ABSTRACT

Based on two single-model initial-condition 50-member ensembles of climate simulations conducted with two generations of Canadian Earth System Models (CanESM2 and its successor CanESM5), we analyze the ensemble mean and spread of the projected trends of wintertime North American surface air temperature (SAT) and extreme indices of cold (TX10) and warm (TX90) days over the next half-century (2021–2070) and explore the contribution of internal climate variability to these trends. The ensemble mean of future climate simulations forced by the high-emissions scenario Representative Concentration Pathway 8.5 (RCP8.5) in CanESM2 and the Shared Socioeconomic Pathway 8.5 (SSP5-8.5) in CanESM5 reveals a poleward intensified warming, high risk of severe warm days over the west coast of North America and northern Canada, and a weakening belt of extreme cold days extending from Alaska to the northeastern United States. The warming trend is stronger in CanESM5 than in CanESM2, likely because of higher climate sensitivity and slightly higher CO₂ emissions in CanESM5. Large ensemble spreads are apparent in the SAT trend and in the historical simulations and future projections of extreme temperatures, especially for the TX10 index. Individual realizations differ from the ensemble mean in both spatial pattern and magnitude of the projected trends. The signal-to-noise ratio reveals strong signals of the SAT and TX90 trends primarily over the west coast of North America and northern Canada, along with relatively strong signals of the TX10 trend over most of the central to eastern parts of North America in CanESM2 and western Canada and the southwestern and eastern United States in CanESM5. The components of the mean and extreme temperature trends generated by internal climate variability exhibit large-scale spatial coherences and are comparable to the externally anthropogenic-forced components of the trends, mostly in the central parts of North America. Overall, similar ensemble mean patterns of North American mean and extreme temperature trends are evident in the two models; CanESM5 tends to be less uncertain in projecting those trends than CanESM2.

摘要

基于对两代加拿大地球系统模型（CanESM2及其后继CanESM5）进行的两个单模型初始条件，由50个成员组成的气候模拟合奏，我们分析了冬季北美地面空气预测趋势的集合均值和传播下半个世纪（2021年至2070年）的温度（SAT）和寒冷（TX10）和温暖（TX90）天的极端指数，并探讨内部气候变化对这些趋势的贡献。CanESM2中的高排放情景代表浓度路径8.5（RCP8.5）和CanESM5中的共享社会经济路径8.5（SSP5-8.5）迫使未来气候模拟的总体平均值显示出极度加剧的变暖，严重暖天的高风险在北美西海岸和加拿大北部，从阿拉斯加一直延伸到美国东北部的极端寒冷天气逐渐减弱。CanESM5的变暖趋势比CanESM2更强，这可能是由于更高的气候敏感性和稍高的CO₂CanESM5中的排放。在SAT趋势以及极端温度的历史模拟和未来预测中，尤其是TX10指数的变化，在SAT趋势中会出现较大的整体扩展。在空间格局和预测趋势的幅度上，个体实现与整体平均有所不同。信噪比揭示了主要在北美西海岸和加拿大北部的SAT和TX90趋势的强信号，以及在CanESM2中在北美大部分中东部的TX10趋势的相对强信号。以及CanESM5中的加拿大西部以及美国西南部和东部。由内部气候变化产生的平均和极端温度趋势的组成部分表现出大规模的空间连贯性，并且可以与趋势的外部人为强迫组成部分相比较，主要是在北美中部地区。总体而言，在两个模型中，北美平均和极端温度趋势的总体平均模式相似。与CanESM2相比，CanESM5在预测这些趋势方面的不确定性较小。