Oscillation of Separation Distance between Paired Typhoon-Scale Vortices

The oscillation of separation distance between centers of interacting binary vortices of typhoon-scale in a quiescent environment is investigated numerically with a vorticity equation. It is found that the separation distance between interacting vortices exhibits a tendency to oscillate with time regardless of whether they merge eventually or repel each other. The amplitude of oscillation depends on the initial separation distance as well as the structure of the vortex. The beta-effect has little influence on it during the first couple of days, but becomes important beyond that, due to the secondary circulaions. The maximum amplitude reaches more than one hundred kilometers for the case of strong vortices with a moderate size. The period of oscillation, typically one day or longer, in general, increases with the initial separation distance and shows an increasing tendency with time for foxed initial separation distance. With a simple analysis on the angular momentum, the oscillation of separation distance is described in terms of the eddy-mean interaction based on the plane polar coordinate system of which the origin is the center of binary vortices. In agreement with the previous studies, the analysis indicates that for paired vortices that rapidly merge together (repel each other) the anomaly or the deviation from the axisymmetric part, in average, has a phase line of inward-right tilt (inward-left tilt) near the origin. However, in a case where the vortices do not readily merge or repel in a short time range but maintain the temporal oscillation in the separation distance with a large amplitude, the phase configuration of the anomaly exhibits the alternation between the inward-right and inward-left phase tilt.