Fujiwhara Effect

Source: DTE

Subject: Geography

Context: Two potential cyclonic storms are forming in the Bay of Bengal, with global forecast models indicating a possible Fujiwhara interaction between them.

About Fujiwhara Effect:

What is the Fujiwhara Effect?

• A rare meteorological phenomenon where two nearby cyclonic systems begin to rotate around a common centre due to interaction of their wind circulations.

• Identified by Sakuhei Fujiwhara (1921), it occurs mostly in the tropical cyclone belt when storms are within ~1,400 km of each other.

Factors Aiding Its Occurrence:

• Proximity of two cyclones within a threshold distance (typically <1400 km in the Indian Ocean).

• Similar rotational direction (counter-clockwise in the Northern Hemisphere).

• Favourable sea surface temperatures >26°C supporting sustained convection.

• Low vertical wind shear allowing stable cyclone structure.

How it forms?

• Close Formation: Two cyclones forming within ~1400 km begin influencing each other’s wind fields and natural movement patterns due to proximity.

• Wind Interaction: Their outer rainbands and upper-level winds overlap, creating deformation zones that gradually pull the systems toward each other.

• Coupled Circulation: The interacting winds generate a shared pivot point, forcing both cyclones to rotate in curved, mutually influenced paths.

• Orbiting: If one storm is stronger, the weaker one revolves around it and may eventually be absorbed due to energy imbalance.

• Merger: When centres move very close, the vortices fuse into a single, larger cyclone with enhanced convection and stronger winds.

• Weakening: Competition for heat and moisture can deprive the weaker cyclone of inflow, triggering rapid weakening or dissipation.

• Deflection: If interaction is weak, storms may push each other onto diverging paths, adding significant uncertainty to forecasts.

Key features:

• Mutual Rotation: Both cyclones circle a common centre anti-clockwise, altering their original trajectories and movement speed.

• Energy Transfer: The stronger system can steal momentum or moisture from the weaker one, reshaping their relative strengths.

• Track Uncertainty: Steering winds get disrupted, making prediction of landfall, intensity, and movement highly challenging for meteorological agencies.

• Possible Fusion: Close approach may cause the cyclones to merge into a more intense system with higher rainfall and damaging winds.

• Stalling: Storms may slow down or stall during interaction, prolonging rainfall events and increasing flood risks.

Implications:

• Forecast Challenges: High uncertainty delays accurate warnings for landfall and complicates planning for evacuation and relief operations.

• Heavy Rainfall: Prolonged interaction increases rainfall over TN, Andhra, Odisha, Bengal, Sri Lanka, and Myanmar, worsening flood potential.

• Intensification Risk: Energy transfer or merger can rapidly strengthen one system, raising threats of severe winds, storm surge, and coastal damage.