Record-Breaking Marine Heatwave over the Central North Pacific in 2021 Summer: Its Formation Associated with Loss of Central Mode Water

Abstract Marine heatwaves (MHWs) are prolonged extremely high sea surface temperature (SST) events. In the 2021 summer, an intense MHW occurred over the central North Pacific; the SST in September 2021 was the highest in September since 1900, and the warming signal was distributed not only near the sea surface but also below the ocean mixed layer (∼300-m depth). Atmosphere reanalysis data showed westward expansion of the North Pacific subtropical high (NPSH) in the 2021 summer, but both an increase in downward shortwave radiation and a decrease in upward latent heat flux were not so large, and ocean mixed layer heat budget analysis, which also used ocean reanalysis data, revealed that the atmosphere-induced heating is insufficient to form the record-breaking MHW. Argo profiling floats indicated that, in the 2021 summer, the Central Mode Water (CMW)—a huge water mass characterized by vertically uniform properties in depths of 100–500 m—decreased extremely, the thickness of which was less than 20% of the normal. Statistical analysis showed that, from the sea surface to the upper boundary of CMW, the heavier isopycnal surfaces are deeper associated with the decrease in CMW, leading to a weakening of the seasonal pycnocline. Then, this causes the weakening of cooling heat flux associated with the entrainment of subsurface waters into the mixed layer, resulting in surface ocean warming, which in turn contributed to form the MHW in the 2021 summer.