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Air pollution fuels killer cyclones

http://www.nature.com/nature/journal/v479/n7371/full/nature10552.html

Arabian Sea tropical cyclones intensified by emissions of black carbon and other aerosols

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Throughout the year, average sea surface temperatures in the Arabian Sea are warm enough to support the development of tropical cyclones1, but the atmospheric monsoon circulation and associated strong vertical wind shear limits cyclone development and intensification, only permitting a pre-monsoon and post-monsoon period for cyclogenesis1234. Thus a recent increase in the intensity of tropical cyclones over the northern Indian Ocean5 is thought to be related to the weakening of the climatological vertical wind shear34. At the same time, anthropogenic emissions of aerosols have increased sixfold since the 1930s, leading to a weakening of the southwesterly lower-level and easterly upper-level winds that define the monsoonal circulation over the Arabian Sea6789. 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 landfall1, our results suggest an additional impact on human health from regional air pollution.

Figure 1:Tropical cyclone tracks, aerosol optical depth and meridional SST trends in the Arabian

http://www.nature.com/nature/journal/v479/n7371/carousel/nature10552-f1.2.jpg

a, Genesis points (circles) and tracks (solid lines) of pre-monsoon tropical cyclones during the period 1979–2010. Storms with an lifetime maximum intensity (LMI) of more than 50 m s−1 are indicated with a filled circle at the genesis point and thick track lines. Shaded contours represent annual long-term mean fine-mode aerosol optical depth (AOD) from the MODIS Terra and Aqua instruments averaged over 2003–2009.b, The 50-year change in observed SST, averaged over 55°–75° E. The SST change is defined as the average of the monthly linear trend from 1955–2004, multiplied by 50.

  1. Figure 2: Distributions of pre-monsoon and post-monsoon LMI and storm-ambient vertical wind shear.http://www.nature.com/nature/journal/v479/n7371/carousel/nature10552-f2.2.jpg

Box plots of LMI (a) and storm-ambient vertical wind shear (b) showing the medians (central lines), inner quartile ranges (boxes), and the 25th and 75th centiles minus and plus 1.5 times the inner quartile range, respectively (whiskers). Shear is calculated from the National Center for Environmental Prediction-Department of Energy Reanalysis20. The significance of the separation of the median and mean values is given in Supplementary Table 1.

  1. Figure 3: Thirty-year trends in pre-monsoon SST and vertical wind shear.http://www.nature.com/nature/journal/v479/n7371/carousel/nature10552-f3.2.jpg

The 30-year trends in vertical wind shear (contours) based on reanalysis data from 1979–2010 (a) and numerical experiments designed to isolate the effect of the ABC on the regional circulation7 (b). Dashed contours indicate negative trends, and solid contours indicate positive trends; the zero contour is not shown. Positive and negative shear contours are in units of 0.5 m s−1. Shading shows the 30-year SST trends over the same period from observations (a), which is relative to the equatorial SST trend, and the aerosol-forced SST change prescribed in the model experiments (b).

SCMP  Scientists blame pollutants from South Asia spewed by factories, diesel exhaust and cooking fires

CLIMATE
Agence France-Presse 
Nov 04, 2011 in SCMP
Airborne pollution from South Asia is helping to brew monster storms in the Arabian Sea that have claimed thousands of lives and cost billions of US dollars, scientists said.

In a paper published on Wednesday in the British journal Nature, researchers pointed the finger at a haze known as the Asian brown cloud, which hangs over parts of the northern Indian Ocean, India and Pakistan. Several kilometres thick, the cloud comprises brownish particles of carbon soot and sulphates spewed by factories, diesel exhaust and cooking fires.

Previous research has implicated it in disrupting monsoon patterns and in glacier loss in the Himalayas.

Environmental scientists led by Amato Evan of the University of Virginia looked at patterns in cyclones in the Arabian Sea from 1979 to 2010.

They found the region historically only averaged two or three cyclones a year and these typically were weak — even though the sea was clearly hot enough to fuel very powerful storms.

The reason for the weakness and infrequency, they discovered, lies in a phenomenon called vertical wind shear which occurs in July and August during the hot months of the monsoon season.

Vertical wind shear occurs when strong winds flow in the upper and lower atmosphere in opposite directions. In the lower levels, it blows from the southwest, and in the upper atmosphere, from the east.

The shear rips the top off a would-be cyclone, preventing it from developing the circular winds that are its muscular hallmark.

As a result, the few cyclones that occurred in the Arabian Sea typically happened before or after the monsoon season — usually one in May or June and a couple more in August to December — when the wind shear was far less.

But in the past dozen or so years, the pattern has changed, with the emergence of storms in the weeks immediately before the monsoon season.

They include a cyclone that killed nearly 3,000 people in Gujarat, India, in June 1998. In June 2007, cyclone Gonu, a category-five storm, killed 49 people in Oman and Iran, causing more than US$4 billion in damage. It was the first documented storm ever to enter the Gulf of Oman.

And in June 2010, 26 people were killed in Pakistan and Oman by a category-four cyclone, Phet, inflicting losses of nearly US$2 billion.

The team says “brown cloud” particulates have grown sixfold in volume since the 1930s and the pollution is now a disruptive climatic phenomenon in its own right.

“This study is a striking example of how human reactions, on a large enough scale — in this case, massive regional air pollution caused by inefficient fuel combustion — can result in unintended consequences,” said Anjuli Bamzai of the US National Science Foundation.

“These consequences include highly destructive summer cyclones that were rare or non-existent in this monsoon region 30 or so years ago.”

Download full report : nature10552 (1)

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