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. 2021 Sep 21;118(38):e2104105118.
doi: 10.1073/pnas.2104105118.

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

Matthew B Osman  1   2 , Sloan Coats  3 , Sarah B Das  4 , Joseph R McConnell  5 , Nathan Chellman  5
Affiliations

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

Matthew B Osman et al. Proc Natl Acad Sci U S A. .

Abstract

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 ([Formula: see text]O) 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.

Keywords: Greenland; North Atlantic; climate change; ice core; jet stream.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
Relationship between geometric and statistic NAJ descriptions. (A) Geometric description of the North Atlantic zonal wind profile, created by zonally averaging mean annual near-surface zonal wind speeds over the North Atlantic (yellow boxed region on inset globe; Materials and Methods). Blue and red lines correspond to years with the maximum and minimum intensity (blue) and position (red) indices from the NOAA20C product, while the globe inset shows climatological mean annual near-surface zonal winds (1900 to 2015 CE). (B) Statistic description (14), showing the leading two modes of variability for the North Atlantic zonal wind profile (Jet-PC1 and -PC2). (C and D) Bilinear prediction of position (C) and intensity (D) from Jet-PC1 and -PC2 (circles and triangles denote NOAA20C and ERA20C data, respectively).
Fig. 2.
Fig. 2.
Extraction of NAJ signals from Greenlandic ice-core proxies. (A) Variance explained by GrIS-PC1 (Top), δ18O-PC1 (Middle), and annually accumulated snowfall (Accum.)-PC1 (Bottom) following bootstrap analysis. Shaded bands represent the 2.5th to 97.5th percentile range and the bold line the 50th percentile. Also shown are null distributions of explained variance (gray) following 1,000 PCAs conducted using power-spectrum-preserving surrogate datasets, revealing each PC1 series to be significant at the P< 0.001 level (SI Appendix). Map inset shows the GrIS-PC1 spatial loading pattern (unit normalized). (B) As in A, but for GrIS-PC2. (C) Correlation analyses for all three PC1 time series (GrIS-, δ18O-, Accum.-PC1) vs. Jet-PC1 following 1,000 bootstrap correlation tests for the overlapping interval AD 1900 to 2000. Null distributions represent 1,000 correlations conducted using pseudorandom surrogate time series. (D) As in C, but for all three PC2 time series vs. Jet-PC2. Note that only GrIS-PC1 and PC2 (i.e., combining δ18O and annually accumulated accumulation records) allow significant extraction of both Jet-PC1 and -PC2 signals.
Fig. 3.
Fig. 3.
Skillful reconstruction of the North Atlantic zonal wind profile during the last millennium. (A) Availability of Greenland δ18O and annually accumulated snowfall ice-core records over time. (B) Coefficient of efficiency (C.E.) significance levels for NAJ position, intensity, and the North Atlantic zonal wind profile for all nested models. Note that all nests have C.E. values greater than 0, signifying skill above climatology in the reconstruction. (C) NAJ position (red) and (D) intensity (blue) derived from the North Atlantic zonal wind profile reconstruction. The dark lines show NAJ position and intensity from the NOAA20C reanalysis, and the bold smooth lines show 30-y low-pass-filtered time series. Underlying dashed and dotted levels show the ±1σ range and the middle 95% range, respectively. (E) Hovmöller diagrams of the North Atlantic zonal wind profile for four selected time periods; note that observed and reconstructed NAJ positions are illuminated as dark (right panel) and light (all panels) red lines, respectively. Purple arrows at top of E denote years shown in F. (F) Selected annual snapshots of the North Atlantic zonal wind profile, as discussed in the text. All results shown are calibrated to the NOAA20C reanalysis (23) over 1900 to 2000 CE.
Fig. 4.
Fig. 4.
Projected NAJ intensity and position changes under high-emissions scenarios. CMIP-modeled mean annual NAJ intensity (A) and position (B) for the RCP8.5 and SSP5-8.5 emissions scenarios, smoothed for visualization at 30-y low-pass resolution. All CMIP5 and CMIP6 model runs are recentered to have the same mean NAJ intensity and position as the NOAA20C reanalysis between 1900 and 2000 CE. Shown for comparison (light gray shading) is the reconstructed range (95% CI) of (30-y low-pass) NAJ intensity and position during the last 13 centuries. The cross-model onset timing for sustained, significant (P< 0.05) northward NAJ migration (SI Appendix) is shown in yellow (median and 95% CI range; n = 48) in B. Projected 21st century NAJ position and intensity trends (median and 95% CI range) are shown relative to the last millennium (all 100-y periods) and CMIP-historical period (1850 to 2005 CE), at the top. Shown at the bottom in purple is the percent of years per decade whose CMIP-modeled NAJ position and intensity are significantly different (P< 0.05) than the NAJ position and intensity range exhibited by the reconstruction.
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