The Mediterranean was not so refreshing last August. Up to 30.8°C was measured on August 4, on the surface of the sea, off the coast of France, during the heat wave. And the warming of the atmosphere during these heat waves has a direct link with that of water, demonstrate researchers from the National Center for Meteorological Research (CNRM) in a study published May 12 in the journal Ocean Science.
The data collected during the meteorological summer of 2022 (June 1 to August 31) "broke records," the researchers point out. The water in the Bay of Biscay was, on average, 1.3°C warmer than normal (between 1982 and 2011), the English Channel 1.4°C and the Mediterranean 2.6°C warmer. And even more so during the heat wave in early August, the temperature anomaly reaching +3.1°C in the Mediterranean.
The "crucial role" of wind
However, the three maritime facades reacted more or less intensely to this heat wave, making it possible to isolate the weather conditions that are taken into account in the warming of the water. Thus, if the Mediterranean saw, on the French side, its temperature increase during the heat wave, the phenomenon was less marked than on the other two coasts. The excess of solar radiation to which the Bay of Biscay was exposed contributed to a rise in the thermometer, while the Mediterranean reacted less because the sunshine is already high in normal times.
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If the heat wave quickly warmed the water on the Atlantic coast, the temperature then fell sharply before climbing again when that of the atmosphere remained very high, note the researchers, attributing this to strong surface winds, greater than normal. This facilitated the mixing of the water and therefore its cooling. Winds would therefore play "a crucial role" in stopping runaway temperatures underwater. Conditions that did not benefit the Mediterranean last summer.
The frequency of these marine heat waves has already doubled since 1982, says Météo France, adding that they are expected to multiply with global warming. In its 2019 report on the oceans, the IPCC estimates that they have absorbed "more than 90% of the excess heat accumulated in the climate system". A warming that represents a direct threat to underwater biodiversity whose natural environment is disrupted.