North Atlantic jet stream projections in the context of the past 1,250 years

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 (delta O-18) 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.

Automated summary

The reconstruction of the North Atlantic jet stream using last-millennium climate model simulations and Greenland ice core records shows that late 20th- and early 21st-century variations were likely not unique compared to natural variability. Insights from the 1,250 year reconstruction highlight the significant role of natural variability in masking the response of midlatitude atmospheric dynamics to anthropogenic forcing. Under high greenhouse gas emissions scenarios, the model projected annual mean position of the jet stream is expected to emerge as distinct from natural variability by as early as 2060 AD.