CHENNAI:Good news: Cyclone Gaja has spared Chennai. Bad news: We aren’t very sure about its behaviour.Uncertainty over sea surface temperature , erratic movement of winds and presence of another cyclone in the southern hemisphere have led to models finding it tricky to predict the trajectory and intensity of cyclone Gaja, weather experts say. After lying stationary for 24 hours in a region between two opposing winds, Gaja has shifted course further down south.Prof Sridhar Balasubramanian, climate expert from IITBombay, said the presence of cyclone Bouchra may have led to the difficulty in models predicting the trajectory of cyclone Gaja. The two storms did not interact, he said, but may have influenced each other and they got pushed away at the equator, where winds swirling in opposite directions converge.“When two cyclones form within a certain distance — which is not very common — they either merge or move away from each other. In this case, they didn’t merge; they got pushed away,” he said. “But not everyone agrees with this theory.”IMD DDG S Balachandran said Bouchra had no effect on Gaja, as they were far away. “Gaja was lying stationary for 24 hours in between two opposing wind regions. Because of the opposing winds, the cyclone shifted it course down south. Usually when a system is far away, it is difficult to predict the exact location of its landfall,” he said'No clear results over intensity of Gaja’Bouchra is more than 2400km from Bay of Bengal. Gaja originated more than 1500km away from Tamil Nadu coast near Andamans.Models show contradicting results over the intensity of Gaja during its landfall. An advisory by the Regional Specialised Meteorological Centre-Tropical Cyclones of IMD said different models predict different intensity at the time of landfall. “While models like EC ECMWF, IMD GFS and NCEP models indicate the system to be a depression at the time of landfall, NCUM and HWRF models indicate the system will cross coast as a cyclonic storm,” it said.Experts observed that Gaja exhibited unusual behaviour as it moved south and executed a looping. “Storms in the Bay of Bengal usually move towards the north and the west. In this case, it was rare for it to move south towards the equator. There could be various reasons for the behaviour, and interaction with another cyclone is one. In this case, however, there was no such interaction,” said weather expert YEA Raj. “Different models project different pictures, but in this case most of them have predicted southerly movement.”Models also find it hard to accurately predict sea surface temperature of the Bay of Bengal as there is very little understanding among researchers over the causes. “The Bay of Bengal is a basin full of mystery, unlike other seas and oceans. Sea surface temperature (SST) is an important parameter in tracking the intensity of any cyclone. Ocean mixing is a process that influences SST. Since ocean mixing is not clearly understood, models cannot predict SST accurately. Now, that leads to trouble because we are going to use that data to forecast further,” said Subramanian. He said that models cannot accurately predict the winds along the equator that push and pull the cyclone increasing the uncertainty.He also pointed out that with low resolution, models are unable to predict smallscale processes that can change ocean dynamics rapidly. “Unlike above the Arabian Sea, where prediction is robust, models specific to Bay of Bengal have to be improved,” Subramanian said.

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