**1. Introduction**

The Mediterranean Sea is located in a transitional climatic area between Europe, Africa, and Asia and its climate is widely known as "the Mediterranean climate". The Mediterranean climate is generally characterized by considerable seasonal variations in almost all climatic parameters, for example air temperature, cloudiness, precipitation, lightning activity, etc. [1–5]. Although these seasonal variations appear over the whole Mediterranean region, they present significant differences among the various subregions. These differences are connected to various atmospheric and geographical factors. The position and the variability of the large-scale atmospheric circulation systems (e.g., the subtropical anticyclone of the North Atlantic and the south Asian summer low) and the global atmospheric oscillations affecting the region are dominant atmospheric circulation factors connected to the significant spatial variability of climate within the Mediterranean region [6–8]. The North Atlantic Oscillation (NAO), which refers to the sea level pressure seesaw between the Icelandic low and the subtropical anticyclone of the North Atlantic, affects the atmospheric conditions over most of the Mediterranean region. Specifically, positive/negative values of the NAO index are generally connected to below/above normal cyclonic activity and precipitation over the Mediterranean [9,10]. The Arctic Oscillation (AO) is connected to the variation of the intensity of the polar low. Positive values of the AO index are generally associated with anticyclonic conditions and dryness over the Mediterranean, while negative AO index values are connected to the transfer of cold masses towards

the south and the prevalence of cyclonic activity and above normal precipitation over the same region [11,12]. The North Sea–Caspian Pattern (NCP) is a pressure seesaw between the North Sea and the Caspian Sea. A positive phase of NCP is accompanied by northeasterly flow over a part of the Eastern Mediterranean (Balkans and Western Turkey), while a negative phase is accompanied by southwesterly flow over the same regions [13,14]. Furthermore, the spatial variability of climate characteristics is enhanced by the complicated geographical features of the region (coastline, relief, land–sea alteration), which affect significantly most of the climatic parameters including temperature, cloudiness, precipitation, and wind. For example, the windward or leeward character of a specific region, as well as its distance from the sea, plays an important role on its cloudiness and precipitation regimes. These regimes are strongly affected by the associated atmospheric humidity and static stability levels and the frequent prevalence of adiabatic sink (warming) or rise (cooling) [3,15]. Considering the above atmospheric and geographical features of the Mediterranean region and the associated high spatial variability of its climate, climatological studies, which can be carried out with the use of high-resolution meteorological data, would lead to interesting results referring to the detailed spatial characteristics of the Mediterranean climate. This is further supported by the fact that the future climatic changes, as they are forecasted by the climatic models, are also expected to be characterized by high spatial variability over the same region [16,17].

The climate of the Balkans, a Mediterranean subregion of great climatological interest, has been extensively studied by means of the spatial regimes of specific climatic parameters [18,19] and their connection to specific atmospheric circulation modes [20]. The impacts of the acting teleconnections, global warming, and local surface warming (land–atmosphere interactions) reveal high spatial complexity over the region. Most studies involve station meteorological data and/or Reanalysis data of spatial resolution up to 1◦ × 1◦. Moreover, an attempt for a climate regionalization of the Balkan Peninsula has been made with the application of Cluster Analysis on station sea level pressure data over the region [21]. Nowadays, the recently introduced ERA5 high resolution meteorological data set [22] provides the scientific community with an extra useful tool for studying the spatial variability of climate over a region with relatively complex relief. The southern Balkans is such a region and the present study aims at examining this variability by defining climatologically homogeneous subregions within it, i.e., subregions with characteristic seasonal variations of the main climatic parameters. This is achieved with the application of a multivariate statistical scheme including Principal Component Analysis and Cluster Analysis on the high-resolution ERA5 meteorological data set. The temporal availability of the data set is at the moment restricted to the recent 10-year period of 2008 to 2017. Although this period is relatively short, not being sufficient for the full establishment of the statistical parameters connected to the climate of the region, the general characteristics of the spatial variability of climate, especially those that are significantly influenced by the complicated relief, can be highlighted by means of the high spatial resolution and the reliability of the recently introduced data set.
