Physical Oceanographic and Acoustic Observations of the Beaufort Sea and its Subsurface Duct Sensitivity to Deterministic Ocean Features
The Arctic Ocean is rapidly transforming due to climate change, and this thesis analyzes two, year-long ocean-acoustic data sets to quantify variability in the thermohaline, current and ice structure and examine the implications for acoustic propagation in the Beaufort Sea. The two field efforts are the Canada Basin AcousticPropagation Experiment (CANAPE) and the Coordinated Arctic Acoustic Thermometry Experiment (CAATEX). These results are important since acoustics offers a unique tool for the Arctic under ice communication, navigation, and remote sensing.
A key acoustic feature of the Beaufort Sea is the Beaufort duct created by the Pacific winter water (PWW), which is sandwiched between the shallow Pacific summer water (PSW) and the deeper Atlantic water (AW). This is important because this duct allows for long-range transmission without lossy interactions with sea ice or surface waves. In general, we find this duct can trap from 2-6 acoustic modes in the frequency range between several tens of Hz and several hundreds of Hz. Since fluctuations can alter the number of trapped modes and affect Transmission loss, we have analyzed variability due to spice, internal waves, eddies, and near-inertial waves, and we find spicy thermohaline structure is observed to be the most significant source of variability in the top 100 m, followed by eddies and internal waves.
Acoustic interaction with the ice cover is another critical factor affecting arctic acoustics, so we have analyzed high-frequency acoustic scattering statistics from the CANAPE. Five important surface scattering epochs were identified over the seasonal cycle: open water, initial ice formation, ice solidification, ice thickening, and ice melting. The most significant changes in statistics are seen during the formation, solidification, and melting. The statistical features are comparable throughout the CANAPE region, implying similar ice properties.
The stability of acoustic propagation in the Beaufort duct is another aspect of this thesis. We analyzed the oceanographic measurements from the CANAPE and CAATEX, and focused on the problem of mode coupling in the Beaufort duct,induced by deterministic ocean features such as eddies and intrusions. Here we find that deterministic variability in the PSW due to spice and stronger halocline eddies can result in enhanced coupling between acoustic modes in and out of the BD. Here we use the mode interaction parameter (MIP), which is a non-dimensional quantity, Γmn that quantifies the mode coupling strength between mode m (a duct mode) and all the other n modes. Strong/moderate-weak coupling is determined by MIP greater-than/less-than 1. The MIP is a function of acoustic frequency and horizontal structure, which we define as the typical half-width of Gaussian perturbation (Δ) in this thesis. Our result showed that for both large and small Δ, Γmn goes to zero, and maximal coupling occurs when Δ ranges between 1.5 and 2.2 km.