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

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.

海表温度（Sea-surface temperature, SST）极端事件是珊瑚礁生态系统面临的主要胁迫因子，但当前多数全球海表温度产品的空间分辨率不足，难以量化影响珊瑚礁暴露风险的近岸精细尺度梯度。本研究采用5公里分辨率的美国国家海洋和大气管理局（National Oceanic and Atmospheric Administration, NOAA）极轨融合海表温度数据集，以及1985—2014年的30年气候学基准期，更新了基于像元与月份特异性阈值的局地极端热指数（local extreme heat index, LEHI），以此量化美国太平洋群岛533个沿海网格单元内极端热事件发生频率的变化。1985—1994年至2015—2024年期间，局地极端热指数均值提升14倍；相较于2005—2014年，该指数增幅达6.4倍，这意味着曾被视作罕见的极端热环境如今已较为常见。局地极端热指数的均值以每十年0.07个单位的速率增长（区间为0.01—0.11），按线性趋势推算，等价于每约15年增加1个月的热胁迫时长。2015—2024年的区域均值范围为0.18±0.03（威克岛）至0.42±0.08（马里亚纳群岛），其中关岛在2020年达到峰值0.67，当年12个月中有8个月处于极端热状态。年度空间局地极端热指数与美国国家海洋和大气管理局珊瑚礁观测站（NOAA Coral Reef Watch）的指标具有极强相关性：7天最大白化预警区域的零滞后相关系数r=0.70，热积温周的相关系数r=0.65。交叉相关分析在滞后-1至-4年时出现峰值，表明局地极端热指数升高通常先于累积热胁迫出现，可作为更早出现的统计预警信号。高分辨率局地极端热指数可对现有业务化产品形成有效补充：其可识别持续受热胁迫影响的区域（如帕尔米拉环礁）、潜在的热避难所（如威克岛），并可为管理措施延长预警提前期。