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Coupled Ocean-Acoustic Prediction of Transmission Loss in a Continental Shelfbreak Region: Predictive Skill, Uncertainty Quantification and Dynamical Sensitivities
MASSACHUSETTS INST OF TECH CAMBRIDGE DEPT OF MECHANICAL ENGINEERING
Pagination or Media Count:
In this study, we explain and quantify the dynamical causes of striking differences in the acoustic transmission data collected on the shelf and shelfbreak in the north-eastern Taiwan region within the context of the Quantifying, Predicting and Exploiting QPE uncertainty 2008 Pilot Experiment. To do so, we employ our own coupled oceanographic4D-acousticNx2D modeling systems with ocean data assimilation and a best-fit depth-dependent geo-acoustic model. For the first time, predictions are compared to the measured acoustic data, showing skill. We also study the sensitivity of our results to uncertainties in several factors, including geo-acoustic parameters, bottom layer thickness, bathymetry and ocean conditions. Our simulations first reveal that the lack of signal received on the shelfbreak is due to a dramatic 20 dB increase in transmission loss TL caused by bottom trapping of sound energy during up-slope transmissions over the complex and deeper bathymetry. Sensitivity studies on sediment properties show larger but isotropic TL variations on the shelf and smaller but more anisotropic TL variations over the shelfbreak. Diverse thicknesses of sediments lead to only limited effects on the TL. The small bathymetric data uncertainty are modeled and also shown to lead to small TL variations. We discover that the initial transport conditions in the Taiwan Strait can affect acoustic transmissions downstream more than 100 km away, especially above shelfbreak. Simulations also reveal internal tides and quantify their spatial and temporal effects on the ocean and acoustic fields.
APPROVED FOR PUBLIC RELEASE