The ocean’s centre of mass has shifted – affecting climate and ecosystems
The world’s oceans have become increasingly stratified over recent decades due to global warming. This is shown by a new study from the University of Gothenburg. The reduced mixing of layers can both disrupt marine ecosystems and reduce the ocean’s ability to store heat and carbon dioxide.
The ocean’s water mass is organised in different layers that must be constantly mixed for the ocean to function. But as stratification increases, the lighter surface water remains at the surface and no longer mixes with the heavier deep water. The ocean becomes stagnant.
“This is an important indicator of ongoing climate change in the ocean. What we see is not just that stratification is increasing, but that it is doing so steadily over time,” says lead author Fabien Roquet, Professor of Oceanography at the University of Gothenburg.
Linked to climate change
The study demonstrates that the increase in stratification is primarily driven by warming in the upper layers of the ocean, as the ocean absorbs more and more heat from the atmosphere.
“That also shows that the increased stratification is closely linked to global warming,” says Prof. Roquet.
The consequences of increased stratification are many. It becomes more difficult for the ocean to absorb heat and carbon dioxide because the surface water is not transported down into the depths but remains in the upper layer. Also, the nutrient-rich and cold deep water does not reach the surface to sustain marine ecosystems.
“It also affects major ocean currents, biogeochemical cycles, and oxygen conditions,” says Prof. Roquet.
No collapse of AMOC
There are significant regional differences in the study’s findings. The most significant increases are found in the tropical regions of the Indian Ocean and the Pacific Ocean, in major western boundary current systems, and in the Arctic. At the same time, stratification has weakened in much of the subpolar North Atlantic, that is, the waters between northern Europe, Greenland, and the British Isles.
“It’s interesting, because present research suggests that increased stratification would weaken the AMOC current system, of which the Gulf Stream is a part. But here, we have found a decrease in stratification over the last few decades, which is unexpected. Our research therefore does not indicate any imminent collapse of the AMOC; quite the opposite, in fact,” says Prof. Roquet.
New method measures ocean's centre of mass
In the study, the researchers used a completely new method to measure stratification. They measured where the ocean’s centre of mass was located. In other words: the point that indicates whether more of the ocean’s water mass is sitting relatively high or deep.
This is because the centre of mass shifts as stratification in the ocean increases – it lies deeper. The greater the stratification, the more dense water sits at the bottom, pushing the centre of mass down. This method provides a more stable measure of long-term stratification change than conventional approaches.
“The ocean sends clear signals. By measuring the ocean’s centre of mass, we make the ocean’s complex climate signals easier to see and explain. This is important for researchers to be able to track how the ocean is changing in line with climate change,” says Fabien Roquet
Research article in Tellus: Global Stratification Trends Diagnosed Using the Center of Mass of the Ocean
Contact: Fabien Roquet, Professor of Oceanography at the University of Gothenburg, telephone: +46 760-81 48 68, email: fabien.roquet@gu.se
Writer: Annika Wall
