One of the largest uncertainties in future climate projections is the interplay between internally generated and externally forced changes. This study investigates the changes in the link between the North Atlantic Oscillation (NAO) and Mediterranean winter extreme rainfall and dry days by the end of the 21st century compared to present day. We compare two different future pathways and estimate the extent to which the NAO imprint is affected by the global warming level using the latest EC-Earth3 large ensemble historical and future experiments. It is shown that the expected range of winter extremes changes due to internal and unpredictable fluctuations of the NAO largely overcomes the signal associated with externally-forced NAO variations. The NAO is found to exert a similar control on European climate variability, regardless of the amount of warming. For most of the Mediterranean region, magnitude and even sign of projected changes in the NAO-congruent precipitation indices vary substantially across the individual ensemble members according to the corresponding evolution of the NAO. Internal variability provides an average basin-wide contribution of up to 90% or more to the total NAO-driven variability in SSP1–1.9, and of about 80% in SSP5–8.5. Sub-regionally, the anthropogenic component of the NAO link is more evident over the Iberian Peninsula and parts of the central Mediterranean. This emphasises the role of internal variability and related uncertainty in determining the future impact of the NAO via the large spread in the circulation responses. However, the NAO is found to exert a weaker influence on the extreme precipitation total variability in both future scenarios given their future marked increase in total intensity and variance as opposed to the negligible NAO-related trends. Opposite conclusions are drawn for dry days, which are projected to decrease in the future, especially in the northern Mediterranean. Thus, this study also highlights how the variability of future extreme precipitation intensity in the Mediterranean basin will be less dependent on the principal mode of internal climate variability, posing further challenges for prediction and adaptation to weather-related hazards.

中文翻译：

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NAO 对 EC-Earth3 大系综中地中海冬季极端降水影响的未来变化：内部变率的突出作用

未来气候预测中最大的不确定性之一是内部产生的变化和外部强迫的变化之间的相互作用。本研究调查了 21 世纪末北大西洋涛动 (NAO) 与地中海冬季极端降雨和干旱天数之间的联系与现在相比的变化。我们比较了两种不同的未来路径，并使用最新的 EC-Earth3 大型集合历史和未来实验来估计 NAO 印记受全球变暖水平影响的程度。结果表明，由于 NAO 内部和不可预测的波动而导致的冬季极端变化的预期范围在很大程度上克服了与外部强制 NAO 变化相关的信号。人们发现，无论变暖程度如何，国家气象局都会对欧洲气候变化进行类似的控制。对于地中海地区的大部分地区，根据 NAO 的相应演变，NAO 一致降水指数的预测变化的幅度甚至符号在各个集合成员之间存在很大差异。在 SSP1-1.9 中，内部变异性对全流域范围内 NAO 驱动的变异性的平均贡献高达 90% 或更高，在 SSP5-8.5 中则约为 80%。从次区域来看，NAO 联系的人为成分在伊比利亚半岛和地中海中部部分地区更为明显。这强调了内部变异性和相关不确定性在通过环流响应的大范围确定 NAO 未来影响方面的作用。然而，我们发现，在两种未来情景中，NAO 对极端降水总变率的影响较弱，因为它们未来的总强度和方差显着增加，而不是与 NAO 相关的趋势可以忽略不计。对于干旱天数得出了相反的结论，预计未来干燥天数将减少，特别是在地中海北部。因此，这项研究还强调了地中海盆地未来极端降水强度的变化将如何较少地依赖于内部气候变化的主要模式，这对预测和适应与天气相关的灾害提出了进一步的挑战。