INTERVIEW. Anthony Bosse is the leader of WP2 Physical Processes in the BioSWOT-Med campaign. He describes the different physical processes that will be studied during the campaign and the different sampling platforms used with the goal to understand how ocean dynamics from mesoscale (eddies and fronts of 10-50km) to dissipation scale (millimeter scale turbulence) impact the distribution of heat, its evolution with climate change, and how these processes influence the development of phytoplankton.
THE RESEARCH THEMES – Anthony Bosse is an oceanographer with a background in physics, which led him to combine the two disciplines to analyze observations carried out from research vessels or collected by autonomous platforms. The main focus of his research is to understand how the ocean dynamics from mesoscale (eddies and fronts of 10-50km) to dissipation scale (millimeter scale turbulence) impact the distribution of heat and its evolution with climate change, as well as the impact those processes can have on the development of phytoplankton. His research focuses on the two regions: the Mediterranean Sea and the Nordic Seas. He is a researcher at the Mediterranean Institute of Oceanography in Marseille and affiliated at University Aix-Marseille. During the BioSWOT-Med campaign is the leader of WP2 Physical Processes.
In the BioSWOT-Med campaign you are the leader of WP 2 on physical processes and will coordinate the work of other 8 people. What are the main contributions of your team to the campaign?
The BioSWOT-Med cruise is an ambitious research cruise with a strong multidisciplinary approach to study the physics/biology coupling in the context of the SWOT CalVal. In this context, the WP2 dedicated to the physical processes will have the important task to describe the different aspects of the physical context driving the distribution and fluxes of biogeochemical tracers (such as nutrients) and plankton. To do so, a strong team of researchers in WP2 will use Lagrangian surface drifters, shipborne current meters and underway temperature and salinity sensor, as well as underwater gliders, to characterize flows at submesoscale (typically 1-10km) and at high resolution, assess vertical velocities using a vertical velocity profiler and free-fall current profiler, and finally characterize the level of turbulence using dedicated sensors.
What do you refer to when you speak about “flow”?
Flow is a generic term in oceanography referring to streams of water with significant velocities (typically of 0.1 to 1 m/s). They are similar to oceanic river transporting waters of specific physical (temperature and salinity) and biogeochemical properties (nutrients, oxygen, carbon, …). The only difference is that the scale of these rivers is much larger, and the volume transport of a single one of these is about 10 larger than the Amazon.
What are the “vertical velocities”?
Vertical velocities refer to vertical the movements in the oceans. Those are ubiquitous, but usually much smaller in magnitude than horizontal ones: of the order of mm/s, or even smaller, but yet, they are crucial to sustain biological activity and carbon sequestration, as vertical velocities bring nutrients upward to the sunlit layer where they can be consumed to sustain primary production, and carry fresh organic matter to great depths where they will die and eventually fill layers of sediments with carbon.
What is “turbulence”?
Energy is injected in the Ocean at large scale by winds for instance, and atmospheric forcing. Once a water parcel is set in motion, large amount of energy is transported over large distances while being slowly dissipated. This dissipation involves turbulent motions at scale of a few millimeters. Turbulence describes the chaotic tri-dimensional movements of waters at those scale where kinetic energy is converted into heat by the friction of water. We (oceanographer) like to compute the rate of energy dissipation, because it affects vertical fluxes of heat or tracers, and oceanic circulation as a whole. To put it in simple words, turbulence is like the movements created in a cup with a spoon when stirring milk with tea, and the energy dissipation rate controls how fast the homogeneous process happens.
To study these processes, you will use both instruments that are aboard the R/V L’Atalante, as well as deploy instruments in the water. What instruments will you use to measure these processes?
As said before, we will use a broad range of instruments. In collaboration with Italian partners from Italy (CNR-ISMAR and OGS), we will deploy Lagrangian drifters, which record their position in real-time and drift with surface currents, as well as subsurface Argo profiling floats, which drift at depth and every day sink to 2000m and profile the water column measuring temperature and salinity. Some of these floats will even be able to measure vertical profiles of nutrients, oxygen and chlorophyll-a fluorescence. From the ship, we will have three instruments onboard recording currents under the hull down to 1000m. We will tow the MVP (Moving Vessel Profiler) behind the ship while transiting in order to get high resolution properties of the water column from the surface down to 400m. On station, we will deploy classic CTD instruments to measure temperature, salinity, horizontal currents and collect water samples. What is more interesting is the deployment of two new instruments developed at the Mediterranean Institute of Oceanography (MIO) able to track vertical velocities (VVP and FF-ADCP) using two different techniques, as well as a turbulence profiler in order to quantify its impact on vertical fluxes (of nutrients for instance).
BioSWOT-Med is an international campaign with several partners and together with your colleagues from Norway and the USA you will be deploying 4 gliders. What is a glider and how does it work?
Indeed, last but not least, we will be using several autonomous platforms called « gliders » during the cruise. They are long-range autonomous underwater vehicles that can stay in the water for several months while profiling from the surface to a depth and carrying low-power sensors. They dive and climb by modifying their volume using an oil bladder and are equipped with wings that convert their vertical motion into horizontal translation. During the BioSWOT-Med cruise, we will deploy two gliders piloted by the French team at MIO, one glider equipped with a turbulence sensor will be piloted by Norwegian partner (University of Bergen) and another one piloted by American partner (SCRIPPS) carrying acoustic and optic sensors to look at zooplankton. In addition, Spanish colleagues from SOCIB will also deploy 2 gliders in the region as part of another cruise (FastSWOT). So, it is going to be a challenge to coordinate all these platforms to work together, but it is a very exciting time and the data set collected will allow some ground breaking discoveries.
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