Tropical Cyclone Season Peaks Twice in the Northern Indian Ocean

The first peak in the northern Indian Ocean occurs from April through June and the second peak is from September into December. Typically, the strongest cyclones that affect India occur during the fall peak. During the spring, tropical cyclones are more often carried east or northeast into Bangladesh and Myanmar.

In the summer months between these peaks, there is little tropical cyclone activity. The few storms that do form in July and August tend to be weak and move inland quickly.

The reason for this dual peak in the tropical cyclone season is the Asian Monsoon.

A monsoon is a seasonal wind shift, and this part of Asia is famous for having the most pronounced monsoon in the world. Southwest winds from the Indian Ocean feed moisture into India and surrounding countries from late spring through summer, fueling heavy rainfall at times. The other months of the year, winds are typically from the northwest off the dry continent.

Typically, right before the monsoon kicks in (April through June) and just after it fades (September into December), there is a window where vertical wind shear relaxes enough for possible strong tropical cyclone development in the northern Indian Ocean.

During these twin seasons, the Bay of Bengal and Arabian Sea have seen some of the deadliest tropical cyclones. Most of the deaths have been due to extreme storm surge flooding in coastal locations.

The Bhola Cyclone in November 1970 is estimated to have killed at least 300,000 people in Bangladesh. Cyclone Nargis in May 2008 killed more than 130,000 deaths in Myanmar.

Several other ocean basins in the world – the Atlantic, the eastern Pacific and western Pacific, – have single peaks to their tropical cyclone season. In the Atlantic, hurricane season has a single peak in activity from mid-August into early October. More than 60 percent of hurricanes and about 75 percent of major hurricanes form during this time period.