## 3 Results
### Spatial Distribution of Extreme Significant Wave Heights from Altimeter Data  
To understand the spatial distribution of extreme SWHs in the Northwest Pacific, the 100-year return period SWHs within a bin of \(2^{\circ}\times 2^{\circ}\) were estimated using the PoT method (Figure 9). To compare the magnitude of the deviation of 100-year extreme SWHs from the maximum SWHs over the past decades, we computed the mean values of SWHs within the upper 0.1% at each bin (Figure 9a). The number of SWHs within this upper limit generally accounted for approximately 100 out of approximately \(10^{5}\) individual observations in a bin. The spatial distribution of the upper 0.1% mean SWHs showed a clear contrast between the western continental side, with small values of less than 8 m, in the Yellow Sea and the East Sea (Sea of Japan) and the eastern open ocean side of the Northwest Pacific, with high values of over 12-13 m (Figure 9a).  
Figure 8: Distribution of extreme significant wave height estimates (red line) with respect to return periods from 1 year to 100 years near (**a**) the IORS and (**b**) the Donghae buoy, where the dash lines represent the upper and lower limits of the 95% confidence interval.  
Figure 9: Spatial distribution of (**a**) significant wave height (SWH) within upper 0.1% (m), (**b**) 100-year return period SWH (m) estimated by Peak over Threshold (PoT) analysis with a generalized Pareto distribution, (**c**) time of SWH within upper 0.1% in each grid (in months), and (**d**) difference between extreme SWH from PoT analysis and SWH within upper 0.1% (m).  
Compared with the upper 0.1% SWHs, the 100-year return period extreme values based on the PoT analysis were much higher than the maximum SWHs over the period from 1992 to 2016 (Figure 9b). The PoT analysis indicates that the overall distribution of the 100-year return period SWHs ranged from 5 to 22 m. High SWH values were estimated in the open ocean at high-latitude regions above 40\({}^{\circ}\)N. It is noted that the wave heights of 14-15 m in the East China Sea can be found in the results of the PoT analysis. A strong spatial variability of extreme SWHs estimated with PoT is inferred from the high winds with spatially dominant fluctuations in the Northwest Pacific region in winter. In addition, the irregular movement of typhoons in summer and fall is believed to cause an unsystematic imbalance in space. Figure 9c shows the highly frequent occurrence time of high SWH values, by investigating the highest frequency of SWHs within the upper 0.1%. This reveals a contrast distinction between the northeastern part in winter and the southwestern part in summer. This implies that the extreme SWHs of the East China Sea and southern region of the Northwest Pacific might be induced by typhoons in summer and fall. In the northern region, the strong winds in winter seem to be responsible for the high 100-year extreme SWHs. The difference between the 100-year return period SWH and the maximum SWH showed positive values of 6-8 m in the East China Sea, southern (<25\({}^{\circ}\)N) and northern regions (>40\({}^{\circ}\)N) of the Northwest Pacific (Figure 9d). Obvious positive differences greater than 6 m were observed in the typhoon region.