Seasonal Variability of the Global Spectral Wind Wave Climate

Our understanding of the seasonal variability in the global wind wave field is revisited here using a novel analysis that resolves the directional wave spectra. Empirical orthogonal function analysis was applied to modeled wave spectral data from a WAVEWATCH III hindcast across a sparse global grid to identify the main patterns of the climatological wave spectral variability at each grid point. Prior methods have focused on the variability of two modes of the wave fieldlocally generated sea and the primary swell component. Our results also consider additional wave modes at each location, enabling us to track the passage of the less dominant swell modes. Consistent with existing climatological knowledge, the main modes of wave spectral variability at high latitudes are related to eastward propagating waves that disperse equatorward as swell following great circle paths. However, despite being the less energetic mode, the Northern Hemisphere generated swell is found to propagate into the Southern Hemisphere further than the more energetic Southern Hemisphere swell which propagates northward. In the equatorial zone, a complex multimodal wave climate is found, with the spectra variability modulated by remotely generated swell and higher-frequency waves associated with the prevailing winds. The evolution of these patterns throughout the year is clearly depicted. Overall, our approach captures a more complete picture of the seasonal variability of the global wave field, by accounting for all the wave modes observed in the spectra at each location together with their temporal variability. Plain Language Summary Waves generated by the wind blowing over the surface of the ocean are able to travel immense distances and reach distant coasts (e.g., it has been recognized since the 1950s that the coasts of California receive waves generated in the Southern Ocean). This means that the wave climate of a given region can be very complex, having waves generated by local winds but also from many remote places at once (multiple swell fields). In this study, we present a new method to examine seasonal variations in the global wave climate which accounts for the full directional wave spectra and includes wave systems with different frequencies and directions separately, as opposed to using integrated wave parameters, such as significant wave height or mean direction. Our results show how low-frequency swell waves propagate across ocean basins from high to low latitudes of both hemispheres, higher-frequency waves that are a product of the local winds, and the variation in the intensity of these signals throughout the year. We believe future wave studies can benefit from this approach, since a concise and yet more complete representation of the wave climate variability can be achieved.