When two tropical cyclones (hurricanes or typhoons) appear to be on a converging path, the term “collision” is a dramatic misnomer. These powerful, rotating weather systems rarely crash into one another. Instead, when they approach a certain proximity, they begin a dynamic interaction governed by fluid mechanics. This process involves mutual influence, where each storm affects the other’s trajectory and intensity. The outcome can be a change in storm track, a temporary orbit, or even the merger of the two systems.
The Science of Tropical Cyclone Interaction
The interaction between two nearby tropical cyclones is formally known as the binary interaction. This effect, the atmospheric equivalent of two orbiting bodies, was first described in 1921 by Japanese meteorologist Sakuhei Fujiwhara. The mechanism involves the two storms, which are large, rotating columns of air, exerting a mutual influence due to their rotational wind fields.
The interaction becomes noticeable when the centers of the two cyclones come within approximately 870 miles (1,400 kilometers) of each other. At this distance, the outer wind fields overlap and pull on the other storm’s vortex. This causes the two low-pressure centers to begin rotating around a common midpoint.
Variables Determining the Outcome
The specific result of a binary interaction depends on the relative characteristics of the two tropical cyclones.
A primary factor is the disparity in storm intensity, measured by maximum sustained wind speed and minimum central pressure. If one storm is significantly stronger, the common center of rotation shifts closer to the more powerful vortex. This stronger circulation then has a greater influence on the weaker storm.
Another variable is the difference in size, specifically the diameter of the wind fields. A larger storm can dominate a smaller system because its rotational influence extends over a greater area. The location of the common point of rotation, or barycenter, is determined by the relative mass and intensity of the two vortices.
The interaction is also influenced by the background atmospheric flow, known as the environmental steering current. This larger-scale wind pattern acts as an external force, potentially pulling one or both storms away from the binary interaction.
Possible Scenarios of Interaction
The interaction can result in four primary scenarios:
Mutual Orbiting
This is one of the most common outcomes, where the two storms circle a common center without immediately merging. This “dance” can persist for several days, causing the path of both storms to become erratic and difficult to predict. The storms maintain their separate identities but are temporarily locked into a rotational pattern.
Complete Merger
This occurs primarily when the two systems are of similar size and intensity. As they orbit, the two vortices spiral inward toward the barycenter, eventually combining to form a single, larger, and often more intense tropical cyclone. This consolidation results in the combined mass and energy of both systems concentrating into one powerful storm.
Absorption or Destruction
This happens when there is a substantial difference in strength between the two storms. The stronger cyclone’s circulation completely overwhelms the weaker one, effectively tearing the smaller system apart. The remnants of the weaker storm are then incorporated into the circulation of the dominant system.
Mutual Steering
This involves a change in the trajectory of one or both storms without a merger or absorption taking place. The interaction is not strong enough to force the storms into a tight orbit or merger, but the wind fields are sufficient to deflect their original courses. Following the deflection, the storms move far enough apart to lose the mutual influence.
Documented Historical Examples
The binary interaction has been observed across various ocean basins, demonstrating the different outcomes possible.
A clear example of absorption occurred in 1995 when the stronger Hurricane Iris absorbed the weaker Hurricane Humberto in the Atlantic, resulting in a single, more robust system.
In the Pacific, the 2017 interaction between Hurricanes Hilary and Irwin provided a case of a rotational “dance” followed by a partial merger. Hilary, the dominant storm, tugged Irwin along as they orbited before consolidating.
Another instance of significant path alteration without a full merger involved Typhoons Parma and Melor in the western Pacific in 2009. Their complex interaction caused Parma to stall near the Philippines, leading to prolonged heavy rainfall and devastating flooding.