Jetstreaks and Associated Circulations

            A jetstream is a narrow band of strong winds that encircles the Earth in the mid-latitudes, containing regions where locally strong pressure gradients produce exceptionally strong winds, called jetstreaks, which migrate through curved flow patterns. A jetstreak is located in a region of strong pressure gradient and is indicated by the large values of isotachs (lines of constant wind speeds) and the close spacing of the pressure or height contours. As air moves through the jetstreak, air parcels are displaced northward in the entrance region and southward in the exit region. Divergence occurs in the right entrance region (looking in the direction of the flow) while convergence in the left entrance region displaces air from the right (south) to the left (north) side of the jet. Divergence aloft will result in lower surface pressure, whereas convergence will result in higher surface pressure.

            Although the jetstream is not always a single ribbon of fast-moving air encircling the pole. In nature, a single jetstream can split into two branches and then merge again at a downstream location. In fact, the most extreme low pressures associated with cyclones in the middle latitudes usually occur when two (or even three) jetstreaks, each embedded in a different branch, interact with one another as their parent jetstreams merge.