While the summer of 2023 was the hottest ever recorded in the world, according to the Copernicus Institute, despite sometimes gloomy weather in northern France, a late heat wave hit part of France this week. Western Europe and in France, where 47 departments are placed on heat wave yellow alert. Spain and Greece are swept away by torrential rains.
Three people died in Spain, and a year’s worth of rain in Paris fell within 24 hours in Greece. A person also died there on Tuesday in the Volos area, where extensive damage was found. This road, for example, linking Volos to Portaria, in central Greece, was completely destroyed.
The road from Volos to Portaria, at the height of Ano Volos and higher, was destroyed.
These two phenomena are linked within a “complex meteorological structure” called “omega blocking”, explains CNRS research director and deputy director of the dynamic meteorology laboratory at ENS Fabio d’Andrea. Concretely, when we look at a weather map, the Greek letter ? is drawn.
The current
The “O” stands for an anticyclone, an area of ??high pressure, currently placed over France, Germany and Scandinavia, where it is responsible for the high temperatures and sunny weather of the past few days. Two areas of low pressure, depressions, or “cold drops”, are joined to it, forming the “legs” of the omega: one is located on the Mediterranean, in Greece, and the other off Portugal, responsible for the rains in Spain. They are supplied with cold air by a persistent northerly flow, with humid and cool weather.
“Blockages, including omega blockage, are common but complex weather phenomena, the causes of which we don’t yet understand,” continues Fabio d’Andrea. Concretely, the waves which normally propagate in the atmospheric jet, this great river of air which plays a major role in atmospheric circulation at our latitudes, remain frozen for a period which can last from a few days to two weeks. »
The omega blockage we are going through is thus persistent and bears strong anomalies – an abnormal temperature for the season, very intense rains -, which is not always the case.
Scientists are able to predict its arrival and reproduce it in numerical models, but they do not yet understand all its mechanisms. It is therefore difficult to determine the role played by global warming in its formation or its prevalence. “It is a challenge to understand what is the future of this blocking phenomenon in the context of global warming”, underlines Fabio d’Andrea.
“We must think of global warming as a change in frequency and intensity of complex processes, such as these blocking phenomena, and not limit it to the only variables of temperature or precipitation, details the specialist. We are working on it, but if these phenomena become more frequent, and are accompanied by heavier precipitation, as indications suggest, even if there is not yet a scientific consensus on the matter, they could result in both more stable and more intense weather. »
However, omega blockages are nothing new: about 10% of summer days and 20% of winter days are blocked days, resulting in a heat wave or cold snap. The current one is expected to last through Monday or Tuesday, until the low over the Iberian Peninsula eventually moves eastward. The “tripod” will then deform and lose its characteristic shape.
