What is the Fujiwhara effect? Explain its impact on the movement and intensity of tropical cyclones.
10 marks · answer in 150 words · directive: Explain
The core of a good answer
The Fujiwhara effect is the mutual rotation of two nearby cyclonic vortices around a common centre; it makes the tracks of interacting cyclones erratic and can strengthen one storm while weakening, absorbing or merging the other.
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What the question asks
Two parts: a precise definition, then the effects on track and on strength. 'Explain' needs the mechanism (each vortex steers the other) and real examples, ideally including one from the Indian Ocean.
Answer outline
Introduction
Named after Japanese meteorologist Sakuhei Fujiwhara, who described it in 1921, the effect occurs when two cyclones come close enough for their circulations to interact.
Mechanism
- Each vortex's winds advect the other, so both centres orbit a point between them: anticlockwise in the Northern Hemisphere, clockwise in the Southern.
- For tropical cyclones interaction becomes noticeable within about 1,400 km; rotation speeds up within about 650 km; merger or shearing of one storm occurs within about 300 km.
- When the storms differ in size, the larger one tends to dominate and the smaller one circles around it.
Impact on movement
- Tracks become looping or erratic, deviating from steering winds, which complicates forecasting.
- Storms may stall or slow, as Typhoon Parma did near the Philippines in 2009 while interacting with Melor.
- One storm can be flung onto a new track or pulled towards the other.
Impact on intensity
- The larger storm may absorb the smaller and gain energy and moisture (Iris absorbed Humberto, 1995; Seroja absorbed Odette, 2021).
- The weaker storm is often sheared and weakened (Irwin weakened while Hilary strengthened, 2017).
- Initial interaction can also slow a storm's strengthening, as with Seroja before it absorbed Odette.
Why it matters
- Erratic tracks and sudden intensification shorten warning times for coastal populations.
- Merged systems can bring prolonged rain and flooding to areas outside the expected path.
Conclusion
The Fujiwhara effect shows that cyclones interact with one another as well as with steering winds, so binary interactions must be built into forecasting and early warning.
Examples and data to cite
- Typhoons Parma and Melor, West Pacific, 2009.
- Hurricanes Hilary and Irwin, East Pacific, July 2017.
- Cyclones Seroja and Odette, eastern Indian Ocean, April 2021: Seroja fully absorbed Odette on 10 April 2021 and made landfall in Western Australia on 11 April 2021.
- Cyclones Diamondra and Eunice, central Indian Ocean, 2015.
Facts to remember
- Described by Sakuhei Fujiwhara in 1921, building on work by Diro Kitao (1889).
- Interaction threshold for tropical cyclones: about 1,400 km; merger within about 300 km.
- Full merger of tropical cyclones is uncommon; usually one dominates or they separate.
- Seroja caused at least 272 deaths across Indonesia, Timor-Leste and Australia.
Where it sits in the syllabus
- GS1: Geophysical phenomena and changes
Important Geophysical phenomena such as earthquakes, Tsunami, Volcanic activity, cyclone etc., geographical features and their location: changes in critical geographical features (including water-bodies and ice-caps) and in flora and fauna and the effects of such changes.
- GS1: Physical geography of the world
Salient features of world's physical geography.