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The 13th Symposium on Polar Science/Interdisciplinary sessions [IA] Arctic research, Thu. 17 Nov.

The 13th Symposium on Polar Science/Interdisciplinary sessions [IA] Arctic research, Thu. 17 Nov.

The 13th Symposium on Polar Science/Interdisciplinary sessions [IA] Arctic research, Thu. 17 Nov.

The 13th Symposium on Polar Science/Ordinary sessions [OM] Polar meteorology and glaciology, Wed. 16 Nov.

The 13th Symposium on Polar Science/Special session [S] Towards the next six-year Japanese Antarctic Research Project (phase X), Tue. 15 Nov.

Waves in the Marginal Ice Zone in the Okhotsk Sea are less studied compared to the Antarctic and Arctic. In February 2020, wave observations were conducted for the first time in the Okhotsk Sea, during the observational program by Patrol Vessel Soya. A wave buoy was deployed on the ice, and in situ wave observations were made by a ship-borne stereo imaging system and Inertial Measurement Unit. Sea ice was observed visually and by aerial photographs by drone, while satellite synthetic aperture radar provided basin-wide spatial distribution. On 12 February, a swell system propagating from east northeast was detected by both the stereo imaging system and the buoy-on-ice. The wave system attenuated from 0.34 m significant wave height to 0.25 m in about 90 km, while the wave period increased from 10 s to 15–17 s. This anomalous spectral downshifting was not reproduced by numerical hindcast and by applying conventional frequency-dependent exponential attenuation to the incoming frequency spectrum. The estimated rate of spectral downshifting, defined as a ratio of momentum and energy losses, was close to that of uni-directional wave evolution accompanied by breaking dissipation: this indicates that dissipation-driven nonlinear downshifting may be at work for waves propagating in ice. This article is part of the theme issue 'Theory, modelling and observations of marginal ice zone dynamics: multidisciplinary perspectives and outlooks'.

At reconnaissance to feasibility study stages of wave energy development, guidelines and standards recommend wave power assessments to be conducted for long periods of time, albeit at relatively coarse spatial resolutions. However, quite often developers need to make a preliminary site selection based on high-resolution information. Unfortunately, even with abundant computational resources nowadays, running a high-resolution coastal model for more than a decade is still not practical. In this paper, we propose to combine uncertainty estimates from a 21-year-long 1-km-resolution model and a high-resolution (10 m nearshore) coastal model to assess wave energy. Two sites in Japan facing the Pacific Ocean were selected for this study. One site located on the southern coast (site-S) is affected more by the passage of typhoons than another site located on the eastern coast (site-E). The estimated wave power climatology at site-S ranges between 3 and 7 $$\text{kW}\,\text{m}^{-1}$$ with an uncertainty of ± 0.27 to ± 0.51 $$\text{kW}\,\text{m}^{-1}$$ ; whereas for site-E, the climatology varies between 3 and 9 $$\text{kW}\,\text{m}^{-1}$$ with uncertainties of ± 0.12 to ± 0.34 $$\text{kW}\,\text{m}^{-1}$$ . In general, the eastern coastal area of Japan is found to offer higher wave power and lower margins of uncertainty than compared with the southern coastal area.

Ocean surface waves are known to decay when they interact with sea ice. Wave–ice models implemented in a spectral wave model, e.g. WAVEWATCH III® (WW3), derive the attenuation coefficient based on several different model ice types, i.e. how the model treats sea ice. In the marginal ice zone (MIZ) with sea ice concentration (SIC) < 1, the wave attenuation is moderated by SIC: we show that subgrid-scale processes relating to the SIC and sea ice type heterogeneity in the wave–ice models are missing and the accuracy of SIC plays an important role in the predictability. Satellite-retrieved SIC data (or a sea ice model that assimilates them) are often used to force wave–ice models, but these data are known to have uncertainty. To study the effect of SIC uncertainty ΔSIC on modelling MIZ waves during the 2018 R/V Mirai observational campaign in the refreezing Chukchi Sea, a WW3 hindcast experiment was conducted using six satellite-retrieved SIC products based on four algorithms applied to SSMIS and AMSR2 data. The results show that ΔSIC can cause considerable wave prediction discrepancies in ice cover. There is evidence that bivariate uncertainty data (model significant wave heights and SIC forcing) are correlated, although off-ice wave growth is more complicated due to the cumulative effect of ΔSIC along an MIZ fetch. The analysis revealed that the effect of ΔSIC can overwhelm the uncertainty arising from the choice of model ice types, i.e. wave–ice interaction parameterisations. Despite the missing subgrid-scale physics relating to the SIC and sea ice type heterogeneity in WW3 wave–ice models – which causes significant modelling uncertainty – this study found that the accuracy of satellite-retrieved SIC used as model forcing is the dominant error source of modelling MIZ waves in the refreezing ocean.

We have explored tsunami current signals in maritime Automatic Identification System (AIS) data during the 2011 Tohoku, Japan, tsunami. The AIS data were investigated in detail taking into account ship motion and response to tsunami current. Ship velocity derived from AIS data was divided into two components in terms of the ship heading: heading-normal and heading-parallel directions. The heading-normal velocity showed good agreement with the simulated tsunami current, as mentioned in our former research. Here, we found the heading-normal velocity was contaminated by non-tsunami noises that were mostly related to the ship yaw motion around the pivot point. The noises due to the yaw motion were reasonably corrected in the heading-normal velocity. The corrected heading-normal velocity clearly showed better agreement with the simulated tsunami current. Although the heading-parallel velocity is basically the navigation speed, and is mostly controlled by ships’ captain, we could find the heading-parallel velocity was also drifted by tsunami currents. The corrected heading-normal velocity was still a ship response to the tsunami current. Based on an equation of a ship response to tsunami currents, we numerically estimated tsunami current from the corrected heading-normal velocity. We could find very slight improvements in estimating the tsunami currents, which indicated that this operation possibly worked as a secondary correction. Tsunami currents of tens of centimeters per second are expected to be suitably detected using AIS based on discussion on detection limit.

Freak/rogue waves are considered to be the causes of marine accidents and their generation mechanism is closely related to the formation of wave groups. However, observations that capture the spatiotemporal evolution of coherent wave groups in directional windsea are rather limited. The paper presents a new technique known as the surface wave reconstruction by ensemble adjoint-free data assimilation (SWEAD) method that enables reconstruction of a spatiotemporal wave field covering a large area from wave records limited in observational density and spatial extent. We reconstructed spatiotemporal profiles of nonlinear surface gravity waves from virtual observational data using the adjoint-free four-dimensional variational data assimilation (a4DVar) scheme. The higher-order spectral method (HOSM) is used as a forward deep-water nonlinear wave model in a realistic sea state. The a4DVar scheme uses perturbed ensemble simulations to calculate the cost function gradient and Hessian; thus, construction of an adjoint model is not needed. A few extensions of the a4DVar scheme are proposed in this study. For efficient wave reconstruction, perturbed ensemble simulation results are reused by increasing the searching direction dimension at each iteration while assuring conformity to the perturbed model’s linearity. For regularization, Fourier coefficient magnitudes are constrained by a known power spectrum from the phase-averaged wave model. Twin experiments were conducted for a unidirectional wave with virtual wave gauge data and a multidirectional wave with virtual stereo camera imaging data. For both unidirectional and multidirectional cases, nonlinear freak wave–related wave groups were well reproduced, which is impossible using a linear model.

Variability in sea ice conditions, combined with strong couplings to the atmosphere and the ocean, lead to a broad range of complex sea ice dynamics. More in-situ measurements are needed to better identify the phenomena and mechanisms that govern sea ice growth, drift, and breakup. To this end, we have gathered a dataset of in-situ observations of sea ice drift and waves in ice. A total of 15 deployments were performed over a period of 5 years in both the Arctic and Antarctic, involving 72 instruments. These provide both GPS drift tracks, and measurements of waves in ice. The data can, in turn, be used for tuning sea ice drift models, investigating waves damping by sea ice, and helping calibrate other sea ice measurement techniques, such as satellite based observations.

Accurate ocean surface wave knowledge is crucial for ship design. With the significant advancements of model physics and numerical resources, the recent numerical wave hindcast data has a potential to provide environmental conditions for wave load estimation in the ship design process. This study aims to quantify model uncertainty in the state of art numerical wave hindcast products to get insight into the application of wave hindcast for ship design. The model uncertainty is deduced based on the comparison with the wave buoys in the North Atlantic as well as the inter-model comparison with four wave model products. The multiple wave buoys distributed over the Northwest Atlantic and Northeast Atlantic are considered as wave buoy arrays and used for model evaluations. All the four models in general showed very good and similar accuracy in the estimation of the significant wave height $H_s$. The model $H_s$, however, deviates from buoys and also deviate among each other for the extreme wave conditions of $H_s$>10. It is also found that there is disagreement for the mean wave period $T_{m02}$ between numerical wave models and the wave buoys. More specifically, the models underestimate $T_{m02}$ for extreme wave conditions of $H_s$>10 in the Northeast Atlantic. The joint probability density function (JPD) of the significant wave height $H_s$ and mean wave period $T_{m02}$ for the extreme wave conditions are also derived. Among the JPDs, the total number of samples that satisfies the extreme wave conditions of $H_s$>10 and 8, Comment: 15 pages, 11 figures, 3 tables

Uncertainties of wave data and models got increasing attention in the last decades by the marine industry because of their importance for safety at sea. Improved knowledge about wave description and related uncertainties leads to a reduction of safety factors, and consequently building costs of marine structures. The ISSC (International Ship and Offshore Structures Congress) and ITTC (International Towing Tank Conference) organizations have also been concerned with uncertainties associated with wave data and models used for the design and laboratory testing of marine structures. The different emphasis on uncertainty modelling by these organizations led to a somewhat different approach or at least different nomenclature adopted for uncertainty modelling. This study highlights these differences, but also existing similarities, in the context of ocean waves. It is addressing long-term wave description ; the wave data and the models used to extrapolate to extreme values. The investigations point out different sources of uncertainties in existing wave data sets and models which may impact design and operations of ships and offshore platforms and should get attention. The presented results are applicable to all types of marine structures.

The Kuroshio current is well known for generating cold wakes behind islands over Izu Ridge in Northwestern Pacific. Observational data from the geostationary Himawari-8 satellite for 2015–2017 revealed the occurrence of cold waters during the period when the Kuroshio current flows away from the islands. With a focus on tidal currents, this study presents an investigation of dynamical processes responsible for the formation of areas with low sea surface temperature (SST) through the adoption of a high-resolution numerical ocean model for an event that happened in July 2017. Areas with cold water emerged only when tidal currents are included in the numerical model. The model results indicate the cold surface waters are formed in the vicinity of the islands because of upwelling and vertical mixing. Qualitative features of the cold water formation for each island are found to depend on its size, topography, and ambient currents. Near Kozu Island, the tidal excursion is large enough to cause eddy shedding. These shed eddies are stirred by tidal currents to extend the surface cooling effect to wider areas. Near Hachijo Island, a persistent wake is formed by the ambient northward current. Inclusion of tidal currents destabilizes the wake, and consequently leads to the formation of a low SST area, although no clear eddy shedding is detected. The flow patterns around the islands are classified using an additional non-dimensional parameter, defined as the ratio between tidal excursion and island diameter.

This study proposes the detection of large offshore tsunamis by airborne radar. Altimeter measurements were carried out south of Japan by an observational aircraft along the Jason satellite tracks crossing over the Kuroshio Current. A nadir-pointing frequency-modulated continuous wave (FM-CW) radar and a global navigation satellite system (GNSS) based positioning system were concurrently used to measure the sea surface height (SSH). The SSH was validated against the Jason-2 and Jason-3 satellite altimeter SSH. After the removal of the geoid and tidal signals, the observation error was less than 10 cm. This is sufficiently small to capture offshore tsunamis within 10 min after the genesis. This was demonstrated by virtual airborne radar-detected SSH from a numerically simulated hypothetical large offshore tsunami south of Japan. The feasibilities and limitations of utilizing commercial airplanes with altimeters to create a dense tsunami observation network are discussed. The suggested methodology can be adapted to various emerging airborne platforms such as unmanned aerial vehicles. Moreover, a dense coverage of SSH observations is invaluable from an oceanographic point of view.

One physical mechanism that is responsible for the focusing of uni-directional water waves is the modulation instability. This occurs when side-bands around the main frequency are excited either deterministically or randomly and subsequently grow exponentially. In physical space, the periodically-perturbed wave group can reach significant wave amplifications and in the case of infinite modulation period even three times the initial amplitude of the regular Stokes wave train. These periodic wave groups propagate in deep-water with a group velocity half the wave’s phase speed. In this experimental study, we investigate the dynamics of modulationally unstable wave groups that propagate with a velocity that is higher than the packet’s group velocity in deep-water. It is shown that when this additional velocity to the wave group is marginal, a very good agreement with nonlinear Schrodinger hydrodynamics is reached at all stages of propagation and the characteristic wave energy cascade remains symmetric and stationary. Otherwise, a significant deviation is observed only at the stage of large wave focusing of the isolated wave packet. In this case, the wave field experiences an almost perfect return to the initial conditions after the compression, a particular dynamics also known as the Fermi–Pasta–Ulam paradox.

A towing experiment was conducted using a modulated wave train to investigate the vertical bending responses of a hydro-structural container ship model. In the experiment, a spatially periodic modulated wave train, as a model of a freak wave in successive high waves mimicking the so-called three sisters, was generated by the recently established higher-order spectral method wave generation (HOSM-WG) method. HOSM-WG enables us to control the location and timing of the maximum crest height in a wave tank. With precise control of the towing carriage, an experiment was conducted in which the timing of the encounters between the ship model and the modulated wave train was accurately determined. The maximum sagging moment (SM) was found to increase in proportion with the encounter wave height. However, because of differences in the relative depth of the fore and aft troughs, the maximum SM is highly variable for a given wave height. The temporal wave-geometry evolution caused the relative trough-depth to vary significantly within a wave period in the vicinity of the maximum crest height. As a result, depending on the encounter timing, the SM varied considerably for a given wave height. The temporal variation of the wave geometry is a robust feature of a modulated wave train and is common between the spatially periodic and temporally periodic modulated wave trains.

We seek to improve estimation of extreme sea state severities offshore Japan. In this tropical cyclone-dominated region, magnitudes of large values of storm severity (significant wave height, H S ) observed at a location of interest are highly dependent on the trajectories of tropical cyclones relative to the location. As a result, a naive estimate for a return value of storm severity at a location of interest corresponding to a long return period, made using a relatively short period of observational or hindcast data, shows unrealistically large spatial variation. To address this issue, we propose a pragmatic statistical representation for cyclone sea state severity in space and time, consisting of (1) an extreme value model for the maximum of storm severity per cyclone (over space and time), and (2) a model for the “exposure” of a location to a random cyclone event. For a particular location, exposure quantifies the maximum storm severity observed during a cyclone event, expressed as a fraction of the storm peak severity (over space and time). Importantly, exposure is quantified per location on an absolute spatial lattice, independent of cyclone path relative to the location. Numerous statistical diagnostic tests are performed to justify that modelling assumptions made are consistent with data. Resulting return value estimates have plausible magnitudes and show plausible spatial variation, and in particular reflect bathymetric and shielding effects of coastlines and islands.

A new wave generation method, called HOSM-WG, is presented in this paper. The method utilizes the precomputed spatially periodic wave field based on the higher-order spectral method (HOSM) simulation to calculate the wave-maker signals. The spatially periodic wave fields were reproduced in a rectangular basin with 382 wave makers on the peripheral. Upon wave generation, the bound components are excluded from the spectrum and the free waves are generated by applying the Schaffer's second-order correction. The HOSM-WG was first validated for unidirectional spatially-periodic modulated wave trains generated in physical and numerical wave basins. Then, freak waves in unidirectional irregular waves and in directional irregular waves were generated. The time series of surface elevation, frequency spectra, and spatial wave profiles compared well between the HOSM-WG and HOSM simulation even for a broad bimodal directional spectrum.

Two freak waves were observed a day apart in October 2009 at a 5000-m deep moored station in the northwest Pacific Ocean. As the typhoon passed by, the wave system transitioned within a day from a narrow and unimodal spectrum to the broad and bi-modal spectrum. The occurrence probability of a freak wave is known to increase due to a modulational instability; however, whether the modulational instability survives under a realistic directional sea state has not been conclusively determined yet. In this study, a phase-resolving wave model was used to obtain ensembles of realizations based on observed and simulated directional spectra. Unlike previous studies that focused only on the probability of freak wave occurrence, this study focuses on wave shape. It reveals that the front-to-rear asymmetry and crescent shape deformation of the crest are more pronounced for narrower spectrum and longer-lifetime freak waves; this distortion of wave shape and extended lifetime are both characteristics of nonlinear wave groups. This study also shows that the distribution of the lifetime of a freak wave depends on the sea state and that the number of nonlinear wave groups increases for a narrower spectrum. We therefore conjecture that both the four-wave quasi-resonance and dispersive focusing are responsible for freak wave generation, but their relative significance depends on the spectral broadness. Investigating the total kurtosis or occurrence probability alone is insufficient to unravel the underlying mechanisms of individual freak-wave generation.

Wave generation and growth via wind input in a marginal ice zone (MIZ) is not a well-understood process and remains a neglected component in wave-ice models. During the 2020 R/V Mirai observational campaign in the freezing season, a 3-day wave evolution event was captured in the Beaufort Sea MIZ by four drifting wave buoys that were spread zonally over a distance of roughly 60 km. ERA5 surface wind speed over these buoys were 5–10 ms-1, and the direction was off-ice and primarily zonal, i.e., waves grew from ice cover to ice-free waters. The peak significant wave heights were ~0.8 m and over 2 m for the buoys located furthest from and nearest to the ice edge, respectively, with sea ice concentrations between 0.3 and 0.8. The most notable features of the observation are as follows: 1) waves were seemingly generated in ice cover; 2) the wave age was (i.e., wind speed was slower than wave propagation) for the duration of the event at all the buoys. We present analysis results with a physical viewpoint of wave evolution in freezing MIZs.

Sea ice seasonally covers the Sea of Okhotsk, a marginal Arctic basin nested between Russia and Japan, but its extent is predicted to decrease by 40% by 2050 leaving larger ice free areas over which waves can form. In the highly dynamical seasonal ice zone, i.e. where waves and ice interact, ice formation and breakup, and wave attenuation mutually affect each other via complex feedback mechanisms. To shed light into these interactions, wave measurements were conducted in the winter seasonal ice zone in the Southern Okhotsk Sea, North of Hokkaido, from onboard the P/V Soya using a stereo camera system. Data show that wave energy penetrates even in high ice concentration (>85%), where contemporary wave models predict complete attenuation of wind waves. Consistently with physical experiments and field observations of waves in the Arctic and Antarctic marginal ice zones, the measurements also show that the ice cover is more effective in attenuating short wave components and, consequently, the dominant wave period in ice is significantly increased compared to corresponding open ocean waves. The present data can inform calibration of wave models in the rapidly evolving seasonal ice zone in the Sea of Okhotsk.

Wave statistics of the North Atlantic Ocean, instrumental in designing merchant ships, were revisited by an in-house high-resolution 25-year wave hindcast (TodaiWW3-NK). The tail of the exceedance ...

Surface ocean waves are one of the potential processes that influence on the Arctic sea-ice extent. A better understanding of the generation, propagation, and attenuation of ocean waves under the sea ice is necessary to discuss the future Arctic climate change. We deployed two drifting wave buoys in the marginal ice zone in the western Arctic. Since the surface wave observation in the marginal ice zone is rare, the obtained data are useful for validation of the numerical modeling of the surface waves under the sea ice. The first buoy was deployed in the pancake-ice covered area and the second one in the open ocean. The distance between the two buoys at the deployment was about 40km, and the second buoy was deployed approximately 5 hours after the first deployment. The comparison of the wave bulk statistic measured by the two buoys shows the wave transformation under the sea ice. In general, the significant wave height decreases, and the mean wave periods increase by the presence of the sea ice.

We experimentally investigated the influence of the geometries of a modulated wave train on the vertical-bending and torsional moments acting on a container ship in bow-quartering sea conditions. We conducted a towing experiment with a hydro-structural container ship model in the Actual Sea Model Basin (ASMB) (80 m deep, 40 m wide, and 4.5 m deep) at the National Maritime Research Institute. The ship model is made of urethane foam and was designed to have similar vertical bending and torsional vibration mode shapes to an actual ship. A modulated wave train was generated in the ASMB by the higher-order spectral-method wave generation (HOSM-WG) method such that the maxim crest appeared at the center of the basin. The ship model was towed in the modulated wave train with a relative heading angle of 120 degrees. A series of tests was performed by varying the encounter timing of the ship model and the maximum crest of the modulated wave train. In the experiment, fiber-Bragg-grating strain gauges successfully measured whipping vibrations of the ship model due to a slamming impact. The experimental results revealed that the rear wave height Hr and the ratio of the rear and front wave heights Hr/Hf were the dominant parameters governing the maximum sagging and torsional moments of a ship in bow-quartering modulated wave trains.

Extreme value analysis of significant wave height using data from a single location often incurs large uncertainty due to small sample size. Including wave data from nearby locations increases sample size at the risk of introducing dependency between extreme events and hence violating modelling assumptions. In this work, we consider extreme value analysis of spatial wave data from the 109-year GOMOS wave hindcast for the Gulf of Mexico, seeking to incorporate the effects of spatial dependence in a simple but effective manner. We demonstrate that, for estimation of return values at a given location, incorporation of data from a circular disk region with radius of approximately 5° (long.-lat.), centred at the location of interest, provides an appropriate basis for extreme value analysis using the STM-E approach of Wada et al. (2018).

We examine and discuss the spatial evolution of the statistical properties of mechanically generated surface gravity wave fields, initialised with unidirectional spectral energy distributions, uniformly distributed phases and Rayleigh distributed amplitudes. We demonstrate that nonlinear interactions produce an energy cascade towards high frequency modes with a directional spread and triggers localised intermittent bursts. By analysing the probability density function of Fourier mode amplitudes in the high frequency range of the wave energy spectrum, we show that a heavy-tailed distribution emerges with distance from the wave generator as a result of these intermittent bursts, departing from the originally imposed Rayleigh distribution, even under relatively weak nonlinear conditions.