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Atmospheric Sciences & Global Change
Research Highlights

November 2016

Cyclone Identity Crisis in the Bay of Bengal

The El Niño-La Niña weather cycle strengthens Indian Ocean spring cyclones in the north, weakens them in the south

Bay of Bengal tropical cyclone
Tropical Storm Brewing in the Bay of Bengal (inset) in November 2013, just before it was classified as a tropical cyclone. The size of the storm relative to the Bay indicates the scope of the threat these storms can pose for surrounding communities, and why it’s important to track and study them. Inside image captured by EUMESTAT, a global weather and climate satellite agency. zoomEnlarge Cyclone Satellite Image.

Results: In India's potboiling Bay of Bengal, cyclones are routine. But what turns some into epic cyclones and others a mere tropical storm? Researchers at Pacific Northwest National Laboratory and the National Oceanic and Atmospheric Administration discovered that changes in cyclone intensification rates have increased in the northern part of the Bay and decreased in the southern part, resulting in a cyclone identity crisis.

But what fuels these conditions? A north-south battle of the environment.

The research team found that changes in the Bay's cyclone environment were caused by decades-long changes in the El Niño Southern Oscillation, or ENSO, a regular pattern variation of winds and sea surface temperatures affecting the tropics and sub-tropics. The ENSO triggers an atmospheric wave that helps instigate the May-June monsoon circulation over the Bay of Bengal, which then becomes responsible for initiating a north-south dipole—opposite reactions—of tropical cyclone activity.

Why It Matters: The largest bay in the world, the Bay of Bengal, borders India, Sri Lanka, Bangladesh and Myanmar. The Bay is infamous for frequent, large tropical storms and cyclones affecting millions of coastal-region residents who are highly vulnerable to the devastating impacts of storm surge and flooding. The Bay provides a valuable case to study conditions that could help other regions around the world affected by hurricanes and cyclones. To understand the changing conditions of these annual storms, it is important to understand the factors that control cyclone activity and how it will respond to global warming in the future.

Methods: The team analyzed changes in pre-monsoon Bay of Bengal cyclone activity from 1979 to 2013 using tropical cyclone track data from the U.S. Navy's Joint Typhoon Warning Center. They computed the cyclone activity using the Genesis Potential Index (GPI), and the monsoon circulation index using the ERA-Interim atmospheric reanalysis datasets over the same period.

Using a mathematical stand-in for tropical cyclone activity, the team found that the dipole-like changes in tropical cyclone activity are consistent with those in the Bay's tropical cyclone environment.

"In the northern Bay, we found that increased humidity in the mid-troposphere and counter clockwise rotational flow below it in the lower-troposphere we call 'positive vorticity' made the environment more favorable for tropical cyclones," said Dr. Karthik Balaguru, oceanographer at PNNL and lead author of the paper. "But, in the southern Bay, increased vertical wind shear hampers cyclone growth."

Their analysis revealed that variability in the ENSO is likely responsible for the increase in May-June monsoon circulation. Caused by a decadal shift in the El Niño-like pattern of the Pacific climate called the Pacific Decadal Oscillation, or PDO, the variability consequently caused the dipole in tropical activity. Further, using 10 different coupled climate models from the Coupled Model Intercomparison Project, or CMIP5, analysis showed that most models correctly produce the link between ENSO and pre-monsoon Bay of Bengal tropical cyclone activity, verifying the robustness of the findings.

What's Next? In this study, the team used the GPI method to understand the large-scale tropical cyclone environment. They represented the role of the ocean in the GPI, through the sea surface temperature influence. The study doesn't include the effects of upper-ocean temperature stratification; therefore, future studies are needed to more accurately account for the role of the ocean.

Acknowledgments

Sponsors: The work was supported by the U.S. Department of Energy Office of Science, Office of Biological and Environmental Research for the Regional and Global Climate Modeling program and base funds from NOAA's Atlantic Oceanographic and Meteorological Laboratory

Research Team: Karthik Balaguru, L. Ruby Leung, and Jian Lu, PNNL; Gregory R Foltz, NOAA

Research Area: Climate and Earth Systems Science

Reference: Balaguru K, LR Leung, J Lu and GR Foltz. 2016. "A Meridional Dipole in Premonsoon Bay of Bengal Tropical Cyclone Activity Induced by ENSO." Journal of Geophysical Research: Atmospheres 121(12): 6954-6968. DOI: 10.1002/2016JD024936

Related Research: Tropical Tempests Take Encouragement from Environment


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In short...

In one sentence: Tropical cyclone intensification rates in the Bay of Bengal have increased in the northern Bay and decreased in the south, caused by decades-long changes in the El Niño Southern Oscillation provoking atmospheric waves, helping trigger the May-June monsoon.

In 100 characters: Decades of ENSO changes have caused north-south differences in Bay of Bengal tropical cyclone intensification rates

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