Recent seasonal asymmetric changes in the NAO (a marked summer decline and increased winter variability) and associated changes in the AO and Greenland Blocking Index
Recent seasonal asymmetric changes in the NAO (a marked summer decline and increased winter variability) and associated changes in the AO and Greenland Blocking Index
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DOI：
10.1002/joc.4157
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发表时间：
2015-07-01
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影响因子：
--
通讯作者：
Allan, Rob
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作者：
Hanna, Edward;Cropper, Thomas E.;Allan, Rob
北大西洋涛动（NAO）指数的平均值和变率最近发生了变化。自20世纪90年代以来，夏季NAO最近持续显著下降，与此同时，冬季特别是12月NAO的变化显著增加，导致2004-2013年115年记录中5个NAO 12月中的3个（5个中的2个）创纪录的高（创纪录的低）NAO 12月。这些NAO的变化与夏季格陵兰阻塞指数（GBI，格陵兰上空的高压）的增加趋势和12月更易变的GBI有关。增强的初冬NAO变率主要起源于NAO的南部节点，但也与12月更易变的GBI有关。过渡季节（春季和秋季）在过去30年中保持相对不变。这些结果证实了使用几个NAO指数。北极涛动（AO）指数，虽然与北大西洋涛动密切相关，并没有显示最近持续显着的夏季减少，但它确实显示出增强初冬变率。这些最近观察到的变化并不存在于当前一代的全球气候模型中，尽管最新的过程研究确实提供了对其原因的深入了解。我们援引几个合理的气候强迫和反馈来解释最近NAO的变化。
Recent changes are found in the means and variability of the North Atlantic Oscillation (NAO) index. There has been a sustained significant recent decrease in the summer NAO since the 1990s and, at the same time, a striking increase in variability of the winter-especially December-NAO that resulted in three of five (two of five) record high (record low) NAO Decembers occurring during 2004-2013 in the 115-year record. These NAO changes are related to an increasing trend in the Greenland Blocking Index (GBI, high pressure over Greenland) in summer and a more variable GBI in December. The enhanced early winter NAO variability originates mainly at the southern node of the NAO but is also related to the more variable GBI in December. Transition seasons (spring and autumn) have remained relatively unchanged over the last 30years. These results are corroborated using several NAO indices. The Arctic Oscillation (AO) index, although strongly correlated with the NAO, does not show the recent sustained significant summer decrease, but it does show enhanced early winter variability. These recent observed changes are not present in the current generation of global climate models, although the latest process studies do offer insight into their causes. We invoke several plausible climate forcings and feedbacks to explain the recent NAO changes.