While the India Meteorological Department (IMD) has over the years been largely accurate in forecasting the direction and intensity of cyclones into India, data suggest that it takes more time for the agency to accurately forecast the trajectory of storms that originate in the Arabian Sea, than those in the Bay of Bengal.
Historically, most cyclones around India tend to originate in the Bay of Bengal but global warming, as scientists have been pointing out for a while now, is causing the Arabian Sea to be heating up more than average and whetting greater - and increasingly stronger – cyclones like Biparjoy, which barrelled into Gujarat late Thursday.
On the evening of June 9, Biparjoy was situated about 700 km west of Goa and as per the IMD forecast, expected to move north and briefly curve east, and after June 10, shift course west and away from the Gujarat coast to dissipate into the sea without reaching land in either Kutch, Gujarat or Pakistan. Only on June 11, or four days before Biparjoy commenced landfall on June 15, did the IMD first suggest that the storm would strike India.
Contrast this with the most recent storm, Mocha, in the Bay of Bengal. In May, it briefly seemed poised towards India but coursed its way into Myanmar via southeast Bangladesh. On May 9, the IMD forecast five days ahead that Mocha, then located in the South Andaman sea, would progress north-westwards and then “recurve” (sharply change direction) towards the Bangladesh-Myanmar coasts. This was the trajectory that the cyclone largely adhered to when it made landfall on May 14 between Cox’s Bazaar (Bangladesh) and Sittwe (Myanmar), though it turned out to be a stronger cyclone than the IMD had initially predicted.
Or even Cyclone ‘Sitrang,’ again a storm that rose in the south-eastern Bay of Bengal and looked all set to charge into India between Vishakhapatnam and Bhubaneshwar, but ended up taking a sharp eastward turn and crossing the Bengal-Bangladesh coasts on October 25, 2022, just as the IMD predicted four days ahead on October 21. Cyclone Yaas in May 2021, Cyclone Mandous in Dec 2022 and Cyclone Gulab in September 2021 – all major storms in the Bay of Bengal in recent years that made landfall – followed paths predicted by the IMD at least four or more days in advance.
However, the last major cyclonic storm in the Arabian Sea before Biparjoy also threw a surprise. On May 14, 2021, the IMD forecast that a storm fomenting near the Lakshadweep Islands would move north and sharply curve westwards away from the Konkan coast and dissipate into the sea. On May 15, however the IMD updated its forecast and suggested that the cyclone, eventually named Tauktae, would curve the other way and head towards Saurashtra and this is where it eventually landed on May 17. Thus, unlike the general precedent of correctly predicting the path of Bengal cyclones four days ahead, Tauktae’s direction could be gauged only two days before landfall. Tauktae turned out to be one of the most intense cyclones over the Arabian Sea causing destruction in almost every coastal State along the western coast.
Fewer in Arabian Sea
Experts told The Hindu that cyclones in the Bay of Bengal, being far more frequent, were better understood. The Arabian Sea cyclones, historically have been fewer because of relatively colder sea surface temperatures. Nearly 48% of cyclones here never reached land, as opposed to only 13% in the Bay of Bengal.
“It is the winds in the upper reaches of the atmosphere, called steering winds, that influence the direction and recurving, whereas the heat within the ocean layers determined the strength and duration of cyclones. While the latter is better captured in our (prediction) models, the wind component is not always fully captured in our models,” said M Ravichandran, Secretary, Ministry of Earth Sciences – the parent organisation of the IMD. However the nature of the wind patterns is such that it’s “close to impossible” to predict them more than five days ahead –whether in Arabian Sea or Bay of Bengal, he added.
The weather models used to predict Arabian Sea and Bay of Bengal cyclones were the same and the IMD incorporated inputs from Indian as well as several international models to estimate the track and intensity of cyclones, Mr. Ravichandran emphasised.
There are factors unique to the Arabian Sea, but absent in the Bay of Bengal, which influence a cyclone’s intensity and movement. “The Arabian Sea has a much deeper - up to 40 metres – layer of warm water compared to that in the Bay of Bengal. Many times, these sub-surface values aren’t captured in the cyclone prediction models and that’s why, the strength and speed of the cyclones aren’t accurately captured in advance,” said Roxy Mathew Koll of the Indian Institute of Tropical Meteorology, Pune. “Once upon a time we didn’t have the computational resources to incorporate all these factors –particularly ocean and atmospheric interaction – into forecast models. But it’s high time we urgently do.” In a 2021 study, Dr. Koll and colleagues pointed out the increasing prominence of Arabian Sea cyclones with a 52% rise in such cyclones from 2001-2019 and an 8% decrease in those over the Bay of Bengal.
