courant atlantique sud crédit photo © SAGA

Spring SAGA: in the heart of the South Atlantic Ocean!

by oceans connectes, le 09 september 2021

In the spring of 2021 and despite a still complicated health context, scientists spent 42 days criss-crossing the South Atlantic Ocean with the aim of quantifying the most important marine current in the ocean circulation and climate system of our planet.

The SAGA programme (“The South Atlantic GAteway”) was born in 2019. It is the result of a consortium of 4 oceanographic institutes: the University Institute of Oceanography and Global Change of Las Palmas (IOCAG), the Institut de Ciències del Mar of Barcelona (CSIC), the Spanish Institute of Oceanography (IEO), and the Centre for Scientific Research and Higher Education of Ensenada (CICESE) in Mexico.

The scientific purpose of SAGA is to quantify and monitor the AMOC in the South Atlantic Ocean, a major marine current in the thermostatic regulation of our planet.

AMOC, THE PLANET’S THERMOSTAT

The Atlantic Meridional Overturning Circulation (AMOC) is one of the Earth’s major ocean circulation systems. It is driven by surface winds and by convection between water masses of different densities that coexist in the ocean, and it regulates temperature and heat distribution on the planet.

Like a giant conveyor belt, this ocean current carries warm, salty water from the equator northwards. This water cools, becomes denser and, when it is heavy enough, sinks to deeper oceanic layers to return southwards. In total, this current moves nearly 20 million cubic metres of water per second, almost a hundred times the flow of the Amazon River!

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Schématisation de l’AMOC - Crédit © Nature

The South Atlantic Ocean is a major component of the climate system that regulates the intensity of the AMOC. The deep ocean circulation in the South Atlantic connects the deep waters of the North Atlantic with the deep waters of Antarctica, while the surface and intermediate layer circulation connects the very cold regions of the Southern Ocean with the tropical and equatorial regions.

Today, several research studies point to the first signs of a deterioration of the AMOC. Such a destabilisation could accelerate the rise in temperatures and plunge our planet into a vicious circle, until it reaches a breaking point. Monitoring, quantifying and better understanding the AMOC is a priority for oceanographers.

42 days at sea and a series of valuable data

In March-April 2021, forty scientists left on board the ship Sarmiento de Gamboa from Punta Arenas (Chile) to the Canary Islands to carry out the SAGA-10W campaign.

Along the 10° West meridian and for more than a month and a half, they collected abundant data with a single goal: to quantify the return branch of the AMOC in the South Atlantic.

scientifique saga
Photo « team » – L’équipe scientifique embarquée pour la campagne SAGA10W - © SAGA
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Photo “RV-Sarmiento-de-Gamboa “ - © SAGA

Despite its regional and global importance, the South Atlantic Ocean is a region that has been little explored until now. The SAGA-10W campaign has enabled exceptional fieldwork, combining traditional and innovative measurements, sampling and numerical modelling.

Many valuable data were collected during SAGA-10W, including:

  • CTD measurements of pressure, temperature, and salinity made at a given station point between the surface and 6,000 metres depth
  • water sampling with dissolved oxygen measurements
  • the deployment of 15 autonomous ARGO type floats (www.argo.net) allowing collection every 10 days
  • along the float’s trajectory, measurements of pressure, temperature and salinity between the surface and 2000 metres depth. Among the 15 floats deployed, 5 floats were deep floats diving to 4000 metres
  • 3 moorings were set up to ensure the recording of physical and biogeochemical measurements at a fixed point and continuously for one year
  • the 4 PIES (Pressure Inverted Echo Sounder) deployment, an underwater instrument for measuring the average speed of sound in the water column

To round off the SAGA programme and this first SAGA-10W campaign, a second SAGA-34S campaign is being prepared. Its objective will be to obtain measurements similar to the SAGA-10W campaign, but this time along the 34°South latitude. The objective is to have a zonal quantification of the AMOC, in its passage from the North to the South of the Atlantic basin. Conducted by the same scientific consortium, the SAGA-34S campaign is scheduled for February 2022.

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