A classification of binary tropical cyclone-like vortex interactions

The interaction between two tropical cyclones with different core vorticities and different sizes is studied with the aid of a nondivergent barotropic model, on both the f plane and the sphere. A classification of a wide range of cases is presented, using the Dritschel-Waugh scheme, which subdivides vortex interactions into five types: elastic interaction, partial straining out, complete straining out, partial merger, and complete merger. The type of interaction for a vortex pair on the f plane, and the same pair on the sphere, was the same for 77 out of 80 cases studied. The primary difference between the results on the f plane and those on the sphere is that the vorticity centroid of the pair is fixed on the f plane but can drift a considerable distance poleward and westward on the sphere. In the spherical case, the interaction between the cyclone pair and the associated beta-induced cyclonic and anticyclonic circulations can play an important role. The "partial merger" regime is studied in detail. In this regime the interaction between vortices can lead to episodic exchanges of vorticity, with both vortices surviving and entering a stage of continued but weaker interaction. With the aid of passive tracers, it is found that the exchange of vorticity is restricted to the vortex periphery even when the vorticity field within each vortex is flat, so that the vortex core is the last region to be eroded. It is hypothesized that some observed interacting tropical cyclones actually do undergo this partial-merger process.