Abstract
The influence of the Agulhas Current on the wave field is investigated. The study is conducted by performing high resolution spectral wave model simulations with and without ocean currents. The validation of the numerical simulations is performed for the Significant Wave Height (Hs) using all possible satellite altimetry data available in the study region for a winter period of 2018. Wave spectra and extreme waves parameters are examined in places where waves and current are aligned in the Agulhas Current. Sentinel-1 (S1) wave mode Synthetic Aperture Radar (SAR) spectra are used to estimate the composites of the Hs and BFI (Benjamin–Feir Index). SAR computed BFI and Hs were compared with the respective composites obtained from the Simulating Waves Nearshore (SWAN) model. From the Hs composites using SAR data and modeled data, it can be concluded that the Hs maxima values are distributed in the Agulhas Current Retroflection (ACR) and also in the southern limit of the domain that is affected by the strong circumpolar winds around Antarctic. In addition, the BFI composites exhibit the highest values in the ACR and some few values are observed in the southern border as occurred with the Hs. The results of this study indicate that there is direct correlation between the Agulhas Current strength, the Hs and the BFI. It was found that the modeled directional wave spectra are broadened when the ocean current is considered in the simulation. The analysis of the modeled wave spectra is performed over eddies, rings and meanders in the Agulhas Current region. The transformation of the wave spectra due to current refraction is discussed based on the numerical simulations. The effect of the Agulhas Current on the spectral shape is explored. The spectral wave energy grows when the wave and the current are aligned, resulting in peaked, elongated and widened spectra. A decrease of the peak period was observed before the occurrence of maximum values of BFI, which characterize abnormal sea states.
Highlights
Bad weather is not the main cause of ship losses and it may represent a relatively small percentage of the direct causes of ship losses, as described in [1]
The fetch of the southwesterly winds blowing across the Southern Ocean, is of thousand miles and the Hs shows a maximum of 9 m (Figure 4, right)
In the present study the nonlinear mechanisms that might have generated extreme waves are not discussed because the Simulating Waves Nearshore (SWAN) model is a phase-averaged model
Summary
Bad weather is not the main cause of ship losses and it may represent a relatively small percentage of the direct causes of ship losses, as described in [1]. Ship losses by other causes, such as fire or grounding can have bad weather as a causal factor or even as a direct cause. This has been explored by [2] who have analyzed a sample of accident records of Lloyds from 1978 to 1999 in a total of 547 reports and in these reference was made to bad weather in about 53% of the cases, which makes bad weather appear as an important effect in ship losses. Severe sea states have caused the loss of many ships and to improve safety in maritime operations, decision making about ship routing [3] must rely upon wave forecast that require the best available knowledge of hazardous wave environments. Improving the understanding of extreme and abnormal waves [4,5,6,7] is very important to assess the safety of marine operations to avoid ship casualties [8,9,10,11,12,13]
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