INTERVIEW –Maristella Berta is in charge of lagrangian instrument coordination in the BioSWOT-Med campaign. She describes what drifters and floats are, what information can be gathered from them and how real-time data can help inform the adaptative sampling strategy of BioSWOT-Med.
THE RESEARCH THEMES – Maristella Berta is a researcher in physical oceanography at the Institute of Marine Science – National Research Council (CNR-ISMAR) in Lerici, La Spezia, Italy. In the BioSWOT-Med campaign she is charge of of lagrangian instruments coordination. She describes what drifters and floats are and what information can be gathered from them.
What are your research interests besides BioSWOT-Med?
I am a physical oceanographer and I focus on the dynamics of sea currents by analysing observations from field samplings (buoys and sensors at sea), remote platforms (such as satellites and radars) and ocean model outputs. I’m interested in the combination of independent and complementary observation platforms, that is essential to get the overall picture of the ocean processes playing at different scales at the same time. Multidisciplinarity is also a key aspect of the multiplatform approach, since it allow us to investigate the interaction between the physical and biogeochemical component of the ocean. Among the physical processes characterizing the upper layer of the ocean I investigate how marine currents drive the dispersion of tracers and passive particles (such as pollutants like oil or plastic, and biological tracers as microalgae or fish larvae), and the transport of these particles in the vertical (from surface to depth, and viceversa). Understanding these processes can provide guidance for practical applications such as marine accident response or management of marine protected areas.
In the BioSWOT-Med campaign you are in charge of lagrangian instruments coordination. What does lagrangian mean?
Lagrangian instruments are devices that measure sea water properties as they drift with sea currents, this cathegory of instruments differs from the Eulerian ones which observe sea water properties from a fixed-point, such as a device moored to the sea floor. Lagrangian devices need to be equipped with a GPS (Global Position System) tracker and satellite communication in order to retrieve measurements and correspondent location at regular times.
Drifters are Lagrangian buoys usually equipped with drogues, that make them “sail” at a specific depth: CODE and CARTHE drifters follow surface currents within the first meter, while SVP design includes a drogue centered at 15m depth to be representative of the dynamics of that water layer. Other drifters, such as the spotters, are designed to float at the air-sea interface and do not have any drogue.
Floats instruments, on the other hand, are pseudo-Lagrangian in the sense that they are not totally passive to sea currents since they drift at a specific depth and periodically perform vertical cycles measuring water properties from depth to the surface, where they also transmit position and data just recorded. In BioSWOT-Med we will have 30 CARTHE plus CODE drifters, 20 SVPs, 2 spotters, and 6 floats.
What type of sensors are present on the drifters and floats and how do you use the information gathered from these instruments?
Aside from the basic drifter setup (simply the GPS tracker, as for CODE and CARTHE), the other drifter types involved in the experiment are equipped with additional sensors to measure essential ocean variables such as sea water temperature (for the SVPs), waves (for spotters) or biogeochemical properties of the water (in the SVP-BGC prototype). All (six) floats have pressure, temperature and conductivity (for salinity) sensors, four of them have an additional oxygen sensor, while the other two have biogeochemical sensors as well.
In Lagrangian datasets we look at individual drifter trajectories, but more importantly to the relative displacement among drifters within a cluster. This analysis can indicate converging or diverging water masses in the upper sea layer, where intense vertical currents tend to develop and, in turn, enhance the water properties exchange from surface to depth. Combining drifter trajectories with water column sampling from floats can contribute to characterize sea properties in correspondence of intense water mixing spots, that can be associated to high biological productivity.
While on board, you will analyze in real time the data collected by the langrangian instrument. How can this help the sampling strategy of the campaign?
During BioSWOT-Med we will target specific small scale (order of 10km) circulation features, such as vortices or fronts (the interface between two different water masses) identified within the SWOT satellite swaths. The satellite observations will guide the choice of the field of activity that we will start exploring with some water samplings and drifters deployment. Through a first real-time processing, the Lagrangian component (drifters) will contribute to assess and sharpen the target identified by SWOT images and it will provide guidance for the deployment of other instruments at sea able to resolve finer scale processes. In a second phase, the analysis of drifter data can be used to quantify surface divergence and convergence and to provide an estimate for vertical currents magnitude in the targeted feature. The analysis of floats together with the other high resolution observations will contribute to characterize the variability of the biophysical properties in the targeted feature.
Tosca Ballerini (firstname.lastname@example.org)