The Fujiwhara Effect, also known as the Fujiwara interaction or binary interaction, describes the interaction between two nearby cyclonic vortices.

The effect is named after Japanese meteorologist Dr. Sakuhei Fujiwhara, who first described it in 1921. His initial observations were based on the motion of vortices in water.

When two cyclonic systems come within a certain distance of each other, their centers begin to rotate cyclonically around a common midpoint. This rotation is counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The distance for this effect to become noticeable is generally within 1,400 kilometers (870 mi) for tropical cyclones. For extratropical cyclones, this distance is within 2,000 kilometers (1,200 mi).

Several factors influence the outcome of the Fujiwhara Effect:

• Size and Intensity: Larger cyclones tend to dominate smaller ones, potentially leading to a merger.
• Distance: The closer the cyclones, the more intense the interaction.
• Speed: The speed at which the cyclones are moving influences whether they collide or pass each other.
• Ocean Temperature: Warmer waters can intensify cyclones, while cooler waters can weaken them.

The Fujiwhara Effect can result in several outcomes:

• Merger: The two cyclones merge into a single, more powerful storm.
• Diversion: The original path of one or both cyclones may be diverted.
• Spinning: The cyclones orbit around a common center.
• Absorption: One cyclone, usually the larger or stronger one, absorbs the other.
• Change of Direction: Can less commonly result in a mere change of direction of one or both of the cyclones.

There are different ways in which the Fujiwhara Effect can take place:

• 相寄り型 (Approach Type): The weaker tropical cyclone approaches the stronger one and is absorbed.
• 指向型 (Steering Type): One tropical cyclone moves around the other in a circular motion.
• 追従型 (Following Type): One tropical cyclone follows the path of the other.
• 時間待ち型 (Time Waiting Type): The eastern tropical cyclone moves north first, and then the western tropical cyclone starts moving north after the former departs.
• 同行型 (Accompanying Type): Two tropical cyclones move in parallel.
• 離反型 (Separating Type): The eastern tropical cyclone accelerates and moves northeast, while the western tropical cyclone decelerates and moves west.

The Fujiwhara Effect is most common in the Pacific Ocean, particularly the Northwest Pacific, due to the high number of tropical cyclones in the region. It is less frequently observed in the Atlantic Ocean and other basins.

Predicting the Fujiwhara Effect is challenging due to the complexity of cyclone dynamics and the numerous variables involved. Meteorologists use advanced models, satellite imagery, and historical data to anticipate the behavior of interacting cyclones.

Some notable examples of the Fujiwhara Effect include:

• Hurricanes Iris and Luis in the Atlantic Ocean (1995).
• Typhoon Pat and Tropical Storm Ruth in the Northwest Pacific (1994).
• Cyclone Seroja and Tropical Cyclone Odette off the coast of Western Australia (2021).
• Hurricanes Hilary and Irwin in the East Pacific (2017).
• Typhoon Hinnamnor and Tropical Storm Gardo (2022).

Recent studies have explored a "three-dimensional" Fujiwhara effect, where interactions between cyclones induce vertical wind shear, modifying their tracks through asymmetric diabatic heating. This effect can cause cyclones to move apart due to the influence of upper-level winds and convection patterns.

The Fujiwhara Effect makes forecasting cyclone tracks and intensity more challenging. Meteorologists must consider these interactions in their predictions.