When two tropical cyclones, commonly known as hurricanes or typhoons, form or travel close to each other, their massive rotating wind fields do not simply pass by unaffected. Each storm is a powerful, low-pressure vortex that spins across the ocean surface, and when the circulations of two such systems begin to overlap, they exert a mutual influence on one another. This interaction introduces significant complexity into the atmospheric dynamics, fundamentally altering the trajectory and potential strength of both weather systems. This unique atmospheric relationship sets the stage for a spectacular and often unpredictable meteorological phenomenon.
The Fujiwhara Effect
The interaction between two nearby cyclonic vortices is governed by a principle known as the Fujiwhara Effect, named for the Japanese meteorologist Sakuhei Fujiwhara, who first described the process in 1921. While the phrase “hurricane collision” is often used, it is a misnomer, as the systems rarely crash head-on like physical objects. The effect describes how the two low-pressure centers begin to move around a common central point, or barycenter, due to the influence of each other’s rotational flows.
This coupled motion is a result of the storms’ vorticity fields overlapping and inducing movement in one another. In the Northern Hemisphere, where tropical cyclones rotate counter-clockwise, the two systems will also orbit their common center in a counter-clockwise direction. The strength and direction of the interaction are determined by the way their respective wind circulations alter the local steering flows. This phenomenon can drastically change the storms’ paths, making forecasting especially challenging when two systems engage in this atmospheric “dance.”
The location of this invisible common midpoint is not fixed equidistant between the storms. Instead, it is weighted toward the storm that possesses greater rotational mass, which is a combination of its size and intensity. If the two storms are of equal size and strength, the midpoint of rotation will be located exactly halfway between their centers.
Variables Determining the Result
The ultimate fate of the interacting storms depends heavily on a set of physical variables present at the time of their close approach. The separation distance is a primary factor, as tropical cyclones typically need to be within approximately 870 miles (1,400 kilometers) of each other for a recognizable Fujiwhara interaction to begin. The speed of the mutual rotation accelerates considerably when the storm centers close within about 400 miles (650 kilometers).
The relative intensity of the storms, measured by their sustained wind speeds and minimum central pressure, also plays a defining role. A significantly stronger vortex will exert a greater influence on the weaker one, often dominating the interaction. Similarly, the relative size of the circulation area for each storm determines which system possesses more rotational momentum. When one system is substantially larger than the other, it can effectively pull the smaller one into its broader circulation.
If the storm centers get within a much smaller radius, such as 190 miles (300 kilometers), the chance of a complete merger or one system being destroyed by the other increases substantially. The prevailing atmospheric conditions surrounding the storms, including factors like wind shear and surrounding pressure patterns, also modify how the interaction plays out. These inputs collectively determine the specific scenario that unfolds, ranging from a slight course correction to a dramatic combination of the two systems.
Scenarios of Interaction
The interplay of size, intensity, and separation distance results in a few distinct scenarios for the interacting storms.
Stable Mutual Orbit
One possible outcome is a stable mutual orbit, where the two storms rotate around the barycenter while maintaining a relatively consistent distance from each other. This is often observed when the systems are comparable in strength and size but remain farther apart, causing highly erratic and unpredictable storm tracks.
Partial Absorption or Destruction
A second outcome is partial absorption or destruction, often termed “straining out,” which occurs when there is a clear difference in the storms’ strengths. The larger or more intense storm’s powerful outer circulation can effectively tear apart the structure of the weaker vortex. This process often leads to the smaller storm dissipating, with the remnants of its moisture and energy being incorporated into the remaining dominant system.
Complete Merger
The third and most dramatic scenario is a complete merger, which tends to happen when the storms are of roughly equal size and intensity and close the distance between them. In this case, the two vortices spiral inward toward the common center and combine to form a single, larger, and often more powerful tropical cyclone. This process concentrates the energy and moisture from both systems, leading to a single storm with a significantly increased circulation area and potentially higher wind speeds.
Documented Historical Interactions
While the conditions for a perfect Fujiwhara interaction are relatively rare, the phenomenon has been observed in various ocean basins around the world. In the Atlantic in 1995, Hurricane Iris and Hurricane Humberto exhibited a clear interaction before Iris eventually absorbed the weaker Humberto, resulting in a stronger system. Another example occurred in the Pacific in 2017 when Hurricanes Hilary and Irwin rotated around each other off the coast of Mexico.
In the Western Pacific, the interaction is often more common due to the high frequency of tropical cyclones there. Typhoon Parma and Typhoon Melor engaged in a complex “dance” in 2009, which caused Parma’s path to stall near the Philippines, leading to prolonged and devastating rainfall. These real-world events confirm that the mutual influence of tropical cyclones is an occasional, but consequential, factor in global weather patterns.