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13 pages, 7 figures, 3 tables, Two large-scale mass mortality events (MMEs) of unprecedented extent and severity affecting rocky benthic communities occurred during the summers of 1999 and 2003 along the coasts of the NW Mediterranean Sea. These mortality outbreaks were associated with positive thermal anomalies. In this study, we performed an analysis of inter-regional and inter-annual differences in temperature (T) conditions associated with MMEs of the red gorgonian Paramuricea clavata by analyzing high resolution T time series (hourly records for 3 to 8 years) from four regions of the NW Mediterranean with differing hydrological conditions and biological impacts. High resolution records allowed a detailed analysis using classical and new descriptors to characterize T anomalies. We were able to determine that the MMEs were triggered by two main types of positive thermal anomalies, with the first type being characterized by short periods (2 to 5 days) with high Mean T reaching more than 27°C in some regions and being associated with high intra-day and intra-period variability, while the second type of anomaly presented long duration (near one month) at warm T (24°C) with low intra-period variability. Inter-regional patterns arose; some regions displayed both types of anomalies, while others exhibited only one type. The results showed that T conditions should be considered as the main factor that explains the observed inter-regional and inter-annual differences in mortality impacts. In explaining these differences, the late timing of T anomalies, in addition to their magnitude was found to be determinant. Finally, by combining thermotolerance experimental data with the maximal T stress conditions observed in the four regions, we were able to determine the differential risk of mass mortality across regions. We conclude that expanding high resolution T series is important for the development of sound management and conservation plans to protect Mediterranean marine biodiversity in the face of climate change, Funding: This work was supported by the Alßan Program (C. Crisci doctoral fellowship, fellowship code: E06D103213UY; www.programalban.org); Parc National de Port-Cros; The IFB Foundation (Institut Français de la Biodiversite´ ); Agence Nationale pour la Recherche (ANR) (MEDCHANGE Project (http://www.agencenationale-recherche.fr; http://sudek.esil.univmed.fr/medchange/spip); Ministry of Science and Innovation (Biorock project ref. CTM2009-08045). These funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Dr. Nathaniel Bensoussan is employed by IPSO-FACTO SARL; he contributed to the analysis of the data and the writing of the manuscript