Context

• When two hurricanes (or cyclones, depending on where you live), spinning in the same direction, are brought close together, they begin ‘an intense dance around their common center’ – this interaction is called the Fujiwhara effect.

Background

• The United States west coast recently witnessed Hurricane Hilary (a sub-tropical storm by the time it hit the US), prompting the National Hurricane Centre (NHC) to issue its first ever tropical storm watch for parts of Southern California.
• This was the latest incident in a string of unusual weather phenomena in the region.
• Earlier this year, California witnessed an unusually wet winter, with at least twelve ‘atmospheric river’ storms battering the state. ‘Atmospheric rivers’ are vast airborne currents carrying dense moisture and hanging low in the atmosphere.

What is the Fujiwhara effect?

• As per the National Weather Service (NWS), when two hurricanes (or cyclones, depending on where you live), spinning in the same direction, are brought close together, they begin ‘an intense dance around their common center’ – this interaction between two cyclones is called the Fujiwhara effect.
• If one hurricane’s intensity overpowers the other, then the smaller one will orbit it and eventually crash into its vortex to be absorbed.
• On the other hand, if two storms of similar strengths pass by each other, they may gravitate towards each other until they reach a common center and merge, or merely spin each other around for a while before shooting off on their own paths.
• In rare instances, the two ‘dancing’ cyclones, if they are intense enough, may merge with one another, leading to the formation of a mega cyclone capable of wreaking havoc along coastlines.
• There are five different ways in which Fujiwhara Effect can take place.
  • The first is elastic interaction in which only the direction of motion of the storms changes and is the most common case. These are also the cases that are difficult to assess and need closer examination.
  • The second is partial straining out in which a part of the smaller storm is lost to the atmosphere.
  • The third is complete straining out in which the smaller storm is completely lost to the atmosphere. The straining out does not happen for storms of equal strengths.
  • The fourth type is partial merger in which the smaller storm merges into the bigger one and the fifth is complete merger which takes places between two storms of similar strength.
• During a merger interaction between two tropical cyclones the wind circulations come together and form a sort of whirlpool of winds in the atmosphere.
• Fujiwhara effect was identified by Sakuhei Fujiwhara, a Japanese meteorologist whose first paper recognising the Fujiwhara cases was published in 1921.
• The first known instance of the effect was in 1964 in the western Pacific Ocean when typhoons Marie and Kathy merged.

Unpredictable Path

• The occurrence of the Fujiwhara Effect makes cyclones more unpredictable due to their rapid intensification, carrying of more rain and newer ways of moving over warming oceans.
• This is because each of the interactions between the two storm systems is unique and very difficult to capture with current climate models.
• Assessment of Fujiwhara Effect cases over longer periods is a problem.
• Researchers across the world tend to select the Fujiwhara cases they find interesting to investigate, but there is no collated dataset.
• The main problem is that “there is no worldwide accepted technique or recording agency dedicated to recognise and collect the cases of the Fujiwhara Effect as these are rare events and tough to assess.

Source: IE

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