Pollution is making Arabian Sea cyclones more intense, according to a multi-institutional study that included Scripps Institution of Oceanography, UC San Diego, scientists.
Traditionally, prevailing wind shear patterns prohibit cyclones in the Arabian Sea from becoming major storms. A paper appearing in the Nov. 3 issue of the journal Nature, however, suggests the weakening of the winds aloft has enabled the formation of stronger cyclones in recent years - including storms in 2007 and 2010 that were the first recorded storms ever to enter the Gulf of Oman.
The researchers note that the weakening wind pattern over the past 30 years has corresponded with a buildup of aerosols in the atmosphere over India, which deflect sunlight away from the surface, creating dimming at ground level.
The aerosol buildup creates formations known as atmospheric brown clouds (ABCs) in which smog from diesel emissions, soot and other by-products of biomass burning accumulate and become widespread to a degree significant enough to be a force in regional climate.The three-kilometer (1.9-mile)-thick brown cloud has been linked to altered rainfall patterns in South Asia. Because of the large-scale dimming by the ABCs, they have a mitigating effect on the warming of the ocean in the region that is also associated with greenhouse gas-driven climate changes.
"We're 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," said study lead author Amato Evan, an assistant professor in the Department of Environmental Sciences at the University of Virginia. "It underscores the importance of getting a handle on emissions in the region."
Historically, the onset of the monsoon season in summer months has produced strong winds in the lower and upper atmosphere that travel in opposite directions, also known as vertical wind shear, which makes formation of cyclones virtually impossible in July and August. Thus despite warm sea surface temperatures, the Arabian Sea averages two or three cyclones per year that tend to form outside the monsoon season, when the wind shear is diminished.
The researchers note a trend of increasingly strong cyclones in the months immediately preceding monsoon season. A 1998 cyclone that made landfall in Gujarat, India killed nearly 2,900 people. Cyclone Gonu made an extremely rare landfall in Iran in 2007 and caused more than $4 billion in damage. Cyclone Phet in 2010 struck the coastlines of Pakistan and Oman and caused nearly $2 billion in damage. Gonu produced category 5-strength winds in excess of 250 kilometers per hour (156 miles per hour). Phet was a category 4 storm.
"This study is a striking example of how human actions, 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, program director in the National Science Foundation (NSF)'s Division of Atmospheric and Geospace Sciences, which partially funded the research. "These consequences include highly destructive summer cyclones that were rare or non-existent in this monsoon region 30 or so years ago."
"The research shows that pollution can threaten humans in unexpected ways. In this case, by reducing wind shear in the Arabian Sea and making conditions more favorable for tropical cyclones to intensify," added report co-author James Kossin, a climatologist at the National Climatic Data Center.
The scientists used findings from direct observations and model studies of ABCs made by Scripps Oceanography climate and atmospheric scientist Veerabhadran Ramanathan, a study co-author. Among those findings was that brown clouds inhibited summertime warming of the surface, which caused summertime sea surface temperatures in the northern Arabian Sea to more closely match cooler temperatures closer to the equator. The team modeled the effects of brown clouds on atmospheric and oceanic circulation patterns. They found that ABCs changed the circulation of the atmosphere in such a way as to actually reduce the climatological vertical wind shear.
"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," said Ramanathan. "The one silver lining is that the atmospheric concentrations of these pollutants can be reduced drastically and quickly using available technologies."
The other co-author of the study is Chul "Eddy" Chung of the Gwangju Institute of Science and Technology in South Korea. The NOAA Climate Program Office provided additional support for the study.
Scripps Institution of Oceanography at the University of California San Diego, is one of the oldest, largest, and most important centers for global science research and education in the world. Now in its second century of discovery, the scientific scope of the institution has grown to include biological, physical, chemical, geological, geophysical, and atmospheric studies of the earth as a system. Hundreds of research programs covering a wide range of scientific areas are under way today on every continent and in every ocean. The institution has a staff of more than 1,400 and annual expenditures of approximately $195 million from federal, state, and private sources. Scripps operates oceanographic research vessels recognized worldwide for their outstanding capabilities. Equipped with innovative instruments for ocean exploration, these ships constitute mobile laboratories and observatories that serve students and researchers from institutions throughout the world. Birch Aquarium at Scripps serves as the interpretive center of the institution and showcases Scripps research and a diverse array of marine life through exhibits and programming for more than 430,000 visitors each year. Learn more at scripps.ucsd.edu and follow us at Facebook, Twitter, and Instagram.
About UC San Diego
The University of California San Diego is a student-centered, research-focused, service-oriented public institution that provides opportunity for all. Recognized as one of the top 15 research universities worldwide and born of a culture of collaboration, UC San Diego sparks discoveries that advance society, drive economic growth and positively impact the world. Our students, who learn from Nobel laureates, MacArthur Fellows and National Academy members, are committed to public service. For the sixth consecutive year, UC San Diego has been ranked first in the nation based on research, civic engagement and social mobility. We are one campus with multiple pillars of excellence, a top ten public university that is transforming lives, shaping new disciplines and advancing the frontiers of knowledge. Learn more at www.ucsd.edu.