Jet streams are persistent narrow bands of fast-moving high-altitude meandering air currents which flow from West-to-East around the globe in both hemispheres.
The speeds tend to be around 100 > 200 km/h, and the altitude 9–12 km above sea level/
Their positioning and power is very significantly important to lower altitude weather systems.
A 2008 study found that the principal jet streams (the one in the Northern hemisphere and the one in the South) have been 'migrating' slowly towards the poles each year (study period 1979 to 2001).
The movement was attributed to climate change. Since then however, the streams appear to have moved back away from the poles again.
There is currently no agreement as to whether the streams are likely to move pole-ward over the long term.
What is agreed is that any permanent movement will substantially alter global weather patterns.
These changes in jet stream latitude, altitude, and strength have likely affected, and perhaps will continue to affect, the formation and evolution of storms in the mid-latitudes and of hurricanes in the sub-tropical regions. Further observations and analyses are needed to confidently attribute the causes of these changes to anthropogenic climate change, natural variability, or some combination of the two.
See : Historical trends in the jet streams Geophysical Research Letters, Volume 35, Issue 8.
Note: Recent years have seen an increase in the fluctuations of the directions of the streams. The streams now tend to meander more North/South rather than a linear flow directly West to East.