In the Mediterranean, the marine heat wave is accelerating the replacement of species

Are the ecosystems of the Mediterranean Sea irremediably in danger? This is what scientists are beginning to believe, in the light of the sea heat waves which are increasing in the Big Blue. This summer, the phenomenon is particularly worrying.

A differential of 6.5°C compared to the seasonal normal was measured by satellite on July 19 off Marseille. It is since the month of May, i.e. more than seventy days in total, that the sea has "very significant temperature anomalies", notes Samuel Somot, specialist in heat waves in the Mediterranean at Météo-France. This concerns waters between 0 and 20 meters deep, between the Balearic Islands and the Spanish Levant coast, but also facing the French Riviera, in France, all along the Italian boot, from the Ligurian Sea to the Gulf of Taranto, and all around Corsica.

These figures approach – if not exceed – the two previous records recorded in the recent past. That of intensity, observed on August 5, 2018 off Marseille, with a water temperature 6.6°C higher than the seasonal norm, and that of duration, with a heat wave that had spread, in 2003, from August 3 to September 2.

“In the atmosphere, a heat wave never lasts that long. In the sea, yes. On the French coasts, the phenomenon experienced peaks at the end of May, end of June and end of July, which should be followed by a further increase in early August., says the expert from Météo-France. Opposite Marseille, the sea heat wave is "extremely intense", with 28 ºC measured on July 19, instead of a seasonal average of 21.5 ºC. Opposite Bastia, it is “continues since the end of May”, with a water temperature of around 30 ºC. Same thing in the Balearic Islands.

Plus 1°C in twenty-seven years

“These marine heat waves are triggered during a lasting situation of high temperatures and stable conditions, cloudless sunshine, calm winds. The surface water no longer moves and the cold waters at depth can no longer rise, causing enormous suffering to corals, crustaceans, fish…”, explains Rubén del Campo, spokesman for the Spanish National Meteorological Agency. Until the first days of August, the thermostat effect of the Mediterranean – i.e. its ability to regulate outside temperatures – could be annihilated: “Normally, in winter, the sea tempers the cold and in summer, it tempers the hot. Right now it's not, especially at night. »

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