Effect of Negative Vorticity on the Formation of Multiple Structure of Natural Vortices

Abstract

If a cyclonic vortex is bounded by a zero isotach surrounding its center at some radius, it is inevitably accompanied by negative vorticity regions somewhere within it. Since the radial gradient of vorticity changes its sign at the negative vorticity regions, it is thought to cause instability of the vortex and the resultant secondary circulation. It is attempted to attribute the origin of the oscillatory motion and/or the multiple structure observed in natural vortices to this instability. For this purpose, a simple axisymmetric vortex having a negative vorticity region between the solidly rotating core and a potentially rotating outer part is assumed, and its linear stability to asymmetric disturbances is examined with a quasigeostrophic model. The results indicate that the disturbances with azimuthal wavenumber m ≥ 1 are destabilized under certain conditions. The most unstable mode changes from an internal disturbance for m = 1 to an internal or an external disturbance for m ≥ 2 dependent on the magnitude and width of the negative vorticity region. Such effects of the negative vorticity may be relevant to the formation of the oscillatory motion and the multiple structures of natural vortices.