Structural Climate Drivers of Global Coral Bleaching

Mass coral bleaching has become a hallmark ecological signature of anthropogenic climate change, yet the structural mechanisms governing its spatial and temporal evolution remain poorly resolved. Traditional assessments emphasize surface thermal anomalies and strong El Niño phases but increasingly fail to explain the persistence of bleaching, its regional asymmetry, and the instability of ecological thresholds. Here, using long-term datasets (1993-2020) of ocean temperature, bleaching alerts, El Niño-Southern Oscillation (ENSO) indices, coral-algal cover, and subsurface thermal profiles, we identify a regime shift from episodic bleaching toward chronic thermal exposure. This shift reflects a reduced dependence of bleaching on ENSO intensity, as even moderate ENSO phases, including both El Niño and La Niña events, are now associated with widespread bleaching, suggesting progressive erosion of thermal thresholds under sustained ocean warming. We further propose a "Structure-Pathway-Response" framework to capture the structural reconfiguration of bleaching risk: (i) heat convergence along eastern continental margins (e.g., East Asian Seas, Caribbean); (ii) poleward transport and topographic retention of subsurface heat along continental slopes; and (iii) vertical accumulation through isotherm deepening that elevates bleaching risk. We identify two dominant heat-retention regimes: a current-deflection mode in the Northern Hemisphere and a thermal-stacking mode in the Southern Hemisphere. These patterns increase regional vulnerability by broadening the spatial extent and persistence of thermal anomalies, reflecting processes not fully captured by surface Sea Surface Temperature (SST) variability. Our findings highlight the limitations of surface-only monitoring systems and underscore the need for thermodynamically informed, region-specific early-warning frameworks. Protecting structural thermal refugia and managing heat pathways will be critical for sustaining coral reef resilience in a rapidly warming ocean.