An atmosphere of 30.8 ° C, it is more and more frequent in summer. Seawater at 30.8°C is much less so. This record temperature was reached last August in the northwestern Mediterranean, shortly after the third heat wave that the France suffered. This sea area was 2.6°C warmer than normal, on average, over the entire summer season. Locally, we even reached + 7.9%!
By what mechanisms can the sea warm so much? Atmospheric temperature, humidity, cloud cover and solar radiation, wind... Surface heat waves are directly responsible and sometimes in a very short time, details a study Météo France/CNRS published on May 12 in the journal Ocean Science.
"2022 was an exceptional year"
Ocean warming is linked in particular to "atmospheric blockages" on the surface, when a large anticyclone forms and blocks the arrival of any disturbance. Air temperatures — generally measured between 1m50 and 2 m above the ground — above normal and combined with low wind can, on their own, increase the temperature of the water at the surface by 2 to 4 °C.
The evolution of the sun's radiation does not have much impact on the thermometer in the Mediterranean Sea, because it is very often sunny in the south of the France in summer. As a result, exposure to the "star of the day" is regularly close to its maximum level. On the other hand, the sun's rays accentuate the warming of the water in the west and north of the France, less often accustomed to filling up with sunshine.
The researchers were surprised to find that the basins north of the France had also warmed up a lot last summer. "In the English Channel, for example, during the period studied, there were large tides that stir the water a lot, which usually limits surface warming," says Thibault Guinaldo, a researcher in space oceanography at the National Center for Meteorological Research in Lannion and lead author of the study. "The degree of warming of this area has marked us a lot. We can logically say that 2022 was an exceptional year and will become a reference for study," he adds.
More heat also means more thunderstorms
Another striking finding: sea temperature reacts quickly to atmospheric conditions. In other words, a few days of strong heat felt on earth can be enough to make it climb. However, warmer water promotes evaporation and can lead to more moisture in the air, and therefore more stormy Mediterranean episodes that dump a lot of rain.
As heat waves are expected to become more intense and frequent in the coming years, "this will have an impact on the ocean with more frequent and/or more intense marine heat waves," anticipates the researcher. According to an IPCC report on "the ocean and cryosphere in the context of climate change", published in 2019, there could be 20 to 50 times more between 2081 and 2100 than over the period 1850 – 1900, depending on the scenario chosen for greenhouse gas emissions.
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Far from being limited to the formation of storms, the possible consequences of ocean warming are numerous: disappearance of corals, disruption of the migration of certain fish, rise in sea level, because water takes up more space by expanding, etc. "When warm water plunges below the surface, it can lead to ecological disasters," says Thierry Perez, CNRS research director at the Mediterranean Institute of Marine and Continental Biodiversity and Ecology.