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.