Assessing heat wave conditions

The intensity of the thermal forcing observed during the summer 2018 was categorised following Hobday et al.⁶¹, using the climatology of both air and seawater temperature of the study area. Specifically, we categorised the summer 2018 heat wave using multiples of the difference ΔT between the climatological mean and the climatological 90^th percentile, which is the threshold used to identify marine heat waves (MHW; Hobday et al.⁶²). The magnitude of this difference was subsequently used as a descriptor of an observed thermal event as moderate (1 to 2 ΔT), strong (2 to 3 ΔT), severe (3 to 4 ΔT) and extreme (>4 ΔT) heat waves. The climatology of air and sea surface temperatures were respectively based on hourly air temperature recorded by the Météo France (www.donneespubliques.meteofrance.fr) weather station of Boulogne-sur-Mer (50°43′54N, 1°35′53E) from 1949 to 2017 and biweekly sea surface temperature recorded at the inshore station (50°40′75N, 1°31′17E) of the SOMLIT network (Service d’Observation en Milieu LITtoral; www.somlit.epoc.u-bordeaux1.fr) from 1996 to 2017.