Why hasn't summer arrived yet ?

© Joy Stam/Unsplash

For the moment, we cannot say that we are crushed by the Sun, and that is an understatement! Since the end of winter and this spring, we were literally drowned in the rain. Quite exceptional rainy episodes which continue and which have already marked the start of the summer season. Cumulative amounts of more than 200 mm were recorded in the Alps and the Massif Central, floods affected several regions, notably in Gard and Deux-Sèvres. Impressive storms hit several towns in the country on June 19… What is happening ?

Contrasting with recent summers marked by heatwaves and pronounced drought, this summer period is very different from the others, you are not dreaming. This phenomenon is due to a combination of various meteorological and climatic factors that we will dissect here.

A disturbed atmosphere on a large scale

The global atmospheric circulation or general circulation of the atmosphere (GAC) played a major role in the meteorological configuration of this summer. CGA is the large-scale movement of air masses that surround the Earth. This complex circulation is the result of the interaction between several forces: radiation from the Sun, the Coriolis force due to the rotation of the Earth and gravity. This influences the redistribution of heat and humidity on the Earth's surface and has been disrupted by several factors.

Winds from the west have dominated over Europe, bringing with them areas of low pressure that cause disturbances and rain. The latter have generated recurring disruptions and persistent precipitation across the continent.

In addition, large-scale atmospheric variations, such as < strong>the North Atlantic Oscillation (NAO) and the Pacific Multidecadal Oscillation (PDO), influenced these circulation patterns. The NAO is a natural fluctuation in atmospheric pressure over the North Atlantic that directly affects precipitation in Europe. The PDO, for its part, is a natural cycle of variability in the surface temperature of the Pacific Ocean which oscillates between warm and cold phases over a period of approximately 20 to 30 years, influencing the large-scale climate models in the northern hemisphere.

As for ENSO (El Niño – Southern Oscillation), although its influence is less direct, it can also disrupt global weather patterns and atmospheric circulation. Even though it was in neutral this year (sea surface temperatures in the equatorial Pacific Ocean were near normal), it is possible that it had an influence on the climatic situation.

Another big culprit in changing this dynamic is global warming. This, abnormally warming the oceans, leads to an increase in atmospheric water vapor and thus fuels the formation of clouds and therefore the risk of precipitation .

200% Deposit Bonus up to €3,000 180% First Deposit Bonus up to $20,000

An unusually warm Mediterranean Sea

Regional conditions played a determining role in this summer's unique weather pattern. The abnormally high temperature of the Mediterranean surface generated a positive anomaly, intensifying evapotranspiration (the combined passage from liquid water to ;#8217;gaseous state on the earth's surface through evaporation and transpiration of plants) and increasing the content of atmospheric water vapor.

There too, this phenomenon fueled the formation of clouds and precipitation over France, fitting into a broader context of complex ocean-atmosphere interactions as described previously.

A stubborn atmospheric depression over Western Europe hampered the passage of anticyclones between the end of May and the beginning of June, maintaining humid and rainy conditions over a large part of French territory. These barometric anomalies were a key factor in the persistence of unstable weather conditions.

Local influences

À On a local scale, the relief also shapes the local climate, and French mountainous areas have exacerbated precipitation; we call this phenomenon: the’orographic effect. Imagine a mass of air laden with moisture, sliding across the plains, until it suddenly encounters an obstacle: a mountain. Forced to rise, this air engages in an ascent which gradually transforms it. As it climbs, its temperature drops, causing the water vapor to pass into droplets which, gathered together, form clouds then pour out as rain.

In France, this phenomenon takes on a particular magnitude thanks to the presence of imposing massifs such as the Alps or the Massif Central. These stone giants intercept ocean air flows, usually from the west, forcing them to rise abruptly. This rapid rise triggers intense cooling, causing massive condensation of water vapor. This results in copious precipitation, thus explaining the rainfall records frequently recorded in these mountainous regions.

L&amp ;#8217;summer we are experiencing this year is the perfect illustration of what a combination of factors can cause on a large scale (CGA, ONA, PDO, ENSO), medium scale (warming of the Mediterranean) and small scale (local geographical particularities). Several studies, through the use of climate simulations, show that climate change is likely to increase the intensity of extreme precipitation in France. The east and south of the country would be the areas most affected. However, there is not yet a firmly established scientific consensus on this subject and research continues. We will therefore certainly have to get used to this type of climatic anomaly. Now all we have to do is wait for the next heatwave!

• The start of summer 2024 in France was marked by unusual precipitation and below-normal temperatures.
• Global atmospheric circulation, atmospheric pressure oscillations, and global warming contributed to these unusual conditions.
• Other local conditions also played a role, including France's mountainous terrain that forced moist air to rise and condense.

📍 To not miss any news from Lemon Squeezer, follow us on Google News and WhatsApp.