When the perturbations of the pressure distribution due to rapidly moving cyclones are eliminated by plotting charts of the mea1;1 distribution of the pressure for a number of days, centres of larger dimensions 13,nd of a more permanent character appear, as has been shown by the work carried out by Rossby and his collaborators (7). On winter maps of the northern hemisphere, as a rule at least five of these perturbations are found: the Icelandic and the Aleutian Lows, the Azores, Asiatic and Pacific Highs. They can hardly be considered as mere perturbations of the general circulation, but must rather be regarded as important parts of it, as is evidenced by their semipermanent character. Nevertheless, they will in the following sometimes be referred to as perturbations or disturbances, since they are treated mathematically as perturbations of an undisturbed current.
Haurwitz, B.. 1940. "The motion of atmospheric disturbances on the spherical Earth." Journal of Marine Research 3, (3). https://elischolar.library.yale.edu/journal_of_marine_research/575