Cul-de-sac effect: Why Mediterranean regions are becoming more prone to extreme floods in a changing climate

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In May 2023, devastating floods hit Emilia-Romagna, causing deaths, displacement, and estimated damages of €8.5 billion. Cul-de-sac effect: The CMCC research team described for the first time how a specific configuration of mountain topography and circulation patterns trapped moisture coming from the Adriatic, while a stationary cyclone fueled prolonged rains that lasted for several days over the same area, leading to extreme flooding. From the analysis of the Emilia-Romagna case, the research suggests that the same effect could occur in geographical areas with a similar orographic conformation in the Mediterranean region. Extreme events of this magnitude, which have been very rare so far, could become more frequent in a continuously warming Mediterranean climate, as demonstrated by research analysing trends over the past 40 years. CMCC researchers have developed cyclone density persistence, a new metric that could prove useful in improving early warning systems. From the analysis of past events and in light of the cul-de-sac effect, the study highlights the urgent need for better monitoring, forecasting, and preparedness across Mediterranean regions.  

In May 2023, Italy’s Emilia-Romagna region experienced devastating, if not unprecedented, floods that caused widespread damage to infrastructure, homes, businesses, and farmland. Seventeen people lost their lives, and the disaster caused an estimated €8.5 billion in damages. The persistent rainfall and resulting landslides and flooding displaced tens of thousands of residents, leaving a deep mark on the region’s economy and communities.

New research from the Euro-Mediterranean Center on Climate Change (CMCC) sheds light on why this extreme event occurred, the role that climate change may have played, and what it could mean for the future of the Mediterranean region and beyond.

The study “A cul-de-sac effect makes Emilia-Romagna more prone to floods in a changing climate,” published in Scientific Reports, finds that the extreme flooding was not the result of a single extreme precipitation event, but was fueled by prolonged accumulation of rain over several days.

CMCC scientists describe a “cul-de-sac” effect, where mountains block moisture from the Adriatic Sea, trapping rain over the region and causing extremely rare floods which, statistically, would be expected to happen only once every 500 years.

Researchers identified a stationary cyclone over central Italy as a key driver, channeling humid air from the Adriatic Sea toward the region of Emilia-Romagna. A similar configuration is also believed to have led to flooding in the same area in 2024.

“Our analysis shows that the kind of persistent cyclone that caused the 2023 and 2024 Emilia-Romagna floods is not unique to this region. Other Mediterranean areas with similar geography could face the same risks – and these events may become more frequent as the climate continues to change,” says CMCC senior scientist Enrico Scoccimarro, first author of the study. “This has important implications for how we design and operate early warning systems.”

The study introduces a new metric, cyclone density persistence, which could help develop more effective early warning systems. By tracking the presence and persistence of such cyclones, forecasters may improve predictions of extreme precipitation events, not only for short-term forecasts but potentially for seasonal outlooks as well.

“This work represents the first step in a long-term plan to develop Early Warning Systems for flood events on a seasonal timescale at CMCC,” says Scoccimarro. “We aim to assess the capability of our numerical models and AI-based tools to forecast the large-scale conditions that lead to extreme precipitation events. Our goal is to go beyond current limitations of state-of-the-art numerical models, and aim at higher skills in directly forecasting extreme precipitation, and help communities better prepare for floods.”

Analysis of historical data suggests that conditions favoring these prolonged, extreme rainfall events have been increasing over the past 40 years. With climate change, these “cul-de-sac” floods may become more frequent – highlighting the urgent need for improved monitoring, forecasting, and planning across vulnerable Mediterranean regions.

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