DATE2022-05-13 21:00:35
TITLEComparing and combining different approaches for the objective definition of the Mediterranean seasons
AUTHORSGeorge Kotsias (1) ,Christos J. Lolis (1) ,Piero Lionello (2) ,Nikolaos Hatzianastassiou (1) ,Nikolaos Bakas (1) ,Aristides Bartzokas (1)
  1. 1) University Of Ioannina, Department Of Physics, Ioannina (Greece) ,2) University Of Salento, Bdepartment Of Biological And Environmental Sciences And Technologies, Lecce (Italy)
ABSTRACTMultiple objective methodologies are followed for the purpose of defining the seasons for the Mediterranean region during the period 1950-2018. The first method utilizes the mean intra-annual variations of 12 meteorological parameters obtained from NCEP/NCAR. The application of “spectral analysis”, which includes the successive use of Principal Component Analysis (PCA) and Cluster Analysis (CA), on the matrix of the aforementioned variations lead to the classification of the dates of the year into four distinct and homogenous time periods, which can be characterized as the objectively defined seasons. Another method of seasons’ definition, involves the use of weather types (WTs). As such, spectral analysis is applied on the daily values of the above-mentioned parameters and 8 WTs are determined. Then, spectral analysis is applied again, this time on the mean intra-annual variations of the frequencies of the 8 WTs, resulting in the definition of the seasons and four seasons are defined. In the third approach, a cyclone detection and tracking algorithm is applied for the Mediterranean region by utilizing 6-hourly high spatial resolution mean sea-level pressure data from the ERA5 reanalysis database. Then, spectral analysis is performed in order to classify the detected cyclone trajectories into groups and 12 cyclone clusters are constructed. In the next step, spectral analysis is applied on the mean intra-annual variations of the frequencies of the 12 cyclone clusters and four seasons are defined. Finally, a composite method is performed which combines the preceding three methods. According to the composite method the objectively defined seasons are: "winter" (November 16 - March 25) with a duration of more than 4 months, "spring" (March 26 - June 11) which lasts approximately 2.5 months, "summer" (June 12 - September 12) with a duration of about 3 months and finally "autumn" (September 13 - November 15) which lasts about 2 months. Long-term changes of the limits and duration of the seasons are also investigated.