Herlé Mercier is a physical oceanographer and CNRS research director at the Physical and Spatial Oceanography Laboratory(CNRS/Ifremer/IRD). He is also the designer and first mission leader in 2002 of the Ovide programme, which stands for an Observatory of Interannual and Decadal Variability. This programme describes and tracks the North Atlantic Current, a component of the Atlantic Meridional Overturning Circulation, a determining current for our temperate European climate.
The windows of the building of the Physical and Spatial Oceanography Laboratory (LOPS), based at the Ifremer centre in Plouzané (Finistère, France), offer an unobstructed view of the Brest Narrows, although it is a little blocked by mist on this late afternoon in early April. Here, at the gateway to the North Atlantic Ocean, we find Herlé Mercier, a physical oceanographer and CNRS research director in this laboratory (created in 2016) made up of researchers from the CNRS, Ifremer and the IRD. After completing his PhD in 1983 and a post-doctorate at MIT in Boston, he spent his entire career there, interspersed with long periods at sea on French, Spanish, American and British oceanographic vessels. Here Herlé Mercier looks back over his career and shares his vision of the ocean and the changes that lie ahead for his discipline.
Interview by Maud Lénée-Corrèze
Why did you choose physical oceanography?
Herlé Mercier: Having grown up on the coast and started sailing at a very young age, I developed a strong attraction to the marine environment. And I loved physics. It was quite natural for me to enrol for a DEA in physical oceanography at the University of Brest. I liked it, and my passion for the sea led me to turn to in situ observations rather than modelling, so that I could continue to go out on the water and see how the ocean really works.
What major programmes have you been involved in as part of your research?
H. M. : The first was the World Ocean Circulation Experiment (WOCE) in the 1990s, which gave us our first, albeit somewhat blurred, snapshot of global thermohaline circulation (circulation driven mainly by changes in temperature and salinity, which cause water masses to sink or rise). I was in charge of the Romanche experiment, named after the Romanche fault (between 2 degrees North and 2 degrees South) which allows the circulation of bottom waters between the western and eastern basins of the equatorial Atlantic.
Since 2002, I have been working on Ovide in the North Atlantic, a major programme of hydrographic surveys and biogeochemical tracers [chemical elements or isotopes that make it possible to trace the history of a process (nutrient salts, oxygen, salinity, chlorofluorocarbons, which are fluorinated gases of anthropogenic origin), editor’s note] between Portugal and Greenland. Thanks to this project, we are observing the variability of the Atlantic Meridional Overturning Circulation (AMOC), which is part of the thermohaline circulation and responsible for our temperate climate in Europe. We are also monitoring the carbon cycle, to see where it is absorbed by the ocean and where it is buried, as well as changes in the properties of water masses and deep circulation.
In parallel with the Ovide programme, I also took part in the Reykjanes Ridge experiment led by Virginie Thierry, the aim of which was to study how the interaction of currents with the topography of the seabed at the Reykjanes Ridge, to the south-west of Iceland, modified the course of the North Atlantic Current, a component of the AMOC that brings warm waters up from the south towards the north. We put current meters in the water for two years and carried out hydrographic surveys to build up a significant database for this particular area.
The bathysonde, or rosette, was used during the first Ovide campaign in 2002. It is a frame on which instruments for measuring CTD (salinity, temperature, depth) and bottles for taking water samples at given depths are fixed. This is one of the instruments most commonly used by physical oceanographers during their in situ campaigns. Credits: Pascale Lherminier/Thalassa
What do you mean by ‘experience’ ?
H. M. :
These experiments allow us to focus on a local phenomenon. In my approach as a researcher, it has always been important to combine observatory-type experiments like Ovide with experiments where we set out to understand a process, like the last one I took part in, Boundary Layer Turbulence in the Rockall Basin in Ireland, in collaboration with Great Britain and the United States [MIT, Scripps, University of Exeter and the National Oceanography Centre in Southampton, editor’s note]. The aim was to see how the water rises up from the depths, how, in short, this thermohaline circulation is completed: the North Atlantic Current takes the water northwards to the level of Greenland, then the water, which has become cold, plunges down and, having reached the depths, is re-exported southwards. But where do they come up, and by what mechanism? That was the objective at Rockall.
The section being studied by the OVIDE programme is a repeat of a section explored during the WOCE global programme in the late 1990s. This is where a number of currents, both surface and deep, pass through, forming part of the world’s largest thermohaline circulation. Herlé Mercier also travelled to the north-west of Ireland to carry out surveys in the Rockall Basin, off the coast of Ireland, and made a more detailed study of deep-water exchanges in the vicinity of the Reykjanes Fault. Credits: H. Mercier, P. Lherminier, F. Fernandez Perez/Ovide/Seanoe
What are the challenges facing physical oceanography today?
H. M. : It’s a young science: the first theories to explain the whys and wherefores of certain currents, such as the Gulf Stream, date back to the 1950s. Einstein had already discovered relativity! And these were still just theories: we were only able to observe these currents a few years later, thanks to technological advances. These have been decisive in the development of physical oceanography, particularly in the 1990s with GPS for precise survey points and the first satellites to measure sea surface height, temperature and phytoplankton production, followed by the Argo and DeepArgo profiling float programme, which allows us to acquire data continuously rather than just during one-off campaigns.
Today, the focus of development is on autonomous biogeochemical measurements: the Argo programme has taken this on board with Argo-BGC, to measure oxygen, nutrients, pH, etc. But we also need to train people capable of interpreting these measurements, checking for any inconsistencies due to a potential malfunction of the sensor – in the ocean it is subject to such pressure that this can happen -, in order to obtain quality data. Because that’s also the challenge: maintaining data quality over time, despite the mass of information we are acquiring and will acquire in the future. It’s not necessarily easy, because you need funding, in particular.
The Provor BGC profiling float to be tested in 2022 at the Ifremer centre in Plouzané (Finistère). This range of profiling floats in the Argo programme is equipped with sensors for biogeochemical parameters, the autonomous measurement of which is an important area of development for physical oceanography. Credits: Olivier Dugornay/Ifremer
As an oceanographer, what is your view of the state of the ocean and the current human response to the challenges of climate change?
H. M. : The oceans are warming, with the well-known consequences of melting ice, ecosystems and fish species moving northwards. It is also absorbing anthropogenic carbon through ocean circulation: it is therefore moderating climate change. In return for this absorption, it is becoming more acidic, so we urgently need to change our way of life and move towards a zero-carbon economy.
But I don’t think we should be totally negative. When I gave a seminar ten years ago and talked about climate change, there were a lot of sceptics in the room. The message that human activity is changing the climate has gone. Now we’re being told ‘but are you sure it’s as dangerous as all that, are you sure we won’t be able to adapt? Are you sure that if we did geo-engineering, it wouldn’t work? The discourse and attitudes have changed, even if it’s not yet enough.
What can an oceanographer do to change attitudes and encourage action?
H. M. :
I think we need to start by explaining what oceanography is, and the vital role that the ocean plays in our climate system. The ocean is the Earth’s memory. It stabilises the current climate, and this circulation in the Atlantic makes our climate temperate at our latitudes. We then show how climate change is upsetting this balance, and what is likely to happen in Northern Europe, for example.
To achieve this, I really believe in training young researchers, teachers, journalists and politicians to raise their awareness, and I believe in informing the public. Researchers from the laboratory and the European University Institute of the Sea welcome secondary school teachers to introduce them to physical oceanography, write books and publish in journals on our disciplines, answer journalists’ interviews, appear on broadcasts and present their work at science days and open days.