Spatiotemporal Characteristics and Long-Term Variability of Large-Wave Frequency in the Northwest Pacific

This study provides a systematic analysis of the spatiotemporal distribution and trends in the frequency of significant wave height (SWH) exceeding level 5 (SWH > 2.5 m) and level 7 (SWH > 6 m) in the Northwest Pacific (NWP) for 1993–2024, which are defined as f5 and f7, respectively, as well as their correlations with major climate indexes. Our results indicate that (1) the high-value zones for the annual mean f5 and f7 are both located in the south waters of the Aleutian Islands, with maximum values of 58.0% and 6.4%, respectively. Winter’s contribution is greatest (maximum values of 96.9% and 16.8% per year), while summer’s is the smallest. (2) f5 exhibits a significant decline trend across the entire NWP basin (of −0.15 to −0.30%/yr), with the steepest decline occurring in autumn (−0.69%/yr) and the shallowest in summer. f7 exhibits a significant linear decrease in the open ocean east of Japan (−0.08%/yr) while showing a significant linear increase in the waters east of the Kamchatka Peninsula (0.08%/yr). Both variations peak in winter (maximum values of −0.27% and 0.30% per year) and are smallest in summer. (3) Seasonal and regional variations in climate index–f5 and f7 relationships reflect large-scale atmospheric modulation of waves. For example, the Oceanic Niño Index shows a predominantly negative correlation with f5 in winter (maximum correlation coefficient rm = −0.70) around the Luzon Strait, shifting to a significant positive correlation in summer (rm = 0.70) across the extensive region east of Taiwan Island and the Philippines. The Pacific Decadal Oscillation index shows a significant positive correlation with f7 in summer and autumn (rm = 0.69) east of Taiwan Island and a strong negative correlation in winter (rm = −0.77) to the east of Kamchatka Peninsula.