## 2 Data and Methods
### In-Situ Data  
The Iedo Ocean Research Station (IORS), located at 125.18\({}^{\circ}\)E, 32.12\({}^{\circ}\)N in the East China Sea and south of the Korean Peninsula, has been operating since 2003 [43]. It was constructed on an underwater rock with a depth of approximately 40 m. It is far from land or islands, approximately 149 km southwest of Marado, Korea, 276 km from the west of Dorisima, Japan and 287 km from the nearest island of China, as shown in Figure 2b. The SWH measurements from IORS were used from 2005 to 2016 to evaluate the accuracy of the altimeter SWH data in regions with the greatest frequency of extreme conditions such as typhoons. The tracks of satellite altimeters around IORS are presented in Figure 2c-k, which contains all the regular tracks and transitional tracks during the study period from 1992 to 2016. \begin{table}
\begin{tabular}{c c c c} \hline
**Satellite** & **Operational Period** & **Data Period** & **Repeat Period (Days)** \\ \hline ERS-1 & 17 July 1991–10 March 2000 & 2 January 1992–2 June 1996 & 3, 35, 168 \\ \hline TOPEX/Poseidon & 10 August 1992–9 October 2005 & 25 September 1992–8 October 2005 & 9.9156 \\ \hline ERS-2 & 21 April 1995–5 September 2011 & 15 May 1995–2 July 2011 & 35 \\ \hline GFO & 10 February 1998–22 October 2008 & 7 January 2000–7 September 2008 & 17 \\ \hline Jason-1 & 7 December 2001–21 June 2013 & 15 January 2002–20 June 2013 & 9.9156 \\ \hline Envisat & 1 March 2002–8 April 2012 & 14 May 2002–8 April 2012 & 35 \\ \hline Jason-2 & 20 June 2008–9 October 2019 & 4 July 2008–31 December 2016 & 9.9156 \\ \hline Cryosat-2 & 8 April 2010–Present & 16 July 2010–31 December 2016 & 30 \\ \hline SARAL & 25 February 2013–Present & 14 March 2013–31 December 2016 & 35 \\ \hline \end{tabular}
\end{table} 
Table 1: Information on satellite altimeters used in this study, including the operational period, data period, and repeat period. As IORS is a tower, the wave heights are observed using a radar instrument on a platform-based remote sensing system, in contrast to the conventional buoy using an accelerometer to measure the wave height. The observed SWH data might contain abnormal values due to various atmospheric and marine environments and instrumental errors. The Korea Hydrographic and Oceanographic Agency, which distributes the observation data from IORS, has developed a series of quality control procedures for wave height measurements based on the techniques presented by the Intergovernmental Oceanographic Commission [44,45] and [PERSON] et al. [46]. The agency distributes the SWH data from IORS with quality control information. In this study, quality controlled data at 1-h interval with quality flags were used for analysis. In addition, the SWH measurements from the Donghae buoy of the Korea Meteorological Administration (KMA) were utilized. The buoy is located relatively far from the coast (129.95\({}^{\circ}\)E, 37.54\({}^{\circ}\)N), in an area with less influence from typhoons than IORS. The data were collected from 2011 to 2016. The SWH measurements at 1-h interval from Donghae buoy were also quality controlled by applying quality flags provided with the data. As shown in Figure 3a, SWHs higher than 8 m were observed at IORS. The monthly average is a mixture of seasonal variation in which the SWH increases in winter and  
Figure 2: (**a**) Bathymetry (m) in the Northwest Pacific; (**b**) an enlarged map corresponding to the yellow box of (**a**), showing the locations of leedo Ocean Research Station (IORS) (125.18\({}^{\circ}\)E, 32.12\({}^{\circ}\)N) and Donghae buoy (129.95\({}^{\circ}\)E, 37.54\({}^{\circ}\)N). (**c**–**k**) Satellite altimeter tracks around IORS marked with a red circle, and (**l**–**t**) tracks around the Donghae buoy, marked with a green circle. decreases in summer, and the highest SWH in August due to frequent passage of typhoons (Figure 3c,e). The SWHs from Donghae buoy show more pronounced seasonal variability than those of IORS (Figure 3d,f), but they also exhibit high values exceeding 4.5 m due to typhoons in August-September.