| Summary: | 5th International Jellyfish Bloom Symposium, 30 May to 3 June 2016, Barcelona. Even if the Portuguese Man-of-War (Physalia physalis) is not a native species in the Mediteranean basin, the region experienced a high number of swarms in 2010 that had dramatic consequences, including the first human fatality caused by a jellyfish sting. In this work, a comprehensive analysis of the meteorological and oceanographic conditions of the NE Atlantic Ocean during the months previous to the appearance of P. physalis was performed. A virtual experiment of the drifting of the individuals using a hydrodynamical model, consisting of a ROMS based numerical simulation forced with realistic winds (ASCAT) and heat fluxes from ERA-Iterim, together with an Individual Based Model simulations was implemented. The values during the winter of 2009-2010 of the North Atlantic Oscillation (NAO) were among the most negative since recordings began (150 years ago). The intensity of westerly winds was between one and a half to four times higher than average. The study concludes that these unusually strong winds, along with the prevailing ocean currents, are likely to have moved the P. physalis population from the open Atlantic Ocean towards the mainland, through the Strait of Gibraltar, and into the Mediterranean basin. Field data and modelling simulations confirm this hypothesis. Sightings of P. physalis have fallen dramatically since 2010, indicating that they will not pose a continuous problem along Mediterranean beaches and, unless the extreme negative values of the NAO become more frequent due to climate change, the 2010 exceptional occurrence of P. physalis is unlikely to reoccur on a regular basis. No
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