Event Description

The Mediterranean depression named Daniel has brought, on 5 September 2023, extreme rainfall to the southeastern Mediterranean region, affecting Greece, Turkey, and Bulgaria. Red warnings were issued by the Hellenic National Meteorological Service for "particularly severe" weather with heavy rain and storms, the highest category of warning for dangerous weather phenomena in Greece. Depression Daniel is a result of an Omega blocking pattern. Omega blocks are characterized by a high-pressure system forming in the shape of the Greek letter "Omega," resulting in a deflection or weakening of the large-scale climatological west to east atmospheric winds -- known as jet stream. This disrupts the normal west-to-east movement of weather systems, effectively creating stagnant or slow-moving weather conditions in the affected region. In the case of depression Daniel over Greece and surrounding countries, the Omega block has caused the depression to remain in the region for an extended period. As a result, the depression continuously drew in moist air from the Mediterranean Sea, allowing it to intensify and result in extremely heavy rainfall. It has also triggered waterspouts off the coasts of Greece and southern Italy. The depression has caused fatalities and widespread flooding, as well as setting a new national daily precipitation record for Greece of 754 mm.

Depression Daniel has wreaked havoc across Greece, Turkey, and Bulgaria, leading to widespread flooding and at least seven casualties. The deluge of rain triggered flash floods, including in Istanbul, Turkey's largest city. In Greece, the rainfall led to fatalities and extensive property damage. 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 over a large part of continental Greece.

Climate and Data Background for the Analysis

The IPCC AR6 report - which addresses the Mediterranean region in Sections 10.6.4, 12.4.1 and 12.4.5 - acknowledges that Mediterranean cyclones are occurring increasingly frequently in summer. The IPCC AR6 report underscores that sea-surface temperatures (SSTs) in the Atlantic Ocean and Mediterranean have risen by 0.25°C to 1°C since 1982–1998, serving as a crucial energy source for these powerful convective events. Despite the substantial uncertainty and natural variability associated with convective events, the report points to a noticeable correlation between summer Mediterranean cyclones and warmer SSTs, alongside heightened marine heatwaves.  The report refrains from providing historical trend statements on extreme precipitation in the Mediterranean area.  Moreover, Chapter 11 of the IPCC AR6 report emphasizes the complexity of synthesizing trends in severe convective storms across regions, as definitions vary and observations of phenomena like tornadoes, hail, and lightning associated with such storms lack robust detection due to limited long-term coverage. Consequently, specific statements about convective storm changes in the Mediterranean region are withheld.  

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

ClimaMeter Analysis

We analyse here (see Methodology for more details) how events similar to 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). The Surface Pressure Changes show that the surface pressure over the affected area has become lower, favouring more intense depressions in the present than in the past. Precipitation Changes show that similar events produce larger (between 4 and 12 mm/day) amounts of precipitation over continental Greece and the Peloponnese, which are among the areas mostly affected by extreme rainfall on Septemrer 5th. Increased precipitation is also observed in Greek urban areas targeted for the analyses with an increase of 4 mm/day or more in Athens and Thessalonik, while no significant changes in precipitation are detected for Istanbul. Although not included in our analysis, we note that the general increase in precipitation is compatible with a thermodynamic contribution provided by the increasingly warm Mediterranean Sea, which provides ample potential energy to fuel convective systems over the region. We also find that Similar Past Events have become more frequent in June, while they previously occurred chiefly in July. No change in the frequency of events like Daniel is observed in September.

Finally, we find that sources of natural climate variability did not influence the event. This means that the changes we see in the event compared to the past may be primarily due to human driven climate change.

Conclusion

Based on the above, we conclude that Mediterranean depressions like Daniel show lower atmospheric pressure and higher precipitation in the present than in the past. We interpret depression Daniel as an unusual event whose characteristics can be ascribed to human-driven climate change.

Contact Author

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