Lee, S. H., P. D. Williams, and T. H. A. Frame, 2019: Increased shear in the North Atlantic upper-level jet stream over the past four decades. Nature, https://doi.org/10.1038/s41586-019-1465-z.
- What is the key result of the study?
Our study shows that the annual-mean vertical west-to-east (zonal) wind shear over the North Atlantic at aircraft cruising altitudes (within the jet stream) has increased by 15% (with a range of 11-17%) over the last four decades (1979-2017).
- What is vertical wind shear?
Vertical wind shear is the name given to the change in wind speed (or direction) with height. In our study, we consider the change in the speed of the west-to-east (“zonal”) wind with height.
- What data were used to get these results?
We used three reanalysis datasets (ERA-Interim, JRA-55, and NCEP/NCAR). These are our best estimates of the past evolution of the atmosphere, based on observations. We use data from 1979 onwards, as satellites have been routinely used to monitor the state of the atmosphere since then, increasing the reliability of the data.
- Why has wind shear increased?
On large scales like the North Atlantic, the vertical wind shear is driven by the difference in temperature as you move from the equator to the pole. When this is larger, the wind shear is larger. At aircraft cruising altitudes (around 34,000 feet), this temperature difference has increased, causing an increase in wind shear.
- Why has the temperature difference increased at aircraft cruising altitudes?
At low latitudes (less than about 50N), temperatures are warming at this level in response to increasing greenhouse gases. Poleward of 50N, temperatures are in fact cooling. At these latitudes, this level (34,000 feet) is in the stratosphere, not the troposphere where we live. Increasing greenhouse gas concentrations actually cause this layer of the atmosphere to cool. There is also a cooling effect from a decline in concentrations of ozone.
- Why should we care?
Wind shear is a key driver of clear-air turbulence (CAT), which is a significant aviation hazard as it is invisible and difficult to predict. Turbulence in general has been found to play a role in the majority of weather-related aviation incidents. Climate models project a large increase in CAT over the North Atlantic under future climate change (Storer et al. 2017). Our study shows observation-based evidence that a key driver has already increased by a large amount, supporting these projections.
- The study finds shear has increased. Have encounters with turbulence increased?
Unfortunately, it is very difficult to say this with any degree of confidence. Over the last four decades, airplane designs have changed, flight numbers have increased, and we are now able to identify regions of possible turbulence and avoid them – so it is difficult to produce any consistent record of turbulence observations. However, our study suggests it may have, as a key driver has increased by a large amount.
- So, is the jet stream strengthening or weakening?
We find that, in the lowermost ~5 km or so, the temperature difference between the equator and pole is weakening, thanks to the rapidly warming Arctic. This contrasts with the increasing difference at higher levels. We find that these effects are approximately equal and opposite, like a balanced tug-of-war. The overall speed of the jet stream aloft depends on the total effect (the vertical integral), so there has been no significant change to the speed of the jet stream at aircraft cruising altitudes. As our study only looks at the annual-mean, it is possible that different weakening or strengthening trends occur in different seasons, as other studies have found.
- Is the jet stream becoming wavier?
We do not look at this in our study. Some previous studies have suggested this, primarily as a result of the decreasing temperature difference in the lower part of the atmosphere. Our results neither prove nor disprove these findings.
- What does this mean for the future?
We focus on what has been observed to occur, rather than what is projected. However, most studies suggest the upper-level “tug” will win, leading to a strengthening jet stream, with impacts on transatlantic flight times (Williams 2016). As increasing greenhouse gas emissions continue to increase temperatures in the troposphere and cool those in the stratosphere, we expect shear to continue to increase at flight levels (though large interannual variability is likely), consistent with model projections of increasing turbulence.