth on the

lifetime of the anticyclones is found, with longer-lived

systems having greater radii and depths.

FIG. 13. Central pressure for (a) winter and (b) summer. The

contour interval is 2 hPa.FIG. 14. Anticyclone radius for (a) winter and (b) summer. The

contour interval is 0.5 deg.lat.

FIG. 15. Anticyclone depth for (a) winter and (b) summer. The

contour interval is 0.5 hPa.

9286 JOURNAL OF CL IMATE VOLUME 27

In general, two major anticyclonic routes were found

parallel either to the northern (from the Iberian toward

the Balkan Peninsula) or to the southern (North Africa

coast) Mediterranean boundaries. The frequency in-

creases over the northern route in winter and spring and

over the southern route in summer and autumn. During

summer, the tracks are shifted to the north and also

follow maritime paths. The anticyclones exhibit greater

transitional velocities in winter as compared to summer

and it can be derived that themovement of the systems is

determined by their thermal character.

The above compilation provides a comprehensive

picture of the climatology of the Mediterranean anti-

cyclones and lead to deductions regarding the processes

that govern the behavior of the anticyclonic systems,

although this picture should be verified by the exami-

nation of the vertical thermal profiles of the systems to

obtain information on their structural features, such as

their baroclinic character and their cold- or warm-core

structure. Furthermore, the processes that lead to the

energy feeding of the systems during anticyclogenesis and

system evolvement will help to understand the dynamic

mechanisms responsible for the development and move-

ment of anticyclones over the Mediterranean region.

Acknowledgments. This research project is imple-

mented within the framework of the Action ‘‘Supporting

Postdoctoral Researchers’’ of the Operational Program

‘‘Education andLifelong Learning’’ (Action’s Beneficiary:

General Secretariat for Research and Technology), and is

co-financed by the European Social Fund (ESF) and the

Greek State. Parts of the analysis were made possible by

a grant from the Australian Research Council.

REFERENCES

Ammar, K., M. El-Metwally, M. Almazroui, and M. M. A. Wahab,

2014: A climatological analysis of Saharan cyclones. Climate

Dyn., 43, 483–501, doi:10.1007/s00382-013-2025-0.

Barriopedro, D., R. García-Herrera, A. R. Lupo, and E. Hernández,2006: A climatology of Northern Hemisphere blocking. J. Cli-

mate, 19, 1042–1063, doi:10.1175/JCLI3678.1.

Bartholy, J., R. Pongracz, and M. Pattantyus-Abraham, 2009:

Analyzing the genesis, intensity, and tracks of western Medi-

terranean cyclones. Theor. Appl. Climatol., 96, 133–144,

doi:10.1007/s00704-008-0082-9.

Bitan, A., and H. Saaroni, 1992: The horizontal and vertical ex-

tension of the Persian Gulf pressure trough. Int. J. Climatol.,

12, 733–747, doi:10.1002/joc.3370120706.

Bluestein, H. B., 1992: Principles of Kinematics and Dynamics.

Vol. 1, Synoptic–DynamicMeteorology inMidlatitudes,Oxford

University Press, 431 pp.

Buzzi, A., and S. Tibaldi, 1978: Cyclogenesis in the lee of the Alps:

A case study. Quart. J. Roy. Meteor. Soc., 104, 271–287,

doi:10.1002/qj.49710444004.

Campins, J., A. Genovés, M. A. Picornell, and A. Jansà, 2010:Climatology of Mediterranean cyclones using the ERA-40

dataset. Int. J. Climatol., 31, 1596–1614, doi:10.1002/

joc.2183.

Dee, D. P., and Coauthors, 2011: The ERA-Interim reanalysis:

Configuration and performance of the data assimilation sys-

tem. Quart. J. Roy. Meteor. Soc., 137, 553–597, doi:10.1002/

qj.828.

Favre, A., and A. Gershunov, 2006: Extra-tropical cyclonic/

anticyclonic activity in north-eastern Pacific and air tem-

perature extremes in western North America. Climate

Dyn., 26, 617–629, doi:10.1007/s00382-005-0101-9.

Flocas, A. A., 1984: The annual and seasonal distribution of fronts

over central southern Europe and the Mediterranean. J. Cli-

matol., 4, 255–267, doi:10.1002/joc.3370040304.

——, 1988: Frontal depressions over the Mediterranean Sea and

central southern Europe.Méditerranée, 66, 43–52, doi:10.3406/medit.1988.2580.

Flocas, H. A., P. Maheras, T. S. Karacostas, I. Patrikas, and

C. Anagnostopoulou, 2001: A 40-year climatological study

of relative vorticity distribution over the Mediterranean. Int.

J. Climatol., 21, 1759–1778, doi:10.1002/joc.705.

——, I. Simmonds, J. Kouroutzoglou, K. Keay, M. Hatzaki, D. N.

Asimakopoulos, and V. Bricolas, 2010: On cyclonic tracks

over the eastern Mediterranean. J. Climate, 23, 5243–5257,

doi:10.1175/2010JCLI3426.1.

——, P. Kountouris, J. Kouroutzoglou, M. Hatzaki, K. Keay, and

I. Simmonds, 2013: Vertical characteristics of cyclonic tracks

over the eastern Mediterranean during the cold period of the

year. Theor. Appl. Climatol., 112, 375–388, doi:10.1007/

s00704-012-0737-4.

Godev, N., 1971: Anticyclonic activity over south Europe and its

relationship to orography. J. Appl. Meteor., 10, 1097–1102,

doi:10.1175/1520-0450(1971)010,1097:AAOSEA.2.0.CO;2.

Haak, U., and U. Ulbrich, 1996: Verification of an objective cy-

clone climatology for the North Atlantic.Meteor. Z., 5, 24–30.Hannachi, A., A. Awad, and K. Ammar, 2011: Climatology and

classification of spring Saharan cyclone tracks. Climate Dyn.,

37, 473–491, doi:10.1007/s00382-010-0941-9.Hodges, K. I., 1994: A general method for tracking analysis

and its application to meteorological data.Mon. Wea. Rev.,

122, 2573–2586, doi:10.1175/1520-0493(1994)122,2573:

AGMFTA.2.0.CO;2.

Holton, J. R., 2004: An Introduction to Dynamic Meteorology. 4th

ed. Academic Press, 535 pp.

Hoskins,B. J., andK. I.Hodges, 2002:Newperspectiveson theNorthern

Hemisphere winter storm tracks. J. Atmos. Sci., 59, 1041–1061,

doi:10.1175/1520-0469(2002)059,1041:NPOTNH.2.0.CO;2.

Ioannidou, L., andM. K. Yau, 2008: A climatology of the Northern

Hemisphere winter anticyclones. J. Geophys. Res., 113,

D08119, doi:10.1029/2007JD008409.

Jones, D. A., and I. Simmonds, 1994: A climatology of Southern

Hemisphere anticyclones.ClimateDyn., 10, 333–348, doi:10.1007/

BF00228031.

Katsoulis, B. D., T. D. Makrogiannis, and Y. A. Goutsidou, 1998:

Monthly anticyclonicity in southern Europe and the Medi-

terranean region. Theor. Appl. Climatol., 59, 51–59,

doi:10.1007/s007040050012.

König, W. R., R. Sausen, and F. Sielmann, 1993: Objective iden-

tification of cyclones in GCM simulations. J. Climate, 6, 2217–

2231, doi:10.1175/1520-0442(1993)006,2217:OIOCIG.2.0.CO;2.

Kouroutzoglou, J., H. A. Flocas, K. Keay, I. Simmonds, and

M. Hatzaki, 2011a: Climatological aspects of explosive cy-

clones in the Mediterranean. Int. J. Climatol., 31, 1785–1802,

doi:10.1002/joc.2203.

15 DECEMBER 2014 HATZAK I ET AL . 9287

——, ——, I. Simmonds, K. Keay, and M. Hatzaki, 2011b: As-

sessing characteristics of Mediterranean explosive cyclones

for different data resolution. Theor. Appl. Climatol., 105, 263–

275, doi:10.1007/s00704-010-0390-8.

——, ——, K. Keay, I. Simmonds, and M. Hatzaki, 2012: On the

vertical structure of Mediterranean explosive cyclones. Theor.

Appl. Climatol., 110, 155–176, doi:10.1007/s00704-012-0620-3.——, ——, M. Hatzaki, K. Keay, and I. Simmonds, 2014: A high-

resolution climatological study on the comparison between sur-

face explosive and ordinary cyclones in the Mediterranean. Reg.

Environ. Change, 14, 1833–1846, doi:10.1007/s10113-013-0461-3.

Kurz, M., 1998: Synoptic Meteorology. 2nd ed. Deutscher Wetter-

dienst, 200 pp.

Leighton, R. M., 1994: Monthly anticyclonicity and cyclonicity in

the Southern Hemisphere averaged for January, April, July,

and October. Int. J. Climatol., 14, 33–45, doi:10.1002/

joc.3370140103.

Leonard, S. R., J. Turner, and A. van Der Wal, 1999: An assess-

ment of three automatic depression tracking schemes.Meteor.

Appl., 6, 173–183, doi:10.1017/S135048279900119X.

Leroux, M., 1998: Dynamic Analysis of Weather and Climate.

Wiley-Praxis, 365 pp.

Le Treut, H., and E. Kalnay, 1990: Comparison of observed and

simulated cyclone frequency distribution as determined by an

objective method. Atmósfera, 3, 57–71.Lim, E.-P., and I. Simmonds, 2007: Southern Hemisphere winter

extratropical cyclone characteristics and vertical organization

observed with the ERA-40 reanalysis data in 1979–2001.

J. Climate, 20, 2675–2690, doi:10.1175/JCLI4135.1.

Lolis, C. J., A. Bartzokas, and B. D. Katsoulis, 2004: Relation be-

tween sensible and latent heat fluxes in theMediterranean and

precipitation in the Greek area during winter. Int. J. Climatol.,

24, 1803–1816, doi:10.1002/joc.1112.

Maheras, P., H. A. Flocas, I. Patrikas, and C. Anagnostopoulou,

2001: A 40 year objective climatology of surface cyclones in

the Mediterranean region: Spatial and temporal distribution.

Int. J. Climatol., 21, 109–130, doi:10.1002/joc.599.Makrogiannis, T. J., 1976: Tracks of the anticyclonic systems over

Greece (inGreek). Ph.D. thesis,University ofThessaloniki, 80 pp.

——, and B. D. Giles, 1980: Frequencies, individual, and mean

tracks of moving anticyclones over south-east Europe.

J. Meteor., 5, 240–248.

——,——, andA.A. Flocas, 1981: The problem of the extension of

the Siberian anticyclone towards southeast Europe and its

relation to atmospheric circulation anomalies over the

Northern Hemisphere. Arch. Meteor., Geophys. Bioklimatol.,

30A, 185–196, doi:10.1007/BF02257842.Mesquita, M. D. S., D. E. Atkinson, I. Simmonds, K. Keay, and

J. Gottschalck, 2009: New perspectives on the synoptic de-

velopment of the severe October 1992 Nome storm.Geophys.

Res. Lett., 36, L13808, doi:10.1029/2009GL038824.

Metaxas,D.A., andA.Bartzokas, 1994: Pressure covariability over

the Atlantic, Europe and N. Africa. Application: Centers of

action for temperature, winter precipitation and summer

winds in Athens, Greece. Theor. Appl. Climatol., 49, 9–18,

doi:10.1007/BF00866284.

Meteorological Office, 1962:General Meteorology. Vol. I,Weather

in the Mediterranean, 2nd ed. Her Majesty’s Stationery Office,

362 pp.

Murray, R. J., and I. Simmonds, 1991a: A numerical scheme for

tracking cyclone centres from digital data. Part I: De-

velopment and operation of the scheme. Aust. Meteor. Mag.,

39, 155–166.

——, and ——, 1991b: A numerical scheme for tracking cyclone

centres fromdigital data. Part II:Application to January and July

circulation model simulations. Aust. Meteor. Mag., 39, 167–180.

Musk,L.F., 1988:WeatherSystems. CambridgeUniversityPress, 160pp.

Navarra, A., and C. Tubiana, Eds., 2013:Air, Sea, and Precipitation

and Water. Vol. 1, Regional Assessment of Climate Change in

the Mediterranean, Springer, 338 pp.

Neu, U., and Coauthors, 2013: IMILAST: A community effort to

intercompare extratropical cyclone detection and tracking

algorithms.Bull. Amer.Meteor. Soc., 94, 529–547, doi:10.1175/

BAMS-D-11-00154.1.

Palmén, E., and C. W. Newton, 1969: Atmospheric Circulation

Systems: Their Structure and Physical Interpretation. Aca-

demic Press, 606 pp.

Pelly, J. L., and B. J. Hoskins, 2003: A new perspective on blocking.

J.Atmos. Sci., 60, 743–755, doi:10.1175/1520-0469(2003)060,0743:

ANPOB.2.0.CO;2.

Petterssen, S., 1956: Weather Analysis and Forecasting. 2nd ed.

Vol. 1, McGraw Hill, 269 pp.

Pezza, A. B., and T. Ambrizzi, 2003: Variability of Southern

Hemisphere cyclone and anticyclone behavior: Further

analysis. J. Climate, 16, 1075–1083, doi:10.1175/

1520-0442(2003)016,1075:VOSHCA.2.0.CO;2.

——, I. Simmonds, and J. A. Renwick, 2007: SouthernHemisphere

cyclones and anticyclones: Recent trends and links with de-

cadal variability in the Pacific Ocean. Int. J. Climatol., 27,

1403–1419, doi:10.1002/joc.1477.

Pinto, J. G., T. Spangehl, U. Ulbrich, and P. Speth, 2005: Sensi-

tivities of a cyclone detection and tracking algorithm: In-

dividual tracks and climatology. Meteor. Z., 14, 823–838,

doi:10.1127/0941-2948/2005/0068.

Prezerakos, N. G., 1978: Contribution to the study of blocking over

Greek area (inGreek). Ph.D. thesis, University ofThessaloniki,

191 pp.

——, 1985: Some aspects of the existence of the so-called extension

of the Siberian anticyclone towards the Balkans and Greece.

Meteor. Z., 35, 373–378.

Quadrelli, R., V. Pavan, and F. Molteni, 2001: Wintertime vari-

ability of Mediterranean precipitation and its links with large-

scale circulation anomalies. Climate Dyn., 17, 457–466,

doi:10.1007/s003820000121.

Rodwell, M. J., and B. J. Hoskins, 2001: Subtropical anticyclones

and summermonsoons. J. Climate, 14, 3192–3211, doi:10.1175/

1520-0442(2001)014,3192:SAASM.2.0.CO;2.

Rudeva, I., and S. K. Gulev, 2007: Climatology of cyclone size

characteristics and their changes during the cyclone life cycle.

Mon. Wea. Rev., 135, 2568–2587, doi:10.1175/MWR3420.1.

Serreze, M. C., 1995: Climatological aspects of cyclone de-

velopment and decay in the Arctic. Atmos.–Ocean, 33, 1–23,

doi:10.1080/07055900.1995.9649522.

Simmonds, I., 2000: Size changes over the life of sea level cyclones

in the NCEP reanalysis. Mon. Wea. Rev., 128, 4118–4125,

doi:10.1175/1520-0493(2000)129,4118:SCOTLO.2.0.CO;2.

——, and R. J. Murray, 1999: Southern extratropical cyclone be-

havior in ECMWF analyses during the FROST special ob-

serving periods. Wea. Forecasting, 14, 878–891, doi:10.1175/

1520-0434(1999)014,0878:SECBIE.2.0.CO;2.

——, and K. Keay, 2009: Extraordinary September Arctic sea ice re-

ductions and their relationships with storm behavior over 1979–

2008.Geophys.Res. Lett., 36,L19715, doi:10.1029/2009GL039810.

——, R. J. Murray, and R. M. Leighton, 1999: A refinement of

cyclone tracking methods with data from FROST. Aust. Me-

teor. Mag., 1999 (special ed.), 35–49.

9288 JOURNAL OF CL IMATE VOLUME 27

——,K.Keay, and J. A. T. Bye, 2012: Identification and climatology

of Southern Hemisphere mobile fronts in a modern reanalysis.

J. Climate, 25, 1945–1962, doi:10.1175/JCLI-D-11-00100.1.

Sinclair, M. R., 1994: A climatology of anticyclones and blocking

for the Southern Hemisphere.Mon.Wea. Rev., 122, 2239–2256,

doi:10.1175/1520-0493(1994)122,2239:AOCCFT.2.0.CO;2.

——, 1997: Objective identification of cyclones and their circula-

tion intensity and climatology. Wea. Forecasting, 12, 595–612,doi:10.1175/1520-0434(1997)012,0595:OIOCAT.2.0.CO;2.

Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K. B.

Averyt, M. Tignor, and H. L. Miller, Eds., 2007: Climate

Change 2007: The Physical Science Basis. Cambridge Uni-

versity Press, 996 pp.

Sutcliffe, R. C., 1960: Depression, front and air mass modification

in the Mediterranean. Proc. UNESCO/WMO Seminar on

Mediterranean SynopticMeteorology,Rome, Italy,WMO, 13–

143.

Trenberth, K. E., 1991: Storm tracks in the Southern Hemisphere.

J.Atmos. Sci.,48,2159–2178,doi:10.1175/1520-0469(1991)048,2159:

STITSH.2.0.CO;2.

Trigo, I. F., T. D. Davies, and G. R. Bigg, 1999: Objective cli-

matology of cyclones in the Mediterranean region. J. Cli-

mate, 12, 1685–1696, doi:10.1175/1520-0442(1999)012,1685:

OCOCIT.2.0.CO;2.

——, G. R. Bigg, and T. D. Davies, 2002: Climatology of cyclo-

genesis mechanisms in the Mediterranean. Mon. Wea. Rev.,

130, 549–569, doi:10.1175/1520-0493(2002)130,0549:

COCMIT.2.0.CO;2.

Trigo, R. M., I. F. Trigo, C. C. DaCamara, and T. J. Osborn, 2004:

Climate impact of the European winter blocking episodes

from the NCEP/NCAR reanalyses. Climate Dyn., 23, 17–28,

doi:10.1007/s00382-004-0410-4.

Tyrlis, E., and B. J. Hoskins, 2008: Aspects of a Northern Hemi-

sphere atmospheric blocking climatology. J. Atmos. Sci., 65,

1638–1652, doi:10.1175/2007JAS2337.1.

Wallace, J. M., G.-H. Lim, andM. L. Blackmon, 1988: Relationship

between cyclone tracks, anticyclone tracks and baroclinic

waveguides. J. Atmos. Sci., 45, 439–462, doi:10.1175/

1520-0469(1988)045,0439:RBCTAT.2.0.CO;2.

Wiedenmann, J. M., A. R. Lupo, I. I. Mokhov, and E. A. Tikhonova,

2002: The climatology of blocking anticyclones for the Northern

and Southern Hemispheres: Block intensity as a diagnostic.

J. Climate, 15, 3459–3473, doi:10.1175/1520-0442(2002)015,3459:

TCOBAF.2.0.CO;2.

Zolina, O., and S. K. Gulev, 2002: Improving the accuracy of

mapping cyclone numbers and frequencies.Mon.Wea. Rev.,

130, 748–759, doi:10.1175/1520-0493(2002)130,0748:

ITAOMC.2.0.CO;2.

15 DECEMBER 2014 HATZAK I ET AL . 9289