A ship sinks in the Mongla river in Bangladesh. (AFP)

New Delhi, Nov. 16: Scientists today attributed the shift of Cyclone Sidr towards Bangladesh to factors long known to influence cyclone paths, but asserted that its scale and severity justified the warnings issued to Bengal.

India Meteorological Department officials said they had anticipated an eastward shift which began about three hours before the cyclone hit Bangladesh about 80km east of the initially predicted landfall zone.

The 250kmph winds killed around 600 people in Bangladesh, left thousands injured or missing, flattened houses and unleashed a 15-foot tidal surge that destroyed three coastal towns and forced the evacuation of 32 lakh people. Unofficial reports put the toll above 1,000.

Three factors could have contributed to the shift — Earth’s rotation, the interaction of the cyclone’s outermost winds with land just as they began to graze the coastline and high-altitude winds called the westerlies.

“A (cyclonic) system in the northern hemisphere’s atmosphere is deflected slightly towards the east because of Earth’s rotation,” said H.R. Hathwar, additional director-general of the IMD. “As it neared the coast, the balance of the cyclonic system could have been disturbed and caused the winds to change direction a bit.”

Scientists are yet to fathom the mechanics of these land-wind interactions, but the tendency of cyclones in the northern Bay of Bengal to veer towards Bangladesh indicates that the effects are consistent over time.

The cyclone may also have come under the influence of long-range winds blowing from west to east called the westerlies.

Yesterday, the IMD had initially predicted landfall (the cyclone hitting land) at 89°E longitude on the Bengal-Bangladesh border. But it struck the coast at 89.8°E, about 80km eastwards.

This is within the acceptable error margin when dealing with an event on the scale of a cyclone, a scientist said.

“A cyclone has dimensions of 300km to 500km (Sidr had a 450km diameter), and an 80km shift would still mean severe winds on either side of the border,” said Mrutyunjaya Mahapatra, director, cyclone warning division, IMD.

Scientists used a synthesis of satellite imagery, ground data and a weather radar in Calcutta to track the cyclone and wind speeds. Despite the eastward shift, parts of Bengal experienced 90kmph winds that damaged over 1,000 thatched houses in North and South 24-Parganas.

Predictions that Bengal would be hit had led many tourists to cancel or truncate seaside trips and software companies to arrange guesthouse rooms for staff. Some people suggested the Met office should have issued hourly bulletins instead of a sweeping cyclone forecast.

“The warning was justified because such high-speed winds had the potential to cause extensive damage,” Hathwar said. “In such situations… it’s better to be overcautious.”

The weather radar is mainly used for real-time wind tracking and “nowcasting” — forecasting for the next two to three hours. But a Met warning issued two-three hours before a cyclone strikes would be too late for evacuation or effective public advisories.