Heavy precipitation in Medicane Daniel mostly strengthened by human-driven climate change

Press Summary (First published 2023/09/13, Updated 2023/11/22)

• Mediterranean depressions similar to Medicane Daniel are between 5 and 9 mm/day wetter in the present than they would have been in the past along the eastern Libyan coast.  

• Medicane Daniel was a largely unique event: similar events are Mediterranean depressions which do not display the tropical characteristics typical of medicanes. 

• Natural climate variability likely played a role in driving the pressure pattern linked to the landfall of Medicane Daniel.

Event Description

After affecting Greece, Turkey and Bulgaria with extreme rainfall, causing devastating floodings on 5 September 2023, Mediterranean Depression Daniel slowly moved to the South and acquired sub-tropical features, strengthening into a medicane. Medicanes are Mediterranean cyclones whose characteristics resemble those of tropical cyclones. Every year, on average 1-2 medicanes form, and are often associated with hurricane-force winds and heavy precipitation. On September 10, Daniel made landfall near the Libyan city of Benghazi, and then moved to the East, heavily affecting the Libyan coast, including the cities of Tobruk and Darnah. The Libyan National Meteorological Centre issued a 72-hour advance warning for Daniel's landfall, prompting the declaration of State of Emergency in the eastern regions. Despite the early warning, Daniel's severe precipitations caused catastrophic flooding over the eastern Libyan coast, with 5000 fatalities and more than 10000 estimated missing people as of September 13, as estimated by the Red Cross. Precipitation totals reached 150-200 mm in several areas, peaking over 400 mm in Al-Bayda, an all-time record for the city. This amount of precipitation is exceptional for Libya, and the dramatic toll has been due to dam failure following the exceptional rainfall.

Initially developed as a Mediterranean cyclone part of an omega-blocking pattern, Medicane Daniel has caused catastrophic damage and loss of life in Libya, before dissipating over Egypt on September 11. The Surface Pressure Anomalies pattern associated with the event consists of a deep low pressure area over the Libyan coast, South of Crete. Precipitation Data show daily accumulated values above 60 mm over a large region of the eastern Libyan coast.

Climate and Data Background for the Analysis

The IPCC AR6 addresses the influence of climate change on medicanes in Section 11.7. The report states with medium confidence that the frequency of medicanes is projected to decrease under warming, while the strongest medicanes will become stronger, underlining that this trend is also consistent with expected global changes in TCs under warming. In Section 12.4, the report also highlights that the same negative trend in frequency and positive trend in intensity concerns in general cyclones reaching North Africa, again with medium confidence. 

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For Medicane Daniel, we have low confidence in the robustness of our approach given the available climate data, as the event is largely unique in the data record.

ClimaMeter Analysis

We analyse here (see Methodology for more details) how events similar to the landfall of Medicane Daniel have changed in the present (2001–2022) compared to what they would have looked like if they had occurred in the past (1979–2000) in the region [17°E 35°E 25°N 38°N]. The Surface Pressure Changes show that the surface pressure over the affected area not changed significantly. Precipitation Changes show that similar events produce larger (between 5 and 9 mm/day) amounts of precipitation the eastern Libyan coast, which has been severely affected by Daniel's severe precipitation on September 10-11. Considering the affected urban areas, Darnah and Benghazi sees an increase in precipitation in the present (+1.5 mm/day), while Tobruk is not affected by any change. We also find that Similar Past Events have become less frequent in September and November, and slightly more common in October and December. We remark here that our analysis is not capable of capturing the tropical-like nature of Medicane Daniel, which explains why its impacts where much more severe than we find in its analogues. 

Finally, we find that sources of large-scale natural climate variability may have influenced the event. Although not included in our analysis, we however hypothesise that the changes we see in precipitation amounts compared to the past may be partially due to human driven climate change, in keeping with the potential for heavier precipitation in a warmer climate.

Conclusion

Based on the above, we conclude that medicanes like Daniel show similar atmospheric pressure, higher precipitation in the present than in the past. We interpret the landfall of Medicane Daniel as a largely unique event for which natural climate variability played a role.

Contact Author

Flavio Pons, IPSL-CNRS, France  📨flavio.pons@lsce.ipsl.fr  🗣️French, Italian, English