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

The Fujiwhara effect, first described by Japanese meteorologist Sakuhei Fujiwhara in 1921, is the phenomenon in which two nearby tropical cyclones begin to rotate around a common center of rotation, or barycenter, once their separation narrows to roughly 1,400 km or less. The stronger, larger system typically sits closer to the barycenter while the weaker one orbits farther out.

Impact on Movement 

1. Mutual Cyclonic Orbiting: Both vortices rotate around a common barycenter (anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere), overriding the regional steering influence of the subtropical ridge.
2. Erratic Looping and Stalling: Kinematic interaction causes sharp track reversals and stalling.
   1. Example: Typhoon Parma (2009) made three distinct landfalls in the Philippines as it was dragged into a looping track by the stronger Typhoon Melor.
3. Slingshot Ejection: Large disparities in vortex size can accelerate the smaller cyclone outward onto an erratic, high-speed path once the systems disengage.
4. Vortex Coalescence (Merger): If the separation distance contracts to under 300km, the circulations collapse into a single combined vortex.

Impact on Intensity

1. 1. Asymmetric Vertical Wind Shear: Outflow from the dominant storm imposes severe upper-level shear on the companion vortex, tilting its convective chimney and triggering rapid decay.

• Example: Hurricane Wilma (2005) exerted strong wind shear on Tropical Storm Alpha, tearing apart its circulation.

1. Moisture and Vorticity Cannibalization: The stronger low-pressure core pulls the primary moisture inflow and angular momentum away from the peripheral storm, starving it of latent heat energy.
2. Cold-Wake Upwelling & Weakening: Mutual stalling drives continuous Ekman pumping, bringing cold sub-thermocline water to the surface. This drops Sea Surface Temperatures (SST) below the 26.5°C threshold, weakening both systems.
3. Expanded Post-Merger Footprint: While mergers rarely double core wind speeds, they significantly broaden the Radius of Maximum Winds (RMW), widening coastal storm surge zones and rainfall footprints.

Regional relevance and way forward

While binary interactions are more frequent in the Western Pacific, warming trends in the Bay of Bengal and Arabian Sea increase the probability of overlapping cyclogenesis, requiring specialized disaster risk reduction protocols: 

1. Strengthening IMD's Doppler Weather Radar network and leveraging INSAT-3DS for finer-resolution tracking of binary systems.
2. Coupled ocean–atmosphere models to capture the SST-cooling feedback these interactions cause.
3. Flexible, rolling evacuation protocols for coastal Odisha, Andhra Pradesh, and West Bengal, given the high track-forecast uncertainty such interactions introduce.

The Fujiwhara effect illustrates the non-linear, chaotic character of tropical cyclone dynamics. As warming oceans raise the odds of overlapping cyclogenesis, integrating high-resolution radar, satellite, and coupled models into India's early-warning architecture is essential for coastal disaster risk reduction.