Monitoring polynias with ocean acoustic tomography: a feasibility study in Terra Nova Bay

This study addresses the feasiblity of Ocean Acoustic Tomography for long-term monitoring of polynyas. OAT applicability has been investigated by means of the available observations in Terra Nova Bay polynya (Ross Sea, Antarctica) and by simulations with a range dependent, multi-layered adiabatic normal-mode acoustic propagation model. The summer sound speed profile is characterised by surface values around 1450 m/s, a minimum of 1441 m/s around 50 m depth and a linear increase with depth up to 1453 m/s at the bottom, thus leading to ducted propagation in the near surface layer and upward refraction below, while during winter, due to water cooling and mixing process, the subsurface minimum disappears and surface sound speed is about 1440 m/s. The feasibility study of acoustic inversion was approached by means of normal mode Match Field Tomography and the results from simulations demonstrate that ocean acoustic tomography is able to reproduce quite well the vertical sound speed profile, in particular the temporal evolution of summer stratification and winter mixing processes, thus providing information on the upper layer, where direct measurements are not possible.