The oxygen (O2) present in water is essential for the respiration of marine organisms. A significant proportion of this O2 comes from the transfer of this gas from the atmosphere to the ocean, where it dissolves in the surface water. This "ventilation" of the ocean can only be achieved at depth by phenomena that carry this surface water to the bottom, where it mixes and enriches the deep waters with O2. The north-western Mediterranean Sea - and more specifically the Gulf of Lion - is one of the few regions in the world where this phenomenon occurs.
In winter, the cold, dry Mistral and Tramontane winds cause the surface water to cool and evaporate, increasing its density. This is when deep convection is triggered: the surface water, which has suddenly become denser, "sinks" and mixes with the water at depths of up to 2,000 m, providing it with large quantities of O2. This new deep water then spreads throughout the western Mediterranean, completing the ventilation process.
To gain a better understanding of this phenomenon, an intensive deployment of new robots, capable of measuring and transmitting O2 concentrations in real time, was launched in the north-western Mediterranean in 2012-2013 during an intense convection episode as part of the MISTRALS DEWEX project. Combined with several at-sea campaigns and a coupled three-dimensional physics and biogeochemistry numerical model, these ground-breaking observations have shown that this region does indeed act as a sink for atmospheric O2 during the two months of winter convection. In addition, we now know that the quantity of atmospheric O2 absorbed by surface waters is equivalent to that breathed in by marine organisms throughout the Mediterranean.
Also as part of MISTRALS, the PERLE project has launched a series of similar observations from 2018 to 2021 in the eastern Mediterranean to study a second region of water convection located between Crete and Cyprus. However, the initial results show that the intensity of the phenomenon is much weaker there than in the Gulf of Lion and that the winter mixing only ventilates the water column at intermediate levels (a few hundred metres).
A major concern is how the ventilation of the Mediterranean's intermediate and deep waters will evolve over the coming decades. It could become less efficient as a result of the warming of surface waters linked to climate change. If their temperature rises, the capacity of surface waters to capture atmospheric O2 will be reduced, and convection will be less intense. The volume of O2-poor water in the north-western basin could then increase within 25 years, which would have a considerable and lasting impact on marine ecosystems.
Authors
Laurent Coppola, Caroline Ulses, Dominique Lefèvre, Thibaut Wagener