Summer marine heatwaves in the South China Sea:Trend,variability and possible causes

Marine heatwaves(MHWs)have been frequently observed worldwide,causing devastating impacts on marine organisms and ecosystems,but the trend of MHWs is still unclear in the South China Sea(SCS).Here,the long-term trend and inter-annual variability of the summer SCS MHW events are investigated based on the high-resolution daily satellite data.The results revealed remarkable increases in the duration,intensity,coverage,and severity during 1982-2019,indicating that the SCS MHW events have become more frequent,intense,extensive,and serious.The probability ratio of SCS MHW events is four times during the 2010s of that during the 1980s.The increasing trend can be largely attributed to the long-term increase in the mean SCS temperature.The inter-annual variability of the SCS MHWs is linked closely to the El Niño and Southern Oscillation,with more/less MHW events occurring during the following summer after the El Niño/La Niña events.A diagnosis of synoptic-scale heat budget suggests that the extreme SCS warming can be explained by the combined effects of positive surface heat flux largely due to the enhanced shortwave radiation and convergence of oceanic advection in association with an anomalous upper-ocean anticyclone.The effect of surface heat flux seems to be predominant over the large spatial coverage,whereas oceanic heat transport is also important in some specific regions.The large-scale anticyclonic circulation anomalies over the northwestern Pacific accompanying the westward-extending western Pacific subtropical high during the El Niño decay summers play an essential role in the building-up and persistence of the extreme warming,which has important implications for the prediction of the SCS MHWs.

Causes of 2022 summer marine heatwave in the East China Seas

Recent occurrences of marine heatwaves(MHWs)in coastal China seas have caused serious impacts on marine ecosystem services and socio-economics.Nevertheless,the underlying physical process,including local drivers and remote associations,remains poorly understood,thereby hindering accurate predictability.In this study,we reported an extreme MHW event in the East China Seas(ECSs,including the Bohai,Yellow,and East China Sea),lasting for 75 d with a maximum intensity of 1.96℃relative to 1982-2011 during the summer 2022.This ECSs MHW event was triggered by a combination of anomalous atmospheric and oceanic conditions,including enhanced insolation,weakened surface wind speed,suppressed latent heat loss from ocean,a shallower mixed layer,and upper ocean current anomaly.Mixed-layer temperature budget diagnosis suggested that changes in the ECSs temperature were dominated by the surface net heat flux,largely due to strong shortwave radiation flux,during the development and decay of the MHW event.Oceanic advection also created favorable conditions for the maintenance of the MHW.These physical drivers were further regulated by the westward expanded and intensified western Pacific subtropical high(WPSH),potentially linked to the negative phase of Indian Ocean Dipole(IOD).Despite the three years(2020-2022)consecutive La Niña events,the ECSs summer MHWs appeared to be more closely linked to negative IOD events,with a lagging period of 1-3 mon.The seasonal precursor signals of the negative IOD have the potential to affect local physical drivers of ECSs MHWs through regulating the strength and position of WPSH,thus serving as a promising predictor for the ECSs MHWs.The future likelihood and intensity of the ECSs MHWs are projected to increase substantially in the coming decades,largely due to broad-scale warming attributed to anthropogenic climate change.Consequently,there is an urgent need to develop MHW forecasting and early warning systems,and robust approaches to address climate change.