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Underwater acoustics has developed into a powerful tool for monitoring the climate of ocean basins. Acoustic travel-times are a direct indication of ocean temperature and give a large-scale average of heat content. A recent experiment called the Coordinated Arctic Acoustic Thermometry Experiment (CAATEX) is a prime example of the method in a region known to be experiencing the most rapid response to anthropogenic forcing. In CAATEX, two 35 Hz acoustic transceiver moorings, one in the Nansen Basin and one in the Beaufort Sea, were deployed along with four other receiving moorings. The travel-times between these moorings provide an accurate baseline of the environment during the 2019–2020 deployment year that can be compared to past, present, and future measurements and climatologies. Furthermore, other acoustic observables, such as transmission loss and the ambient soundscape, provide important insight into the ice conditions and the changing environment.

An ocean acoustic tomography array with a radius of 150 km was installed in the central Beaufort Gyre during 2016–2017 for the Canada Basin Acoustic Propagation Experiment (CANAPE). Five transceivers were deployed in a pentagon shape with a sixth transceiver at the center and a long vertical receiving array northwest of the central mooring. At least 12 refracted-surface-reflected (RSR) ray arrivals with lower turning points at depths between 500 and 3500 m were resolved in the acoustic receptions at all receivers. Travel-time anomalies were computed relative to a range-dependent sound-speed reference made by objectively interpolating annual-average sound-speed profiles constructed from the temperature data at each mooring. The travel time anomalies were inverted to estimate the 3-D sound-speed anomaly, including corrections to the positions of sources and receivers consistent with the uncertainty from long-baseline acoustic navigation systems at each mooring. Although the deep water in the Canada Basin is nearly homogeneous in temperature and salinity and highly stable (slowly warming in response to geothermal heating), it proved necessary to allow for a sound-speed change in the deep ocean to obtain consistent inversions, suggesting that the sound-speed equation at high pressure and low temperature is in error by about 0.1–0.2 ms−1.

The Arctic Ocean is undergoing dramatic changes in response to increasing atmospheric concentrations of greenhouse gases. The decreases in ice extent, the near disappearance of multiyear ice, and changes in the stratification of the ocean all have important implications for underwater acoustic propagation. During the 2016–2017 Canada Basin Acoustic Propagation Experiment (CANAPE), a long vertical receiving array was embedded within an ocean acoustic tomography array of six acoustic transceivers with a radius of 150 km. The impulse response of the ocean was measured every four hours using broadband signals centered at about 250 Hz. The observed travel-time variability was extraordinarily low, reflecting both the low internal-wave energy level and sparseness of mesoscale eddies in the Canada Basin. The peak-to-peak travel time variability of the early, resolved ray arrivals was only a few tens of milliseconds, and the standard deviations over the entire year were only a few milliseconds. 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. The travel-time fluctuations are roughly an order of magnitude smaller than is typical in midlatitudes at similar ranges.

The hydroacoustic environment of a rapidly warming Arctic Ocean will be impacted by changing thermohaline structure, increased marine traffic, changes in sea ice coverage, and likely increases in microseism/storm noise. This will lead to obsolescence for today’s Arctic Ocean acoustic models just as the need for understanding and monitoring the Arctic acoustic environment becomes more critical. To make sophisticated predictions for coming conditions, we need fully-coupled Earth system models. Los Alamos National Laboratory is contributing to developing the Department of Energy’s Energy Exascale Earth System Model (E3SM), which can potentially predict the ocean and sea ice conditions necessary to drive an acoustics model in a rapidly-evolving Arctic Ocean. We present a preliminary analysis of a comparison of acoustic travel times in the Canada Basin calculated from E3SM simulations with measured travel times from the 2016–2017 Canada Basin Acoustic Propagation Experiment (CANAPE) and travel times computed from ice-tethered-profiler measurements of acoustic properties in the water column. The goals of this effort are to provide boundary ocean conditions to an acoustic model, to quantify the ocean acoustic implications of climate change, as well as to create a climate-aware atlas of global acoustic noise that could be applied, for example, in signal detection.

Sustained in situ ice-ocean observations are sorely lacking in the Arctic, limiting research on climate, weather, ice-ocean processes, and geophysical hazards. A sustained network of advanced multipurpose underwater moorings and drifting buoys in the Nansen and Amundsen Basins that included acoustic and other instrumentation would make a substantial contribution to a high Arctic Ocean observing system. Such a network would provide point measurements of ocean parameters, large-scale temperature measurements using acoustic thermometry, acoustic geo-positioning of underwater floats and gliders, and passive acoustic measurements for detection of marine mammals, geohazards, and human generated noise. Optimal design of such a network of fixed moorings and drifting platforms requires accurate knowledge of the ice-ocean environment to determine the acoustic properties. Such a sustained network would build on the successful basin-wide coordinated arctic acoustic thermometry experiment (CAATEX). In this presentation, the focus will be on the oceanographic and acoustic characteristics of the Nansen and Amundsen Basins using observations made during CAATEX. The ability of several climate models and reanalysis products to describe the oceanographic characteristics as well as their usefulness in predicting low-frequency acoustic propagation will be evaluated.

Acoustic signals transmitted from a 75-Hz broadband source near Kauai as part of the North Pacific Acoustic Laboratory (NPAL) experiment were recorded on an array of receivers near California at a range of 3890 km, and on a vertical line array at a range of 3336 km in the Gulf of Alaska. Because the source is approximately 2 m above the seafloor, and the bottom depth at the receivers near California is approximately 1800 m, acoustic interaction with the bathymetry complicates the identification of the recorded arrivals with those present in numerical simulations of the experiment. Ray methods were used to categorize acoustic energy according to interactions with the sea bottom and surface and to examine the significance of seafloor geometry. A modal decomposition was also used to examine the role of range-dependent bathymetry and to associate the effects on the acoustic field with seafloor features at specific ranges. Parabolic-equation simulations were performed in order to investigate the sensitivity of the received signal to geoacoustic parameters; shear excitations within the seafloor were modeled using a complex-density, equivalent-fluid technique. Incorporation of bottom interaction into models of the propagation enables an identification between experimental and simulated arrivals.

A method to obtain coherent acoustic wave fronts by measuring the space–time correlation function of ocean noise between two hydrophones is experimentally demonstrated. Though the sources of ocean noise are uncorrelated, the time-averaged noise correlation function exhibits deterministic waveguide arrival structure embedded in the time-domain Green’s function. A theoretical approach is derived for both volume and surface noise sources. Shipping noise is also investigated and simulated results are presented in deep or shallow water configurations. The data of opportunity used to demonstrate the extraction of wave fronts from ocean noise were taken from the synchronized vertical receive arrays used in the frame of the North Pacific Laboratory (NPAL) during time intervals when no source was transmitting.