Over the past few years the working hypothesis among many climate scientists is that global warming—specifically strong polar warming—is causing an increase in extreme weather events at midlatitudes. The leading proposed mechanism revolves around two main factors: the midlatitude jet stream and atmospheric blocking. Although the idea has been well covered in public discussions and in the media, the actual physical basis behind it is still under considerable debate. In their new study, Hassanzadeh et al. suggest that previous research has overlooked one crucial detail, and in the process gotten the connection between polar warming and midlatitude extreme weather backwards.
Based on previous research, the proposed mechanism tying polar warming to extreme weather worked like this: By increasing the temperature in the Arctic faster than at the equator, climate change is reducing the meridional pressure and temperature gradients, and the speed of west-to-east zonal winds. Slower winds cause a more “wavy” jet stream and more incidences of atmospheric blocking at midlatitudes. Atmospheric blocking causes regions of high or low atmospheric pressure to become stuck in place, causing weather patterns to stagnate for days or even weeks at a time, resulting in heat waves, cold spells, long, deep droughts, or periods of seemingly endless precipitation.
Using a highly idealized model of atmospheric circulation, however, the authors of the current study show that this previous research had overlooked one important factor: polar warming doesn’t just reduce the meridional pressure and temperature gradients; it also reduces the variabilities of pressure and temperature at the jet stream’s altitude. Polar warming, they find, stabilizes the atmospheric circulation.
In their idealized model, which accounts for changes in pressure and temperature variabilities, the authors found that rather than making the jet stream wavier and causing more atmospheric blocking, polar warming is actually streamlining the jet stream and reducing the incidence of blocking events. (Geophysical Research Letters, doi:10.1002/2014GL060764, 2014)
—Colin Schultz, Freelance Writer
Citation: Schultz, C. (2015), Polar warming makes the jet stream stable, not wavy or blocked, Eos, 96, doi:10.1029/2015EO023125. Published on 3 February 2015.