Wind-driven Ocean Waves Research Articles

Nangaghneghput – our way of life

Like all Indigenous people, I believe my home – the land and waters of St Lawrence Island in the Bering Strait – is the most special and sacred place on Earth. For thousands of years, my ancestors thrived here. Even throughout the millennia when there were periodic changes in climate, the variety and quantity of the natural resources sustained us. Whales, walrus, seals, seabirds, and millions of fish migrate through the strait between southern oceans and Arctic seas as the sea ice retreats in spring and advances in fall. However, we are now experiencing astonishing environmental change that is causing an unsettling shift in the traditional, ageless rhythms of our communities. Siku, sea ice, remains the most real and powerful presence in our relationship with our world in the Arctic. It defines our seasons and activities. For instance, our sea ice is important even as it begins to break apart and retreat north. It calms the wind-driven ocean waves and provides the perfect platform for finding, harvesting, and safely retrieving seals and walrus. The spring hunting season remains the most critical time for ensuring our communities’ food security, which depends on the complex interaction between the quality of the sea ice, wind direction and strength, general weather conditions, and marine mammals’ migration timing and location. However, as the condition of our sea ice weakens, our traditional hunting seasons are dissolving and blending together, with hunter safety becoming an increasing concern. We are traveling farther with less sea ice and with what seems like more disruptive, stormy weather during our hunting trips. Also with warmer seas, we are experiencing growing threats to the Arctic marine ecosystems on which we depend, including harmful algal blooms, ocean acidification, an unusual degree of mass mortality events involving marine mammals, and, perhaps most seriously, the destabilization of the Bering Sea food web. As the ice-free season lengthens to allow increasing commercial fishing and large vessel traffic, we are contending with oil and gas contamination, extensive marine debris, noise pollution, and increased risk of catastrophic accidents. Not only does this all challenge Arctic Indigenous people’s way of life, it also presents major unanswered national and international governance questions. Arctic Indigenous Knowledge (IK) and communities are indispensable to understanding and responding effectively to an evolving Arctic. Once commonly known as traditional ecological knowledge, IK goes beyond observations and ecological research. It is a systematic way of thinking applied to phenomena across biological, physical, cultural, and spiritual systems. It has developed over millennia and continues to do so as a living process, including knowledge acquired today and in the future, passed on from generation to generation. It is not something simply to be documented as data to be used by others for their own purposes. Instead, its true value is found when it is applied by our IK experts and knowledge bearers to questions about the health, condition, and well-being of their world – the land, waters, air, and all who inhabit it. It offers a unique “way of knowing” that can help complete humankind’s understanding of the Arctic, and which is necessary for informing scientific research and public policy. Indigenous Knowledge is grounded in the fundamental cultural belief of respecting and conserving the gifts the Creator provides us. Only by cherishing the blessing of a harvested walrus (or whale, or seal) are we worthy of continued successful hunting. Even our thoughts while accepting the gift offered to us must be proper and respectful, because they reflect how well we care for and conserve what is given. We give thanks. The cultural practice of humbly sharing our harvest is an expression of this understanding. While it is beyond translation, this profoundest IK and way-of-knowing in my language is Esla. It encompasses and connects all things. One of my mentors, the late Caleb Pungowiyi, would share a lesson from his grandmother to “listen to the whispers of the grass”. We must realize the importance of being closely connected to our environment, to open our ears to hear, or our eyes to see, or any of our senses to experience. We must open ourselves even to what may seem irrelevant. So while the natural world is acting strangely and the traditional rhythms of our lives are unsettled, the Arctic will always be our home, eternal and sacred. It seems that we now are struggling with our land and waters and are not in balance with them, as before; but our communities will continue adapting and will rely on our IK to maintain our relationship properly with our world. VERA K METCALF Eskimo Walrus Commission, Nome, AK

Directional Spectra-Based Clustering for Visualizing Patterns of Ocean Waves and Winds

The energy distribution of wind-driven ocean waves is of great interest in marine science. Discovering the generating process of ocean waves is often challenging and the direction is the key for a better understanding. Typically, wave records are transformed into a directional spectrum which provides information about the wave energy distribution across different frequencies and directions. Here, we propose a new time series clustering method for a series of directional spectra to extract the spectral features of ocean waves and develop informative visualization tools to summarize identified wave clusters. We treat directional distributions as functional data of directions and construct a directional functional boxplot to display the main directional distribution of the wave energy within a cluster. We also trace back when these spectra were observed, and we present color-coded clusters on a calendar plot to show their temporal variability. For each identified wave cluster, we analyze wind speed and wind direction hourly to investigate the link between wind data and wave directional spectra. The performance of the proposed clustering method is evaluated by simulations and illustrated by a real-world dataset from the Red Sea. Supplementary materials for this article are available online.

Computational modeling of the effect of wind-driven ocean waves on the underwater light field distributions

We have developed a numerical model, which is a combination of ray-tracing and optical propagation, to study underwater downwelling irradiance fluctuations, which occur due to the focusing of light by wind-driven surface waves, at different depths and under various conditions. In particular, to have more realistic results, we utilized the Gerstner model to simulate sea surface waves and to take account of water effects such as scattering and absorption, the point spread function (PSF) of water is used. The effects of physical factors such as wind speed, water depth, and light wavelength on the underwater irradiance spatial distribution, probability density function (PDF) and coefficient of variation (CV) of the irradiance are investigated. Irradiance fluctuations occur at high frequencies immediately below the surface, which means they are related to surface waves with short wavelengths. As we look deeper, the effect of surface waves with longer wavelengths becomes apparent. The simulation also shows the dependence of the average of the downwelling irradiance on the surface profile, depth and incident wavelength. The variance of the irradiance reaches a maximum at a certain depth and then rapidly decreases.

A sea-state based source function for size- and composition-resolved marine aerosol production

Abstract. A parameterization for the size- and composition-resolved production fluxes of nascent marine aerosol was developed from prior experimental observations and extrapolated to ambient conditions based on estimates of air entrainment by the breaking of wind-driven ocean waves. Production of particulate organic carbon (OCaer) was parameterized based on Langmuir equilibrium-type association of organic matter to bubble plumes in seawater and resulting aerosol as constrained by measurements of aerosol produced from productive and oligotrophic seawater. This novel approach is the first to parameterize size- and composition-resolved aerosol production based on explicit evaluation of wind-driven air entrainment/detrainment fluxes and chlorophyll-a as a proxy for surfactants in surface seawater. Production fluxes were simulated globally with an eight aerosol-size-bin version of the NCAR Community Atmosphere Model (CAM v3.5.07). Simulated production fluxes fell within the range of published estimates based on observationally constrained parameterizations. Because the parameterization does not consider contributions from spume drops, the simulated global mass flux (1.5 × 103 Tg y−1) is near the lower end of published estimates. The simulated production of aerosol number (1.4 × 106 m−2 s−1) and OCaer (29 Tg C y−1) fall near the upper end of published estimates and suggest that primary marine aerosols may have greater influences on the physicochemical evolution of the troposphere, radiative transfer and climate, and associated feedbacks on the surface ocean than suggested by previous model studies.

Characteristics of short period secondary microseisms (SPSM) in Taiwan: The influence of shallow ocean strait on SPSM

Taking advantage of a unique opportunity provided by a dense array of coastal short-period seismic stations and the diverse bathymetry around Taiwan, we examine how the long-range coherent ambient noises are influenced by surrounding ocean settings using the cross-correlation functions (CCFs) between pairs of stations. The effective energy of the CCFs derived from three components of short-period seismometer data falls within the frequency range of the short period secondary microseism (SPSM). The spatial variations mapped from the amplitude asymmetry of CCFs and source migration images evidently demonstrate that the SPSM strengths are closely linked to the drastic changes in offshore ocean characteristics and result in much stronger SPSM in the shallow and narrow Taiwan Strait than in deep open seas of eastern Taiwan. The temporal variations of the CCF strengths exhibit very good correlations with the wind speeds and wave heights, explicitly indicating the observed SPSM is dominated by local sources generated from wind-driven ocean waves around offshore Taiwan.

Multi-Gaussian Representation of the Cox–Munk Distribution for Slopes of Wind-Driven Waves

Abstract The Cox–Munk probability density function (PDF) for slopes (γx, γy) of wind-driven ocean waves was obtained about 50 years ago and until now has remained the most complete result. This PDF is widely used in different applications. With respect to the problem of wave scattering from a sea surface, this PDF completely determines the scattering lidar cross sections. For Ku and C bands this PDF is important for radars and thermal radiation at small grazing angles. Unfortunately, the analytical representation of the Cox–Munk PDF has an imperfection because in certain regions of slopes it leads to negative probabilities. This feature is a consequence of the general theorem proven by Marcinkiewicz that any truncated cumulant expansion (except in the case of Gaussian distribution) cannot represent a true PDF. An alternative representation of a non-Gaussian multivariate PDF is used. Any non-Gaussian PDF is presented as a linear combination of Gaussian functions having different weights, mean values γx, γy...

Horizontal asymmetry and steepness distributions for wind-driven ocean waves from severe storms

Results on horizontal asymmetry and steepness distributions from analyses of ocean wave data collected during 10 severe storms in the northern North Sea are presented. The data have been collected at a sampling rate of 5 Hz using laser altimeters mounted on a fixed platform permitting the shapes of individual waves to be quite closely defined. This has allowed the steepness of the fronts and backs of wave crests and troughs to be examined. It is found that, on an average, as the non-dimensionalised wave height increases, the horizontal asymmetry becomes more pronounced. That is, the fronts of large wave crests tend to be steeper than their backs. Regression has been used to establish relationships between individual non-dimensionalised wave heights and steepness measures. The generalised Pareto distribution has then been used to establish a simple model for predicting the probability of extreme wave steepness conditional on the non-dimensionalised wave height.

Fractal structure of large-scale variability of wind-driven waves according to laser-scanning data.

Results are presented for experimental laser-scanning investigations of the statistical characteristics of wind-driven ocean waves. The method involves counting the number of specular points during scanning of the sea surface by a narrow laser beam on a moving ship. The data analyzed are the set of specular points recorded along a track traced out by the laser beam as a result of the motion of the ship and the scanning beam. A prominent feature is the large-scale variability of the number of specular points and the self-similar nature of the process over a rather wide range of spatial scales. A fractal analysis of the process shows a clear power-law interval in the spatial spectrum of the distribution of specular points.

Relationship between thermally forced surface wind and sea surface temperature gradient

An important part of the influence of the oceans on the atmosphere is through direct radiation, sensible heat flux and release of latent heat of evaporation, whereby all of these processes are directly related to the surface temperature of the oceans. A main effect of the atmosphere on the oceans is through momentum exchange at the air-ocean interface, and this process is directly related to the surface wind stress. The sea surface temperature (SST) and the surface wind stress are the two important components in the air-ocean system. If SST is given, a thermally forced boundary layer atmospheric circulation can be simulated. On the other hand, if the surface wind stress is given, the wind-driven ocean waves and ocean currents can be computed. The relationship between SST and surface wind is a coupling of the atmosphere and the oceans. It changes a one-way effect (ocean mechanically driven by atmosphere, or atmosphere thermally forced by oceans) into two-way air-sea interactions. Through this coupling the SST distribution, being an output from an ocean model, leads to the thermally forced surface winds, which feeds back into the ocean model as an additional forcing. Based on Kuo's planetary boundary layer model a linear algebraic equation is established to link the SST gradient with the thermally forced surface wind. The surface wind blows across the isotherms from cold to warm region with some deflection angle α to the right (left) in the Northern (Southern) Hemisphere. Results from this study show that the atmospheric stratification reduces both the speed and the deflection angle of the thermally forced wind, however, the Coriolis' effect increases the wind speed in stable atmosphere (Ri>10−4) and increases the deflection angle.

Wave intensification for seaside power

Wind driven ocean waves appear to be an ideal means of concentrating some of the sun's energy to provide man with power, but even wind driven waves coming from a long ocean reach still require equipment to harness them for the generation and storage of useable power. The wave intensifier analyzed here shows that the intermittent low hydraulic head energy from these wind driven seas against a shoreline can be increased in pressure and stored for use in a standard small hydroelectric plant to supply power when needed. This system is simple, practical, environmentally benign and within the economic capabilities of many poor nations that are literally awash with unused power. Small installations in the 100 kW range can be made for about $500 per kilowatt of peak output—larger units are even more economical.