Fukui, H.

Degree Heating Weeks (DHW) is the standard metric for global coral bleaching prediction, yet its performance varies markedly across regions for unexplained reasons. We analyse five years (2020-2024) of standardised bleaching surveys from Japan's Monitoring Site 1000 program (26 sites, 113 site-years; balanced panel n = 105) across a 24-35 degrees N gradient to diagnose why DHW fails in subtropical waters. Only 4 of 113 site-years (3.5%) reached the DHW >= 4 alert threshold, while bleaching (>0%) was recorded in 65 site-years (57.5%). DHW sensitivity for detecting any bleaching was 6.2%. A simple absolute-temperature metric (days with SST >= 30 degrees C) outperformed DHW in discriminating >=50% bleaching (AUC = 0.926 vs 0.667, DeltaAUC = 0.260, 95% CI [0.154, 0.355], p < 0.001), largest at low latitudes (DeltaAUC = 0.293, p < 0.001). The Maximum Monthly Mean (MMM) was strongly correlated with latitude (r = -0.914), compressing the thermal gap available for HotSpot accumulation at low-latitude sites and eliminating HotSpot events at high-latitude sites. This structural insensitivity -- arising from the anomaly-based design of DHW rather than from threshold miscalibration -- operated through two distinct mechanisms across the latitudinal gradient. At low latitudes, where MMM approaches 30 degrees C, HotSpot signals were compressed below detection thresholds despite widespread bleaching; at high latitudes, SST rarely exceeded MMM, rendering HotSpot events absent altogether. These findings demonstrate that DHW's standard alert framework is structurally non-functional across Japan's coral monitoring network and that regional assessment requires metrics independent of the MMM-relative anomaly architecture.