Linking Annual Maximum Sea Surface Temperature to Summer Marine Heatwave Occurrence in the Eastern China Seas

Marine heatwaves (MHWs) in the Eastern China Seas exert profound ecological and economic impacts, highlighting the need for reliable indicators to support early prediction. Based on observations from 1982 to 2022, this study identifies three characteristic patterns linking annual maximum sea surface temperature (Tmax) with summer MHWs: in July, the northern region follows a pattern where earlier Tmax favors more frequent MHWs; in August, the whole study area is dominated by a mode where Tmax coincides with the seasonal threshold peak, driving widespread MHWs; and in September, the southern region exhibits a pattern where later Tmax favors more frequent MHWs. A threshold-based method integrating both Tmax and its timing demonstrates strong skill in assessing MHW occurrence and exhibits practical utility when validated with independent observations from 2023 to 2024. Long-term warming of Tmax, together with regionally divergent trends in its seasonal timing, closely aligns with observed increases in MHW days. Significant correlations between Tmax and preceding monthly mean SST suggest that Tmax integrates accumulated thermal conditions and carries seasonal memory, offering a potential pathway from seasonal SST prediction to early MHW risk assessment. These findings clarify the structured and regionally differentiated Tmax–MHW relationship, demonstrate the feasibility of a Tmax-based assessment framework, and provide a scientific basis for improving seasonal monitoring and early warning of MHWs under sustained climate warming.