A severe coral bleaching event occurred in the Beibu Gulf during boreal summer 2020. This event was caused by record-breaking warm sea surface temperature (SST) anomalies in the northern South China Sea (NSCS). Anomalous anticyclones (AACs) over the NSCS linked to the Indian Ocean warming and La Niña induced the extreme NSCS warming. In May–July 2020, anomalous anticyclonic-shear easterlies appeared over the tropical western Pacific as a Kelvin wave response to the Indian Ocean warming. These easterlies resulted in an AAC over the NSCS and tropical northwestern Pacific via the Ekman dynamics. Besides, the anomalous easterlies increased moisture fluxes from the Pacific to the Maritime Continent (MC), enhancing the MC convection and thus supporting the NSCS AAC via an anomalous Hadley circulation. In this stage, the increased incident shortwave radiation induced by the NSCS AAC was the major contributor to the NSCS warming. In September 2020, anomalous equatorial Pacific easterlies associated with the La Niña could trigger an NSCS AAC by enhancing the MC convection. The resultant AAC warmed the NSCS SST anomalies mainly by increasing downward latent heat flux and shortwave radiation. Downward heat advection associated with the NSCS AAC also contributed to the extreme NSCS warming.

In the Northwest Pacific (NWP), where a unique monsoon climate exists and where both typhoons and extratropical storms occur frequently, hazardous waves pose a significant risk to maritime safety. To analyze the 20-year variability of hazardous waves in this region, this study utilized hourly reanalysis data from the ECMWF ERA5 dataset covering the period from 2001–2020, alongside the wave risk assessment method. The ERA5 data exhibits better consistency, in both the temporal and spatial dimensions, than satellite data. Although hazardous wind seas occur more frequently than hazardous swells, swells make hazardous waves travel further. Notably, the extreme wave height (EWH) shows an increasing trend in high- and low-latitude areas of the NWP. The change in meridional wind speeds is the primary reason for the change in the total wind speed in the NWP. Notably, the maximum annual increase rate of 0.013 m/year for EWH exists in the region of the Japanese Archipelago. This study elucidated the distributions of wave height intensity and wave risk levels, noting that the EWHs of the 50-year and 100-year return periods can reach 20.92 m and 23.07 m, respectively.