A Climatology of Filaments Emanating from the Stratospheric Polar Vortex in the Southern Hemisphere

Abstract

Using 43-year reanalysis data, the time evolution, three-dimensional structure, and climatology of the Ertel potential vorticity (PV) filaments associated with the stratospheric polar vortex from winter to spring in the Southern Hemisphere are examined. The filaments are formed when a part of the polar vortex extends equatorward, mainly due to the breaking of planetary waves originating from the troposphere. In early winter, the polar vortex occasionally has a structure with multiple PV steps. The filaments are continuously observed during the abrupt disappearance of the lowest latitude PV step and followed by shrinking of the polar vortex. A small-scale barotropic vortex forms at the tip of the filament when the filament is sufficiently zonally elongated. The climatology of the filament occurrence frequency is examined for an 850 K isentropic surface. The filament root, which is robustly determined and used as a reference of the filament location, is mainly distributed over 25°S-45°S in the western hemisphere in July–October with a slight eastward movement in October. The momentum flux associated with small-scale disturbances, including vortices arising from the filaments, is largely negative in 30°S-50°S, giving westward (eastward) forcing in the equatorward (poleward) side to diminish the PV maximum including filaments.