On Saturday March 26th, the R/V L’Atalante started a first transect of the Gulf of Lion, from Marseille to Menorca. The plan was to deploy a towed instrument, the SeaSoar, together with acoustic Doppler velocimeters on the ship.
The SeaSoar is equipped with Conductivity-Temperature-Depth (CTD) probes. Mechanically, it is able to tilt its wings to dive up and down profiling the water column up to 400m depth. The data are collected in real time through a connected cable. This instrument allows us to sample the hydrological properties along long sections. This information is relevant for the organization of the water masses and enables to infer the geostrophic currents across the ship trajectory.
However, doing field experiment is full of twists and turns, as we combined both meteorological and technical issues. A broken cable was quickly repaired on board, but a strong Mistral blow of about 30 m/s generated waves of about 3 m on Sunday, then we had to give up using SeaSoar on the way back. A storm implies difficulties to access the deck, to work on a computer, risk of damaging the sensors. It also impacts all aspects on life on a ship because everything needs to be secured: block all the doors and drawers, secure the computers, attach the chairs and even blocking the trays in the dining hall.
Nonetheless, a very nice acoustic velocimetry transect was recorded. Leaving Marseille we first crossed the Ligurian-Provencal North current, with current speed about 0.6 m/s, then later several smaller fronts (transitions from lower to higher chlorophyll concentration). On this chlorophyll 1km resolution satellite image, fronts match quite well velocities variations indeed observed from the ship. Close to Menorca in the South we also crossed a small sub-mesoscale cyclonic eddy (radius smaller than 10km) well marked by chlorophyll but completely missed by altimetry. This kind of structure is precisely what we expect to resolve better with SWOT data.
