In late June, 2021, a devastating heatwave affected the US Pacific Northwest and western Canada, breaking numerous all-time temperature records by large margins and directly causing hundreds of fatalities. The observed 2021 daily maximum temperature across much of the U.S. Pacific Northwest exceeded upper bound estimates obtained from single-station temperature records even after accounting for anthropogenic climate change, meaning that the event could not have been predicted under standard univariate extreme value analysis assumptions. In this work, we utilize a flexible spatial extremes model that considers all stations across the Pacific Northwest domain and accounts for the fact that many stations simultaneously experience extreme temperatures. Our analysis incorporates the effects of anthropogenic forcing and natural climate variability in order to better characterize time-varying changes in the distribution of daily temperature extremes. We show that greenhouse gas forcing, drought conditions and large-scale atmospheric modes of variability all have significant impact on summertime maximum temperatures in this region. Our model represents a significant improvement over corresponding single-station analysis, and our posterior medians of the upper bounds are able to anticipate more than 96% of the observed 2021 high station temperatures after properly accounting for extremal dependence.

翻译：2021年6月下旬，一场毁灭性热浪侵袭美国太平洋西北地区及加拿大西部，大幅刷新多项历史最高气温纪录，直接导致数百人死亡。在考虑人为气候变化影响后，美国太平洋西北地区大部分观测到的2021年日最高气温仍超过基于单站点温度记录估算的上限值，这意味着该事件在标准单变量极值分析框架下无法被预测。本研究采用灵活的空间极值模型，该模型考虑太平洋西北区域内所有站点，并纳入多站点同时经历极端温度的事实。我们的分析融合了人为强迫与自然气候变率的影响，以更好地刻画日极端温度分布的时变特征。研究表明，温室气体强迫、干旱条件及大尺度大气环流模态均对该地区夏季最高气温产生显著影响。相比对应单站点分析，我们的模型实现了显著改进，在合理考量极值依赖性后，其后验上限中位数能够预测超过96%的2021年观测站点高温值。