New floats probe the ocean at a depth of 4,000 meters

New floats probe the ocean at a depth of 4,000 meters

The Argo program constitutes a revolution in the history of ocean observation, essential for the study of climate change. It relies on an army of more than 4,000 profiling floats constantly floating in the ocean, measuring its temperature and salinity (and sometimes biogeochemical properties like oxygen, pH, or chlorophyll concentration).

Capable of diving from the surface to depths of 2,000 meters, these autonomous robots give scientists a robust picture of half the volume of the global ocean. But how to cross the mysteries of the abyss? Thanks to a new generation of floats, called Deep Argo floats, which can descend to 4,000 or even 6,000 meters deep.

The Deep Argo profiler developed by Ifremer and its sensors (head) – Stéphane Lesbats / Ifremer (via The Conversation)

They are now deployed by scientists as part of the “OneArgo” flagship mission which aims, over the next twenty years, to map and study the global ocean and its marginal seas, at all depths and integrating the measurement of biogeochemical parameters, essential for monitor changes in the health of marine ecosystems or better understand the carbon cycle.

Withstand the pressure of the seabed

To plumb such depths, researchers and engineers had to overcome a number of technological challenges. The pressure under 2,000 meters of water column is really very high – at 4,000 meters deep it is 400 times greater than on the surface – and there the signal of global warming is very attenuated. Therefore, they had to improve the performance of the sensors or even adapt the hydraulic system that would allow the float to go down or up in the water column, while limiting the on-board power needed 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 attached to the end of an electrocarrier cable that researchers uncoil at specific locations on ships. oceanographic. These occasional high resolution and accurate measurements will be a considerable asset in verifying and possibly correcting the data transmitted over the water by the Deep Argo floats.

Launch of a profiler on the Brest test raft
Launch of a profiler in the Brest test basin – Stéphane Lesbats / Ifremer (via The Conversation)

​Deep Argo, the thermometer of the abyss

The Deep Argo floats will soon make it possible to precisely quantify the increase in average global ocean temperature between the ocean surface and the ocean floor, but also to identify the regions and ocean layers most affected 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.

Accumulation of anthropogenic excess heat in the different compartments of the Earth's climate system from 1960 to 2018. The measurement of heat accumulated from the deep ocean (in dark blue) has large uncertainties, because it is based since the 1990s on the poor sampling.  of batondas.  That is why it is urgent to explore it in more detail thanks to the Deep Argo floats.
Accumulation of anthropogenic excess heat in the different compartments of the Earth’s climate system from 1960 to 2018. The measurement of heat accumulated from the deep ocean (in dark blue) has large uncertainties, because it is based since the 1990s on the poor sampling. of batondas. This is why it becomes urgent to explore it in more detail with Deep Argo floats – Schuckmann et al., 2021 (via The Conversation)

Therefore, the ocean significantly buffers atmospheric warming and climate imbalance. On the other hand, this heat storage causes almost half of the current rise in sea level. In fact, 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, the 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?

Early estimates from the “bathysonde” rosettes reveal that the ocean below 2,000 meters depth would provide 10 to 15% of this storage, with the southern ocean showing the strongest warming so far. But strong uncertainties remain, because the cost of these deep shipborne measurements makes them relatively rare and fairly dispersed. Only Deep Argo floats will be able to provide access to geographically detailed and temporally continuous maps of water temperatures below the 2,000-meter mark, and will allow scientists to make adequate global and regional heat budgets.

In the Brazil basin, for example, a fleet of 35 Deep Argo profiling floats deployed by a US team has already made it possible to accurately quantify ongoing abyssal warming, possibly induced by a slowdown in the supply of cold, southern-origin water to this basin. region. Being able to explain such temperature changes in the deep, in particular by dissociating anthropogenic causes and natural ocean movements, is a major challenge to which Deep Argo will greatly contribute.

Mapping the highways of the abyss

Deep Argo also aims to characterize large-scale ocean circulation through its drift paths at depths of 2,000 meters or 3,000 meters, or through its temperature and salinity measurements.

This will allow a better understanding of what happens to the stored heat by understanding, among other things, the transport and circulation of the two great masses of water that border the great oceanic basins: the deep waters of the North Atlantic, formed around 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 oceanic circulation, known as “thermohaline”, plays an active role in regulating the climate by connecting the surface -and therefore the atmosphere- and the great depths, and slowly redistributing key properties, such as heat, oxygen or carbon. over thousands of kilometers.

Simplified thermohaline circulation diagram
Simplified diagram of the thermohaline circulation – Lynn D. Talley, Oceanography, vol. 26, no. 1, 2013 (via The Conversation)

The simple trajectory of a Deep Argo float can already break certain prejudices, by revealing deep ocean routes as yet unsuspected. One of the Deep Argo profiler buoys deployed by Ifremer between 2015 and 2017 in the Charlie Gibbs Fracture Zone in the North Atlantic, for example, directly attached to the continental slope of Newfoundland in Canada, instead of taking a more “classic” direction. “, in the direction of Iceland and Greenland.

In the longer term, when Deep Argo’s network of profiling floats has achieved homogeneous and durable global coverage, a map of the deep circulation, such as that resulting from first-generation floats drifting through the oceans, could finally see the light of day. 1000 meters deep, 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 are still struggling to numerically represent deep-sea currents and the complex dynamical processes that govern them.

Deep Argo to better predict “the ocean that will do”

The program should also enrich the world of ocean and climate modelling. In fact, the measurements will improve the realism of operational oceanographic simulations, to predict “what the ocean will be like” from season to season or year to year.

The more observations scientists have, the more accurately they can model deep ocean dynamics and their impact on the ocean surface and other components of the climate system, such as the atmosphere. Researchers at 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% when 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. With the deployment of fifteen floats per year for the next decade, France will contribute, together with Europe, to the acceleration of the OneArgo mission, whose objective is to keep 1,200 Deep Argo floats in operation in all the oceans.

The growing number of groundbreaking scientific publications based on these new autonomous robots offers a glimpse of the scope of knowledge and the societal benefits to come.

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This analysis was written by Damien Desbruyères, a researcher in physical oceanography at the French Research Institute for the Exploitation of the Sea (Ifremer). Marine Bollard (Euro-Argo) is a co-author of this article.

The original article was published on the site of The conversation.

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