Wave refraction on Southern Ocean eddies

Motivated by differences in direction of propagation between the modelled and measured waves observed at the Southern Ocean Flux Station (SOFS), we investigated the possible effects of wave refraction on a Southern Ocean current field. We implemented a numerical model to describe the refraction of wave trains when propagating over a spatially varying current field and to see how they would be affected as they approach the location of the SOFS. The model was applied to three typical swell directions observed in the Southern Ocean, coming from south (S), southwest (SW) and west (W). The regions of convergence (increasing energy) and divergence (decreasing energy) of the wave rays are defined for each situation, as well as the relative changes in wave energy, and hence wave height. It is shown that a specific eddy located at southwest from the buoy can have strong influences in the wave propagation and, in the case of westerly and southwesterly swells, can generate a clear divergence of the wave rays that would otherwise have reached the buoy location if there were no currents

Wave modulation by tidal currents near Cook Strait

Wave-current interactions are studied through the WAve and current MOnitoring System (WAMOS II) based on an HR Radar, installed at Makara, near Cook Strait (NZ). At Makara, the tidal regime generates currents up to 2.5 m/s parallel to the coastal line. Waves propagate through the strait mainly from the S quadrant. We observed that the waves are strongly modulated by the tidal currents and undertake a clear refraction pattern. The refraction is further explained and discussed exclusively through the geometrical optics. The modulation is also seen for significant wave height, absolute peak period and peak wavelength. When waves approach shallow regions they become steeper and eventually break. In these circumstances the currents are in turn modulated by the waves and WAMOS reveals a localised strong current jet. A wave model which fully represents the wave action balance equation (SWAN model) was also implemented. The mean WAMOS wave spectrum was applied as a constant boundary condition and propagated over the varying current field. The wave parameter modulations are well represented by the model. It is concluded that the current field is effectively inhomogeneous, and because of this generates wave height and wavelength modulations, and unsteady, which fact is responsible for absolute peak period oscillations. Therefore currents are an important factor to be considered when modelling waves near Cook Strait

Identificação de Período e Direção de Ondas Monocromáticas Através do Processamento de Imagens Ópticas

Uma nova metodologia para a obtenção do período e direção de propagação de ondas de gravidade monocromáticas é apresentada baseada no processamento digital de sequencias de imagens. A análise fundamenta-se nos padrões espaçotemporais apresentados pelo brilho no plano da imagem. Foi empregado apenas um ponto de vista da cena visualizada, ou seja, processou-se imagens de somente uma câmera. A principal motivação é apresentar uma configuração simplificada e prática que permita a implementação operacional da medição, tanto em laboratório quanto em plataformas de petróleo ou no alto de prédios. Os resultados obtidos pela técnica aqui proposta são considerados promissores e correspondem a experimentos realizados em um tanque de ondas monocromáticas. O erro máximo na determinação do período de onda obtido foi de 0.06 segundos e na direção de propagação 7.80°. O processamento de imagens demonstra ser uma ferramenta valiosa para a medição dos parâmetros de período e direção de ondas

Observation of wind-waves from a moored buoy in the Southern Ocean

The Southern Ocean is an important component in the global wave climate. However, owing to a lack of observations, our understanding of waves is poor compared to other regions. The Southern Ocean Flux Station (SOFS) has been deployed to fill this gap and represents the first successful moored air-sea flux station at these southern hemisphere latitudes. In this paper, we present for the first time the results from the analysis of the wave measurements, focused on statistics and extremes of the main wave parameters. Furthermore, a spectral characterization is performed regarding the number of wave systems and predominance of swell/wind-sea. Our results indicate a high consistency in terms of wave parameters for all deployments. The maximum significant wave height obtained in the 705 days of observation was 13.41 m. The main spectra found represent unimodal swell dominated cases; however, the dimensionless energy plotted against dimensionless peak frequency for these spectra follows a well-known relation for wind-sea conditions. In addition, the Centre for Australian Weather and Climate Research wave hindcast is validated with the SOFS data

Técnicas Ópticas para Identificação de Mares Regulares Cruzados

Um método de medição óptica de ondas superficiais de gravidade é desenvolvido e aplicado para ondas regulares bimodais. O método consiste na análise dos padrões espaciais e temporais do brilho no plano da imagem ao longo de uma sequência de frames. As medições são configuradas objetivando simplificar sua aplicação operacional. Para tanto emprega-se apenas um ponto de vista da cena visualizada, o que representa uma considerável vantagem em detrimento das configurações estereoscópicas usualmente apresentadas na literatura. Os resultados demonstram que é possível identificar o período e a direção de propagação de cada uma das componentes de onda