An objective definition of seasons for the Mediterranean region based on synoptic weather types

An objective definition of seasons for the Mediterranean region is performed based on the intra-annual variation of the frequencies of eight (8) objectively defined synoptic Weather Types (WTs). The data used for the determination of the 8 WTs are daily NCEP/NCAR Reanalysis grid point values of 2m air temperature, total cloud cover, 2m zonal and meridional wind components, 500hPa and 1000hPa geopotential height, 500hPa and 850hPa air temperature, 850hPa specific humidity and precipitable water over the Mediterranean region, for the period 1949-2018. Firstly, Principal Component Analysis (PCA) is applied to the inter-annual variations of the above parameters in order to reduce the dimensionality and then k-means Cluster Analysis (CA) is applied to the resultant Principal Components (PCs) in order to group dates with similar patterns of the above parameters, resulting in 8 synoptic WTs. Then, PCA and CA are applied again, now on the intra-annual variations of the frequencies of the resultant WTs and group dates of the year (seasons) with similar frequency distribution among the 8 WTs. According to the results, four (4) seasons are defined for the 70-year period 1949-2018. The same methodology is applied also for the five overlapping 30-year sub-periods 1949-1978, 1959-1988, 1969-1998, 1979-2008 and 1989-2018, leading to 4 seasons too. Although the characteristics of these seasons generally correspond to the ones of the four conventional seasons, there are differences regarding the onset and cessation dates and the duration. In general, it is found that winter and summer last about 4 months (115 and 114 days respectively), spring has a duration of about 2.5 months (72 days) and autumn lasts about 2 months (64 days). The most remarkable long-term changes of the seasons’ characteristics are: (i) the colder winters and warmer autumns during the last decades, (ii) the shortening of winter and spring due to later onset and earlier cessation dates, respectively, and (iii) the extension of autumn and summer due to later cessation and earlier onset dates, respectively.