Sea Wind Waves Research Articles

On the need to integrate interannual natural variability into coastal multihazard assessments

The co-occurrence of multiple hazards can either exacerbate or mitigate risks. The interrelationships between multiple hazards greatly depend on the spatiotemporal scale and can be difficult to detect from large to local scales. In this paper, we identified coastal regions worldwide where the leading tropical (El Niño-Southern Oscillation, ENSO) and polar (Arctic Oscillation, AO; Southern Annular Mode, SAM) modes of climate variability simultaneously modify the seasonal conditions of multiple hazards, including the near-surface wind speed and swell and wind-sea wave powers. We classified the results at the national and municipal levels, with a focus on multiple hazards simultaneously occurring in space and time. The results revealed that the ENSO modulates multiple hazards, affecting approximately 40% of coastal countries, while the polar annular modes affect approximately 30% of coastal countries. The ENSO induced a greater diversity of multiple hazards, with Asian countries (e.g., Indonesia experienced increases of + 2% in wind and + 7% in swell) and countries in the Americas (e.g., Peru exhibited increases of + 1.5% in wind and + 6% in wind-sea) the most notably affected. The SAM imposed a greater influence on swells in the eastern countries of ocean basins (+ 2.5% in Chile) than in other countries, while the influence of the AO was greater in Norway and the UK (+ 12% for wind-sea and 8% for swell). Low-lying islands exhibited notable variations in pairwise hazards between phases and seasons. Our results could facilitate the interpretation of multihazard interactions and pave the way for a wide range of potential implementations of different coastal industries.

SEAKEEPING EVALUATION OF SHIPS FOR OPERATIONAL PLANNING IN THE GULF OF GUINEA

This paper evaluates the sea-keeping characteristics of a Platform Supply Vessel (PSV) for operational planning in the Gulf of Guinea according to the respective international requirements or standard criteria. The ship’s design and operation characteristics were examined, the characteristics of the specified sea environment in which the ship should perform its mission were assessed, the impact of the specified sea environment on the design and operation of the ship was evaluated and ultimately, the operational criteria that should not be exceeded, so that the ship can perform its mission effectively was determined. Structural Analysis and Design Computer-Aided Design (SACS CAD) Software was employed to model the ship design and it was subjected to the environment in which it is to operate. The Joint North Sea Wave Project (JONSWAP) spectrum was used to model the local wind sea wave with Orca Finite Element Analysis of Flexible Systems (ORCAFLEX). This analysis showed the dynamic movement of the Platform Supply Vessel (PSV) in the various degrees of freedom (heave, surge, sway, yaw, roll, and pitch). Broad-based results showed that the PSV is fit to be operational. This was because of the various tests that were conducted for normal conditions. The various degrees of freedom offset exerted by the PSV were below the tolerance limit of platform supply vessels (10% of water depth of 100m=10m). The results obtained were for the south, the heave offset of 1.6032m and surge offset of 2.1507m. For the southwest, the heave offset of 1.5033m, the surge offset of 1.0712m, and the sway offset of 1.0753m. For the west, the heave offset of 1.068m, and the sway offset of 1.2114m. Local Sea wind wave in the south direction (180°) exerts the most environmental loading on the designed PSV. This was because of the result obtained for the extreme condition (heave offset of 1.9571m and surge offset of 1.4107m. The PSV stability parameters were also obtained as summarized in Appendix C and known to be within the limit of acceptability and validated by comparing the righting lever, GZ, of the designed PSV in Figure 16 with that obtained by marine.info (2021). Both righting levers, GZs were above the 30 degrees requirement of UKMCA (2008) criteria for PSVs. The GZ obtained was 62.8 degrees which should not be exceeded to prevent the PSV from capsizing. The study contributes to the body of knowledge surrounding seakeeping evaluations of PSVs and serves as a foundation for evaluating the design and performance of these vessels in the demanding offshore environment

Nonlinear Interaction of Infragravity and Wind Sea Waves

In this paper, the authors analyze data obtained from a supersensitive detector of hydrosphere pressure variations which was positioned on the shelf of the Sea of Japan at a depth of 25 m for several months. When processing this data, the main attention was paid to studying nonlinear hydrophysical disturbances of “rogue waves” type: “one sister”, “two sisters”, “three sisters”, and “potential well”, the origin of which is associated, apparently, with the interaction of the hydrophysical wave field in gravity range and disturbances in the infragravity range. Analysis of synchronous data of the laser strainmeter and laser nanobarograph, installed at Shultz Cape, with synchronous records of the supersensitive detector of hydrosphere pressure variations confirmed these conclusions.

A coupled wave-hydrodynamical model to assess the effect of Mediterranean storms under climate change: The Calabaia case study

Climate change will have an undeniable influence on coastal areas, resulting in increased rates of both sea level rise and storm-related impacts. In this context, it is crucial to estimate the local probable extreme sea wave conditions, to properly reproduce the sea state and the coastal hydrodynamic, and to investigate the effectiveness of sea defenses under sea level rise. This work describes the first steps towards an innovative fully coupled modeling system composed of a wind-sea wave (SWAN) and hydrodynamic model (2DEF). Numerical simulations, focusing on the shoreline of Calabaia, Cosenza, Italy, have been compared to the MIKE model outcomes in the same area. The simulations have been performed to investigate the inshore sea wave characteristics, to assess the effectiveness of the actual sea defense interventions, and to identify the impact of extreme storms, by combining sea level rise and extreme sea wave scenarios with the most recent georeferenced territorial data. The models are two-way coupled at half-hourly intervals exchanging the following fields: 2D sea level, surface currents and bottom elevation are transferred from 2DEF to SWAN; sea wave characteristics computed by SWAN is then passed to 2DEF by modifying the radiation stress.

Space-time statistics of extreme ocean waves in crossing sea states

The study of extreme ocean waves has gained considerable interest in recent years, due to their importance for offshore design and navigation safety, and several theoretical approaches have been developed for their statistical description. However, in the case of crossing seas, where two or more wave systems of different characteristics are present, a full understanding of the main physical mechanisms responsible for the occurrence of very high individual waves is still lacking. As a consequence, the prediction of extremes in such conditions currently relies on integrated parameters of the total sea state, such as the spectral wave steepness. In this study, to gain further insight into the role of the crossing wind sea and swell wave systems in producing extreme individual waves, we investigate realistic sea states during typhoon Kong-rey (2018) using an ensemble of numerical simulations obtained from a phase-resolving wave model based on the High-Order Spectral (HOS) method. The reliability of the numerical fields is assessed for the first time with stereo wave measurements of the sea surface elevation field collected from an offshore platform in the area of interest. We show that, in specific conditions, space-time extreme crest heights in crossing seas can be larger than in unimodal seas due to second-order bound wave interactions between the wind sea and the swell. To improve existing prediction capabilities, we propose a novel formulation for the wave steepness in crossing seas, which includes nonlinear effects up to the second order and accounts for the spectral parameters of the interacting wave systems.

Drift simulation of a floating offshore wind turbine with broken mooring lines in a dynamic sea condition

This study investigates the aero-hydro-mooring coupled dynamics of the National Renewable Energy Laboratory (NREL) offshore 5-MW baseline wind turbine supported by the Offshore Code Comparison Collaboration Continuation (OC4) DeepCwind semisubmersible floating platform by considering the effects of suddenly broken mooring behaviour. The wind and wave loads can be selected as the Cummins time-domain equation inputs based on the Norwegian Petroleum Directorate wind and Joint North Sea Wave Project wave spectrums. The nonlinear viscous drag was estimated using the quadratic damping matrix rather than Morison's element. Additionally, the quadratic transfer function matrix was used to calculate the slow-drift force. Compared to the multi-segmented quasi-static mooring model, the lump-mass method is a dynamic mooring model that can solve real-time motion on the platform. Once a single or a pair of mooring lines is intentionally disconnected from the floating platform at a certain time, the transient responses of mooring line tensions and turbine performance will be evaluated. The novelty of this study is that the drift trajectories and power performances of OC4 DeepCwind semisubmersible floating offshore wind turbine under different conditions of environmental loads and broken mooring lines can be approximately estimated in the offshore wind farm. The analysis can be used for the environmental impact assessment in harsh sea conditions.

Impact of the ENSO phenomenon on wave variability in the Pacific Ocean for wind sea and swell waves

Using wave data from 1960 to 1990 from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis ERA-40 dataset and the ERA-5 reanalysis dataset (1990–2018), this paper presents a comprehensive study on the spatial and temporal variability of wind sea wave height (Hw), swell wave height (Hsw) and mixed significant wave height (Hs) in the Pacific during the warm (El Niño) and cold (La Niña) ENSO phases. The main findings show that During DJF, the Hsw is dominated by strong winds and large waves from extratropical cyclones in the North Pacific, especially from 1960 to 1990. However, from 1990–2018 these maximum values extend into the subtropical crest of the Pacific Ocean and the southeast trade winds increase during La Niña. The annual cycle of Hw, Hsw and Hs for specific points located from the west center toward the east center along the equatorial Pacific for the analyzed decades show a strong transition in the locations of the maximum and minimum values during the year. In this sense, the eastern point is more highly influenced by the swell systems that flow from the northwest and southwest of the southern hemisphere associated with the intensification of winds during the occurrence of La Niña. It is therefore not possible to determine a single period of maximum and minimum values. During the decades of 1990–2018, the difference in swell wave height during La Niña compared to El Niño increased throughout the year compared to 1960–1990. During the 1960–1990 period, the maximum values were detected in JJA for both El Niño and La Niña in an area that extends from Perú and ends toward the eastern center of the Colombian Pacific Basin.

Frequency-dependent wave damping by tidal wetlands under storm conditions

Climate change is increasing the threat of coastal flooding worldwide, and the incorporation of tidal wetlands into coastal protection is being widely promoted. These circumstances have motivated extensive research on wetland-induced wave damping, while its distribution across the frequency scales remains poorly understood. Here, field observations during Typhoon Rumbia were adopted to examine the frequency-dependent wave damping by a Changjiang estuarine wetland, which consists of a salt marsh followed seaward by an unvegetated mudflat, under storm conditions. The results showed that the typhoon produced waves with approximately trimodal spectra comprising low-frequency swell (0.02–0.15 Hz), moderate-frequency wind-sea (0.15–0.48 Hz) and high-frequency components (0.48–0.70 Hz) at the mudflat edge. As the waves propagate landward across the mudflat and salt marsh, the integral wave heights are successively attenuated by 16 % and 32 % and the wind-sea waves continuously experienced the largest height attenuation. Mechanisms for the frequency-dependent wave damping were explored by discriminating energy dissipation and nonlinear transfer and reconstructing wave spectra induced by each of them. The frequency-dependent wave damping by unvegetated mudflats is controlled by nonlinear energy transfer, which shifts 10 % of the gross energy from the wind-sea to high-frequency components; the wind-sea waves’ attenuation is increased by 66 % but the high-frequency wave attenuation is reduced by 88 %, compared with the regime controlled solely by direct dissipation. Direct dissipation regulates the frequency-dependent wave damping across the salt marsh, and the contribution of nonlinear energy transfer to height attenuation of the wave components is below 15 %. The similar regime but differing mechanisms by which the studied wetland units damp waves of different frequencies provide new insights for wetland-wave interactions.

Multiparametric sea state fields from synthetic aperture radar for maritime situational awareness

This paper introduces a method for estimating a series of sea state parameters from satellite-borne synthetic aperture radar (SAR). The method was realized in a near real time (NRT) application which allows for the processing of data from different satellites and modes. The algorithm estimates the total significant wave height Hs, dominant and secondary swell and windsea wave heights, first, and second moment wave periods, the mean wave period and the period of wind sea. The algorithm was applied to the Sentinel-1 (S1) C-band Interferometric Wide Swath Mode (IW), Extra Wide (EW) and Wave Mode (WV) Level-1 (L1) products and also extended to X-band TerraSAR-X (TS-X) StripMap (SM) products. The scenes are processed in raster and result in continuous sea state fields, with the exception of S1 WV, where averaged values for sea state parameters for along-orbit imagettes of 20 km × 20 km are presented.The developed empirical algorithm consists of two parts: a first CWAVE_EX (extended CWAVE) part, based on a linear regression approach, and a subsequent machine learning part using the support vector machine (SVM) technique. A series of new data preparation steps (i.e. filtering, denoising) and new features estimated from SAR images are also introduced. The algorithm was tuned and validated using two independent global wave model hindcasts, WaveWatch-3 and MFWAM as well as National Data Buoy Center (NDBC) measurements. The achieved root mean squared errors (RMSE) for CWAVE_EX for the total Hs are 0.60 m for low-resolution modes S1 IW (10 m pixel) and EW (40 m pixel) and 0.35 m for S1 WV and TS-X SM (pixel spacing ca.1–4 m) in comparison to model predictions. The RMSEs of the retrieved wave periods are in the range of 0.45–0.90 s for all of the satellites and models considered. Similarly, the dominant and secondary swell, and wind sea wave height RMSEs are in the range of 0.35–0.80 m. The SVM postprocessing improves the accuracy of the initial results of CWAVE_EX for Hs and reaches an RMSE of 0.25 m for S1 WV. Comparisons to 64 NDBC buoys, collocated at distances shorter than 50 km to S1 WV imagettes worldwide, result in an RMSE of 0.41 m. All results and the methods presented are novel in terms of the accuracy achieved, combining the classical approach with machine learning techniques, and performing an automatic NRT processing of multiparametric sea state fields from L1 data with automatic switching for different satellites and modes.The complete archive of S1 WV L1 Single Look Complex products from December 2014 until February 2021 was processed to create a sea state parameter database and validated using model hindcast and buoy measurements. The derived parameters are available to the public within the scope of the European Space Agency's Climate Change Initiative.

Global trends in oceanic wind speed, wind-sea, swell, and mixed wave heights

The climatic trends of Wind Speed (WS) and wave height play a key role in wind and wave energy assessments, climate change analyses, and air-sea interactions, among many others. Using ERA5 reanalysis, this study reveals the climatic trends of global oceanic WS and wave height for 1979–2018, including the overall trends, regional and seasonal differences of the trends, with a special focus on the differences and similarities between the trends in wind-sea wave height (Hwind) and swell wave height (Hswell), as well as the contributions of climate indices to WS, Hwind, Hswell and Significant Wave Height (Hs) respectively, by employing linear regression and correlation analysis. The results show an overall global oceanic increase for 1979–2018 in WS (+0.47 cm/s/yr), with increases of +0.13, +0.28 and +0.32 cm/yr in Hwind, Hswell and Hs respectively, and a stronger increasing trend in the Southern Hemisphere than in the Northern Hemisphere. There is good agreement between the spatial distribution of annual and seasonal trends in WS and those of Hwind, as well as between Hswell and Hs. Areas with strong increasing trends of WS and Hwind are mainly located in the tropical South Indian Ocean and tropical Pacific Ocean. Hswell and Hs exhibit significant increases in most global oceans. The months with the broadest and strongest increase in Hswell and Hs are June-July-August (JJA). There is a close relationship among the WS, wave height climatology and the modes of climate variability. The wind has the strongest response to climate indices, followed by the wind-sea, with swell having a minimal response.

A multi-dimensional long-term joint probability model for environmental parameters

Reliability-based design of offshore structures usually requires joint probabilistic models to describe the long-term environmental scenario, however, directional variables are rarely included. This paper presents a multi-dimensional joint model alternative for the probabilistic long-term environmental data description, including directional variables. The joint model aims to statistically describe the main environmental parameters for the probabilistic design of marine structures, such as floating production, storage, and offloading vessels (FPSOs) systems. The model targets full long-term response analyses and simplified and efficient design methodologies. Wind sea, swell and current parameters are investigated for a location southeast of the Brazilian coast. Modeled environmental parameters include significant wave heights, peak periods and incidence directions of the wind and swell waves, as well as wind and current velocities and respective directions. To represent parameters statistical dependencies, the conditional modeling approach was employed. Kernel density estimation is used in the probabilistic description of directional variables and is shown to well represent data characteristics. Case study results show the model's suitability to well-represent wind sea and swell waves, wind and current parameters at the studied location.

Brazil Wave Climate from a High-Resolution Wave Hindcast

A detailed climatology of ocean wind waves in the South Atlantic Ocean, based on ERA-5 reanalysis and in a higher-resolution wave hindcast (ERA-5H), both developed by the European Centre for Medium-Range Weather Forecasts, is presented. The higher resolution of the wave fields in the ERA-5H (22 km) allowed for a better description of the wind sea and swell features compared to previous global and regional studies along the Brazilian coast. Overall, it is shown that swell waves are more prevalent and carry more energy in the offshore area of the study area, while wind sea waves dominate the nearshore regions, especially along the northern coast of Brazil. The influence of different climate indices on the significant wave heights patterns is also presented, with two behavioral groups showing opposite correlations to the North Atlantic Oscillation and Southern Annular Mode than to the Southern Oscillation Index. The analysis of the decadal trends of wind sea and swell heights during the ERA-5H period (1979–2020) shows that the long-term trends of the total significant wave height in the South Atlantic Ocean are mostly due to swell events and the wave propagation effect from Southern Ocean storms.

Ocean-Bottom Laser Seismograph.

This paper describes an ocean-bottom laser seismograph, based on the modified laser meter of hydrosphere pressure variations, and designed to record vertical bottom displacements at the place of its location. Its measuring accuracy is about 1 nm, limited by the stability of the laser emission, which can be improved by using more advanced lasers. The purpose of this instrument is to measure the displacements of the seabed’s upper layer in the low-frequency sonic and infrasonic ranges. Theoretically, it can operate in the frequency range from 0 (conditionally) to 1000 Hz; the upper limit is determined by the operating speed of the digital registration system. We demonstrated the capabilities of the ocean-bottom laser seismograph while registering vertical bottom displacements caused by sea wind waves and lower frequency processes—seiches, i.e., eigenoscillations of the bay in which the instrument was installed. Comparison of experimental data of the bottom laser seismograph with the data of the laser hydrosphere pressure variations meter and the velocimeter—installed in close proximity—shows good efficiency of the instrument.

Determination of Spatial-Temporal Characteristics of Sea Wind Waves

The nonlinearity of sea surface waves is an important property of them. Despite the fact that the nonlinearity is weak, it must be taken into account when solving many applied and fundamental problems. In this work, we study the effect of nonlinearity on the space-time structure of sea waves. The results of determining the spatial-temporal structure from the data of measurements by an array of wave sensors and from optical images of the sea surface are compared. It is shown that there is a discrepancy between the wave characteristics determined in different ways. Partially, but not completely, these discrepancies are explained by incomplete consideration of the directional spreading function.

Assessment of ocean wave spectrum using global Envisat/ASAR data and hindcast simulation

Wave mode of spaceborne synthetic aperture radar (SAR) is designed for the global ocean wave observations. Despite the fact that the significant wave height inferred from SAR measurements has been validated against model output and in-situ data, SAR's primary and unique capability for operational 2-dimensional spectral description of sea state remains to be fully evaluated. In this study, we extended the previous assessment approaches by introducing a new SAR image spectral parameter, the Mean rAnge Cross-Spectrum (MACS) that focuses on the isolated wave scales along the radar line-of-sight direction. MACS is an efficient variable in that it characterizes the local wave spectra properties without need of the non-linear wave inversion procedure. The assessment is based on the multiple-year data acquired by Envisat/ASAR wave mode, along with the collocated WaveWatch III (WW3) hindcast and the in-situ buoy-observed wave spectra, for which the SAR forward transformation is systematically performed to obtain the simulated image spectra. Inter-comparison between SAR-measured and WW3-simulated MACS demonstrates that the consistency is wavelength (or wavenumber) dependent. Three typical wavelengths, around 62 m for windsea, 168 m for intermediate waves and 342 m for swell, are selected to present the MACS comparison in detail. Comparable magnitude of SAR-measured and the simulated MACS is observed for the intermediate waves and swell, while larger simulation values are predicted for the windsea waves. Spatial distribution of MACS agrees well between these two data sets for all wavelengths with high correlation coefficients (>0.8) in most of the global ocean. One exception is in the extratropics where the quantitative difference is particularly notable. In the contrary, when comparing SAR-measured and buoys-simulated MACS, the agreement increases towards the shorter (<100 m) wavelengths. We also found that the large-scale atmospheric/oceanic features persistent on SAR images lead to the overestimate of SAR MACS at long wavelengths, which is expected to bias the wave inversion. The wave spectra retrieval performance shall advance as long as such impact is properly resolved.

Hydrodynamic Сonditions of the Bakalskaya Spit Degradation (Western Crimea) over the Past 30 Years

urpose. The paper is aimed at studying the morphodynamic features of the Bakalskaya Spit evolution being influenced by the sea wind waves and swell, namely assessment of inter-annual variations in the alluvial (erosion) areas of the Bakalskaya Spit coastline, analysis of inter-annual variability of the wind wave parameters, determination of the surface wave characteristics (or a combination of a few ones) responsible for the processes of the bottom material erosion or accumulation in the coastal zone. Methods and Results. Based on the analysis of satellite images for 1984–2016, the areas of the bottom material accumulation or erosion of the Bakalskaya Spit coastline were determined. Application of the spectral wave model permitted to obtain time series of the main parameters of wind waves and swell (significant wave heights and propagation directions) in the Bakalskaya Spit coastal zone with the 1 hr time resolution for the period from 1984 to 2016. The characteristics of surface waves responsible for the coastline deformation were revealed using the discriminant analysis. Conclusions. Analysis of satellite images of the spit made it possible to distinguish three periods in the history of the Bakalskaya spit evolution: 1985–1997, 1998–2007 and 2007–2016. The first period was characterized by relative stability. The strongest erosion took place in 1998; after that the alluvial and erosion cases alternated for 10 years weakly tending to general erosion that constituted the second period. The third one that began in 2007 can be defined as the period of spit degradation accompanied by the irreversible loss of beach material. The basic parameters conditioning hydrodynamics of the Bakalskaya Spit water area are: total duration of storms; mean and maximum values of significant heights of wind waves and swell. Statistical characteristics of the wind waves’ parameters are of a fairly strong inter-annual variability. According to the mean and maximum indices, the wind waves directed close to the normal to the coastline (WSW and WNW) are the most developed. The applied discriminant analysis permitted to draw a statistically reliable conclusion that the direction of the final (mean annual) wave impact on the coastal zone, conditioning the processes of sand accumulation or erosion was set by the waves directed to NNW, at that the swell contribution was dominant. The impact degree is determined by strong storms with the directions close to the normal to the coastline, namely, the WSW ones.

Gidrodinamicheskiye usloviya degradatsii Bakal'skoy kosy (Zapadnyy Krym)

Purpose. The paper is aimed at studying the morphodynamic features of the Bakalskaya Spit evolution being influenced by the sea wind waves and swell, namely assessment of inter-annual variations in the alluvial (erosion) areas of the Bakalskaya Spit coastline, analysis of inter-annual variability of the wind wave parameters, determination of the surface wave characteristics (or a combination of a few ones) responsible for the processes of the bottom material erosion or accumulation in the coastal zone. Methods and Results. Based on the analysis of satellite images for 1984–2016, the areas of the bottom material accumulation or erosion of the Bakalskaya Spit coastline were determined. Application of the spectral wave model permitted to obtain time series of the main parameters of wind waves and swell (significant wave heights and propagation directions) in the Bakalskaya Spit coastal zone with the 1 hr time resolution for the period from 1984 to 2016. The characteristics of surface waves responsible for the coastline deformation were revealed using the discriminant analysis. Conclusions. Analysis of satellite images of the spit made it possible to distinguish three periods in the history of the Bakalskaya Spit evolution: 1985–1997, 1998–2007 and 2007–2016. The first period was characterized by relative stability. The strongest erosion took place in 1998; after that the alluvial and erosion cases alternated for 10 years weakly tending to general erosion that constituted the second period. The third one that began in 2007 can be defined as the period of spit degradation accompanied by the irreversible loss of beach material. The basic parameters conditioning hydrodynamics of the Bakalskaya Spit water area are: total duration of storms; average and maximum values of significant heights of wind waves and swell. Statistical characteristics of the wind waves’ parameters are of a fairly strong inter-annual variability. According to the average and maximum indices, the wind waves directed close to the normal to the coastline (WSW and WNW) are the most developed. The applied discriminant analysis permitted to draw a statistically reliable conclusion that the direction of the final (average annual) wave impact on the coastal zone, conditioning the processes of sand accumulation or erosion was set by the waves directed to NNW, at that the swell contribution was dominant. The impact degree is conditioned by strong storms with the directions close to the normal to the coastline, namely, the WSW ones

Projected incremental changes to extreme wind-driven wave heights for the twenty-first century

Global climate change will alter wind sea and swell waves, modifying the severity, frequency and impact of episodic coastal flooding and morphological change. Global-scale estimates of increases to coastal impacts have been typically attributed to sea level rise and not specifically to changes to waves on their own. This study provides a reduced complexity method for applying projected extreme wave changes to local scale impact studies. We use non-stationary extreme value analysis to distil an incremental change signal in extreme wave heights and associate this with a change in the frequency of events globally. Extreme wave heights are not projected to increase everywhere. We find that the largest increases will typically be experienced at higher latitudes, and that there is high ensemble model agreement on an increase (doubling of events) for the waters south of Australia, the Arabian Sea and the Gulf of Guinea by the end of the twenty-first century.

The Turbulent Structure of the Marine Atmospheric Boundary Layer during and before a Cold Front

AbstractThe turbulent structure within the marine atmospheric boundary layer is investigated based on four levels of observations at a fixed marine platform. During and before a cold front, the ocean surface is dominated by wind sea and swell waves, respectively, affording the opportunity to test the theory recently proposed in laboratory experiments or for flat land surfaces. The results reveal that the velocity spectra follow a k−1 law within the intermediate wavenumber (k) range immediately below inertial subrange during the cold front. A logarithmic height dependence of the horizontal velocity variances is also observed above the height of 20 m, while the vertical velocity variances increase with increasing height following a power law of 2/3. These features confirm the attached eddy model and the “top-down model” of turbulence over the ocean surface. However, the behavior of velocity variances under swell conditions is much different from those during the cold front, although a remarkable k−1 law can be observed in the velocity spectra. The quadrant analysis of the momentum flux also shows a significantly different result for the two conditions.

Projected future changes in tropical cyclone-related wave climate in the North Atlantic

Tropical cyclones are a major hazard for numerous countries surrounding the tropical-to-subtropical North Atlantic sub-basin including the Caribbean Sea and Gulf of Mexico. Their intense winds, which can exceed 300 km h−1, can cause serious damage, particularly along coastlines where the combined action of waves, currents and low atmospheric pressure leads to storm surge and coastal flooding. This work presents future projections of North Atlantic tropical cyclone-related wave climate. A new configuration of the ARPEGE-Climat global atmospheric model on a stretched grid reaching ~ 14 km resolution to the north-east of the eastern Caribbean is able to reproduce the distribution of tropical cyclone winds, including Category 5 hurricanes. Historical (1984–2013, 5 members) and future (2051–2080, 5 members) simulations with the IPCC RCP8.5 scenario are used to drive the MFWAM (Meteo-France Wave Action Model) spectral wave model over the Atlantic basin during the hurricane season. An intermediate 50-km resolution grid is used to propagate mid-latitude swells into a higher 10-km resolution grid over the tropical cyclone main development region. Wave model performance is evaluated over the historical period with the ERA5 reanalysis and satellite altimetry data. Future projections exhibit a modest but widespread reduction in seasonal mean wave heights in response to weakening subtropical anticyclone, yet marked increases in tropical cyclone-related wind sea and extreme wave heights within a large region extending from the African coasts to the North American continent.