INTERVIEW – Louise Rousselet, SWOT AdAC Science Officer, is in charge of producing the daily bulletin that informs BioSWOT-Med adaptive sampling strategy as well as the strategy of other SWOT AdAC campaigns. Here she describes what information is provided within the bulletin.
THE INSTRUMENTS OF OCEANOGRAPHERS – Louise Rousselet is a research engineer employed by LOCEAN (Laboratoire d’Océanographie et du Climat: Expérimentations et Approches Numériques, Paris, FRANCE) financed by CNES. She’s SWOT-AdAC science officers and describes how she contributes to the BioSWOT-Med campaign.
What are your research interests besides SWOT AdAC campaigns?
My research focuses on the influence of small scale features (eddies, fronts and filaments) on the 3D distribution of elements (tracers, nutrients and phytoplankton communities) in the ocean. Fine-scale circulation shapes the horizontal distribution of these elements through transport and stirring processes. They can also create favorable conditions for phytoplankton bloom to occur by bringing together limiting nutrients either through horizontal or vertical transport. Therefore they have a strong impact on the biological growth, primary production and thus global biogeochemical cycle. Several field studies already observed instances of these influences but many questions remain: Are these influence similar in oligotrophic vs nutrient-rich regions ? Can we quantify these influences? What is the global variability of fine-scale features ? How biogeochemical fluxes are impacted by fine scales ? All these questions are part of my current research field and can be assessed in the framework of SWOT mission and SWOT-AdAC consortium.
You are the SWOT AdAC science officer and in the BioSWOT-Med campaign you are in charge of the software SPASSO and of the Lagrangian and adaptive sampling strategy. What is the software SPASSO and how do you use it?
The BIOSWOT-Med campaign aim at targeting a fine scale front separating water masses with contrasting conditions, especially in terms of biological activity (i.e. productive vs. poor water masses identified using Chlorophyll-a concentration). However due to their small scale and ephemeral characteristic, it is a real challenge to sample such features like fronts. The software package SPASSO has been developed to overcome this issue. SPASSO provides maps of dynamical and biogeochemical, structures such as fronts, eddies, filaments, in addition to chlorophyll and SST, based on near-real-time acquisition of satellite sensors. The fine-scale features are detected by Lagrangian diagnostics computed from numerical particle trajectories advected with daily near-real-time surface currents derived from altimetry. All these maps are generated automatically every day and sent to the BIOSWOT-Med team to analyze them in order to determine the best sampling site.
What other software and analysis will you perform to provide information for the cruise?
I re-wrote and developed a Lagrangian software allowing for advecting numerical particles within any 2D current field. The Lagrangian software computes some Eulerian diagnostics such as Kinetic energy or Okubo-Weiss parameter but also series of diagnostics from the particle trajectories: a) Lyapunov exponents to detect fronts; b) longitude/latitude advection fields to identify water masses origin; c) retention parameter to detect trapping eddies; d) tracer advection fields (SST, SSS or Chlorophyll); e) water masses age since last contact with a topographic feature. This tool is included in the SPASSO software but can be used with any current data. It will be surely used during the cruise to compare with drifters/float trajectories and predict water masses motion.
Sampling activities during the SWOT AdAC campaigns will be carried out day and night. You will not be on board, but I know that you will be in contact with them and will provide them with a daily bulletin. What is inside the bulletin?
The daily bulletin includes all maps computed by SPASSO software. In particular, SPASSO uses standard Copernicus satellite-derived data for surface currents, SST and Chlorophyll but also a bunch of innovative and high-resolution satellite-derived data for surface currents, SST and chlorophyll produced by CLS, with support from CNES, for SWOT-AdAC campaigns. These latter data are very useful because they minimize clouds coverage. Maps of Eulerian and Lagrangian diagnostics described earlier are also included. Some useful information such as SWOT passing time over the sampling area are also inserted in the bulletin. Every day I will analyze these maps and write down my sampling strategy suggestions to the on-board team.
You have developed a tool to optimize the route of the ship. Can you explain how it works?
Indeed, the LATEXtools toolbox includes several tools to simulate the vessel route, compute sailing time and provide waypoint coordinates. When analyzing SPASSO maps, the on-board team might decide to sample in the middle of a chlorophyll patch and perform back and forth transect to cross a front detected by Lyapunov exponents. To easily get the precise coordinates of the projected waypoints the LATEXtools toolbox retrieves SPASSO maps, superimposes variables of interest and allow the user to click on the map to choose manually the target points while visualizing the satellite-derived data. When the user is done, LATEXtools provides a file, including longitude/latitude of the waypoints and the sailing time, that can be given to the vessel captain to prepare following operations. This tool is very useful especially in the context of adaptive sampling strategy. The tool also allows for plotting some near-real-time in situ data collected on-board such as currents from ADCP, surface temperature and salinity from TSG or surface phytoplankton group abundances from flow-cytometry. Comparing satellite and in situ data validates the use of satellite data and also improve features detection.