On Tuesday March 29th, the R/V Tethys II joined the C-SWOT campaign and will provide measurements in parallel with R/V Atalante until the end of the cruise. This 25-meter vessel is manned by a crew of 10 people, including 3 scientists.
Leaving La Ciotat bay where R/V L’Atalante was sheltered from the Mistral blow, we started joint transects in the Gulf of Lion with 2 vessels, along and across SWOT satellite swath. During the first transect, temperature data could not be obtained from the Seasoar. Indeed Navy exercise zones east of 5⁰E were not authorized for underwater towed probes, we then had to shift to expendable bathythermographs (XBT). XBT are lighter but less precises probes launched in free fall while the ship is underway.
Measuring ocean current with two vessels in parallel tracks will enable to see finer scales and current variations, down to the scale of the distance between the 2 vessels. In the next week, this distance will be fixed to 2nm (~3.5km), because it on the order of the spatial resolution expected from SWOT interferometer altimetry data. Few days after a strong wind blow, we expect to see inertial waves in the upper water column, not yet balanced in density. Measuring at the same time the true current from Acoustic Doppler velocimeter and density from the Seasoar will allow to investigate this.
However what is fantastic for science becomes a nightmare for navigation. These two vessels have a quite different size and tow different instruments. The navigation crew has to adapt the speed frequently to navigate jointly parallel tracks and at the same time continue underway measurements. When another constraint appears, like a ship with a collision route, it becomes even more complex. With their experience and anticipation, the crew was able to avoid collision without any trouble for the experiment, and we greatly acknowledge them ! Other unexpected obstacles can force the ship to deviate from a straight line, such as a whale probably sleeping !