The flood that hit the area was extremely violent Valenciain Spain, on the night between Tuesday 29 October and Wednesday 30 October 2024. To date there are at least 70 confirmed victimsdozens missing and many people trapped at home, as reported by the mayor of Utiel Ricardo Gabadion. The images and videos published by the inhabitants of the area on social media were dramatic, with streets transformed into rivers of water, mud and debris that overturned cars and trucks and dragged with them everything they encountered. Extreme bad weather has hit southern Spain with torrential rain hitting the 490 mm in less than 8 hours in Chiva, in the Valencian Community, of which 340 mm in 4 hours. They are extreme accumulations, comparable to those expected in an entire year in the area (450-500 mm) but fell within a few hours. The rainfall caused floods and floods of extreme violence, with bridges knocked down and cars piled up along the roads. Several tornadoes have also been recorded and there is currently a maximum alert in Barcelona due to the risk of hail and tornadoes. Since this morning the rainfall has decreased (but will continue until Thursday according to the forecasts of the Spanish national meteorological service) and the army is working to rescue citizens, while the Spanish Government has established three days of national mourning for the victims of the disaster .
What is a DANA, the phenomenon at the origin of the violent floods in Valencia
From a meteorological point of view, the cause of this extreme event is what the Spanish meteorological service calls DANA (acronym for Depresion Aislada en Niveles Altoin Italian “isolated depression in high levels”). This is a low pressure system separated from the general circulation of the atmosphere which moves independently of the latter. It is formed when a jet stream which proceeds at high altitude from west to east at speeds even exceeding 200 km/h, ripples and forms an increasingly “strangled” bend until it physically detaches itself from the jet stream.
It is therefore a similar phenomenon to “cold drop”a term by which the phenomenon is best known, especially at a journalistic level. Simplifying a bit, we can say that “DANA” is a technical term that describes a well-defined phenomenon, while “cold drop” is an informal and less specific term regarding the causes and consequences of the phenomenon itself.
Specifically, the DANA that caused floods and floods in Spain is the same disturbance that in previous days caused bad weather in some Italian regions, mainly Liguria, Sicily and Sardinia. A similar high-altitude depression had also been responsible for the ASD Cyclone Daniel which devastated Libya in September 2023. Indeed this phenomenon is not unusual in the western Mediterranean.
The meteorological dynamics of extreme bad weather in Spain: what happened
We have seen that a DANA is essentially a “bubble” of cold low-pressure air no longer controlled and guided by the jet streams. In this specific case, this bubble is dropped in altitude and encountered warmer, more humid air on the groundencountering among other things a Mediterranean Sea with surface waters decidedly warmer than the average for the period (even of 3-4 °C above average). The mild sea supplies the atmospheric disturbance with water vapor, while the pre-existing warm air (which has a greater capacity to accumulate water vapor compared to cold air) in the area is disturbed by cold currents of North Atlantic origin.
Here is the recipe for disaster: atmospheric instability has generated so-called violent thunderstorms V-shaped. This name comes from the characteristic V shape that these structures show in satellite images, due to the particular dynamics of the confluence between cold air and hot air.
These storms were – as often happens – self-healinglike those that plagued the Italian North-West in June 2024. Basically, in self-healing storms the air is “sucked in” from low altitudes and, as it rises, it cools rapidly, causing the water vapor to condense which is continually replenished the storm cell becomes material as the water is released to the ground in the form of precipitation. The self-healing V-shaped storms that hit the Valencian Community were therefore capable of dropping enormous quantities of rain. Plus they were stationaryand this means that they can demonstrate their violence even for hours in the same area. Which is exactly what happened in Valencia.
Be careful, this does not mean that every time a DANA is formed there will be extreme weather phenomena with deaths and destruction. DANA are common events, more frequent in the summer or autumn months, when cold currents of North Atlantic origin disturb areas characterized by high pressure, perhaps near still warm seas. In this case the event was so violent for a series of reasons, including the high sea and air temperatures and the arrival of cold North Atlantic currents.
Torrential rains in the Iberian Peninsula: the role of climate change
This very violent episode happened less than two weeks after yet another flood in Emilia-Romagna. In that case the public debate focused a lot on infrastructure management, but an event as dramatic as the one that hit Spain reminds us that we can’t blame manholes or riverbanks forever. The data indicates that the autumn season in the Mediterranean area is increasingly resembling the monsoon season typical of the Tropics.
In the case of DANA, like the one that caused the floods in Valencia, we reiterate that they are common phenomena and have always happened. We cannot therefore say that climate change isolated a low-pressure zone over southern Spain. However, we can say that climate change increases the probability of DANA frequency (because it weakens the polar jet streams, making the formation of vertical motions more likely which can translate into bottlenecks and “cold drops”) but above all that it causes more and more frequent atmospheric disturbances in conditions of causing enormous damage, mainly by supplying the atmosphere and seas with thermal energy.
