How a meteotsunami contributed to the exceptional flood in Venice on 12 November 2019

Extreme sea levels at the coast result from the combination of astronomical tides with atmospherically forced fluctuations at multiple time scales. On 12 November 2019, an exceptional flood event occurred in Venice, second only to the one that occurred on 4 November 1966. The maximum recorded sea-level height of 189 cm above local datum determined the flooding of more than 85% of the pedestrian surface of the historical city. The event that struck Venice and the northern Adriatic Sea on 12 November 2019, although having certain conditions seemingly typical of the events causing exceptional high waters, had some peculiar characteristics not observed before, which deserved an in-depth analysis. Several factors made this event exceptional. Among these, a fast-moving local depression - and the associated wind perturbation - travelled in the north-westward direction over the Adriatic Sea along the Italian coast. The speed of the depression was about 12 m s-1 and it propagated along the Italian coast characterized by offshore depths of O(10 m). High- resolution numerical simulations indicated that the atmosphere-ocean resonance was at work on 12 November 2019 and that the depression moving along the Italian coast generated a meteotsunami-like wave with a maximum amplitude of 28 cm that propagated into the Lagoon of Venice. Moreover, sea level in the City of Venice was further raised by the strong south-westerly wind blowing over the lagoon after the passage of the perturbation. Wind and waves have hit the southern side of Venice, where Saint Mark's Square and the classical promenade are located, causing damage to boats and infrastructure. According to the analysis of the 1940-2019 sea-level dataset, the meteotsunami that hit the coast in front of Venice on 12 November 2019 represents an extremely rare event.