Listening to the Oceans - Effective Techniques for Acoustic Imaging of Oceanic Structure

Abstract

Submesoscale processes are known to play an important role in the vertical and lateral exchange of water masses, along with tracers such as carbon, atmosphere-ocean exchange, ocean productivity, and the mixing budget necessary to complete the overturning circulation. The challenge is to observe submesoscale variability on sufficiently fine space and time scales. One promising approach is seismic oceanography, which applies acoustic reflection techniques, as originally developed by the hydrocarbon industry, to image temperature and salinity gradients within the water column. Here we present the first multichannel seismic images of ocean fine-structure on the eastern Falkland Plateau region of the subantarctic South Atlantic Ocean, a highly energetic confluence zone where Pacific and Antarctic waters flow via the Antarctic Circumpolar Current (ACC) to merge with waters from the Atlantic Ocean and contribute to the global overturning circulation. High-resolution (O(10m)) sections of sub-surface thermohaline structure reveal an intricate and complex pattern of oceanic fine-structure near the Polar Front: internal waves, lenses and filaments with length scales of 100m-10km in intermediate depths (up to 800 m); steep continuous filaments in deeper sections (up to 2000 m) that are influenced by interactions with bathymetry. Spectral analysis of seismic data reveals maps of increased diapycnal mixing near fronts. Furthermore, a novel approach to quantify dynamic instabilities through estimating Ertel’s Potential Vorticity and balanced Richardson angles, is presented. Another key region for the global thermohaline circulation is the Mozambique Channel, where a strong southward propagating eddy field modulates the strength of the Agulhas Current system that transfers warm, salty water into the South Atlantic Ocean. With the analysis of a 4D industry seismic data variability within the Mozambique Channel is analysed through temporal and spatial imaging of submesoscale features. Coincident closely spaced profiles provide information about the three-dimensional geometry of such features. Alternative ways to capture thermohaline fine-structure are hull-mounted echo sounders. Echo sounder data from a research cruise along the de-glaciating margin of the West Antarctic Peninsula is presented, which captured thermohaline structure and features including internal waves. A significant change in internal wave amplitude before and after an ice front collapse in Börgen Bay, Anvers Island, was quantified for the first time through echo sounder data. Lastly, the capabilities of different acoustic surveys are compared and future paths for acoustically imaging thermohaline structure discussed, especially a possible future implementation of autonomous technology for acoustically mapping oceanic structure.