Fujiwhara effect: When cyclones dance in the Bay

For UPSC aspirants, the Fujiwhara effect is a classic example of how atmospheric dynamics can turn predictable weather patterns into chaos. Named after Japanese meteorologist Sakuhei Fujiwhara, who described it in 1921, this phenomenon explains what happens when two cyclonic vortices wander too close and start influencing each other’s movement.

What exactly happens?

When two cyclonic systems drift within a critical proximity — usually under 1,300-1,400 km — their wind fields begin to overlap. Instead of travelling independently, the systems start to orbit a shared pivot point, almost like a cosmic dance under tropical skies.

Conditions favouring the effect

• Geographical closeness: Cyclones must be near enough for their outer circulation bands to interact.

• Same spin direction: In the Northern Hemisphere, both rotate anti-clockwise, enabling a shared motion.

• Warm waters: Sea-surface temperatures above 26°C help maintain strong convection.

• Low vertical wind shear: Ensures both systems remain structurally intact long enough to interact.

How the interaction evolves

1. Initial tug: Their outer rainbands deform as each vortex tugs at the other’s trajectory.

2. Mutual rotation: Both begin orbiting a common centre; the weaker system usually circles the stronger one.

3. Outcomes vary:

Merger: The two cores collapse into a single, more powerful cyclone.

Absorption: The dominant system consumes the weaker vortex.

Deflection: If interaction is mild, they push each other onto altered, often unpredictable paths.

Weakening: Competition for moisture can drain the smaller system.

Why it matters for India

• Forecasting becomes tricky: Models struggle as steering winds are disrupted, delaying clarity on landfall and intensity.

• Rainfall surges: The prolonged interaction can stall one or both systems, amplifying rainfall over coastal states like Tamil Nadu, Andhra Pradesh, Odisha, West Bengal and even neighbouring Sri Lanka and Myanmar.

• Risk of rapid intensification: A merged or moisture-favoured system may strengthen suddenly, increasing storm-surge and wind damage potential.