On the intra-annual variation of cloudiness over the Mediterranean region

The seasonal variability of cloudiness in the Mediterranean region is studied, through the examination of the mean day-to-day intra-annual variation of gridded NCEP/NCAR total cloud cover for the period 1948-2014. A dimensionality reduction process is followed by using a multivariate statistical methodology and three main modes of seasonal variation are found. The first mode is characterized by a winter maximum and a summer minimum and prevails mainly over the Mediterranean and Black Sea areas; a weak see-saw teleconnection over the Alps represents the opposite intra-annual variation. The second mode presents maxima in early autumn and late spring and minima in late summer and winter and prevails over the SW Europe and NW Africa inland regions. The third mode shows a maximum in June and a minimum in October and prevails over the eastern part of central Europe. The main characteristics of these modes are attributed to the seasonal variation of: i) the temperature and humidity of the lower and the middle troposphere affecting static stability and ii) the intensity and the position of the main circulation systems affecting the region, as for example the Azores subtropical anticyclone and the Mediterranean depressions. The examination of inter-decadal changes in the seasonal variation of cloud cover in the above core regions reveals a long-term decrease of cloudiness during the 66-year period 1948-2014. A comparison with contemporary satellite MODerate resolution Imaging Spectroradiometer (MODIS) and surface based European Climate Assessment & Dataset (ECA&D) station cloud cover data is carried out for the above core regions, evidencing a similarity among the data sets as to their seasonal variability, which supports the validity of the NCEP/NCAR reanalysis cloud cover results.