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The oceans are mixing less and less and this is not good news

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The ocean is warmer at the surface than at depth, which makes it stable from a buoyancy point of view. However, this stability has only increased over the past 50 years due to global warming. New work shows that the pace is even much faster than previously estimated. An evolution with multiple implications since it makes the vertical mixing of the oceans more difficult. The results are published in the journal Nature this March 24.

Unlike the atmosphere, the ocean is a fluid envelope heated from above. An arrangement that makes it much more stable than the atmosphere above it. They say the ocean is strongly laminate, like oil on water. In this regard, physicists tend to separate it into two well differentiated layers. A hot sphere going from the surface to a few hundred meters deep and a cold sphere – much thicker – located below.

However, with climate change, the stratification of the oceans is increasing. Indeed, the accentuation of the heating from the top and the increasing arrival of fresh water Going through melting ice and increasing precipitation creates an even more stable arrangement of ocean layers. A consequence long anticipated by physical calculations. However, recent observations show that evolution is six times faster than we thought.

Schematic representation of the ocean, with hot surface and cold depth spheres. The red area indicates the transition between the two – called pycnocline. Credits: Jean-Baptiste Sallée & al. 2021.

Oceans: a decoupling between surface and depth

The researchers involved, from the CNRS, of Sorbonne University andIFREMER studied the evolution of summer stratification between 1970 and 2018. The 50-year series obtained covers a large part of the oceans and shows an increase in stability of around 9% per decade. Moreover, the border between the hot and the cold sphere has deepened by 3% per decade. A value that corresponds to a few meters every 10 years. In addition to the processes mentioned above, the intensification of surface winds has contributed to this trend.

A more stratified ocean articulates an increasingly less efficient coupling between the top and bottom layers. In other words, heat, carbon and oxygen are less easily transmitted to deep water. In practice, it is as if the storage capacity of the ocean were decreasing. A vicious circle that further amplifies the speed of climate change. In addition, the depth of the mixing layer “Shapes marine ecosystems by housing the major part of primary production (…)”, notes the paper in its summary. Thus, the results also have strong implications for the field of living organisms.



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