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July 2014

Tropical Tempests Take Encouragement from Environment

Scientists find 30-year trends in conditions that ripen Bay of Bengal tropical cyclones

Cyclone Nargis 2008
In 2008, when Cyclone Nargis made landfall in Myanmar (formerly Burma), it caused large devastation. The second deadliest of all time, this Bay of Bengal cyclone was whipped up by environmental conditions favorable to tropical cyclones. Scientists are working to identify the environmental trends that favor such cyclones. Image courtesy of NASA Earth Observatory. Enlarge Image.

Results: Mix some warm ocean water with atmospheric instability and you might have a recipe for a cyclone. Scientists at Pacific Northwest National Laboratory and the Atlanta Oceanographic and Meteorological Laboratory found that the intensity of post-monsoon tropical cyclones in the Bay of Bengal has increased over the 30-year period from 1981-2010. The culprit? Trending increases in certain environmental conditions that brew up these storms: increased sea surface and upper ocean temperatures and atmospheric instability. These particular changes are prominent in the eastern Bay of Bengal where the strongest tropical cyclones have traditionally formed.

The research shows a consistent progression in these conditions, suggesting that the Bay of Bengal will be the location of increasingly intense post-monsoon cyclones.

Why It Matters: The Bay of Bengal, located in the Northern Indian Ocean, is marginally conducive to tropical cyclone formation. Tropical cyclones in the Bay of Bengal, however, have devastating consequences when they hit land due to a combination of factors such as a low-lying and flat coastal terrain, a shallow ocean floor, and a high population density of surrounding countries. For example, Tropical Cyclone Nargis in 2008 was responsible for the death of nearly 138,000 people in Burma (now Myanmar), and also affected Bangladesh, India and Sri Lanka. The environmental and societal toll makes it important to understand the future of tropical cyclone activity in this region.

Methods: While a few studies in the past investigated the influence of climate phenomena such as the El Niño Southern Oscillation and the Madden-Julian Oscillation on the inter-annual variability of tropical cyclones in the post-monsoon Bay of Bengal, changes in long-term cyclone activity are less well understood.

To attack this information gap, the research team analyzed 30 years of tropical cyclone track data obtained from the U.S. Navy's Joint Typhoon Warning Center. Also, they used sea surface temperature observation data from Britain's weather and climate data organization, the UK Met Office's Hadley Center and the National Oceanographic and Atmospheric Administration (NOAA), ocean reanalysis data from the Geophysical Fluid Dynamics Laboratory at NOAA, and atmospheric reanalysis data from the European Centre for Medium-Range Weather Forecasting and NOAA's National Centers for Environmental Prediction for that 30-year period.

They found increases in sea surface temperature and upper ocean heat content made the ocean more conducive to tropical cyclone intensification, while enhanced convective instability made the atmosphere more favorable for the growth of these storms.

What's Next? Scientists are working to understand the large-scale climatic conditions responsible for the observed changes in the various environmental parameters controlling tropical cyclone development.


Sponsor: This research was supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research as part of the Regional & Global Climate Modeling program.

Research Team: Karthik Balaguru, Sourav Taraphdar, and L. Ruby Leung of PNNL; Gregory R. Foltz of Atlanta Oceanographic and Meteorological Laboratory

Research Area: Climate & Earth Systems Science

Reference: Balaguru K, S Taraphdar, LR Leung, and GR Foltz. 2014. "Increase in the Intensity of Postmonsoon Bay of Bengal Tropical Cyclones." Geophysical Research Letters 41(10):3594-3601. DOI:10.1002/2014GL060197.

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Cyclones, Hurricanes and Typhoons, Oh My

Yes, there is a difference between the three storm systems, but it's in name and location only. Each involves torrential winds and rain, and often massive destruction of anything in its path. In the Northwest Pacific, they are called typhoons. In the South Pacific and Indian Ocean, they call them cyclones. Hurricanes are located in the Atlantic and Northeast Pacific. All three have the same ingredients: warm tropical ocean water, atmospheric moisture, and light winds. If these conditions continue long enough, the winds whip up and the torrential rain begins. According to the American Meteorological Society, the word hurricane comes from the ancient peoples of the West Indies, who called this storm a "huracan" after their storm god.