RESEARCH ARTICLE An objective definition of seasons for the Mediterranean region George Kotsias 1 | Christos J. Lolis 1 | Nikolaos Hatzianastassiou 1 | Piero Lionello 2 | Aristides Bartzokas 1 1 Laboratory of Meteorology, Department of Physics, University of Ioannina, Ioannina, Greece 2 Department of Biological and Environmental Sciences and Technologies, University of Salento, Lecce, Italy Correspondence George Kotsias, Laboratory of Meteorology, Department of Physics, University of Ioannina, 45110 Ioannina, Greece. Email: g.kotsias@uoi.gr Abstract 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 conven- tional 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 Medi- terranean region, for the period 1949–2018. Firstly, Principal Component Anal- ysis 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 win- ter 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. KEYWORDS cluster analysis, definition of seasons, Mediterranean climate, principal component analysis Received: 17 February 2020 Revised: 21 July 2020 Accepted: 24 August 2020 DOI: 10.1002/joc.6819 Int J Climatol. 2020;1–17. wileyonlinelibrary.com/journal/joc © 2020 Royal Meteorological Society 1