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It is well known that warm sea-surface temperatures can boost the intensity of hurricanes (known in the Indian Ocean as cyclones). For many decades water in the Arabian Sea has been warm, which should enhance the strength of cyclones. However, regional winds in the upper and lower atmosphere tend to blow briskly in opposing directions, resulting in vertical wind shear [see http://en.wikipedia.org/wiki/Wind_shear]. This phenomenon can have a major impact on the strength of storms [see, for example, http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/comp/wind/home.rxml]. Thus, mega-cyclones were relatively rare, until recently.
Researchers analyzed every regional tropical cyclone for the last 30 years. None had occurred during the monsoon season, and before 1998, cyclones in the months before and after that rainy season seldom reached wind speeds exceeding about 80 kph. But since 1998, five large storms reached wind speeds double the "norm" (sometimes exceeding 185 kph). The researchers found that in premonsoon months there had been a slow decrease in the average vertical wind shear, from 11 meters per second to 8 meters per second; they concluded that the lower speed appeared to represent some type of threshold. At the lower range, mega-cyclones were possible.
The researchers ultimately linked the steady decrease in vertical wind shear in cyclone seasons with a 3-kilometer-thick brown cloud of soot and other pollutant particles (e.g., aerosols [see http://en.wikipedia.org/wiki/Aerosol]) in the region’s lower atmosphere. This pollution has increased sixfold in 80 years, and now filters roughly 10% of sunlight, preventing it from reaching the sea surface. Computer analyses indicated that the pollution cloud’s cooling effect on the ocean below has slowed wind speeds in the atmosphere, and allowed storm intensities to increase.
Critics could not unreasonably focus on the modest data set of mega-cyclones that were available for analysis. However, the researchers appear to have identified a reasonable causal relationship.
This analysis demonstrates a fascinating interplay between global warming and air quality.
The report can be found at http://www.nature.com/nature/journal/v479/n7371/full/nature10552.html.
For more information on related topics, see http://www.agu.org/pubs/crossref/2005/2005GL023125.shtml and http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html.
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