Autonomous underwater vehicles (AUVs) offer a platform to sample on time/space scales unattainable by more traditional ship-based sampling campaigns. As illustrated in the Stommel plot below, many processes that we believe are integral in surface ocean biogeochemical cycling are difficult to capture using traditional techniques, but can be sampled using AUVs.
For instance, during interaction with the shelf/slope topography, a portion of tidal energy is lost to turbulence during conversion into internal waves, which lose further energy to turbulence as they move onshore. Although this mechanism likely increases the rate of vertical nutrient flux into the surface ocean over the continental shelves, we have yet to fully quantify the effect this process has on biological productivity, in part because these processes occur over time/space scales that we are unable to capture using more traditional tracer-based approaches.
I have collaborated with N. Nidzieko and A. Santoro (UCSB) to use a propelled AUV (Hydroid REMUS 600; below) capable of making simultaneous in-situ measurements of vertical turbulent mixing rate (via a microstructure shear, temperature and conductivity probes) and nitrate concentration profiles (via a SUNA nitrate sensor) to characterize internal wave structure, vertical turbulent mixing rate, and the vertical diffusive nitrate flux across the thermocline in coastal Southern California. Our goal is to quantify the effect of internal waves on vertical turbulent nutrient flux over the continental shelf edge, testing the hypothesis that tidal energy dissipation caused by the interaction with the shelf edge will increase the rate of vertical nutrient flux near the base of the euphotic zone. Check out this video produced by the URI Inner Space Center about some of our work in the North Atlantic.