Dynamics, predictability, impacts and climate change considerations of the catastrophic Mediterranean Storm Daniel (2023)

Abstract. In September 2023, Storm Daniel formed in the central Mediterranean Sea, causing significant socioeconomic impacts in Greece, including fatalities and severe damage to agricultural infrastructure. Within a few days, it evolved into a tropical-like storm (medicane) that made landfall in Libya, likely becoming, to our knowledge, the most catastrophic and lethal weather event ever documented in the region. This study places Storm Daniel as a centerpiece of the disasters in Greece and Libya. We conducted a comprehensive analysis that links a cyclone system with hazardous weather conditions relevant to extreme precipitation, floods and significant sea wave activity. In addition, we examine Daniel's predictability in different development stages and draw connections with previous case studies. Given the climatologically extreme precipitation produced by Daniel, we examine the capacity of numerical weather prediction models to capture such extremes, and we finally investigate potential links to climate change. Daniel initially developed like any other intense Mediterranean cyclone, including medicanes: due to upper-tropospheric forcing followed by Rossby wave breaking. At this stage, it produced significant socioeconomic impacts in Greece. As it intensified and attained tropical-like characteristics, it developed markedly just prior to landfall, reaching peak intensity over land. Considering the short lead times (around 4 d), the cyclone formation exhibited low predictability, whilst landfall in Libya was more predictable. Our analysis of impacts highlights that numerical weather prediction models can capture the extreme character of precipitation and flooding in both Greece and Libya, providing crucial information on the expected severity of imminent flood events. We also examine moisture sources contributing to extreme precipitation. Our findings indicate that large-scale atmospheric circulation was the primary driver, drawing substantial water vapor from the eastern Mediterranean, the Black Sea and continental Europe. The intensification of Storm Daniel was likely driven by anomalously warm SST in the Mediterranean and Black Sea, enhancing evaporation and contributing to the extreme precipitation along the Libyan coast. Finally, our analysis supports the interpretation of its impacts as characteristic of human-driven climate change but also highlights the exceptionality of this cyclone, especially in its medicane phase, which complicates the comparison with other cyclones.