Ocean of sound: underwater gliders observing the oceanic environment

Ocean gliders play an increasingly important role in the Global Ocean Observing System. They are now routinely used to monitor the ocean, along repeated transect lines from the coast to the open ocean, in remote locations and during severe weather events. They offer persistent presence at sea, collecting high-resolution scientific measurements during months- to year-long missions and over thousands of kilometres. The ocean glider com-munity continuously develops new sensors, new navigation capabilities and new usage for underwater gliders, increasing their observation range. This thesis investigates the opportunity offered by addition of passive acoustic monitoring (PAM) capability on ocean gliders and the associated technical challenges. Ocean gliders’ specificities, such as quiet propulsion, low speed and vertical profiling make them highly suitable for PAM applications. Ocean gliders were equipped with PAM systems during 12 missions in different conditions, in polar regions, in open ocean remote lo-cations and along routine coastal transect lines. This thesis reviews the currently avail-able PAM glider solutions, identifies technical challenges and desirable developments and presents pathways to improved scientific PAM glider observations. Intense ocean glider presence in the northwestern Mediterranean basin provided an experimental framework to demonstrate the ability to collect valuable scientific information from PAM glider surveys. Wind speed measurements obtained from glider-borne acoustic recordings, up to 20m s−1, colocated with collection of oceanographic profiles, can improve air-sea interaction studies. Sperm whale acoustic activity detected on PAM glider recordings provides information on population distribution and behaviour along the glider tracks. Wide addition of PAM systems on the ocean glider fleet would benefit for its global time and space coverage, enabling long-term observations in key areas, critical for conservation, monitoring of anthropogenic pressure and assessment of ecosystems health