Compared to the Northern Hemisphere, the Southern Hemisphere has a lot more open ocean. Notably, there is a band of water North of Antarctica and below Australia, Africa and South America that is completely uninterrupted by land masses. This open band allows for a strong circumpolar ocean current around Antarctica that is unique to the Southern Hemisphere. The strong circumpolar current creates atmospheric circulation patterns throughout the Southern Hemisphere that are more “zonally symmetric”—a meteorological term that means more consistent across latitudes.
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.