An objective definition of seasons for the Mediterranean region. (14th September 2020)

• Record Type: Journal Article
• Authors: Kotsias, George; Lolis, Christos J.; Hatzianastassiou, Nikolaos; Lionello, Piero; Bartzokas, Aristides
• Is Part Of: International journal of climatology. Volume 41(2021)Supplement 1
• Page Start: E1889
• Page End: E1905
• Publication Date: 2020-09-14
• Subjects: cluster analysis -- definition of seasons -- Mediterranean climate -- principal component analysis; Climatology -- Periodicals; Climat -- Périodiques; Climatologie -- Périodiques; 551.605
• DOI: 10.1002/joc.6819
• Languages: English
• ISSNs: 0899-8418

Abstract

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 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 : An objective definition of seasons based on the climatic characteristics of the Mediterranean region for the period 1949–2018 is achieved. The four objectively defined seasons generally correspond to the conventional ones (winter, spring, summer and autumn), but they differ in the duration and the onset and cessation dates. Shorter winter and spring and an extended autumn, along with a warming trend are found during the recent decades.