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Α new method, based on the definition of weather types (WTs), for the determination of seasons in the Mediterranean region is adopted. This method combines principal component analysis and k‐means Cluster Analysis on daily grid point meteorological data for the 70‐year period 1949–2018 obtained from the NCEP/NCAR database. At first, the method is applied to the classification of WTs and subsequently to the definition of seasons based on the mean intra‐annual variations of the WTs' frequency. The resulting seasons are compared to a previous method of the seasons' definition, based on the long‐term mean intra‐annual variations of climatological parameters. Furthermore, a synthesis of the two methods is proposed. The analysis is applied not only to the 70‐year period 1949–2018, but also to the five overlapping 30‐year sub‐periods 1949–1978, 1959–1988, 1969–1998, 1979–2008, 1989–2018 in order to investigate possible long‐term changes of the seasons' characteristics. Four seasons are identified for all periods and for both methodological approaches. These seasons broadly correspond to the four conventional ones, but they differ in their onset and cessation dates as well as their duration. For the composite method, it is found that winter lasts about 4 months, summer is a little longer than 3 months and spring and autumn last around 2.5 months. The most remarkable findings regarding the seasons' characteristics are: (a) the start/end dates and duration of winter are almost identical in all three approaches, while summers' characteristics differ; (b) shortening of winter and spring in recent periods, associated with the delayed start and expedited end dates, respectively; and (c) the prolongation of autumn and summer, due to the delay of its end date, and the earlier onset and delayed end dates, respectively. [ABSTRACT FROM AUTHOR]

An objective definition of seasons for the Mediterranean region is performed using the mean intra‐annual variations of 12 climatological parameters. The aim is to achieve a climatologically appropriate determination of the onset and cessation dates, as well as the duration, of each season, better than the conventional seasons (winter, spring, summer and autumn). Moreover, possible changes of the defined seasons during the 70‐year study period 1949–2018 are investigated over the climatologically sensitive Mediterranean region. The data used are daily NCEP/NCAR Reanalysis grid point values of precipitation rate, convective precipitation rate, 2 m temperature, total cloud cover, 2 m zonal and meridional wind, 500 and 1,000 hPa geopotential height, 500 and 850 hPa temperature, 850 hPa specific humidity and precipitable water over the Mediterranean region, for the period 1949–2018. Firstly, Principal Component Analysis is applied to the mean intra‐annual variations of the above parameters in order to reduce the dimensionality and then k‐means Cluster Analysis is applied to the resultant Principal Components in order to group dates with similar regimes of the above parameters. This methodology is applied for the 70‐year period 1949–2018 as well as for the five overlapping 30‐year subperiods 1949–1978, 1959–1988, 1969–1998, 1979–2008, and 1989–2018. According to the results, four seasons are defined for all periods. Although these seasons correspond to the four conventional seasons, there are differences regarding the onset and cessation dates and the duration. In general, it is found that winter lasts about 4 months, spring and summer are a little shorter than 3 months and autumn lasts about 2.5 months. The most remarkable long‐term changes are: (a) the warming during the last 30‐year period, which agrees with the ongoing climate change, (b) the shortening of winter and spring due to the delay of their onset date, and (c) the extension of autumn due to the delay of its cessation date. [ABSTRACT FROM AUTHOR]

ABSTRACT The connection between atmospheric circulation over Europe and snowfall in Ioannina, Northwestern ( NW) Greece, is examined. The study deals with 160 snow events, referring to the 56-year period 1956-2011. For each of the events, the patterns of 500 and 1000 hPa geopotential height, 500 and 850 hPa air temperature, 1000-500, 1000-700 and 700-500 hPa thickness are constructed for the European area, whereas static stability K-index, relative humidity at 500, 700 and 850 hPa levels and finally relative vorticity at 500 and 1000 hPa are also used for the Southeastern ( SE) Mediterranean region, for the previous day (D-1), the starting day (D) and the day following the cessation of snowfall ( END). Factor analysis and cluster analysis are applied to the above datasets and the evolutions of the above circulation parameters are classified into eight clusters. Snowfall in NW Greece is generally associated with a low pressure system over the Ionian Sea combined with an anticyclone over Western or NW Europe, causing cold northwesterly flow in the lower troposphere over NW Greece. In the middle troposphere, a deep 500 hPa trough usually extends from Western Russia to Italy and the Ionian Sea. The eight atmospheric circulation structures differ mainly in the exact location and/or the intensity of the depression and/or the anticyclone, in the orientation of the 500 hPa trough axis and in the trajectories of the above synoptic systems between D-1 and END days. [ABSTRACT FROM AUTHOR]

The evolution of the atmospheric circulation over Europe and the Mediterranean during fog events in Ioannina, north-western Greece, is examined for the period 1957-2002. A 24-h period starting at 12:00 UTC is defined as a fog day, when at least one of the present and/or past weather codes of the corresponding SYNOPs refers to fog. A sequence of such fog days is defined as a fog event. In total, 1019 fog events are found during the period under study. A methodology scheme using S-mode factor analysis and K-means cluster analysis is applied to the patterns of 850 hPa air temperature and 1000 and 500 hPa geopotential heights for the day before the initiation, the first day of occurrence and the day after the dissipation of fog, resulting in the classification of the 1019 fog events into ten clusters. For each one of the ten clusters, the mean 00:00 UTC patterns of the above parameters are constructed for 3 days. These patterns correspond to the evolution of the atmospheric circulation associated with fog formation and dissipation in the Ioannina region. Most of the patterns are characterized by anticyclonic activity or a weak southerly flow over Greece. The differences among the ten clusters refer mainly to the position and the intensity of the surface and upper air synoptic systems involved. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

The variability of sensible and latent heat fluxes in the Mediterranean Sea and their effect on precipitation in the Greek area during the winter months are investigated for the 39 year period 1959–97 by using multivariate statistical methods. First, factor analysis is applied, mainly as a data reduction tool, and then canonical correlation analysis is applied on the factor scores time series of the two pairs, i.e. sensible heat flux–precipitation and latent heat flux–precipitation, to reveal their interrelations. The results show there is a statistically significant relationship between sensible and latent heat fluxes over the western Mediterranean Sea and precipitation in the western windward areas of Greece. The sensible and latent heat fluxes in the western Mediterranean Sea contribute significantly to depression development. The depressions formed advect cold and dry air masses over the western Mediterranean, reinforcing these sensible and latent heat fluxes. Furthermore, these depressions lead to a southerly or southwesterly airflow over Greece. This flow, along with the existence of potential instability, significantly contributes to precipitation formation in the windward areas of western Greece and also in the islands of the eastern Aegean Sea. Copyright © 2004 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

In this work, the relationship between atmospheric circulation over Europe and precipitation in Greece is studied during high winter (January–February). Specifically, the 850 hPa relative vorticity centres, affecting or related to precipitation in the various areas of Greece, are defined and the corresponding temporal variability of both parameters is examined. Factor analysis is first applied on both fields, in order to reduce the dimensionality of the original data sets. Canonical correlation analysis is then applied on the factor analysis results, in order to interrelate the two parameters. It is found that: (i) precipitation over western and northern Greece, the eastern Aegean islands and the western Turkish coasts is mainly controlled by vorticity over Italy and the Gulf of Genoa; (ii) southern Aegean Sea and Crete precipitation is controlled by relative vorticity west of Crete; and (iii) eastern mainland areas precipitation is significantly affected by a vorticity seesaw between Tunisia and the northern Aegean. The temporal variations of precipitation and relative vorticity parameters over the above areas show a precipitation reduction over most Greek stations, combined with enhanced anticyclonic activity over almost the whole Mediterranean during the late 1980s and the early 1990s, a period characterized by high North Atlantic oscillation index values. Finally, cluster analysis is applied on the factor scores time series of precipitation, in order to classify the winter precipitation anomaly patterns into objectively defined clusters. Six main precipitation patterns were revealed and the corresponding relative vorticity mean anomaly regimes over southern Europe indicate the atmospheric circulation characteristics causing these specific patterns. Copyright © 2003 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

In this work, the variability and covariability of winter temperatures in the middle and the lower troposphere are studied over Europe and the North Atlantic Ocean. Temperature variations of the middle and the lower troposphere are examined in terms of (a) 500–700 hPa and 700–l000 hPa thickness and (b) air temperature on the isobaric surfaces of 500 hPa and 700 hPa. At first, factor analysis (FA) defined areas with characteristic temperature variability in each layer (and on each isobaric surface) and then, canonical correlation analysis (CCA) revealed areas in both layers (and on both isobaric surfaces) presenting common temperature variations. A temperature see-saw between N Europe and W Asia was revealed for both layers and isobaric surfaces implying that temperature changes in these areas are vertically spread. Another well-defined area, appearing in both analyses, is the area of the Labrador Sea and S Greenland. This region is also teleconnected to other regions, though not very clearly in every height. These temperature patterns are mainly attributed to the Eurasian (EU) and North Atlantic Oscillation (NAO) circulation patterns, which are responsible for large air mass exchanges in the area, being vertically extended in the middle and the lower troposphere. Copyright © 2001 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

In this work, 12 monthly climatic parameters of 49 Meteorological stations in Greece are classified according to their spatial covariability, in order to define groups of climatic elements having the same, characteristic, spatial distribution. The grouping of the parameters is achieved objectively by using the method of factor analysis. The analysis is applied to the winter months of January and February and to the summer months of July and August. In winter, the parameters are classified into four groups as follows: (i) maximum, mean and minimum temperature and, negatively, pressure and number of frost days; (ii) wind and, negatively, diurnal temperature range; (iii) precipitation; and (iv) cloudiness and relative humidity. In summer, the parameters are classified into three groups, namely: (i) precipitation, cloudiness and, negatively, minimum temperature; (ii) maximum and mean temperature, relative humidity; and (iii) wind. From the above groups the most interesting is group (i) of summer, as its parameters belong to different groups in winter. Finally, the study of the spatial distribution of the above groups showed that some depend on latitude, others on the continentality and others on the leeward or windward character of the various areas. Copyright © 1999 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

In this paper, using 20 year mean monthly precipitation totals from 54 stations, the general statistics for the time and space distribution of precipitation in northwest Greece is at first described. Precipitation varies from about 1000 mm at the coast to 2500 mm on the mountains, with a large variability. Then, the intra-annual variation is studied, using Fourier analysis. This variation is described well by two harmonics, explaining about 90% of the total variance. The first harmonic, over 80% of the variance, shows a maximum from late December (coastal areas) to early January (continental areas) and is caused by the northwest Mediterranean depression activity. The second harmonic exhibits maxima in May and November, also delayed in the continental areas, and is mainly caused by upper air troughs and instability. The spatial distribution of precipitation is then studied by using factor analysis. The 12 months of a year are classified in two parts corresponding to two statistically significant factors, explaining 88.5% of the total variance. Finally, the combination of positive and negative standardized scores of the two factors showed that northwest Greece is characterized by four precipitation regimes, depending on the combination of depression activity, the effects of the sea and of topography. Copyright © 1999 Royal Meteorological Society [ABSTRACT FROM AUTHOR]