On the interaction of tropical‐cyclone‐scale vortices. III: Continuous barotropic vortices

The interaction of cyclonic vortices in spherical geometry is investigated using a shallow‐water model on an adaptive grid. It is found that the mutual approach and merger of binary cyclones support the compound vortex‐patch findings of Part II. As interaction commences, distortion of the weak outer vorticity fields leads to a change in the advecting flow over each vortex core, which leads to mutual approach or retreat depending on the shape of the vorticity fields. Raped merger occurs when the cyclone cores approach within the critical separation distance defined in Part II. The core merger of cyclones seems to the largely independent of environment and the rotation of the earth. The initial approach can be quite sensitive, however. Non‐merging binary cyclones sweep equatorial parcels of air over long trajectories towards the pole to create exceptionally strong anticyclonic gyres. These can be of sufficient strength to capture the nearest cyclone and induce the escape mode described in Part I. Some sensitivity to latitude (and hence strength of the earth‐vorticity gradient), therefore, is found. A large anticyclone in a subtropical ridge location is found to be sheared and torn apart by the interacting cyclones, with little effect on merger. The observations in Part I that interactions of three cyclones could be broken down into separate binary interaction sequences is supported by experiments with three vortices. When two of the vortices merge, the interaction evolves to a small/large cyclone situation. When no merger occurs, each pair of cyclones proceeds through a distinctive orbit/escape cycle.