11:30 AM - 11:45 AM
[ACG30-04] Formation of an intense marine heatwave in the central North Pacific during 2021 summer favored by the atmospheric and oceanic conditions
Keywords:marine heatwave, subtropical high, mode water
Marine Heatwaves (MHWs) are prolonged extreme warm ocean events, which are one of the characteristic events in summertime surface ocean (e.g., Hobday et al., 2016; Amaya et al., 2021). MHWs are caused by various factors, such as strengthened High, reduced cloud cover, suppressed surface winds, and thinned ocean mixed layer. We found that an intense MHW occurred in the central North Pacific in 2021 summer; SST averaged in the central North Pacific (CNP; 180°–160°W, 30°N–40°N) was the highest in the last 120 years. We investigated a cause and effect 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 then 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 from August to October were favorable conditions for the occurrence of MHW. However, these heat fluxes were not noticeably larger over the last 60 years when the data was available. In addition, the ocean mixed layer was not exceptionally thin in 2021 summer. To assess the cause of the MHW occurrence quantitatively, we performed a heat budget analysis in the ocean mixed layer. The result 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 throughout the recent three years of 2019–2021. The lighter variety of the Central Mode Water (L-CMW) usually exists in the subsurface ocean in CNP (Oka & Qiu, 2012), which was absent in 2021 summer. It was implied that the L-CMW thickness was related to the surface ocean temperature. There was a negative correlation between the L-CMW thickness and the ocean heat content from the sea surface to 100 m depth. 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.
In the present study, we focused on an intense MHW in the central North Pacific favored by the atmospheric and oceanic conditions. MHWs are expected to become more frequent under global warming. To predict the occurrence of MHWs and their devastating impact on society, it is necessary to understand the mechanism of occurrence of MHWs.
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 then 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 from August to October were favorable conditions for the occurrence of MHW. However, these heat fluxes were not noticeably larger over the last 60 years when the data was available. In addition, the ocean mixed layer was not exceptionally thin in 2021 summer. To assess the cause of the MHW occurrence quantitatively, we performed a heat budget analysis in the ocean mixed layer. The result 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 throughout the recent three years of 2019–2021. The lighter variety of the Central Mode Water (L-CMW) usually exists in the subsurface ocean in CNP (Oka & Qiu, 2012), which was absent in 2021 summer. It was implied that the L-CMW thickness was related to the surface ocean temperature. There was a negative correlation between the L-CMW thickness and the ocean heat content from the sea surface to 100 m depth. 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.
In the present study, we focused on an intense MHW in the central North Pacific favored by the atmospheric and oceanic conditions. MHWs are expected to become more frequent under global warming. To predict the occurrence of MHWs and their devastating impact on society, it is necessary to understand the mechanism of occurrence of MHWs.