Aerological investigations conducted during the last few years have revealed that the long, meandering disturbances characteristic of the middle latitude belt of westerlies in the upper troposphere frequently increase in amplitude until troughs and ridges finally are cut off from the main current, thus forming cold cyclonic vortices on the equatorial side of the west-wind belt and warm anticyclonic vortices on the north side thereof. True vortices, with nearly vertical axes, are thus characteristic phenomena not only of equatorial regions where they appear as tropical cyclones but also of middle latitudes. The middle latitude vortices seem to be phenomena of great stability which have sometimes been observed to persist for several weeks.
Rossby, C. G.. 1948. "On displacements and intensity changes of atmospheric vortices." Journal of Marine Research 7, (3). https://elischolar.library.yale.edu/journal_of_marine_research/660