Sunday, January 1, 2012

Cyclone Intensity in North Indian Ocean linked to increasing air pollution

A recent study linked increasing air pollution over the Indian sub-continent to an increase in the intensity of Tropical cyclones in the Arabian Sea. The air pollution over the Indian sub-continent changes wind circulation patterns reducing wind shear in the Northern Indian Ocean providing more potential for intense cyclones to form. This is combined with warm sea surface temperatures in the region, especially in the Bay of Bengal, to provide the conditions for more intense and destructive cyclones.

Image by NASA Earth Observatory - Category 4
Tropical Cyclone Gonu in the Arabian Sea in 2007

About 5 tropical cyclones form in the North Indian Ocean each year, about 5-6% of the global cyclones (also known in other regions as hurricanes, typhoons) total of about 80 cyclonic storms per year. Cyclone formation frequency in the region favours the Bay of Bengal rather than the Arabian Sea by a ratio of 4:1 with bi-modal peaks in May and November when there is less wind shear.

Even though the region experiences a number of cyclones below the global average, the Indian sub-continent is very vulnerable to extreme weather disasters from cyclonic storms. With high population levels clustered along low lying coastal areas - areas likely to be highly impacted by cyclones. The shallow seas and coastal topography also increases the storm surge hazard to a greater degree than other regions of the world. (See Vulnerability to Cyclones in Cyclones & their Impact in India)

Global warming induced Sea level rise will have a greater impact on the coastal areas bordering the Bay of Bengal due to more pronounced thermal expansion due to the shallowness of the waters. Add in the storm surge potential associated with tropical cyclones which exacerbates extreme sea level rise during storms. Historical cyclones have produced storm surges as high as 13 metres, the most recent being a Bangladesh cyclone in November, 1970. More recent severe cyclones have achieved storm surge of up to 6 metres.

James B. Elsner and others in a 2008 study on The increasing intensity of the strongest tropical cyclones (abstract) found that "Our results are qualitatively consistent with the hypothesis that as the seas warm, the ocean has more energy to convert to tropical cyclone wind." adding to the potential for increased tropical cyclone intensity.

The current cyclone season is bimodal between April and December with severe cyclones likely to occurr in May and November. A more regional study by Jinhua Yu and Yuqing Wang - Response of tropical cyclone potential intensity over the north Indian
Ocean to global warming
(PDF) indicated "the Tropical Cyclone season is likely to become longer in a warmed climate in the North Indian Ocean" and increase cyclone intensity particularly in May. "Our results regarding to the potentially more intense TCs and possible longer TC seasons seem to be consistent with recent observational studies" concludes the study.

India Meteorology Department FAQ - frequencies of Cyclonic systems
over north Indian Ocean during 1891-2006

A paper by K. Muni Krishna in Global and Planetary Climate Change in 2009 - Intensifying tropical cyclones over the North Indian Ocean during summer monsoon — Global warming (Abstract) discusses the decreasing trend in the Tropical easterly jet (TEJ) reducing wind shear increasing tropical cyclone formation in the North Indian Ocean. The abstract for this paper concludes:

The vertical wind shear shows a positive correlation with the number of severe storms suggesting that a decrease in easterly shear is favourable for the formation of severe storms. Here I show that the sea surface temperature change over a long period is a key parameter to control the vertical wind shear over the NIO, an important quantity for cyclone activity. The stronger warming of the tropical North Indian Ocean during recent years drove reduced vertical wind shear. Thus if the present decreasing trend of TEJ intensity continues, which is highly probable in view of presently occurring green-house warming, there is a strong likelihood of the formation of tropical cyclones of hurricane intensity even during the summer monsoon. Presently the intense systems are known to form only in the pre and post-monsoon seasons, when the vertical wind shear is small.

Pollution intensifying Arabian Sea Cyclones

A recent change in the Cyclone intensity over the Arabian Sea has been studied by Amato T. Evan et al. and is connected to a six fold increase in anthropogenic emissions of aerosols leading to a weakening of the southwesterly lower-level and easterly upper-level winds that define the monsoonal circulation over the Arabian Sea.

A trend was identified by the researchers of increasingly strong cyclones in the months immediately preceding monsoon season in the region. In June 1998, a significant cyclone struck Gujarat, India, killing nearly 2,900 people. In June 2007, Cyclone Gonu, a category 5 storm with winds in excess of 251km per hour, caused $4 billion in damage when it made landfall in Iran. In June 2010, a category 4 storm, Cyclone Phet, struck the coastlines of Pakistan and Oman, causing nearly $2 billion in damage.

The abstract for this research - Arabian Sea tropical cyclones intensified by emissions of black carbon and other aerosols published in Nature in November 2011 concludes:

In principle, this aerosol-driven circulation modification could affect tropical cyclone intensity over the Arabian Sea, but so far no such linkage has been shown. Here we report an increase in the intensity of pre-monsoon Arabian Sea tropical cyclones during the period 1979–2010, and show that this change in storm strength is a consequence of a simultaneous upward trend in anthropogenic black carbon and sulphate emissions. We use a combination of observational, reanalysis and model data to demonstrate that the anomalous circulation, which is radiatively forced by these anthropogenic aerosols, reduces the basin-wide vertical wind shear, creating an environment more favourable for tropical cyclone intensification. Because most Arabian Sea tropical cyclones make landfall, our results suggest an additional impact on human health from regional air pollution.

In a media release lead author Amato Evan, an Associate Professor of Environmental Science at the University of Virgina, said "There has been a real uptick in the intensity of tropical cyclones in the Arabian Sea. We wanted to understand why, and we think we found the reason."

"We are showing that pollution from human activity – as simple as burning wood or driving a vehicle with a diesel engine – can actually change these massive atmospheric phenomena in a significant way. It underscores the importance of getting a handle on emissions in the region." said Evan.

"The net effect of this pollution is it cools the water," Evan said. "You can think of it as being like a sponge that absorbs sunlight that otherwise would have reached the ocean."

The team found that the brown clouds changed the circulation of the atmosphere in such a way as to reduce the climatological vertical wind shear, thus leading to more intense storms.

"It's a fairly recent change in the circulation, the vertical wind shear, that seems to be allowing storms to intensify now, rather than break up as they normally would do," Evan said.

"The only thing that's been systematically changing in this part of the world is pollution," he added. "There's been a huge growth in pollution from human activity from the Indian subcontinent over the last 60 or so years, a six-fold increase in emissions of pollutions like black carbon and sulfates."

Study Co-author Ramanathan said, "This study adds another major dimension to a long list of negative effects that brown clouds have, including rainfall reduction, Himalayan glaciers melting, significant crop damages and deaths of a million or more annually. The one silver lining is that the atmospheric concentrations of these pollutants can be reduced drastically and quickly using available technologies."

Evan emphasised that strong cyclones in the Arabian Sea may become more commonplace unless air pollution emitted from the Indian subcontinent is reduced. More frequent and more intense cyclones has the potential to cause more loss of human life, additional billions of dollars in damage, and disruptions to shipping traffic in the Arabian Sea, the Gulf of Oman and the Persian Gulf.

Australian CSIRO scientists recently reported a trend in the East Indian Ocean for Cyclone frequency declining while intensity increasing.


  • India Meteorological Department - Frequently Asked Questions on Tropical Cyclones
  • James B. Elsner, James P. Kossin & Thomas H. Jagger, Nature (2008) - The increasing intensity of the strongest tropical cyclones (abstract) doi:10.1038/7209xb
  • Jinhua Yu and Yuqing Wang - Geophysical Research Letters (2009) Response of tropical cyclone potential intensity over the north Indian Ocean to global warming (PDF) doi:10.1029/2008GL036742
  • K. Muni Krishna, Global and Planetary Climate Change (2009) - Intensifying tropical cyclones over the North Indian Ocean during summer monsoon — Global warming (Abstract) doi:10.1016/j.gloplacha.2008.10.007
  • Amato T. Evan, James P. Kossin, Chul ‘Eddy’ Chung & V. Ramanathan, Nature (2011) - Arabian Sea tropical cyclones intensified by emissions of black carbon and other aerosols (abstract) doi:10.1038/nature10552
  • University Of Virgina media release, November 3, 2011 - Arabian Sea Tropical Cyclones Are Intensified by Air Pollution, Study Shows
  • Image by NASA Earth Observatory - Category 4 Tropical Cyclone Gonu in the Arabian Sea in 2007