Earth’s equator-to-pole temperature gradient drives westerly mid-latitude jet streams through thermal wind balance1. In the upper atmosphere, anthropogenic climate change is strengthening this meridional temperature gradient by cooling the polar lower stratosphere2,3 and warming the tropical upper troposphere4,5,6, acting to strengthen the upper-level jet stream7. In contrast, in the lower atmosphere, Arctic amplification of global warming is weakening the meridional temperature gradient8,9,10, acting to weaken the upper-level jet stream. Therefore, trends in the speed of the upper-level jet stream11,12,13 represent a closely balanced tug-of-war between two competing effects at different altitudes14. It is possible to isolate one of the competing effects by analysing the vertical shear—the change in wind speed with height—instead of the wind speed, but this approach has not previously been taken. Here we show that, although the zonal wind speed in the North Atlantic polar jet stream at 250 hectopascals has not changed since the start of the observational satellite era in 1979, the vertical shear has increased by 15 per cent (with a range of 11–17 per cent) according to three different reanalysis datasets15,16,17. We further show that this trend is attributable to the thermal wind response to the enhanced upper-level meridional temperature gradient. Our results indicate that climate change may be having a larger impact on the North Atlantic jet stream than previously thought. The increased vertical shear is consistent with the intensification of shear-driven clear-air turbulence expected from climate change18,19,20, which will affect aviation in the busy transatlantic flight corridor by creating a more turbulent flying environment for aircraft. We conclude that the effects of climate change and variability on the upper-level jet stream are being partly obscured by the traditional focus on wind speed rather than wind shear.
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The NCEP/NCAR reanalysis data may be obtained from the National Oceanic and Atmospheric Administration (NOAA) Oceanic and Atmospheric Research (OAR) Earth System Research Laboratory (ESRL) Physical Sciences Division (PSD), Boulder, Colorado, USA (https://www.esrl.noaa.gov/psd/). The ERA-Interim and JRA-55 reanalysis data may be obtained from the Research Data Archive at the National Center for Atmospheric Research (NCAR), Computational and Information Systems Laboratory, Boulder, Colorado, USA (https://doi.org/10.5065/D6CR5RD9 and https://doi.org/10.5065/D6HH6H41, respectively).
The analytical computer codes are publicly available at https://doi.org/10.5281/zenodo.3238842.
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S.H.L. acknowledges support through a PhD studentship from the Natural Environment Research Council SCENARIO Doctoral Training Partnership (reference NE/L002566/1).
The authors declare no competing interests.
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Peer review information Nature thanks Darryn Waugh and Elizabeth A. Barnes for their contribution to the peer review of this work.
Extended data figures and tables
Extended Data Fig. 1 Vertical profiles of annual-mean trends in wind characteristics in the North Atlantic over the period 1979–2017.
a–c, Trends in the vertical shear in the zonal wind. d–f, Trends in the zonal wind speed. Linear trends are calculated from the ERA-Interim (a, d), NCEP/NCAR (b, e) and JRA-55 (c, f) reanalysis datasets. Red and blue colours represent positive and negative trends, respectively. Error bars represent the 95% confidence intervals in the slope of the ordinary least-squares regression (two-tailed t-test; n = 39).
Extended Data Fig. 2 Annual-mean regional-maximum six-hourly vertical shear in zonal wind in the North Atlantic at 250 hPa over the period 1979–2017.
Data are presented from the ERA-Interim, NCEP/NCAR and JRA-55 reanalysis datasets. Also shown are the mean of the three reanalysis datasets and the linear trend in the mean.
Extended Data Fig. 3 Annual-mean latitude of the core of the polar jet stream in the North Atlantic at 250 hPa over the period 1979–2017.
a, Annual-mean latitude of the regional-maximum six-hourly vertical shear in zonal wind. b, Annual-mean latitude of the regional-maximum six-hourly zonal wind speed. Data are presented from the ERA-Interim, NCEP/NCAR and JRA-55 reanalysis datasets. Also shown are the mean of the three reanalysis datasets and the linear trend in the mean, which has a statistically insignificant slope of –0.1° per decade (two-tailed t-test; P = 0.54; n = 39) (a) and 0.01° per decade (two-tailed t-test; P = 0.76;n = 39) (b).
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Lee, S.H., Williams, P.D. & Frame, T.H.A. Increased shear in the North Atlantic upper-level jet stream over the past four decades. Nature 572, 639–642 (2019). https://doi.org/10.1038/s41586-019-1465-z
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