Several thousand profiling floats are constantly drifting in the ocean and measuring its temperature and salinity, according to our partner The Conversation.
Next-generation floats, called Deep Argo floats, can now unlock the secrets of the abyss by reaching depths of 4,000 to 6,000 meters.
This analysis was conducted by Damien Desbruyères, researcher in physical oceanography at the French Research Institute for the Exploitation of the Sea (Ifremer).
The Argo program constitutes a revolution in the history of ocean observation, essential to the study of climate change.
It relies on an army of more than 4,000 profiling floats that constantly drift in the ocean and measure its temperature and salinity (and sometimes biogeochemical properties such as oxygen, pH, or chlorophyll a concentration).
These autonomous robots, capable of diving from the surface to 2000 meters deep, provide scientists with a robust description of half the volume of the global ocean.
But how to pierce the mysteries of the abyss?
Thanks to a new generation of floats, called Deep Argo floats, which can go down to 4000 or even 6000 meters deep.
They are now deployed by scientists as part of the flagship "OneArgo" mission which aims, over the next twenty years, to map and study the global ocean and its marginal seas, at all depths and by integrating the measurement of parameters biogeochemicals, essential for monitoring changes in the health of marine ecosystems or better understanding the carbon cycle.
Withstand the pressure of the seabed
To probe such depths, researchers and engineers had to overcome a series of technological challenges.
The pressure under 2,000 meters of water column is indeed very high – at a depth of 4,000 meters it is 400 times greater than at the surface – and the global warming signal is very attenuated there.
They therefore had to improve the performance of the sensors or even adapt the hydraulic system allowing the float to lower or raise in the water column, while limiting the onboard energy necessary for autonomous operation for several years.
The only other devices capable of measuring the physical and biogeochemical properties of the deep ocean are "bathysonde" rosettes, a set of sensors and sampling bottles placed at the end of an electrocarrying cable that researchers unroll at specific locations from oceanographic vessels.
These occasional high-resolution and precise measurements will be a considerable asset for checking and possibly correcting the data transmitted over the water by the Deep Argo floats.
Deep Argo, the thermometer of the abyss
Deep Argo floats will soon make it possible to accurately quantify the increase in the average temperature of the global ocean between the surface and the ocean floor, but also to identify the regions and ocean layers most impacted by ongoing climate change.
It is estimated that the global ocean absorbs more than 90% of the excess heat generated by human activities, due to a storage capacity approximately 1000 times greater than that of the atmosphere.
The ocean therefore significantly dampens atmospheric warming and climate imbalance.
On the other hand, this heat storage causes almost half of the current rise in sea level. Indeed, the more the temperature of the water increases, the more the molecules that compose it move and take up space. , the more it occupies a large volume.
In addition, warming accentuates the stratification of the ocean by increasing the temperature contrast between the warm surface layers and the deep cold layers.
This stratification affects certain key mechanisms for climate regulation, such as the mixing of water masses and large-scale ocean circulation, the transfer of gases between the ocean and the atmosphere, and even primary biological production.
Heat storage, how deep?
The first estimates from the "bathysonde" rosettes reveal that the ocean below 2000 meters depth would provide 10 to 15% of this storage, and that it is the southern ocean which would show the strongest warming so far. .
But strong uncertainties still persist, because the cost of these deep measurements by boats makes them relatively rare and rather dispersed.
Only Deep Argo floats will be able to provide access to geographically detailed and temporally continuous mapping of water temperatures below the 2000 meter mark, and allow scientists to draw up adequate global and regional heat budgets.
In the Brazil Basin, for example, a fleet of 35 Deep Argo profiling floats deployed by an American team has already made it possible to precisely quantify an abyssal warming in progress, possibly induced by a slowdown in the cold water supply of southern origin to this region.
Being able to explain such temperature changes in the abyss, in particular by dissociating anthropogenic causes and natural movements of the ocean, is a major challenge to which Deep Argo will strongly contribute.
Mapping the highways of the abyss
Deep Argo also aims to characterize large-scale ocean circulation through their drift trajectories at 2000 meters or 3000 meters depth, or through their temperature and salinity measurements.
This will make it possible to better understand what happens to the stored heat by apprehending, among other things, the transport and circulation of the two large masses of water that line the large ocean basins: North Atlantic deep water, formed around the Greenland, and Antarctic bottom water, formed on the continental shelves of Antarctica.
Both participate in the great "conveyor belt" of ocean currents, generated in part by the different densities of seawater. This ocean circulation, known as "thermohaline", plays an active role in climate regulation by connecting the surface – and therefore the atmosphere – and the great depths, and by slowly redistributing key properties, such as heat, oxygen or carbon, over thousands of kilometres.
The simple trajectory of a Deep Argo float can already break certain prejudices, by revealing still unsuspected deep ocean routes.
One of the Deep Argo profiling floats deployed by Ifremer between 2015 and 2017 in the Charlie Gibbs fracture zone in the North Atlantic, for example, directly joined the continental slope of Newfoundland in Canada, instead of taking a more "classic", in the direction of Iceland and Greenland.
In the longer term, when the network of Deep Argo profiling floats has achieved global, homogeneous and lasting coverage, a map of the deep circulation could finally see the light of day, like the one from the first generation floats drifting at 1000 meters depth, or from satellite altimetry data for the surface layer.
This would represent a colossal source of information and validation for ocean and climate modellers, who still struggle to numerically represent deep-sea currents and the complex dynamic processes that govern them.
Deep Argo to better predict "the ocean it will make"
The program should also enrich the world of ocean and climate modelling.
The measurements will indeed improve the realism of operational oceanography simulations, in order to predict “what the ocean will be like” from one season to another or from one year to another.
The more observations scientists have, the more accurately they can model deep ocean dynamics and its impact on the surface ocean and other components of the climate system, such as the atmosphere.
Researchers from Mercator Ocean International have also shown that by adding the observations of 1,200 floats to a digital simulation of the ocean, it is possible to reduce errors by 50% in estimating global and regional variations in temperature and salinity.
Today, around 200 Deep Argo profiling floats make it possible to measure the global ocean, between the surface and the bottom.
By deploying approximately fifteen floats per year over the next decade, France will contribute, alongside Europe, to the ramp-up of the OneArgo mission, which aims to maintain 1,200 Deep Argo floats in operation in the whole oceans.
OUR “OCEAN” FILE
The growing number of innovative scientific publications based on these new autonomous robots provides a glimpse of the extent of knowledge and societal benefits to come.
This analysis was written by Damien Desbruyères, researcher in physical oceanography at the French Research Institute for the Exploitation of the Sea (Ifremer). Marine Bollard (Euro-Argo) is co-author of this article.
The original article was published on The Conversation website.
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