We have rightly spoken of a “cold drop” phenomenon, associated with a Mediterranean episode.
On Tuesday, October 29, when storms stalled in the Spanish region of Valencia, they ravaged entire suburbs, killing more than 200 people and leaving dozens of people missing.
A pocket of cold air broke away from the upper altitude currents of northern Europe and reached the Iberian Peninsula, further south.
The thermal contrast between this cold mass and a warmer atmosphere caused extreme instability. The storms locally poured more than 618 mm of water per square meter in a few hours. To put it in perspective, we can say that this quantity of water is the equivalent of a year of rain in some of our Var or Riviera localities.
Near Valencia, we have to go back to September 11, 1996 “to find such an intense episode. We measured 520 mm of precipitation in 24 hours”note the Météo France services. Here are four key points to identify the causes.
Because of the climate?
Around the world, researchers mobilize around each major meteorological event. These are so-called “attribution” studies. Climate change is neither a systematic nor a single cause. Its influence should not be denied either. A very first response has just been given by the WWA (World Weather Attribution). The collective does not yet comment on a precise attribution, but recalls a context. “Rainfall concentrated on a single day is 12% more intense in today’s climate than it would have been in the climate of the pre-industrial era.” As for the probability of frequency of these rains, it is “multiplied by two”.
Expected in the coming weeks (or months), the attribution study will say precisely whether Spain had a greater risk of being affected, because of the effects of climate change.
Observations ou projections?
If climate models anticipate a worsening of the risk for decades to come, reality is already catching up with us.
Météo France counted the number of days with high accumulations (above 150 mm of water per square meter). “Since 1961, we have observed an increase in intense precipitation events on the French Mediterranean rim, describes Hélène Correa, head of the south-east climatology service at Météo France. That is to say rain which gives significant accumulations in 24 hours.”
The number of days with more than 200 mm of precipitation has already doubled, between 1961 and 2023. What will happen in the future? “The intensity and frequency will increase in the northern part of the Mediterranean basin.” A trend that is all the stronger the higher the warming.
The Mediterranean and the atmosphere
Where does this water come from? First of all, there is a purely physical principle. “It depends on the temperature. The warmer the air, the more water vapor it can hold, dissects Hélène Correa. For each additional degree, the air can be enriched with 7% more water vapor.”
This water comes from the nearby environment. “Warmer air causes greater evaporation of soils, which dries them out.” But it’s not just the floors… “In this case, we have the Mediterranean, so it’s easy to have evaporation. The air masses that arrive above our heads, from the sea, will often be loaded with water vapor. “
When the vapor condenses and falls, it makes rain “potentially very intense, with staggering quantities of water”. In October, we experienced numerous sequences of this type – Roquebrune-sur-Argens, Le Luc, Vidauban, La Motte, Fréjus, Mandelieu, Vence… With more or less serious consequences.
An acceleration
The driving mechanism behind all these phenomena is, and remains, the increase in air temperature. Whose evolution is accelerating. “By averaging the temperatures over a period of ten years, we see that the slope is accelerating”notes Hélène Correa.
For thirty years, “each new decade is above average, it’s visible to the naked eye” (reference 1961-1990). Not a single decade is colder, or fresher.
With +2.2°C, our Region is already suffering from particularly marked warming, compared to other regions of the globe.
But where does this excess heat from the atmosphere go? 90% of it is absorbed by the Ocean. A semi-enclosed sea, the Mediterranean is already overheating – a new record of 28.9°C was recorded in mid-August 2024. The warmer the water and air are, the more there is this evaporation mechanism.
At the other end of the chain, greenhouse gas emissions caused by human activities continue. And broke a world record in 2023 (1).
1. With 57.1 gigatons of CO2 equivalent, or +1.3%. Calculation by Citepa, Interprofessional Technical Center for Atmospheric Pollution Studies.
**Interviewer:** Thank you for joining us today, Dr. Correa. The recent storms in Valencia have been devastating, resulting in significant loss of life. Could you explain the meteorological phenomena that led to this catastrophe?
**Dr. Correa:** Thank you for having me. What we experienced is a classic case of a “cold drop” phenomenon. Essentially, a pocket of cold air from northern Europe detached and moved southward into the Iberian Peninsula. This cold air created a thermal contrast with the warmer atmosphere, resulting in extreme atmospheric instability.
**Interviewer:** It sounds like the intensity of the rainfall was extraordinary. Can you put that in context?
**Dr. Correa:** Absolutely. Near Valencia, we saw rainfall amounts exceeding 618 mm in just a few hours, which is equivalent to a year’s worth of rain for many localities. This is a stark reminder of the severity of these weather events; we haven’t seen such intensity since 1996 when 520 mm fell in a day.
**Interviewer:** There seems to be an ongoing debate about the role of climate change in such events. What insights can you provide on this issue?
**Dr. Correa:** The connection between climate change and these extreme weather events is complex. Attribution studies are currently being conducted to assess the influence of climate change on this specific episode. While it’s difficult to ascribe any singular event to climate change, we do know that rainfall intensity has increased by about 12% today compared to pre-industrial times, and the likelihood of these types of downpours has doubled.
**Interviewer:** With this increase in extreme weather, what trends are we observing in long-term climate data, especially around the Mediterranean?
**Dr. Correa:** Our analysis shows a marked increase in intense precipitation events in the French Mediterranean region since 1961. Days with over 200 mm of rain have doubled in frequency in that time. Looking ahead, we expect that both the intensity and frequency of such events will continue to rise, particularly in the northern Mediterranean as temperatures increase.
**Interviewer:** And could you explain how warmer temperatures are influencing the amount of water vapor in the air contributing to these storms?
**Dr. Correa:** Certainly. Warmer air has a greater capacity to hold water vapor; specifically, for each additional degree of warming, the air can hold about 7% more moisture. This means that evaporation rates from soils and bodies of water, like the Mediterranean, increase, leading to more humid air and, subsequently, more intense rainfall when conditions are right.
**Interviewer:** Thank you, Dr. Correa. Your insights are invaluable as we seek to understand the relationship between climate change and our changing weather patterns.
**Dr. Correa:** Thank you for having me. It’s crucial that we continue to monitor these changes and adapt our responses to prevent future disasters.
