Cyclone Arnab likely drew moisture from Tamil Nadu system, but did not merge

Cyclone Arnab has weakened into a depression over south Chhattisgarh and adjoining south Odisha, with the India Meteorological Department (IMD) saying it is “very likely” to weaken further into a well-marked low-pressure area over north-Chhattisgarh and adjoining northwest Odisha by Thursday evening.

The remnant of the very severe cyclonic storm moved nearly northwards at 18 kmph over the past six hours. At 8.30 a.m. on Thursday, it lay about 40 km north-northwest of Jagdalpur in Chhattisgarh, 180 km southwest of Titlagarh in Odisha, 200 km south of Raipur and 310km southwest of Sambalpur.

Arnab had crossed the Andhra Pradesh and Yanam coasts between Bapatla and Visakhapatnam early on October 28 as a very severe cyclonic storm, with sustained winds of 125-135 kmph, gusting to 150 kmph. The IMD said the landfall process was completed by 8 a.m. that day.

Moisture link

Weather experts said a lingering low-pressure system near Tamil Nadu may have supplied additional moisture to Arnab, potentially helping sustain its cloud bands and rainfall as it approached the Andhra coast. The interaction also raised the possibility of Arnab becoming more powerful had the two systems combined, though there was no indication that a Fujiwara interaction had occurred.

“Seriously, seeing something like this for the first time! Cyclone Arnab is intensifying like a monster, while the earlier low pressure system that gave Tamil Nadu nearly 10 days of widespread rains is still hanging around as a separate system!” wrote independent weather analyst Ajith Kumar (Chennai Weatherman) on X on October 27.

“ATCF [Automated Tropical Cyclone Forecasting System] has now tagged that system as Invest 90B, previously 99A. Seriously bizarre thing!! It is still lingering around Tamil Nadu’s latitude, but remains far east of the coast to bring significant rains right now. If those both would have combined by now, Arnab could potentially have become an even more powerful system,” Kumar added.

The possibility of the two systems combining highlights how interactions between weather systems can influence a cyclone’s intensity.

In theory, the interaction of two sufficiently developed tropical cyclones can produce a Fujiwara effect, in which they rotate around a common centre and may eventually merge.

However, Arnab was a much stronger system than the lingering low-pressure area near Tamil Nadu. Instead, moisture feeding or partial is more likely to have occurred as they interacted, per weather research.

The Fujiwara effect occurs when two nearby cyclonic systems orbit each other and interact, merging into a single, larger cyclone. The phenomenon is named after Japanese meteorologist Sakuhei Fujiwara, who first described it in the 1920s.

The IMD had earlier noted an interaction between Arnab and a low-pressure area over the southwest Bay of Bengal, but its advisories did not describe the event as a Fujiwara merger.

New system in Bay of Bengal

Even as Arnab weakens inland, the IMD is tracking a fresh low-pressure area over the south Andaman sea and neighbourhood. The system is likely to move west-northwestwards and intensify into a well-marked low-pressure area over the south Andaman sea and adjoining Andaman and Nicobar islands within 24 hours.

It may subsequently concentrate into a depression over the southeast Bay of Bengal, adjoining Andaman and Nicobar Islands and the Andaman Seaduring the following 24 hours. Weather experts said conditions over the Bay of Bengal are becoming favourable for the system to intensify further, possibly into a cyclonic storm.

If it strengthens, it could move towards northern Tamil Nadu, with a potential landfall between Chennai and the Andhra Pradesh coast. Chennai could receive significant rainfall on November 1 and 2 if the system follows that track.