Blog post by Francesca Doglioni with Lara Aschenbeck
PIES are our friendly Pressure-Inverted Echo Sounders: chirping instruments contained in a round glass casing which will remain anchored for a few years at the bottom of the ocean. PIES are the smallest possible version of moored instruments in oceanography, they measure pressure, temperature (and in some cases currents) at the bottom of the ocean, and travel time of an acoustic signal to the ocean surface and back. This simple set of measurements provide a whole lot of information about local changes in the sea surface height and, if used in pairs, can be used to calculate ocean transport. Unlike CTD profiles (see CTD blog post), which provide a very detailed snapshot of the water column at a specific time, PIES measure proxies for integrated changes in the water column (pressure, travel time) every hour for a long period of time (usually 2+ years). This means that they are particularly useful to study large-scale long-term processes, just like the interaction between the DWBC and the NAC in the EPOC project.
During expedition M212, as part of the EPOC project we are interested in monitoring the variability and interaction of two currents in the western North Atlantic. One of them is the Deep Western Boundary Current (DWBC), a bottom current carrying cold and dense waters southwards; the other is the North Atlantic Current (NAC), a broad surface intensified current carrying warm and light water towards Europe and the Arctic Ocean. These currents are crucial components of the Atlantic Meridional Overturning Circulation (AMOC), the system of ocean currents which ensures the redistribution of heat between the equator and the poles. We want to study how these two currents interact at the Transition Zone between the subtropical and subpolar gyre, to understand whether and how their interaction might influence the AMOC.
Above: Deploying and recovering the PIES. Images courtesy K. Scheliga
Plan for this cruise and components of the observational platform
In the frame of the EPOC project, cruise M212 is a continuation of the work done previously during cruise MSM121. In this cruise we recovered a total of 16 PIES around Flemish Cap (Canada), of which 8 were then re-deployed further south. Just like the instruments’ measurements, we can say that also our work on the ship was a sustained effort. Starting on day one, we mounted all the necessary infrastructure on the ship to communicate with the PIES during the recovery and download the data, and we organised the materials for re-opening them, providing maintenance and prepare them for the next deployment. The excitement started on the second cruise week, when we first tried to communicate with one of the instruments that have been sitting at the bottom for two years and … it replied: “I’m here, ready for release!” From then on, our work has been establishing communication with the instruments, spotting them when they emerged at the surface (in all kinds of weather conditions!), and chasing them with the ship, for the fun of the ship’s crew. Once recovered, data were downloaded and finally brought to safety. It was then time for the delicate task of providing maintenance and sealing them again, ready to measure for another two to three years. In this moment, after the external casing was removed, the instrument was showing all the beauty of its electronics contained in a thick glass sphere, with radio antenna and flashing led included. “This looks so futuristic!” commented Henry from the crew, talking about the PIES. During the last week, it is finally time for re-deployment: once attached to their anchor they’re slowly lowered to the water level and, swishhhhhh…. released into the darkness. See you soon!