The interaction of binary tropical cyclones (TC) is investigated using a coupled TC-ocean movable nested-grid model. The model consists of an eight-layer atmospheric model in the sigma coordinate system and a three-layer primitive equation ocean model. There are five meshes in the TC model. The outermost domain (3840 km × 3840 km) is motionless. For the description of each TC in a TC pair, two telescopically nested meshes of finer resolution are used. The pair of the middle (1600 km × 1600 km) and innermost (800 km × 800 km) meshes move with the center of a corresponding TC. The space increments of the outermost domain and the middle and finest meshes are 160, 80, and 40 km. The oceanic domain contains 107 × 107 grid points, with the spatial increment of 40 km. In all numerical experiments a pair of equal strength axisymmetric vortices was located at different separation distances.
Experiments show that the rate of development of interacting TCs is different, mainly due to the difference in the velocities of TC movement. There is a “critical” separation distance between the centers of TCs, so that in case the separation distance is less than this critical value, attraction and merger of the TCs were observed. The critical separation distance depends on the structure of the vorticity field created by the binary TCs. Because of the changes in the structure of a TC during its life cycle the critical separation distance should also change. Two mechanisms related to the mutual vorticity advection and to the activity of irrotational velocity components seem to contribute to the attraction and repulsion of binary TCs.
The impact of the TC-ocean interaction on the evolution and trajectory of binary TCs is much stronger than in the case of a single TC. A decrease in TC strength is related not only to a TC response to seawater cooling caused by the TC itself but also to the crossing of the cold water wakes created both by the other TC and by the TC itself. A decrease in strength loads to a decrease in the mutual rotation velocity and, consequently, to a marked change in the trajectories of each of the interacting TCs. Changes in the structure of binary TCs caused by the TC-ocean interaction lead to an increase of the critical separation distance. Binary TCs cause seawater cooling over vast ocean areas and lead to the formation of a spotted sea surface temperature pattern.