Air pollution makes cyclones stronger: US study

Weather scientists looking to devise better models for tropical cyclone prediction could take a cue from a recent US study which found that air pollution makes these storms more destructive. Current numerical weather prediction models do not take into account air pollution.

But a team of scientists led by Amato T Evan from University of Virginia in the US has linked the increase in the intensity of  tropical cyclones over the Arabian Sea between 1979 and 2010 with a rise in soot and aerosol emissions over the Indian subcontinent.

On average, the Arabian Sea experiences two to three cyclones over a year, which is very less as compared to those over the Bay of Bengal. However, most of the Arabian Sea cyclones hit land and this makes the study significant.

Pollutants weaken cyclone inhibtor 

Emissions from burning of fossil fuels have formed a layer of atmospheric brown cloud (ABC), mainly consisting of sulphate emissions and black carbon, over the northern Indian Ocean.  The onset of the south-west monsoon sets up very strong vertical wind patterns called wind shear across the Arabian Sea, inhibiting the development of strong cyclones during July and August. The build-up of ABC over the northern Indian Ocean in the past three decades has coincided with the weakening of the wind shear, leading to intensification of cyclones.

Evan, assistant professor at the department of environmental sciences at University of Virginia, says: “An area of high vertical wind shear would be characterised by winds in the upper and lower atmosphere being fast and moving in opposite directions. If a cyclone interacts with such an environment, it will literally be torn in half, with the upper half being pulled in one direction while the lower half is pushed in the opposite direction.” The study, published in Nature on November 2, notes that aerosol build up reflects solar radiation, causing a net cooling over the northern Indian Ocean. The impact of this cooling or dimming effect is so significant that it weakens the wind shear.

Warning for post-monsoon cyclones

For the study, the scientists used a combination of observational and modelling tools. The team looked at lifetime maximum intensity (maximum speed attained by the cyclone during its entire course) of pre- and post-monsoon cyclones between 1979 and 2010. The analysis tracked 20 pre-monsoon cyclones formed over this period, and the team noticed an increasing trend in the intensity of cyclones towards the period between 1997-2010. However, no significant trend was noticed in the post-monsoon activity over the same period. “We think this is because the wind shear during the post-monsoon season is, on average, twice that of the pre-monsoon period. Therefore, although there is a downward trend during each period, in the post-monsoon season, the shear has to decline further before the environment becomes favourable for the storm intensity to really jump. In the paper we suggest something like this could happen in the near future, though,” Evan says.

A K Srivastava, senior scientist at India Meteorological Department office in Pune, who's researched cyclones, says that studies like the one cited rely on model simulations. “Their accuracy is questionable,” he says. “The intensification of tropical cyclones is a worldwide phenomenon, and it's not just restricted to the Arabian Sea. It's early to say that air pollution is the only factor responsible here.”

The authors conclude that since aerosols have a short life span, “decreasing emissions should have a nearly immediate effect on the propensity of pre-monsoon tropical cyclones to reach their maximum potential intensity.”

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