Heavy precipitation in Mediterranean depression Elias likely strengthened by both  human-driven climate change and natural variability  

Press Summary (First published 2023/09/28, Updated 2023/11/22)

• Low pressure systems similar to that producing September 2023 Mediterranean depression Elias are more intense (1 hPa deeper) and locally wetter in Eastern Greece than they would have been in the past. 

• Mediterranean depression Elias was a somewhat uncommon event.

• Natural climate variability likely played a role in driving the pressure pattern and the associated increase in precipitation linked to Elias.

Event Description

From September 25th to 28th, Greece experienced severe weather conditions, resulting in significant disruptions and adverse impacts on the region. The National Meteorological Service and National Observatory of Athens issued a high-level warning, mainly for flash floods and large hail. EUMETNET has named the storm "Elias".

The establishment of a persistent high pressure anticyclonic area over Western Europe on 24 September opened the way for intrusion of cold air from the Alps and the Balkans towards Italy. This low pressure system got isolated over the Ionian Sea and created favourable conditions for thunderstorm development over Greece. Multiple disturbances and the complex topography of Greece resulted in persistent thunderstorm activity causing widespread flooding.

Hail up to 6 cm in diameter was reported in many parts of Greece on Monday 25/9, and between Tuesday 26 and Thursday 28 September severe flooding was reported in Central Greece. In Euboea, the NOANN/meteo.gr weather stations in Istiaia report up to 486 mm of accumulated rainfall within less than 48 hours. Multicell thunderstorms in Volos on 27 September resulted in severe flooding with more than 315 mm of rainfall within less than 24 hours. This is the second flooding event in the area after Storm Daniel on 5-7 September, and the monthly accumulated rainfall has reached 933 mm (mean annual value: 480 mm). Reports of road closures, mudslides, power cuts and lack of potable water have caused significant disruptions to Euboea and Volos area. The Surface Pressure Anomalies pattern associated with the event consists of a low pressure area over the Ionian Sea. Precipitation Data show daily accumulated values above 100 mm/day over a large part of continental Greece.

Climate and Data Background for the Analysis

The IPCC AR6 report, with a focus on the Mediterranean region in Sections 10.6.4, 12.4.1, and 12.4.5, highlights the increasing occurrence of Mediterranean cyclones during the summer season. These cyclones are fuelled by rising sea-surface temperatures (SSTs) in the Atlantic Ocean and Mediterranean, which have climbed by 0.25°C to 1°C since 1982–1998, contributing significantly to the intensity of these convective events. Despite the report's acknowledgment of the correlation between warmer SSTs, marine heatwaves, and summer Mediterranean cyclones, it refrains from making historical trend statements about extreme precipitation in the Mediterranean region. Furthermore, Chapter 11 of the IPCC AR6 report underscores the challenges in summarizing trends in severe convective storms across regions due to varying definitions and limited long-term data on phenomena such as tornadoes, hail, and lightning associated with such storms. Consequently, the report opts not to provide specific statements regarding changes in convective storms in the Mediterranean region. 

Our analysis approach rests on looking for weather situations similar to those of the event of interest observed in the past. We have medium-high confidence in the robustness of our approach given the available climate data, as the event is similar to other past events in the data record.

ClimaMeter Analysis

We analyse here (see Methodology for more details) how events similar to the low pressure systems leading to the Mediterranean depression Elias floods 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 [14°E 28°E 30°N 41°N]. The Surface Pressure Changes show that low pressure systems have deepened compared to the past. Precipitation Changes show that similar events now produce locally higher amounts of precipitation in Eastern Greece. Considering the affected urban areas, Karditsa, Volos, and Chalkida see an increase in precipitation in the present (about 1 mm/day). We also find that Similar Past Events have become more frequent in all summer while in the past they mostly occurred in September. 

Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal Oscillation and El Niño Southern Oscillation may have partly influenced the event. This means that the changes we see in the event compared to the past may be due to natural climate variability.

Conclusion

Based on the above, we conclude that low pressure systems leading to heavy rainfall similar to that caused by Mediterranean depression Elias are locally 1 mm/day wetter in Greece than they would have been in the past. We interpret the Mediterranean depression Elias flood as an unusual event for which natural climate variability played a role.

Contact Authors

Stavros Dafis, Data4Risk & National Observatory of Athens/meteo.gr 📨sdafis@noa.gr  🗣️Greek, English

Davide Faranda, IPSL-CNRS, France  📨davide.faranda@lsce.ipsl.fr  🗣️French, Italian, English