The severe impacts of extreme ocean waves, including flooding in coastal areas (Melet et al., 2020), loss of life and property damage (Hansom et al., 2015), and severe coastal erosion (Masselink and Gehrels, 2014) have been recorded worldwide. The increasing rate of extreme ocean waves in northern latitudes, which also cover East Asian (EA) waters such as the East Sea (ES), the Yellow Sea (YS) and the East China Sea (ECS), is higher than that of the mean condition (Young et al., 2011) and is predicted to become more extreme in the future (Meucci et al., 2020). While extreme ocean surface waves are generally induced by storms and are associated with increasing surface wind speeds (Chun and Ahn, 2017; Heo et al., 2020), the EA region is notably sensitive to seasonal atmospheric phenomena, such as monsoonal systems (Chen et al., 2019; Son et al., 2023). Several wave climate studies conducted on a seasonal basis have shown that extreme wave heights in the EA seas have been increasing during the summer season (Liang et al., 2016; Li et al., 2016).
Moreover, extreme ocean wave events in the Western North Pacific (WNP) have been linked to tropical storm (TS) activities (Sasaki and Hibiya, 2007; Sasaki et al., 2005; Woo and Park, 2021) and associated with large-scale natural climate variability such as the El Niño Southern Oscillation (ENSO) (Chien et al., 2014; Yang and Oh, 2018). This is because ENSO events, which are indicated by the anomalous ocean warming tongue structure in the Central-Eastern Pacific Ocean, could influence the global climate system, including wind patterns in the EA region (Wang et al., 2000). In addition, ENSO can modulate TS (Camargo et al., 2005; Iizuka and Matsuura, 2008) through atmospheric pressure changes, with a stronger relationship occurring during summer (Zhao and Wang, 2019).
Based on the decomposition signal (Figure S1), significant wave height (Hs) in EA shows a decadal signal. This condition implies that the leading mode of anomalous sea surface temperature (SST) in the North Pacific, namely the Pacific Decadal Oscillation (PDO; Mantua et al., 1997) could contribute to Hs variation in the WNP. Wang et al. (2008) reported that the PDO could also modulate ENSO. Consequently, numerous studies have assessed the effects of the ENSO–PDO on anomalous variables, including heavy precipitation (Chan and Zhou, 2005; Wu and Mao, 2016) and atmospheric circulation (Kim et al., 2014; Lee et al., 2019). Although information on extreme ocean waves under the effect of climate modes remain limited, previous studies have attempted to improve our understanding of extreme ocean wave variation with respect to climate variability. A global study by Kumar et al. (2016) showed that the phase (in or out) of ENSO and PDO occurrence could strengthen or weaken the maximum wave height during winter. Results consistent with this were obtained by Yang and Oh (2020), who investigated the response of extreme anomalous summer wave power density in the WNP during the in phase of ENSO–PDO. They argued that the PDO modulation of the ENSO effect on extreme wave power was linked to southeast trade wind variations.
However, the response of extreme event attributes, such as the duration and the intensity of events under the El Niño effect during summer, has not been addressed thus far. This information is essential because the extreme wave impact on coastal regions is determined by the duration and strength of extreme events. For example, extreme wave height that occurred for 14 h caused significant damage to the breakwaters of Seogwipo harbour on Jeju Island (Chae et al., 2013). The duration of extreme waves plays a crucial role in determining the breakwater stability. Therefore, investigating how the PDO modulates the El Niño effect on extreme wave duration is necessary. This study aimed to assess the extreme wave climate over 42 years and quantify the duration and intensity. The combined effects of ENSO and PDO during summer were determined. The influence of TS activity was also evaluated. The findings of this study contribute to improving the understanding of extreme wave variations in EA under the climate-mode effect.