Summer Marine Heatwaves in the Kuroshio-Oyashio Extension Region

During 1982-2021, the highest sea surface temperature (SST) variability over the North Pacific was in the Kuroshio-Oyashio Extension (KOE) region, with more intense marine heatwaves (MHWs), especially during summertime. In this study, we explored the evolution and driving factors of the strongest summer MHWs based on their cumulative intensity using satellite observations and reanalyzed model results. Strong summer MHWs in 1999, 2008, 2012, and 2016 were initiated and peaked around summer. The more recent summer MHW events in 2018, 2020, and 2021 appeared to be associated with intermittent MHW events in the previous winter that extended to boreal summer. Based on a mixed layer temperature budget analysis from March to their peaks in summer, MHWs in 1999, 2008, 2012, and 2016 were primarily driven by the air-sea heat flux anomalies, with anomalous shortwave radiation due to reduced cloud cover being the dominant factor. Summer MHWs in 2018, 2020, and 2021 were mainly contributed by the ocean memory of winter warming. The northward shift of the Kuroshio Extension axis, the northward intrusion of the anticyclonic eddies, and the decadal warming trend may contribute to the positive sea surface height anomalies and increased upper ocean heat content in the KOE to increase winter SST and precondition the summer MHWs. Understanding MHW variability and the underlying mechanisms will help manage the marine ecosystem of the KOE region, as well as predict climate change impacts.

Automated summary

This study explores the evolution and driving factors of summer marine heatwaves (MHWs) in the Kuroshio-Oyashio Extension (KOE) region of the North Pacific. The results indicate that MHWs in 1999, 2008, 2012, and 2016 were primarily driven by air-sea heat flux anomalies, while MHWs in 2018, 2020, and 2021 were mainly influenced by the ocean memory of winter warming. The northward shift of the Kuroshio Extension axis, northward intrusion of anticyclonic eddies, and decadal warming trend may contribute to the occurrence of these summer MHWs.