Observations of sound-speed fluctuations in the Beaufort Sea from summer 2016 to summer 2017

Impact of vertical resolution in a regional ocean circulation model of the northern Gulf of Mexico for acoustic predictions in the upper ocean

This study evaluates the impact of ocean model vertical resolution in representing three-dimensional (3D) sound speed variability used for acoustic predictions in the upper ocean. Sound propagation is investigated for two configurations of the same regional ocean circulation model of the De Soto Canyon in the northern Gulf of Mexico. Both configurations employ a submesoscale-permitting horizontal resolution of 0.5 km but differ in vertical resolution, featuring 30 or 200 terrain-following layers. The higher vertical resolution is found to better represent oceanographic features, such as submesoscale fronts and salt-fingering staircases, which are crucial for accurately predicting 3D sound speed variability in the upper ocean, especially below the mixed layer. Additionally, ray-tracing and coupled normal mode simulations indicate that such oceanographic features, predicted by the higher vertical resolution configuration, can significantly affect acoustic propagation in the upper ocean for the tested frequencies in the band [500-1500 Hz], even at relatively short ranges (<30 km). These results indicate the potential of high-resolution regional ocean models for improving the accuracy of acoustic forecasts in the presence of submesoscale ocean variability, notably for operational oceanography and naval applications.

Transmission loss of surface-reflected ray arrivals underneath seasonally varying sea ice in the Canada Basin during 2016-2017

During the 2016-2017 Canada Basin Acoustic Propagation Experiment, an ocean acoustic tomography array with a radius of 150 km measured the impulse responses of the ocean every 4 hr at a variety of ranges and bearings using broadband signals with center frequencies from 172.5 to 275 Hz. Ice-profiling sonar data showed a gradual increase in ice draft over the winter with daily median ice drafts reaching maxima of about 1.5 m and daily standard deviations reaching maxima of about 1.2 m. The travel-time variability of early, resolved arrivals from refracted-surface-reflected rays with lower turning depths below 500 m was reported in a previous paper [Worcester et al. (2023). J. Acoust. Soc. Am. 153, 2621-2636]. Here, the transmission loss of these same ray arrivals is analyzed. The transmission loss was lowest when open water was present and increased as the ice draft increased. The excess transmission loss per surface reflection, defined as the increase in transmission loss relative to open water conditions, increased with center frequency and surface grazing angle. The combination of transmission loss measurements for resolved ray arrivals and ice drafts from the ice-profiling sonars provides an excellent dataset for testing ice-scattering models.

Observations of ocean spice and isopycnal tilt sound-speed structures in the mixed layer and upper ocean and their impacts on acoustic propagation

A 400-m deep and 970 km long conductivity, temperature, depth section in the Northeast Pacific Ocean is decomposed into sound-speed variations associated with tilting isopycnals and ocean spice. The vertical distribution of sound-speed variance from these two processes shows significant fluctuations in the mixed layer (ML) and transition layer (TRL) below. Acoustic simulations at 400 and 1000 Hz are conducted with the decomposed fields to quantify their relative impact on upper ocean propagation for source locations in the ML and TRL. The low frequency simulations show that localized scattering processes dominate the propagation while higher frequencies experience more diffuse scattering. For propagation in the ML, spice generates the most loss while tilt can reduce loss when combined with spice. Statistics further show that energy can couple into and out of the ML duct depending on source depth and frequency.

Acoustic travel-time variability observed on a 150-km radius tomographic array in the Canada Basin during 2016-2017

The Arctic Ocean is undergoing dramatic changes in response to increasing atmospheric concentrations of greenhouse gases. The 2016-2017 Canada Basin Acoustic Propagation Experiment was conducted to assess the effects of the changes in the sea ice and ocean structure in the Beaufort Gyre on low-frequency underwater acoustic propagation and ambient sound. An ocean acoustic tomography array with a radius of 150 km that consisted of six acoustic transceivers and a long vertical receiving array measured the impulse responses of the ocean at a variety of ranges every four hours using broadband signals centered at about 250 Hz. The peak-to-peak low-frequency travel-time variability of the early, resolved ray arrivals that turn deep in the ocean was only a few tens of milliseconds, roughly an order of magnitude smaller than observed in previous tomographic experiments at similar ranges, reflecting the small spatial scale and relative sparseness of mesoscale eddies in the Canada Basin. The high-frequency travel-time fluctuations were approximately 2 ms root-mean-square, roughly comparable to the expected measurement uncertainty, reflecting the low internal-wave energy level. The travel-time spectra show increasing energy at lower frequencies and enhanced semidiurnal variability, presumably due to some combination of the semidiurnal tides and inertial variability.

Observations of the space/time scales of Beaufort sea acoustic duct variability and their impact on transmission loss via the mode interaction parameter

The Beaufort duct (BD) is a subsurface sound channel in the western Arctic Ocean formed by cold Pacific Winter Water (PWW) sandwiched between warmer Pacific Summer Water (PSW) and Atlantic Water (AW). Sound waves can be trapped in this duct and travel long distances without experiencing lossy surface/ice interactions. This study analyzes BD vertical and temporal variability using moored oceanographic measurements from two yearlong acoustic transmission experiments (2016-2017 and 2019-2020). The focus is on BD normal mode propagation through observed ocean features, such as eddies and spicy intrusions, where direct numerical simulations and the mode interaction parameter (MIP) are used to quantify ducted mode coupling strength. The observations show strong PSW sound speed variability, weak variability in the PWW, and moderate variability in the AW, with typical time scales from days to weeks. For several hundreds Hertz propagation, the BD modes are relatively stable, except for rare episodes of strong sound speed perturbations. The MIP identifies a resonance condition such that the likelihood of coupling is greatest when there is significant sound speed variability in the horizontal wave number band 1/11<kh<1/5 km-1. MITgcm ocean model results are used to estimate sound speed fluctuations in this resonance regime.

Introduction to the special issue on ocean acoustics in the changing arctic

This paper introduces the Special Issue of The Journal of the Acoustical Society of America on Ocean Acoustics in the Changing Arctic. The special issue includes papers on ocean (and in one case atmospheric) acoustics. Changes in both the ice cover and ocean stratification have significant implications for acoustic propagation and ambient sound. The Arctic is not done changing, and papers in this special issue, therefore, represent a snapshot of current acoustic conditions in the Arctic.

Transmission loss for the Beaufort Lens and the critica frequency for mode propagation during ICEX-18

Complexities of acoustic propagation in ducts have long been known, e.g., shallow water environments and deep waters off Gibraltar. The "Beaufort Lens" (Lens) is a duct north of Alaska with nominal depths between 60 and 200 m and is reachable by oceanographic instruments and underwater unmanned vehicles and submarines. Propagation within the ducts is governed by waveguide physics. The frequencies must be high enough to support the modes within them such that there is a "critical frequency" (CF) where modes start to "detach" from surface loss mechanisms. Therefore, transmission losses (TLs) can abruptly decrease once a mode "fits" within a duct. This paper describes an experimental part of Ice Exercise 2018 supported by the U.S. Navy's Arctic Submarine Laboratory. The signals were transmitted from Camp Sargo north of Prudhoe Bay to the submarines SSN Hartford, SSN Connecticut, and HMS Trenchant. The data indicate low TLs near 100 Hz and an abrupt 10 dB decrease in TLs 244-280 Hz, both suggesting CFs. Modeling suggests CFs for modes 1 near 100 Hz and a higher CF when modes 3-6 "cascade" into the Lens starting near 250 Hz. There are also abrupt increases in TLs at other frequencies, which are explained by nulls in the product of the mode functions.

Beaufort Sea observations of 11 to 12.5 kHz surface pulse reflections near 50 degree grazing angle from summer 2016 to summer 2017

Sea-surface acoustic scattering is investigated using observations from the 2016-2017 Canada Basin Acoustic Propagation Experiment. The motions of the low-frequency acoustic source and/or receiver moorings were measured using long-baseline acoustic navigation systems in which the signals transmitted once per hour by the mooring instruments triggered high-frequency replies from the bottom-mounted transponders. The moorings recorded these replies, giving the direct path and single-bounce surface-reflected arrivals, which have grazing angles near 50°. The reflected signals are used here to quantify the surface scattering statistics in an opportunistic effort to infer the changing ice characteristics as a function of time and space. Five scattering epochs are identified: (1) open water, (2) initial ice formation, (3) ice solidification, (4) ice thickening, and (5) ice melting. Significant changes in the ice scattering observables are seen using the arrival angle, moment of reflected intensity and its probability density function, and pulse time spread. The largest changes took place during the formation, solidification, and melting. The statistical characteristics across the experimental region are similar, suggesting consistent ice properties. To place the results in some physical context, they are interpreted qualitatively using notions of the partial and fully saturated wave fields, a Kirchhoff-like approximation for the rough surface, and a thin elastic layer reflection coefficient model.

Effects of Pacific Summer Water layer variations and ice cover on Beaufort Sea underwater sound ducting

A one-year fixed-path observation of seasonally varying subsurface ducted sound propagation in the Beaufort Sea is presented. The ducted and surface-interacting sounds have different time behaviors. To understand this, a surface-forced computational model of the Chukchi and Beaufort Seas with ice cover is used to simulate local conditions, which are then used to computationally simulate sound propagation. A sea ice module is employed to grow/melt ice and to transfer heat and momentum through the ice. The model produces a time- and space-variable duct as observed, with Pacific Winter Water (PWW) beneath a layer of Pacific Summer Water (PSW) and above warm Atlantic water. In the model, PSW moves northward from the Alaskan coastal area in late summer to strengthen the sound duct, and then mean PSW temperature decreases during winter and spring, reducing the duct effectiveness, one cause of a duct annual cycle. Spatially, the modeled PSW is strained and filamentary, with horizontally structured temperature. Sound simulations (order 200 Hz) suggest that ducting is interrupted by the intermittency of the PSW (duct gaps), with gaps enabling loss from ice cover (set constant in the sound model). The gaps and ducted sound show seasonal tendencies but also exhibit random process behavior.

Sensitivity of mixed layer duct propagation to deterministic ocean features

Here, the problem of mode coupling in a mixed layer (ML) surface duct is considered where the coupling is induced by deterministic upper ocean features such as eddies, filaments, and/or density compensated temperature and salinity anomalies (spice). The single scatter Dyson series solution for mode energy is used to define a non-dimensional mode interaction parameter Γ_mn that quantifies the strength of coupling between modes m and n as a function of environmental factors and frequency. Direct coupled mode simulations at 400 and 1000 Hz show weak, first order coupling and small ML transmission loss (TL) variability when Γ_mn<1, while for Γ_mn>1, there is strong, higher order coupling with large changes in ML TL. Importantly, there is a frequency dependent resonance condition associated with the range width of the perturbations, Δ, such that Γ_mn→0 as Δ→0 and ∞.