Extensive measurements of mid-frequency sound propagation and oceanographic variability were collected on the Washington continental shelf during a month-long experiment in June 2025. A moored sound source transmitted signals with center frequencies of 4.0 and 6.0 kHz with a bandwidth of 1000 Hz to several moored receiver arrays located 500 m to 20 km in range from the source. These signals were used in alternating sequences such that each signal was transmitted every 13 s for 2 h and every 10 min for 16 h for approximately 18 days during the experiment. These measurements enable examination of the variability in mid-frequency propagation at timescales ranging continuously from tens of seconds to several weeks. Temporal and spatial oceanographic data were also collected on five dedicated oceanographic moorings, as well as using a ship-towed profiler and a shipboard echosounder. Using the combined acoustic and oceanographic measurements and a regional ocean model, the goal is to determine the degree to which various oceanographic processes impact mid-frequency propagation. This talk presents a preliminary analysis of the temporal variability of the sound propagation over the course of the experiment. [Work supported by the Office of Naval Research under Task Force Ocean.]

Monthly mean reanalysis from assimilating global ocean circulation models spanning 27 years is used to study subsurface secondary acoustic ducts, which provide waveguides for the transmission of mid-frequency sound. A systematic diagnosis of secondary ducts from monthly mean temperature and salinity fields characterizes their distribution and properties in two global ocean models. Results from both models are compared against a monthly gridded product derived from Argo float observations to evaluate the climatology, distribution, and formation mechanisms of these ducts. Geographical and seasonal patterns reveal two distinct formation mechanisms for subsurface ducts. Regions dominated by subducted pycnostads, associated with mode waters, exhibit well-mixed layers with weak stratification dominated by temperature. In contrast, ducts formed within the permanent pycnocline are characterized by stratification dominated by salinity, especially in subpolar regions. A constraint limiting bulk stratification of the upward-refracting layer as a function of density ratio or of Turner angle across the layer is obtained from linearized equations of state for density and sound speed. Subsurface ducts diagnosed from nonlinear equations for density and sound speed conform to this approximated constraint, which accounts for the global decomposition of modeled ducts into two partially overlapping branches: one with the upward-refracting layer stratified primarily by salinity and the other, more weakly stratified. The distribution of weakly stratified layers largely conforms to known mode waters. The formation of salinity-dominated upward-refracting layers in ducts is linked to stratification generated annually by one-dimensional processes at the base of deep winter mixed layers, freshened by precipitation and runoff.

A widespread prevalence of subsurface acoustic ducts impacting mid-frequency sound propagation was observed over the outer shelf and the continental slope during a field experiment in July-August 2022 in the Pacific Northwest coastal ocean of North America. Simulations of the coastal shelf ocean using LiveOcean, a tidally driven operational model (MacCready et al., 2021), based upon a widely used variant of the Regional Ocean Modeling System (ROMS), were compared with observations of the thermohaline stratification layers responsible for the ducts, and found to have a nearly complete absence of these acoustic features due to excessive parametrized mixing. After implementing additional realistic constraints in the 'k-epsilon' second moment closure (SMC) to control instabilities in the turbulence mixing model with low background mixing, the source of instabilities was identified in a coding error for the default, third-order upstream advection of the turbulence parameters for TKE k and its dissipation epsilon, a longstanding and significant bug impacting mixing parametrization, and one also found in the older SMC 'Mellor-Yamada 2.5' mixing parametrization option in ROMS. With code improvements, LiveOcean was able to successfully simulate the production of observed subsurface acoustic ducts. The primary process for generating the ducts along the outer shelf involves the southward transport of low sound speed water during upwelling, combined with the cross-shelf displacement of higher sound speed water from offshore beneath this layer in bottom-driven Ekman transport.

Olympic Coast National Marine Sanctuary (OCNMS) off Washington State was designated by the National Oceanic and Atmospheric Administration (NOAA) in 1994 and is embedded in the northern California Current (NCC) system, which is affected by climate fluctuations such as marine heat waves, El Niño, hypoxia, ocean acidification, and changes in timing of the spring transition to upwelling conditions. There is a need to better understand the climatological conditions in the Sanctuary, especially considering the presence of four Coastal Treaty Tribes with treaty-protected rights to marine resources on the Olympic Coast. Oceanographic moorings at five cross-shelf lines along the Olympic Coast have measured surface and subsurface water properties from 2000 to present. Measurements focus on the summer upwelling season, when hydrographic conditions fluctuate on the time scale of local wind events and remotely-generated coastal trapped waves, and biogeochemical stressors like hypoxia and ocean acidification tend to worsen. Additionally, temperature sensors deployed at the Teahwhit Head site and, in more recent years, at the Makah Bay site in 42 meters water depth remain in the water throughout the year to measure winter temperatures. This article provides a description of the OCNMS mooring program and our efforts to combine 26 years of water temperature, salinity, dissolved oxygen, pressure, and velocity data into quality-controlled time series available via Zenodo at https://doi.org/10.5281/zenodo.19751759.

A joint oceanography and acoustics experiment was conducted on the Washington continental shelf in the summer of 2022. A towed system measured the in situ sound speed field along a 20 km track between acoustic sources and receivers. A weak but persistent subsurface duct was found with its sound speed minimum generally in the 50–100 m–depth range. The duct exhibited range and time dependence due to the internal tide, internal waves, and possibly other oceanographic processes. Mid-frequency (3500 and 6000 Hz) transmission loss (TL) was measured at 10 and 20 km ranges. The subsurface duct has a 10–13 dB effect on TL, depending on whether the sound source is inside or outside the duct. Measurements were also made using a bottom-mounted source, with transmissions every 3 min over several days. The sound intensity varies about 10 dB over a few minutes, while the scintillation index fluctuates between 0.5 and 1.5. Overall, it is found that mid-frequency sound propagation is variable at several temporal scales, ranging from minutes to hours, to days, or longer. Reducing the impact of these variabilities in acoustic applications would benefit from knowledge of the ocean processes at these different time scales.

Phenotypic plasticity can improve an organism’s fitness when exposed to novel environmental conditions or stress associated with climate change. Our study analyzed spatiotemporal differences in phenotypic plasticity and offspring performance in Olympia oysters Ostrea lurida. This species is an ecosystem engineer and is of great interest for commercial and restoration aquaculture. We used a multidisciplinary approach to examine acute and long-term physiological differences in O. lurida in response to in situ oceanographic conditions in a dynamic inland sea. We outplanted oysters to different areas in Puget Sound, Washington, affixing cages to anchor lines of oceanographic monitoring buoys. This allowed us to couple high-resolution oceanographic data with organism’s phenotypic response. To assess spatiotemporal differences in oyster physiological performance, we collected oysters after six-months and one year of acclimatization at four field sites. During each collection period we evaluated changes in shell properties, diet, metabolism, and reproduction. Adult growth, δ13C and δ15N isotopic signatures, and gametogenesis were affected by both seasonal and environmental conditions. In the winter, oysters from all sites had higher respiration rates when exposed to acute thermal stress, and lower respiration response to acute pH stress. Lipid content, sex ratio and shell strength were unchanged across locations. Offspring growth rates between sites at experimental temperature 20°C closely reflected parental growth rate patterns. Offspring survival was not correlated with growth rates suggesting different energetic trade-offs in oyster offspring. The metabolic response (respiration) of larvae reached its highest point at 20°C but sharply decreased at 25°C. This indicates that larvae are more sensitive to temperature stress, as adults did not exhibit a reduction in metabolic response at 25°C. By deploying genetically similar oysters into distinct environments and employing a wide range of physiological methodologies to examine performance and fitness, our results indicate that Olympia oysters exhibit a high degree of phenotypic plasticity and show evidence of parental carryover.

Strong mid-frequency sound intensity fluctuations were found on the Washington shelf during an experiment in the summer of 2022. The Scintillation Index (SI) of integrated sound energy on single channels at 20 km range was near unity for both 3.5 kHz and 6.0 kHz signals. Accompanying oceanographic measurements from moorings and a towed profiling system revealed sound field variability at multiple time and spatial scales caused by linear and nonlinear internal waves, internal tides, and coastal trapped waves. While the 2022 dataset provides valuable information for addressing issues on intensity fluctuation, additional experimental data are needed to understand the impacts on active and passive mid-frequency sonar systems. To that end, a second Washington Shelf experiment is being planned for the summer of 2025. This talk will discuss the planned measurements and the range of issues to be addressed. These include the frequency-dependence of SI over 1–10 kHz, broadband fading, vertical and horizontal coherence, and the impacts of variability on propagation in subsurface ducts. These measurements are important for investigating fundamental oceanographic causes of sound fluctuation as well as for the design of appropriate signal processing methods. (Work supported by the Office of Naval Research.)

This study investigates the subsurface sound channel or acoustic duct that appears seasonally along the U.S. Pacific Northwest coast below the surface mixed layer. The duct has a significant impact on sound propagation at mid-frequencies by trapping sound energy and reducing transmission loss within the channel. A survey of the sound-speed profiles obtained from archived mooring and glider observations reveals that the duct is more prevalent in summer to fall than in winter to spring and offshore of the shelf break than over the shelf. The occurrence of the subsurface duct is typically associated with the presence of a strong halocline and a reduced thermocline or temperature inversion. Furthermore, the duct observed over the shelf slope corresponds to a vertically sheared along-slope velocity profile, characterized by equatorward near-surface flow overlaying poleward subsurface flow. Two potential duct formation mechanisms are examined in this study, which are seasonal surface heat exchange and baroclinic advection of distinct water masses. The former mechanism regulates the formation of a downward-refracting sound-speed gradient that caps the duct near the sea surface, while the latter contributes to the formation of an upward-refracting sound-speed gradient that defines the duct's lower boundary.

Secondary subsurface acoustic ducts, waveguides that are shallower and weaker than the primary deep ocean channel, depend critically on details of variations in upper ocean temperature and salinity stratification. In a joint oceanographic-acoustic field experiment July-August 2022, the oceanography of subsurface secondary acoustic ducts impacting the propagation of mid-frequency (1–10 kHz) sound in the Washington coastal ocean was surveyed broadly and then sampled intensively along acoustic paths concurrent with transmission loss (TL) measurements. This acoustically focused data set extends and connects measurements from mooring and glider records collected and archived by regional ocean observatory projects in the Washington coastal ocean, to provide a more comprehensive picture of regional subsurface duct structure impacting mid-frequency sound propagation from spring through late fall. An initial wide-ranging survey of the Washington coastal ocean, carried out using CTD and dissolved oxygen sensors mounted on the SWIMS towed body platform, shows a widespread and contiguous presence of subsurface ducts over the outer continental shelf and slope. The ducts were observed at shelf ocean depths greater than ∼100 m, and extending offshore from the shelf break to over the slope, in layers 10–100 m thick, with sound speed minima at depths between 30–80 m. Pervasive layers of subsurface ducts were also locally ubiquitous during the 2-week acoustic transmission experiment, conducted in the context of an array of five instrumented moorings distributed across the outer shelf. These measurements are used to assess sound speed along acoustic experiment transmission paths and provide a basis for (1) directly testing simulations of acoustic propagation based on detailed concurrent sound speed observations; (2) assessing the skill of ocean models to predict sound speed and relevant small-scale coastal ocean processes; and (3) assessing the capacity for the combination of ocean and acoustic models to predict transmission loss in this coastal environment.

A joint Oceanography/Acoustics experiment was conducted 15 July–13 August, 2022 in Washington’s shallow coastal shelf waters to investigate mid-frequency sound intensity fluctuations and the oceanographic mechanisms driving them, including subsurface ducts, internal tides, and internal waves. Acoustic pulses centered at 3.5 kHz and 6 kHz were transmitted up-shelf from a stationary source and recorded by receivers on two moorings at 10 km and 20 km ranges, respectively. A ship-towed Shallow Water Integrated Mapping System (SWIMS) repeatedly sampled the ocean between the source and receiver moorings, providing sound speed measurements as a function of range, depth, and time. In addition, five oceanographic moorings strategically placed around the acoustic path took additional ocean data over time. The sound speed profiles feature a surface mixed layer underneath which a subsurface duct with local sound speed minima of 1–2 m/s is frequently present. Analysis shows that (1) when the sound source is inside the subsurface duct, transmission loss is reduced by 15 dB compared to when the source is outside and (2) the statistics of the acoustic intensity are consistent with strong scattering. [Work supported by the Office of Naval Research.]

The northern portion of Washington’s outer coast—known locally as the Olympic coast—is a dynamic region characterized by seasonal upwelling that predominates during summer interrupted by occasional periods of downwelling. We examined spring-to-fall water temperature records collected along this coast from 2001–2015 from April to October at four nearshore locations (Cape Elizabeth to Makah Bay) that span one degree of latitude and are located within 15 km of the shore. When compared against a long-term climatology created for 2001–2013, seven-day smoothed temperature anomalies of up to 4.5°C at 40 m depth during 2014 and 2015 show short-term warm events lasting 10–20 days. These periods of warming occurred within the well documented marine heatwave in the Northeast Pacific and were about twice the seasonal temperature range in the climatology at that depth. These warm events were strongly correlated with periods of northward long-shore winds and upper ocean currents, consistent with what is expected for the response to downwelling-favorable winds. While our focus a priori was on 2014 and 2015, we also found large positive temperature events in 2013, which were potentially related to the early stage of the marine heatwave, and in 2011, which did not have a documented marine heatwave. This indicates that near-shore short-term warm events occur during periods of large-scale offshore marine heatwave events, but also can occur in the absence of a large-scale marine heatwave event when downwelling-favorable winds occur during the summer/early fall.

In this study, we investigate quantitative characteristics and the statistics of secondary subsurface ducts on a global scale using sound speed $c$ determined from operational global ocean model reanalysis products from the Hybrid Coordinate Ocean Model HYCOM for the purpose of underwater sound applications. Systematic diagnosis of secondary subsurface ducts from monthly averaged model hydrography is used to characterize their distribution and properties, including the depth of their central axis and duct strength. The results show that the secondary subsurface duct, with a typical central axis depth range of 10–300 meters, is prevalent in numerous regions of the worldwide ocean, particularly at mid-latitudes within the thermally stratified permanent pycnocline. The thickness of the duct exhibits significant variation across geographically separated regions of prevalence and undergoes seasonal fluctuations due to atmospheric forcing. Some areas may feature relatively thick O(100 m) ducts, while others may have thinner ones, measuring less than 50 m in thickness, potentially constrained by local model resolution. The acoustic cutoff frequency of the duct, which is a function of duct prominence and thickness, also exhibits regional variability, with higher values in the low-latitude regions and lower values in the high-latitude regions. The climatology of annual variations in monthly duct prevalence and properties presents a distinct seasonal cycle in the duct thickness, with thicker ducts in the winter and thinner ducts in the summer, corresponding to the impact of surface heat fluxes. Significant regional variations in duct thickness and depth diagnosed from HYCOM may therefore arise directly from variations in surface heat and freshwater fluxes. These variations may be expected to differ between models with different surface forcing, vertical resolution or model physics. In addition to responding to ocean surface fluxes, geographic and seasonal variability of duct prevalence and properties that are due to variations in upper ocean thermohaline stratification also reflect advection and modification by ocean currents, circulation, and mesoscale eddies, as well as mixing by smaller submesoscale and turbulence processes parameterized in ocean models. Both seasonal and interannual variabilities of duct properties correspond strongly to variations in surface fluxes of heat and salinity, driving the evolution of ocean surface mixed layer density and sound speed, combined with advection by ocean currents. We find weak correlations between duct properties and ENSO, an index of climate variability, with marginal impacts of note in a few regions.

Pacific Summer Water eddies and intrusions transport heat and salt from boundary regions into the western Arctic basin. Here we examine concurrent effects of lateral stirring and vertical mixing using microstructure data collected within a Pacific Summer Water intrusion with a length scale of ∼20 km. This intrusion was characterized by complex thermohaline structure in which warm Pacific Summer Water interleaved in alternating layers of O(1 m) thickness with cooler water, due to lateral stirring and intrusive processes. Along interfaces between warm/salty and cold/fresh water masses, the density ratio was favorable to double-diffusive processes. The rate of dissipation of turbulent kinetic energy (ε) was elevated along the interleaving surfaces, with values up to 3×10−8 W kg−1 compared to background ε of less than 10−9 W kg−1. Based on the distribution of ε as a function of density ratio Rρ , we conclude that double-diffusive convection is largely responsible for the elevated ε observed over the survey. The lateral processes that created the layered thermohaline structure resulted in vertical thermohaline gradients susceptible to double-diffusive convection, resulting in upward vertical heat fluxes. Bulk vertical heat fluxes above the intrusion are estimated in the range of 0.2-1 W m−2, with the localized flux above the uppermost warm layer elevated to 2- 10 W m−2. Lateral fluxes are much larger, estimated between 1000-5000 W m−2, and set an overall decay rate for the intrusion of 1-5 years.

The dramatic decrease in Arctic sea ice has resulted in a corresponding increase in the seasonal freshwater flux due to melt water in the Canada Basin. This source of freshwater can be quite patchy as sea ice breaks aparts and melts, resulting in freshwater fronts that are strained and stirred by the mesoscale eddy field. We would like to understand the relevant processes that determine the evolution of these freshwater fronts and how heat and salt are exchanged between the fresh melt water and the background water masses. In particular we investigate the importance of submesoscale processes for the lateral and vertical exchange of heat and salt, using high resolution observations of a freshwater front in the Arctic to initialise idealised simulations of frontal evolution. We isolate the effect of submesoscale dynamics by comparing high resolution submesoscale-resolving simulations with lower resolution simulations permitting only larger-scale eddies. Comparisons with observed temperature wavenumber spectra will be presented to investigate whether the simulated dynamics are representative of observations. Heat and salt budgets are presented for the simulations and the impact of submesoscale dynamics on the balance between across-front ageostrophic and geostrophic transports will be discussed. We will also discuss the implications of these results on the seasonal redistribution of heat over the upper ocean, specifically do submesoscale dynamics lead to an increase in the vertical transport of heat across the base of the summer mixed layer, therefore increasing the heat content within the winter mixed layer and delaying the formation of sea ice in the fall?

Destratification and restratification of a similar to 50-m-thick surface boundary layer in the North Pacific Subtropical Front are examined during 24-31 March 2017 in the wake of a storm using a similar to 5-km array of 23 chi-augmented EM-APEX profiling floats (u, upsilon, T, S, chi(T)), as well as towyo and ADCP ship surveys, shipboard air-sea surface fluxes, and parameterized shortwave penetrative radiation. During the first four days, nocturnal destabilizing buoyancy fluxes mixed the surface layer over almost its full depth every night followed by restratification to N similar to 2 x 10(-3) rad s(-1) during daylight. Starting on 28 March, nocturnal destabilizing buoyancy fluxes weakened because weakening winds reduced latent heat flux. Shallow mixing and stratified transition layers formed above similar to 20-m depth. A remnant layer in the lower part of the surface layer was insulated from destabilizing surface forcing. Penetrative radiation, turbulent buoyancy fluxes, and horizontal buoyancy advection all contribute to its restratification, closing the budget to within measurement uncertainties. Buoyancy advective restratification (slumping) plays a minor role. Before 28 March, measured advective restratification integral(u(z)b(x) + upsilon(z)b(y)) dt is confined to daytime; is often destratifying; and is much stronger than predictions of geostrophic adjustment, mixed-layer eddy instability, and Ekman buoyancy flux because of storm-forced inertial shear. Starting on 28 March, while small, the subinertial envelope of measured buoyancy advective restratification in the remnant layer proceeds as predicted by mixed-layer eddy parameterizations.

Multiple stressors are affecting the Pacific Northwest (PNW) coastal ocean, including harmful algal blooms (HABs), ocean acidification, marine heatwaves, and hypoxia (low oxygen). While these conditions or events are tied to seasonal cycles such as upwelling periods and multiyear cycles such as El Niño/La Niña, they are becoming increasingly frequent and intense. Additionally, they can have devastating impacts on ecosystem health and human wellbeing, shutting down fisheries, stifling the local economy, threatening food security, and inhibiting cultural practices. For example, increasing ocean acidification has affected shellfish growers’ capability to secure reliable product. In 2015, a HAB associated with a marine heatwave shut down crab fisheries from Alaska to Baja for commercial and tribal fishers (McCabe et al., 2016), a closure so impactful that the US Congress included the Fishery Disaster Relief Program for Tribal Fisheries in the Budget Act of 2018. And, an unpredicted hypoxia event in 2015 resulted in the Quinault Indian Nation pulling up crab pots with dead crab. Regional projections indicate increases in warming, ocean acidification, and hypoxia by the end of the century (Siedlecki et al., 2021), so solutions are needed. The challenge of multi-stressor impacts can be addressed by engaging a variety of partners to collect multi-variable observing and forecast data while increasing both scientific knowledge and application of data and information to real-world needs. The Northwest Association of Networked Ocean Observing Systems (NANOOS, http://www.nanoos. org/) helps sustain long-term observations and forecast models to help communities adapt to and plan for variable and changing ocean conditions, thus increasing resilience. NANOOS is the PNW regional coastal ocean observing system of the US Integrated Ocean Observing System (IOOS). It was recently designated a nexus organization for the UN Decade of Ocean Science for Sustainable Development because of its work to sustain and integrate ocean observations and modeling to produce publicly accessible regional data products that help diverse coastal communities ensure safety, build economic resilience, and increase understanding of the coastal ocean. NANOOS, in collaboration with regional partners, provides observations of temperature, salinity, oxygen, chlorophyll, carbon dioxide, pH, and HABs from buoy assets off the PNW coast (Figure 1). These observations also support several models such as LiveOcean, which provides 72-hour projections of ocean variables such as temperature, salinity, Multi-Stressor Observations and Modeling to Build Understanding of and Resilience to the Coastal Impacts of Climate Change

Efforts to identify in situ the mechanisms underpinning the response of harmful algae to climate change demand frequent observations in dynamic and often difficult to access marine and freshwater environments. Increasingly, resource managers and researchers are looking to fill this data gap using unmanned systems. In this study we integrated the Environmental Sample Processor (ESP) into an autonomous platform to provide near real-time surveillance of harmful algae and the toxin domoic acid on the Washington State continental shelf over a three-year period (2016–2018). The ESP mooring design accommodated the necessary subsystems to sustain ESP operations, supporting deployment durations of up to 7.5 weeks. The combination of ESP observations and a suite of contextual measurements from the ESP mooring and a nearby surface buoy permitted an investigation into toxic Pseudo-nitzschia spp. bloom dynamics. Preliminary findings suggest a connection between bloom formation and nutrient availability that is modulated by wind-forced coastal-trapped waves. In addition, high concentrations of Pseudo-nitzschia spp. and elevated levels of domoic acid observed at the ESP mooring location were not necessarily associated with the advection of water from known bloom initiation sites. Such insights, made possible by this autonomous technology, enable the formulation of testable hypotheses on climate-driven changes in HAB dynamics that can be investigated during future deployments.

In the Beaufort Sea in September of 2015, concurrent mooring and microstructure observations were used to assess dissipation rates in the vicinity of 72°35′N, 145°1′W. Microstructure measurements from a free-falling profiler survey showed very low [ (10 − 10 ) W kg −1 ] turbulent kinetic energy dissipation rates ε . A finescale parameterization based on both shear and strain measurements was applied to estimate the ratio of shear to strain R ω and ε at the mooring location, and a strain-based parameterization was applied to the microstructure survey (which occurred approximately 100 km away from the mooring site) for direct comparison with microstructure results. The finescale parameterization worked well, with discrepancies ranging from a factor of 1–2.5 depending on depth. The largest discrepancies occurred at depths with high shear. Mean R ω was 17, and R ω showed high variability with values ranging from 3 to 50 over 8 days. Observed ε was slightly elevated (factor of 2–3 compared with a later survey of 11 profiles taken over 3 h) from 25 to 125 m following a wind event which occurred at the beginning of the mooring deployment, reaching a maximum of ε = 6 × 10 −10 W kg −1 at 30-m depth. Velocity signals associated with near-inertial waves (NIWs) were observed at depths greater than 200 m, where the Atlantic Water mass represents a reservoir of oceanic heat. However, no evidence of elevated ε or heat fluxes was observed in association with NIWs at these depths in either the microstructure survey or the finescale parameterization estimates.