In three consecutive years from 2016 to 2018, extreme ocean warming events, or marine heatwaves (MHW), occurred during boreal summers in the East China Sea (ECS) and South Yellow Sea (SYS), which was unprecedented in the past four decades based on the satellite record. In this study, we used a high-resolution hydrodynamic model based on FVCOM (Finite Volume Community Ocean Model) to simulate the evolution of these warming events. An upper ocean temperature budget (0-20m) analysis based on the model results shows that the shortwave radiation and the ocean advection anomalies jointly contributed to the anomalous warming in the three successive summers (June-August) in the SYS and the north part of the ECS. In addition, the reduction of surface wind speeds during the 2016 and 2017 summers further weakened the vertical mixing, thereby enhancing the anomalous warming in the north part of the ECS adjacent to the SYS. During the three summers, the increases of shortwave radiation were closely related to the East Asian Summer Monsoon variability, which reduced the cloud cover in the ECS and SYS, whereas the advection anomalies were mostly associated with regional wind anomalies. In summer 2018, upper ocean heat was transported into the central trough of the South Yellow Sea, accumulated in an anticyclonic eddy generated by the anomalous wind stress curls. Understanding the drivers of the MHWs can help MHW predictions in the coastal region, in order to help the fisheries and aquaculture industries to better manage the environmental risks under a warming climate.

In three consecutive years from 2016 to 2018, extreme ocean warming events, or marine heatwaves (MHW), occurred during boreal summers in the East China Sea (ECS) and South Yellow Sea (SYS), which is unprecedented in the past four decades based on the satellite record. In this study, we used a high-resolution hydrodynamic model based on FVCOM (Finite Volume Community Ocean Model) to simulate the evolution of these warming events. An upper ocean temperature budget (0-20m) analysis based on the model results shows that the shortwave radiation and the ocean advection anomalies jointly contributed to the anomalous warming in the three successive summers (June-August) in the SYS and the north part of the ECS. In addition, the reduction of surface wind speeds during the 2016 and 2017 summers further weakened the vertical mixing, thereby enhancing the anomalous warming in the north part of the ECS adjacent to the SYS. During the three summers, the increases of shortwave radiation were closely related to the East Asian Summer Monsoon (EASM) variability, which reduced the cloud cover in the ECS and SYS, whereas the advection anomalies were mostly associated with regional wind anomalies. In summer 2018, upper ocean heat was transported into the central trough of the South Yellow Sea, accumulated in an anticyclonic eddy generated by the anomalous wind stress curls. Therefore, despite the primary driver of the MHWs is the EASM variation, regional processes are critical to driving the spatial pattern of the MHW intensity in the ECS and SYS.

Long-term temperature changes drive coastal Marine Heat Waves (MHW) trends globally. Here, we provide a more comprehensive global analysis of cross-shore gradients of MHW and SST changes using an ensemble of three satellite SST products during recent decades. Our analysis reveals depressed onshore SST trends in more than 2/3 of coastal pixels, including both eastern and western boundary current systems. These were well correlated with depressed trends of MHW exposure and severity, ranging from a -2 to -10 decrease in MHW days per decade and a –2.5 to –15°C.days per decade decrease in cumulative intensity. Results were consistent across all satellite products, indicating that these cross-shore gradients are a robust feature of observations. ERA reanalysis data shows that neither air-sea heat fluxes nor wind driven upwelling were found to be consistent drivers. Global ocean circulation models (OFAM3 and ACCESS-OM2) have limited ability to simulate the depressed onshore trends. A heat budget analysis performed in the Chilean coast region, where models agree with observations, showed that the gradient of temperature change was controlled by an onshore increase of longwave radiative cooling, despite an increase in upwelling. This highlights the complexity of small-scale coastal ocean-atmosphere feedbacks, which coarser resolution climate models do not resolve. Here, we show that global coastal regions may act as thermal refugia for marine ecosystems from aspects of climate change and pulsative (MHW) changes. Contrary to the literature, our results suggest that driving mechanisms are region dependant, stressing the necessity to improve climate models resolution.