Recent studies indicated that the internal climate variability plays an important role in various aspects of projected climate changes on regional and local scales. Here we present results of the spreads in projected trends of wintertime North American surface air temperature and extremes indices of warm and cold days over the next half-century, by analyzing a 50-member large ensemble of climate simulations conducted with CanESM2. CanESM2 simulations confirm the important role of internal variability in projected surface temperature trends as demonstrated in previous studies. Yet the spread in North American warming trends in CanESM2 is generally smaller than those obtained from CCSM3 and ECHAM5 large ensemble simulations. Despite this, large spreads in the climate means as well as climate change trends of North American temperature extremes are apparent in CanESM2, especially in the projected cold day trends. The ensemble mean of forced climate simulations reveals high risks of warm days over the western coast and northern Canada, as well as a weakening belt of cold days extending from Alaska to the northeast US. The individual ensemble members differ from the ensemble mean mainly in magnitude of the warm day trends, but depart considerably from the ensemble mean in spatial pattern and magnitude of the cold day trends. The signal-to-noise ratio pattern of the warm day trend resembles that of the surface air temperature trend, with stronger signals over northern Canada, Alaska, and the southwestern US than the midsection of the continent. The projected cold day patterns reveal strong signals over the southwestern US, northern Canada, and the northeastern US. In addition, the internally generated components of mean and extreme temperature trends exhibit spatial coherences over North America, and are comparable to their externally forced trends. The large-scale atmospheric circulation-induced temperature variability influences these trends. Overall, our results suggest that climate change trends of North American temperature extremes are likely very uncertain and need to be applied with caution.

最近的研究表明，内部气候变化在区域和地方尺度的预计气候变化的各个方面都起着重要作用。在这里，我们通过分析使用CanESM2进行的由50个成员组成的大型气候模拟合集，介绍了下一个半个世纪北美冬季地表气温的预估趋势和冬季的极端指数的分布结果。CanESM2模拟证实了内部可变性在预计的表面温度趋势中的重要作用，如先前的研究所示。但是，CanESM2在北美变暖趋势中的传播范围通常小于从CCSM3和ECHAM5大型集成模拟获得的范围。尽管如此，在CanESM2中，北美气候极端现象的气候均值和气候变化趋势非常明显，尤其是在预计的寒冷天气趋势中。强迫气候模拟的整体平均值显示，西海岸和加拿大北部出现温暖天气的风险很高，而从阿拉斯加到美国东北部的寒冷天气也在减弱。各个合奏成员与合奏平均值的差异主要在于暖日趋势的幅度，但与合奏平均值的空间格局和冷日趋势的幅度相差很大。暖日趋势的信噪比模式类似于地表气温趋势，加拿大北部，阿拉斯加和美国西南部的信号强于大陆中部。预计的寒冷天气模式揭示了美国西南部，加拿大北部和美国东北部的强烈信号。此外，平均温度和极端温度趋势的内部产生的分量在北美地区表现出空间连贯性，并且与它们的外部强迫趋势相当。大气环流引起的大规模温度变化会影响这些趋势。总体而言，我们的结果表明，北美极端温度的气候变化趋势可能非常不确定，应谨慎应用。大气环流引起的大规模温度变化会影响这些趋势。总体而言，我们的结果表明，北美极端温度的气候变化趋势可能非常不确定，应谨慎使用。大气环流引起的大规模温度变化会影响这些趋势。总体而言，我们的结果表明，北美极端温度的气候变化趋势可能非常不确定，应谨慎使用。