Observations of the ocean waves directional spreading during the HIGHWAVE project and SUMOS campaign.

The directional distribution of ocean waves is of great importance for a better understanding of air-sea interactions. Countless applications in science and engineering, such as, offshore energy production, microseisms prediction, wave climate modelling, coastal erosion, among many others, require precise information about the wave directionality. However, in spite of its importance, this quantity is poorly understood and difficult to accurately model. This study presents observations of the directional spreading parameters obtained from a set of low-cost GPS-based buoys during highly energetic conditions. One of the buoys was anchored off the west coast of Ireland during the HIGHWAVE project. These observations are compared with the measurements of 20 freely drifting buoys deployed in the Bay of Biscay during the SUMOS campaign. Spreading parameters were compared in the framework of widely used parameterisation for the directional distribution. The directional spreading is narrower at the spectral peak and broadens as the frequency moves away towards higher and lower scales. There is a particularly sharp increase in the spreading for f < f_p. The results showed that buoy-based observations significantly differ from spatial-based measurements for frequencies around half the spectral peak. The measruements obtained by the drifting buoys show that for 2 < f/f_p < 6, the spreading appears to be approximately constant with the frequency and tends to increase again for f > 6f_p. The results showed that the directional spreading seems to be independent of the wave age, roughly across the entire range of frequencies.  This may imply that the shape of the directional spectrum is primarily controlled by the non-linear wave-wave interactions rather by the wind forcing.  In the vicinity of the spectral peak, a weakly linear relationship between the directional spreading and the significant wave height was observed.  The results show that as the significant wave height increases by one meter, the spreading decreases by about 4.5°. The preliminary results presented here contribute to the understanding of the directional distribution of ocean waves. However, further observations and comparisons are needed to fully capture the complexity of this phenomenon. Despite being preliminary, these results provide valuable insights and add to the ongoing discussion on this topic. This work was funded by the European Research Council (ERC) under the EU Horizon 2020 research and innovation programme (grant agreement no. 833125-HIGHWAVE). We are very grateful to the scientific team behind the SUMOS campaign for providing the drifting buoys data.