  Commercial electricity production from marine renewable sources is becoming a
necessity at a global scale. Offshore wind and solar resources can be combined
to reduce construction and maintenance costs. In this respect, the aim of this
study is two-fold: i) analyse offshore wind and solar resource and their
variability in the Mediterranean Sea at the annual and seasonal scales based on
the recently published ERA5 reanalysis dataset, and; ii) perform a preliminary
assessment of some important features of complementarity, synergy, and
availability of the examined resources using an event-based probabilistic
approach. A robust coefficient of variation is introduced to examine the
variability of each resource and a joint coefficient of variation is
implemented for the first time to evaluate the joint variability of offshore
wind and solar potential. The association between the resources is examined by
introducing a robust measure of correlation, along with the Pearson's r and
Kendall's tau correlation coefficient and the corresponding results are
compared. Several metrics are used to examine the degree of complementarity
affected by variability and intermittency issues. Areas with high potential and
low variability for both resources include the Aegean and Alboran seas, while
significant synergy (over 52%) is identified in the gulfs of Lion, Gabes and
Sidra, Aegean Sea and northern Cyprus Isl. The advantage of combining these two
resources is highlighted at selected locations in terms of the monthly energy
production.
