What is the Fujiwhara Effect? Why Do Dual Typhoons "Dance" Together?
An analysis of the interaction mechanisms between dual typhoons.
What is the Fujiwhara Effect?
When two typhoons (tropical cyclones) approach within approximately 1,000–1,200 kilometers of each other, they begin to rotate around and pull on each other relative to a "center of mass" located on the line connecting their circulation centers. This phenomenon is known as the Fujiwhara Effect, discovered in 1921 by Japanese meteorologist Sakuhei Fujiwhara through water tank experiments.
Three Outcomes of the Dual Typhoon "Dance"
The Fujiwhara Effect is not simply a graceful "pas de deux." The outcome depends on the relative size and intensity of the two typhoons:
- Significant Strength Disparity → Merger: The stronger typhoon "absorbs" the circulation of the weaker one. The weaker system dissipates, while the stronger one may intensify as a result. Similar to a big fish eating a small fish.
- Comparable Strength → Mutual Rotation then Separation: Both systems rotate counter-clockwise around their common center of mass for a period (in the Northern Hemisphere), before being carried away by their respective steering flows.
- One System Ejected → Sudden Path Change: The weaker typhoon may be "flung" onto a trajectory that deviates significantly from forecasts. This is the primary reason for major forecast errors during dual-typhoon events.
Classic Cases
- Maria and Jelawat (2018): Early-stage mutual rotation caused forecast tracks to change three times in a single day.
- Nesat and Haitang (2017): Haitang was pulled by Nesat, leading both to make landfall in the same area of Fujian province sequentially, resulting in a "double typhoon strike."
- Saola, Haikui, and Kirogi (2023): The coexistence of three typhoons caused Saola to trace a rare "looping" path near the coast of Guangdong. The main causes were the Fujiwhara Effect combined with weakening steering flows.
Why Does the Fujiwhara Effect Give Forecasters Headaches?
- The intensity and distance between two typhoons change every six hours, making it difficult to precisely calculate the angular velocity of their mutual rotation.
- Numerical weather prediction models often have significant biases in depicting the circulation of weaker typhoons. Inaccurate calculations of the "pulling force" amplify discrepancies in track forecasts.
- When three typhoons coexist (as in September 2023), the interactions become a "three-body problem," causing uncertainty to rise exponentially.
What Should You Pay Attention to During Dual Typhoon Events?
During dual typhoon periods, please increase the frequency of checking track updates (it is recommended to check the latest forecast every 6 hours). Do not base decisions on track maps from two days prior. Also, pay close attention to the wording used by meteorological authorities—when phrases like "significant uncertainty in the track" appear, prepare your defenses based on the worst-case scenario.