On Sunday August 6, on the beach of La Ciotat, near Marseille, many bathers were surprised: the temperature of the Mediterranean showed 13.5 degrees. Far, very far, from its heat record, set just a few weeks ago at 28.71 degrees. This recurring and natural phenomenon, at the origin of this sudden drop in temperature, is called “upwelling”.
From English, it literally means “rise of water” in French. We speak of upwelling when a strong wind, from the land towards the sea, pushes the surface water offshore, heated by the sun, which brings up the colder water from the depths. This phenomenon is proportional to the depth of the water, the more space there is, the fresher the rising water. In the case of La Ciotat, an episode of strong and constant mistral ended the sea heat wave of the last few days by stirring the water, causing the temperature to drop by more than 10 degrees.
For the marine fauna on the contrary, it is a godsend. This mixing allows water from the seabed, rich in nutrients, to rise. In addition to abnormally cold water, upwelling phenomena are also characterized by a high phytoplankton content. In other words, it creates an area conducive to the appearance and development of many fish, corals, marine plants.
This phenomenon is even rather frequent in the Gulf of Lion, in the south-east of France. Regularly subject to constant episodes of mistral and tramontane (wind coming from the northwest), the region has been swept since this weekend by particularly strong windy episodes on the coast, favoring upwelling. According to meteorologist Guillaume Séchet, winds recorded at more than 75 km / h crossed Marseille Saint-Loup, enough to push back the warm waters. For those who are not fond of freezing swimming, “the stopping of the mistral and the arrival of warm currents” at the end of the week will put an end to this episode of sea cold, said Alexandre Isgro, meteorologist at Le Figaro.
