Heatwave: the cascading consequences of an overheated Mediterranean Sea

To escape the stifling heat, there is no point in jumping into the Mediterranean Sea. The big blue also suffered an exceptional heat wave: 30 degrees were recorded on the surface of the sea off Corsica at the end of July, an extremely rare event, while temperatures were 4 to 6 degrees above normal. between Italy and the Balearic Islands, reports Météo-France.

The situation is such that scientists speak of a "sea heat wave", when the sea temperature is abnormally warm with a impact on ecosystems sailors. Started at the end of May, this heat wave first affected an area located between Italy and northern Corsica, before spreading to other areas of the sea. In early August, the sea heat wave continued around from Corsica and Sardinia and near the Balearic Islands.

“Tropicalization” of the Mediterranean

A set of favorable conditions led to the overheating of the big blue. The absence of clouds for several days in a row, the high temperatures but also "the absence of wind which allows the surface of the sea to remain isolated from the deeper, colder waters" led to the overheating of the waters, explains Samuel Somot, CNRM researcher. And “in the ocean, heat waves last longer than on Earth,” explains the researcher. According to Météo-France, in the current climate, marine heat waves in the Mediterranean last an average of 20 days and cover around 40% of the sea surface at their peak.

This overheating threatens in the first place marine ecosystems also faced with other threats like overfishing. In a June 2021 report , the environmental NGO WWF evokes a "tropicalization" of the warmest waters of the Mediterranean Sea, a "disaster" for the sea. Tropical invasive species arrive from the Suez Canal and settle in waters that were once too cold to survive. them. The rabbitfish, for example, devours the underwater flora, and therefore the natural habitat of other species. Others, like the lionfish, devour Mediterranean fish larvae, enough to "create a big shock on the ecosystems", warns Samuel Somot.

Impacts on tourism and fishing

The heat of the Mediterranean Sea leads to a "reorganization of biodiversity" according to Samuel Somot. The consequences relate not only to ecosystems, but also to tourism and fishing, notes the WWF in its report. The proliferation of jellyfish in a warmer sea bothers fishermen, who "spend hours getting rid of them instead of catching fish", describes the NGO. Their presence on Mediterranean beaches can also repel tourists and hurt the local economy.

Previous sea heat waves, such as that of 2003, have also led to “mass mortality events” marine flora , recalls the CNRM researcher. “All ecosystems such as corals or Posidonia seagrass beds suffer enormously from heat when heat waves last a long time”. However, this flora constitutes a vital habitat for other marine species and is essential for the climate, with certain plants functioning as natural carbon sinks.

More heavy rains

An overheating of the Mediterranean can also affect the weather. The warmer the water in the sea, the more it evaporates and the more water there is available in the atmosphere. This phenomenon can therefore favor intense rains, such as the Mediterranean episodes that generally occur in autumn and can cause flooding.

However, “it rains very little in the Mediterranean in summer. So having a sea heat wave at the beginning of August does not mean much for the events of this fall”, tempers Samuel Somot. “The sea can still cool down this summer with the mistral. For the moment, there is no link between the heavy rains and the sea heat wave”, continues the researcher, especially since other elements participate in the triggering of these meteorological episodes.

But if sea overheating continues into late summer, it could influence autumn weather events in the region. And this, in a context of global warming where ocean heat waves are longer, more extensive and more intense, according to Météo-France.

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