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.

A document by Jim Thomson. Click on the document to view its contents.

A document by Hannah Glover. Click on the document to view its contents.

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 fleet of surface drifters (SWIFTs) equipped with down-looking high-resolution ADCPs were used to estimate the turbulent kinetic energy (TKE) dissipation rate (ε) within highly stratified diurnal warm layers (DWLs) in the Southern California bight. Over a 10-day period, five instances of DWLs were observed with strong surface temperature anomalies up to 3 °C and velocity anomalies up to 0.3 m s-1. Profiles of ε in the upper 5 meters suggest turbulence is strongly modulated by the DWL stratification. Burst-averaged (8.5 minutes) ε is stronger than predicted by law-of-the-wall boundary layer scaling within the DWL and suppressed below. Predictions for ε within the DWL are improved by a shear-production scaling using observed shear and linearly decaying turbulent stress. However, ε is still under-predicted. Examination of the un-averaged acoustic backscatter data suggests elevated ε is related to the presence of turbulent structures in the DWL which span the layer height and strongly modulate TKE. Evolution in the bulk Richardson number each day suggests the DWLs become unstable to layer-scale overturning and entrainment each afternoon, thus the turbulent structures may result from shear-driven instability. This interpretation is supported by a conditional average of the data during a burst characterized by strongly periodic structures. The structures resemble high-frequency internal waves with strong asymmetry in the along-flow direction (steepening) which suggests they are unstable. Coincident asymmetric patterns in upwelling/downwelling and corresponding regions of strong vertical convergence/divergence suggest both vertical transport and local TKE generation are plausible sources of elevated ε in the DWL.

Drifting buoy observations in Hurricane Idalia (2023) are used to investigate the dependence of ocean surface wave mean square slope on wind, wave, and storm characteristics. Mean square slope has a primary dependence on wind speed that is linear at low-to-moderate wind speeds and approaches saturation at high wind speeds (> 20 m s-1). Inside Hurricane Idalia, buoy-measured mean square slopes have a secondary dependence on wind-wave alignment: at a given wind speed, slopes are higher where wind and waves are aligned compared to where wind and waves are crossing. At moderate wind speeds, differences in mean square slope between aligned and crossing conditions can vary 10% to 15% relative to their mean. The dependence on wind-wave alignment is robust up to 30 m s-1, but can be obscured at the highest wind speeds near the center of the storm where wind and wave directions change rapidly. These changes in wave slopes may be related to the reported dependence of air-sea drag coefficients on wind-wave alignment.

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.

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.