Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling
03 October 2016

Bay of Bengal May-June Cyclones Grow Stronger in the North, Weaker in the South

Scientists found that the El Niño weather pattern changed monsoon circulation in the Bay of Bengal, causing May-June cyclones to grow stronger in the north and weaker in the south.

Tropical cyclone forming and heading toward the Bay of Bengal.

The Bay of Bengal—the northeast portion of the Indian Ocean bordering India, Bangladesh, and Myanmar—experiences tropical cyclones both before and after the monsoon season. Scientists discovered that the El Niño Southern Oscillation, an anomalous weather pattern in the Pacific Ocean, is a major factor in changing pre-monsoon (May-June) cyclone activity, increasing the intensity of northern cyclones while weakening southern storms. This leads to a north-south disparity, or a meridional dipole, that may have intensified cyclones preferentially in the northern Bay in the past two decades.


Coastal populations surrounding the Bay of Bengal are highly vulnerable to the devastating impacts of storm surge and coastal flooding, with yearly death tolls often rising to hundreds of thousands. To protect against the effects of cyclones, it is important to understand how the frequency of intense storms changes with weather patterns from global warming.


Using data from 1979 to 2013, the Department of Energy scientists at Pacific Northwest National Laboratory found that pre-monsoon tropical cyclone (TC) intensity has been increasing in the northern part of the Bay of Bengal and decreasing in the southern part. They found these changes are because of changing conditions in the Bay of Bengal: increases in mid-troposphere humidity and lower-troposphere relative vorticity encourage northern TCs, but stronger vertical wind shear hampers growth in the southern Bay. In addition, they found that changes in TC environment are associated with a strengthening of the May-June monsoon circulation strength over this period. The strengthened monsoon is due to variability in the El Niño Southern Oscillation, which in turn is influenced by the Pacific Decadal Oscillation. The researchers checked their results against ten different climate models and verified the robustness of the El Niño pattern’s control over pre-monsoon Bay of Bengal TC activity through the monsoon circulation.

L. Ruby Leung
Pacific Northwest National Laboratory (PNNL)
Balaguru, K, L Leung, and GR Foltz.  2016.  "A meridional dipole in premonsoon Bay of Bengal tropical cyclone activity induced by ENSO."  Journal of Geophysical Research: Atmospheres 121: 6954-6968.