NOAA/WDS Paleoclimatology - Greenland Ice Cores 1,250 Year d18O, Accumulation, and North Atlantic Jet Stream Reconstruction

Reconstruction of the North Atlantic jet stream (NAJ) presents a critical, albeit largely unconstrained, paleoclimatic target. Models suggest northward migration and changing variance of the NAJ under 21st-century warming scenarios, but assessing the significance of such projections is hindered by a lack of long-term observations. Here, we incorporate insights from an ensemble of last-millennium water isotope–enabled climate model simulations and a wide array of mean annual water isotope (δ18O) and annually accumulated snowfall records from Greenland ice cores to reconstruct North Atlantic zonal-mean zonal winds back to the 8th century CE. Using this reconstruction we provide preobservational constraints on both annual mean NAJ position and intensity to show that late 20th- and early 21st-century NAJ variations were likely not unique relative to natural variability. Rather, insights from our 1,250 year reconstruction highlight the overwhelming role of natural variability in thus far masking the response of midlatitude atmospheric dynamics to anthropogenic forcing, consistent with recent large-ensemble transient modeling experiments. This masking is not projected to persist under high greenhouse gas emissions scenarios, however, with model projected annual mean NAJ position emerging as distinct from the range of reconstructed natural variability by as early as 2060 CE.

北大西洋急流（North Atlantic Jet Stream, NAJ）的重建是一项关键却在很大程度上尚未得到充分约束的古气候研究目标。气候模式显示，在21世纪变暖情景下，NAJ会发生北移且方差发生改变，但由于缺乏长期观测数据，评估这类预测的意义受到了阻碍。本研究整合了近千年水同位素耦合气候模式的集合模拟结果，以及大量来自格陵兰冰芯的年平均水同位素（δ¹⁸O）与年累积降雪记录，将北大西洋纬向平均纬向风的重建序列追溯至公元8世纪。基于该重建序列，我们对年平均NAJ的位置与强度提供了观测前约束，结果表明：20世纪末与21世纪初的NAJ变化，相对于自然变率而言很可能并非独有。相反，基于我们1250年重建序列的分析结果凸显了自然变率的压倒性作用——迄今为止，自然变率掩盖了中纬度大气动力学对人为强迫的响应，这与近期的大集合瞬变模拟试验结果一致。不过，在高温室气体排放情景下，这种掩盖效应预计不会持续存在：模式预测的年平均NAJ位置最早可在公元2060年，就与重建得到的自然变率范围出现显著偏离。