Diversity of South China Sea Marine Heatwave Intensity Trends

Abstract This study investigates the seasonal characteristics and physical drivers of marine heatwave (MHW) intensity trends in the South China Sea (SCS) during 1993–2022. Results reveal a robust, seasonally consistent north-south divergent pattern: MHW intensity has intensified pronouncedly in the northern and northwestern SCS while declining on decadal timescales in the southwestern basin, forming a "rich-get-richer, poor-get-poorer" contrast. Mixed-layer heat budget diagnostics show that enhanced net surface heat flux (Qnet), driven by increased shortwave radiation (SSR) and reduced latent heat loss, primarily causes MHW intensification in the northern and northwestern SCS. In contrast, changes in the South China Sea West Boundary Current (SCS WBC) dominate in the southwestern SCS, where a weakening northward WBC in summer and strengthening southward WBC in winter shift horizontal advection toward net cooling, and thereby weakens MHWs. Teleconnection analyses link this regional disparity to different large-scale climate modes across seasons. Strengthening of the Indian Ocean Dipole (IOD) and Atlantic Multidecadal Oscillation (AMO) respectively increase SSR over the northern SCS during summer and winter, while the negative phase shift of the Pacific Decadal Oscillation (PDO) and ENSO weakens anticyclonic forcing that modulates the SCS WBC in the southwestern SCS in both seasons. These findings highlight the importance of seasonal and regional heterogeneity in MHW dynamics for improving the prediction of extreme ocean warming in marginal seas.