Non-typhoon extreme waves in the Northeast Asian Marginal Seas: Observation-based identification, synoptic conditions, and classification

This study presents an observation-based approach for identifying, characterizing, and classifying non-typhoon extreme waves in the Northeast Asian Marginal Seas using in-situ wave and wind measurements, with reanalysis data used to interpret the associated synoptic conditions. Extreme-value analysis of wave power identifies hundreds of non-typhoon extreme waves across the East Sea, Yellow Sea, and East China Sea. These extreme waves are characterized and classified using quantitatively defined criteria based on the energy period and the correlation between significant wave height and local wind speed, distinguishing wind-sea-dominated, swell-influenced, and mixed wave conditions. Spectral-partition analysis supports the physical distinction among the identified extreme wave classes. Analysis of representative atmospheric fields reveals two predominant synoptic conditions associated with the occurrence of non-typhoon extreme waves: expansion of the Siberian High and extratropical cyclone. The Yellow Sea is predominantly characterized by locally forced wind-sea extremes associated with the expansion of the Siberian High, whereas the East Sea exhibits both wind-sea- and swell-influenced extremes under different synoptic conditions. The East China Sea similarly exhibits both wind-sea- and swell-influenced extremes associated with extratropical cyclone activity and atmospheric forcing over adjacent basins. These results demonstrate pronounced regional differences in non-typhoon extreme wave characteristics and their associated atmospheric conditions, with implications for regional wave modeling and coastal forecasting.