4. Results

Animation 1: Time evolution of elliptical vortex (GIF animation; 351kB).
The contour position is displayed with a thick lines from t = 0.0 to t = 14.0 at an interval of 0.1 . Dashed lines are the directions of the axes of the ellipse in the case of no perturbation.

- Until t = 5.0 , the vortex keeps its shape and rotates with a nearly constant angular velocity γ .

- After that, the ellipstical vortex is deformed gradually with the developing perturbation. A part of the vortex patch is put out from one edge and is elongated thinner and thinner. This part is called a streamer. Interpreting the situation by the breaking of the wave trapped on the contour, this is similar to the breaking pattern of the planetary waves in the stratosphere in winter ( McIntyre and Palmer,1983 ).

- As the time goes on, the streamer gets longer and thinner, and surrouds the vortex. The main vortex comes close to a circle in its shape and increases its angular velocity ( cf. (2.13) ).