[Vendée Globe 2024 – #4] –
With great humour, he has christened it the ‘Flamingo Argo’! Vendée Globe skipper Sébastien Marsset (Foussier) caused quite a stir this week by deploying his Argo float. And he wasn’t the only one, as the arrival in the Southern Ocean is synonymous for many with the deployment of floats, often filmed ‘live’ by the sailors. A 25-year-old ocean observation system that has totally revolutionised the way scientists study the oceans.
As a scientific partner of the Vendée Globe, océans connectés provides scientific insights into the oceans during this 2024 race. Each week, we’ll be highlighting the particular ocean features crossed by the skippers and the key events in the commitment of some of them to science.
By Carole Saout-Grit
Cover photo: ARVOR-type Argo float deployed from the oceanographic vessel Pourquoi Pas? operated by the French Oceanographic Fleet © Argo Program, https://argo.ucsd.edu
Sébastien Marsset (Foussier) deploys a rather special buoy on 29 November 2024 © Vendée Globe 2024
The birth of a revolutionary idea for oceanography
SCRIPPS Institution in Oceanography, University of San Diego California. 1995. Dean Roemmich and Russ Davies, two experienced American oceanographers, are having a chat over a coffee break. Their conclusion was clear: it was high time to find an effective way of monitoring the interior of the oceans 24 hours a day, from the surface to the bottom, every day of the year. But the task is not an easy one!
Throughout history, explorers and scientists have observed the ocean at certain precise locations, at certain precise times, using ships, buoys and other oceanographic tools such as the bathysonde. Given the vast expanse of the oceans, it has long been difficult to gather comprehensive data on the physical state of the water column, and in particular on the reference ocean parameters of temperature and salinity.
Dean Roemmich, 2018 winner of the Alexander Agassiz Medal established since 1913 by the Académie Nationale des Sciences for an original contribution to oceanography© Alan Blacklock, National Institute of Water and Atmospheric Research (NIWA, New Zealand)
We really had to wait until the 1970s and 80s to see the emergence of the first satellites and the first measurements at the ocean surface. But these were still a long way from being able to observe currents and water properties at depth. In the 1990s, the international WOCE ( World Ocean Circulation Experiment) experiment on global ocean circulation led to the first massive collection of observations on each ocean basin. A large number of radials and routes were covered methodically by research vessels, based on highly effective international coordination. It was also during the WOCE experiment that the first autonomous explorers of Lagrangian circulation (following a particle as it moves) were used to map currents in entire ocean basins. The ALACE is launched into the sea, adjusts its buoyancy and sinks to a pre-programmed depth where it remains for a pre-programmed period. It then readjusts its buoyancy, returns to the surface and is positioned by satellite.
Very quickly, the idea naturally arose of equipping these ALACE floats with instruments for observing temperature, then later salinity, as they ascended, eventually becoming PALACE (Profiling ALACE). It was finally on the basis of this experience that a plan was presented in 1998 to set up a global network of floats, with initial proposals made by D.Roemmich and R.Schmitt. The name Argo was chosen to underline the strong complementarity between the global float network and the altimetry mission of the Jason satellite launched in 1985. In Greek mythology, Jason sailed a boat called ‘Argo’ to capture the Golden Fleece.
A concentrate of innovative technologies
Measuring around 1.50 m and weighing around 20-30 kg, each float is a concentrate of technology. From bottom to top, you can see a bladder to act as ballast, a hydraulic pump with an internal tank to rise and fall in the water, a battery to ensure the float’s autonomy, physical sensors to measure oceanic parameters (pressure, temperature and salinity), computer equipment with software and an internal programming card defining all the mission parameters, and finally a satellite antenna to send the data collected to the data centres on land where the oceanographers can use them.
View of the inside of an Argo float (source: oceanOPS).
From the moment it is deployed from a ship, the Argo float becomes autonomous for a specific period of 5 years. It begins its first cycle by diving from the surface to a depth of 1,000 metres, then drifts at this depth for around 9 days, following the ocean currents it encounters. On the 9th day, it dives to a depth of 2,000 metres and then returns to the surface in a few hours, measuring pressure, temperature and salinity every 25 metres along the water column. Once at the surface, it transmits all the data it has been able to record over ten days by satellite to the data centres on land where the scientists can process it.
Cycle after cycle, the float progresses vertically, inflating or deflating its external bladder to modify its buoyancy. While it drifts freely at a depth of 1,000 metres, following the currents and taking a few measurements, the grail of these measurements is the temperature and salinity ‘profile’ measured from a depth of 2,000 metres to the surface every 10 days.
Diagram of the stages (from 1 to 7) in the cycle of an Argo float © Thomas Haessig
A scientific revolution
In 1998, Argo was conceived with the aim of deploying an initial network of 3,000 floats in all the world’s oceans, with one float deployed every 3 degrees (300 kms), within the limits of the zones between 60°N and 60°S and outside the marginal seas.
In November 2007, this objective was reached thanks to a growing number of countries involved, from 10 to 35 today. In 2012, the milestone of one million P/T/S profiles collected by Argo floats was reached. Today, the Argo network has more than 3,800 active floats and more than 2 million profiles collected, with more than 120,000 profiles recorded every year.
Status of the Argo active float network on 30/10/2024 by national contributions (source: oceanOPS).All the data from the Argo programme is freely available to anyone who wishes to use it. This policy has led to a revolution in oceanography. Since the first major oceanographic campaign by the British Challenger expedition in the 1870s, more than 500,000 temperature and salinity profiles (down to at least 1,000 metres) have been collected by onboard instruments. This means that in just over two decades, Argo has quadrupled the number of deep ocean profiles.
Not only are Argo observations much more numerous than those from other instruments, but they also provide a more complete picture of the ocean. They are not limited to a few maritime routes, but sample all seasons, whatever the weather conditions. Argo has a particularly important impact on hard-to-reach areas such as the Southern Ocean and regions of the Southern Hemisphere, especially in winter. Since 2009, oceanographers have been working on extending the network to high latitudes, marginal seas and biogeochemical observations.
Worldwide trend in the number of scientific publications obtained from Argo data since 1999, by year. Source: Argo Project OfficeArgo has radically accelerated our ability to monitor ocean conditions and predict the ocean-atmosphere interactions that regulate climate. Proof of this is the exponential increase in scientific publications obtained thanks to these new Argo profiles, which expose the ocean.
The environmental impact of these floats was recently assessed by a study conducted by Euro-Argo. It concludes that there is currently no other method of observing the interior of the ocean that has such a low impact on the environment and is so crucial to our scientific understanding of ocean dynamics and their role in the climate machine.