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

Press Summary (Firt Published 2024/06/17

• Mediterranean depressions similar to Medicane Ianos are locally up to 10 mm/day (up to 15%) wetter in the present than they would have been in the past 

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

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

Event Description

Medicane Ianos, which occurred to the east of Greece (Ionian Sea) from 15 to 21 September 2020, was one of the most intense Medicanes ever recorded. "Medicanes" (contraction of Mediterranean hurricanes) exhibit characteristics similar to tropical storms, including a well-defined cloud eye, sustained winds of tropical-storm strength and high waves and sea surge.

Ianos severely impacted western and central Greece from September 17th to 18th. It generated wind gusts up to 117 km per hour and brought record-breaking rainfall to several areas, such as the Ionian Islands and the mainland region of Thessaly. The 48-h accumulated rainfall exceeded 600 millimeters in Cephalonia and reached up to 300 millimeters in central Greece. The storm resulted in four fatalities and significant structural damage. For example, landslides and house flooding, primarily due to the intense rainfall, were recorded in the mountainous region near Karditsa. The flooding disrupted power supplies, destroyed roads, and necessitated the closure of schools and nurseries in heavily affected areas like Karditsa and the inland Greek town of Mouzaki​​​​​​​​.

Surface Pressure Anomalies show a depression with values reaching -5 hPa centered on the Ionian sea. Near Surface Temperature anomalies to the north of the depression reach 6 degrees Celsius. Precipitation Data   take (locally) values over 80 mm/day and Windspeed Data show the signature of the cyclone with values up to 40 km/h.  

Climate and Data Background for the Analysis

The IPCC AR6 addresses the impacts on the intensity of medicanes of climate change in Section 11.7. The report states that it is likely that the strongest medicanes will become stronger under warming scenario projections. However, considering the limited observations records, climate model deficiencies, and the poor understanding of the effect of climate change on medicane genesis, confidence on the trend in the frequency is low. The report also states that it is very likely that tropical cyclone total rain rates increase with warming. It is likely that the peak rain rates will increase at even a higher rate than the average (greater than the Clausius–Clapeyron scaling rate of 7% per 1°C of warming) in some regions due to increased near surface moisture convergence caused by regional increases in tropical cyclone wind intensity.

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For this event we have low confidence in the robustness of our approach given the available climate data, as the event is very different from other past events in the data record.

ClimaMeter Analysis

We analyze here (see Methodology for more details) how events similar to Mediterranean Cyclones Ianos in September 2020 changed in the present (2001–2023) compared to what they would have looked like if they had occurred in the past (1979–2001) in the region [17°W 25°W 35°N 41°N]. 

The Surface Pressure Changes show that the surface pressure over the affected area has not changed significantly. The Temperature Changes show a positive temperature anomaly over sea and the Balkans (up to +2°C). The Precipitation Changes show that similar present events have become up to 10mm/day (up to 15%) wetter with respect to past events. These changes are however localized over the sea at the border between Greece and Albania. Changes in the Urban Areas show that the cities of Ioannina, Brindisi and Sarande see an increase in precipitation in the present (from 2.5mm/day to 7 mm/day). The Windspeed Changes show less windy conditions (up to -4km/h) south of the Peloponnese region,  and no changes over Albania.

We also find that Similar Events have become more frequent in September and less frequent in October compared to past events. We remark here that our analysis is not capable of capturing the tropical-like nature of Medicane Ianos, which explains why its impacts were much more severe than we find in its analogues.

Finally, we find that sources of natural climate variability, namely the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation may have influenced the event. Although not included in our analysis, we however hypothesize 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 mediterranean depressions like Ianos show similar atmospheric pressure and up to 15% higher precipitation in the present than in the past. We interpret the landfall of Medicane Ianos as a largely unique event for which natural climate variability played a role.

Contact Authors

This work was made possible by Institut Pascal at Université Paris-Saclay with the support of the program “Investissements d’avenir” ANR-11-IDEX-0003-01.

-Juan Jesus Gonzalez Aleman, AEMET, Spain 📨jgonzaleza@aemet.es

-Alice Portal, University of Bern, Switzerland  📨alice.portal@unibe.ch 🗣️ Italian, English, French

-Greta Cazzaniga, IPSL-CNRS, France 📨greta.cazzaniga@lsce.ipsl.fr 🗣️Italian, English

-Chen Lu, ICTP, Italy 📨clu@ictp.it 🗣️Italian, Chinese

-Marco Reale, OGS, Italy 📨mreale@ogs.it 🗣️Italian, English

-Carmen Alvarez-Castro, UPO, Spain 📨 mcalvcas@upo.es 🗣️Spanish, English, Italian, French

-Lisa Bernini, University of Genoa, Italy, 📨 lisa.bernini@cimafoundation.org 🗣️ French, English, Italian

-Florian Pantillon, CNRS-LAERO, France, 📨 florian.pantillon@aero.obs-mip.fr 🗣️ French, English, German

-Michael F. Wehner, Lawrence Berkeley National Laboratory 📨 mfwehner@lbl.gov 🗣️ English

-Stavros Dafis, NOA, Greece 📨 sdafis@noa.gr 🗣️ Greek, English

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

-Tommaso Alberti, INGV, Italy 📨tommaso.alberti@ingv.it 🗣️Italian, English