Climate Variations of Heat Waves on the Croatian Adriatic Coast for the Period 1961–2018

Abstract An analysis of characteristics of the boreal summer season June-July-August (JJA) measured daily maximum 2-m air temperatures (BSSDMATs) and associated heat waves (HWs) for 1961–2018 was conducted for three locations on the Croatian Adriatic coast representing its northern (Rijeka), middle (Split) and southern extents (Dubrovnik). Larger values occurred in the second part of the period (1990–2018) compared to the first part (1961–1989), including significant (α = 0.01) trends in mean seasonal averages (0.44 to 0.69°C per decade), extremes, frequencies, duration, and intensity. Exceedances and HWs spanning from 10 June to 24 August were determined in 53 years (out of 58 years) by the 95th and in 9 years by the 99th percentile criteria. Since heat stress frequently affects health at the beginning of a HW, exceedances of one or more days were all considered irrespective of any minimum length or separation. In 30 years, the exceedances appeared at all three locations in the same year. There were 122–147 (30–36) HW cases lasting 245–259 (51–54) days for the 95th (99th) percentiles. The maximum event duration ranged from 9 to 12 (5) days for the 95th (99th) thresholds. Weather conditions for the longest-duration events were characterized by propagation of a strong and wide ridge from the Azores High extending to southern Europe and blocking lows from the north. Based on these results, the Croatian coast is part of a Mediterranean hot spot that has been experiencing significant increasing warming trends and associated frequency of HWs that will likely continue in the future.