Regional divergence of extreme marine heat in Pacific Island coastal waters: a high-resolution extreme heat index as an earlier statistical signal

Abstract Sea-surface temperature (SST) extremes are a major stress to coral reef ecosystems, yet most global SST products lack the spatial resolution to measure fine-scale nearshore gradients that influence reef exposure. Using the 5 km NOAA geo-polar blended SST record and a 30 year climatological baseline (1985–2014), we updated the local extreme heat index (LEHI), a metric based on pixel- and month-specific thresholds, to quantify changes in the frequency of extreme thermal events across 533 coastal grid cells in the U.S. Pacific Islands. The mean LEHI increased 14-fold from 1985–1994 to 2015–2024, and 6.4-fold relative to 2005–2014, indicating that extreme thermal conditions once considered rare are now common. The average LEHI increased by 0.07 units per decade (range: 0.01–0.11), equivalent to one additional month of heat stress every ∼15 years under a linear trend. Regional means for 2015–2024 ranged from 0.18 ± 0.03 (Wake) to 0.42 ± 0.08 (Marianas), with Guam peaking at 0.67 in 2020 (8/12 extreme months). Annual spatial LEHI correlated strongly with NOAA Coral Reef Watch metrics (zero-lag r = 0.70 for the seven-day maximum bleaching alert area; r = 0.65 for degree-heating weeks). Cross-correlation peaks at lags −1 to −4 years indicate that elevated LEHI generally precedes accumulated heat stress, representing an earlier statistical signal. High-resolution LEHI complements existing operational products by highlighting areas of persistent exposure (e.g., Palmyra), identifying potential thermal refugia (e.g., Wake Island), and extending lead times for management actions.