Do CMIP5 Models Reproduce Observed Low-Frequency North Atlantic Jet Variability?
Do CMIP5 Models Reproduce Observed Low-Frequency North Atlantic Jet Variability?
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DOI：
10.1029/2018gl078965
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发表时间：
2018-07-28
影响因子：
5.2
通讯作者：
Woollings, Tim
中科院分区：
文献类型：
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作者：
Bracegirdle, Thomas J.;Lu, Hua;Woollings, Tim
冬季观测到的北大西洋涛动(NAO)的数十年变化幅度处于气候模式模拟的范围的上端，对此仍难以得出明确的解释。最近的研究表明，观测到的数十年冬季NAO变率与北大西洋(NA)急流强度的关系比与纬度的关系更强，因此促使我们在耦合模式比较项目第五阶段(CMIP5)模式中全面比较NA急流和NAO变率。我们的结果表明，与133次历史CMIP5模拟的模型模拟范围相比，数十年急流强度变率的观测峰值比NAO变率更不寻常。一些CMIP5模型似乎能够再现观测到的喷流强度的低频峰值，但对每个模型的模拟太少，无法清楚地识别哪一个。研究还发现，自19世纪中叶以来，急流强度和北大西洋涛动之间存在着很强的年代际相关，这可能是这一时期所特有的。北大西洋海平面气压变化的主要模式是北大西洋涛动(NAO)，它与西欧的气候变化密切相关。然而，当前研究的主要焦点是，气候模式总体上不能再现足够大的低频(长于30年)的NAO变率，这意味着十年天气预报技能的显著改进是可能的。受此启发，我们分析了44个世界领先模式的数据，并更深入地研究了冬季NAO低频变化的主要潜在特征，特别是(1)从美国东部吹到西欧的盛行西风带的强度和纬度。我们的研究表明，是西风的强度，而不是纬度，是模型在再现低频NAO变化方面的困难所在。这张更详细的图片在对欧洲气候的影响方面也非常重要，因为主要与更强的西风相关的更正的NAO与主要与更偏南的西风相关的NAO对气候的影响不同。一个进一步的发现强调了这一点，即西风带的任何一个组成部分都可以在不同的几十年甚至几个世纪内主导NAO的变化。
The magnitude of observed multidecadal variations in the North Atlantic Oscillation (NAO) in winter is at the upper end of the range simulated by climate models and a clear explanation for this remains elusive. Recent research shows that observed multidecadal winter NAO variability is more strongly associated with North Atlantic (NA) jet strength than latitude, thus motivating a comprehensive comparison of NA jet and NAO variability across the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. Our results show that the observed peak in multidecadal jet strength variability is even more unusual than NAO variability when compared to the model-simulated range across 133 historical CMIP5 simulations. Some CMIP5 models appear capable of reproducing the observed low-frequency peak in jet strength, but there are too few simulations of each model to clearly identify which. It is also found that an observed strong multidecadal correlation between jet strength and NAO since the mid-nineteenth century may be specific to this period.Plain Language Summary The dominant pattern in sea level pressure variability over the North Atlantic (NA) is the North Atlantic Oscillation (NAO), which is strongly associated with climate variations over western Europe. However, major focus of current research is that climate models broadly do not reproduce large enough low-frequency (longer than 30years) NAO variability, with the implication that significant improvements in the skill of decadal weather prediction are possible. Motivated by this, we analyzed data from 44 of the world's leading models and delved more deeply into major underlying characteristics of low-frequency winter NAO variability, specifically (1) strength and (2) latitude of the belt of prevailing westerly winds that blow across the NA from the eastern United States to western Europe. Our study revealed that it is the strength of the westerlies, rather than latitude, that underlies models' difficulty in reproducing low-frequency NAO variability. This more detailed picture also really matters in terms of impacts on European climate, since a more positive NAO associated mainly with stronger westerlies has different climate impacts compared to one associated mainly with more poleward westerlies. This is emphasized by a further finding that either component of the westerlies can dominate NAO variability across different decades and even centuries.