Impacts of Atmospheric Rivers in extreme precipitation on the European Macaronesian islands

Extreme precipitation events in Europe during the winter half of the year have major socio-economic impacts associated with floods, landslides, extensive property damage and life losses. In recent years, a number of works have shed new light on the role played by Atmospheric Rivers (ARs) in the occurrence of extreme precipitation events in Europe as was the case in major historical floods in Duero (Pereira et al., 2016) and Tagus (Trigo et al., 2015) rivers in Iberia. The flash flood event that occurred on the Madeira Island on the 20 February 2010 with a deal toll of near 50 was due to an AR (Couto et al., 2012).

However, regarding the European Macaronesia islands (Azores, Madeira, and Canary) no comprehensive study has been made concerning the relationship between ARs and extreme precipitation. To do so, we have analysed recorded daily precipitation from different locations in the different European Macaronesia islands and computed the percentiles into 10% bins.

Regarding the ARs database we have used the method of Guan and Waliser et al., (2015) applied to 4 different reanalysis datasets (ERA-Interim; CFSR; MERRA2 and NCEP/NCAR). The use of multiple reanalyses is necessary to ensure more robust results since the Atlantic islands are relatively small and therefore changes in the resolution of the reanalysis datasets could influence the results.

It is shown that the ARs influence over extreme precipitation (above the 90^th percentile) is higher in the Azores islands when compared with the Madeira or the Canary Islands. In the Azores, for the most extreme precipitation days, the presence of ARs is particularly high between 60% and 70% of the days) while for Madeira the importance of the ARs is reduced to 50-60% of the most extreme precipitation days. For the Canary Islands, the occurrence of ARs on extreme precipitation is usually below 50% of the days.

Acknowledgements: This work was supported by the project FORLAND – Hydrogeomorphologic risk in Portugal: driving forces and application for land use planning (PTDC/ATPGEO/1660/2014) funded by Fundação para a Ciência e a Tecnologia, Portugal (FCT). A. M. Ramos was also supported by a FCT postdoctoral grant (FCT/DFRH/ SFRH/BPD/84328/2012).