Coastal disasters induced by low frequency swells with massive energy are a serious problem along Japanese coasts and have been frequently occurring in recent years such as those reported in Toyama bay (the so-called Yorimawari-waves). Prediction and monitoring of these swells is a pressing task for the coastal engineering community. Swells originate from the evolution of windseas of distant weather systems. Due to the spectral downshifting by resonant interactions, wind waves transfer spectral energy to the lower frequency bands. They travel long distance with less attenuation, far away from where the wind waves are actually generated. To understand these physical processes, it is first necessary to clearly separate windsea and swell from the viewpoint of spectral components, because it is quite common for windsea and swell to coexist in the ocean. Next we need to investigate the impact of swell energy in terms of coastal disasters. Wave steepness (H/L), which is defined by a set of wave heights and periods, has been widely used for practical applications because of its relative simplicity. However, this parameter is limited in its applicability to distinguish between windsea and swell since it does not provide the degree of swell energy inside the total wave energy. Instead, H/L just represents whether the sea state is windsea or swell.
In this study, we propose a Swell Index (SI) based on the wave growth rate in order to decompose the measured wave spectra into windsea and swell components. Then we investigate historical ocean wave data from 2007 to 2016 observed by the NOWPHAS observation network. Based on analysis of the wave fields in Toyama bay, we show that the SI can clearly separate swell energy from total energy and that the use of SI successfully explains the characteristic wave fields in the bay. We also discuss swell properties along the Japanese coast and their seasonality in relation to the atmospheric forcing. We conclude that SI is a useful metric to quantify swells for practical purposes because of their simplicity and robustness.

Key Words: Ocean swells, Swell Index, NOWPHAS, Yorimawari-wave, Swell properties along the Japanese coast