The attenuation of ocean surface waves during seasonal ice cover is an important control on the evolution of many Arctic coastlines. The spatial and temporal variations in this process have been challenging to resolve with conventional sampling using sparse arrays of moorings or buoys. We demonstrate a novel method for persistent observation of wave-ice interactions using distributed acoustic sensing (DAS) along existing seafloor telecommunications cables. The DAS measurements span a 36-km cross-shore seafloor cable on the Beaufort Shelf from Oliktok Point, Alaska. DAS measurements of strain-rate provide a proxy for seafloor pressure, which we calibrate with wave buoy measurements during the ice-free season (August 2022). We apply this calibration during the ice formation season (November 2021) to obtain unprecedented resolution of variable wave attenuation rates in new, partial ice cover. The location and strength of wave attenuation serve as a proxy for ice coverage and thickness, especially during rapidly-evolving events.

Wind, wave, and acoustic observations are used to test a scaling for ambient sound levels in the ocean that is based on the relative penetration depth of active bubbles during surface wave breaking. The focus is on acoustic frequencies in the range 1-10 kHz, which are typically scaled by wind speed alone. Wind and wave information are combined in a parametric form to describe the depth of the active bubble layer (which produces sound) relative to the depth of the passive bubble layer (which attenuates sound). The relative depth scaling has a primary dependence on wind speed and a secondary dependence on any departure of significant wave height from fully-developed, open-ocean conditions. The scaling is tested with long time-series observations of winds and waves at Ocean Station Papa (North Pacific Ocean), as well as with a case study with fetch limitation near the island of Jan Mayen (Norwegian Sea). When waves are less developed (e.g., limited by fetch) at a given wind speed, the attenuating layer is relatively thin and the sound levels are higher. The scaling is a plausible explanation for the observed reduction in sound levels during high wind events (winds greater than 15 m/s).

In the past decade, two large marine heatwaves (MHWs) formed in the northeast Pacific near Ocean Station Papa (OSP), one of the oldest oceanic time series stations. Physical, biogeochemical and biological parameters observed at OSP from 2013 to 2020 are used to assess ocean response and potential impacts on marine life from the 2019 northeast Pacific MHW. The 2019 MHW was preceded by calm and stratified surface conditions, lower dissolved inorganic carbon, and higher pH of surface waters relative to the 2013-2020 period. A spike in the summertime chlorophyll followed by a decrease in surface macronutrients suggests increased productivity in the well-lit stratified upper ocean during summer 2019. More blue whale calls were recorded at OSP in 2019 compared to the prior year. Large subsurface temperature anomalies were also found, suggesting that the earlier northeast Pacific MHW (2013-2015, previously referred to as “Blob”) as well as the long-term increase in sea surface temperatures in the region contributed to the intensity of the 2019 MHW. This study shows how the utility of long-term, continuous oceanographic datasets and analysis with an interdisciplinary lens is necessary to understand the potential impact of MHWs on marine ecosystems.

Capes and cape-associated shoals represent sites of convergent sediment transport, and can provide points of relative coastal stability, navigation hazards, and offshore sand resources. Shoal evolution is commonly impacted by the regional wave climate. In the Arctic, changing sea-ice conditions are leading to (1) longer open-water seasons when waves can contribute to sediment transport, and (2) an intensified wave climate (related to duration of open water and expanding fetch). At Blossom Shoals offshore of Icy Cape in the Chukchi Sea, these changes have led to a five-fold increase in the amount of time that sand is mobile at a 31-m water depth site between the period 1953-1989 and the period 1990-2022. Wave conditions conducive to sand transport are still limited to less than 2% of the year, however - and thus it is not surprising that the overall morphology of the shoals has changed little in 70 years, despite evidence of active sand transport in the form of 1-m-scale sand waves on the flanks of the shoals which heal ice keel scours formed during the winter. Suspended-sediment transport is relatively weak due to limited sources of mud nearby, but can be observed in a net northeastward direction during the winter (driven by the Alaska Coastal Current under the ice) and in a southwestward direction during open-water wind events. Longer open-water seasons mean that annual net northeastward transport of fine sediment may weaken, with implications for the residence time of fine-grained sediments and particle-associated nutrients in the Chukchi Sea.

Wave energy propagating into the Antarctic marginal ice zone affects the quality and extent of the sea ice, and wave propagation is therefore an important factor for understanding and predicting changes in sea ice cover. Sea ice is notoriously hard to model and in-situ observations of wave activity in the Antarctic marginal ice zone are scarce, due to the extreme conditions of the region. Here, we provide new in-situ data from two drifting Surface Wave Instrument Float with Tracking (SWIFT) buoys deployed in the Weddell Sea in the austral winter and spring in 2019. The buoy location ranges from open water to more than 200 km into the sea ice. We estimate the attenuation of swell with wave periods 8-18 s, and find an attenuation coefficient  α = 4 · 10-6 to 7 · 10-5 m-1 in spring, and approximately five-fold larger in winter. The attenuation coefficients show a power law frequency dependence, with power coefficient 3.3 in spring and 4 in winter. The in-situ data also shows a change in wave direction, where wave direction tends to be more perpendicular to the ice edge farther into the sea ice. A possible explanation for this might be a change in the dispersion relation caused by changing sea ice composition. These observations can help shed further light on the influence of sea ice on waves propagating into the Marginal Ice Zone, aiding development of coupled wave-sea ice models.

A document by Morteza Derakhti. Click on the document to view its contents.

The effects of instrument noise on estimating the spectral attenuation rates of ocean waves in sea ice are explored using synthetic observations in which the true attenuation rates are known explicitly. The spectral shape of the energy added by noise, relative to the spectral shape of the true wave energy, is the critical aspect of the investigation. A negative bias in attenuation that grows in frequency is found across a range of realistic parameters. This negative bias decreases the observed attenuation rates at high frequencies, such that it can explain the rollover effect commonly reported in field studies of wave attenuation in sea ice. The published results from four field experiments are evaluated in terms of the noise bias, and a spurious rollover (or flattening) of attenuation is found in all cases. Remarkably, the wave heights are unaffected by the noise bias, because the noise bias occurs at frequencies that contain only a small fraction of the total energy.

Increasing extent and duration of seasonally ice-free area in the western Arctic Ocean suggests increased air-sea coupling, specifically fluxes of momentum and heat between the lower atmosphere and the upper ocean. The dependence of these fluxes on ice concentration and its dynamical characteristics is still uncertain. As part of the Stratified Ocean Dynamics of the Arctic (SODA) project, year-long time series of upper-ocean velocity profiles were obtained across a range of ice conditions and are used to infer momentum fluxes. We consider the structure of observed current profiles as a function of sea state and ice cover. During the summer and in open water with minimal stratification, the wind forcing is in local equilibrium with surface gravity waves, and there is a direct transfer of momentum from the atmosphere to the upper ocean. The presence of ice modifies the momentum budget through both the inclusion of ice-atmosphere and ice-ocean stresses, and by damping short surface gravity waves and thus changing the surface roughness that the atmosphere acts upon. Ice presence is also associated with increased near-surface stratification, which can act to decouple the sub-surface ocean from atmospheric forcing. Our observations show frequent decoupling of a thin surface layer (<10 m depth), including case studies in which the relatively fresh surface waters formed by “ice puddles” have entirely different motion from the relatively salty water a few meters below. Ice formation in the fall affects both the ocean stratification and the ice characteristics, leading to competing effects affecting momentum transfer. Initial results across the annual cycle are presented.

The retreat of Arctic sea ice is enabling increased ocean surface wave activity at the sea ice edge, yet the physical processes governing interactions between waves and sea ice are not fully understood. Here, we use a collection of in situ observations of waves in ice to evaluate a recent global climate model experiment that includes coupled interactions between ocean waves and the sea ice floe size distribution. Observations come from subsurface moorings and free-drifting buoys spanning 2012-2019 in the Beaufort Sea, and we group the data based on distance inside the ice edge for comparison with model results. Locally generated wind waves are relatively prevalent in observations beyond 100 km inside the ice but are absent in the model. Low-frequency swell, however, is present in the model, while subsurface moorings located more than 100 km inside the ice do not report any swell with significant wave height exceeding the instruments' detection limits. These results motivate further model development and future observing campaigns, suggesting that local wave generation inside the ice edge may play a significant role for floe fracture while demonstrating a need for more robust constraints on wave attenuation by sea ice.

Interaction between surface gravity waves and sea-ice in the marginal ice zone is complex, and most of the prior research focus has been in deeper oceans. Here, the regional wave model Simulating WAves Nearshore (SWAN) is configured to simulate reduced wind-generation and wave dissipation in the presence of sea-ice. The wind-generation process is modified by scaling the generation terms with the open-water fraction, while wave dissipation in the presence of sea-ice is simulated as an exponential energy decay as function of ice concentration, wave frequency and empirical coefficients determined from prior experiments. Modified SWAN is used to simulate interaction between regional sea-ice and a swell event in the Barents Sea. The simulation accounting for wave-ice interaction reasonably agrees with field measured significant wave height and the energy spectral density. Additional simulations are conducted for the shallow seas of Gulf of Bothnia, located in the northernmost reach of the Baltic sea. Modeled wave dynamics in this region agrees well with satellite altimetry based measurements. This model setup is further investigated to understand fetch scaling in the marginal ice zone, and non-dimensional energy scales well with a non-dimensional fetch determined from a cumulative fetch dependent on ice concentration. Additional implications for Stokes drift and Stokes drift shear are also discussed for the Bothnian bay. Finally recommendations for including dissipation due to ice thickness, and plans for future model coupling are considered.

The retreat of Arctic sea ice coincides with increased ocean surface wave activity, and wave-ice interactions are consequently poised to have a growing influence on the Arctic climate system. Recent field campaigns have focused on rectifying the scarcity of wave measurements inside the marginal ice zone, and work is now underway to incorporate wave-ice interactions in global climate models. Here, we apply a collection of in situ wave observations spanning multiple years in the Beaufort Sea and including wave activity beyond 100 kilometers inside the sea ice edge. To better understand waves in the presence of sea ice, we connect the in situ data with satellite-derived ice concentrations across the Arctic and compare the observations with a recent global climate model experiment that includes coupled interactions between waves and a sea ice floe size distribution. We present a series of comparisons focused on wave energy and wind-wave relationships in partial ice cover. These analyses provide a framework for assessing the impact of uncertainty in wave-ice physics on the marginal ice zone in new experiments in the coupled wave-ice model. Our work guides further model development and future observational campaigns.

Cross-shore transport of larvae, pollutants, and sediment between the surf zone and the inner shelf is important for coastal water quality and ecosystems. Rip currents are known to be a dominant pathway for exchange, but the effects of horizontal temperature and salinity gradients are not well understood. Airborne visible and infrared imagery performed on the California coast show warm and cool plumes driven by rip currents in the surf zone and extending onto the shelf, with temperature differences of approximately 1$^\circ$C. The airborne imagery and modeled temperatures and tracers indicate that warm plumes exhibit more lateral spreading and transport material in a buoyant near-surface layer, whereas cool plumes move offshore in a subsurface layer. The average cross-shore extent of warm plumes at the surface is approximately one surfzone width larger than for cool plumes. Future work may explore the sensitivity of nearshore plumes to density patterns, wave forcing, and bathymetry.