Seasonal Variability of Key Climate Parameters in the Mediterranean Region: Distinct Features From A Long‐Term (1940–2023) Analysis

ABSTRACT The intra‐annual variability of atmospheric circulation and static instability in the Mediterranean region for the 84‐year period 1940–2023 is examined with the use of a multivariate statistical approach applied to the mean intra‐annual variations of 500 hPa Relative Vorticity and K‐index. This long‐period analysis is considered necessary for the identification or confirmation of specific peculiarities in the seasonality of the Mediterranean climate. The data used are 1 0 × 1 0 mean daily values, obtained from the ERA5 reanalysis data set. A data reduction is performed on the long‐term mean intra‐annual variations of both parameters with a 5‐day time resolution. For 500 hPa Relative Vorticity and K‐index, four and three modes of mean intra‐annual variation are revealed, respectively, corresponding to specific sub‐areas of the greater Mediterranean region. Some of the maxima and minima appearing in these modes are directly connected to the main well‐known features of the Mediterranean climate, while some others cannot be clearly attributed to specific dynamic, geomorphological or thermal factors. The mean intra‐annual variations of the above parameters in the defined areas are also calculated for the two 42‐year sub‐periods 1940–1981 and 1982–2023 to detect possible long‐term changes in the seasonal variability. The results show that the main differences between the two sub‐periods refer to K‐index, which in the recent 42‐year period becomes higher and lower in the warm and cold periods of the year respectively. These changes in static instability characteristics can be considered as one of the main symptoms of the ongoing climatic change.