Marine Heat Waves in East Asia: its Characteristics and Relation to Climate Variability

Marine heat wave (MHW) denotes an anomalously warm surface seawater event, which could be prolonged for several months and has a devastating effect on the marine ecosystem and biodiversity. We investigated the spatial distribution and seasonal to inter-decadal variation of the MHW in East Asia based on 39 years of Sea Surface Temperature (SST) data. The persistent MHW events occurred predominantly in East Korea Bay (EKB), East of Japan (EJ), and the Western North Pacific (WNP). During the MHW events, the maximum temperature anomaly was about half over the WNP compared to those in EKB and EJ. The number of MHW days was almost invariant between the warm and cold seasons in the WNP, whereas those were maximum during the warm season in EKB and EJ. It was revealed that the MHW in WNP was primarily caused by an increase of the downward latent heat flux and short wave radiation, accompanied by enhanced western North Pacific subtropical high. On the other hand, oceanic circulation was responsible for the MHW in EKB and EJ. Further, a spectral separation was performed on the timeseries of MHW days to distinguish the effects of different climate variabilities on the MHW in East Asia. The high-frequency component (1-5 years) peaks when the El-Niño to La-Niña transition occurs, whereas the low-frequency component (5-13 years) is correlated with the Pacific Decadal Oscillation index with a coefficient of -0.49, led by 18 months. We also examined whether the MHW, defined based on the SST, accompanies subsurface warming. The warming associated with the high-frequency component of MHW was confined near the surface layer in WNP, whereas the warming extended to the subsurface in EKB and EJ. Low-frequency component of MHW was found to accompany significant subsurface warming over all three regions. The residual component (13-39 years) accounts for the recent intensification of the MHW events, which coincides with the reacceleration of East Asia surface warming.