Context 

• The world is seeing a rise in cyclonic formations and in recent years, several storms have come close to undergoing the Fujiwhara effect. 

Fujiwhara effect 

• The Fujiwhara Effect was first described by a Japanese meteorologist, Dr. Sakuhei Fujiwhara in 1921. 
• It is a phenomenon that occurs when two nearby cyclonic vortices move around each other and close the distance between the circulations of their corresponding low-pressure areas. 
• When the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will circle around it.

 Mechanism 

• There are five different ways in which Fujiwhara effect can take place. Refer to the image provided below.

• Elastic Interaction (EI): Interaction of vortices (storms) of same or different sizes, resulting in changes only in the direction of motion. This is the most commonly seen interaction 
• Partial Straining-Out (PSO): Interaction of vortices of unequal sizes. Part of the smaller vortex is lost to the atmosphere. 
• Complete Straining-Out (CSO): Interaction of vortices of unequal sizes. The smaller vortex completely lost to the atmosphere 
• Partial Merger (PM): Interaction of vortices of unequal sizes Part of the smaller vortex merged to the bigger vortex. 
• Complete Merger (CM): Interaction of vortices of same or different sizes, resulting in complete merger of both the storms. 

Is the frequency of the Fujiwhara effect increasing? 

• Warmer oceans and stronger cyclones drastically increase the possibility of Fujiwhara effect.  
• There has been a 35 per cent increase in the strength of typhoons that have hit Taiwan between 1977 and 2016, due to 0.4 to 0.7°C rise in the sea surface temperature 
• This shows how global warming is responsible for making cyclones stronger, and thus increasing the chances of the Fujiwhara effect.  

Way Forward 

• Current climate models being used for tracking tropical cyclones around the world should take the Fujiwhara effect into account. This will help scientists to know if a mega storm is gathering momentum.