The subtropical cyclone

Upper-level ‘cut-off’ cyclones, first identified in middle latitudes, are found as well in lower latitudes (where they are known as subtropical cyclones), often extending to the surface and persisting for long periods. A nearly steady-state subtropical cyclone photographed by Tiros I on April 4, 1960, and an earlier subtropical cyclone which developed near Lihue in February 1951 provide data for a subtropical cyclone model. Development occurs as in middle latitudes. The mature cyclone comprises a direct ascending circulation in the upper troposphere overlying a direct subsiding circulation in the middle troposphere. In the surface layers a regime resembling the trade winds prevails. Because of its energy-exporting character and the insignificant effect of surface friction, a subtropical cyclone apparently does not decay, being usually absorbed by a large-amplitude trough in the polar westerlies.