Change in Northern Hemisphere Circulation
Global warming affects regional temperature and humidity structures, and winds respond by changing the intensity and structure of the circulation. Changes that have taken place in recent decades in Northern Hemisphere atmospheric circulation structure and its associated winds are best described as poleward displacements of major wind and pressure systems. Trends associated with these displacements are important signals of climate change. Observed trends include the poleward expansion of the Hadley cell, the poleward shift and increase in mid-latitude westerly winds and contraction of the northern polar vortex.
The IPCC states, “it is likely that circulation features have moved poleward since the 1970s, involving a widening of the tropical belt, [and] a poleward shift of storm tracks and jet streams.”
Depending on the indicator under investigation, scientists have observed tropical widening by 0.3 to 3.1° latitude per decade since 1979, with a consensus widening of about 1.4°. (Note: Part of the wide range is because the boundary between the tropics and extratropics is not well-defined and depends on the definition of specific indicators of tropics width—such as the position of the jet stream, where surface winds change from westerly to easterly or physical indicators like ozone concentration and humidity.)
Research indicates that extratropical cyclones have shifted toward the poles in both hemispheres over the past 50 years, with further but limited evidence showing a decrease in wind storm frequency at mid-latitudes. Mid-latitude westerly winds have generally increased in both hemispheres.
In the Northern Hemisphere, annual precipitation has likely increased since 1901.