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Abstract

Marine Heatwaves (MHWs) are prolonged extreme warm ocean events, which are one of the characteristic events in summertime surface ocean. We found that an intense MHW occurred in the central North Pacific (CNP) in 2021 summer; SST averaged in the CNP was the highest in the last 120 years. We investigated a cause of this record-breaking warming. In August, atmospheric reanalysis data showed that the intensified North Pacific Subtropical High (NPSH) over the eastern North Pacific caused a weakening of surface winds and induced a decrease in upward heat release in latent heat flux. In September–October, the NPSH expanded westward and covered over the CNP, which resulted in an increase in downward shortwave radiation. These heat exchanges between the atmosphere and ocean were favorable conditions for the occurrence of MHW. However, a heat budget analysis in the ocean mixed layer showed that the entrainment process had a dominant role in determining the ocean temperature in September. We explored the temporal behavior of ocean temperature by using gridded-Argo data. It was found that positive temperature and negative density anomalies were distributed not only near the sea surface but also below the ocean mixed layer. The lighter variety of the Central Mode Water (L-CMW) usually exists in the subsurface ocean in the CNP, which was absent in 2021 summer. The series of results indicated that a deepening of upper isopycnals attributable to the absence of L-CMW caused a decrease in surface ocean density, which contributed to the occurrence of the MHW.