Strength of the winter North Atlantic jet stream has deviated from its natural trend under anthropogenic warming

The North Atlantic jet stream (NAJ) has a profound impact on the climate of the North Atlantic-European sector, especially in winter. Observations show that the winter NAJ (NAJw) has strengthened over the past similar to 140 yr. However, it remains unclear whether this long-term trend has deviated from the natural variability. Here, we present a 2500-yr-long reconstruction of NAJw strength using high-quality stalagmite delta 18O records from southeastern Europe. Our results show that the NAJw weakened during both the Roman Warm Period (300 B.C.- A.D. 200) and the Medieval Warm Period (A.D. 900-1250) but that it has strengthened under anthropogenic warming (since A.D. 1850). This indicates that its current trend has already deviated from the natural variability. The best explanation for this present anomalous trend of NAJw strength is that it was triggered by the appearance of the North Atlantic warming hole under anthropogenic forcing. This anomalous trend suggests that continued global warming may further strengthen the NAJw in the future.