The synergy between drought and extremely hot summers in the Mediterranean

In the last years a large number of weather driven extreme events has occurred worldwide with unprecedented socio-economic impacts and are expected to increase, in both frequency and intensity, under future global-warming conditions. In this context early identification and predictability of such events are paramount as they mostly affect several socio-economic activities. Despite the effort in monitoring and evaluation of these extreme events, a quantitative assessment of their interaction is still a challenge. We propose to analyze if the occurrence of extremely hot days/nights in the summer is preceded by drought events in spring and early summer throughout the Mediterranean area. This was investigated by correlating the number of hot days and nights in the regions hottest months with a drought indicator on the prior months. Drought characterization was performed using both the Standardized Precipitation Evaporation Index (SPEI) and the Standardized Precipitation Index (SPI) for the 3-, 6- and 9-months time scales, considering the period 1980–2014 with a spatial resolution of 0.5°. The number of hot days and nights per month (NHD and NHN, respectively) is determined for the same period and spatial resolution. Results show that the most frequent hottest months for the Mediterranean region occur in July and August. Most regions exhibit statistically significant negative correlations, i.e. high (low) NHD/NHN following negative (positive) SPEI/SPI values, and thus a potential for NHD/NHN early warning. This analysis allowed to identify the Iberian Peninsula, northern Italy, northern Africa and the Balkans as the main hotspots of predictability of extreme hot temperatures in the summer preceded by the occurrence of drought events in the spring or early summer.