What is the Fujiwhara effect? Explain its impact on the movement and intensity of tropical cyclones.

What it is

The Fujiwhara effect is the interaction of two nearby cyclonic vortices that begin to orbit a common centre of mass. Described by the Japanese meteorologist Sakuhei Fujiwhara in 1921, it typically begins when two tropical cyclones of comparable size come within roughly 1,200–1,400 km of each other. Each storm advects the other around the midpoint between them, producing a cyclonic (anticlockwise in the Northern Hemisphere) dance.

Possible outcomes

• Mutual rotation: both storms circle the centroid and then separate, each with a sharply altered track.
• Merger: if the systems close to within about 300 km, they can combine into a single larger circulation.
• Absorption: the stronger system draws in and consumes the weaker one.
• Straining out: the weaker vortex is sheared into an elongated band and dissipates.

Impact on movement

• Tracks become erratic — looping, stalling, or reversing — because the steering flow is no longer the large-scale environment alone.
• A storm can be pulled off a climatologically expected recurvature path, so landfall point and timing forecasts lose skill.
• Stalling extends the duration of rainfall and storm surge over one coastal stretch, raising flood risk even without higher wind speeds.

Impact on intensity

• Weakening is more common. The two circulations compete for the same warm ocean surface and moisture, and the interaction increases vertical wind shear, disrupting the vertical alignment of the warm core.
• Strengthening can follow a merger, when a single system inherits the combined mass, moisture and angular momentum and reorganises over warm water.
• The weaker partner is usually the loser: it is sheared, absorbed or starved of inflow.

Examples and relevance

Typhoons Marie and Kathy (1964) and Hurricanes Hilary and Irwin (2017) are classic cases; interaction between concurrent Bay of Bengal and Arabian Sea systems, and between a Bay of Bengal depression and a residual monsoon low, occasionally produces the same behaviour near India. Since the North Indian Ocean produces relatively few simultaneous cyclones, the effect is rarer here than in the western Pacific, but it matters for IMD track forecasting whenever twin systems form on either side of the peninsula.

Conclusion

The Fujiwhara effect is a major source of track and intensity uncertainty. Understanding it — alongside the standard mechanics of tropical and temperate cyclones — is essential for lead-time-critical evacuation planning.